Loss of Utility Protection

[Schmoleskin]

A quote from an email I recently saw, "The load on this feeder may at some times be less than the capacity of the generator."

The utility is requiring this cogen provider to supply an SEL 700G protective relay. Can anyone help me understand how it's possible to sense loss of utility if the connected load doesn't challenge the overcurrent/underfrequency elements? By sensing a change in Power Factor?

The "At some times" part is somewhat worrisome if the utility is expecting this line to be dead when they open its feeder, and there's still a perfectly happy generator on.

 

[davidbeach]

As the utility I'd be requiring transfer trip in that case. Probably best to talk to utility.

 

[ScottyUK]

Google the one or more of the following: "G59 relay"; "ROCOF relay"; "vector shift relay".

G59 is a UK document detailing requirements for detecting loss of utility. The other two are methods of achieving it.


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

 

[rbulsara]

This is a common interconnect requirement for synchronous generators. (by most utility co. in the USA).

I interpret the utility's statement as them requiring reverse power relay. When the gen is capable of producing more power than the load on the feeder, it will back feed the utility, which is not always permitted by utility or may not be in the agreement. It does not have to be dead.

Even all relays need to be utility grade. Many mfr's built-in protection may not cut it.

Yes, talk to the utility co in the end.

Rafiq Bulsara

http://www.srengineersct.com

 

[pwrtran]

Even there is a load match but the frequency may more or less slip away. So the basic anti-islanding protection shall have the following elements:
1) 59
2) 27
3) 81O
4) 81U

You may required to have transformer trip if the generation greater than the threshold size specified by the utility.

 

[ScottyUK]

Normally those functions are too slow to prevent a prime mover stall under gross overload, which is why a vector shift or rate-of-change-of-frequency relay is used instead. It detects the slowing of the set before it reaches the point of no return.


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

 

[slavag]

What Scotty say.
ROCOF or df/dt or 81R .
vector shift is more...problem,.
posssible use logical functionality, like to 81U or 81O with df/dt.

 

[slavag]

btw, G59 is great document, one of the best on the LOM ( loss of main) issue.

 

[stevenal]

As far as "expecting the line to be dead", I would hope the utility would use lockout tagout on all sources and personal safety grounding to ensure the line is dead when needed for work.

It is common utility practice to use automatic reclosing to ensure customers don't have a lengthy outage for a transient fault. This is the real risk to your co-gen. Like David said, transfer trip is recommended.

 

[slavag]

Its not simple provide transfer trip in the distribution network

 

[davidbeach]

No, it is simple enough, just not inexpensive.

 

[slavag]

Yep, David, you are right.

 

[dfdt]

where can i find G59 document?

 

[ScottyUK]

Here:

http://2010.energynetworks.org/link-to-engineering-documents/

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