Answer MET Question 33
Question: With reference to A.C generators:
B.
During parallel operation if a prime-mover failure occurred the
generator would act as a motor. Generators intended to operate in
parallel must have reverse power protection (RP). A reverse power relay
monitors the direction of power flowing between the generator and the
load. The reverse power relay detects this fault and acts to trip the
generator circuit-breaker. The pick-up power level setting and
time-delay setting are adjustable and are pre-set to suit the
prime-mover. If the prime mover is a turbine, very little power is
absorbed when motoring and a reverse-power pick-up setting of 2-3% is
usual. If the prime mover is a diesel then a setting range of 5-15% is
usually adopted. A time delay range of about 0.5-3 s is usual. The RP
relay operation is easily checked during a generator changeover. The
outgoing generator is gradually throttled down so that it motors causing
the reverse power relay to trip its generator circuit-breaker.
A. Give a brief outline of the care maintenance that should be given to the stator and rotor of an A.C.
generator;
B. Explain what is likely to occur if the driving power of one A.C. generator suddenly fails when two
generators are running in parallel. What safety devices are usually provided for such events?
Answer:
A. Regular inspection and the correct maintenance of generators and
their associated control gear is essential to prevent failure and
inefficient operation.
1. Ensure that the generator prime-mover is shut down and locked off before you begin any maintenance. Also ensure that the generator
circuit breaker is locked off, auto-start circuits are disabled and electric heaters are switched off and isolated.
2. All wiring to the generator should be inspected for damage or frayed insulation and tightness of terminal connections.
3. Particularly check for signs of oil and water contamination of cable insulation within terminal boxes.
4. Check that the cooling air intake and exhaust openings are not blocked and are free of dirt and dust.
5. Inspect and clean the generator rotor and stator windings by removing dust with a dry lint-free cloth.
6. Low pressure, dry compressed air may be used to dislodge heavier dirt but be careful not to drive the dirt deeper into the windings.
7. Use vacuum cleaner for removing dirt from the windings.
8. Use a rubber or plastic coated nozzle on the vacuum cleaner tube to prevent abrasive damage to the sensitive winding insulation.
9. Oil on the surface of winding insulation will reduce the insulation resistance and shorten its life. The oily deposits can be removed by washing the windings with special degreasant liquids.
10. Minor abrasions to winding insulation can be repaired, after cleaning, by the application of a suitable air-drying varnish.
11. Rotor sliprings must be checked for uniform (even) wear and that the carbon brushes have free movement in their boxes.
12. Correct brush pressure can be checked using a pull-type spring balance and compared with the manufacturer's instructions.
A pull of around 1-1.5 kg is usual. If the brushes become too short (below about 2 cm) the reduced spring pressure will cause sparking at the slipring contact. Replace brushes with the correct type and bed them to the curvature of the slip rings. This can be done by placing a thin strip of glass paper (not emery paper) over the slip ring with its cutting surface under the carbon brush. Pull the glass paper around the slip ring until the brush surface has the same contour as the ring. The last few passes of the glass paper should be made in the same direction as the normal rotor direction.
13. Remove all traces of carbon dust with a vacuum cleaner Generator excitation transformers. AVR components and rotating diodes must
be kept free of dirt, oil and dampness. A special contact grease is used between the diode connections to prevent electrolytic action occurring between dissimilar metals. Check such contacts for tightness but do not disturb them unnecessarily.
14. Measure the insulation of the stator and rotor windings to earth and between stator phases (assuming that the neutral point is available for disconnection at the terminal box). Remember to disconnect or short-circuit any electronic circuit components which are likely to be damaged by u high voltage insulation test. Consult the wiring diagrams and the manufacturer's instructions before testing.
15. Record the IR values and note the prevailing temperature and humidity. Compare with previous test results.
A minimum IR value is usuallv taken to be 1 MΩ but a lower value may be acceptable to a surveyor based on 1 kΩ/volt, e.g. 450 kΩ or 0.45 MΩ for a 450 V generator. However, it is the historical trend of the machine IR values which will give a better picture of the insulation condition. Generators with very low IR values (less than 0.5 MΩ) should be given a thorough cleaning then dried out.
16. If the IR has recovered to a reasonable value which has become steady during the drying period, its windings should be covered with high-quality air-drying insulating varnish. Should the IR value remain low during a dry-out, the machine insulation needs to be completely reimpregnated or rewound (generally by a specialist contractor).
After maintenance, no-load running, checks should precede synchronising and loading. On load, particularly check for excess temperature rise and loadsharing stability when running in parallel. Finally, if a generator is to be left idle or a long time, make sure that its windings are suitably heated to prevent internal condensation forming on its insulation.
1. Ensure that the generator prime-mover is shut down and locked off before you begin any maintenance. Also ensure that the generator
circuit breaker is locked off, auto-start circuits are disabled and electric heaters are switched off and isolated.
2. All wiring to the generator should be inspected for damage or frayed insulation and tightness of terminal connections.
3. Particularly check for signs of oil and water contamination of cable insulation within terminal boxes.
4. Check that the cooling air intake and exhaust openings are not blocked and are free of dirt and dust.
5. Inspect and clean the generator rotor and stator windings by removing dust with a dry lint-free cloth.
6. Low pressure, dry compressed air may be used to dislodge heavier dirt but be careful not to drive the dirt deeper into the windings.
7. Use vacuum cleaner for removing dirt from the windings.
8. Use a rubber or plastic coated nozzle on the vacuum cleaner tube to prevent abrasive damage to the sensitive winding insulation.
9. Oil on the surface of winding insulation will reduce the insulation resistance and shorten its life. The oily deposits can be removed by washing the windings with special degreasant liquids.
10. Minor abrasions to winding insulation can be repaired, after cleaning, by the application of a suitable air-drying varnish.
11. Rotor sliprings must be checked for uniform (even) wear and that the carbon brushes have free movement in their boxes.
12. Correct brush pressure can be checked using a pull-type spring balance and compared with the manufacturer's instructions.
A pull of around 1-1.5 kg is usual. If the brushes become too short (below about 2 cm) the reduced spring pressure will cause sparking at the slipring contact. Replace brushes with the correct type and bed them to the curvature of the slip rings. This can be done by placing a thin strip of glass paper (not emery paper) over the slip ring with its cutting surface under the carbon brush. Pull the glass paper around the slip ring until the brush surface has the same contour as the ring. The last few passes of the glass paper should be made in the same direction as the normal rotor direction.
13. Remove all traces of carbon dust with a vacuum cleaner Generator excitation transformers. AVR components and rotating diodes must
be kept free of dirt, oil and dampness. A special contact grease is used between the diode connections to prevent electrolytic action occurring between dissimilar metals. Check such contacts for tightness but do not disturb them unnecessarily.
14. Measure the insulation of the stator and rotor windings to earth and between stator phases (assuming that the neutral point is available for disconnection at the terminal box). Remember to disconnect or short-circuit any electronic circuit components which are likely to be damaged by u high voltage insulation test. Consult the wiring diagrams and the manufacturer's instructions before testing.
15. Record the IR values and note the prevailing temperature and humidity. Compare with previous test results.
A minimum IR value is usuallv taken to be 1 MΩ but a lower value may be acceptable to a surveyor based on 1 kΩ/volt, e.g. 450 kΩ or 0.45 MΩ for a 450 V generator. However, it is the historical trend of the machine IR values which will give a better picture of the insulation condition. Generators with very low IR values (less than 0.5 MΩ) should be given a thorough cleaning then dried out.
16. If the IR has recovered to a reasonable value which has become steady during the drying period, its windings should be covered with high-quality air-drying insulating varnish. Should the IR value remain low during a dry-out, the machine insulation needs to be completely reimpregnated or rewound (generally by a specialist contractor).
After maintenance, no-load running, checks should precede synchronising and loading. On load, particularly check for excess temperature rise and loadsharing stability when running in parallel. Finally, if a generator is to be left idle or a long time, make sure that its windings are suitably heated to prevent internal condensation forming on its insulation.
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