Neutral Voltage Displacement (Nvd) Protection Applied To Condenser Bushings (Capacitor Cones); Figure 115: Residual Voltage For An Impedance Earthed System - GE MiCOM P40 Agile Technical Manual

P14x
N
Z
E
V - G
C
E00801

Figure 115: Residual voltage for an impedance earthed system

An impedance earthed 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.
5.3.3 NEUTRAL VOLTAGE DISPLACEMENT (NVD) PROTECTION APPLIED TO CONDENSER
BUSHINGS (CAPACITOR CONES)
Voltage Transformers are not fitted at distribution levels, due to their expense. Instead, capacitor cones, or
condenser bushings, may be used at 11kV and 33kV substations to provide a neutral voltage displacement output
to a suitable protection device.
Often, bushings are starred together, and the star point used to provide the displacement voltage to the device, as
seen in the diagram, below.
Warning:
As protection method requires the device to be placed in a primary circuit location, all
relevant safety measures must be in place.
P14xEd1-TM-EN-1
S
E
Z
S
V - G
A
S
G,F
V - G
C
V - G
B
V
RES
V - G
B
V - G
A
V - G
C
V =
RES
2Z + Z
S1
Chapter 10 - Voltage Protection Functions
IED
Z
L
V - G
R
A
G,F
V - G
C
V - G
B
V
RES
V - G
B
V - G
A
V - G
C
Z + 3
S0 ZE
+ 2Z + Z + 3Z
S0 L1 L0
E
F
G,F
V - G
B
V
RES
V - G
B
V - G
C
X 3 E
221