Environmental Engineering Reference
In-Depth Information
3
Power Balance / Frequency
Control
3.1 Introduction
The essential function of an electrical power system is to meet the energy demand of consum-
ers. This chapter describes the power demand characteristics of consumers and the technical
and operational principles involved in the reliable delivery of power to them from a variety
of conventional and renewable energy generators. The detailed regulatory issues of control
and operational requirements adopted in different countries where electricity systems have
been liberalized are addressed in Chapter 7.
3.1.1 The Power Balance Issue
Consider fi rst the case in which the power system consists of a single generator driven by a
prime mover and supplying a load. In Chapter 4 it will be shown that the frequency of the
generated voltage is directly proportional to the rotational speed of the generator.
The generator has rotational inertia and it is assumed that the prime mover is fi tted with a
governor
. The function of a governor is to sense any changes in speed and to adjust the fuel
supplied to the prime mover so that the speed (and therefore the frequency) is controlled.
The law of energy conservation requires that at any instant the power demanded by the
load, usually referred to as
demand
or just
load
, is supplied by the generator and/or by energy
stored within the system. If the load is suddenly increased, the extra energy demand is initially
supplied by the rotational inertia of the generator through a decrease of its speed. This
decrease in speed is also refl ected by a proportionate decrease in frequency. The governor
senses this decrease in speed and increases the fuel supply to arrest the fall in speed and fre-
quency. Exactly how far the frequency falls, how quickly it recovers and the frequency
of the new equilibrium state depends on the governor characteristics and the frequency
