Electrical Power Systems

186 Electrical Power Systems
Symmetrical ault
8.1 INTRODUCTION
8
This chapter is devoted to the analysis of symmetrical three-phase fault or balanced fault. This
type of fault can be defined as the simultaneous short circuit across all the three phases. This
type of fault occurs infrequently, for example, when a mechanical excavator cuts quickly
through a whole cable, or when a line, which has been made safe for maintenance by clamping
all the three phases to earth is accidentally made alive or when due to slow fault clearance, an
earth fault spreads across to the other two phases. This type of fault generally leads to most
severe fault current flow against which the system must be protected. ault studies form an
important part of power system analysis and the problem consists of determining bus voltage
and line current during faults. The three phase fault information is used to select and set phase
relays. ault studies are used for proper choice of circuit breakers and protective relaying. A
power system network comprises synchronous generators, transformers, transmission lines
and loads. During fault, loads current can be neglected because voltages dip very low so that
current drawn by loads can be neglected in comparison to fault currents. The magnitude of the
fault current depends on the internal impedance of the synchronous generator and the impedance
of the intervening circuit. We have seen in Chapter-4 that for the purpose of fault studies,
generator behaviour can be divided into three different periods: (i) the subtransient period,
lasting only for the first few cycles; (ii) the transient period, covering a relatively longer time
and (iii) steady state period.
Another important point is that the circuit breakers rated MVA breaking capacity is based
on three phase fault MVA. In fact high precision is not necessary when calculating the three
phase fault level because circuit breakers are manufactured in standard sizes, e.g., 250, 500,
750 MVA etc. Generally for three phase fault calculation, following assumptions are made:
1. The emfs of all generators are 1 0º pu. This assumption simplify the problem and it means
that the voltage is at its nominal value and the system is operating at no load at the time
of fault. Since all emfs are equal and in phase, all the generators can be replaced by a single
generator.
2. Charging capacitances of the transmission line are ignored.
3. Shunt elements in the transformer model are neglected.
Example 8.1: A synchronous generator and a synchronous motor each rated 20 MVA, 12.66 KV
having 15% subtransient reactance are connected through transformers and a line as shown in
ig. 8.1. The transformers are rated 20 MVA, 12.66/66 KV and 66/12.66 KV with leakage
reactance of 10% each. The line has a reactance of 8% on a base of 20 MVA, 66 KV. The motor