Alstom MiCOM P545 - Page 487

Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-43
AP
A - G
E
N
G
Z
E
S
Z
S
Z
L
R
F
V
A - G
S
G,F
V
C - G
V
B - G
V
C - G
V
B - G
R
V
A - G
G,F
G,F
V
B - G
V
C - G
V
RES
V
A - G
V
C - G
V
B - G
V
RES
V
A - G
V
C - G
V
B - G
V
RES
V
C - G
V
B - G
V
RES
2Z
S1
Z
S0
2Z
L1
Z
L0
+++
=x 3 E
Z
S0
3Z
E
+
3Z
E
+
P0118ENa
Figure 13 Residual voltage, resistance earthed system
Figure 13 shows that a resistance ea
rthed system will always generate a relatively large
degree of residual voltage, as the zero sequence source impedance now includes the
earthing impedance. It follows then, that the residual voltage generated by an earth fault on
an insulated system will be the highest possible value (3 x phase-neutral voltage), as the
zero sequence source impedance is infinite.
The detection of a residual overvoltage condition is an alternative means of earth fault
detection, which does not require any measurement of zero sequence current. This may be
particularly advantageous at a tee terminal where the infeed is from a delta winding of a
transformer (and the delta acts as a zero sequence current trap).
Note: Where residual overvoltage protection is applied, such a voltage will
be generated for a fault occurring anywhere on that section of the
system and hence the NVD protection must co-ordinate with other
earth/ground fault protection.