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A
B
C
I
U2
I
+....
U3
Healthy
feeder
V
cn
Figure B2 - Residual Current Flow in an Unearthed System
It can be shown that the residual current measured in the faulty feeder is 180° out of phase with
that in the healthy feeder, as illustrated in Figure B2 This fact can be used to apply a GRD150
directional sensitive earth fault relay. The polarising voltage used for directional earth fault
relays is normally -V
(I
) for the faulty feeder leads this voltage by 90°. For the healthy feeders the residual current
R1
lags the voltage by 90°. Therefore, the GRD150 sensitive earth fault protection should be
applied with a characteristic angle of +90° so as to provide discriminatory protection.
The residual current in the faulted phase is equal to three times the per phase charging current,
and the sensitive earth fault element should be set well below this value to ensure operation
(30% of this value is typical).
3. Impedance earthing
In between the two extremes of solidly earthed and unearthed systems there are a variety of
compromise solutions, which normally involve connecting the system neutrals to earth via a
resistance or reactance.
3a. Resistance earthing
In the case of resistance earthed systems, GRD150 directional earth fault relays can normally be
applied in a similar manner to that for solidly earthed systems, with the exceptions that current
settings will be lower and the characteristic angle setting will probably be different. In the event
of a fault, it is the resistance in the neutral which predominates in the source impedance, and so
51N
I
F
V
an
Earth (e)
I
b
I
(=I
)
U2
R2
n
I
I
=I
c
F
V
bn
(the residual voltage inverted), and it can be seen that the residual current
R
⎯ 284 ⎯
F
I
U1
-V
R
Faulty
feeder
I
R1
V
an
I
U1
+I
+I
+...
n
U1
U2
U3
V
cn
6 F 2 S 0 8 4 2
51N
I
U1
V
bn
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