excit_and_avr.pdf

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Excitation systems and automatic voltage regulators


ELEC0047

November 2012

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Objectives

of chapter

describe the excitation systemsthere exists a wide range of systems, depending upon manufacturer

short description of main systems without going into detailsgive examples of models

simpleincluding limiters

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Overview

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Excitation systems and automatic voltage regulators Description of main excitation systems

Description of main excitation systems

Purpose:provide the power required by the eld winding of generatormake the eld voltage quickly vary in response to network disturbances.

Two main categories:1 rotating machine: excitation power taken from mechanical power turbine

mounted on the same shaft as turbine and generatorDC machineAC machine with rectier

2 static excitation system: excitation power taken from network throughtransformer and rectier.

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d l l f


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DC generato r

non negligible time constant of exciter, to be compensatedDC generator self-excited or separately excited (requires a pilot exciter =separate permanent magnet DC machine)large units: speed below brushes too large and current too important

has been replaced by power electronics5/21

E it ti t d t ti lt l t D i ti f i it ti t


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alternator with non-controlled rectier

diodes and thyristor ring very fastexciter still introduces a time constanteld current and voltage cannot go negative (during large transients)

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alternator with controlled rectier

thyristor ring very fastto avoid delays, exciter alternator operates at full voltagehence it is dimensioned to operate permanently at ceiling eld voltagethyristors allow reverting the eld voltage (during large transients)

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rotating dio des or brushless system

no contact between stator and rotortime constant of inverted generatorno access to eld current i f of main generator; excitation current of inverted

generator used instead8/21



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potential-source controlled-rectier or static system

very fast excitation systemin case of short-circuit close to the main generator, drop of voltage of thetransformer feeding the excitation system; counteracted by compoundingcircuits

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Excitation systems and automatic voltage regulators Modelling excitation systems, regulators and limiters


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c tat o syste s a d auto at c vo tage egu ato s ode g e c tat o syste s, egu ato s a d te s

Modelling excitation systems, regulators and limiters

Per unit system

The following bases are usually chosen:V fB : the eld voltage that produces the nominal voltage V B at the terminalof the open-circuited generatorI fB : the eld current that produces the nominal voltage V B at the terminal of the open-circuited generator

In steady state, in Volt:v f = R f i f

and in per unit:

v fpu =v f

V fB =

R f i f R f I fb

= i fpu R fpu = 1

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y g g g y , g

Generic model of automatic voltage regulator and excitation system

OEL : OverExcitation Limiter UEL: UnderExcitation LimiterT m 0. 05 s T a 0. 05 s T e a few10 2 1 s

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Other facts to be taken into account in specic models:diode rectier: drop of v f voltage increases with increasing i f brushless system: internal compensation does not use the (unavailable) v f voltage

v minf = 0 for the diode rectier, v

minf < 0 for the thyristor rectier

v max f sensitive to generator terminal voltage in the static excitation systemsaturation of exciteretc.

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Overexcitation limiter acting on summation point of AVR (non-takeover):

Bloc 1: u = 1 if i f i limf d < 0= 0 if d < i f i limf 0

= i f i limf if i f i

limf > 0

A value i f > i limf is tolerated during a time such that:

i f i limf =

|L1 |K 1

=|L1 |K 1

1i f i

limf

inverse-time characteristic. Fixed-time obtained with block 4 instead of 1.13/21



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Overexcitation limiter acting through min gate of AVR (takeover):

In steady state, after OEL action:

v f

= G aG

e K

3(i

f i lim

f ) i

f = v

f =

G a G e K 31 G a G e K 3

i limf

=G a G e |K 3 |

1 + G a G e |K 3 |i limf

and, since G a G e 1 and |K 3 | > 1:

i f i limf

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Underexcitation limiter

Aimed at preventing:

i f from becoming lower than a minimum, orreactive power Q from becoming lower than a minimum (which depends onactive power P ).

Example: limiter of second category, acting on summation point of AVR

forces:K P P + K Q Q + K O 0

after a short delay dictated by L1.

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Excitation systems and automatic voltage regulators Case study No. 1


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Case study No. 1

System : same as in Case study of Chapter 1 except:different operating pointgenerator equipped with AVR (st-st open-loop gain 50, time constant 0.2 s)

Fault on line 1-3 at t = 1 s, cleared at t = 1.1 s by opening the faulted circuit.

Distribution transformer equipped with a load tap changer keeping the load (MV)voltage in the deadband [0. 99 1. 01] pu

delay on rst tap change: 20 sdelay on each subsequent tap change: 10 s. 16/21



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short-term response

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short-term response

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long-term response

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C d

2


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Case study No. 2

System : same as in Case study of Chapter 1 except:

generator equipped with AVR (st-st open-loop gain 50, time constant 0.2 s)and OverExcitation Limiter (type shown in slide 13, inverse-time).

Disturbance in external system represented by Thevenin equivalent 13 % drop of Thevenin e.m.f.

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excit_and_avr.pdf