Coordination with Utility settings

[victa1288]

I am doing a coordination study for a ficility. I got a TCC curve from a Utility company.They set their Curve way below our main incomer curve. This a radial distribution sytem. A 5MVA transformer (69kV/4.16kV) is supplying power to a 4160V SWGR, which is a single power source.Secondary cable is underground buried.Primary is overhead. The transformer is delta/wye(high resistance)connected. Utilty comapny set their curve based on frequent fault and WYE sencondary grounded so that the curve was shifted to the left to be 58% of normal TCC curve. My understanding is, the transformer damage curve should be based on infrequent fault and normal curve, because ony when the transformer is wye solid grounded, primary side will see 58% fault current.58%rule doesn't apply to high resistance grounded system.

We can't move our curve down, but how can we convince the utility company to accept our suggested curve? Possibly we will have to get our client to be involved and get this conflict to be solved.

Who have a better suggetion for this?

ThanX in advance.

 

[dpc]

This is a common situation. You can ask the utility to increase their setting, but don't count on them doing it.

The use of frequent vs. infrequent damage curves is subject to debate and interpretation. If the utility owns the transformer, they will decide how to protect it. If your client owns the transformer, you should be able to negotiate this.

ANSI defines "frequent" as 10 or more through faults in the life of the transformer.

You're correct regarding the shifted damage curve - for a resistance grounded system, this is a non-issue.

I would try to discuss with the utility and offer them some alternative settings. It won't hurt to ask.

And actually, you CAN move your curve down - you just may have to compromise on downstream coordination.

 

[victa1288]

Thank you. DPC.
I think I will get our client to be involved to solve this issue.

 

[stevenal]

Are other utility customers affected? The way to convince the utility is to show how mis-coordination unnecessarily causes outages on the un-faulted lines feeding other loads.

 

[victa1288]

Actually. no other customers will be affected. This transformer is owned by Utility company and the facility is owned by an American Company. This transformer is dedicated for this ficilty.

 

[stevenal]

Okay, plan B. Ask utility if they wish to be responding to outages due to the (ANSI frequent) faults that can be cleared by your facility. Ask about their response times during regular and after hours. Make sure you get past the customer service folks to the engineering department.

 

[victa1288]

Good idea.