Brushless Exciter

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Brushless exciter model

S.M.L.

Kabir

R. Shuttleworth

Indexing term s: Exciter , Power system beh aviour, Simulation

Abstract: The IEEE model of a brushless exciter

can be found in many software packages for the

simulation of power system behaviour. Yet the

model is simplistic and does not represent the

exciter alternator accurately. This paper describes

the reasons for the inaccuracy of the IEEE repre-

sentation and proposes an alternative model.

Goo d agreement is obtained between results from

a micromachine test system and the alternative

model.

1

Introduction

Generator exciters and automatic voltage regulator

play an important part in determining power system sta-

bility during transients. Nevertheless, general purpose

computer models describing their performance over a

wide range of operating conditions have not yet emerged.

This is because interactions between AVRs, exciters and

generators are complicated and not thoroughly under-

stood.

In

1968

an IEEE working party published a set of

excitation system models for use in large scale stability

studies. These mod els were devised in an attem pt to

establish a unified approach to power system analysis. In

1981,

the set was updated with the publication of a

second paper

[l]

reflecting changes in excitation tech-

nology and modelling methods. These models have

become widely used in industry, as was intended, to the

point where they are now considered a standard in the

specification and testing of excitation equipment as well

as in power system analysis.

Unfortunately the form of the IEEE models makes

them inappropriate for the universal role gradually being

forced upon them. This is evident from some of the con-

flicting results they yield in practice. These arise mainly

from misuse, since the models, designed for small signal

analysis are frequently used in the pred iction of response

following a large disturbanc e. H owever, the lim its of

applicability remain unknown as there has been no

precise guidance from the working party or any other

published source.

This indeterminate situation has led to unnecessary

conflicts between the users and suppliers of equipment.

The former are tending to apply IEEE models, often

embedded in standard software packages, to check

on

the

IEE, 1994

Paper

9704C (PlO),

first received 1st October 1992 and in revised

form

17th May 1993

S.M.L . Kabir is with B.U.E.T. ,Bangladesh

R. Shuttleworth is with the Electrical Engineering Laboratories, The

University, Manchester

M13 9PL,

nited Kingdom

I E E Proc.-Gener. Transm . Distrib.,

Vol.

141,

No.

January 1994

specification of equipment supplied by the latter a nd to

predict its response to large scale disturbances. One of

the commonest excitation systems, the brushless exciter,

is particularly badly simulated by the IEEE model and

has caused confusion.

For

this reason the authors have

addressed the problem of brushless exciter modelling.

This paper describes a model which, whilst based on the

IEEE arrangement, takes better account of exciter behav-

iour and is able to predict responses more accurately.

2

In essence a brushless exciter is, as shown in Fig. 1, an

inside-out three-phase synchronous generator, the field

winding of which is mounted on the stator housing, the

The brushless exciter ecti fi er system

generotor

e x c i te r

f i e l d

winding

a

f i e l d

w l n d l n g g e n e r a t o r

t h r e e - p h a s e

e x c i te r thr e e -pha s e

i n d 3

k

rotating

s e c t

ion

F ig . 1 Brushless exciter-generator

three-phase windings being attached to the rotor. The

three-phase outp ut voltage is rectified by diodes mo unted

on the rotating shaft and applied directly to the main

generator field winding. Thereby, sliprings and brushes

are eliminated, and maintenance costs reduced. In most

cases the rectifier is a three-phase full wave bridge.

2.1 Effect of rect i f icat ion on the exci ter

It is generally known that, as a consequence of source

inductance, rectifying systems suffer from overlap. The

inductance in each phase of the supply opposes transfer

of current from rectifier to rectifier creating temporary

phase to phase short circuits, or overlaps, during the

cycle. The overall effect of these repetitive intervals of

overlap, each occurring for an angular duration

U,

is to

reduce the mean ou tput voltage of the rectifying system.

For

a

three-phase bridge rectifier supplying an induct-

ive load, the rectification process can be divided into

The authors wish to thank Dr. R.D.M. Whitelaw

and Mr.

B.

Aranyos of CEC-Alsthom Turbine

Generators Ltd., and Prof. D.W. Auckland of

Manchester University, for their help and suppo rt.

61


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three distinct modes,

1,

2, and 3, which occur in numeri-

cal sequence as load current increases from zero.

A

full

description of the phenomenon is given by Gayek

[Z]

and Witzke et

al. [3].

In brief, mode

1

is characterised by intervals of no

overlap, where two diodes conduct, interspaced with

intervals of overlap and the conduction of three diodes.

As the rectifying system output current increases, so does

the overlap angle U, until U reaches

60',

which occurs at

the point of transition from mode 1 to mode 2. At this

transition point overlap becomes continuous and thus

three diodes are always conducting. In mode 2,

U

remains

constant at 60 , and a delay angle, known as Y, appears

which retards the start of each overlap period. The angle

ct

increases from

0

to 30 as the rectifying system moves

through mode 2 owing to increasing load current. When

ct

equals 30 , which signifies the transition from mode 2

to mode 3, an increase in load current will again increase

U,

while ct remains fixed at

30 .

However as

U

increases, so

intervals of three-phase short circuits occur during the

overlap process. Increasing the load current causes both

U

and the intervals of three-phase short circuit to

increase, until at the end of mode 3, a complete three-

phase short circuit is imposed upon the source, and U is

equal to

120 .

Thus for this level of load current and for

higher levels, the b ridge becom es, for the load, a free-

wheeling path. This final short circuit condition will be

referred to as m ode

4.

It follows that, as rectifier load current increases from

zero,

so

the power factor impressed upon the AC source

worsens, moving from almost unity at the beginning of

mode 1 to zero lagging at the end of mode 3 and

throughout mode

4.

3

The

IEEE model

Fig.

2

shows the exciter and rectifier components of the

IEEE model. The voltage applied to the field winding of

the exciter is represented by V, on the left of the diagram

whereas

E,,

is the voltage applied to the main generator

field winding on the right of the mod el.

t

K E ' 5 E

NMCm lVE

:L


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applied to a rectifier bridge simulation which performs

the necessary logic to determine the connections of the

phases to the

DC

load as follows.

At

the end of every time step a comm utation integer is

set with a value which indicates the state of the rectifier.

The values assigned to the integer indicate which phases,

if any, are being subjected

to

commutat ion, or, alterna-

tively, if a three-phase short circuit is occurring.

O n the following time step this integer is used to

connect the load to the correct phases of the Canay

model. After calculating the new parameters a check is

made

to

see if the co mm utatio n integ er has change d. If

i t

has not,

I ,

and

I ,

are returned to the alternator model.

If, however, a change has occurred, then the com-

mutation integer, and hence load connection, is updated,

and the recalculated values of

I ,

and

I ,

are returned to

the alternator model. The program is slow in operation,

as it must, for accuracy, be time stepped at

0.1

ms inter-

vals for a 50

Hz

alternator. Typically it takes some

3 hours to simulate seconds of exciter operation using

an IBM compatible PC. No doubt the model could be

improved in speed of operation, but this was not

attempted as its purpose was to give a benchmark for

development

of

the faster model

to

be described in the

next Section.

~

10

5 Simple

digital

model

Fig. 4s a block diagram of the model.

As

in the complex

model, a Canay representation of alternator behaviour is

used, but terms for rate of change

of

direct and quadra-

ture axis flux are omitted. Thus direct current terms in

the stator current cannot be simulated, but the model can

be time stepped with longer periods than the previous

model.

An assumption m ade is that rectifier operation,

although highly nonlinear, can be treated as a steady

state phenomenon since the repetitive line-to-line short

O f A 1

and

81

.

l D a n d 10 mode mode I F

-

.

81 U .O: calculat ion

of

-

l o X Q

I

circuiting due to comm utation oc curs equally to all three

phases.

The Canay alternator simulation has applied to it, at

each time step, values of

I , , I ,

and input voltage V,,

from which terminal voltages U , and U, are calculated.

Commutating reactance is assumed to be equal to sub-

transient reactance [4] and the phasor values of voltage

behind this, U, , and U , , , are thus determined as shown

in Fig. 5. The per unit alternator output voltage, V,,

follows. The sim ple rectifier simulation pro posed by the

IEEE is used, with the inclusion of a fourth mode

of

operation which accounts for freewheeling of generator

field currents through the bridge rectifier, as explained in

Section

2.

Fig. 5 Phasor diagram

I ,

and I , can be determined from the real and reactive

currents, A I and B1, taken by the rectifier, providing

these can be determined. Formulas for calculating

A 1

and B1 can be found in the literature [2,

8,

91 but only

for modes

1

and 2. Mode 3 has been neglected in the

literature with the exception of Ferguson et

al

[4] who

provide a graphical solution for a machine of particular

subtransient reactance. In order to obtain a mathemati-

Fig.4 Simple digital model

I E E

Proc-G ener. Transm. Distrib . , Vol

141,

N o . I

January

19Y4

63


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cal representation, suitable for use in a computer,

of

real

and reactive currents in mode 3, it was necessary to

perform a Fourier analysis

of

the alternating current

taken by the rectifier. This analysis was based upon the

description

of

mode 3 operation given by Witzke et al.

[3]

The formulas derived for mode 3 are unlike the

equivalent formulas for modes 1 and

2

as they include the

terms

V,/X,

as

a

result of the repetitive three-phase short

circuits.

Having determined A 1 an d E1 for three modes

of

rec-

tifier operation,

I ,

a n d

I ,

can be found from the phasor

diagram

of

Fig.

5

as U D 0 ,U Q o and

VE

are known. In the

case of

mode

4

he alternator supplies reactive current to

the short circuiting rectifier so that A1 = 0 and E1

=

V,/X,.

A complete list

of

the formulas used is given in

Appendix 11.1.

6

Testsystem

Obtaining test results from

a

brushless exciter

is

of

course

difficult as it is an integral part

of

the main generator.

For this reason, two microalternators were used to simu-

late a brushless exciter and main generator, as shown in

Fig.

6

The machines were rated at

220 V, 3

kVA and

50 Hz,

an d were driven by DC motors.

I I

round rotor

machine

h

MOSFETrectiiier

sirnulotion

shadow

winding

Fig.6

Diagram ojmicromuhin e tes t system

The first microalternator had a salient pole rotor with

wound direct and quadrature axis damper circuits and

was used to represent the brushless exciter alternator.

The second machine had

a

round rotor with wound

direct and quadrature axis damper circuits an d was used

to simulate the field winding of the main generator.

So

as

to minimise the complexity

of

the physical model, the

damper circuits of the second microalternator were open-

circuited, thereby ensuring the field winding represented

a single time con stant load. It was thus possible a t a la ter

stage to reconnect the damper circuits and simulate a

complete brushless exciter-generator system. This

allowed the impact

of

disturbances at the main generator

terminals on the exciter rectifier system to be invest-

igated.

Because the two microalternators had identical ratings

it was necessary t o ope rate the salient pole machine with

a low field current to avoid over exciting the round rotor

machine. At this current level brush drop is significant

and so the graphite brushes of the salient pole micro-

alternator were replaced by low voltage drop copper

loaded brushes. Losses in the field brushes of the round

rotor machine were minimised by the same method.

Standard silicon diodes were not used in the bridge

rectifier as their voltage drop is significant. Instead rec-

64

tifying elements comprising two power

MOSFETs in

bilateral configuration driven by an operational amplifier

clamp circuit were used. These introduced a low resist-

ance of

0.1 R

which is about

2.5%

of the load resistance.

It is usual for microalternator windings to be fitted

with time constant regulating equipment (TCRs) [lo,

1 I].

Although TCRs were available for the salient pole

machine it was decided not to use these as their band-

width

of

100H z, which is adequate for normal power

system studies, would cause undue attenuation

of

the rec-

tifier induced harmonic currents. However, it was neces-

sary to enhance the natural time constant of the round

rotor field circuit as this, being about looms, was

untypical of

a

large generator. A continuously acting

TCR was used to increase this time constant by a factor

of

50.

The amplifier used had a gain

of 50

and a band-

width of

2

kHz, which calculations showed was enough

bandwidth to ensure adequate time constant com-

pensation for harmonics. This was proven by tests at a

lower amplifier gain

of

10 performed with

10

k H z a n d

2 kHz bandwidths which produced identical results,

validating the choice of

2

kHz.

7

Test results

A step inpu t of 1 p.u. excitation voltage was applied to

the field winding of the salient pole alternator and the

consequent variations in field and load current were

recorded. These results were plotted and the responses

of

the two digital models, and the standard

I E E E

model,

compared with them.

The parameters used in the digital models were deter-

mined from

a

three-phase sudden short circuit test per-

formed on the salient pole machine at a current level

approximately equal to that used in the step test above.

Th e parame ters are given in Appendix 11.2. A flux

linkage versus current plot for the field winding

of

the

round rotor machine, from zero to the appropriate

current level and back to zero, is shown in Fig.

7.

For

increasing current, the curve has, approximately, a slope

current

Fig.

7

Flux

linkage versus current plotfor

the DC

load

I E E

Proc .-Gener. Transm. Distrib . , Vo l .

141,

N o .

I ,

January 1994


5/7

equal

to

an inductance

of

2 5 . 6 H .

Fig.

8

compares the

responses of the two computer models with the step test

described. Despite the simplifications involved in the

simple model it follows closely the results of the comp lex

model.

/

0 - .

. .

, I

1 I I

0

0 5

1 0 1 5 2 0

2 5

3 0

3 5

4 0 4 5

5 0

time, s

Fig. 8

held

uoltage

a

field current

of

simple model

field current ofcomplex model

Brushless Exciter

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