STATIC EXCITATION EQUIPMENT FOR SYNCHRONOUS GENERATORPRESENTED BY:RANDHIR KUMAR VERMA AWADHESH KUMAR SINGH ARAVIND KUMAR YADAV ABHAY KUMAR (EEE, 8TH SEMESTER ,SOE, CUSAT )

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PRESENTATION OUTLINEy Introduction . y Excitation principle & Types . y Excitation transformer . y Field flashing & Field breaker . y Automatic voltage regulator (AVR) . y Renovation And Modernization. y Conclusion . y References .

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OVERVIEW OF A POWER STATIONHV SYSTEM CONTROL ROOM STEP UP TRANSFORMER LV SWITCHGEAR AC & DC AUXILIARY SYSTEMS

HV- BREAKER

GOVERNOR 1 GENERATOR BREAKER

AUX. TRANSF .

PROTECTION 1

CONTROL SYSTEMS

SYNCHRONIZING

PT's & CT's

TURBINE

SYNCHRONOUS GENERATOR

EXCITATIO NSYSTEM

STAR POINT CUBICLE

EXCITATION TRANSFORMER

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WHAT IS EXCITATION SYSTEMy Creating and strengthening the magnetic field of the

generator by passing DC through the filed winding .

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TYPES OF EXCITATION SYSTEMy Conventional DC (up to 100/110 MW units). y AC (High frequency) Excitation system. y Brushless system (used in 500 MW units). y Static excitation system (used in 200 MW units).

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EXCITATION PRINCIPLE

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EXCITATION PRINCIPLE

Flux in the generator rotor is produced by feeding DC supply in the field coils, thus forming a 2 pole magnet of rotor7

Slip Rings

From ExciterBrush

P O L E

P O L E

P O L E Shaft

P O L E

P O L E

P O L E

EXCITATION DC DISRIBUTION8

EXCITATION PRINCIPLE Stator induced Voltage E = K. L. d / dt K = constant L = length exposed to flux d / dt = rate of change of flux Frequency of induced Voltage F = NP / 120 Magnitude of flux decides generated voltage and speed of rotation decides frequency of generated voltage .9

Regulator Power supply Rotor current limiter

X-mer prot >1 X-mer prot >1 Pulse supervision

Rectifier (exct) transformer 11kv/415v

415/380 415/380

Stator current limiter Filter Rotor angle limiter Filter

THYRISTOR Fan supplyGate ckt Pulse amplifier Pulse final Stage

PT CTSlip stabilizer M

Automatic voltage regulator Auto/Man M

Auto channel Grid control

DC sourceField flashing

Manual channel Grid control Pulse amplifier

o/ v protComparator& Follow up

AC source

GField breaker

STATIC EXCITATION SYSTEM: BLOCK DIAGRAM10

BLOCK DIAGRAM OF SEE(static excitation equipment)

AVR maintains the Generator terminal voltage.11

Static excitation systemSupply tapped from generator terminals. Brushes are used to supply field current . Thyristor bridge is used with full wave rectification to supply DC voltage . At the time of starting field is supplied from external source called field flashing .12

Cont .y Field forcing provided through 415 v aux supply . y Thyristors are cooled by air . y The static excitation system is the most commonly used

excitation system for hydro generators .

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EXCITATION TRANSFORMERy It is directly connected to the generator & feeding power to the machine via thyristor convertor. y Oil & clophen filled transformers are still adopted for large rating. y But oil & clophen filled transformer are banned. y Dry type transformers were taken up for large power & high voltage rating.

TRANSFORMER CUBICAL RATINGy KVA-850 y Primary Voltage - 11 KV y Secondary voltage - 380 V y NO. of phases - 3

50 HZ y Basic impulse level 75 KV y Connection DYN 5 y Type - CAST RESIN COIL DRY TYPEy Frequency

SALIENT FEATURESy Short circuit proof . y High over load capacity . y Resistant against temperature fluctuation . y Moisture proof . y Immediate switch on .

Conty Non-inflammable y Partial discharge y Compact installation y Operating condition

FIELD FLASHINGy It is required for initial excitation as no power is y

y y y

available to excitation system. For start up DC excitation is fed to the field from external source like station battery or rectified AC from station Ac supply . Filed flashing is used to build up voltage up to 30 %. From 30 to 70 % both flashing and regulation remains in circuit. 70 % above flashing gets cut-off.

FIELD BREAKERy The occurrence of a fault on the generator terminals the fault current has to be reduced as quickly as possible to limit the resulting damages. y In this, it is necessary to disconnect the excitation source to reduce the fault current quickly & also to avoid high voltage across the rotor which may feed fault current

AUTOMATIC VOLTAGE REGULATOR(AVR)

Automatic voltage regulator (AVR) maintains the Generator terminal voltage at a given value automatically by changing the excitation current to the Generator field. The AVR supplies the required D.C. to the Generator field depending on the load, power factor etc. to maintain a constant terminal voltage.

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CONTROL SYSTEMS OF AVR1. Auto control 2. Manual control

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TYPES OF AVR1.Single channel AVR

Two controllers one is automatic and the other is manual Both the controllers are fed from the same supply22

2. Dual channel AVR system One automatic voltage controller and one manual controller Different power supply, gate control and pulse amplifier units for each of the controllers

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3.Twin channel AVR system

Two automatic voltage regulators

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Function of AVR

compares the Generator terminal voltage with a preset reference voltage. If the Generator terminal voltage is less than the reference voltage, the AVR increases D.C. voltage across the Generator field. Maintaining the constant voltage as per the setting.25

EXCITATION AS A CLOSED LOOP CONTROL SYSTEM

If

Synchronous Generator

VgGrid

Excitation

System

The automatic voltage control system

OPERATIONAL AMPLIFIERFEED BACK GAIN7

I N P U T

2

6OP AMPOUTPUT

3

REF

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Block diagram of AVR

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AVR Inputs

Generator voltage feedback signal Generator voltage reference

AVR provides the following functions and signals

Summing and amplification of signals to provide the exciter rectifier firing angle control signal. A signal for manual follow up.29

AVR Action

Two inputs are of opposite polarity. When the magnitudes are equal, the net input is zero. When unequal-the error is amplified and reversed to obtain a correction signal which goes to the GPG(gate pulse generator)

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AVR CIRCUIT

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+-50 V SUPPLY

To power the regulator amplifier as well as the optional accessories such as manual follow up and power integral stabilizer

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AVR CIRCUIT

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Regulator Auxiliary Card

3 phase voltage from the generator PT s are stepped down and applied to the regulator auxiliary card which converts it to ripple free dc voltage suitable for the input of regulator amplifier.

Balance Voltmeter

A 30-0-30 V dc voltmeter is connected across the output of regulator amplifier to read the magnitude and polarity of the output voltage of OP-AMP at any instant.

AVR CIRCUIT

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AVR Voltage Setting

From the +-50 V source ,-50 V is applied and taken across a motorized potentiometer which can be operated from the control bench board. The variable output from the pot. is applied to the input of the regulator OP-AMP.

Manual Voltage Control

In the absence of AVR the voltage can be manually controlled. The stabilized 15 V from the MPS is applied across a second potentiometer. The variable output goes to the GPG together with output of the regulator OP-AMP.

AVR CIRCUIT

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MANUAL FOLLOW UP

When there is a noticeable output from the regulator OP-AMP, depending on its polarity, either of a pair of relays is automatically closed to drive the manual potentiometer motor in the required direction.

THYRISTOR RECTIFIER RECTIFIER PRODUCES THE DC VOLTAGE TO BE FED TO THE GENERATOR FIELD AS PER THE SIGNAL PROVIDED BY THE AVR. THREE PHASE AC IS CONVERTED INTO DESIRED DC VOLTAGE BY PHASE CONTROL OF SCRs. MORE THAN ONE BRIDGE CIRCUITS ARE USED IN PARALLEL TO INCREASE THE DC CURRENT LIMITS OF EXCITERS

THYRISTOR SIGNALSAnode

PJ1

NJ2

PJ3

Gate

NCathode

3 PHASE SCR BRIDGE

R Y B

R

B

Y

3 PHASE SCR OUTPUT WAVEFORM

FIRING ORDERTime/ Degree

0

60

120

180

240

300

360

Thyristor

1

2

3

4

5

6

1

Thyristor

6

1

2

3

4

5

6

Current Limiting Resister From Exciter Transformer

Blocking Diode F

g1

g3

g5

I E L D

g4

g

g6 2

C O I LField Breaker

RENOVATION AND MODERNISATION Ancient excitation method was simple electro-

magnetic type. Due to the increase in unit sizes of generators and commutation problems associated with dc exciters. when suitable high current and PIV rated thyristors become available, S.E.S have been established.

Advantages of Static Excitation EquipmentResponse: Fast response system leads to better

system stability during both transient and dynamic condition. the machine does not lose its synchronism during severe disturbances.Redundancy: S.E.S works when problems in the

main circuits. circuits are planned to offer many options for running the unit without shutdown even during faults

Technology: There is absolutely no problem regardingspares availability and at the time the Equipment will be highly reliable with many novel features helping the operating personnel. Interface: In this there are several optional facilities for interfacing various types of other plant equipments for erasing out extra burden on operating staff. Uprating: SEE can be normally designed with certain margins whenever required and can be used during small uprating of about 10% if required in future.

Comparison of Features of Static Excitation System and Brushless Excitation system

Very fast response and high

initial response systems.

Fast response

Generator field voltmeter and

rotor temperature indicator can be provided due to the presence of slip rings. slip rings for current collection for large TG sets generally limited for single units up to 600MVA rating.

Generally generator field

voltmeter and rotor temperature indicator cannot be provided due to absence of slip rings.

Due to problems in design of

For TG unit with single machine

size of over 600MVA and where elimination of current collection slip rings and brushes is important, this is preferred.

CONCLUSIONy Static Excitation System using shunt connected, completely

static with thyristor control system or brushless excitation systems are being used. along with the fast operation of the excitation system. transient stability of the power system.

y The use of SEE results in considerable energy savings at the plant y The fast response of the excitation system assists in maintaining y Quality improvement in design and manufacturing is gradually

strengthening. and it has large positive future scope of developments.

REFERENCEy Journal on Design of cost effective excitation system for synchronous machines , y Published in European journal of scientific research, Nov. 2007. y Power Apparatus and Systems, IEEE Transaction, Jan. 2007. y The transmission and distribution reference book, B.M Weedy, Washington house y y y y y y y

Elect. Corp. BHEL manual for Static Excitation Equipment. Electrical power by Dr. S L Uppal. Electrical Power System by C L Wadhwa. Modern Electrical Power by Soni Gupta, Bhatnagar and Chakroburthy. Generalised Theory of Electrical Machines by Dr. P S Bimbhra. Modern Power System Analysis by I.J. Nagrath & D.P. Kothari. power electronics by P S Bimbhra.

y LOWER PERIYAR HYDROELECTRIC POWER PLANT, KERALA, INDIA .

(under sri jayasankar, assistant executive engineer,lower periyar,kothamangalam)

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