Electrical Power Systems

96 Electrical Power Systems
Power System Components and
Per Unit System
5.1 INTRODUCTION
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A complete circuit diagram of a power system for all the three-phases is very complicated. It is
very much practical to represent a power system using simple symbols-for each component
resulting in what is called a single-line diagram.
Power system engineers have devised the per-unit system such that different physical
quantities such as current, voltage, power and impedance are expressed as a decimal fraction
or multiple of base quantities. In this system, the different voltage levels disappear and a power
network consisting synchronous generators, transformers and lines reduces to a system of
simple impedances.
Another important component of a power system is the synchronous machine and it greatly
influences the system behaviour during both steady state and transient conditions. The
synchronous machine model, in steady state and transient conditions is given in chapter 4.0.
5.2 SINGLE PHASE REPRESENTATION O A BALANCED
THREE PHASE SYSTEM
igure 5.1 shows a simple balanced three-phase network. As the network is balanced, the
neutral impedance Z n does not affect the behaviour of the network.
ig. 5.1: Balanced three phase network. ig. 5.2: Single-phase representation of a
balanced three phase network
of ig. 5.1.