Restricted Earth Fault Relays for Transformer Protection 5

[smithm]

Can anyone tell me how to select an appropriate setting for restricted earth fault protection relays used to protect low voltage distribution transformers?<br>
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Is there a rule of thumb method?<br>
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Dear Friends,

I am an electrical + Electronics engineers involved in Power System Protection.

I have a doubt on generator protection.

Where should we connect the protective relay for generator ( O/C+ E/F relay)? Should it be to the CT which is near the breaker or should we connect the three phase CT on the neutral side of the generator and connect the O/C + E/F relay to this ?

Many schemes from ABB, Alstom, GE show the relay on the breaker side. Siemens show the relay on the neutral side .

Can one you explain what is correc,why, and why the other is not correct. ?

Thanks

R.Seshadri

 

[jbartos]

Suggestion: With the advent of integrated protective relaying, the integrated relay processor, which has its inputs from CTs and PTs, is the clue. The processor programming and internal design will probably be kept proprietary. Siemens integrated protective relays tend to be an example; however, others follow closely.

 

[bigamp]

It depends on what you want the relay to do.

If you are looking for a flow of fault current from the CB side into the generator then connect the relay to CT's at the CB end. The thing is, if your generator can run in island mode all by itself an OC relay connected to CT's at the CB end will not be able to detect an internal generator fault as (neglecting contribution from in-service motors) there will be no flow of fault current for the relay to &quot;see&quot;.

If you want the OC relay to be able to detect an internal generator fault and there is any chance that the generator can run in island mode all by itself, then connect the OC relay to CT's at the starpoint end. A relay so connected will also detect faults external to the generator.

Also, take care. The OC relay may need to have a voltage dependent characteristic, it depends on what you want it to do.

As to how a modern multi-function relay is configured, I concur with jbartos, it depends on the model and the manufacturer. I have just completed a generator protection relay installation using Alstom P343 relays. These relays utilise phase CT's at both the CB and the starpoint ends, with the CT's at the starpoint end being used for OC protection.

Alstom's less capable generator protection relay, for smaller machines that typically are not fitted with CT's at the starpoint is the P342. This relay has provision for CB end CT's only so (naturally) the OC protection will utilise CT's at the CB end.

So, what is correct? I think definately use CT's at the starpoint end if the generator can ever operate in island mode. You should also have a look through IEEE C37.102, a very useful standard, and have a think about exactly what protection you are trying to provide. Also, many manufacturers have excellent information on their websites and you should check these out.

Regards

 

[busbar]

(j)bartos' Jul 3, 2000 entry bears investigation, and Warrington is an indispensable text to have for protective relaying.

 

[peterb]

Bigamp is correct in suggesting the neutral end CTs for islanded generators. Another factor to take into account is that the neutral end CT location provides overcurrent protection before the generator is synchronized to the grid. This is the preferred location for larger unit- connected generators.
A word of caution on using integrated multifunction relays - don't put all of your eggs in that one basket. Prudent relay engineering practice dictates that at least some of the functions should be provided (or duplicated) by separate devices. As an example, I typically provide separate ground relaying and backup overcurrent (or duplicate differential) protection when using a multifunction relay.

 

[peterb]

Bigamp is correct in suggesting the neutral end CTs for islanded generators. Another factor to take into account is that the neutral end CT location provides overcurrent protection before the generator is synchronized to the grid. This is the preferred location for larger unit- connected generators.
A word of caution on using integrated multifunction relays - don't put all of your eggs in that one basket. Prudent relay engineering practice dictates that at least some of the functions should be provided (or duplicated) by separate devices. As an example, I typically provide separate ground relaying and backup overcurrent (or duplicate differential) protection when using a multifunction relay for generator protection.