1 OVERVOLTAGES DURING

POWER SYSTEM FAULTS

1 • 1 INTRODUCTION

Faults occur in a power system because insulation at the point of

fault is momentarily subjected to voltage stress in excess of its di-

electric strength. In the past, lightning has probably been the most

frequent cause of fault initiation and subsequent line and apparatus

outages. Switching surges also have been frequent causes of over-

voltages of sufficient magnitude to cause insulation to be dielectrically

overstressd, resulting in system faults. Birds, snakes, kites, airplanes,

and rifles in capable, though morally misdirected, hands have been

known to initiate faults. Mechanical failure of insulation may and

occasionally does result in a power system fault.

The list of causes of these faults is long and varied. However, it is

not the purpose of this chapter to concern itself further with the

causes, but rather to consider the consequences of the occurrence of a

fault as far as the system itself is concerned. In considering these

consequences, it will be preferable to think of the power system as an

electrical circuit made up of linear impedance elements of resistance,

inductance, and capacitance. The circuit is normally energized and

carrying load until a fault suddenly occurs. The fault then corre-

sponds to the closing of a switch (or switches, depending on the type

of fault) in the electrical circuit. The closing of this switch (or

switches) changes the circuit so that a new distribution of currents

and voltages is brought about. This redistribution is accompanied in

general by a transient period during which the resultant currents and

voltages may momentarily be relatively high.

There are two components of voltage in such linear circuits due to

the occurrence of a system fault (or due to closing the switch in the

electrical network), viz., (1) fundamental-frequency voltages, and (2)

l

Genera

ted o

n 2

01

5-0

2-1

9 1

6:3

5 G

MT /

htt

p:/

/hd

l.hand

le.n

et/

20

27

/mdp.3

90

15

00

60

51

68

7Public

Dom

ain

, G

oog

le-d

igit

ized

/

htt

p:/

/ww

w.h

ath

itru

st.o

rg/a

ccess

_use

#pd-g

oogle