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EXCITATION SYSTEM Presentation Transcript
1.PURPOSE OF EXCITATION SYSTEM
REGULATE TERMINAL VOLTAGE OF THE MACHINE.
MEET EXCITATION POWER REQMTS UNDER ALL NORMAL OPERATING CONDITIONS.
ENABLE MAXIMUM UTILISATION OF MACHINE CAPABILITY.
GUARD THE MACHINE AGAINST INADVERTANT TRIPPING DURING TRANSIENTS.
IMPROVE DYNAMIC & TRNSIENT STABILITY THEREBY INCREASING AVAILABILITY.
REGULATE TERMINAL VOLTAGE OF THE MACHINE.
MEET EXCITATION POWER REQMTS UNDER ALL NORMAL OPERATING CONDITIONS.
ENABLE MAXIMUM UTILISATION OF MACHINE CAPABILITY.
GUARD THE MACHINE AGAINST INADVERTANT TRIPPING DURING TRANSIENTS.
IMPROVE DYNAMIC & TRNSIENT STABILITY THEREBY INCREASING AVAILABILITY.
2.EXCITATION SYSTEM
EXCITATION TRANSFORMER 16,500/575 VOLT
FOUR THYRISTER BRIDGE
FIELD FLASHING UP TO 70% OF RATED VOLTAGE
AVR
LIMITERS
SLIP RINGS
FIELD BREAKER
INVERSION
6 THYRISTER CONVERTOR
EXCITATION TRANSFORMER 16,500/575 VOLT
FOUR THYRISTER BRIDGE
FIELD FLASHING UP TO 70% OF RATED VOLTAGE
AVR
LIMITERS
SLIP RINGS
FIELD BREAKER
INVERSION
6 THYRISTER CONVERTOR
3.PROTECTIVE CHANGEOVER FROM AUTO TO MANUAL
AVR PT SPLY FAIL
AUTO PULSE FAILURE
HIGH AUTO REF
EX TRAF O/C ST-1
SUPPLY A FAILURE
SPLY A1 FAILURE
4.CONSEQUENCES OF NEGATIVE PH SEQUENCE
ROTOR LOSSES INCREASES
NO CONSTRUCTIVE OUT- PUT
TRY TO ROTATE IN OPPOSITE DIRECTION
AVR PT SPLY FAIL
AUTO PULSE FAILURE
HIGH AUTO REF
EX TRAF O/C ST-1
SUPPLY A FAILURE
SPLY A1 FAILURE
4.CONSEQUENCES OF NEGATIVE PH SEQUENCE
ROTOR LOSSES INCREASES
NO CONSTRUCTIVE OUT- PUT
TRY TO ROTATE IN OPPOSITE DIRECTION
5.TRIPPING OF FIELD BREAKER
EXCITATION TRANSFORMER TEMP HIGH
(N-3) BRIDGE FAIL
MANUAL PULSE FAIL IN MANUAL SELECTION
MAIN-1(186G) OPERATE
MAIN-2(286 G) OPERATE
EXCITATION TRANSFORMER O/C STAGE-2
ROTOR OVER VOLTAGE +Ve HI
ROTOR OVER VOLTAGE -Ve HI
REGULATOR SUPPLY FAIL
VOLTAGE BUILT UP NOT EXCEED 70% WITHIN 20 SECS
FF DISTURBED
S-12 CHANGED TO TEST POSITION
BOTH 48 VOLT SUPPLY FAIL (48 V1 & 48V2)
EXCITATION TRANSFORMER TEMP HIGH
(N-3) BRIDGE FAIL
MANUAL PULSE FAIL IN MANUAL SELECTION
MAIN-1(186G) OPERATE
MAIN-2(286 G) OPERATE
EXCITATION TRANSFORMER O/C STAGE-2
ROTOR OVER VOLTAGE +Ve HI
ROTOR OVER VOLTAGE -Ve HI
REGULATOR SUPPLY FAIL
VOLTAGE BUILT UP NOT EXCEED 70% WITHIN 20 SECS
FF DISTURBED
S-12 CHANGED TO TEST POSITION
BOTH 48 VOLT SUPPLY FAIL (48 V1 & 48V2)
6.CAUSES OF THYRISTER BRIDGE FAIL
LOSS OF COOLING
FAN SUPPLY FAILURE
PULSE FINAL STAGE POWER SUPPLY FAILURE
THYRISTER FUSE FAILURE
ISOLATOR OFF
LOSS OF COOLING
FAN SUPPLY FAILURE
PULSE FINAL STAGE POWER SUPPLY FAILURE
THYRISTER FUSE FAILURE
ISOLATOR OFF
7. ADVANTAGES OF STATIC EXCITATION EQUIPMENT
REDUNDANCIES INCREASE RELIABILITY AND AVAILABILITY.
MONITORING AND DIAGNOSTICS REDUCES DOWN TIME.
ABSENCE OF ROTATING PARTS ENABLE MAINTENANCE WITH EASE EVEN WHEN THE EQPNT IS IN SERVICE.
UPRATING OF THE M/C BY USING UP THE MARGINS
OR BY ADDING ADDITIONAL CIRCUITS.
EASE OF EQUIPMENT LAYOUT.
BETTER OPTIONS FOR PLANT R&M.
REDUNDANCIES INCREASE RELIABILITY AND AVAILABILITY.
MONITORING AND DIAGNOSTICS REDUCES DOWN TIME.
ABSENCE OF ROTATING PARTS ENABLE MAINTENANCE WITH EASE EVEN WHEN THE EQPNT IS IN SERVICE.
UPRATING OF THE M/C BY USING UP THE MARGINS
OR BY ADDING ADDITIONAL CIRCUITS.
EASE OF EQUIPMENT LAYOUT.
BETTER OPTIONS FOR PLANT R&M.
8.ROTOR EARTH FAULT
The field circuit of generator is not earthed and therefore single earth fault is not considered to be a dangerous condition. However presence of such a fault increases the risk of a second earth fault developing due to increased stresses between rotor winding and earth. The resulting double earth fault will cause part of the rotor winding to become short circuited, with consequence unbalance of the magnetic field produced by the rotor and possibly serious damage to the machine
The field circuit of generator is not earthed and therefore single earth fault is not considered to be a dangerous condition. However presence of such a fault increases the risk of a second earth fault developing due to increased stresses between rotor winding and earth. The resulting double earth fault will cause part of the rotor winding to become short circuited, with consequence unbalance of the magnetic field produced by the rotor and possibly serious damage to the machine
9.STATOR EARTH FAULT
It is generally agreed that there is no need to provide 100% stator earth fault protection on industrial generators , since the likelihood of an earth fault close to the star connection of the machine is considered to be extremely remote and that 95% coverage of the stator winding is perfectly acceptable
It is generally agreed that there is no need to provide 100% stator earth fault protection on industrial generators , since the likelihood of an earth fault close to the star connection of the machine is considered to be extremely remote and that 95% coverage of the stator winding is perfectly acceptable
10.In generators, the occurrence of phase to phase and three phase faults are rare and less common than phase to earth faults.when they do occur they are much more severe in intensity and require high speed clearance, if considerable damage to the stator and rotor is to be avoided.
11. GENERATOR BACKUP PROTECTION
The purpose of the generator backup protection is to disconnect the generator from the bus-bars, if a system fault has not been cleared by the main protection after a predetermined time delay has elapsed. There are two types of backup relays available to perform this function. A voltage dependent overcurrent relay and a impedence relay.
The purpose of the generator backup protection is to disconnect the generator from the bus-bars, if a system fault has not been cleared by the main protection after a predetermined time delay has elapsed. There are two types of backup relays available to perform this function. A voltage dependent overcurrent relay and a impedence relay.
12. REVERSE POWER PROTECTION
Failure of the prime mover of a generator set ,will keep the set running as a synchronous compensator, taking the necessary active power from the net work and could be detrimental to to the safety of the set, if maintained for any length of time. The amount of power taken will depend on the type of prime mover involved. It ranges from 5% to 25%.
Failure of the prime mover of a generator set ,will keep the set running as a synchronous compensator, taking the necessary active power from the net work and could be detrimental to to the safety of the set, if maintained for any length of time. The amount of power taken will depend on the type of prime mover involved. It ranges from 5% to 25%.
13. Negative phase sequence protection
While balanced load conditions produce balanced currents which are equal in magnitude and displaced by 120 degrees and the field which they create rotates at synchronous speed with the motor and no eddy currents are induced.however when the load is unbalanced, negative phase sequence currents are produced which are also in magnitude and displaced by 120 degree but rotates in the opposite direction at twice the synchronous speed. These double frequency eddy currents induced in the rotor of the machine may cause excessive heating, mainly in the surface of the cylindrical rotor.
While balanced load conditions produce balanced currents which are equal in magnitude and displaced by 120 degrees and the field which they create rotates at synchronous speed with the motor and no eddy currents are induced.however when the load is unbalanced, negative phase sequence currents are produced which are also in magnitude and displaced by 120 degree but rotates in the opposite direction at twice the synchronous speed. These double frequency eddy currents induced in the rotor of the machine may cause excessive heating, mainly in the surface of the cylindrical rotor.
14. Field failure protection
Loss of generator field excitation under normal running conditions may arise due to any of the following condition.
1. Failure of brush gear.
2.unintentional opening of the field circuit breaker.
3. Failure of AVR.
When generator on load loses it’s excitation , it starts to operate as an induction generator, running above synchronous speed.cylindrical rotor generators are not suited to such operation , because they donot have damper windings able to carry the induced currents, consequently this type of rotor will overheat rathyer quickly.
15. Over-voltage protection
The field excitation system of generators is usually arranged so that over voltage conditions at normal speeds possibly can not occur.the conditions where over-voltage , other than transient over-voltage, do occur is when the prime mover speed increases due to sudden loss of load.the control governors of industrial prime movers are inherently very sensitive to speed changes and the resulting increase from any sudden loss of load is normally checked before any dangerous over-voltage conditions can arise. An instantaneous high set over-voltage relay can be provided to trip the generator quickly in in case of excessive over--voltage following a sudden loss of load.
Loss of generator field excitation under normal running conditions may arise due to any of the following condition.
1. Failure of brush gear.
2.unintentional opening of the field circuit breaker.
3. Failure of AVR.
When generator on load loses it’s excitation , it starts to operate as an induction generator, running above synchronous speed.cylindrical rotor generators are not suited to such operation , because they donot have damper windings able to carry the induced currents, consequently this type of rotor will overheat rathyer quickly.
15. Over-voltage protection
The field excitation system of generators is usually arranged so that over voltage conditions at normal speeds possibly can not occur.the conditions where over-voltage , other than transient over-voltage, do occur is when the prime mover speed increases due to sudden loss of load.the control governors of industrial prime movers are inherently very sensitive to speed changes and the resulting increase from any sudden loss of load is normally checked before any dangerous over-voltage conditions can arise. An instantaneous high set over-voltage relay can be provided to trip the generator quickly in in case of excessive over--voltage following a sudden loss of load.
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