generator excitation system

WHAT IS EXCITATION?

IT IS THE REACTIVE POWER REQUIRED BY A SYNCHRONOUS MACHINE FOR ITS OPERATION

IT IS THE ARRANGEMENT THROUGH WHICH THE REACTIVE POWER TO THE GENERATOR IS SUPPLIED WHEN IT IS CONNECTED WITH THE GRID.

EXCITATION SYSTEM RESPONSE

RELIABILITY STABILITY QUICK RESPONSE

EXCITATION SYSTEM RESPONSE

RESPONSE RATIO CEILING VOLTAGE STEADY STATE AND TRANSIENT

STABILITY

EXCITATION SYSTEM RESPONSE

RESPONSE RATIO= RATE OF RISE OF EXCITATION VOLTAGE(V/S)/NOMINAL VOLTAGE

GENERALLY CALCULATED FOR .5S AND COMES TO 2x MAXIMUM VOLTAGE APPLIED/RATED VOLTAGE

voltage

A

timeO

D

C

B

Actual build up of voltage

.5 S

EXCITATION SYSTEM RESPONSE

CEILING VOLTAGE

IT IS THE MAXIMUM VOLTAGE APPLIED ACROSS THE FIELD WINDING FOR VERY

SHORT DURATION (APPX. 10S) AND GENERALLY 1.5 TO 2 TIMES OF RATED

VOLTAGE.

EXCITATION SYSTEM RESPONSE

STEADY STATE STABILITYCAPACITY OF THE MACHINE TO PRODUCE FORCE EQUAL TO

DISTURBING FORCES SO THAT SYNCHRONISM COULD BE

MAINTAINED.

POWER

LOAD ANGLE

90

EXCITATION SYSTEM RESPONSE

TRANSIENT STABILITY

POWER

LOAD ANGLE

TYPES OF EXCITATION

DC EXCITATION SYSTEM USING DC EXCITER AND VOLTAGE REGULATOR

AC EXCITATION SYSTEM USING PILOT EXCITOR AND DC CONTROLLER

BRUSHLESS EXCITATION SYSTEM STATIC EXCITATION SYSTEM

DC EXCITATION USING DC EXCITOR

AND VOLTAGE REGULATOR THE EXCITATION SUPPLY IS TAKEN FROM

STATION DC AND FED TO INDIVIDUAL MACHINE THROUGH RHEOSTAT.

THE RHEOSTAT IS CONNECTED IN SERIES WITH FIELD WINDING OF THE MACHINE.

THE TERMINAL VOLTAGE IS ADJUSTED USING THIS RHEOSTAT.

SUITABLE FOR LOW CAPACITY MACHINE.

AC EXCITATION SYSTEM

A SHAFT DRIVEN AC PILOT EXCITOR HAVING ROTATING FIELD AND STATIONARY ARMATURE.

THE AC OUT OF PILOT EXCITOR IS FED TO FIELD WINDING OF MAIN EXCITOR THROUGH CONTROLLED RECTIFIER.

THE HIGH FREQUENCY AC OUTPUT OF MAIN EXCITOR IS FED TO THE STATIONARY DIODES AND THE RECTIFIED DC IS FED TO GENERATOR FIELD THROUGH SLIP RINGS.

BRUSHLESS SYSTEM

DIODE RECTIFIERS ARE MOUNTED ON THE GENERATOR SHAFT.

RECTIFIER OUTPUT IS DIRECTLY FED TO GENERATOR FIELD.

THIS AVOIDS SLIP RING AND CARBON BRUSH ARRANGEMENT.

STATIC EXCITATION SYSTEM

THE AC POWER IS TAPPED OFF FROM GENERATOR TERMINALS.

STEPPED DOWN BY RECTIFIER TRANSFORMER. RECTIFIED BY FULLY CONTROLLED THYRISTOR BRIDGE. FED TO GENERATOR TERMINALS BY SLIP RING AND

CARBON BRUSH ASSEMBLY. THE GENERATOR FIELD INPUT CONTROLS THE

GENERATOR OUTPUT VOLTAGE.

Automatic voltageregulator

Rotor angle limiter

Slip stabilizer

Regulator Powersupply

Filter

Grid control

Grid control

Pulseamplifier

Pulse amplifier

Pulse supervision

X-mer prot>1

X-mer prot>1

Pulse finalStage

Gate ckt

Stator current limiter

Rotor current limiter

M

MAuto/Man

DC source

o/ v prot

Fan supply

Rectifier transformer

Comparator&Follow up

Auto channel

Manual channel

GField breaker

AC source

Field flashing

STATIC EXCITATION SYS: BLOCK DIAGRAM

PT

CT

Filter

575/380

575/380

THYRISTOR

15.75kv/575

PARTS OF STATIC EXCITATION SYSTEM

RECTIFIER TRANSFORMER SCR OUTPUT STAGE EXCITATION START UP AND FIELD DISCHARGE

EQUIPMENT REGULATOR AND OPERATIONAL CONTROL

CIRCUITS.

RECTIFIER TRANSFORMER

THIS IS RESIN CAST COIL, DRY TYPE X-MER WITH NATURAL/ FORCED AIR COOLING.

15.75/0.580 KV, DYn5 TRANSFORMER. PRIMARY IS CONNECTED WITH GENERATOR OUT PUT. SECONDARY IS FEEDING TO 580V 3-PHASE AC BUS

FEEDING TO SCR BRIDGES THROUH FUSES.

SCR OUTPUT STAGE

CONSISTS OF SUITABLE NO OF BRIDGES CONNECTED IN PARALLEL.

EACH BRIDGE COMPRISES OF SIX THYRISTORS WORKING AS SIX PULSE FULLY CONTROLLED BRIDGE.

CURRENT CARRYING CAPACITY OF EACH BRIDGE DEPENDS ON THE RATING OF INDIVIDUAL THYRISTOR.

BY CHANGING FIRING ANGLE, VARIABLE OUTPUT IS OBTAINED.

SCR OUTPUT STAGE

EACH BRIDGE IS CONTROLLED BY ONE FINAL PULSE STAGE AND COOLED BY FAN.

N-1 BRIDGE OPERATION IS ENSURED. FAILURE OF TWO BRIDGES LIMITS THE EXCITATION

CURRENT. FAILURE OF THREE BRIDGES ENSURES GENERATOR

TRIPPING.

SHUNT CIRCUIT

THIS TAPS THE POWER FROM GENERATOR TERMINALS AND FEEDS DC POWER TO SLIP RINGS THROUGH FIELD BREAKER.

FOR INITIAL EXCITATION, A SEPARATE SUPPLY SOURCE IS REQUIRED AS THE REMANENCE VOLTAGE OF GENERATOR IS TOO LOW.

THIS SOURCE GETS DISCONNECTED AUTOMATICALLY WHEN THE THE GENERATOR OUTPUT VOLTAGE REACHES TO 70% OF ITS RATED VALUE.

EXCITATION CURRENT LIMITER PREVENTS THE THERMAL OVERLOADING OF ROTOR BY

LOWERING THE OUTPUT VALUE OF THE VOLTAGE REGULATOR AND THUS THE FIELD VOLTAGE.

LOAD ANGLE LIMITER

INCREASES THE FIELD VOLTAGE AND THUS STOPS THE GENERATOR FROM FALLING OUT OF STEP.

STATOR CURRENT LIMITOR

THIS REDUCES THE EXCITATION CURRENT WHEN THE STATOR CURRENT EXCEEDS A SAFE PERMISSIBLE LIMIT IN INDUCTIVE DIRECTION.

PULSE MONITOR

THIS IS CONNECTED TO THE BUS BAR. IF FAULTY PULSES OR PULSE FAILURE ARE DETECTED, THE CHANNEL CHANGES TO ANOTHER STANDBY CHANNEL .

PULSE COMPARING STAGE

IT COMPARES PULSE POSITION OF BOTH CHANNELS.

MATCHING IS AFFECTED WITH FOLLOW-UP CONTROL DEVICE.

PROTECTIONS PROVIDED

O/C RELAY WITH INST. TRIP. O/C RELAY WITH DELAYED TRIP. TEMPERATURE PROTECTION. ROTOR E/F PROTECTION

EXCITATION START UP

FIELD FLASHING IS REQUIRED FOR INITIAL BUILD UP OF GENERATOR VOLTAGE.

BOTH DC & AC FIELD FLASHING ARE PROVIDED FOR RELIABILITY.

FIELD DISCHARGE EQUIPMENT

ON TRIPPING OF GENERATOR, ITS STORED ENERGY IS REQUIRED TO BE DISCHARGED.

THIS IS DONE THROUGH A NON LINEAR DISCHGARGE RESISTENCE.

DIGITAL VOLTAGE REGULATOR

THE PRESENT DAY MICROPROCESSOR BASED EXCITATION SYSTEM NOT ONLY MEET THE VOLTAGE REGULATION FUNCTION BUT ALSO PROVIDES ENHANCED INTERNAL CONDITION MONITORING AND EXTENSIVE FAULT DIAGNOSIS BESIDES PROVIDING OPERATING FLEXIBILITY AND OTHER ADVANTAGES ASSOCIATED WITH DIGITAL SYSTEM.

DVR OPERATION

THE DVR PERIODICALLY CALCULATES THE CONTROL SIGNAL FROM THE MEASURED AND REFERANCE VALUES.THE CALCULATION IS REPEATED AT VERY SHORT TIME INTERVALS (= 3.3 ms) RESULTING IN APPARENTLY CONTONUOUS REGULATOR OUTPUT SIGNAL.

DVR FUNCTIONS

CONTROL OF GENERATOR VOLTAGE LIMITATION OF FIELD CUURENT LIMITATION OF LOAD ANGLE LIMITATION OF STATOR CURRENT SLIP STABILISATION FORMATION OF GENERATOR VOLTAGE

ACTUAL VALUE SIGNAL REFERENCE /ACTUAL VALUE DIFFERENCE PID ALGORATHIM

ADVANTAGES OF DVR

FAST RESPONSE(<20ms) TO NETWORK DISTURBANCES. SYSTEM REDUNDENCY AT INTERNAL POWER SUPPLY,

CONTROLS & EXCITATION POWER OUTPUT. SELF MONITORING & DIAGNOSTIC FEATURE. MAINTAINS DRIFT FREE PARAMETERS AVAILABILITY OF SINGLE/DUAL AUTO CONTROL

CHANNEL.

ADVANTAGES OF DVR

FACILITY TO PROVIDE ADOPTIVE TYPE PSS.

LOW FAILURE RATE OF CKT COMPONENTS.

FACILITY TO PROVIDE SECURITY MEASURE FOR UNWANTED OPERATION.

POSSIBILITY OF ON LINE REPAIR.