Page 118 - Power Electronic Control in Electrical Systems

//SYS21/F:/PEC/REVISES_10-11-01/075065126-CH004.3D ± 106 ± [106±152/47] 17.11.2001 9:54AM

4

Power flows in

compensation and

control studies

4.1 Introduction

The main objective of a power flow study is to determine the steady state operating
condition of the electrical power network. The steady state may be determined by
finding out the flow of active and reactive power throughout the network and the
voltage magnitudes and phase angles at all nodes of the network.
The planning and daily operation of modern power systems call for numerous
power flow studies. Such information is used to carry out security assessment
analysis, where the nodal voltage magnitudes and active and reactive power flows
in transmission lines and transformers are carefully observed to assess whether or not
they are within prescribed operating limits. If the power flow study indicates that
there are voltage magnitudes outside bounds at certain points in the network, then
appropriate control actions become necessary in order to regulate the voltage mag-
nitude. Similarly, if the study predicts that the power flow in a given transmission line
is beyond the power carrying capacity of the line then control action will be taken.
Voltage magnitude regulation is achieved by controlling the amount of reactive
power generated/absorbed at key points of the network as well as by controlling the
flow of reactive power throughout the network (Miller, 1982). Voltage regulation is
carried out locally and, traditionally, the following devices have been used for such a
purpose:
1. Automatic voltage regulators, which control the generator's field excitation in
order to maintain a specified voltage magnitude at the generator terminal.
2. Sources and sinks of reactive power, such as shunt capacitors, shunt reactors,
rotating synchronous condensers and SVCs. Shunt capacitors and reactors are