Electrical Installations

1.Introduction

An electrical installation work is complete only when the prescribed pre-commission tests are conducted on different components and the test results are found satisfactory. By carrying out the pre-commission tests, it is ensured that different components of the system have their declared specifications/characteristics and also the system is fit for safe and reliable operation. The precommission tests for different components of an electrical system are prescribed by the Bureau of Indian Standards and International Electrotechnical Commission. The manufacturers also recommend certain site tests before commissioning their equipments. The Electrical Inspector who is the authority to certify the safety of an electrical installation may insist on the owner of the installation to carry out such additional tests as may be necessary before commencing the supply. At present there is no codified book or publication covering the various standards and guidelines for the testing of various industrial equipments. In this book, it is attempted to cover the methods and procedures of all important pre-commission tests of industrial electrical equipments.In the ensuing chapters, the requirements and procedures for testing of different components of an electrical system are given under the headlines of Transformers, Motors, Generators, Switch Board Assemblies, Cables, Relays and Protective earthing. The last Chapter deals with the preparation of completion reports. The pre-commission tests are broadly classified into pre-commission checks meant for initial inspection covering visual inspection for damages, quality of workmanship, mechanical operation etc. and the pre-commission tests meant for measurement or testing of various safety parameters like insulation resistance, earth resistance, breakdown voltage of insulation, relay characteristics etc. In addition to describing the procedure for checks and tests, attempt is also made to give essential information about the specifications and characteristics of different parts and components. For measurement of insulation resistance and earth resistance, detailed description of the measuring instruments, test procedures and interpretation of the test results are given under the chapter on Transformers. While carrying out the measurement of insulation resistance and earth
resistance of other parts and components, readers are advised to refer to the chapter on Transformers also for better understanding of the instruments, test procedures and interpretation of the test results.Check lists for pre-commission checks of each component/equipment is given at the end of the respective chapter. These check lists are not exhaustive and the users have the option either to add items or delete those found not relevant. The users may modify the given checklist as warranted by the requirements of the situation.
The list of Indian Standards relevant to each chapter is given at the end of the corresponding chapter.For further details of the equipments, testing instruments, specifications, test procedures, etc, readers are advised to refer to the relevant Indian Standards also.Before carrying out the pre-commission tests it is always advisable to refer to the manufacturers’ instructions. Products of different manufactures may have special features and characteristics and it is necessary to have a good knowledge of these special features before carrying out the tests.

2.Power transformers

2.1 Introduction
Transformer is the most important unit in an electrical distribution network. All transformers are subjected to thorough tests at the manufacturer’s works before despatch to the destination of erection.Due to limitations in transport, large capacity transformers are dis-assembled into various components before despatch. At site, the transformers are re-assembled with the various components like bushings, coolers, conservator etc. and then the internal body is dried out to remove the surface moisture sticking to the paper insulation during exposure at site. As erection of transformers involve assembly of various components, pre-test inspection of transformers have greater importance than other parts of an electrical system. The following paragraphs explain the pre-test inspections/pre-commission checks and the pre-commission tests to be conducted on power transformers prior to energisation of the unit.
2.2 Recording the salient parameters
As the service life of a transformer is expected to cover very many years, it is necessary to record the salient parameters of the transformer for future reference. Rated capacity, rated voltage ratio,connection, make, maker’s serial number, year of manufacture, date of completion of erection,insulation dry out details at site etc. may be documented in a register as permanent record. It is also necessary to record the serial number, rating and make of various components like bushings, tap changer, tap changer control cubicle, cooler control cubicle, cooling fans, oil pumps, Buchholtz relay,temperature indicators, heat exchangers, oil flow meter, water flow meter, pressure gauges, oil level gauge etc. For easy reference, the details of the main body and various components may be recorded in separate pages of a register. This register will serve as a record of the service of the transformer.Details of replacement of components may also be recorded in the same register.
2.3 Pre-commission checks
Before commencing the pre-commission tests, it is necessary to visually inspect various parts,components and accessories of the transformer and also to conduct operational check for various protective devices. Check lists may be followed for the visual inspection and the operational checks so that the pre-commission checks are conducted in a systematic manner and also that no check/test is omitted. Model check lists for General checks and Functional checks are given at the end of thischapter in Appendix 2.1 and 2.2 respectively.
2.3.1 General checks
(i) General arrangement
The General arrangement of the electrical installation shall be checked for concurrence with the
scheme approved by the Department of Electrical Inspectorate. Special emphasis may be given to the
following:
· size of cables
· size of bus bars
· size of bus trunking
· size of earthing conductors
· adequacy of various clearances
· spacing between supports
· ventilation
· oil drain facilities
· fire protection walls
· fire fighting arrangements
(ii) Terminations
The transformer terminal connections may be checked for the following:
· flexibility and area of cross section of flexible connections at bushings
· clearances of live jumper connections from transformer tank and accessories
· socket size
· perfection of crimpings
· tinning of contact surfaces to prevent bimetalic action
· clearances inside cable end box
· clearances of bus bar trunking
· conformity of cable end box with the relevant IP( Ingress Protection) classification
· correctness of cable glanding and adequacy of cable gland earthing or pig tail
· support of cables at terminations and unsupported lengths
(iii) Perfection of connections
Connections to the following shall be checked for proper surface contact, seating and tightness.
· to bushings
· to the tap changer
· to earth leads
· to control and protective cables
· to thermometers
(iv) Earthing
Check the size of earthing conductors, tinning of contact surfaces, area of contact and seating,effectiveness of bolting, socketing, riveting, welding etc.for the earthing of the following:
· Duplicate earthing for neutral and body
· Main tank and top cover
· Fan motors
· Pumps
· On Load Tap Changer (OLTC) chamber
· Tap changer driving gear
· Divertor switch
· Cable glands/termination
· Marshalling box
(v) Control cable connections
Check the control cable connections between the following
· transformer accessories and marshalling box
· marshalling box and sub-station panel
· tap changer control cubicle and sub-station panel
(vi) Radiator
Check the radiator for release of air and position of valves. The valves shall be in open position.
(vii) Main conservator and OLTC conservator
Check the oil level in the main conservator and OLTC conservator . The conservator shut off valve in the Buchholtz relay pipe line shall be in open position.
(viii) Bushings
Check the oil level in the bushings if sealed bushings are used. Release air from bushings if air release plugs are provided.
(ix) Breather
Check the oil level in the oil seal of the breather. Check the colour of the silica gel in the breather .
(x) Cooler units, fans and pumps
Check fans and pumps for proper mounting. The number of fans and their position on the radiators shall be in conformity with the general arrangement drawings.
· Check the direction of rotation of cooling fans and pumps
· Check the direction of oil flow
· Check flow of water in heat -exchangers
· Measure the Insulation Resistance (IR) of fans and pumps
· Check the settings for operation of fan motors and oil pumps
· Check the cooler unit for correct indication of oil flow and setting of the thermometer
(xi) Winding Temperature Indicator (WTI) and Oil Temperature Indicator (OTI)
· Check whether thermometer pocket is filled with oil
· Check whether the connections of the CT for winding temperature indicator to the thermometer pocket is properly made as per the instructions given on the WTI terminal box.
· Check whether the contacts of WTI and OTI for alarm and trip are set at required temperatures depending upon ambient temperature and loading conditions. For oil filled transformers, the maximum permissible temperature rise above the ambient temperature is usually taken as 450C for oil and 550C for winding. In the case of cast resin transformers, the alarm contact of the winding temperature relay is usually designed to operate at 1400 C and trip contact at 1600 C for transformers upto 1000 kVA. For higher ratings, the temperatures are 1600 C and 1800C respectively.
· Calibration of the WTI/OTI may be checked with hot oil. Working of the WTI/RTD repeaters shall be checked at the control room.
(xii) Buchholtz relays
· Check the angle of mounting of the Buchholtz relay using a spirit level
· Check the floats for free movement
· Release air in the Buchholtz relay
· In the case of forced oil cooled transformers, make sure that the Buchholtz relay does not operate when the pumps are switched on
xiii) Magnetic Oil Level Gauge
Move the float level of the oil level indicator up and down between the end positions to ensure that the mechanism does not get stuck at any point. The low oil level alarm of the gauge shall be checked.
(xiv) Arcing horn gap
Check arcing horn gaps of bushings for conformity with standard values. The standard values are given below:
(xv) Tap changer
Check the sequence of operation of the tap changer for the following:
· manual operation
· local electrical operation
· remote electrical operation
· parallel operation
(xvi) General inspection
i. Heaters in cubicles, conservator, etc. shall be checked
ii. Any other alarm/trip, contacts of flow meters, differential pressure gauges, etc. shall be checked
iii. In the case of water cooled transformers, the pressure gauge readings on water and oil sides shall be checked to ensure that the water pressure is less than the oil pressure. The quantity of oil and water flow shall not be less than what is specified
iv. The angle of protection of the lightning shield provided for outdoor transformers shall be checked. The angle shall be less than 30 degrees
v. Check whether roller blocks are provided for the rollers of the transformer
2.3.2 Functional checks
After the visual inspection is complete, it is necessary to test proper functioning of various protective
relays and instruments. The following functional checks may be carried out.
1. Check the operation of the Buchholtz alarm and trip by injecting air through the test pet cock.
2. Test the OTI for alarm and trip.
3. Test the WTI for alarm and trip.
4. Check the working of the WTI / RTD (Resistance Temperature Device) repeaters at the
control room.
5. Test the OLTC – Oil surge relay for trip.
6. Check alarm for low oil level .
7. Check the REF relay for current setting
8. Check the differential relay for main and bias settings
9. Check the back up over current and earth fault relays for current and time.
10. Check the over voltage relay for voltage and time.
11. Check the instantaneous over voltage relay for voltage.
12. Check the over fluxing relay for voltage, frequency and time.
13. Check the cooler unit for
· over current setting of fans
· over current setting of oil pumps
· cooler supply failure alarm
· fan/pump trip alarm
.any mal- operation of the transformer Buchholtz relay when all the oil pumps are switched on simultaneously in forced oil cooled transformers
2.4 Pre-commissioning tests
2.4.1 Insulation Resistance test
Insulation Resistance test is the simplest and most widely used test to find out the soundness of insulation between two windings or between windings and ground. Insulation resistance is measured by means of insulation testers popularly known as ‘Megger’. The ‘Megger’ consists of a D.C power source (hand operated or electrically driven D.C generator or a battery source with electronic circuit ) and a measuring system. Microprocessor based insulation testers are also now available. The insulation test reveals the condition of the insulation inside the transformer. The insulation resistance values are affected by temperature, humidity and presence of dirt on insulators and bushings.
Selection of Insulation Testers
Insulation testers with test voltage of 500, 1000, 2500 and 5000 V are available. The recommended
ratings of the insulation testers are given below:
Factors influencing IR value
The IR value of transformers are influenced by
1. surface condition of the terminal bushing
2. quality of oil
3. quality of winding insulation
4. temperature of oil
5. duration of application and value of test voltage
Different IR values monitored in transformers
The following IR values are monitored in transformers
1. winding to ground. eg. HV to LV and earth connected together LV to HV and earth.
2. winding to winding. eg. HV to LV
3. all windings to ground .eg. HV and LV to earth.
Steps for measuring the IR
1. Shut down the transformer and disconnect the jumpers and lightning arrestors.
2. Discharge the winding capacitance.
3. Thoroughly clean all bushings
4. Short circuit the windings.
5. Guard the terminals to eliminate surface leakage over terminal bushings.
6. Record the temperature.
7. Connect the test leads (avoid joints).
8. Apply the test voltage and note the reading. The IR. value at 60 seconds after application of the test voltage is referred to as the Insulation Resistance of the transformer at the test temperature.
Minimum value of IR
The following values of IR at 30deg. C can be considered to be the minimum requirement for new oil
filled transformers.
The transformer IR values in oil drained condition will be 15 to 20 times more than in oil filled condition.
Influence of temperature on IR
IR. values decrease sharply with the rise in temperature of the oil. The following correction factors may be used for arriving at the IR value with difference in temperature.
Interpretation of Insulation Resistance value.
While interpreting IR values, importance shall be given to the variation of the values over a period of time rather than absolute values. For conclusive analysis, use only results from tests performed at identical conditions as IR values are affected by value of test voltage, temperature of oil, duration of application of voltage, humidity, extent of stress applied etc. IR values recorded over a period of time may be plotted as a curve to study the history of the insulation resistance. A curve showing a downward trend indicates a loss of IR due to unfavourable conditions such as oildeterioration, excessive moisture in paper, deterioration/damage to terminal bushings etc. A very sharp drop is a cause for concern and action shall be taken to ascertain the exact cause of insulation failure and for corrective steps.
Points to note
1. Transformers with OLTC have lower IR values when compared with transformers with off circuit tap changer.
2. Auto transformers have lower IR when compared to two winding transformers.
3. Transformer windings with graded insulation have lower IR when compared to fully
insulated windings.
4. If the non-measured winding terminals are not guarded, the megger will give a low reading.
5. Avoid meggering when the transformer is under vacuum.
2.4.2 Dielectric absorption and polarisation index tests
Dielectric absorption and polarisation index tests give a good indication of the condition of the insulation. This test is based on the comparison of absorption characteristics of good insulation versus absorption characteristics of humid or contaminated insulation.
Instruments/materials required
Motorised or electronic insulation testers of appropriate voltage
Stop watch
Logarithmic paper
Procedure for test
In this test, a test voltage is applied for an extended period of time, usually thirty minutes, using a megger of appropriate voltage. The megger readings are taken every 10 seconds for the first minute and thereafter every minute – upto 30 minutes. The procedure for measurement of IR under para 2.4.1. is followed here. Hand cranked instruments are not suitable as continuous application of voltage is not possible with such instruments. Motorised or battery operated insulation testers are used for the test. A curve is drawn showing the variation in the value of IR. against time on a logarithmic paper. The resultant curve is known as dielectric absorption curve. A typical dielectric absorption curve is shown in fig. 2.1.
Polarisation Index is the ratio of Insulation Resistance at 10 minutes to Insulation Resistance at 1 minute of application of test voltage.
Polarisation Index =Insulation Resistance at 10 minutes/Insulation Resistance at 1 minute.
Interpretation of Polarisation Index and Dielectric Absorption Curve
A steady increase in insulation resistance with continuous application of test voltage indicates that the insulation is clean and dry. Flat or ambulated curves demand reconditioning of the insulation. Polarisation index is a good appraisal of the condition of the insulation.
The following are the guidelines for evaluating the condition of transformer insulation with respect to Polarisation Index values.
Polarisation indices with respect to insulation resistance between HV and LV + earth , LV and HV+ earth , earth and HV+LV are evaluated to ascertain the real condition of the transformer insulation.
2.4.3 Two Voltage Test (Step Voltage Test)
This test is an extension of the dielectric absorption test. This has been recommended as a more conclusive indication of presence of moisture. Two separate dielectric absorption tests made at different voltages help to detect moisture in the winding. The higher test voltage should be about 4 to 5 times the lower one, (eg.2500 V and 500 V) but should not be so high as to damage the insulation. A wide spread between the two dielectric absorption curves indicates presence of moisture. If the insulation resistance value decreases substantially at a higher voltage, say more than 25 percent, it is a reasonable indication of presence of moisture in the insulation system.
2.4.4 Measurement of Tan delta
Various insulation resistance tests explained above indicate mainly the surface conditions and presence of moisture in the insulation. Measurement of loss factor, commonly referred to as tan delta reveals the internal condition of the insulation. With alternating currents, the absorption of the dielectric is intimately connected with the loss of power in the dielectric. This loss within an insulation structure is associated with the oscillation of polar molecules trying to orient themselves with the alternating electric field. Hence current flowing through the insulation leads the voltage by some angle which is slightly less than 90 degrees. This small angle between pure capacitive current and actual current represented by d (delta) is known as loss angle. The dielectric loss in an insulation is given by V2 w C tan d and hence proportional to tan d. If the insulation is perfect, the characteristic of tan delta versus the applied voltage is almost a horizontal line. If voids have crept in the insulation during manufacture or service, there will be substantial increase in tan delta with the applied voltage. Hence the absolute values of tan delta in a commercially manufactured equipment have comparatively little practical significance. But the variation in tan delta – ie. D tan delta – with respect to time is very important. The values found during maintenance testing should be compared with the initial values recorded before commissioning the equipment. A stable value of tan delta is indicative of insulation stability and small increase is indicative of normal ageing. Tan delta is
measured using tan delta measuring equipment.
2.4.5 Transformer Ratio Test
Transformer ratio test is conducted to ensure that the turns ratio tally with the name plate details and also that tap changer connections are done correctly. Ratio test is done using a transformer turns ratio tester or with voltmeters. With the turns ratio tester, the turns ratio is directly read on the tester for each tap and for each phase of the winding.The turns ratio can also be tested by applying a single phase ac voltage (approximately 230V) on the HV side and measuring the voltage on the low voltage side at all tap positions.
The results of the voltage ratio test may be recorded in tabular form as given below:
Voltage ratio test ( by voltmeter )
2.4.6 Short circuit current measurement
This test is carried out as a check for any loose contact in the tap changer, lead connections etc. In this test, all the 3 windings in the LV side of the transformer are short circuited. All contacts in the tap changer, lead connections and terminals are checked for proper contact. From a variable voltage source, a 3 phase balanced low voltage a.c supply is applied to the HV winding at rated tap and the current measured. The current measured at rated tap should tally with the calculated value of HV current at the applied voltage.
The value of HV current at the applied voltage is calculated as follows:
Repeat the test at different tap positions by lowering and raising taps. The current measured in the HV winding should tally with the calculated value of HV short circuit current.
Wide difference between the measured and calculated values of HV short circuit current is an indication of loose contact in tap changer or lead connections.
2.4.7 Measurement of Magnetising current
The magnetising current is measured to test any fault in the magnetic circuit and winding. The measured values are compared with the factory test values. A balanced three phase 415V ac supply is applied to the LV winding and the simultaneous current readings of the three phases are taken using low range a.c ammeters of the same accuracy class. For a core type transformer, the middle phase magnetising current will be approximately half that in other windings. In YyO, Dy1 and Dy11 connections, the currents in ‘u’ and ‘w’ phases will be nearly double the current in ‘v’ phase. In a Yd1 connected transformer, currents in ‘v’ and ‘w’ phases will be nearly equal and the current in ‘u’ phase more than that in ‘v’ and ‘w’ phases. In a Yd11 connection, currents in ‘u’ and ‘v’ phases will be nearly equal and the current in ‘w’ phase more than that in ‘u’ and ‘v’ phases. If the measured values widely differ from the above values or from the factory test values, there is reason to suspect some defect in the transformer core and the manufacturer may be consulted. The
measured values of magnetising current may be used as bench marks for the service life of the transformer. Sample format for recording the magnetising current is given below:
2.4.8 Test for magnetic balance
This test is done to find out the condition of stacking of core laminations, tightness of core bolts and perfection of magnetic circuit. The HV and LV sides are isolated by removing the bushing connections. A single phase supply of nearly 230V is applied to one phase of the star connected winding and the induced voltage in othertwo phases are measured. The voltage may preferably be applied on the HV winding, as applying voltage to LV winding may induce very high voltage in the HV winding. If the HV winding is connected in delta, the test may be conducted on the LV side after taking necessary precautions against accidental contact with the HV bushings.
When the voltage is applied to the middle phase, the induced voltage measured on the two other phases should be approximately equal. Where the voltage is applied to an extreme phase, the induced voltage on the middle phase should be substantially high when compared to the voltage induced in the other extreme phase. In each test, the sum of the induced voltages in two phases should be nearly equal to the applied voltage.
Tests may be carried out by connecting a series lamp (say 25 watts) at supply side to restrict higher current, if any. If the series lamp glows brightly or the induced voltage readings in different phases indicate zero or very low value or if the induced voltages show abnormal variation from the expected values, fault in the winding can be suspected.
For measuring the voltages, high impedance voltmeter like digital multimeter should be used. The test may be repeated by applying voltage to the second and third phases and measuring the induced voltages in other phases. When the magnetic circuit is balanced, there would be symmetry in the value of measured induced voltages. The measured voltages may be recorded in the sample format given below:
2.4.9 Phasor Group Test
Phasor relationship between HV and LV voltages is checked by this test. Without earthing the winding neutral points, interconnect one phase of HV winding – say 1U – to the corresponding phase of LV winding -2U and apply a balanced 3 phase low voltage to the HV winding. The phase sequence of the supply should be the same as the specified phase sequence of the transformer winding. Connections for Dy 11 and Dy1 transformers and the corrsponding vector groupings are given in figures 2.2 and 2.3. Measure the voltage between the primary and the secondary terminals. The following requirements shall be fulfilled depending on the vector group of the transformers.
If two transformers are available for test, the phasor groups can be compared easily by applying voltage from same source to identical bushings on the HV side and by measuring the voltage between identically marked terminals on the LV sides with single interconnection between either the neutrals or any one phase.
2.4.10 Test for Transformer Oil
Transformer oil is of petroleum origin and is used as a coolant and dielectric in transformers. Transformer oil in good condition and conforming to relevant standards will prevent deterioration of transformer insulation. As the transformer oil is, to some extent, exposed to air at site, it is always necessary to test the oil for various characteristics before the transformer is put to service. As the various tests for transformer oil are laboratory tests, details of these tests are beyond the scope of this book. Test procedures for various tests are given in the relevant standards of BIS, list of which are given at the end of this chapter in para 2.6.
From the point of view of field tests, what is important is the method of and precautions for collecting the transformer oil and the limit values of various characteristics. However a rough test on the moisture content of the oil can be made at site by conducting a simple test, popularly called the crackle test. In this test, a piece of steel tube of approximately 25 mm dia is closed at one end and the closed end is heated to just under red hot . Now the hot end is plunged into the oil sample with the ear close to the open end. If the oil contains large quantity of moisture, a sharp crackle will be heard.Dry oil will only sizzle.
Sampling of oil – General precautions
Since the results of the tests prescribed for transformer oil largely depend on the impurities in the sample sent for testing, it is essential to keep the oil free from any contamination. The following precautions shall be taken while collecting samples of transformer oil.
1. For collecting the sample, glass containers with glass stoppers are preferred over metal type. Wax shall not be used for sealing the containers. The stopper may be covered with a piece of cloth packed with silica gel.
2. The container may be warmed to above the ambient air temperature in order to avoid any condensation of moisture.
3. Before collecting the sample, all equipments used for handling the oil must be washed with clean transformer oil. The oil used for washing must be discarded.
4. Flexible steel hose may be used for handling the oil. Some kinds of synthetic hoses are also suitable. Ordinary rubber hose should not be used as oil dissolves the sulphur in the rubber and thereby gets contaminated. The hose used must be clean and free from dust, rust and scale.
5. The operator shall take special care to see that his hands do not come in contact with the sample or the internal surface of the container.
6. The transformer oil shall be protected against all kinds of light radiation during transportation and storage.
Sampling procedure
1. Remove the valve shield if fitted.
2. Remove all visible dirt and dust from the valve with a lintfree clean cloth.
3. Run off sufficient quantity of oil- say 1 litre – to eliminate any contaminant that might have accumulated in the drain cock.
4. Rinse the container with the oil being sampled.
5. Fill the container by allowing the oil to flow against the side of the containers to avoid air traps.
6. Close and seal the container and store the samples in a dark place.
Evaluation of test results
Three samples of oil from the top and bottom of the tank are tested for various characteristics. For transformers of capacity below 1 MVA and where very high reliability is not expected the five characteristics given in table 2.1 shall be invariably tested and it shall be ensured that the test results are within the minimum/maximum limits.
In the case of transformers where very high reliability is required and in all cases where the capacity is 1 MVA or above, the additional characteristics given in table 2.2 shall also be tested. The test results shall be within the given limits.
2.4.11 Relay Tests
All protective relays , CTs, PTs and control wiring shall be tested as explained under the chapter for protective relays. The relays shall be set to suit the operating conditions and to coordinate with other sections of the system. The relay test results shall be documented for future reference.
2.5 Commissioning
After completing all the pre-commission tests given under section 2.4, the pre-test checks under section 2.3 are redone once again. All the protective relays and circuit breakers are tested for proper working. The relay settings are kept at a low value so that the transformer will get isolated if there is any internal fault.Allow a settling time of at least 24 hours for oil and then release air from all points. Now the transformer may be test charged from the incoming side on no-load and operated for about two hours.
Observe the transformer hum for any abnormality. Any vibration or abnormal magnetising current may also be observed. After continuous operation for about two hours, isolate the transformer and check the gas operated Buchholtz relay for any gas collection. Any dissolved air or air bubble that may be collected in the Buchholtz relay may be released and the transformer charged again on noload. All connected instruments may be checked for any abnormal indication. Now gradually load the transformer to full capacity and keep it under constant observation for at least 24 hours of operation. Check the oil and winding temperature at full load and compare with factory test values. After four or five days of service, test the oil for various characteristics, especially for BDV. Any gas collection in the Buchholtz relay may also be observed. If the test results and observations are found normal, the transformer may be cleared for regular service. After the transformer is put in service for some weeks with normal working temperature, all sealed joints shall be re -tightened.
The results of the various tests shall be recorded and kept in the station as a permanent record for future reference. Details such as place of erection, date of commissioning, protection given to the transformer etc. may be furnished to the manufacturer after commissioning.
2.6 List of Indian Standards relevant to testing of Power transformers
Appendix

3. Motors

3.1 Introduction
The reliability and trouble free service of a motor depend on the superiority of its electrical and mechanical design, the accuracy and care with which the components are manufactured and assembled and on the stringency with which the motor is tested at the manufacturer’s works and at the site of installation. The operational duty requirements of motors vary from application to application.For common applications like fans, pumps, small flour mills etc. the motors may have standard characteristics. But for special applications like cranes, hoists, lifts, power press, machine tools,power station auxilliary etc. and for industries like coal mining, oil refineries, paper plants, sugar industry, cement industry, lift irrigation etc. specially designed motors with appropriate electrical and mechanical characteristics are available . The main mechanical features of such specially designed motors relate to the method of cooling (IC classification) and degree of ingress protection (IP classification). The methods of cooling and their designation are classified under I.S.6362-1971 and the degree of protection outlined in I.S. 4691-1968 . The method of cooling and their designations are given in table 3.1 below. The first characteristic numeral designates the mode of circulating the coolant and the second characteristic numeral designates the method of supplying the power necessary to circulate the coolant. Table 3.2 illustrates the capability of each degree of protection.The electrical characteristics of motors include voltage, frequency, insulation, temperature, class of duty ratings etc. The operation of nearly all driven machines can be classified into eight types of duties ranging from S1 to S8 as given in table 3.3.Testing of electric motors is a wide subject as there is large variation in the mechanical and electrical characteristics of motors depending on the type of industry, type of application etc. It is beyond the scope of this book to cover the pre-commission checks and tests of the wide variety of motors . What is attempted is to cover testing of motor installations of common applications.
3.2 Pre-commission checks
More than seventy percent of the motors employed in industries are induction motors. SCR controlled variable speed drives are also used where speed control is required. These high speed rotating dynamic machines call for testing of both the mechanical and electrical characteristics before commissioning the machine. The following pre-commission checks are recommended for industrial motors.
Mechanical Checks
· Check the alignment of the driving motor with the driven machine.
· Check the adequacy of foundation and size of foundation bolts and tightness of the motor to the foundation.
· Check the alround working clearance for safe operation and easy maintenance.
· Check whether the vibration is within permissible limits.
· Check the method of cooling and ensure that the requirement under IC classification is satisfied.
· Check the degree of protection against ingress of solid bodies/particles and liquids and ensure that the requirement under IP classification is satisfied.
· Check the bearings and air gap between stator and rotor.
Electrical checks
· Check whether facility to isolate the supply is provided within 3 metres. If push button switch is provided as isolation device, it should be provided with facility to lock.
· Check the size of cable for conformity with the approved drawing.
· Check the cable termination for
i. Proper crimping of cable conductors – check whether cable glands are of proper size and glanding is done properly.
ii. Proper seating of conductor sockets at motor terminals- check whether spring washers are used.
iii. Check the size of earthing conductor of glands. The size should be same as that used for the motor .
iv. Clamping of the cable at a distance of 45 cms from the termination.
v. Check whether cables are taken straight from the termination at the terminal box – there shall not be any tangential stress at the glands.
vi. Check whether double compression glanding is used in hazardous areas.
· Check whether duplicate earthing is provided for the body of the motor from diametrically opposite sides – check the size of the earthing conductors for adequacy.
· In hazardous areas, check whether earthing is done from inside of terminal boxes.
· Check the earth lead terminations and connections for
i. socketing of the earth lead ends (if round conductor)
ii. proper surface contact.
iii. removal of paint on the body of the equipment
iv. tinning of copper flats at terminations
v. effectiveness of bolting
· Check whether starting devices as per the approved scheme are provided for the motor.
· Check the cable connection from slip ring motors to rotor resistance cubicle.
· Check the enclosure of rotor resistance. The rotor resistance shall be inaccessible and enclosed for safety.
· Check the length of flexible connections to movable motors. The length shall be limited to 60 cms.
· Check whether flexible cables are enclosed in steel re-inforced PVC flexible conduits.
· Check the terminations for use of proper couplings and for adequate mechanical strength.
· Check the rating of capacitors for conformity with the approved scheme.
· Check the connections of motor capacitors. Capacitors shall either be connected from the motor terminal box or from the outgoing side of the motor starter. Suitable adapter boxes shall be provided for making the capacitor connection.
· Check whether the capacitor is provided with duplicate earthing.
· In the case of MV consumers, check whether ammeters of adequate rating is provided in the capacitor circuit.
· Check the utilisation category of contactors used in starters. The standardized utilisation categories of contactors for common applications of ac motors are given below:
AC-1 Non inductive or slightly inductive loads – resistance furnaces.
AC-2 Slip ring motors: starting-switching off.
AC-3 Squirrel cage motors: Starting, switching off during running.
AC-4 Squirrel cage motors: Starting, plugging, inching.
· Check the current rating of the contactors and ensure that it is suitable for the motor for which it is used.
· Check whether the settings of over current relays in starters correspond to the motor loading.
· Check whether the protective arrangements provided are as per the approved scheme.
· Check the motor protection relays for the number of elements/ protections and their settings.
· Check whether relays are tested and set to suit the operating conditions.
· Check the rating of back up fuses for conformity with the approved scheme.
3.3 Pre commission tests
3.3.1 Measurement of insulation resistance
The insulation resistance between phases, between each phase and earth and between all phases together and earth are measured using insulation testers of appropriate voltage. (For detailed description of insulation testers and procedure for testing refer para 2.4.1.) The recommended voltage ratings of insulation testers is as given below:
MV motors – 1000 V tester
HV motors – 2.5 kV/5 kV tester
The measured value of insulation resistance should be more than 1 meg ohm for M.V motors and 20 meg ohm for HV motors. If satisfactory values are not obtained, the motor may be cleaned and dried and the insulation resistance measured again. The results may be recorded for future reference.
Polarisation Index
(Please refer para 2.4.2 also)
It is necessary to measure and record the polarisation index of HV motors. Measurement of polarisation index is a conclusive test for ascertaining the quality of insulation of HV motors. The value of insulation resistance after 1 minute and 10 minutes of starting the measurement are measured using HV insulation testers of appropriate voltage. The polarisation index (P.I. value) which is the ratio of the 10 minute value to the 1 minute value shall be more than 2.0.
3.3.2 High Voltage Test
High voltage test of motors are normally done at the manufacturer’s work site. This is done only once and not repeated at the site of installation. However in some special cases, HV test is conducted as a supplementary to the test conducted at the manufacturer’s work site. The manufacturer shall be consulted before such tests. The value of the test voltage shall be limited to 75-80 % of the value prescribed for the HV test. The insulation resistance may be measured before and after the HV test and recorded for future reference.
3.3.3 Measurement of tan delta for HV motors
(Please refer para 2.4.4 also)
The dielectric loss factor ‘tan delta’ is a measure of the quality of insulation of HV motors. As a standard practice, measurement of ‘tan delta’ is carried out for motors of 5 kV and above. A low dielectric loss factor is an indication of good insulation of the motor. Measurement of tan delta is done using ‘tan delta’ measuring equipment and the results are recorded for future reference. Future test results during periodical or break down maintenance may be compared with the initial values. The variation in the value of tan delta at different periods of interval shall be within reasonable limits.
3.3.4 No load test
The no load test is conducted to compare the actual values of current, power, power factor and losses with the designed values. The test results give an indication of the condition of the magnetic circuit
and also the mechanical losses. The motor is decoupled from the driven machine and run on no load. The direction of rotation may be checked and the no load speed measured using a tachometer. Measure the current and voltage of the three phases and compare with the factory test results. There shall not be notable variation between the measured values and the factory test results.The measured values of no load current, voltage, power and power factor may be documented for future reference.
3.3.5 Open circuit voltage ratio test
In the case of slip ring motors, the open circuit voltage ratio may be measured and recorded. The value is compared with the name plate details and recorded for future reference.
3.3.6 Earth continuity test
Earth continuity of the motor frame, cable glands, capacitor body and control gears shall be tested using an earth tester or an ohm meter for very low resistance measurement. Earth continuity of various non-current carrying metal parts with the main earth bus may be checked by touching the common lead of the tester/ohm meter on the body of the respective metal parts.
3.3.7 Measurement of shaft voltage and insulation resistance of bearing
This test is conducted only for very large capacity motors and generators. Due to non-uniformity in the air gaps and magnetic circuits, a very low voltage (of the order 0.5V to 2V) may be induced in the motor/generator shaft which will circulate a current (shaft current) through the bearings and external frame work. It is necessary to prevent this shaft current as it can cause damages to the costly bearings of large capacity motors. Shaft current is prevented by insulating the bearing support. The insulation resistance between the two sides of the bearing insulation is measured using 250V/500V insulation tester. The measured value of the insulation resistance shall be more than 1 meg ohm. The shaft voltage is measured between the shaft and the bottom of the insulated bearing with a low voltage voltmeter of range 0-5 V. The normal value of shaft voltage is below 1 volt. The measured values of shaft voltage and insulation resistance of the bearing are recorded for future reference.
3.3.8 Oscilloscopic Tests
In the case of HV motors, it is necessary to record the transient inrush current at the instant of starting and the subsequent steady state starting current. In the case of large capacity HV. motors (1500 kW and above) it is also necessary to record the “Snap shot” voltage dip at the instant of starting. This is done using storage type oscilloscopes. For measuring the transient inrush current of HV motors, the secondary from the protection core of the CT (not from the metering core as the metering core may saturate when the current exceeds five times the full load current) is connected to a CIV resistor (current-into-voltage resistor). The voltage developed across the resistor is proportional to the current passed and hence the voltage can be recorded on the oscilloscope, calibrated to read the corresponding current. During transit and usage it is possible that one or more resistors of the CIV network may either get disconnected or open circuited altering the value of the CIV resistor grid. Hence before commencement of the oscilloscope recording, the value of the CIV resistor must be measured accurately with a micro ohm meter or a milli ohm meter. The oscillograms taken during precommission tests are documented for future reference and analysis.
3.3.9 Measurement of capacitor current
Where individual capacitors are installed for power factor compensation, the current drawn by the capacitors in each phase shall be measured and checked with the current rating given in the name plate of the capacitor. The value shall be within reasonable limits of ± 10%. The measured value shall be recorded for future reference. The reading of the ammeter provided in the capacitor circuit shall be checked to ensure accuracy.
3.3.10 Partial Discharge measurement
It is preferable to measure the quantity of partial discharge from windings of large capacity HV motors. Partial Discharge is measured using a partial discharge measuring probe. The measured values are recorded for future reference. The quantity of partial discharge in subsequent years gives an idea of the ageing of the insulation. In the case of substantially increased partial discharge, corrective action shall be taken by replacing coils susceptible to failures. The assessment on the condition of the insulation is made by comparing the quantity of partial discharge with the test results of previous years.
3.3.11 Testing and Setting of Relays
Smaller motors are provided with over current relays only. Generally, thermal over current relays are used. These are tested and set to correspond to the actual loading condition. Larger MV motors and HV motors are provided with composite motor protection relays of many elements. A motor is said to be satisfactorily protected if the relay has atleast five elements. The operation of all these elements are tested using a secondary injection relay test kit and the relay is set to suit the operating conditions of the motor. The motor protection relay is tested and set in consultation with the manufacturer of the particular relay. The details of the thermal characteristic of the motor may be obtained from the motor manufacturer.
3.4 Commissioning of large capacity motors
After completing the pre-commission tests, repeat the checks once again and ensure the following before starting the motor for trial run.
1. The surrounding space is clear and free from unwanted objects.
2. The rotor is free to rotate.
3. Adequate resistance is inserted in the rotor circuit in the case of slip ring motors.
4. Brushes are properly bedded to the curvature of the slip rings with correct pressure and that the brush holders are sufficiently tight on the supporting rods.
5. The space heater supply is switched off.
6. Oil has been let into the bearing in sufficient quantity (in the case of forced or combined lubricated bearing)
7. The driven equipment is ready.
Now switch on to start the motor. The trial run must be on No Load. During the trial run the temperature of bearings shall be observed every 10-15 minutes till constant temperature is attained, but in no case for less than 8-10 hours. If the temperature rises above the prescribed limits, the motor hall be stopped and the corresponding bearing checked.
If the motor runs satisfactorily on No Load for sufficiently long time, the motor may be put on Load. Immediately after the motor is put on Load, the vibrations at bearings, frame etc. shall be measured using a Vibrometer. The vibration shall be within specified limits. The readings of all the indicators on the control board shall be noted.
Motors are assigned a maximum permissible number of start which is related to the heating of the stator and rotor windings at the end of each cycle of operation. Hence too frequent trial starts can cause undesirable over heating of the motor windings and consequent damage to the insulation of the stator and rotor windings. The starting cycle shall be repeated only after complete cooling of the motor to the original condition unless the motor is designed for frequent starting. For any motor, there is a limit for the number of starts per hour which depends on the moment of inertia and other facors of the motor and dirven load. A fair idea of the state of the motor is given by the reading of various thermometers mounted on the motor. The mean temperature rise of the winding and also the maximum temperature rise of the winding and the stator core shall not exceed the specified limits.
The state of insulation of the stator winding shall be assessed by regular measurement of the insulation resistance in hot state of the winding and on comparison of the dielectric absorption coefficient K which is usually taken as the ratio of insulation resistance at 60 seconds to the value at 15 seconds.






4.Generators

4.1 Introduction
Installation of generators involves coupling of rotating machines and hence requires high degree ofengineering skill and competence, both mechanical and electrical. In order to ensure long and satisfactory life, it is necessary that the machine foundations have adequate strength, there is perfect alignment between the prime mover and the alternator and that the vibration of the machines are within permissible limits. The generator shall be installed in a cool, well ventilated place devoid of moisture, oil, dust, grease, carbon and metallic dust. It shall not be mounted in an atmosphere containing inflammable gases, corrosive acid fumes or other injurious gases unless specially made for such applications. Installation of the generator and the prime mover calls for a wide range of mechanical tests and measurements. Such tests covering the mechanical characteristics of the generator and prime mover are not included in this book. During the pre-commission checks and tests, each component of the generator installation is checked for proper installation and correct assembly. The electrical connections are verified for conformity with the approved diagram at every stage of the installation. Pre-commission tests of generators include testing of the protective devices and relays, auxilliary equipments and control circuitry. The following paragraphs explain the pre-commission checks and tests to be conducted on generator installations.
4.2 Precommission checks
General Checks
· Inspect the Generating Station/Generator room and premises for conformity with the approved site plan.
· Verify the inside layout of the Generating station/Generator room for conformity with the approved scheme.
· Check whether the building/buildings are of fire proof construction
.Check the physical layout of the generator, prime mover, control panels, auxiliary equipments etc. for conformity with the approved plan. Measure the alround clearances of equipments and control panels.
· Measure the height of the exhaust pipe/chimney and ensure that the minimum requirements prescribed for residential, commercial and industrial areas are maintained.
· Verify the fire safety aspects of fuel storage. Where separate fuel tank is provided, it shall be located outside the generator room with adequate fire protection.
· Inspect the generator foundation for proper construction.
· Check the mounting of the prime mover and generator installations for anti-vibration arrangements.
· Check the acoustic control arrangements of the Generator/Generator room. The sound level in residential and commercial areas shall be within permissible limits.
· Check the acoustic barriers provided between the Generator room and control room.
Sound level in the control room shall be within permissible limits.
· Check the acoustic barriers between the generator room and plant rooms.
· Check whether the Generator floor/Generator room is visible from the control room/switch room. There should be standard arrangement for visibility by providing glass panels.
· Check the arrangement for ventilation of the Generator room. There should be provision for free flow of air.
· In the case of container type generator sets, check the facilities provided for maintenance and heat dissipation.
· In the case of generator installations above ground floor, check the facilities for speedy oil drain and remote engine stopping.
· Check the fire fighting arrangements and devices in the Generator room and ensure their adequacy.
Electrical checks
· Verify all electrical connections to ensure that they are as per the approved electrical schematic diagram.
· Check the tightness of electrical connections particularly those associated with heavy currents.
· Check whether the armature and field connections are made as per the terminal marks.
· Check the connections of excitation system for correctness and tightness.
· Inspect the slip rings (where provided) for smooth surface. It should be free from rough spots
. Inspect the brush holders to ensure that they are sufficiently tight on the supporting rod and that the brushes are properly bedded to the curvature of the slip rings with correct pressure.
· In brushless alternators, check the rotor connections.
· Check the air gap between the rotor and the stator. The air gap shall be uniform and there shall not be any rubbing between the rotating and stationary parts.
· Check the connections to the automatic voltage regulator (AVR). The connections should be as per the diagram of connections and the instructions of the supplier of AVR.
· Check the power cables provided for the generator for conductor size, voltage grade, and other specifications.
· Check the power cable connections at the generator terminal box and the control panel for proper clamping and termination.
· Check the control cable connections for proper termination and glanding.
· Check whether the neutral formation and neutral earthing have been done correctly.
· Check whether an easily detachable neutral link is provided in the neutral circuit.
· Check the Neutral Grounding Resistor (NGR)/Neutral Grounding Transformer (NGT). The specifications should be as per the approved scheme.
· Check the voltage grade of the cable used for the NGR/NGT. The voltage grade of the cable used for NGR/NGT shall be same as that for unearthed systems. The supply and neutral side cables for a 11 kV generator with NGR shall have a voltage grade of 11 kV/11 kV.
· Where neutral switching is provided with NGR/NGT, check whether the neutral switching gear associated with the NGR/NGT is rated for the same voltage as that of the generator.
· Check whether remote switching facility is provided for the neutral switching device.
· Check whether the frame work of the NGR is insulated from its enclosure for the same voltage as that of the generator.
· Check the connections of electronic governors, if provided. The connections should be as per the diagram supplied by the manufacturer.
· Inspect the control panels for the following:
(a) Layout of panels, alround clearances, breaker rating etc. for conformity with the
approved scheme.
(b) Proper connections of power cables (Please refer to chapter 6 on cables also
(c) Internal arrangement of panels, control wiring, control cable trays, bunching of control cables, segregation of potential and current leads, working of control contactors etc. (please refer to chapter 5 on Switch Board Assemblies also).
(d) Proper working of relays ( Please refer to chapter 7 on Relays also ).
(e) Proper working of meters. Separate fuses shall be provided for the pressure coil leads of the energy meter so that testing of the energy meter is possible by removing the fuses ( Please refer para 4.3.6 ).
(f) Finish and neatness of the panel assembly.
(g) Working clearance inside and around the panel assembly.
(h) Clearances of live parts.
(i) Facility for maintenance and testing.
(j) Safety aspects such as finger proofing, shrouding, earthing of panel doors etc.
· Check the various protection schemes provided for the generator. The protection arrangements shall be as per the approved scheme.
· Check all the embedded temperature detectors and indicating instruments to ensure that they are connected properly and are in working condition.
· Check whether window annunciation arrangement is provided for both the prime mover and generator protection schemes.
· Check sounding of hooter and visual indication of window annunciators.
· Check the secondary circuits of instrument transformers and measuring instruments for proper connections.
· Check the manual interlock for trouble free and safe operation.
· Check whether electrically and mechanically interlocked contactors are used in AMF panels.
· Check the interlocking between generator and grid supplies. Check whether 4 pole breakers/switches are provided for the interlocking.
· Where generators are intended for parallel operation, check the arrangements for parallel operation.
· Check whether electrically operated remote switching breakers are used for synchronisation.
· Check whether the speed control and excitation control arrangements are suitable for synchronisation.
· Check whether remote control is available at the synchronisation panel.
· Where more than 2 generators are to be synchronised, check whether the synchronoscope is visible from various synchronisation panels.
· Where generators are synchronised directly without using generator transformers, check the neutral switching of generators. Only the neutral of the largest capacity generator shall be connected to the system and earthed. Neutrals of other generators shall be kept
floating.
· Check the condition of the starting battery and the battery of the protective relays.
· Check the earthing of the neutral and body of the generator, generator transformer and main switchgears. The earth connections shall be done direct to the earthing system/earth mat.
· In the case of HV generators
(i) Check whether anti – condensation heaters are provided.
(ii) Check the thermostats and supply arrangement for the space heaters.
· Check temperature indicators such as bearing temperature indicator, cooling water temperature indicator etc. for proper functioning.
Auxiliaries of large capacity generators
· Inspect the cooling system- examine the heat exchanger and cooling water pump – check
the temperature and pressure settings- examine the temperature and pressure gauges and
their settings.
· Check the capacity of the cooling water storage to meet emergency situations.
· Check the provisions for emergency cooling water supply.
· Check whether spare cooling water pumps are provided.
· Check the cooling water system by operating the motors and checking the pressure gauges and their settings.
· Check the operation of the lubricating systems. Check whether a spare bearing oil pump is provided.
· In the case of servo motor type governors, check the governor oil pumping system for proper functioning.
· Check the adequacy of redundancy of governor oil pumps.
· Check all pressure pipes and return pipes for approved colouring.
· In the case of petroleum based fuels, check the pumping system. Flame proof pumping arrangements shall be used wher ever required.
· Check the complete installation of the Station Auxiliary Transformer and connected switch boards, cables, motors and earthing (Please refer the respective chapters for detailed checks and tests).
· Check the dc supply arrangement of the generating station for the following:
a) Whether station type batteries are used.
b) Whether the battery room is ensured not to communicate directly to the generator and control rooms.
c) Whether emergency dc lighting is provided in the generator floor, control room, battery room etc.
· Check the adequacy of fire fighting arrangements. Check the operation of smoke detectors and working of fire fighting system.
4.3 Pre-Commission tests
4.3.1 Measurement of insulation resistance of Armature windings
The insulation resistance between phases, between each phase and earth and between all phases together and earth are measured using insulation testers of appropriate voltage. Detailed description of insulation testers and procedure for testing is given in para 2.4.1. The recommended voltage ratings of insulation testers are as given below:
MV Generators -1000 V tester
HV Generators – 2.5 kV/5 kV tester
If the measured value of insulation resistance is 1 meg ohm or less, it is an indication of presence of moisture and condensation of vapour in the winding. The windings should therefore be dried out as per the instructions of the manufacture. The measured values of insulation resistance may be recorded for future reference.It may be noted that the AVR connections shall be removed before measuring the insulation resistance. The insulation resistance of HV generators shall be more than 20 meg ohms or as recommended by the manufacturer.
Polarisation Index
(Please refer para 2.4.2 also)
It is necessary to measure and record the polarisation index of HV generators. Measurement of polarisation index is a conclusive test for ascertaining the quality of insulation of HV generators. The value of insulation resistance after 1 minute and 10 minutes of starting the measurement are measured using HV insulation testers of appropriate voltage. The polarisation index (PI value) which is the ratio of the 10 minute value to the1 minute value shall be more than 2.
4.3.2 Insulation resistance measurement of Field winding
Insulation resistance of the field winding is measured using a 500 V insulation tester. The insulation value shall be more than 1 meg ohm. Care shall be taken to measure the insulation resistances on both sides of the rectifier unit in the case of brushless alternators. Insulation resistances of the excitation transformer shall be measured and verified in the case of static excitation systems. Insulation resistances of excitors and pilot excitors may be measured and verified where these are installed.
4.3.3 High Voltage Test
High Voltage test of generators is normally done only once at the manufacturer’s work site and not repeated at the site of installation. However, in some special cases, HV test is conducted as a supplementary to the test conducted at the manufacturer’s work site. Value of the test voltage shall be limited to 75-80% of the test voltage prescribed for the HV test. The manufacturer shall be consulted. before conducting the HV test. The insulation resistance may be measured before and after the HV test and the measured values recorded for future reference.
4.3.4 Measurement of tan delta
(Phase refer para 2.4.4 also)
The dielectric loss factor ‘tan delta’ is a measure of the quality of insulation of the generator. As a standard practice, measurement of tan delta is carried out for HV generators. A low value of tan delta is an indication of good insulation of the generator. Measurement of tan delta is done using tan delta measuring equipment and the results are recorded for future reference. The variation in the value of tan delta at different periods of interval shall be within reasonable limits.
4.3.5 Measurement of shaft voltage and bearing insulation resistance
A low voltage of the order 0.5V to 2 V may be induced in the generator shaft due to non- uniformity in the air gaps and magnetic circuits. This voltage will circulate a current (shaft current) through the bearings and external frame work. This shaft current can damage the costly bearings of the generator. Shaft current is prevented by insulating the bearing support. The insulation between the two sides of the bearing insulation is` measured using a 250V/500V insulation tester. The value shall be more than 1 meg ohm. The shaft voltage is measured between the shaft and the bottom of the insulated bearing with a low range voltmeter (0 – 5V). The normal value of shaft voltage is below 1 volt. Measurement of shaft voltage and bearing insulation resistance is usually done for large capacity generators. The measured values are recorded for future reference.
4.3.6 Test for the direction of rotation and phase sequence
The standard phase sequence of generators is u – v – w for clock wise rotation at the driven end. However the phase sequence and direction of rotation of the machine will be given in the name plate. When generators are operated in parallel, the phase sequence of all the machines shall be the same. Phase sequence is tested using a phase sequence indicator.
4.3.7 Test for correctness of energy meter connection
Correctness of the energy meter connections is checked to ensure that the energy meter records the quantum of energy correctly. This is done by checking the polarities of the current and pressure coil connections. Separate control fuses are provided in the pressure coil leads of the energy meter for the
above purpose.
Remove fuses from all the three pressure coil circuits of the energy meter. Load the generator balanced to about 25 to 30% of the rated load and ensure that the energy meter does not record any consumption. Now insert the control fuse in one of the phases, say “u” phase, with the fuses in the other two phases kept removed. Ensure that the disc rotates in the right direction or an electronic meter gives a positively increasing reading. Remove the fuse from “u” phase and insert in “v” phase and check whether the disc rotates in the positive direction. Repeat the test with the fuse in “w” phase. When the pressure coil and current coil connections are correct the disc will rotate in the positive direction in all the three cases or an electronic meter will give a positively increasing reading. Now insert the fuses one by one and ensure that there is increase in the speed of rotation of the disc at the instant of insertion of each fuse.
4.3.8 Measurement of partial discharge
In the case of large capacity HV generators with cast resin insulation, it is preferable to measure and record the partial discharge from windings. Partial discharge is measured using a partial discharge measuring probe. The quantity of partial discharge gives an indication of the internal condition of the insulation. In the case of substantially high partial discharge, corrective action shall be taken by replacing the windings susceptible to failures.The quantity of partial discharge in subsequent years gives an idea of ageing of the insulation. Assessment of the condition of the insulation is made by comparison of the measured value of partial discharge with that of previous years.
4.4 Commissioning of generators
After completing the pre-commission tests, the pre-commission checks shall once again be conducted to ensure that the generator is fit for starting. The auxiliary motors of the cooling system, lubrication system and ventilation system are started. Check the readings of the pressure gauges, thermometers and flow meters and ensure that the values correspond to the recommendation of the manufacturer. Before starting the generator, see that both the main and field breakers are open. Generators are always started with these breakers open.
Run the unit slowly and note the direction of rotation. Check for any mechanical rubbing or undue vibration and noise. If any abnormality is noticed, the generator should be stopped immediately and the causes closely investigated and rectified. When the generator reaches its rated speed, mechanical balancing of the machine may be ascertained by measuring vibrations at the bearing, frame etc. A vibrometer is used to measure the vibration. The vibration shall be within the limits specified in I.S.11727.
Carefully examine whether there is any oil leakage from bearings. Oil should not get sprayed towards slip rings.
Check the brush holders for tightness and position of brushes on slip rings. Measure the steady state temperature of the bearing and lubricating oil at the rated speed. If the oil temperature is found to be more than 70o C , remedial steps should be taken.
The voltage can now be generated across the armature terminals by closing the field breaker. If the generator voltage does not build up, field flashing across the field terminals by means of a battery is necessary. Check the phase sequence of the generated voltage and adjust the generator voltage to the required value by means of the excitation control. Run the generator on no load at least for eight hours and see that the lubricating and bearing oil temperatures are less than the values prescribed by the manufacturer for the machine running on no load. Examine the cooling water system for pressure, flow and temperature.
The generator is now ready for loading. The main circuit breaker may be closed and the generator gradually loaded in steps of 25, 50 and 100 per cent of full load. When the machine is delivering various loads, repeat the checks for vibration, noise, oil leakage, position of brushes, temperature etc. After eight hours of running at full load, the generator may be shut down and the tightness of the frame and mounting bolts checked. Any unusual localised heating in the windings and bearings may also be checked. Load throw off test shall also be conducted for generators intended for parallel operation for testing the system and generator stability.
Once the trial run is completed satisfactorily, the generator is ready to be certified fit for regular service. The date of commissioning, name of the commissioning engineer, details of checks and tests conducted, the results thereof etc. may be recorded. The date of commissioning may be intimated to the manufacturer.

 


5. Switch Board Assemblies

5.1 Introduction
Switching centres are the nerve centres of any electrical distribution system. These centres generally comprise of power control devices, bus chambers, switchgear chambers, cable alleys, control bus chamber, control wiring and protective relays and devices. The switching centres in industrial power distribution are usually classified as
(i) Power Control Centres (PCC)
(ii) Motor Control Centres (MCC)
(iii)Power and Motor Control Centres(PMCC)
(iv) Fuse Distribution Board (FDB)
In small installations, the switching centres are designated as Main Switch Board (MSB), Sub Switch Boards (SSBs) and Fuse Distribution Boards (FDBs).High voltage switching centres are generally classified according to the level of voltage whereas medium voltage switching centres are classified on the basis of current rating. In large industrial plants, power may be availed at extra high voltage and the internal distribution done at high and medium voltages (11 kV, 6.6 kV, 3.3 kV and 415 V). Very large capacity plants have internal distributions at 33 kV and 22 kV. The present day practice is to have compact cubicle type factory built switch board assemblies as switching centres. These assemblies, commonly termed as factory built assemblies are intended for indoor use but outdoor versions are also available. The switching devices together with associated control, measuring, signalling, protective and regulating equipments are assembled with all the internal electrical and mechanical interconnections under the responsibility of the manufacturer. The manufacturer shall give all informations regarding the electrical characteristics of the assembly and also necessary markings inside the assembly to identify individual circuits and protective devices.
The instructions for installation, operation and maintenance are also furnished by the manufacturer.Before transporting the switch board assemblies to the place of installation, the manufacturer is required to conduct certain routine and type tests at the manufacturer’s work site to verify the electrical characteristics and also to verify compliance with the requirements laid down in various standards. However, before commissioning the switch board installation, it is necessary to carryout certain pre-commission checks and tests in order to ensure trouble free service. The following paragraphs explain the pre-commission checks and tests to be carried out on switch board assemblies prior to energisation.
5.2 Pre-Commission Checks
Before carrying out pre-commission tests on Switch Boards, it is necessary to ensure that the switch board has been assembled as per the general assembly lay out and erected as per the physical lay out approved by the Department of Electrical Inspectorate. It is also necessary to ensure that there are no manufacturing defects in the assembly. A thorough visual inspection of the assembly is conducted to locate any deviation from the approved scheme and also to detect any defect in the assembly. Any defect detected during such visual inspection shall be rectified before carrying out the precommission tests. The pre-commission checks may be carried out in a systematic manner covering all the components of the assembly. Recommended format of check list is given in Appendix 5.1.
(i) General Checks
· Measure the clearances on all sides of the Switch Board assembly for conformity with the approved scheme.
· Measure the dimensions of various compartments, cable alleys and busbar chambers to ensure conformity with the approved scheme.
· Check the width and depth of trenchs, positioning of the switch boards over the trenches, facility for easy replacement of cables and alteration.
· Check positions of cable trays and racks for OH cabling.
· Inspect all compartments, cable alleys and bus bar chambers. Make sure that no tools and foreign equipments are present.
· Check whether the inside surfaces especially insulation surfaces are clean and dry.
· Check whether all unused gland openings have been blocked with dummy plates/bushes to prevent vermin entry and ingress of dust.
· Check whether all temporary connections made during the installation have been removed.
· Check meters, relays and other components for visual damage during transport and
installation.
· Check whether the switch board assembly conforms to the Ingress Protection (IP)classification recommended for the particular location.
· Check whether adequate number of fire fighting appliances are provided in the room
(ii) Switches and Switch gears
· Check the circuit breakers, switches and switch gears for proper mounting.
· Check the rating of circuit breakers, switches, switch gears and HRC fuses for conformity with the approved scheme.
· Operate all switching mechanisms at least five times to ensure correct operation and alignment . Check the direction of operation and switching position.
· Check the interlocking and pad locking of withdrawable parts to ensure that they can be withdrawn / reinserted only after the main circuit has been interrupted.
· Check the alignment of draw out switch gears and assemblies.
· Check whether the incomer live terminals at switchgears are shrouded with 3 mm. thick SMC, DMC, FRP or acrylic sheets to prevent accidental contact when switchgear chamber is opened and maintenance work attended.
(iii) Doors
· Check the doors for any dimensional defect, deflection , hanging , non-uniform gap,rubbing etc.
· Check the door interlocking. The doors shall not open when the handle is in “on” position.
· Check the quality and type of door handles.
· Check the quality and type of door hinges.
· Check whether open live terminals at the back side of doors in which instruments are mounted have been covered or made finger- proof. If covered, the shrouding shall be minimum 3 mm thick SMC( Sheet Moulded Compound), DMC( Dough Moulded Compound), FRP (Fibre Reinforced Plastic) or acrylic sheets. The shrouding should properly cover live accessible parts when the doors are opened.
(iv) Bus bars
· Check the number of runs, cross sectional area and the spacing between runs of bus bars for conformity with the approved scheme.
· Check the tightness and perfection of bus bar connections and section joints.
· Check the tightness and perfection of connections on switches, contactors and accessories.
· Check the clearances between phases and between phases and earth through out the bus length, tap offs and droppers. The minimum clearance between phases shall be 19 mm for MV and 120 mm for 11 kV and between phases and earth 16 mm for MV and 120 mm for 11 kV.
· Check the number of tap offs from a point. There shall be only one tap off from a bolted tee off.
· Check all bolt projections and ensure that minimum clearances are not infringed.
· Check the dimensions and spacing of bus bar supports for conformity with the approved scheme.
· Check the tightness of bus bar supports.
(v) Power Cables
· Verify the size, number of runs and voltage grade of cables for conformity with the approved scheme. The minimum voltage grade shall be the recommended insulation voltage of the concerned supply voltage level.
· Check the socketing of power cables. Ensure that correct lug sizes are used and that
crimping is proper and no strand is cut off at socket.
· Check for proper seatings, tightness and terminations of conductors.
· Check whether insulated cable cores rest against bare live parts or on sharp edges of entry holes / openings. Beedings / grommets shall be provided at such entry holes / openings.
· Check whether cable cores are connected according to R Y B markings.
· Check whether power cables are adequately segregated from control and metering cables.
· Check whether cables inside the cable alley are properly dressed and clamped.
· Check all cable bends for excessive bending radius and stress.
· Check whether name / identification tags are provided for the cables.
· Check whether the ratings of terminal blocks are in conformity with the approved scheme and the current ratings of feeders.
· Check whether cable alley compartmentalisation using metallic barriers is provided between live sections.-All live terminals should be shrouded.
· Check cable glandings at gland plates and earthing of glands. Size of gland earthing conductor is checked in relation to the size of cable.
(vi) Instrumentation and Control wiring
· Where control supply is provided with a separate bus, check whether the bus chamber is totally segregated from the power bus chamber and tap offs taken as per standard practice.
· Check whether there is proper segregation between power, measurement and control cables.
· Check whether there is proper segregation between power, measurement and control cable terminal blocks.
· Check whether the polarity of the terminal blocks for external d.c supply (if provided) has been marked.
· Check the size of the control and measurement cables. Copper cables of minimum 2.5 sq.mm for current leads and 1.5 sq.mm for voltage leads shall be used.
· Check for correct connections of measurement CT leads to meters and protection CT leads to relays.
· Check whether the connections are made as per ferrule numbers.
· Check whether wiring troughs are fixed properly and cables taken in a neat manner.
· Check whether crimping portions are covered with sleeves.
· Check whether there are any intervening splices or soldered joints. Insulated cables shall not have intervening splices or soldered joints.
· Check whether bunching of cables is done in a neat manner.
· Check whether there is any chance of supply leads to apparatus and measuring instruments mounted on covers and doors getting damaged due to movement of the said covers or doors.
· Check for proper seating, tightness and termination of conductors. Usually only one conductor is connected to a terminal unless the terminals are designed for multiple connections.
· Check the identification of conductors by colour. If identified by colour, the PE conductor shall be green and yellow and the neutral black in colour.
· Check whether recommended alphabets and numbers are used for ferrule numbering.
. Check whether the ferrule numbering corresponds to the control wiring diagram.
· Check whether voltage leads and current leads are segregated and bunched separately.
· Check whether beedings / grommets are provided at entry holes and chamber cross over points through which control cables are taken.
· Check whether DIN rail mounted terminal blocks are with finger proof terminals.
(vii) Switch Board chambers and compartments
· Check whether primary compartmentalised bus chamber is provided in the switch board assembly. The size of the bus chamber should be as per the approved scheme.
· Check the thickness of the GI sheets used for the assembly. Minimum 2 mm thickness is required.
· Check switch gear chambers and measure the size of the enclosures to verify the adequacy.
· Check the clearances and creepage distances of various apparatuses forming part of the switch board assembly.
· Check the clearances and creepage distances of bare live conductors and terminations of bus bars, connections between apparatus, cable lugs etc.
· Check the provisions for cooling and ventilation.
· Check the provisions for heating if provided. Check thermostats and space heaters for proper connection and placement.
· Check the partitions and barriers for
(a) protection against contact with live parts in adjacent units.
(b) protection against accidental initiation of arcs.
(c) protection against the passage of solid foreign bodies from one unit to adjacent units.
· Check whether shrouding is provided as per requirement.
· Check whether entry holes / openings between bus chambers and switch gear chambers are properly covered using 6 mm SMC, DMC of FRP sheets. Only the minimum opening necessary to pass rigid droppers need be given.
· Check the RYB coloured sleeving of bus bars and dropper connections . Sleeves may be provided for the full length except for bolted joints. Moulded joint covers may be used at bolted joints of HV busbars.
(viii)Earthing
· Check the size of the earth bus for conformity with the approved scheme.
· Check whether earth connections are provided for switching apparatus and the doors on which components have been mounted.
· Check whether all non-current carrying metal parts / cases of switches and control gears have been earthed using vertical runs of earth strip and horizontal earth bus.
· Check whether continuous earthing is provided for bus bar trunking and connected to the earth bus at both sides. Check the size of earthing conductor of bus trunking.
· Check whether earth buses of sections of assemblies disconnected for transportation have been reconnected.
· Check all joints of earth strip connections for tinning of connection surfaces, seating and workmanship.
(ix) Identification labelling, appearance and workmanship.
· Check for identification of cables and marking of direction of power flow at cable entry and take off.
· Check for labelling of cable sizes at appropriate places.
· Check for labelling of fuse ratings, CT / PT specifications etc.
· In the case of floor mounted assemblies, measure the height of indicating instruments. The maximum and minimum allowable heights are 2 metres and 0.3 metre respectively.
· Measure the height of operating devices such as handles, push button etc. The maximum allowable height is 1.8 metre.
· Check for any deflection, defect in dimension, defect in levelling etc. for doors, panels, top and bottom covers etc.
· Check whether neoprene rubber/sponge rubber beedings have been provided for doors and panels.
· Check the quality of welding, grinding, sanding and filing.
· Check the quality of paint, paint peel off, scratches etc.
· Check whether electrical grade rubber mats are provided in front of all switch boards.
5.3 Pre-commission tests
After completing the installation work and after carrying out the pre-commission checks, switch board assemblies shall be subjected to the following tests irrespective of the fact that routine tests were conducted on the switch board at the manufacturer’s works.
1. Insulation resistance test
2. Power frequency high voltage withstand test
3. Functional/operational test
4. Test for protective arrangements
5.3.1 Insulation resistance test
The insulation resistance of switch gear assemblies and control gears are measured using insulation testers of appropriate voltage rating. The rating of the insulation tester depends on the insulation voltage level of the installation. The recommended ratings of insulation testers are given below:
The reading of the insulation tester after one minute of application of the test voltage gives the insulation resistance of the switch Board. The minimum value of insulation resistance for LV and MV switch gears and switch board assemblies shall be more than 1 meg ohm at the working temperature. For HV switch gears and assemblies IR of 200 meg ohms at 30 0 C is considered as satisfactory value.
The insulation resistance is measured between phases, each phase and earth and between neutral and earth. The measured values shall be more than the recommended values and recorded for future reference. The ambient temperature at the time of insulation resistance measurement shall also be recorded.
Switch Board assemblies are subjected to high voltage test only if the insulation resistance is more than the above values After the high voltage test, the insulation resistance is again measured and it shall be ensured that the value is satisfactory. The values of insulation resistance before and after the high voltage test shall be recorded for future reference.
5.3.2 Power frequency high voltage withstand test
The high voltage withstand test is conducted to verify the dielectric strength of parts of the Switch Board assembly. In this test, test voltage as per table 5.1 below is applied between
(i) all live parts and the frame of the switch board assembly
(ii) each pole and all other poles connected to the frame of the assembly
The high voltage tests are conducted using HV test sets of adequate rating. Standard high voltage test sets of different ratings are available. The test voltage shall be practically sinusoidal with frequency between 45 Hz and 65 Hz.
For control circuits and auxilliary circuits which are unsuitable for connections to the main circuit, the value of the HV test voltage shall be as follows:
The value of the test voltage at the moment of application shall not exceed 50 per cent of the values given above. The test voltage is now increased steadily to the specified value within a few seconds. The test voltage is maintained for one minute. There shall not be any puncture or flash over.The high voltage test shall be conducted only if the insulation resistance measured as per para 5.3.1 is found satisfactory.
5.3.3 Functional/operational test
All operations of switch gears and connected assemblies shall be done for a minimum of five times to ensure smooth and trouble free operation. Switching operation including draw out may be done in the ‘service’, ‘test’ and ‘isolate’ positions to verify the correct alignment of withdrawable parts and cubicles and interlocked shutters and switch gears. It shall be ensured that every part of the assembly function satisfactorily.
5.3.4 Test for protective arrangements
It is necessary to ensure that there is no deviation from the approved protection scheme and also that the different components of the protective system operate in the correct sequence. The following tests may be conducted on the protection arrangements.
1. Check the settings of relays and releases for the intended co-ordinated operation. The relay and release settings shall correspond to actual operating conditions.
2. Check the fuse ratings to confirm that there is marginal overload protection in addition to short circuit protection.
3. Test all ELCBs to ensure fool proof operation.
4. Test the emergency safety switches provided in High Rise Buildings, Neon sign circuits etc. to ensure the envisaged protection.
5. Test all the relays and releases manually and see that all control gears are operative.
6. Check whether the required a.c and d.c supply is available at the supply points of the control supply. Check the condition of the battery and battery charger.
7. Test all the relays for various characteristics ( see chapter 8 on Relays ).
8. Test control wiring circuits for correctness of connections.

6. Cables

6.1 Introduction
Cables for power distribution are mainly characterised by the material of the conductor, voltage grade, type of insulation, type of sheathing, type of armouring etc. Cables are designed to safely carry the current it is required to carry under normal operating conditions as well as abnormal conditions. To carry a current safely means (1) the temperature rise and voltage drop should be within the prescribed limits and (2) the insulation of the cable should withstand the normal operating voltage as well as higher voltages that may arise under abnormal conditions. In some situations, the cable may encounter with severe mechanical stress or highly contaminated environment in addition to the electrical stress. Hence it is necessary to ensure by means of pre-test inspections and various field tests that the cable is capable of withstanding the various types of stresses it may come across during its service life.
6.2 Cable installation plan
Cables are usually laid according to a plan prepared for the purpose. However, after completing the cable laying, it is necessary to prepare a revised plan incorporating the changes made during execution of the work. This plan will help in locating faults and in carrying out tests during the service life of the cable. The cable installation plan should contain the following details.
(1) Location of cables and joints with reference to some fixed landmarks like buildings, boundary stones etc.
(2) Type of cables, cross sectional area, rated voltage and details of cable core.
(3) Length of cables – between joints and ends.
(4) Year and month of laying.
(5) Date of making joints.
(6) Results of measurements and tests carried out on the cable.
While designating the cable the following code shall be used.
6.3 Pre-Commission checks
The following points may be checked as pre-test verification before carrying out the pre-commission tests.
(i) Check for concurrence with the approval issued by the Department of Electrical Inspectorate for
a) the type/designation of cables.
b) the cross sectional area of the conductors.
c) length of cables.
d) voltage grade of cables.
e) direction of power flow.
(ii) Check the cable terminations and cable entry boxes for
a) proper crimping, seating, surface contact etc. of core connections.
b) proper selection of entry boxes to avoid bending.
c) straightness of cable for a minimum length of 45 cms.
d) clamping of cables at 45 cms. from cable termination and subsequent intervals.
(iii) Check the cable gland for
a) proper size and fixing of the gland.
b) size of the earthing conductor.
c) size of earthing clamps.
d) proper surface contact of the earth clamps and for continuity of earth connection.
e) tinning of contact surfaces.
f) perfection of fixing the armour for continuity.
g) double compression glands in hazardous locations.
(iv) Check the cable bends for minimum bending radius as per Table 6.1 and clamping of the cable at both ends of the bend.
(v) Check the cable route for
a) enroute cable clamping.
b) cable dressing in trays and racks.
c) cable identification tags. In the case of UG (Under Ground) cables, the identification tags shall preferably be at intervals of 10 metres minimum.
d) mechanical protection at cable crossings and take off from ground.
(vi) In the case of High Rise Buildings, examine
a) the mechanical protection at each floor crossing.
b) fire prevention barrier in cable ducts at each floor crossing.
(vii) Check for segregation of cables of different voltages.
(viii) Check for segregation of control and power cables.
(ix) Examine the cable alleys, cable trays and racks for
a) segregation of cables
b) cable alley compartmentalisation
c) support in the alleys, trays and racks
d) running of earth continuity strips, adequacy of size of the strip, tap off, joints etc.
(x) Check for the mechanical protection of cables rising from
a) floors
b) trenches
c) ground
(xi) Check for the manufacturer’s identification throughout the length of the cable. Manufacturer’s name or trade mark, the voltage grade and year of manufacture shall be indented, printed or embossed on the cable.
(xii)Check whether separate racks are provided for control and power cables of different voltages in cable trenches of OH (Over Head) trays. Check whether sufficient moving space is provided between racks and side walls of the trench.
(xiii)Check whether the bottom most rack is used for the highest voltage cable and upper racks for lower voltages. Check whether d.c and a.c control cables are segregated.
(xiv)Check whether fire resistant Low Smoke Cables are used in hazardous areas.
(xv) Check whether cable trenches are filled with dry sand in hazardous locations
6.4 Pre-commission tests
6.4.1 Insulation Resistance Test
All new cables shall be tested for insulation resistance before making joints / connections. IR of cables is measured by means of insulation resistance testers. It is preferable to use motorised insulation testers for measuring the IR of cables. The recommended ratings of insulation tester is given below.
Insulation Resistance values to be measured for cables.
In the case of non-screened cables, the IR of each core is measured against all other cores and armour/metal sheath connected to earth. With screened constructions, the IR of each core is measured against all other cores and the metal screen connected to earth. IR of cable is measured after application of test voltage for 60 seconds. The minimum value of the insulation resistance depends on the type of construction of the cable and hence it is necessary to compare the test readings with the factory test results Reasonable variation from the factory test results need not be a cause for concern as the insulation resistance value varies with parameters like length of the cable and temperature. The above variation is more predominant in PVC cables. Once satisfactory results are obtained, cable jointing and termination can be made. Repeat the test after completion of the joints and terminations. Insulation resistance values of cables of various sections of the entire installation shall be measured and recorded in a register for future reference. The ambient temperature at the time of measurement shall also be recorded.
6.4.2 Conductor Resistance Test
The conductor resistance test is carried out for long cables especially where there are joints in the cable. The resistance is measured using a suitable bridge. The conductors at the far end are connected together to form a loop. The connecting bond shall have a cross section atleast that of the conductor.
The measured value of the loop resistance is converted to resistance in ohms per kilometer per conductor using the formula
R = Rl /2L
R is the resistance in ohm per conductor per km
Rl is the measured value of resistance of the cable loop
L is the length of the cable (not loop ) in km
The conductor resistance calculated above may be converted to resistance at 20 deg.C by the formula
The above value of conductor resistance may be compared with the values given in the test certificate of the manufacturer. Wide difference between the two values is an indication of improper joints. Contact resistances are kept minimum by properly clamping or bolting connections.
6.4.3 Measurement of Capacitance
Capacitance of cables is measured for voltages above 11 kV. The measurement is made using a suitable capacitance bridge. In the case of screened cables the capacitance is measured between the conductor and the screen. For unscreened cables, the capacitance is measured between one conductor and others with the metal sheath/armour connected to earth. The measured values are compared with the values given in the test certificate.
6.4.4 High Voltage Test
After making joints and terminations, cables are subjected to High Voltage Test. The condition of the insulation of the cable is evaluated by applying a voltage higher than the rated voltage for a short duration. The cable shall withstand the test voltages given in table 6.2 when applied for a period of 5 minutes.
The test can be conducted by applying dc voltage also. DC test equipments are compact and portable and they require less power.
Test Procedure
The high voltage source is connected to the conductor of the core under test. The cores not under test, screen and armour are connected to the earth terminal, depending on the mode of connection (please see the figures below).
The voltage is now raised slowly, but not so slowly as to cause unnecessary prolongation of the stress near the test voltage. The test voltage is raised to the specific value given in table 6.2 and is maintained for the specified time of 5 minutes. After the specified time, the voltage is suddenly decreased but without sudden interruption to avoid the possibility of switching transients. Insulation resistance values of the cable before and after the HV test shall be measured and compared. There shall not be wide variation in the IR values. Fig. 6.1
Points to note
1. The Cable cores must be discharged on completion of HV test and the cable earthed before it is put into service.
2. For old cables, the test voltage may be limited to 1.5 times the rated voltage. But in no case shall the test voltage be less than the rated voltage.
3. It is not desirable to conduct frequent HV tests on cable installations. The test shall be carried out when the cable is first installed and thereafter only when essential.
4. During the HV test on cables, other electrical equipments such as switches, breakers etc. shall be earthed and adequate clearance maintained from the test equipment and the cable under test to prevent any flash over.
5. In each test, the metallic sheath, screen and armour shall be connected to earth.
6.4.5 Partial discharge and Tan delta measurement
Partial discharge and tan delta values of HV and EHV cables are measured as bench mark references after the cable installation is complete. Partial discharge is measured using a partial discharge measurement instrument and tan delta with a tan delta measuring instrument. The values are compared with the factory test values and recorded for future reference.
6. 5 List of Indian Standards relevant to testing of cables


7.Protective earthing

7.1 Introduction
Earthing is a general term broadly representing grounding of power systems and bonding of equipment bodies to grounded electrodes. Earthing associated with current carrying power conductors, usually neutral conductor, is normally essential for the stability of the system and is generally known as system earthing. Earthing of non-current carrying metal works of equipment bodies is essential for the safety of life and property and is generally known as safety equipment earthing. The basic requirements of any earthing system are
(i) It should consist of equipotential bonding conductors capable of carrying the prospective earth fault current and a group of pipe/rod/plate earth electrodes for dissipating the current to the general mass of the earth without exceeding the allowable temperature limits in order to maintain all non-current carrying metal works reasonably at earth potential and to avoid dangerous contact potentials being developed on such metal works.
(ii) It should limit earth resistance sufficiently low to permit adequate fault current for the operation of protective devices in time and to reduce neutral shifting.
(iii) It should be mechanically strong, withstand corrosion and retain electrical continuity during
the life of the installation. Earth electrodes, which form part of the earthing system, are provided to dissipate fault current during earth fault and to maintain the earth resistance to a reasonable value so as to avoid rise of potential of the earthing grid. The resistance to earth of an electrode of given dimensions is dependent on the electrical resistivity of the soil in which it is installed. In addition to the measurement of soil resistivity at the design stage, it is essential to repeat the measurement at the pre-commission stage
also, as the effectiveness of the earthing system depends on the value of soil resistivity . Hence before energising electric supply lines and apparatus it is necessary that all components of the earthing system including the soil are inspected and tested to ensure efficient functioning of the system.
7.2 Pre – commission inspection and checks
(i) General Layout
· Check whether the layout of earthing is as per the scheme approved by the department of Electrical Inspectorate.
· Check whether the number of plate electrodes and pipe electrodes are as per the approved scheme.
· Check whether the spacing between the electrodes are as per the approved scheme- 5 metres for pipe electrodes and 8 metres for plate electrodes.
(ii) Earth electrodes
· Check whether all the earth electrode terminals are visible and numbered. The numbering shall be done both on the top of trough cover and inside the trough.
· Check the size of earth electrodes used – 1200 x 1200 x 12.6 mm for cast iron plate and 600 x 600 x 6.3 mm for copper plate – One cast iron plate is equivalent to 4 copper plates of standard size.
· Check the dimension of earth electrode trough – 1000 x 500 x 600 mm for plate electrodes and 500 x 400 x 350 mm for pipe electrodes – This is for easiness of connections and convenience of testing .
· Check the class of pipe used for pipe electrodes – at least class B pipes shall be used.
· Check whether permanent watering arrangement is provided at sub-stations and where earth resistivity is relatively high.
· Check whether funnels are provided for watering the electrodes.
· Check the size of the earth mat of EHT stations with the designed values (spread of earth mat, mesh size, conductor size, size of risers, depth of laying etc.)
· In the case of earth mats, check the size of blue granite jelly, its depth and area of spread. The area of spread shall extend beyond fencing atleast by 1.5 metres.
· Check whether all the earth electrodes are interconnected to form a closed mesh.
(iii) Earth continuity strips and earthing conductors.
· Check whether two continuity strips have been taken from the plate electrodes to the top connector link.
· Where GI is used for earthing, check whether hot dip galvanized GI strips and conductors are used. GI is allowed where corrosion factor is within permissible limits and earth resistivity is more than 100 ohm-metre.
· Check the size of main earth bus for conformity with the approved size.
· Check the size of the sub earth buses and their interconnections.
· Check the size and effectiveness of connections of horizontal and vertical earth buses of cubicle type switch board sections.
· Check the interconnection of earth bus sections in switch boards.
· Check whether duplicate earthing of adequate size is provided for switches, isolators and control gears.
· Check whether duplicate earthing is provided for body of transformers, motors and other equipments. The two connections shall be taken from opposite sides.
· Check whether duplicate earthing is provided for the neutrals of transformers and generators . There shall be one direct connection from each neutral to a separate earth electrode but interconnected with the earthing system.
· Check whether continuous earth strip is run from the top of lattice type towers and structures of EHT stations and lines.
· Check whether the bottom of each High voltage bushing is earthed using earthing strip.
· Check whether outdoor CT, PT, breaker units, isolators, lightning arrestors etc. are directly earthed to the risers of the earth mat / earthing grid.
(iv) Connections and joints
· Check whether connections in the earthing system are made properly . The contact surfaces shall be properly tinned and contacts perfectly bonded and seated – Riveting, bracing or bolting shall be done effectively.
· Inspect the welded joints of GI earth strips and conductors – The welded surfaces shall be covered with zinc dichromate painting / bituminous coating.
· Check the quality of galvanisation of bolts and nuts used for earth lead connections – Hot dip galvanised rust free bolts and nuts shall be used.
· In the case of earth mats, check the perfection of welding of mesh joints.
7. 3 Precommission tests
7. 3.1 Earth Testers and principle of measurement
The most commonly used earth tester is the four terminal tester. The tester comprises of a current source and a meter in a single instrument. The resistance is directly read in the tester from which the earth resistivity is computed. Wenner’s four electrode method is followed for earth resistivity measurement. When four electrodes are driven along a straight line at equal intervals and a current is passed through the two outer electrodes, the current flowing into the earth produces an electric field proportional to the current density and the resistivity of the soil. The voltage measured between the two inner electrodes is therefore proportional to the field. Consequently the resistivity will be proportional to the ratio of the
voltage to current. The earth resistivity of the soil is given by
7 .3.2 Earth Resistivity – Test Procedure
The resistivity of soil varies over a wide range depending on the composition and moisture content of the soil. It is therefore advisable to conduct earth resistivity tests during dry season in order to get conservative results. In the case of sub – stations and generating stations, at least eight test directions shall be chosen from the centre of the station to cover the entire site. For very large station sites this number may be increased. In the case of transmission lines, the measurements shall be taken along the direction of the line throughout the length, at least once in every 4 kms. The connections for the test are given in fig 7.1
The four electrodes are driven into the earth along a straight line at equal intervals . The depth of driving the electrodes in the ground shall be of the order 10 to 15 cms. The earth megger is placed on a steady and approximately level base. The links between the terminals are opened and the four electrodes connected to the instrument terminals. Appropriate range in the instrument is selected to obtain accurate readings. The readings are taken while turning the crank at around 135 revolutions per minute. The resistivity is calculated by substituting the value of R obtained from the test in the equation in para 7.3.1.
If the resistance of the electrodes (two inner potential electrodes) is comparatively high, a correction of the test result is necessary depending on its value. For this purpose, the resistance of the voltage circuit of the instrument Rp is measured by connecting the instrument as shown in fig. 7.2.
Average earth resistivity at the site
The resistivity of the soil at many sites have been found varying with the depth of the soil and also with horizontal distances. Variation of the resistivity with depth is mainly due to stratification of earth layers and is found predominant when compared to the variation with horizontal distances. For the correct computation of earth resistivity, it is desirable to get information about the horizontal and vertical variations of earth resistivity at the site under consideration. The vertical variations may be detected by repeating the measurements at a given location in a chosen direction with different electrode spacing. The spacings may be increased in steps of 2, 5, 10, 15, 25 and 50 metres or more. The horizontal variations are studied by taking measurements in various directions from the centre of the station. If the variation in the earth resistivity readings for different electrode spacings in a direction is within 20 to 30 percent, the soil is considered to be uniform. When the spacing is increased gradually from low values, a stage will be reached at which the resistivity readings become more or less constant irrespective of the increase in the electrode spacing. This value of the resistivity is noted as the resistivity in that direction. Similarly, the resistivity for at least eight equally spaced directions from the centre of the site are measured. These resistivities are plotted on a graph sheet. A closed curve is plotted on the graph sheet joining the resistivity points to get a polar resistivity curve (see fig. 7.3). The area inside the polar curve is measured and the circle of the equivalent area is found out. The radius of the equivalent circle is the average earth resistivity of the site under consideration. The value will be reasonably accurate when the soil is homogeneous. If the soil is not
homogeneous, a curve of resistivity versus electrode spacing shall be plotted and this curve further analysed to decide stratification of the soil into two or more layers of appropriate thickness or a soil of gradual resistivity variation. Computation of earth resistivity of heterogeneous soil is highly involved and reference to text books may be made.
7.3.3 Measurement of earth electrode resistance
The same four terminal earth tester described under para 7.3.1 can be used for measurement of earth electrode resistance. One of the current and potential terminals are shorted to form a common terminal which is connected to the test electrode and the other current and potential terminals connected to two auxilliary electrodes. Alternately, 3 terminal earth testers with common terminal to be connected to the test electrode and independent current and potential terminals for connections to auxiliary electrodes are available for measurement of earth electrode resistance. Two standard auxiliary electrodes supplied with the instrument are used for the measurement. The depth of driving of auxiliary electrodes shall be low compared to the spacing between the electrodes. Generally, the auxilliary electrodes are driven at 15 metres and 30 metres from the test electrode. The connections may be checked before taking the measurement .
Resistance of individual electrodes
Resistance of individual electrodes is measured after disconnecting all interconnections to the electrode. Earth leads from the earth bus, neutral of transformers/generators and interconnections from other earth electrodes are disconnected before taking the measurement. Connections to the earth tester are made as described above. The cranking lever of the earth tester is rotated at the specified speed. The reading of the earth tester gives the earth resistance of the particular electrode. Resistance of all earth electrodes shall be measured in the above manner and the values recorded in a register for future reference.
Effective earth resistance of the station
After measuring the resistance of individual electrodes, reconnect all earth leads including interconnection of earth electrodes. Now measure the earth resistance at the outer most electrode, driving the auxiliary electrodes in the outward direction. The value so measured gives the effective earth resistance of the station.
Points to note
· The test electrode and the auxiliary electrodes shall be in a straight line.
· The spacing between the electrodes shall be approximately equal.
· The auxiliary electrodes shall be driven to approximately the same depth and the depth shall be very low compared to the spacing between electrodes.
· The tester shall be cranked at the specified and uniform speed.
Acceptable limits of earth resistance
The acceptable limits of earth resistance values for various systems are given below:
7.3.4 Earth Continuity Test
Non-current carrying metal parts of equipments, control gears and devices are provided with duplicate earth connections to ensure effective equipotential bonding with the earth bus and thereby to the earth electrodes. Duplicate earth leads are provided to ensure that failure of one lead does not result in the disconnection of the equipment from the earthing system. In order to ensure the effectiveness of the protective earthing system, it is necessary to test the continuity of various earthing conductors in the system. The test procedure for earth continuity test is the same as that for the measurement of earth electrode resistance. The earth tester is set ready with the auxilliary electrodes driven at 15 m. and 30 m. from the outer most earth electrode of the earthing grid. The common terminal of the earth tester is connected to a long flexible copper cable. The other end of the cable is connected or held tight to the body of the equipment under test. The tester is now cranked to the specified speed and the reading noted. The reading shall be very low and near to the combined earth resistance of the system. Repeat the test for all equipments and devices connected to the system.
A high value of earth resistance in the earth continuity test is an indication of loose contact in the terminations/joints or a break in the earth leads/conductors. A thorough check shall be carried out to locate the fault and corrective action taken.
7.3.5 Measurement of Earth Loop Impedance
When a line to earth fault occurs, the fault current shall have a value sufficient to discriminately operate the protective devices. The value of the fault current is determined by the impedance of the closed loop available for the fault current to circulate. The earth loop impedance includes the impedance of the line conductors, fault, earth continuity conductors, earth leads, earth electrodes etc.
Measurement of earth loop impedance is of greater importance in the case of HT and EHT installations where system earthing and equipment earthing are connected to the same grid or bus. Connections for measurement the earth loop impedance is shown in fig. 7.5. When HRC fuses are used to protect the circuit, approximately five times the rated current of the fuse is taken as the minimum required current for fast clearing of the fault. When protective relays are used, a fault current of around two times the setting of the relay is considered the minimum required current. The measured value of earth loop impedance shall be low enough to produce the above fault currents. The earth loop impedance may be measured at different levels of distribution i.e. at the fag end, DB level,
SSB, MCC, MSB etc.

Protective Relays

8.1 Introduction
Relays are devices intended to protect electrical systems and equipements against damages caused due to abnormal operating conditions. In more technical terms, relays are devices designed to produce sudden pre-determined changes in one or more physical systems on the appearance of certain abnormal conditions in the physical systems controlled by them. Protective relays act as sensors of abnormalities and actuate control gears when required. Relays may be suitably set to operate with the required discrimination between sections in order to isolate only the faulty section /sections or equipment / equipments. A relay will have one or more energising quantities and one or more characteristic quantities in terms of which the relay is calibrated (eg. voltage for over voltage relays, time for definite time lag relays, time and current for inverse time lag current relays, angle for directional relays, power for reverse power relays etc.) A relay should watch the system changes and operate when it is called for. Testing of protective relays before commissioning and thereafter periodically is very important as failure of relays can cause danger to life and damage to material.
The testing procedure of common types of relays used in industrial applications are explained in this chapter.
8.1.1 Classification of Relays
Relays are broadly classified into two, based on the principle of operation.They are
1. Conventional electro magnetic relays
2. Static relays
Microprocessor based PLC (Programmable Logic Controlled) multifunctional relays are now
available as a replacement for a large number of independent unifunctional relays. These are
intelligent relays programmed to the requirements.
Relays can also be classified based on the time of operation of the relay as
1. Instantaneous relays
2. Time lag relays
Almost all relays fall under the categories of either instantaneous relays or time lag relays. Instantaneous relays operate and reset without any intentional time delay. But instantaneous relays have inherent time delay and based on this inherent time delay (operating time) they are sub classified as given in table 8.1.
8.1.2 Accuracy of relays
The manufacturer of the relay shall specify the accuracy of the relay at specified setting
values. The standard accuracy classes which correspond to the maximum percentage error is given in
table 8.2.
8.1.3 Characteristics of Time lag relays
Time lag relays are relays, the operation or resetting of which are intentionally time delayed. The time delay of the relay may be fixed or adjustable. Time lag relays are intended to operate after a specified time on the appearance of the energising quantity. The time lag of operation depends on the designed characteristics of the relay. Usually standard inverse, very inverse, extremely inverse and long time delay relays are used in practice. Many of the relays have definite minimum time of operation which will help to attain proper time grading between sections.
· Current operated relays
The time-current characteristics of current operated Inverse Definite Minimum Time Lag (IDMTL) relays without directional feature is given in tables 8.3.a to 8.3.e and in figures 8.1 to 8.5.
1. IDMTL 3 second relay
2. IDMTL 1. 3 second relay

Preparation of completion Reports

After completing the electrical installation works of a new HV/EHV installation or making additions to an existing HV/EHV installation, the electrical contractor who carried out the installation works shall conduct the necessary pre-commission checks and tests and prepare the completion report in the prescribed format. The completion report shall be signed by the electrical contractor and the owner of the installation and submitted to the Electrical Inspectorate and the supply authority. A model form ofthe completion report is given at the end of this chapter.
The completion report shall necessarily contain the following details
1. Full name plate details of all electrical machines and equipments covered in the completion report.
2. Test results of main equipments and switch boards.
3. Measured values of earth resistivity at the site before commencing and after completing the installation.
4. Test results of protective relays
5. Test results of insulating oil in the case of all oil containing equipments.
Copies of the following documents shall accompany the completion report.
1. Power allocation from the supplier
2. Sanction under Sec.44 of Electricity (Supply) Act 1948 in the case of generators
3. Approval from the Electrical Inspectorate.
On receipt of the completion report, the Electrical Inspector will inspect the installation and carry out such additional tests as may be found necessary. If the installation is found fit for energisation, the Electrical Inspector will issue the sanction for energisation under R.63 of the IE Rules. Commencement of supply to a new installation is made by the supplier on the basis of the sanction for energisation issued by the Electrical Inspector.
After satisfactory commissioning of an electrical installation, the date of energisation and date of commercial operation shall be intimated to the Electrical Inspectorate department and the electricity Supply authority. In the case of major equipments, the manufacturer of the equipment may also be intimated of the test results and the date of energisation.
Form of Completion Certificate
I/We certify that the installation detailed below has been installed by me / us and tested and that to the
best of my/our knowledge and belief, the installation complies with the provision of IER 1956 and
also IS 3043, IS-732 and other relevant codes of practice for electrical installations.
1. Name of installation
2. Voltage and system of supply
3. Particulars of work and test results
(a) Transformers (Give details of each transformer)
· Rating
· Voltage
· Make
· Serial number
· Year of manufacture
· Results of Insulation Tests
· Results of earth resistance and continuity tests
· Results of other relvant tests depending on the voltage level and rating of the transformer
(b) Generators (Give details of each Generators)
· Rating
· Voltage
· Make
· Serial number
· Year of manufacture
· Results of Insulation Tests
· Results of earth resistance and continuity tests
· Results of other relvant tests depending on the voltage level and rating of the
Generator
(c) H.V motors
· Number of motors
· Name plate details of each motor
· Result of insulation tests
· Result of earth resistance and continiuity tests
· Results of other relvant tests depending on the voltage level and rating of the Motors.
(d) M.V Motors
· Number of motors
· Name plate details of each motor
· Results of insulation tests
· Results of earth resistance and continuity tests
(e) EHV and HV cables (Give details of each cable)
· Length of cable and number of terminations/joints
· Results of Insulation Tests
· Result of H.V withstand test result
· Results of earth resistance and continuity tests
· Results of other relevant tests
(f) M.V cables
· Results of Insulation resistance tests
· Results of Earth resistance and continuity tests
(g) Relays
· Results of Relay and control wiring tests
· Results of tests of protection and measurement CTs
· Results of tests of PTs
(h) Protective earthing
· Earth resistivity at the site
· Earth resistance of individual earth electrodes
· Result of earth continuity test
· Combined earth resistance of the installation
(i) Special Equipments
· Name plate details of each equipment
· Results of insulation tests
· Results of earth resistance and continuity tests
· Results of other relevant tests depending on the voltage and rating of the equipment
Signature of the Supervisor
Signature of the owner
Signature of the Contractor
Name and address with permit number of the Supervisor.
Nameand address of the owner
Name and address with Contract Licence number of the Contractor.

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