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The Residual Over voltage/Neutral Voltage Displacement Protection (59N) is not tripping when it should with a residual ct.

Issue
The Residual Over voltage/Neutral Voltage Displacement Protection (59N) is not tripping when it should with a residual ct.

Product Line
MiCOM P34x

Environment
MiCOM S1 Studio

Cause
The Residual Over voltage/Neutral Voltage Displacement Protection (59N) is derived instead using the measurement from the residual ct.

Resolution
The neutral voltage displacement protection function of the P342/P343 relays consist of two stages of derived (VN>1, VN>2) and two stages of measured (VN>3, VN>4) neutral overvoltage protection with adjustable time delays. 
The P344/P345 has an additional two stages of measured (VN>5, VN>6) neutral overvoltage protection as it has a dedicated second neutral voltage input, VN(2).

The relay derives the neutral/residual voltage operating quantity from this equation:
Vneutral = Va + Vb + Vc

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