E00801
IED
E
S F
Z
S
Z
L
Z
E
V
A
- G
V
A
- G
S
G,F
V
B
- G
V
B
- G
V
B
- G
V
C
- G
V
C
- G
V
C
- G
G,F
G,F
R
V
C
- G
V
C
- G
V
C
- G
V
B
- G
V
B
- G
V
B
- G
V
RES
V
RES
V
RES
V
RES
=
Z
S0
+ 3
ZE
2Z
S1
+ Z
S0
+ 2Z
L1
+ Z
L0
+ 3Z
E
X 3 E
V
A
- G
V
A
- G
N
Figure 115: Residual voltage for an impedance earthed system
An impedance ear
thed system will alw
ays 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 (CAPA
CITOR 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 pr
ovide a neutral v
oltage 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 pr
ot
ection method requires the device to be placed in a primary circuit location, all
relevant safety measures must be in place.
P14x Chapter 10 - Voltage Protection Functions
P14xEd1-TM-EN-1 221
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