Figure 44: Phasors For A Phase C Earth Fault In A Petersen Coil Earthed System; Figure 45: Zero Sequence Network Showing Residual Currents - GE P24DM Technical Manual

P24xM
I
H3
I
L
I
H2
I
H1
I
A1
N
C
a) Capacitive and inductive currents
V00633

Figure 44: Phasors for a phase C earth fault in a Petersen Coil earthed system

It can be seen that:
The voltage in the faulty phase reduces to almost 0V
●
The healthy phases raise their phase to earth voltages by a factor of Ö3
●
The triangle of voltages remains balanced
●
The charging currents lead the voltages by 90°
●
Using a core-balance current transformer (CBCT), the current imbalances on the healthy feeders can be measured.
They correspond to simple vector addition of I
by exactly 90º.
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 45: Zero sequence network showing residual currents

In practical cases, however, resistance is present, resulting in the following phasor diagrams:
P24xM-TM-EN-2.1
A
I
B1
B
A1
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
3V
0
I = I
R1 H1
I
b1
I
a1
V = -3V
res o
b) Unfaulted line
and I , I and I , I and I
B1 A2 B2 A3
- I - I (C). This is shown in the zero sequence network
L H1 H2
I
OF
Faulty feeder
Healthy feeders
1
Chapter 6 - Current Protection Functions
I
L
-I
H1
-I
H2
I I
R3 R3
V = -3V
res o
c) Faulted line
, and they lag the residual voltage
B3
I = Residual current on faulted feeder
ROF
I = Residual current on healthy feeder
ROH
I = I – I – I – I
OF L H1 H2 H3
I = I + I
ROF H3 OF
so:
I = I – I – I
ROF L H1 H2
= I + I
F H3
= I - I - I
L H1 H2
117