Are Relay Overcurrent 50/51 elements senstive to CT polarity?

[mull982]

I was curious weather or not most relay the provide 50/51 and 50/51N protection elements are sensitive to the polarity or direction of the CT's that are connected to the relay? For most relays will the relay trip on pickup current going either direction through the CT? An application example would be a generator connected in a Co-Gen arrangement where a CT and relay can see fault current in one direction from the utility, or in the other direction from the generator depending on fault location.

I am thinking that the CT polarity would not matter since there it is just looking at current from one CT and there are no other CT's or PT's involved.

I appreciate any other insight.

Thanks

 

[JoeStillman]

Some applications use two CT's to measure 3 phase currents. Polarity matters for that.

If the same set of CT's are used to do 50/51 protection as well as 51G, then polarity matters.

I would pay attention to CT polarity no matter what.

 

[mull982]

Some applications use two CT's to measure 3 phase currents. Polarity matters for that.

If the same set of CT's are used to do 50/51 protection as well as 51G, then polarity matters.

I would pay attention to CT polarity no matter what.

Yes I understand the when looking at CT's for metering or residual ground fault calculations then CT's must have same polarity.

I'm referring to a case where all (3) CT's have the same polarity but current may go through the CT's in either the forward or reverse direction. Will the relay still pickup for current flowing either way through the CT's or only for one way if the relay is polarity sensitive?

 

[GoldDigger]

When looking at AC, the concept of polarity must necessarily be relative to some reference.
That reference could be either some other current (not used by the overload relay) or voltage (also not used by the overload relay).
Does that answer your question?

Now if the relay were measuring load power rather than just load current, the polarity of the CT might be relevant.

 

[big john]

Short answer: No, power direction does not influence a standard 50/51 setup.

For example if you had a business with a cogen plant, they could trip the same overcurrent relay regardless of whether they were consuming or exporting power.

 

[Phil Corso]

Mull192...

Of course polarity affects performance. Take the usual 50/51 combination of 3 CTs monitoring balanced line-currents! If connected properly, the 51's will 'see' egual currents and the 50 zero current!

But if one CT is reverse-connected, then, as above, the 51's will still 'see' equal currents, but the 50 current will double!

Regards, Phil Corso

 

[mull982]

Short answer: No, power direction does not influence a standard 50/51 setup.

For example if you had a business with a cogen plant, they could trip the same overcurrent relay regardless of whether they were consuming or exporting power.

This is what I thought and was the example I was looking at. I should have used the term "power direction" instead of "polarity" which seemed to have caused some confusion. Sorry for any confusion.

 

[Phil Corso]

Mull982...

CT polarity inversion will have major impact for power measuring instruments.

For example, if line-currents are equal, but one CT reversed, then in a 3-Wattmeter setup, total power measured will be just 1/3 of expected value!

Similarly, if wired to a 3-element power measuring protective device it will initiate an warranted trip-signal!

Phil

 

[Bugman1400]

I hope we are not confusing the OP with a bunch of hypothetical scenarios. As big john alluded to, for 50/51 & 50/51N relaying, assuming the CTs are connected correctly, the relays will trip for a fault or overload in either direction. If you add direction then the same relaying is typically called 67 or 67N.

 

[Phil Corso]

Bugman..."
I agree. But I tried to not get too "numeric" so I referred to current measuring protective-devices, those you mentioned, and power related devices, e.g., 32, 32N, that use a similar CT configuration!

Phil

 

[Phil Corso]

Oops... delete '32N'!

Phil

 

[remedios]

I was curious weather or not most relay the provide 50/51 and 50/51N protection elements are sensitive to the polarity or direction of the CT's that are connected to the relay? For most relays will the relay trip on pickup current going either direction through the CT? An application example would be a generator connected in a Co-Gen arrangement where a CT and relay can see fault current in one direction from the utility, or in the other direction from the generator depending on fault location.

I am thinking that the CT polarity would not matter since there it is just looking at current from one CT and there are no other CT's or PT's involved.

I appreciate any other insight.

Thanks

Relay 50/51 will not be affected if any one of the CTs polarity is reversed, but it matters to 50/51N, will expect nuisance breaker tripping when the load is starting to rise.

 

[Bugman1400]

Relay 50/51 will not be affected if any one of the CTs polarity is reversed, but it matters to 50/51N, will expect nuisance breaker tripping when the load is starting to rise.

Good point. If the 50/51N has its own CT then polarity will not matter but, I think you are referring to when the 50/51N is connected in the residual circuit of the 3 phase CTs. A CT or CTs that is(are) connected in reverse of the other will result in less than 3I0 current on the residual.

 

[remedios]

Good point. If the 50/51N has its own CT then polarity will not matter but, I think you are referring to when the 50/51N is connected in the residual circuit of the 3 phase CTs. A CT or CTs that is(are) connected in reverse of the other will result in less than 3I0 current on the residual.

ah, ok.... I am talking of a configuration of 3 CTs connected to a 3 OCRs (phase ABC) and a 50/51N.....

 

[Phil Corso]

Bugman...

Surely, by 50/51N, you don't mean the phase-element is instantaneous, while the neutral-element is delayed!

Phil

 

[Bugman1400]

Bugman...

Surely, by 50/51N, you don't mean the phase-element is instantaneous, while the neutral-element is delayed!

Phil

Typically, the 50/51 is listed together to show an instantaneous and timed element in the same relay with either a P for phase or an N for Neutral. So, you'll see 50/51P or just 50/51 for phase and 50/51N for neutral. If for some weird reason you did have an instantaneous phase with a timed neutral, I would show it as 50P, 51N. I would NOT show it as 50P/51N.

So, Yes, Phil, you are correct.....and, don't call me Surely.