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Directional Earth Fault Protection and Power System Earthing
Power systems may be solidly earthed, impedance earthed or unearthed (insulated). Depending
on the method used, faults to earth have widely differing characteristics, and so methods of earth
fault protection differ greatly between the various types of system.
1. Solidly earthed systems
In a solidly earthed system the neutral points of the power transformers are connected directly to
earth, for the purposes of reducing overvoltages and facilitating fault detection. The
disadvantage of solid earthing is that fault currents can be very high, and must be disconnected
quickly.
Since the impedance of the source is normally very low, fault current varies greatly in
magnitude depending on the location of the fault. Selective isolation of a faulty section is
therefore possible via time/current graded earth fault overcurrent protection. Fault current is
detected by measuring the system residual current.
On an interconnected system, where fault current can flow in either direction, then directional
earth fault relays are applied. The fault causes a residual voltage to be generated, and this can be
used for directional polarization. Residual current and voltage can be measured as shown in
Figure B1.
Residual current I
R
faulted phase voltage by a considerable angle due to the reactance of the source. Directional
control is achieved by polarising against the system residual voltage, which may be found either
by summating the phase voltages, or it may be extracted from the open delta connected
secondary (or tertiary) winding of a five limb VT, as shown in the diagram.
A directional earth fault relay protecting a solidly earthed system is normally connected to
measure V
inverted. If GRD150 is applied to derive residual voltage from the phase voltages
R
then the inversion of V
Figure B1 - Directional Earth Fault Protection for Solidly Earthed Systems
is equal in magnitude and direction to the fault current. It typically lags the
is performed internally.
R
51N
V
an
Pre-
I
a
fault
Earth
n
V
V
cn
bn
⎯ 282 ⎯
F
67
I
R
V
Post-fault
an
n
V
V
cn
bn
V
R
6 F 2 S 0 8 4 2
A
B
C
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