Generator Course

8/10/2019 Generator Course

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Generator Principles

Generator Construction

Excitation System

Operational aspectsGenerator Protections



Alternator:

An Alternator is an equipment which

converts Mechanical Energy into

Electrical Energy.



Sources of Mechanical

Energy

Steam Turbines

Gas TurbinesHydraulic Turbines

Other sources



This forms the basis for the conversion ofMechanical Energy into Electrical Energy.

This can be presented in two forms whichare applicable to the operation of

Generators.



•When a Rotating Magnetic Field cuts the conductors,

Voltage is induced in them.

N

S

RYB

•

This is applicable to the main generator



•When a Rotating conductor cuts the Magnetic Field ,

Voltage is induced in the conductor.

N N

S

S

+

_

•This applicable to the Exciter.



A DC Voltage is applied to the Rotor and itbecomes a Magnet

When this magnet is rotated,it inducesvoltage in the stator windings



STATOR

ROTOR

EXCITER

SLIP RINGS

BRUSHES

BEARINGS

COOLING SYSTEM



PROVIDED FOR EXTERNAL

EXCITATION

USED FOR ROTOR EARTH FAULT

PROTECTION



SLEEVE BEARINGS ARE USED

THEY ARE MADE OF WHITE METALS

NDE SIDE BEARING IS INSULATED TO AVOID SHAFT CURRENT CURRENTCIRCULATION



Stator frame : Mild steel

Stator core : Silicon steel

laminationsStator windings : Copper strips

End shield : Aluminum alloy

Rotor poles : Alloy steel

Rotor coils : Silver alloyed copper



To take away the heat dissipated in thestator and the rotor

Maximum loading of the machine dependson the effectiveness of cooling system.



AIR COOLING - For low ratings

WATER COOLING - For medium ratings

HYDROGEN COOLING - For higher ratings



TO SUPPLY THE FIELD WINDING OF THEGENERATOR WITH DC CURRENT ANDCONTROL IT

THIS KEEPS THE TERMINAL VOLTAGECONSTANT

ENSURES PROPER REACTIVE POWERSHARING

IMPROVES STABILITY



AVR

STATOR

ROTOR

STATOR

ROTOR

STATOR

STATOR

ROTOR

ROTOR

DIODE

RECTIFIER

+

-

11 KV

MAGNET

MAGNET

STATOR

STATOR

SHAFT

PoweringPT

Feed back



AC - DC

CONVERTER

+

- TO EXCITERPOWERING

REFERENCE

PROCESSING

FEEDBACK

PROCESSING

CONTROLLER

FIRING PULSE

GENERATOR

Reference Feedback

Aux. power

Aux. power



There are two identical AVRs present

The AVR will not come into line unlessthe turbine reaches 85% of full speed

One of the AVR will be on line and theother one will remain as standby.

If the main AVR fails , then the standby AVR will automatically come ontoline

A manual excitation control provisionalso exists



Parameters List:

Voltage

Frequency

Active power

Reactive power

Excitation voltage / current

Power Factor



Frequency - Real power

f

PFL

FL

NL

FSR 1

FSR 3

FSR 2



Voltage - Reactive Power:

QFL

FL

NL

V

IF 1

IF 2

IF 3



TURBINE REACHES FULL SPEED

CLOSE THE FIELD BREAKER

BUILD UP VOLTAGE TO 11KV

SYNCHRONISATION

INCREASE THE LOAD

ISLAND OPERATION PARALLEL OPERATION



GT-A

LOADS

Real and Reactive power supplied will be the amount demanded by the Load.

GOVERNOR SET POINT DECIDES THE FREQUENCY

FIELD CURRENT DECIDES THE TERMINAL VOLTAGE

6 MW

6 MW



More load in shared condition.

Increase in Reliability.

Flexibility in Maintenance.



LOADS

GT-A GT-B

ARE THE VOLTAGES SAME?

ARE THE PHASE SEQUENCES SAME?

ARE THE FREQUENCIES SAME?



LOAD

GT-A GT-B

7 MW

4 MW3 MW

LOAD DEMAND IS CONSTANT

7 MW

LOAD HAS TO BE SHARED BETWEEN THEM



PP

GT-BGT-A

1MW6 MW

f

3 MW 4 MW



TO SUMMARISE

Load demand is constant

Increase in Governor set point of one Generator

increases the system frequency and Real poweron that Generator

Increase in Field current of one Generator

increases the system voltage and Reactive poweron that Generator



WHEN INCOMING FREQUENCY IS GREATER THAN GRID:

f

PP

GT-AAPTRANSCO

PGTPAPTRANSCO

GT-A TAKES PART OF LOAD



P

f

P

GT-AAPTRANSCO

PAPTRANSCO -PGT

WHEN INCOMING FREQUENCY IS LESSER THAN GRID:

GT-A ACTS AS LOAD



Voltage and Frequency are fixed by GRID

Governor set point decides the RealPower

Field current decides the Reactive power



To prevent the equipment from severedamages during fault conditions

To continuously track the variousparameters and isolate the equipment

when they deviate from the set value



Over current Protection

Stator Earth Fault Protection

Differential Protection

Unbalance Protection

Reverse power Protection

Loss of Excitation ProtectionOver / Under Frequency Protection

Over / Under voltage Protection



R

Y

B

Winding

Winding

Winding

NGR RELAY

O/C OCCURS

I > 540 A or 1941 A

Relay operates in ‘ t ‘ sec

CTS



It monitors the overloading pattern of the generator

and trips incase of heavy loadingIt follows an Inverse curve and so it trips in less time

for a higher current

• The set point initiates the relay typically at 540 A or 1941 A

t

I



R

Y

B

Winding

Winding

Winding

NGR

CT

I >15.2 A or

20 A

Relay operates



The star point of the Generator isearthed through a Resistor to limitEarth fault current

So whenever an Earth fault occurs , thefault current flows through NGR

When the neutral current exceeds theset value , the relay operates



R

B

YWinding

Winding

Winding

RELAY

CTS CTS

I > 105 A (OR) 350 A

Relay operates



This relay is to protect the stator

winding phase wiseThis is the fastest relay available

Under normal condition both the CT

currents are same and the differentialcurrent is zero.

But when there exists a fault in the

stator winding both CT currents differleading to a differential current

When this differential current is more

than set value the relay operates



To avoid reverse flow of power i.e powerinto the generator

In this state Generator will act as a motorIf continues to operate , this will cause

overheating of turbine

Typically 3% ( 5% ) reverse flow



To avoid running the machine withoutexcitation

It works on the principle,Impedance=V/I.

The terminal voltage falls if a suddenheavy load demand or AVR failureoccurs, this will cause the current toincrease and the impedance falls.

The relay is an Impedance relay and itoperates when Impedance falls below

certain limit.



UNDER VOLTAGE = 7.4 KV ; 7.5 KV

OVER VOLTAGE ALARM = 12 KV ; 11.5 KV

OVER VOLTAGE TRIP = 12.5 KV , 12.5 KV

OVER FREQUENCY = 51.5 Hz , 52 Hz

UNDER FREQUENCY = 47 Hz , 47.5 Hz



DIFFERENTIAL RELAY ACTS

STATOR EARTH FAULT OCCURS

LOSS OF EXCITATION HAPPENS

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