Coordination with Utility

[swiminelectricity]

Hi, Folks,

I am working on a project and I am getting into trouble coordinating with Utility Company.
My problem is, I finished our station curve plot and sent it to this Utility company.They sent back their utilty realy curve, which is SR750 50/51.It ended up overlaping with most of our curves and it is even located below their utilty transformer inrush point.They were asking us to speed up our SR750 to leave some margin to place their HV backup protection. But that would result in the loss of our station protection coordination. I am pretty new in this field. Could you give me a good suggestion on how to deal with this case?
Thank you.

 

[DiscoP]

There is always trouble co-ordinating with the utility !!

You'll need to find a way to reduce your protection operating times, as you will find it difficult to get the utility to change theirs.

Some suggestions are
- Reduce your grading margins down as low as possible (many text books have suggestions for this_
- identify points where co-ordination isn't absolutely essential (eg HV & LV Transformer Protection)
- introduce differential protection on short lines
- Blocking or dynamic relaying schemes (downstream relay either blocks or adds delay to the upstream relay once it picks up)

The solutions depend a lot on your network, and the equipment you have (or can afford).

I think it probably needs one on one discussion with someone with expertise in protection.

 

[dpc]

Yes, this is a common problem. As DiscoP mentions, all you can do is tighten up your settings as much as possible, then request that the utility modify their settings to provide an adequate coordination time interval.

It sometimes help if you will stipulate that an independent third-party testing company will be brought in to calibrate and functionally test your relaying to verify that it actually operates as you are promising.

Sometimes, you just have to live with it.

 

[Bung]

Normally, the utility will give one "level" of grading margin below theirs - they have to for their own downstream protection, if nothing else!

But you won't get more than level - if you have two levels, eg "instantaneous" (ie around 100ms total clear time), and the next up is 300ms above it (0.25t +0.25 traditionally), that is 400ms clear time. That means the utility breaker is set for 750ms at the fastest, and its backup to over a second. Not a good look if the fault level is 13kA (for eg)! No utility (nor anybody else) is going to tolerate a fault burning for at least 3/4 of a second.

All you can do is to use some form of "unit" protection or a blocking scheme of some sort to eleiminate the need for time grading. Costs money, but that is the only way out. It is only required until you have enough impedance between the protection points to bring the fault level down enough to get the grading you need.

Bung
Life is non-linear...