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Chapter 6 - Current Protection Functions
Resistive component
in grounding coil
Operate
V
= -3V
res
V00641
Figure 46: Phase C earth fault in Petersen Coil earthed system: practical case with resistance present
If the residual voltage is used as the polarising voltage, the residual current is phase shifted by an angle less than
90° on the faulted feeder, and greater than 90° on the healthy feeders. With an RCA of 0°, the healthy feeder
residual current will fall in the 'restrain' area of the characteristic while the faulted feeder residual current falls in
the 'operate' area.
Often, a resistance is deliberately inserted in parallel with the Petersen Coil to ensure a measurable earth fault
current and increase the angular difference between the residual signals to reinforce the directional decision.
Directionality is usually implemented using a Wattmetric function, or a transient earth fault detection function
(TEFD), rather than a simple directional function, since they are more sensitive. For further information about TEFD,
refer to Transient Earth Fault Detection in the Current Protection Functions chapter.
7.5.3
SETTING GUIDELINES (COMPENSATED NETWORKS)
The directional setting should be such that the forward direction is looking down into the protected feeder (away
from the busbar), with a 0° RCA setting.
For a fully compensated system, the residual current detected by the relay on the faulted feeder is equal to the coil
current minus the sum of the charging currents flowing from the rest of the system. Further, the addition of the two
healthy phase charging currents on each feeder gives a total charging current which has a magnitude of three
times the steady state per phase value. Therefore, for a fully compensated system, the detected unbalanced
current is equal to three times the per phase charging current of the faulted circuit. A typical setting may therefore
be in the order of 30% of this value, i.e. equal to the per phase charging current of the faulted circuit. In practise,
the exact settings may well be determined on site, where system faults can be applied and suitable settings can
be adopted based on practically obtained results.
118
(I
+ I
+ I
)'
AH1
H2
H3
A
I'
L
N
C
a) capacitive and inductive currents with resistive components
Restrain
I
= I
R1
H1
0
Zero torque line for 0° RCA
b) Unfaulted line
Resistive component
in feeder
B
I
L
Zero torque line for 0° RCA
-I
- I
H 1
H2
I
R 3
I
= I
+ I
= I
- I
- I
R3
F
H3
L
H1
H12
V
= -3V
res
0
Operate
c) Faulted line
3V
0
Restrain
P24xM-TM-EN-2.1
P24xM
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