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operation. The excitation impedance of the saturated CT is considered to approximate a
short-circuit.
Healthy CT
Figure 2.1.23
The voltage across the relay circuit under these conditions is given by the equation:
×(R
V
= I
+ R
S
F
CT
L
where:
V
= critical setting voltage (rms)
S
I
= maximum prospective secondary through fault current (rms)
F
R
= CT secondary winding resistance
CT
R
= Lead resistance (total resistance of the loop from the saturated CT to the relaying point)
L
A series stabilising resistor is used to raise the voltage setting of the relay circuit to V
margin is needed since the extreme assumption of unbalanced CT saturation does not occur in
practice. The series resistor value, Rs, is selected as follows:
R
= V
/ I
S
S
S
Is is the current setting (in secondary amps) applied to the GRD150 relay. However, the actual
fault setting of the scheme includes the total current flowing in all parallel paths. That is to say that
the actual primary current for operation, after being referred to the secondary circuit, is the sum of
the relay operating current, the current flowing in the varistor, and the excitation current of all the
parallel connected CTs at the setting voltage. In practice, the varistor current is normally small
enough that it can be neglected. Hence:
≦ I
I
/ N – 4I
S
P
mag
where:
I
= setting applied to GRD150 relay (secondary amps)
S
I
= minimum primary current for operation (earth fault sensitivity)
P
N = CT ratio
I
= CT magnetising (excitation) current at voltage V
mag
More sensitive settings for Is allow for greater coverage of the transformer winding, but they also
require larger values of Rs to ensure stability, and the increased impedance of the differential
circuit can result in high voltages being developed during internal faults. The peak voltage, Vpk,
developed may be approximated by the equation:
2 ×
V
×
V
= 2×
k
pk
where:
Transformer
Circuit
Varistor
Stabilising
GRD150
Resistor
R
S
Maximum Voltage under Through Fault Condition
)
(
)
I R
−
V
F
S
k
⎯
⎯
32
Saturated CT
I
F
≈0
Z
M
V
S
R
L
S
6 F 2 S 0 8 4 2
R
CT
. No safety
S
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