ByKirti Ratwa

As synchronous machines is constants speedy machine for a constant frequency supply, the output voltage of the machine depends on the excitation current.

RELIABILITY STABILITY QUICK RESPONSE

The excitation system must be reliable, stable & must respond quickly to excitation current requirements.

Nominal exciter response is defined as the ratio: Rate of increase of exciter voltage in volts per second

nominal excitation voltage of generator

USE IN MOST OF OLDER UNIT IN THE COUNTRY. D.C. EXCITERS ARE BASICALLY ROTATING SHUNT WOUND

MACHINES THIS EXCITER CAN BE DRIVEN FROM MAIN GENERATOR

SHAFT EITHER BY DIRECT COUPLING OR THROUGH REDUCTION GEARS.

THE GENERATION OF D.C. SUPPLY IS ACHIVED IN TWO STAGES,

Controlled either manually or via an AUTOMATIC Voltage Regulator(AVR)

The AVR measure the generator terminal voltage & compares it to desired set value.

The A.C. exciter gained favour of the problems & constraints associated with DC excitor.

Commutator & DC brush gear were eliminated giving place to simple rotor slip rings & associated brush gear.

Types A.C. excitation system

Rotating System : This is high frequency excitation system with an AC exciter, permanent magnet piolt exciter & an uncontrolled rectifier.

Static Excitation : This is generator potential fed static excitation system with controlled rectifier (SCR) & a rectifier transformer.

• For maintain system stability, it is to necessary to have fast excitation system for large synchronous machine which means the field current must be adjusted extremely fast to changing operational condition.

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

RECTIFIER TRANSFORMER

SCR OUTPUT STAGE

EXCITATION START UP AND FIELD DISCHARGE EQUIPMENT

REGULATOR AND OPERATIONAL CONTROL CIRCUITS.

1. The excitation power is taken from generator o/p & fed to the exaction transformer(rectifier ) which is step down to the required voltage for SCR bridge & fed through the field breaker to generator field.

• Its dry type cast coil type transformer.

• Its provide current for over current protection also its capable of supplying ceiling currents at the ceiling excitations for a short period of ten seconds.

The thyristor Rectifier Bank consists of a suitable number of bridges connected in parallel.

Each thyristor bridge comprises six thyristors , fuses and snubber ckt for protection.

Current Carring capacity of eacg bridge depends on the rating of indivusual thyrisor

By changing the firing angle of thyristors, variable o/p is obtained.

Each Bridge is contolled by one final pluse stage & cooled by fan.

N-1 Bridge operation is ensured. These bridges are euipped with protection

devices & failure of one bridge causes alarm.

Failure of two tyristor bridges excitation current will be limited to a predetermined value lesser than normal current.

Failure of three bridges result in generator tripping & rapid de-excitation of the generator.

As residual magnetism of generator is not sufficient to start voltage built up.

This type of system needs separate equipment for excitation start up( Field Flashing).

The field flashing is done with Station supply / Dc voltage(battery) for reliability.

At the moment closing the field breaker ,field flashing breker also closes and generater voltage builds up to 30% of rated voltage.

The field flashing breaker is automatically switch off as the generator voltage reaches to 70% of full voltage of generator.

On Tripping of generator, Its stored energy is required to be discharged.

This is done through a non linear discharge resistance.