Is an earthing transformer necessary?

[veritas]

I have attached a simple SLD of a 22kV generator connected to a transmission network via a 22kV/132kV gen trfr. 22kV side of gen trfr is delta connected and 132kV side is star connected.

Synchronisation to the grid is done at the GCB. Prior to synchronisation, with the GCB open, the section of 22kV network from the trfr to the GCB is unearthed. This is allowed for in the governing standards provided there is a protection system applied to detect an EF on this unearthed network. Thus I have applied neutral voltage displacement (NVD) as is the norm in many power stations here. Along comes the client's new engineering manager and says that it is unsafe. It is not so much what the standard allows but whether it is safe. The hazard of an EF on an unearthed network with only one protection function to detect it is not good enough and so an earthing transformer (ET) with neutral earthing resistor (NER) has to go in. CT in ET neutral to be wired to an EF relay. Even a 2nd backup NVD relay is not good enough says he.

This has ignited a raging debate on the issue and I am very interested to hear the thoughts of others. My thoughts are along the lines of is there really a difference between the two? In the sense that an EF with either scenario will result in a serious hazard. Both are dependent on only one relay function to detect and remove the unsafe condition. Yes, EF's on an unearthed network result in very nasty voltage transients due to the capacitive coupling particularly if there is arcing. However, the section of the network in question is very short (around 30m) and it is normally run earthed at the generator end with the GCB closed. Network will only be unearthed for very short periods of time just prior to synchronisation.

Of course the total fault current will increase by the ET contribution if one is introduced but this is not a show-stopper.

Thanks in advance.

 

[davidbeach]

The existing installation as you describe sounds like what's been done at 10's of thousands generators around the world. If full redundancy of protection is desired add a second set of VTs and a second NVD relay. If one's really paranoid about it just synch on the high side of the GSU.

 

[Mbrooke]

NER is enough, no need for an earthing transformer. And what David Beach said, sink on the high side. One less zero sequnce source to worry about when the generator is not in use.

 

[veritas]

Mbrooke,

What do you mean NER is enough, no need for earthing transformer? There can be no NER without an ET, so I'm a bit puzzled.

 

[cranky108]

The NER should be on the generator neutral. That should be your ground reference.

 

[veritas]

Oh, yes right, generator NER - makes sense. Thanks.

 

[davidbeach]

The generator NER may be directly in the neutral or it may be on the secondary of a grounding transformer.

 

[veritas]

Yes, we have a NER in the secondary of the generator grounding transformer.

One further thought - what if the distance of the unearthed network (from delta winding of gen trfr to GCB (which is open)) is increased from 30m to say 100m? 300m? Either overhead line or cable could be used?

Would everyone still be happy to only stick to NVD? Is there a cut-off point? I know things are seldom that black and white but if there is to be a cut-off point what would that be?

Thanks.

 

[Marmite]

Be mindful of the phase to earth insulation level for the circuit breaker, line, cable, transformer and any arresters. With the unearthed system the equipment could experience full line voltage to earth.
Is the unearthed system legal in your jurisdiction? It wouldn't be in some.

Regards
Marmite

 

[veritas]

Yes, it is legal provided that the hazards of an EF is suitably catered for. And yes, I have factored in the voltage rises on the healthy phases with EF's on impedance earthed systems - the unearthed case being an infinite impedance.

 

[prc]

Veritas, Normally there is a surge capacitor and lightning arrester on both sides of the Generator circuit breaker. This is part of breaker, inside it. Transformer manufacturers usually insist for such protection to take care of transferred surges from HV when breaker is open condition.

 

[veritas]

prc, yes, this is same in my case s well. I am still interested though to hear others opinions on how long an unearthed section of network consisting of OHL or cable will need to be before an earthing transformer is considered necessary.

Thanks.