PPT ON EXCITATION SYSTEM

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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.

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

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

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)

6.CAUSES OF THYRISTER BRIDGE FAIL
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.

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

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

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.

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%.

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.  

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.