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.
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