Gen./Transformer Differential Prot.

[burntcoil]

Can anyone suggest a differential relay for a Generator and its step up transformer. One set of differential CTs are on the neutral side of the 13.8kV generator and the other set is on 115kV Bus of a step-up transformer. So the zone of protection includes both Gen. and Transformer. Which relay (Make and Model) will be best suited for this application?

 

[ScottyUK]

ABB's REG670 can provide an overall differential function in addition to stator differential. I assume this is an additional protection function and not in lieu of the stator differential relay: you will not have enough sensitivity to adequately protect the stator if you have to use a biased diff relay to accommodate the GSU tapchanger.

 

[DTR2011]

I have had experience with the SEL 700G in a similar application. There was not a tap changer on the GSU.

SEL 700G

 

[ZapSib]

P34x may be use as block protection as well :

http://www2.schneider-electric.com/...~!NNM2:Generation+Protection~!NNM3:MiCOM+P34x

 

[ScottyUK]

A bit more information from the OP about his intended application would help.

 

[burntcoil]

Both the Generator (13.8kV, 160MVA) and Step-up Transformer (13.8/115kV, 166MVA) have their dedicated differential protection. The overall differential protection is named as "Gen. Step-up Transformer Overall Differential Protection".

 

[burntcoil]

@ScottyUK

In my application, there is a tap changer on 115kV side for GSU transformer. Does it makes difference for Differential Protection? Because I believe the tap changer variation will be compensated in the relay settings for differential protection.

 

[ScottyUK]

A biased differential relay allows a larger imbalance current to flow without tripping the relay as load current increases. This is probably the most common type of differential relay used in transformer protection. In contrast a stator differential relay usually has a very low setting which is independent of load current.

If you use a biased scheme I don't think you will be able to achieve as low a setting and the protection afforded to the machine will be reduced. The cost of an additional relay is likely to be a few thousand pounds or dollars, to protect a machine worth perhaps 1000 times as much.

 

[DTR2011]

"Both the Generator (13.8kV, 160MVA) and Step-up Transformer (13.8/115kV, 166MVA) have their dedicated differential protection. "

I see this as added protection as back up to the primary units. If you can live with lower sensitivity if it is a back up unit (high slope / second slope) settings most modern relays will satisfy your initial requirements. You will want to also pay attention to the CT's for matching performance and lead length / burden issues for a run that long. What you don't want is your back up protection to nuisance trip for a through fault, due to saturation issues. On a 160 MVA unit that could be quite a distance between the Gen Neutral CTs and the Transformer HV side or HV breaker, if you choose to further extend the back up protection to cover the breaker.

 

[ScottyUK]

Thanks smallgreek, I missed the first of the two consecutive posts from burntcoil. . I agree with your comments.

 

[slavag]

Possible use any transformer differential protective terminal, if it's only overall differential protection.
If you need also back-up protection to generator protection, a good style for such application use protective terminals like to ABB REG670/REG650, Alstom P34x, Siemens 7UMx.

In your case, a best way use same terminal as exist transformer differential, less spare parts .