Where If is the total fault current which would flow in an NVD relay (neglecting the impedance of the relay itself),
then knowing this current (If
) and the input impedance of the relay (Rr) we can calculate the voltage produced
across it (Vr) during a fault condition:
Vr = If x Rr
Therefore, we would recommend setting the relay to less than half this voltage:
Vs < Vr/2
Practical Application
In practice, the device’s input impedance varies with voltage, which will have some effect on actual settings.
Therefore, we recommend the use a 23½kΩ resistor combination in parallel with this input, to fix the impedance.
This value is achieved by the use of two 47kΩ resistors in parallel. Utilising two resistors in parallel also gives
increased security.
The resistors used must have a continuous working voltage rating of 5kVdc minimum and a minimum power
rating of 1W.
Warning:
There is the risk of high v
oltage developing on removal of the device or PCB from its
case. Fixed resistors on the device input will prevent this, but we would also
recommend use of an externally connected shorting contact.
Note:
A suitable shorting contact is av
ailable on each device. Please see diagram, below.
Neutral Voltage
Displacement Relay
(NVD)
E00822
47kΩ 47kΩ
Figure 119: Device connection with resistors and shorting contact
Voltage Setting (P141, P142, P143, P145)
The device has a minimum setting on the r
esidual OV/NVD of 1V, which should provide a sensitive enough setting
for most applications. The operating voltage to be applied can be calculated for various capacitor ratings, shown
in calculations provided above.
Chapter 10 - Voltage Protection Functions P14x
224 P14xEd1-TM-EN-1
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