Handbook of Electrical Installation Practice - BeKnowledge

624 Handbook of Electrical Installation Practice
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400/1
Fig. 22.17 Duobias M system.
90MVA 132/33kV +10%
–20% Yd11
400/1
HV Interposing C.T. settings
Con = Yd11,30°
Mult = 0.95
1600/1
Earth
Trans.
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18 17
21 22
22 21
25 26
26 25
13 14
1413
ANI AN2
1600/1
HV Interposing C.T. settings
Con = Ydy0,0°
Mult = 1.0
short-circuit withstand current (and that of the network), and type and protection
of network earthing. It is also related to economics such as cost of protection in relation
to that of machine and the consequences resulting from a breakdown.
Alternators
On occurrence of a short-circuit at the terminals of an alternator the short-circuit
current is initially between five and ten times that of full load of the machine because
the initial stator current is limited only by the subtransient leakage reactance. This
is related to the flux set up by the stator mmf which fails to cross the air gap. The
increase in stator current causes a demagnetising effect by opposing the air gap flux
but it is an appreciable time before a major change in the air gap flux can be effected.
The net effect is a gradual decrease in the short-circuit current over a period
of seconds to a value which may be considerably below full load current of the
machine.
Possible faults on alternators
An alternator may experience various types of faults, the most common being stator
faults which consist of earth-faults, phase-to-phase short-circuits, short-circuits
between turns, open-circuits in windings and overheating. These failures are usually
caused by overvoltage and/or deterioration of the insulation.
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