Alstom MiCOM P545 - 2.13 Residual overvoltage (neutral displacement) protection

P54x/EN AP/La4 Application Notes
(AP) 6-42 MiCOM P543, P544, P545 & P546
AP
2.13 Residual overvoltage (neutral displacement) protection
On a healthy three phase power system, the addition of each of the three phase to earth
voltages is nominally zero, as it is the vector addition of three balanced vectors at 120° to
one another. However, when an earth (ground) fault occurs on the primary system this
balance is upset and a ‘residual’ voltage is produced.
Note: This condition causes a rise in the neutral voltage with respect to
earth which is commonly referred to as neutral voltage
displacement or NVD.
Figure 12 and Figure 13 show the residual voltages that are produced during earth fault
con
ditions occurring on a solid and impedance earthed power system respectively.
E
S
Z
S
Z
L
F
A - G
V
A
V
B
V
C
V
A
V
B
V
C
V
C
V
B
V
B
V
C
V
RES
V
RES
V
A
V
C
V
B
V
A
V
B
V
C
Residual voltage at R (relay point) is dependant upon
Z
S
/
Z
L
ratio.
V
RES
2Z
S1
Z
S0
2Z
L1
Z
L0
+++
=x 3 E
Z
S0
G
R
P0117ENa
Figure 12 Residual voltage, solidly earthed system
As shown in Figure 12 the residual voltage measured by a rel
ay for an earth fault on a solidly
earthed system is solely dependent upon the ratio of source impedance behind the relay to
line impedance in front of the relay, up to the point of fault. For a remote fault, the Zs/Zl ratio
will be small, resulting in a correspondingly small residual voltage. As such, depending upon
the relay setting, such a relay would only operate for faults up to a certain distance along the
system. The value of residual voltage generated for an earth fault condition is given by the
general formula shown.