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CHAPTER 4: SETPOINTS
Neutral Overvoltage
Protection
FASTPATH:
850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
The 850 relay provides one Neutral Overvoltage (also called Neutral Displacement) (Neutral
OV) element per protection group.
The Neutral Overvoltage element can be used to detect asymmetrical system voltage
conditions caused by a ground fault or the loss of one or two phases of the source. The
element responds to the system neutral voltage (3V_0), calculated from the phase
voltages. The nominal secondary voltage of the phase voltage channels entered under
SETPOINTS/SYSTEM/ VOLTAGE SENSING/PHASE VT SECONDARY is the base used when
setting the Pickup level. The Neutral Overvoltage element can provide a time-delayed
operating characteristic versus the applied voltage (initialized from FlexCurves A, B, C or D)
or can be used as a definite time element. The source voltage assigned to this element
must be configured for a phase VT and phase VTs must be wye connected. VT errors and
normal voltage unbalance must be considered when setting this element.
The same curves used for the time overcurrent elements are used for Neutral
Displacement. When using the curve to determine the operating time of the Neutral
Displacement element, substitute the ratio of neutral voltage to Pickup level for the current
ratio shown on the horizontal axis of the curve plot.
Be aware that the Neutral Overvoltage feature should be applied with caution. It would
normally be applied to give line-to-ground fault coverage on high impedance grounded or
ungrounded systems, which are isolated. This constraint stems from the fact that a
measurement of 3V_0 cannot discriminate between a faulted circuit and an adjacent
healthy circuit. Use of a time delayed back-up or alarm mode allows other protections an
opportunity to isolate the faulted element first.
As indicated above, the relay has one Neutral Overvoltage element per protection group.
The settings of this function are applied to 3V_0 calculated from the three phase-to-
ground (wye connected VTs) voltage inputs to produce Pickup and Trip flags per 3V_0
calculated voltage. The Neutral OV Pickup flag is asserted when the calculated 3V_0
voltage is above the PKP value. The Neutral OV Trip flag is asserted if the element stays
picked up for the time defined by the selected inverse curve and the magnitude of the
3V_0 voltage. The element drops from Pickup without operation, if the calculated voltage
drops below 97 to 98% of the Pickup value before the time for operation is reached.
Path:
Setpoints > Protection > Group 1(6) > Voltage Elements > Neutral OV
..\Voltage\Neutral OV 1
Item Name
Function
Pickup
Curves
Pickup Delay
Dropout Delay
Block
Relays
Events
Targets
NTRLOV1
FUNCTION
Range: Disabled, Trip, Alarm, Latched Alarm, Configurable
Default: Disabled
PICKUP
Range: 0.02 to 3.00 x VT in steps of 0.01 x VT
Default: 0.30 x VT
Value
Unit
Disabled
0.30
x VT
Definite Time
1.000
s
1.000
s
Off
Disabled
Enabled
Self Reset
PROTECTION
4–205
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