Generator Course

Generator

Principles Generator Construction Excitation System Operational aspects Generator Protections

Alternator: An Alternator is an equipment which converts Mechanical Energy into Electrical Energy.

Sources of Mechanical Energy Steam

Turbines Gas Turbines Hydraulic Turbines Other sources

This

forms the basis for the conversion of Mechanical Energy into Electrical Energy.

This

can be presented in two forms which are applicable to the operation of Generators.

N

R Y B S

When a Rotating Magnetic Field cuts the conductors, Voltage is induced in them.

•

This is applicable to the main generator

•

S

+  _

 N

 N

S

When a Rotating conductor cuts the Magnetic Field , Voltage is induced in the conductor. This applicable to the Exciter.

•

•

 A

DC Voltage is applied to the Rotor and it becomes a Magnet

When

this magnet is rotated,it induces voltage in the stator windings

STATOR ROTOR EXCITER SLIP

RINGS BRUSHES BEARINGS COOLING SYSTEM

PROVIDED

FOR EXTERNAL EXCITATION

USED

FOR ROTOR EARTH FAULT PROTECTION

SLEEVE

THEY

NDE

BEARINGS ARE USED

ARE MADE OF WHITE METALS

SIDE BEARING IS INSULATED TO  AVOID SHAFT CURRENT CURRENT CIRCULATION

Stator

frame Stator core laminations Stator windings End shield Rotor poles Rotor coils

: Mild steel : Silicon steel : Copper strips : Aluminum alloy : Alloy steel : Silver alloyed copper

To

take away the heat dissipated in the stator and the rotor

Maximum

loading of the machine depends on the effectiveness of cooling system.

 AIR

COOLING

WATER

-

COOLING

HYDROGEN

COOLING

-

For low ratings

For medium ratings

For higher ratings

TO SUPPLY THE FIELD WINDING OF THE GENERATOR WITH DC CURRENT AND CONTROL IT  THIS KEEPS THE TERMINAL VOLTAGE CONSTANT  ENSURES PROPER REACTIVE POWER SHARING  IMPROVES STABILITY 

STATOR STATOR

+

ROTOR

DIODE

ROTOR

SHAFT ROTOR

-

RECTIFIER

ROTOR STATOR

STATOR Powering

11 KV PT

AVR

STATOR

ROTOR

STATOR

+ DIODE

STATOR ROTOR

MAGNET

ROTOR

MAGNET

SHAFT ROTOR

-

RECTIFIER

STATOR STATOR

STATOR Feed back

11 KV PT

AVR

POWERING

Aux. power

AC - DC

+

CONVERTER

-

TO EXCITER

FIRING PULSE GENERATOR

Aux. power

CONTROLLER

Reference

Feedback REFERENCE

FEEDBACK

PROCESSING

PROCESSING

There

are two identical AVRs present The AVR will not come into line unless the turbine reaches 85% of full speed One of the AVR will be on line and the other one will remain as standby. If the main AVR fails , then the stand by AVR will automatically come onto line  A manual excitation control provision

Parameters List:  Voltage Frequency  Active

power Reactive power Excitation voltage / current Power Factor

Frequency - Real power  f  NL FSR 3

FL

FSR 2 FSR 1

P

Voltage - Reactive Power: V

 NL FL

IF 3 IF 2 IF 1

Q

TURBINE REACHES FULL SPEED

CLOSE THE FIELD BREAKER

BUILD UP VOLTAGE TO 11KV

SYNCHRONISATION

INCREASE THE LOAD

ISLAND OPERATION

PARALLEL OPERATION

6 MW

GT-A

6 MW

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

More

load in shared condition.

Increase

Flexibility

in Reliability.

in Maintenance.

GT-A

GT-B

ARE THE FREQUENCIES SAME? ARE THE PHASE SEQUENCES SAME?

LOADS ARE THE VOLTAGES SAME?

3 7 MW

GT-A

4 MW

GT-B

LOAD HAS TO BE SHARED BETWEEN THEM

7 MW

LOAD LOAD DEMAND IS CONSTANT

f 

GT-A

P

6 MW

3 MW

GT-B

1MW 4 MW

P



Load demand is constant



Increase in Governor set point of one Generator increases the system frequency and Real power on that Generator



Increase in Field current of one Generator increases the system voltage and Reactive power on that Generator

Voltage and Frequency are constant i.e not in our control f

V

P

Q

WHEN INCOMING FREQUENCY IS GREATER THAN GRID: APTRANSCO

P

PAPTRANSCO

f 

GT-A

PGT

P

WHEN INCOMING FREQUENCY IS LESSER THAN GRID: APTRANSCO

P

PAPTRANSCO -PGT

f 

GT-A

P GT-A ACTS AS LOAD

WHEN THE GOVERNOR SET POINT OF GT-A INCREASES:

APSEB

P

6 MW 4 MW 2 MW

f 

GT-A

1 MW 3 MW 5 MW

P

 Voltage

and Frequency are fixed by GRID

Governor

set point decides the Real

Power Field

current decides the Reactive power

To

prevent the equipment from severe damages during fault conditions

To

continuously track the various parameters 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 Protection Over / Under Frequency Protection Over / Under voltage Protection

CTS

Winding

R

Winding

Y

Winding

B O/C OCCURS

I > 540 A or 1941 A NGR 

RELAY

Relay operates in ‘ t ‘ sec

It monitors the overloading pattern of the generator and trips incase of heavy loading It 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

Winding

R

Winding

Y

Winding

B

I >15.2 A or 20 A

CT Relay operates

NGR 

The

star point of the Generator is earthed through a Resistor to limit Earth fault current

So

whenever an Earth fault occurs , the fault current flows through NGR

When

the neutral current exceeds the set value , the relay operates

CTS

RELAY

CTS Winding

R

Winding

Y

Winding

B

I > 105 A (OR) 350 A Relay operates

This

relay is to protect the stator winding phase wise This is the fastest relay available Under normal condition both the CT currents are same and the differential current is zero. But when there exists a fault in the stator winding both CT currents differ leading to a differential current When this differential current is more than set value the relay operates

Unbalance

condition arises when the three phase loads are not same

This

relay operates if the percentage of unbalance exceeds the set value

Typically

8%

To

avoid reverse flow of power i.e power into the generator In this state Generator will act as a motor If continues to operate , this will cause overheating of turbine Typically 3% ( 5% ) reverse flow

To

avoid running the machine without excitation It works on the principle, Impedance=V/I. The terminal voltage falls if a sudden heavy load demand or AVR failure occurs, this will cause the current to increase and the impedance falls. The relay is an Impedance relay and it operates when Impedance falls below



UNDER VOLTAGE

= 7.4 KV

; 7.5 KV 



OVER VOLTAGE ALARM = 12 KV



OVER VOLTAGE TRIP

= 12.5 KV , 12.5 KV



OVER FREQUENCY

= 51.5 Hz , 52 Hz



UNDER FREQUENCY

= 47 Hz

; 11.5 KV

, 47.5 Hz