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3

Transmission system

compensation

3.1 Introduction

It has always been desirable to transmit as much power as possible through transmis-
sion lines and cables, consistent with the requirements of stability and security of
supply. Power transmission is limited mainly by thermal factors in cables, short
transmission lines, transformers and generators; but in long lines and cables the
variation of voltage and the maintenance of stability also constrain the power
transmission. The voltage `profile' and the stability of a transmission line or cable
can be improved using `reactive compensation'. In the early days reactive compensa-
tion tookthe form of fixed-value reactors and capacitors, usually controlled by
mechanical switchgear. Synchronous condensers and large generators were used in
cases where it was necessary to vary the reactive power continuously. Since the 1970s
power-electronic equipment has been developed and applied to extend the range of
control, with a variety of methods and products.
BulkAC transmission of electrical power has two fundamental requirements:
1. Synchronism. The basis of AC transmission is a networkof synchronous machines
connected by transmission links. The voltage and frequency are defined by this
network, even before any loads are contemplated. All the synchronous machines
must remain constantly in synchronism: i.e. they must all rotate at exactly the same
speed, and even the phase angles between them must not vary appreciably. By
definition, the stability of the system is its tendency to recover from disturbances
such as faults or changes of load.
The power transmitted between two synchronous machines can be slowly
increased only up to a certain level called the steady-state stability limit. Beyond this
level the synchronous machines fall out of step, i.e. lose synchronism. The steady-
state stability limit can be considerably modified by the excitation level of the
synchronous machines (and therefore the line voltage); by the number and connec-
tions of transmission lines; and by the pattern of real and reactive power flows in the
system, which can be modulated by reactive compensation equipment.