7SR11 & 7SR12 Applications Guide
Page 30 of 48 ©2017 Siemens Protection Devices Limited
2.14 Neutral Overvoltage (59N)
Neutral Voltage Displacement (NVD) protection is used to detect an earth fault where little or no earth current
flows.
This can occur where a feeder has been tripped at its HV side for an earth fault, but the circuit is still energised
from the LV side via an unearthed transformer winding. Insufficient earth current would be present to cause a trip,
but residual voltage would increase significantly; reaching up to 3-times the normal phase-earth voltage level.
If Neutral Overvoltage protection is used, it must be suitably time graded with other protections in order to prevent
unwanted tripping for external system earth faults.
OC/EF
Transformer Feeder
Earth
fault
EHV/HV
HV/MV
HV CB
Tripped
by local
protection
MV CB tripped by:
1) Feeder unit protection or
2) Intertrip from HV feeder protection or
3) NVD protection
NVD
HV CB MV CB
Figure 2.14-1 NVD Application
Typically NVD protection measures the residual voltage (3V
0
) directly from an open delta VT or from capacitor
cones – see fig. 2.13-2 below.
3Vo
3Vo
VT with
Open Delta
Secondary
Capacitor
Cone
Unit
Capacitor
Cone
Adaptor Unit
Relay
Relay
Figure 2.14-2 NVD Protection Connections
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