Neutral Overvoltage (59N) - Siemens 7SR224 Technical Manual

2.13 Neutral Overvoltage (59N)

Neutral Overvoltage Displacement (Residual Overvoltage) 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.
EHV/HV
HV CB
HV CB
Tripped
by local
protection
Figure 2.13-1 NVD Application
Typically NVD protection measures the residual voltage (3V
cones – see fig. 2.13-2 below.
Figure 2.13-2 NVD Protection Connections
©2010 Siemens Protection Devices Limited
Transformer Feeder
OC/EF
Earth
fault
) directly from an open delta VT or from capacitor
0
7SR224 Argus Applications
HV/MV
MV CB
NVD
MV CB tripped by:
1) Feeder unit protection or
2) Intertrip from HV feeder protection or
3) NVD protection
Chapter 7 Page 24 of 50