SPECIFICATION OF EXCITATION SYSTEM

GENERATOR CAPABILITYGenerator converts mechanical energy to electrical energyExcitation System provides DIRECT CURRENT to the synchronous machine Field Winding.

GENERATOR CAPABILITYGenerator supplies both active and reactive power

When connected to grid it either supplies or absorb reactive power

When supplying reactive power it is lagging (Inductive)

When absorbing reactive power it is leading (capacitive)

EXCITATION SYSTEM

Excitation SystemBrushless System (BLE)Static System (SEE)StandardsTrends

EXCITATION SYSTEM

Excitation SystemBrushless System (BLE)Static System (SEE)StandardsTrends

EXCITATION SYSTEM

Excitation System performs CONTROL and PROTECTIVE functions essential to the satisfactory performance of Power System

CONTROL :Control of Voltage and Q flowEnhancement of System Stability

PROTECTION:Ensure Capability Limits of Machine are not exceeded.Other Equipment are within safe operation.

DYNAMIC CHARACTERISTIVCS of Modern EXC SYSTEMSHIGH Initial Response(Voltage Response Time is the time in seconds for EXC SYS to attain 95% of Diff between Vc and Vr). VRT of 0.1 sec or less qualifies as High and therefore fast acting.EXC SYS Ceiling VoltageMaximum direct voltage that EXC SYS is able to supply from its terminals under specified conditions.

EXC SYS Ceiling VoltageMaximum direct voltage that EXC SYS is able to supply from its terminals under specified conditions.

EXC SYS Ceiling CurrentMaximum current that EXC SYS is able to supply from its terminals under specified conditions.

Ceiling Voltage & Response Ratiooedaboa=ed= rated voltage

Eb=ceiling voltage

oe- 0.5 secs.

Generator and its excitation system ratingGenerator and its excitation system shall have a capability at least matching the declared maximum continuous rated output of the associated turbine at

all power factors between 0.85 lagging and 0.95 leading

with +3% to -5% frequency variation

terminal voltage variation of +/- 5%

combined voltage & frequency variation of 5%.

Generator and its excitation system ratingAlso the generator and its excitation system shall be capable of continuous stable operation at the peak output of the associated steam turbine under VWO & HP heater out condition, etc. as available for the secondary cooling water temperature of 39 deg. C

at all power factors between 0.85 lagging and 0.95 leading

with +3% to -5% frequency variation

terminal voltage variation of +/- 5%

combined voltage & frequency variation of 5%.

SPECIFICATION REQUIREMENT OF EXC. SYSTEMThe excitation system shall be designed in such a way that due to any fault in AVR firing circuit pulse transformer, rectifying elements in any channel etc, excitation system shall be available in full capacity. all rectifying elements shall have over voltage and short circuit protection.

Field forcing capability- each excitation system channel shall be capable of supplying without damage to any of the components, the field forcing voltage and current for a period of 10 secs without exceeding the limits of temperature for rectifier junction .

SPECIFICATION REQUIREMENT OF EXC. SYSTEMRedundancy2x100% auto channels including AVRs, power converters & controlseach channel capable of being selected as manual

Interface with ATRS and EHG systemExcitation system response time - 0.5Excitation system response ratio- > 2Excitation system ceiling voltage- >1.5 times rated load excitation voltage

Term Used-Ceiling VoltageIt is the max. voltage that can be impressed on the field under specified conditions.Ceiling voltage ultimately determines how fast the field current can be changed.For normal disturbances, ceiling condition prevails for a 10 seconds max. to either increase or decrease the excitation until the system returns to steady state operating state.

EXCITATION AND VOLTAGE REGULATIONIf(nl)VIaIf(fl)Field currentOCCSCCArmature currentTerminal voltage

VOLTAGE REGULATORAutomatic voltage regulator shall constantly compare a ref voltage signal with the generator terminal voltage feedback signal and shall amplify the error signal which in turn will control the firing circuit of the thyristor bridges. Depending on the firing angle, field voltage in turn field current and machine voltage shall be regulated.

Automatic Voltage Regulator (AVR)Excitation Control Functions:Stator Current limiterRotor Current LimiterLoad Angle LimiterOver-fluxing Limiter / Controller Field ForcingPower System Stabilizer

All limiting mechanisms have an effect only in generator-voltage control mode (automatic mode).

EXCITATION SYSTEM

Excitation System incl AVRBrushless System (BLE)Static System (SEE)StandardsTrends

BRUSHLESS TYPE EXCITATION SYSTEM

Advantages of Brushless excitation system Elimination of slip rings and brushes helps in handling high excitation currents No field flashing circuit for initial voltage build up Rating of power components are smaller, hence the panel requirement is less Independent supply of Excitation power from a shaft driven generator and therefore no impact on excitation in case of line short circuits. No power links to generator No space requirement for excitation transformer as the same is obtained through PMG.

Brushless excitation system- concerns Additional Rotating Exciter and Pilot Exciter.Response is slowerTG deck is longer Large number of power components on shaft eg. Diodes, snubbers, flexibles etc. Failure will lead to longer down time.Longer shaft system, number of bearings.Diode monitoring only. Replacement needs shutdown.

EXCITATION SYSTEM

Excitation System incl AVRBrushless System (BLE)Static System (SEE)StandardsTrends

STATIC EXCITATION SYSTEM

FEATURES OF STATIC EXCITATION SYSTEMTHE EXCITATION SYSTEM SHALL CONSIST OF -RECTIFIER TRANSFORMER-INDOOR TYPE, 3 PH. TRANSFORMER WITH CLASS F INSULATION CONNECTED TO THE GEN. TERMINAL THROUGH ISOLATED PHASE BUSDUCT. -THYRISTOR CONVERTOR CUBICLE ALONG WITH FIRING CONTROL, 3 PHASE, FULL WAVE BRIDGE TYPE-THE CONVERTER SHALL HAVE REDUNDANCY FEATURE.

THYRISTOR BRIDGE (SEE)N+2 redundancyN is the number of bridges can meet the rated field currentN+1 can meet the ceiling voltage

THYRISTOR BRIDGE (SEE)3 phase fully controlled, 6 thyristors, arm fuses and snubber circuitcooling fansfinal pulse stage module for each bridgepulse transformers

FIELD CIRCUIT BREAKER - FEATURES-THE FCB SHALL BE OF DC , MULTIPOLE AIR BREAK TYPESUITABLE FOR OPERATION FROM LOCAL PANEL AS WELL AS FROM UCBFCB SHALL BE DESIGNED TO CARRY CURRENTS FOR CONTINUOUS AS WELL AS SHORT TIME DUTY OF THE EXCITATION SYSTEM-DICHARGE CONTACTS SHALL BE RATED TO DISCHARGE THE FIELD ENERGY CORRESPONDING TO THE HIGHEST FIELD CURRENT WHICH MAY COME DURING ITS ENTIRE OPERATING RANGE.

FEATURES OF STATIC EXCITATION SYSTEM-MAIN FIELD FLASHING DEVICE-INITIAL FIELD FLASHING IS FROM STATIONS 415V SUPPLY. -SUITABLE RECTIFIER AND FILTERS REQUIRED FOR CONVERTING AC TO DC.-GATE FIRING CIRCUIT-SHALL PROVIDE A LINEAR RELATIONSHIP BETWEEN THE CONTROL VOLTAGE AND THE RECTIFIER OUTPUT.

Advantages of Static Excitation system Elimination of all rotating parts -reducing losses-reducing operating cost, Easy maintenance Better redundancy arrangement. No limitation on the degree of redundancy of rectifier. High reliability & availabilityImproved performance during transients and load throw-offs. Due to higher field forcing and absence of rotating exciter - fast excitation response

Advantages of Static Excitation system

Direct measurement of field current and voltage provides better means of monitoring and control of field winding Faster field discharge capability prevents excessive damage to m/c in case of internal fault.Repair work can be started when the m/c is in running condition.Simplified bearing configuration Less balance problem on shaft Shorter TG shaft-shorter TG foundation and thus reduced civil structure

AVR = Autom. Voltage Reg.FCR = Field Current Reg.AVRFCRAVRFCRDual Channel SystemType AFTWith Redundant Twin Converters (cold standby, 100% redundancity, automatic change over)

VOLTAGE REGULATOR-POWER SYSTEM STABILISER.ELECTRO-MECHANICAL OSCILLATIONS. FOLLOW UP CONTROL. SMOOTH CHANGEOVER FROM WORKING TO STANDBY.

-AUTOMATIC CHANGE OVER FROM AUTO-1 TO AUTO-2 AND VICE VERSA AT THE TIME OF. LOSS OF TERMINAL VOLTAGE FEED BACK. LOSS OF AUTOMATIC CHANNEL REF. SIGNAL. MANUAL INTERVENATION.

EXCITATION SYSTEM

Excitation System incl AVRBrushless System (BLE)Static System (SEE)StandardsTrends

IEC-60034-1, IEC-60034-3

IEEE 421.1 to .5 Exc Systems

#IEEE 421.4 covers guidelines for preparation of Excitation System Specifications.

IEEE C37.90.1 Surge Withstand Capability (SWC) TestsOscillatoryFast Transient

Power System Stability and Control: P Kundur

EXCITATION SYSTEM

Excitation System incl AVRBrushless System (BLE)Static System (SEE)StandardsTrends

500 MW sets: mostly BLE660 MW sets: Sipat has BLE. All other manufacturers provide SEE as their standard offer660/ 800 MW BHEL sets are BLE systems except Barh2All other leading manufacturers provide SEE as their standard offer

The Field Circuit Breaker (FCB) of BLE is of lower rating (typ 150 A). Field discharge is by inversion and FCB secondary contacts.Field Circuit Breaker (FCB) of SEE is of high rating (typ 6300 A).Some manufacturers provide FCB on the ac side. Field discharge is by inversion and overvoltage protection by thyristors triggering onto discharge resistor.

**Also here we can get twin configuration for the power converter bridge is possible .