Earthing Transformers vs. NGRs?

[doehl1]

Hi Collegues:

Our standard generator package is provided with low resistance 26Ohm/300A/10sec neutral grounding resistors.

A new customer of ours has specified neutral earthing transformers for each of three, 5MW generators that will be installed and connected to their system (customer is a utility IPP). They have specified a 5A/10sec earthing transformer for the 11kV/50Hz interconnect at each generator to replace the standard NGRs.

What is the motivation for this? Where can I specify and purchase the proper earthing transformers? Is there a good primer document for this application?

Also, it appears their requirement has a resistor on the secondary coil of the earthing transformer which feeds to the protective relaying. What parameter is this looking for or limiting?

Thanks in advance for your comments .

 

[dpc]

They are asking for high-resistance grounding. Your standard is low-resistance grounding.

The transformer is used to allow a much smaller resistor to be used to provide this impedance.

The resistor is limiting ground fault current just like the resistor in your standard system, but the effective resistance seen from the generator side is boosted by the square of the transformer turns ratio.

High resistance grounding is better at limiting generator stator damage in the event of a ground fault in the machine.

J. Lewis Blackburn's Protective Relaying book has a good discussion on this

The transformers will be single-phase with something like a 240 V secondary. There is nothing special about the transformer.

 

[vreijb]

To add to your question
How to size the High Impedence Transformer for the short period of operation of 1 min before clearig the fault

 

[doehl1]

Thanks for the comments, dpc.

When you say there is nothing special about the transformers, does this mean I wouldn't have to use a zigzag transformer with resistor (since the generators is wye connected and has an easily accessible neutral). 11kV/240VAC transformers sound almost like MV PTs, but obviously would need to be rated properly.

 

[dpc]

The transformer would be a single-phase two-winding transformer. The transformer primary is rated for the generator phase-phase voltage normally, but should only see line-neutral voltage during a fault.

There are plenty of references describing how this works. If you are actually designing or proposing on this, I'd advise getting a good reference book.

A zigzag grounding transformer is used to establish a grounded neutral on an otherwise ungrounded system. Not the same thing at all.

The transformer is only there as an impedance transformation device to make the required resistor smaller and cheaper.

If you put the right resistor in, you can accomplish the same thing without any transformer.

You will also need a relay to monitor the voltage across the resistor in order to detect ground faults. Nowadays, this can normally be done with a multi-function generator relay. But if you put a voltage relay here, it needs to have third-harmonic filtering since there is normally some third-harmonic voltage present during normal operation.

 

[dpc]

As for sizing the transformer, conservative approach would use the 5 A current times the system line-neutral voltage:

5 A * 11/1.732 kV = about 32 kVA.

Since they are asking for a 10 sec rating, you can probably use a smaller transformer, but there will some continuous charging current even under normal operation due to system capacitance, as well as constant third-harmonic current, so a PT will generally be too small.

A 10 second resistor rating can be used, with the same caveat regarding being able to dissipate whatever small amount of continuous current you will have.

 

[doehl1]

Much thanks dpc. Very clear.

Can you recommend a good ref text? I have heard good things about the J&P Transformer Book, but availability of this seems difficult in the USA, where I am, and I believe this book may apply more to British/European standards.

Thanks again.

 

[Hoxton]

Why not go to

http://www.areva-td.com

and search for PRAG or NPRAG.

I used to work for them, but it is a good start.

Remember that grounding is a system design issue: I have generators running ungrounded (system is grounded elsewhere).

I used to manufacture 30MVA generators earthed by standard single phse transformers with a resistor across the secondary. As said earlier, earth faults can be detected by a voltage relay across the secondary resistor.

This works because the secondary resistor, refered to the primary is a high impedance - I think by the turns ratio squared (I cannot find my notes at the moment).

These were used where the 11kV generators were each connected to a step up transformer. The high impedance earthing resulted in low fault currents and minimal damage due to fault. Descrimination was poor, as any fault up to the transformer primary would result in immediate relay operation and breaker trip. This may be fine, where you have a close coupled generator / transformer. Poor practice where the generators are paralleled before the step up transformer. (Since any "in zone" fault will trip all generators)

If the generator(s) are connected to a distribution system, at generator voltage, then an earthing resistor, giving a higher fault current may be more appropriate to give discrimination with downstream devices. (often needing one resistor for the site, and one grounding switch per generator)

Each case has to be engineered, there is never a universal answer!

Sorry, this got more complicated as i wrote it, but that is often the problem!

 

[dpc]

doehl1,

See my first response for recommendation on book. IEEE Red Book would be another option.

 

[ScottyUK]

J&P is an excellent reference but it is a British book with British ways of doing things. If you can live with voltages and frequencies which probably look 'odd' to US eyes then there is a mass of information which is applicable to any power transformer. Have a look at faq238-1287.


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If we learn from our mistakes I'm getting a great education!

 

[REM76]

Make sure to order a resistor with multiple taps to be able to adjust the current to be higher than the capacitive charging current of the system. 5 Amps could be lower than the charging current, so it is recommended to spend some time to come up with the accurate current value for the resistor.