Operating with 20A on 1500:5 ratio; Accuracy issues with reverse power relay? 1

[tumbleweed1]

We have a 1500A utility feeder with 1500:5A protection CTs. We are connecting internal generation that will displace the utility power, but not export power to the utility. We will maintain a minimum import, and when we do, we can have as little as 20A incoming from the utility.

For the purpose of reverse power protection, we would like to set a trip such that if we fall less than our desired minimum import, that we will trip off. This will be our "buffer" from exporting.

My concern is operating on the low end of the CT primary rated current. Can we expect large inaccuracies in current measurement? Are there some CT ratings that would be better to use than others? We also have an estimated length of 400ft of 10awg copper to connect the CTs to the relay.

How can I determine an adequate CT selection for this application?

 

[ScottyUK]

Reverse power measurement is notoriously difficult at low currents. Small phase shifts in the instrument transformers can make reactive power appear as active power, and vice-versa. This is a well-known problem in steam turbine protection because the turbine spinning in condenser vacuum with the MSV's closed and the generator acting as a synchronous motor absorbs very little power. The 32R relay doesn't always see the reverse power condition, the reverse power measured by the relay being somewhat dependent on the reactive load on the generator. I think you'll have similar problems.

 

[burntcoil]

I would suggest you to use 1500:1A CT. You cannot do anything to the primary due to the feeder rating but since your relay is 400ft away, i would suggest to use 1A secondary. This will improve things.