Eletronica Potencia

RECTIFIER CHOICES FOR

SYNCHRONOUS GENERATOR EXCITERS

H. S. Barbuy , A. Rocco , L. A. Fernandes

Unisanta - R. Dr. Osvaldo Cruz, 266 Santos, SP, Brazil

[e-mail] ; [e-mail] ; [e-mail]

C. Goldemberg

Escola Politécnica da USP - Av. Prof. Luciano Gualberto, 3-158 São Paulo, SP, Brasil

[e-mail]

Abstract - In synchronous generator exciters there exist

rectifiers both in the actuator and in the AVR (automatic

voltage regulator) feedback sensors. This paper discusses

some of the choices available in each of these sub-systems,

taking into account the overall dynamic performance of the

AVR loop. This dynamic perfomance enables a fast reactive

power dispatch.

KEYWORDS

Rectifier circuits, Synchronous generator control, AVR,

Excitation system, Reactive power dispatch.

I. INTRODUCTION

A fast dispatch of reactive power contributes to the dynamic

stability of the electric energy system’s Q/V loop, avoiding its

collapse.

In 1999, on March 11 there was a black out in the Brazilian

electric energy system because of Q/V loop collapse. Many

national and international experts in this matter (including A.

Rocco that was systems operation manager of Eletropaulo

S.A.) studied what to do to improve the operational security

stability margins. One of the suggestions was to improve the

AVR dynamics of the generators located near the energy

comsumption centers, specifically Henry Borden, L.C.Barreto,

and Porto Colômbia power stations.

Similar voltage collapse events also occurred in Japan,

France, Sweden, USA and other times in Brazil in the last

fifteen years.

These events were caused by local reactive power deficits.

The electrical system was unable to supply, in the required

time, the amount of reactive power necessary to keep the

voltage levels within operational margins. The critical

condition usually happened in the period immediately before

the peak of demand, when the highest demand rate occurs.

This paper discusses some technical details related to the

rectifiers that are used within the synchronous generator

exciters, which affect the AVR dynamic response.

II. AUTOMATIC VOLTAGE REGULATOR MODEL

This paper will discuss only models AC4A (Alternator

supplied controlled-rectifier exciter) and ST1A (Potential

source controlled rectifier exciter) of IEEE standard 421 [1].

The only difference between both models is the energy source

used for feeding the controlled rectifier. An independent

electric network in used in the AC4A model while the

synchronous generator terminals feeds its own rectifier

(usually by means of a step-down excitation transformer) in the

ST1A arrangement. The ST1A unifilar diagram is shown in

Fig. 1.

Usually the automatic control operates as a terminal voltage

controller (AVR-Automatic voltage regulator) although other

operating modes may exist such as a power factor regulator or

even as a VAR regulator. But these other operating modes

usually work around the AVR, which acts as a subordinate

loop inside the exciter system.

II.1 Actuator rectifier

The actuator rectifier is fed by the synchronous machine

through a power transformer (except for low power, low

voltage generators). High power exciter systems usually use a

fully controlled three phase bridge although this is not required

explicitely by the IEEE Std. 421. In some small generators it

is possible to use single phase rectifiers. These two

arrangements for the actuator rectifier are shown in Figs. 2

and 3.

Different ripple levels exist at the machine field terminals

and produce side-effects, even considering that the field circuit

has a large time constant. These ripple components depend on

the excitation transfomer turns ratio (or in other words, of the

ceiling voltage of the excitation system).

Fig. 2 Three phase full bridge rectifier, used for high power

generators.

Fig. 3 Half wave, single phase actuator rectifier, that can be

used for low-power generators.

Fig. 4 Half wave, single phase rectifier with filter.

Fig. 5 Full wave, three phase rectifier with filter.

Fig. 6 Twelve pulse rectifier circuit with filter.