87 Differential tripping main breaker??? 5

[rockman7892]

I have a problem where a SEL 387 differential is tripping our main 5kV breaker. The differential zone is between the secondary bus of our substation transformer and on the load side of our main breaker right after the breaker. The cable that is between the differential zone is 9 runs of 1000MCM for a distance of 1000ft.

The SEL 387 keeps tripping the main and indicating an 87 trip for phases B and C only. The only thing that we did different was added an additional small load in the plant to this breaker however the breaker was closed when we added this load and did not trip. It was not until we opened the main breaker ( unrelated reason) and then tried to close it again that it tripped on differential. After it tripped we tried to close this main again an it tripped on the B and C phase again.

I'm confused b/c nothing that we do on the load side of the breaker (in the plant) should effect the differential zone since what goes in must come out. I'm thinking that there is a problem with the cable run between the substation transformer, or something wrong within the main breaker.

Any ideas???

 

[rockman7892]

The utility came out to investigate the relay and found that indeed the relay was tripping on an 87 trip. They noticed that the magnitude of the trip was at about 1000A, and the phase angles all seemed to make sense. After realizing that this 1000A magnitue was not a fault current (fault levels out here are 42,000kA) we decided to try and close the breaker again. We closed our main breaker, and the picked up load (two 1500 kVA transformers lightly loaded) with our feeder breaker and again tripped the main breaker with a differential trip. We noticed this time that the current magnitude was about 500A.

Figuring that the the fault we were seeing was not a fault related trip, they believed that it was a result of the inrush current from the two transformers. They were saying that this was a characteristic of this inrush that the relay was tripping on. We then disabled the 87 trip element in this relay and tried to close the breaker again. This time the main breaker and feeder breaker both closed and did not trip. Steady state current on the relay with this load was about 25A at 5kV. After we saw the steady state currents and saw that the phase angles were o.k. we re-inserted the 87 trip element back into the relay. With the 87 element enabled nothing trip and the system stayed fine at steady state.

Based on these events it seems that the inrush of these transformers was causing this trip on the 87. This is not a permant fix however b/c there will be many more inrush currents on the system as we bring transformers and motors online. The utility engineer is going to speak with SEL and determine how we can get around this inrush characteristic. There was also a negative sequence element in the relay that they removed which may have been contributing to the trip.

I'm not understanding why this inrush would cause this differential trip to operate? As long both sets of CT's see the inrush current, why would the relay trip? Is there a possible phase shift that is occuring between the CT's as a result of the inrush??

 

[slavag]

I'm not understanding why this inrush would cause this differential trip to operate? As long both sets of CT's see the inrush current, why would the relay trip? Is there a possible phase shift that is occuring between the CT's as a result of the inrush??
Ofcourse, if you have not exactly same sets of CT or
secondary load ( secondary wiring from CT to relay) isn't same and one of sets in inrush time was in partual saturation.
Regards.
Slava

 

[davidbeach]

A properly connected SEL-311L/SEL-387L combination is immune from inrush problems. That is not your problem, but rather a red herring that is distracting you from your real problem. The event reports that are not being downloaded (a computer with a serial port and hyperterminal is sufficient) contain enough information to point out the misconnection.

 

[stevenal]

Out of zone inrush should sum to zero.
With load current going through, I'd suggest looking at at the metering values provided by the relay. Magnitudes should be equal, and phase angles out by 180 degrees from local to remote relay. The vector sum (use the met command) should be close to zero.

 

[jghrist]

Are both relay 87L elements operating or is one relay tripping on transfer trip from the other relay?

 

[rockman7892]

I believe that one relay is tripping on a transfer trip from the other relay. (Master/Slave).

I dont see why the phase angles are 180 degrees out of phase from the local to remote relay. I'm probably missing something simple?

I am asking the utility engineer to send me the event files. He is also going to call SEL about the 87 setting which he is claiming is due to inrush.

 

[davidbeach]

Both relays measure current into the line. At the load end of the line current is out rather than in. Therefore the two ends are 180[°] out from each other.

 

[stevenal]

The inrush may be what is putting you over pickup, so tripping is occuring during energization. This is not the problem, though. The problem is current that does not sum to zero. Here is a sample met command from a 311L. The vector sum of 1 amp primary is close enough to zero.

met

L1-93 Date: 06/24/08 Time: 07:24:32.129
SS106

Local A B C 3I0 3I2 I1
I MAG (A Pri) 30.189 32.598 31.280 0.085 3.864 31.340
I ANG (DEG) 1.30 -118.90 117.40 -80.80 150.10 0.00

Channel X PRIM A B C 3I0 3I2 I1
I MAG (A Pri) 29.816 32.485 31.110 0.186 4.314 31.120
I ANG (DEG) -176.40 63.20 -60.60 7.00 -31.50 -177.90

Channel Y STBY A B C 3I0 3I2 I1
I MAG (A Pri) 29.904 32.626 31.138 0.212 4.185 31.206
I ANG (DEG) -176.50 62.90 -60.80 13.20 -36.40 -178.10

Vector Sum A B C 3I0 3I2 I1
I MAG (A Pri) 1.261 1.198 1.102 0.207 0.465 1.166
I ANG (DEG) -70.30 157.50 37.20 -17.00 -44.90 -78.00

Alpha Plane A B C ZERO-SEQ NEG-SEQ POS-SEQ
RADIUS 0.980 0.990 0.990 0.000 0.000 0.990
ANG (DEG) 177.70 177.90 178.00 0.00 0.00 177.80

 

[jghrist]

Try to get event reports from both relays. If the remote currents are not the same as the local currents on the other relay, then maybe there is a CTR setting error.

I agree with David and stevenal that inrush is very unlikely to be the problem. The relays are very tolerant of CT saturation even if the inrush was enough to cause saturation of one relay. Inrush is unlikely to be enough to cause CT saturation anyway. If it was, the CTs would be pretty useless under fault conditions, wouldn't they.

 

[rockman7892]

I do not believe that the CT's are saturating due to the fact that both sets are 3000:5 CT't and the magnitude of the current we are seeing at time of trip is only 500-1000A.

In the above example of the 311L data I notice that the C Phase currents lead the B phase currents. Does this matter as long as the CT's are matched correctly at both ends?

 

[stevenal]

The columns don't line up very well. A: 1.3 deg, B: -118.9 deg C: 117.4 deg. C lags B which lags A which lags C. This is ABC rotation. Phase rotation must be the same at both ends, and correctly entered in the relay. Note that the magnitude of 3I2 is very low. If I had entered the rotation as CBA, the 3I2 values would approximate the phase values, and I would risk triping on the vector sum of 3I2 which is set more sensitively than the phase differential. I don't know how the setting free 387L handles this. Perhaps it does not do sequence differential. If that is the case, these elements should be probably be disabled in the 311L also.

I agree that saturation is not likely to be a problem.

 

[jghrist]

We agree that saturation is not a problem.

The utility came out to investigate the relay ... they believed that it was a result of the inrush current from the two transformers. They were saying that this was a characteristic of this inrush that the relay was tripping on.

I'd like to hear the utility's reasons for suspecting inrush if neither CT is saturated and the inrush through current is satisfactorily represented by the CT secondary current on both relays. What "characteristic" are they referring to? I would assume that both the SEL-387L and the SEL-311 use the same filtering of the current signal, so harmonics would have the same effect on both ends (probably no effect - filtered out).

 

[rockman7892]

stevenal

So it the sequence rotation in this example considered a positive sequence?

Are the I1, 3I2, and 3I0 parameters listed the positive, negative and zero sequence components of this current?

 

[stevenal]

"So it the sequence rotation in this example considered a positive sequence?"

I cannot say that. The phase currents are just the sums of their components. The majority component is positive, however. To get to the components the relay must have the correct phase rotation information. CBA rotation should also show mostly positive sequence, but if the relay were incorrectly programed the 3I2 component would incorrectly show as the largest component.

In the example above, I1 is the positive sequence current, 3I2 is three times the negative sequence current, and 3I0 is three times the zero sequence current.

 

[rockman7892]

I'm assuming the CBA rotation you mentioned in your last post is the same as ABC rotation. I've always referred to this type of rotation as positve sequence, or positive rotation with respect to the phase angles. Maybe however I am using this terminology wrong.

 

[stevenal]

No, not the same, Still mostly positive sequence under load conditions, though.

 

[stevenal]

From the 387L manual.

"Because most power systems operate on an ABC phase rotation, the
SEL-387L is configured at the factory for an ABC phase rotation. If your
power system operates on an ACB phase rotation, you will need to
interchange, with the same polarity, the B-phase and C-phase CT connections
on the relay terminals."

What I called CBA rotation above SEL calls ACB. If this connection is required on the 387L, the same connection would need to be used on the 311L. The 311L would then be set to ABC, and the user would interpret C labeled records as B and B labeled records as C.

 

[rockman7892]

I have recieved the event report file from the utility for this 311L relay. I do not have the SEL 5601 software to view this event report so I have posted the CEV as requested.

Hopefully somone is able to open and view this file and maybe see something that we are missing.

 

[stevenal]

Remote and local currents look like they are in phase rather than out by 180 when compared to the instant of energization. Not sure why the whole remote curve appears to be shifted by a half cycle in addition.

 

[slavag]

Stevenal. Thanks a lot for the file.
Are possible convert it to the Comtrade format?
Are possible see it as 3 graphs Ia-Ia1, Ib-Ib1 and Ic-Ic1?
And , are possible provide graph of diff currents.
Possible say according to the file, NO SATURATION
But, it's seems as not same CT's on the both sides 3000/5 isn't same
what is a class C100/200/400/800 and what is a secondary load on the CT.
I see somthing like to this in the false trip of 87G protection in the down current time.
Best Regards.
Slava