Differential Protection Coordination
Differential Protection Coordination
(OP)
I am dealing with a nuisance tripping problem on a differential protection scheme and am looking for some help in understanding as this is a bit over my head. (We do have an engineer working on the problem as well)
The situation is we have a line up of GE 15KV switchgear with A and B side feeds and an automatic throw over system which among other things feeds 6 substation transformers (12.47KV to 480/277). A and B systems are redundant with each side having one 3000KVA, one 2500KVA and one 1500KVA transformer. Each breaker has differential protection with the primary CT’s located around the load side bus of each 12KV breaker and the secondary CT’s located around the bus stabs of the breakers in the 480V gear after the transformers. Each 12KV breaker has a T60 relay providing the protection. We ran interconnecting CT wiring from the secondary CT’s to the T60 relays. The problem is the 2 breakers feeding the 1500KVA transformers keep tripping on current, percent differential when only about 175A (@480V) of load is applied. One of the 2500KVA transformers is having the same issue.
What we discovered through some investigation is that the three breakers that are not nuisance tripping have CT ratio’s configured as follows:
2 3000KVA transformers have primary CT’s at 300:1 and secondary CT’s at 4000:1
1 2500KVA transformer has primary CT’s at 300:1 and secondary CT’s at 3200:1
The breakers that are tripping are configured as follows:
2 1500KVA transformers have primary CT’s at 100:1 and secondary CT’s at 2000:1
1 2500KVA transformer has primary CT’s at 100:1 and secondary CT’s at 3200:1.
It looks as though changing the 100:1 CT’s to 300:1 may fix our problem. Does this make any sense??
Thanks in advance for any input.
The situation is we have a line up of GE 15KV switchgear with A and B side feeds and an automatic throw over system which among other things feeds 6 substation transformers (12.47KV to 480/277). A and B systems are redundant with each side having one 3000KVA, one 2500KVA and one 1500KVA transformer. Each breaker has differential protection with the primary CT’s located around the load side bus of each 12KV breaker and the secondary CT’s located around the bus stabs of the breakers in the 480V gear after the transformers. Each 12KV breaker has a T60 relay providing the protection. We ran interconnecting CT wiring from the secondary CT’s to the T60 relays. The problem is the 2 breakers feeding the 1500KVA transformers keep tripping on current, percent differential when only about 175A (@480V) of load is applied. One of the 2500KVA transformers is having the same issue.
What we discovered through some investigation is that the three breakers that are not nuisance tripping have CT ratio’s configured as follows:
2 3000KVA transformers have primary CT’s at 300:1 and secondary CT’s at 4000:1
1 2500KVA transformer has primary CT’s at 300:1 and secondary CT’s at 3200:1
The breakers that are tripping are configured as follows:
2 1500KVA transformers have primary CT’s at 100:1 and secondary CT’s at 2000:1
1 2500KVA transformer has primary CT’s at 100:1 and secondary CT’s at 3200:1.
It looks as though changing the 100:1 CT’s to 300:1 may fix our problem. Does this make any sense??
Thanks in advance for any input.






RE: Differential Protection Coordination
RE: Differential Protection Coordination
Your symptoms indicate a problem with CT polarity and/or phase in the circuit that are tripping. Due to the restraint in the differential relay, the tripping will not occur until there is some load on the transformer. If you observe the current in the relay at low loads before it trips, you will be able to see the differential current, and you can probably get enough information to draw the phasor diagram that will show the problem.
The T60 can provide you with the cause of the trips as well as the current. If it is tripping on differential, the relay will also indicate what it thinks the differential current is.
This should help point you in the right direction. There are about 30 ways to hook up CTs into a transformer differential relay, but only one is correct.
RE: Differential Protection Coordination
For example, the knee point excitation current on a 1200:5 CT might be 0.2A, but the knee point excitation current at a 100:5 tap is roughly 12x as much, or about 2.5A, and is no longer neglible. Furthermore, when you are on low taps, the knee point voltage drops (e.g. by 1/12 in this example) so it is much easier to reach the point where the CT is not a very good CT.
RE: Differential Protection Coordination
We have physically checked polarities and wiring more than once and are confident that that is ruled out, unless of course our connection diagrams are incorrect. The T60 will produce a phasor diagram but it will only do it when there is current flowing. We have had a hard time working with mechanical to get the right number of pumps running so we can see and record it and the relay tripping. Hopefully tomorrow. We also had some software issues we are trying to work out with out IT department as far as the required software being installed.
I don't know the full ratio of the CT's. I will have to check tomorrow but I know 100:5 is at the low end of the scale. That is an interesting point since the 300:5 breakers work fine so far. We might find they still trip just at a higher level.
RE: Differential Protection Coordination
RE: Differential Protection Coordination
It sounds like a fairly straightforward CT polarity issue. Many filed people lose track of the issue of CT polarity in differential circuits.
I have seen the drawings wrong, and I have seen the wiring wrong, and I have seen the CT's installed backward from the drawing, and I have seen the wiring going to the wrong set of CT's. I have also seen the incorrect settings in the relay.
The T60 is a wonderful platform, but some people get confused by the relationship of current inputs and definition of sources in the setup of the relay.
I have also seen problems where the relay logic called for trip on the status of the differential element pickup as opposed to the differential element operation. There's a difference between the two. I made good money finding and explaining the difference to a client and his contract engineer.
I'd love to help you figure this thing out. All it takes is time and money.
old field guy
RE: Differential Protection Coordination
RE: Differential Protection Coordination
In troubleshooting some of these problems, false tripping when we KNOW the transformer is good, I have disabled the tripping function of the differential element for the time you're testing.
On the T-60, this is easily done in the programmable logic. Also, with the T-60, you can route the output, again in the programmable logic, to one of the user-programmable LED's to see when the offending element operates, and you can snag the event record for analysis when it operates.
Then you can load the system up in the configurations that are causing problems and use the relay's own data in the vector analysis functions to see what changes and when, to cause the trip.
Like I said, this is fun stuff!
old field guy
RE: Differential Protection Coordination
One theory that came up is the fact about 98% of the load is VFD's. Someone had an issue with feeding VFD loads with electronic relays. Their solution was to rearrange the loads so more linear load was mixed with the VFD's. We do not have that option.
RE: Differential Protection Coordination
Is this a new installation? And have you tested the differential scheme? (CTs, wiring, etc)
Possible causes, especially on the HV-side's CTs:
Saturating CT/CTs (do a mag-curve test on the CTs)
Fault in the wiring. Something like an intermittent earth fault, flash over, or even a loose connection somewhere in the circuit. (do a megger-test on the wiring and re-check all your connections. Check especially if the test-blocks is fitted securely)
Regards
Ralph
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RE: Differential Protection Coordination
You have not told us what your CT ratings are. Using the ANSI system, the CTs would have a rating like C100, C400 or something.(The european spec would have a VA rating).
The C value is the amount of voltage the CT is designed to sustain (during the duration of a fault detection) with accuracy (i.e. not saturating) across its FULL secondary, so a 1200:5 C200 CT can sustain 200V across its two outside taps (i.e. greatest ratio). You have to ensure that your secondary impedance (including secondary wiring and connections) times your worst-case secondary current does not approach this value.
The ideal formula is that required C > (1+X/R) * (Imax,sec *Zsec)
where X and R are the primary system reactance and resistance.
Note that when you choose a reduced tap, you must derate your CT as well. If you were to choose a 300:5 tap on the CT above, your effective rating on your CT becomes 300/1200* 200 = C50.
However, the 'acceptable' rule-of-thumb for modern relays is that C > 2 * Imax,sec * Zsec
Note that in nearly every real system, this is not usually an issue under load currents, just for fault level currents.
RE: Differential Protection Coordination
You need to verify that the relay is properly set for the actual transformer phase shift, and also verify that the actual phase sequence (ABC or ACB) is correctly set.
I know you are tired of hearing it, but experience tell us that a CT wiring error or relay configuration error is the most likely culprit. Guilty until proven innocent. Often multiple wiring errors can mask themselves under low load conditions. So I would double and triple check all CT polarities, phase, and wiring into the relay.
If the relay is tripping on differential it is because it "sees" a difference in current between primary and secondary. You just need to figure out why. Looking at the relay's computed difference current and plotting that on the phasor diagram can often give clue as to the problem.
RE: Differential Protection Coordination
RE: Differential Protection Coordination
Next option of problem. CT secondary wiring size.
Pls. check T60 CT requerements and calculated n' .
Regards.
Slava
RE: Differential Protection Coordination
Nick
RE: Differential Protection Coordination