Excitation System

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June 28, 2002

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To build-up and maintain constant terminal voltage.

To supply reactive power to the system.

Other sources of reactive power are:

Generator-transformer.

Synchronous condenser.

Series / shunt compensation.

To keep the generator parameters within the

Capability chart.

Functions of an Excitation System

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To protect the transformer from over-fluxing.

To ensure system stability in DYNAMIC state by

providing damping to the electro-mechanical

transients.


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To ensure system stability under TRANSIENT

conditions.

To ensure fast voltage recovery after fault

clearance.

Also, most of the generator protection features can

be incorporated.


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Evolution of Excitation Systems

Over 40 Years of Static Exciter Design Experience

1960 65 70 75 80 85 90 95

00

Static Excitation

Alterrex

Althyrex

Generrex / Shunt SCR

Bus Fed

Digital

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Various Types of Excitation Systems

DC excitation through DC exciter (Figure 1)

AC excitation through AC exciter (Figure 2)

Brush-less excitation through AC exciter and rotating diodes

(Figure 3)

Static excitation system (Figure 4)

Most commonly used Excitation Systems

Brush-less excitation system

Static excitation system

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Excitation System with DC Exciter

SCR

PT

CT

E G

Feedback to

Regulator

Feed from Generator Terminals

or Auxiliary Bus

Figure 1

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Excitation System with AC Exciter

SCR

PT

CT

E G

Feedback to

Regulator


or Auxiliary Bus

Figure 2

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Excitation System with Brush-less Exciter

SCR

PT

CT

E G

Feedback to

Regulator


or Auxiliary Bus

Figure 3

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Static Excitation System

SCR

PT

CT

G


or Auxiliary Bus

Field

Flashing

Figure 4

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+

-

CONTROLLER

FEEDBACK

SYSTEM

SENSOR

REFERENCE

INPUT

CONTROLLED

OUTPUT

ERROR

SIGNAL

DISTURBANCES

A Feedback Control System

S

Figure 5

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General Functional Block Diagram for Excitation Control System

Excitation

Control

Elements

Exciter

Voltage

Transducer /

Load

Compensator

Synchronous

Machine / Power

System

PSS and

Other

Controls

VREF

VUEL

VOEL Vc

VR VT

VS1

EFD

IFD IT

Figure 6

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General Arrangement of Static Excitation System

AC/DC

Field Flashing

Stator

Current

Limiter

Rotor

Angle

Limiter

Power

System

Stabilizer

Auto Control

Rotor

Current

Limiter

Excitation

Transformer

Protection

EXCITATION

TRANSFORMER

Figure 7

FIELD

BREAKER

Inner Loop Control

Auto

Reference

Value

Manual

Reference

SCR

PT

CT

G

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Major Feedback Control Systems Affecting Generator Parameters

Load-Frequency control loop (P-F loop).

Reactive Power-Voltage control loop (Q-V loop).

Relation between Q, V & PF.

Is there any interaction between P-F & Q-V control loops?

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Ceiling Voltage

It is the maximum (positive or negative) voltage that can be

injected on the field under specified conditions. In static

excitation system ceiling voltage ranges from 1.6-2.0 times

the rated one (Figure 8).

Acts for up to 10 seconds.

Decides fastness of change of excitation.

Basic Concepts of Excitation System

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IEEE 421 High Initial Response Definition

Attain 95% of Ceiling Voltage

in 100 m sec. or less

EX2000 reaches 95% of

Ceiling Voltage in 25 m sec.

Vfield

0.95 (Vceiling - Vrated)

Vceiling

Vrated Time (sec.)

0.025 0.5

Conventional

Rotating Exciter

HIR Exciter

0.1

EX2000

Figure 8

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Response

It is the rate of change (increase or decrease) of excitation

system output voltage.

Response ratio is generally obtained as the excitation

system response in volts per seconds measured over first

0.5 seconds (Figure 9).

Steady State Accuracy

This indicates the steady state error offered by the system.


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VO

Vt

VF

Time (Seconds)

VF - VO

RESPONSE RATIO =

0.5

0.5

Figure 9

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General Block Representation

Time Constant (Figure 10).

Integrator (Figure 11).

Lead-Lag Network (Figure 12).


PID Controls

Proportional Control (Output error signal)

Integral Control (Output is Difference between initial & final values)

Derivative Control (Output is rate of change of error signal)

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Simple Integrator with Limiter

1

s U X

Y

B

A

dy / dt = U

If A >= y >= B, then x = y

If y > A, then x = A

If y < B, then x = B

Figure 10

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1

1 + sT U X

Y

B

A

dy / dt = (u - y) / T

If B A, then x = A

If y < B, then x = B

Simple Time Constant

Figure 11

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1+sT1

1+sT2 U X

Y

B

A

If T1 = T2, then y = u

If B

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General Specifications

Overall response (Figure 13)

Overshoot / undershoot which should ideally be limited to one

overshoot and one undershoot.

Rise time should be very small.

Settling time should be as short as possible.

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OVERSHOOT

SETTLING TIME

(95% VALUE)

TIME

RISE TIME

1.0

0.9

0.1

General Specifications

Figure 13

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Technical Features

Maintains constant terminal voltage.

AC power tapped from generator terminals / PMG / Auxiliary

source.

Rectified through 3, full-wave rectification using Diodes /

Thyristors controlled through the Regulator.

Frequency range 47-52 Hz.

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Range of excitation control in Manual channel 75-125%.

Range of Auto control 90-110%.

Voltage control accuracy 0.5%.

Maximum change in terminal voltage after Auto to Manual

changeover 1%.

Technical Features

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Response ratio 3-5.

Fast response (typically 50 milliseconds for analog

systems and ~ 20 milliseconds for digital regulators).

% Transformer compensation up to 15%.

Keeps the machine within the stability region.

Technical Features

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Protects the generator transformer from over-fluxing.

Provides damping to electro-mechanical oscillations.

Technical Features

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Excitation system maintains system stability (Load-Angle Diagram).

AVR provides both Synchronizing & Damping torque.

Damping is important under steady state. Voltage response is

secondary.

Effect of AVR is to reduce stability region in P-Q plane.

Introduction of PSS enhances stability region in P-Q plane.

AVR gain reduction in transient condition (Figures 14 & 15).

Technical Aspects

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Vref

Vt

+ -

+

+

-

1 + s TB

1 + s TC

s KF

1 + s TF

KAVR

1 + s TA

1

1 + s TR

Efd

VS

IEEE Type ST1A Transfer Function Model of Static Excitation System

Transfer function

of terminal

Voltage transducer

Figure 14

S S

Transient Gain

Reduction (TGR)

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Transfer function

of terminal

voltage transducer

Efd + -

+

1

1 + s TR

Vt

Vref

VS

KA

1 + s TA

Simplified IEEE Type ST1A Transfer Function Model of Static Excitation System

(Transient Condition)

Figure 15

S

KA = KAVR * TB / TC

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Capability Chart

Indicates the operating parameters like MW, MVAR, KV, KA, Field volts,

Field current, Power factor, Load angle and Operating margins of the

machine.

Development of Capability Chart for a Turbo-Generator

INFINITE-BUS

V, F = FIXED G

E Xd V0

Xd = Xd Machine + X External

= Power angle for generator

Figure 16

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jIXd

I

A

V

B

E

O

This is a voltage triangle.

Divide all sides by jXd.

I

A

V/X

B

E/X

O

V

This is a current triangle.

VI

A

V2/Xd

B

EV/Xd

O

Ceiling

Excitation 2Per Unit

Excitation

P

Theoretical

Limit

- Q + Q

1 Per Unit

Excitation

Multiplying current triangle by V converts the current triangle into a power triangle.

Figure 17

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Analysis

Point B: operating point of the machine

V/xd indicates If0

If0 is the field current at no load, which produces generation

voltage equal to the terminal voltage.

At no load = 0, E = V

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+Q is the reactive power delivered to system

-Q is the reactive power absorbed from the system

The operating point lies on a certain excitation locus.

For any excitation the maximum power delivered to the system

is achieved for = 90.

The maximum reactive power absorbed = - V2 / Xd.

E V

P = Sin = V I Cos

Xd

Analysis

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Constraints on Capability Chart

Turbine capacity (MW limit)

Stator current limitation

Ceiling excitation limitation

Practical stability limit

Minimum excitation corresponding to 10% of rated power

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VI

A

V2/Xd

B

EV/Xd

O

P

- Q + Q

1 Per Unit

Excitation

Ceiling

Excitation

MW Limit

Field

Current

Limit

Stator

Current

Limit

Theoretical

Limit

Practical

Limit

Minimum

Excitation

Limit

Figure 18

Capability Chart

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Start up Sequence

Turbine start / Speed up

Adjust AC reference

Synchronizing

Power System

Field flashing command Field breaker closing

Voltage build up on AC

Field flashing stops at 70% KV

Full speed & rated voltage

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Constant voltage Constant VAR Constant PF

AVR modes of operation

Different AVR Modes

Figure 19

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Field breaker is used to limit the fault current and also to

avoid high voltage across the rotor. It also discharges the

stored energy through field discharge resistance (Figure 20).

They are generally DC breakers.

Single pole or double pole breakers used.

Generator field breaker

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Over-voltage

Protection

Excitation

System

Field Discharge

Resistance

Generator

Rotor

Generator Field Breaker


Figure 20

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Field discharge resistance (nonlinear) is provided.

They are make-before-break type.

While closing discharge pole opens first and the main pole

closes later.

While opening discharge pole closes first and the main poles

open later.

The rise and decay in field current is not too fast due to large

time constant of the field. So minor overlapping will not matter

much. However, the condition of the contacts must be monitored

during annual overhauls.


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Over voltage protection (thyristorised / vacuum based) to check

induced voltage rise during any fault on stator side.

Basic components of field breakers are:

Main poles

Discharge poles with arc chutes

Closing coil

Trip coil

Auxiliary circuits


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Field Flashing Circuit

Required for starting the excitation system.

Start-up may be through AC and / or DC supply (Figure 21).

During starting voltage builds-up to 30% due to field flashing only.

At 30% voltage level, the excitation system gets activated

sufficiently and starts functioning. The two raise the voltage level to

70%. At 70% field flashing gets cut off and excitation system works

independently. The auto changeover is realized through a voltage

operated relay.

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Generator

Excitation

System

Generator

Rotor

AC

DC

Generator

Field

Breaker

Field Flashing Circuit

Figure 21

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Serves as power source for the excitation system.

Primary is connected to the generator terminals.

Secondary is connected to the thyristors / regulator.

Selection of transformer depends on the field-forcing voltage and

the maximum continuous current in the field winding.

Location of transformer : generally at zero meter level. The free

surrounding area must be sufficient enough to maintain proper

ventilation and ambient temperature.

Excitation Transformer

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Usually a dry, cast coil type of step-down transformer (Rated volts

/ Nominal volts). This results in reduction of size of the transformer.

Have high overload capacity. Heating time constant is 6-10 times

higher than the oil filled transformer.

Impulse strength is comparable to that of oil cooled transformer.

Cast coil resin is non-hygroscopic.

There is no partial discharge.


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Possibility of liquid leaking is absent.

Maintenance-free. No dry out required even after a long shutdown.

The transformer is generally air-cooled with cooling fans which

operate on over-temperature.

Connection DY.

Standards for manufacturing : BS171, IEC76, ANSI, CS7 and VDE

0532.

Enclosure is made to IP 20, IP 21 and IP 23.

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G

Transformer

Protection

Rotor

Current

Limiter

AVR

Thyristors

Power

Supply

REGULATOR

Power

Supply

AUTO

Power

Supply

MANUAL


Figure 22

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Overload/short-circuit protection through excitation transformer

over-current instantaneous and delayed relay (Figure 22).

(Settings: Instantaneous-200% In, Delayed-120% In).

The CTs are mounted on the primary side of excitation

transformer.

The relays must attenuate the harmonics as they receive signal

from machine terminals.

Excitation Transformer Protection

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Excitation Transformer Protection

Two-stage (Alarm / Trip) over-temperature protection achieved

through thermisters embedded in all the three limbs of the

transformer.

Phase unbalance protection to protect uneven loading of all the

phases.

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Dual power supply (DC / AC) with filter

Reference setting controller: DC (Manual) / AC

DC limiter

Inner loop regulator

Pulse generator

Pulse amplifier

Pulse transformer

Thyristors

Manual / DC Control

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Filter Gate

Control

Pulse

Amplifier

Pulse

Final Stage

Manual / DC

Limiter

Manual / DC

Reference

Control

Pulse

Transformer

Thyristors

Manual / DC Control

Figure 23

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Output of excitation system is controlled by controlling the firing

angle of thyristors which in turn controls the conduction of

thyristors (Figures 24, 25 & 26).

Cooled by fans: monitoring through cooling flow monitoring

system. Cooling of thyristors is monitored by:

Monitoring the status of supply continuously.

Through airflow monitoring unit during running.

Thyristor Bridges

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Thyristor Bridges

Thyristors provided with RC networks for protection against hole

storage effect.

Blocking of thyristors is achieved by:

Monitoring the isolators with micro-switches.

Overcurrent protection of thyristors through semi- conductor

fuse.

Healthiness of fuse is checked through micro-switches or

monitoring system.

Reactors are provided for smooth voltage build-up.

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LOAD

3 Phase

AC

R

Y

B

3

5

6

2

1

4

+

-

Thyristor Bridges

Figure 24

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CONDUCTION

OF

THYRISTOR

30 60 90 FIRING ANGLE ()

UST >0 UST=0 UST

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0 1 2 3 4 5 6 7 8 -1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

R(+) Y(+) B(+) R(+)

Y(-) B(-) R(-) Y(-)

AC

SUPPLY

Thyristor Bridges (Diodes)

Figure 26

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Thyristor Bridges More number of thyristor bridges for higher redundancy.

N is the number of bridges.

N-1 meets full load requirements.

N-2 meets partial load (65%).

Rated field current = 2650 amps, Number of thyristors = 4

Capacity of each set = 1000 amps

N = 4, field current = 4000 amps, N = 3, field current = 3000 amps

N = 2, field current = 2000 amps, N = 1, field current = 1000 amps

Best redundancy is obtained when two identical sets of thyristors are

used for obtaining 100% redundancy.

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Dual power supply (DC / AC) with filter.

AC reference setting controller

Automatic voltage regulator (AVR)

Limit acting regulators

Pulse generator

Pulse amplifier

Pulse transformer

Thyristors

Elements of AC Control Circuit

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Receives voltage and current feedback signals through the

respective transducers.

Converts the feedback signals into a proportional DC signal i.e.

generates actual value signal.

Receives the voltage reference signal through AC voltage

reference setting circuits.

Compares the actual value with the reference setting i.e.

performs error detection.

Automatic Voltage Regulator (AVR) Circuits

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Feeds the error to the control amplifier.

AVR control amplifier produces minimum conduction signal if

the error is maximum and vice versa.

Control amplifier adjusts the conduction of thyristors / diodes

depending on the error signal (by shifting the position of firing

pulses).

Output of the thyristor alters according to the control signal.


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Field voltage and the field current vary depending on the thyristors /

diode conduction.

Generator terminal voltage changes, accordingly.

Actual value signal changes and becomes equal to reference signal.

Output of AVR varies to ensure terminal voltage corresponding to the

reference signal.


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AC Control

Pulse

Transformer

Thyristors

AUTOMATIC

VOLTAGE

REGULATOR

(AVR)

AC

Limiter

AUTO

Reference

Control

Comparator and

Follow-up

Figure 27

Power System

Stabilizer

Stator Current

Limiter

Rotor Current

Limiter

Rotor Angle

Limiter

V / Hz

Limiter

PT / CT

Feedback

Pulse

Supervision

Pulse

Amplifier

Pulse

Final Stage

Filter

Gate

Control

Voltage

Supervision

MANUAL

Reference

Control

Gate

Control Pulse

Amplifier

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Ensures unit operation at constant PF / VAR.

If operating in constant PF mode, VAR undergoes changes

during operation.

If operating in constant VAR mode, PF undergoes changes

during operation.

PF / VAR Controller

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V / Hz Limiter

Limit Acting Regulators

Avoids over-fluxing of the transformer.

Protects against over-voltages at normal frequencies.

Protects against normal voltages at lower frequencies.

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Avoids thermal overloading of the stator windings.

In case of inductive current, a time delay is introduced and

excitation is reduced.

In capacitive mode, it acts on AVR instantaneously by

increasing the excitation.

Stator Current Limiter

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Avoids thermal overloading of the rotor windings.

Acts after a delay giving enough time to reach ceiling excitation

limit and allowing sufficient time required for the rate of rise of

the field current.

Rotor Current Limiter

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Acts when the rotor angle exceeds a set point.

Excitation is increased to restrict load angle.

Rotor Angle Limiter

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AVR is allowed to demand reactive power (under-excited

reactive current) from the synchronous machine.

Ensures minimum magnetization to avoid excessive heating of

the armature core and the end structure due to eddy current set

-up by armature reaction flux rotating at synchronous speed.

Original designed as a slow controller but is now applied in

faster control loops.

Under Excitation or Minimum Excitation Limiter

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VI

A

V2/Xd

B

EV/Xd

O

P

- Q + Q

1 Per Unit

Excitation

Ceiling

Excitation

MW Limit

Field

Current

Limit

Stator

Current

Limit

Theoretical

Limit

Practical

Limit

Minimum

Excitation

Limit

Figure 28

Capability Chart

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Documents

Excitation System