7SR210 & 7SR220 Applications Guide
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.
Figure 2.13-1 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.
Figure 2.13-2 NVD Protection Connections
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