Figure 41: Zero Sequence Network Showing Residual Currents; Figure 42: Phase C Earth Fault In Petersen Coil Earthed System: Practical Case With Resistance - GE MiCOM P40 Agile Technical Manual

P14D
The magnitude of the residual current I
the faulted feeder, the residual current is equal to I
shown in the following figure:
I
L
3X
-V
L
0
V00640

Figure 41: Zero sequence network showing residual currents

In practical cases, however, resistance is present, resulting in the following phasor diagrams:
V00641
Figure 42: 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. Hence a directional IED can be used. 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.
P14D-TM-EN-8
is equal to three times the steady-state charging current per phase. On
R1
I
ROF
I
ROH
I
ROH
IH IH IH
3 2
X
CO
(I + I + I
AH1 H2
Resistive component
in grounding coil
I'
L
C
a) capacitive and inductive currents with resistive components
Restrain
Operate
I = I
R1 H1
V = -3V
res 0
Zero torque line for 0° RCA
b) Unfaulted line
- I - I (C). This is shown in the zero sequence network
L H1 H2
I
OF
Faulty feeder
I
I
Healthy feeders
I
I
so:
1
I
Resistive component
in feeder
)'
H3
A
N
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
Chapter 6 - Current Protection Functions
= Residual current on faulted feeder
ROF
= Residual current on healthy feeder
ROH
= I – I – I – I
OF L H1 H2 H3
= I + I
ROF H3 OF
= I – I – I
ROF L H1 H2
3V
0
Restrain
105