Svc Curso Basico

CHAPTER 5

Concepts of SVC Voltage Control

5.1

INTRODUCTION

Static var compensators (SVCs) are used primarily in power systems for voltage control as either an end in itself or a means of achieving other objectives, such as system stabilization [1]–[8]. This chapter presents a detailed overview of the voltage-control characteristics of SVC and the principles of design of the SVCvoltage regulator. The performance of SVC voltage control is critically dependent on several factors, including the influence of network resonances, transformer saturation, geomagnetic effects, and voltage distortion. When SVCs are applied in series-compensated networks, a different kind of resonance between series capacitors and shunt inductors becomes decisive in the selection of control parameters andfilters used in measurement circuits. Various considerations affecting the design of the SVC voltage regulator are discussed in this chapter as well.

5.2 5.2.1

VOLTAGE CONTROL V-I Characteristics of the SVC

The steady-state and dynamic characteristics of SVCs describe the variation of SVC bus voltage with SVC current or reactive power. Two alternative representations of thesecharacteristics are shown in Fig. 5.1: part (a) illustrates the terminal voltage–SVC current characteristic and part (b) depicts the terminal voltage–SVC reactive-power relationship. The dynamic V-I characteristics of SVCs are described in the following text.
5.2.1.1

Dynamic Characteristics

Reference Voltage, Vref This is the voltage at the terminals of the SVC during the floating condition, that is,when the SVC is neither absorbing nor generating any reactive power. The reference voltage can be varied between the maximum and minimum limits—V ref max and V ref min —either by the SVC control system, in case of thyristor-controlled compensators, or by the taps of the
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VOLTAGE CONTROL

143

VSVC

Overcurrent Limit

Steady-State Characteristic

Bmin
Overload Range

V1 Vref V2Linear Range of Control B max

Dynamic Characteristic

ICr
Inductive

I set 0
(a)

ILr Capacitive

I SVC

VSVC
Overcurrent Limit Steady-State Characteristic

Bmin Overload Range V1 Vref V2
Dynamic Characteristic

Linear Range of Control

Bmax

QCr

Q set 0 Inductive (b)

QLr Capacitive

Q SVC

Figure 5.1 (a) The voltage–current characteristic of the SVC and (b) thevoltage– reactive-power characteristic of the SVC.

coupling transformer, in the case of saturated reactor compensators. Typical values of V ref max and V ref min are 1.05 pu and 0.95 pu, respectively.
Linear Range of SVC Control

This is the control range over which SVC terminal voltage varies linearly with SVC current or reactive power, as the latter is varied over its entirecapacitive-to-inductive range. The slope or droop of the V-I characteristic is defined

Slope or Current Droop

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CONCEPTS OF SVC VOLTAGE CONTROL

as the ratio of voltage-magnitude change to current-magnitude change over the linear-controlled range of the compensator. Thus slope K SL is given by K SL where DV DI DV Q DI (5 . 1 )

the change in voltage magnitude (V) the change in current magnitude(A)

The per-unit value of the slope is obtained as K SL DV / V r pu DI / I r (5.2)

where V r and I r represent the rated values of SVC voltage and current, respectively. For DI I r , K SL DV(at I r or Qr ) pu Vr DV(at I r or Qr ) . 100% Vr (5.3)

where Qr represents the rated reactive power of SVC. Thus the slope can be defined alternatively as the voltage change in percent of the ratedvoltage measured at the larger of the two—maximum inductive- or maximum capacitive-reactive-power outputs, as the larger output usually corresponds to the base reactive power of the SVC. In some literature, the reactivepower rating of the SVC is defined as the sum of its inductive and capacitive rating. The slope is often expressed as an equivalent reactance: X SL K SL in pu (5.4)

The slope can be...