Neutral Voltage Displacement Protection Relay Settings 1

[RichardTaylor]

I work for a UK distribution network operator.

We fit Neutral Voltage Displacement protection on our 33/11kV delta-star transformers in order to detect and clear earth-faults backfed from the 11kV busbars.

Traditionally, the NVD protection signal has been derived from a star-connected set of capacitor bushings on the 33kV side of the transformer, feeding into an Edgcumbe Peebles type NVD101 relay via a small transformer / rectifier unit. The capacitor bushings on modern transformers have a value of 150pF.

These relays are set to detect a 33kV system neutral displacement of 5kV.

We have now stopped using the NVD101 relays, as these are proving unreliable and expensive, and started using Alstom MVTU13 relays, until these became obsolete. We are now using Alstom P921 relays.

The question I would like to ask is whether anyone else is using this arrangement? If so, I would like some advice on settings for the relay.
Various people in our organisation have calculated settings, but have come up with settings between 1V and 14V. (The minimum setting available is 5V.)

We have done some injection tests, which suggest that a setting of 5V is about right, but I would like some theoretical cofirmation of this.

Does anyone use any other type of relay?

Can give further details if required.

 

[RRaghunath]

assuming that the voltage displacement is 100 volts for a line end fault, 5V setting should give sensitivity to the relay to detect faults in 95% of the winding (or 5% from the neutral end).
Hope this helps.

 

[RichardTaylor]

Perhaps I should attempt to clarify the situation:

The magnitude of neutral Voltage displacement we are looking to detect on the 33kV system is 5kV. (This is a nominal value, as the 33kV system is effectively unearthed once the source 33kV CB has opened.)

We use NVD protection to guard against backfed earth faults on the 33kV circuit supplying the transformer, not to detect faults on the transformer itself.

We use capacitor VTs (150pF) to detect the dispacement
Voltage. The secondary star point of these VTs is connected to true earth via the NVD relay.

 

[DEPU]

The neutral voltage displacement varies from a maximum value
of 33kv for aground fault at relay location to a aminimum for a ground fault at the 33kv bus of the source station.However setting the relay to detect faults close to the source bus will necisitates large time delay since the relay will respond to ground faults on all outgoing 33kv from the source bus.The best way to set the relay is to simulate a ground at the end of the incoming 33kv feeder at the source bus with line diconnected from the bus and set the relay accordingly.By doing this your are assuming sequancial tripping of your 33kv line.

 

[RichardTaylor]

Thanks, DEPU.

We do indeed employ a log time delay (several seconds) on the NVD relay in order to give time for other earth fault protection to operate.

I am more concerned with the application of a Voltage-measuring relay connected to a set of capacitor VTs. Some of our older sites have capacitor VTs of 60pF or less, and we are struggling to get the necesary sensitivity to detect a neutral displacement of 5kV.

Do any other DNOs out there use this method of NVD protection? Are there any better solutions, other than using open-delta VTs which would not be economic?

 

[Derm]

In Rep. Ireland a similar scheme is used on the 40kV network for earth fault detection. The neutral at the transformers is arc suppressed through Petersen coils and a single VT fed to an ABB SPAU 110C relay set for 30% to avoid residual voltages, this would correspond to 6.5kV on our system.
We previously used the mvtu relay before for similar purpose.