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87 Differential tripping main breaker???
5

87 Differential tripping main breaker???

87 Differential tripping main breaker???

(OP)

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???

RE: 87 Differential tripping main breaker???

2
What do the relay event reports show.  Post a one-line and the event files (CEV format preferred) and maybe somebody can suggest something.

RE: 87 Differential tripping main breaker???

Check polarity of CT's. May be they are reversed.

RE: 87 Differential tripping main breaker???

Hi
As Edison said, polarity of CT's it's first reason of false operation in your case:
1.Load was lower than setting.
2.With additional load, sum. load still was lower than setting.
3. Inrush current in time of open/close of main CB was enough for tripped CB.
Are someone provided stability check of the protection system?

Please send us SLD, as David asked, please with all CT's technical data, maybe some CT's was in saturation

Best Regards.
Slava

RE: 87 Differential tripping main breaker???

(OP)

Attached is a single line for this relay.  It isn't the best SLD, and i'm working on getting another one posted.  The breaker I'm referring to is labeled SECT 1.  You can see the two sets of CT's where are at the breaker and right after the switch on the transformer secondary bus.

I dont think it has anything to do with the CT's b/c like I've mentioned in the post it has worked fine in the past.  When we energize the cables to the line side of the breaker everything is fine with the relay. It isn't until we close the breaker that the relay trips.

I'll try to get the relay files from the utility when they come out tomorrow.

RE: 87 Differential tripping main breaker???

The SLD shows only one set of CTs connected to the 87L that trips SECT1.  Where is the other 87L winding connected?  The SLD shows a second 87L linked to the first by fiber optic and shows one CT lead going off the page.  Where is it connected and what does the fiber optic link do?  Is there no transformer differential?  I would expect a transformer differential zone to include the transformer and the main breaker.

I would not rule out CT polarity or winding connection problems.  It may be that without the added load, the error did not cause enough differential current to operate the relay.  The new load may have a high enough inrush to trip only on energizing.

CT wiring is the most common problem with differential trips.

RE: 87 Differential tripping main breaker???

It would have been nice if you would have said SEL-387L in your first post rather than SEL-387.  The 387 is a transformer differential and the 387L is a line differential; very different application.  Where is the connection of the upper relay and its CTs?  What is the line that goes off to the right about 1/3 up the diagram?

Please post an event report from one, or better yet both, of the relays showing the trip.

RE: 87 Differential tripping main breaker???

Additional Q about SLD.
What is a some connection before main CB ( right side with a truck symbol)?
Regards.
Slava

RE: 87 Differential tripping main breaker???

Hi Rockman 7892:
I agree with slavag I would check the settings of this relay and confirm the CT specs and wiring. You may have a rolled CT, but more likely the additional load is exceeding the relay set points during inrush.
I would test the following:
1. SEL relay for proper pick-up at required set points.
2. CT, ratio, polarity and wiring to relay.  Make sure that all the CT's are on the same tap.
3. Cable tests.  Do a Quick Megger check phase-ground and phase-phase.  You do not have to diconnect any cabling to do this test in this scheme, Just rack-out the c/b and open the isolating switch.  If the cables indicate no fault you then have to concentrate on the relay and the CT circuitry.
Good Luck.

RE: 87 Differential tripping main breaker???

For those not familiar with the SEL-387L, it has no protection settings; it is just line differential in communication with its mate at the other end of the fiber optic link.  CTs are absolutely critical, both must be connected at the same ratio and have the same performance characteristics as there is nothing to tell the relay(s) about the CTs.

My guess is that there is a CT problem and that load was low enough that it didn't trigger a differential, but the added bit of load was enough that now inrush currents cross the differential threshold.  That's why I keep asking for an event report; it will clearly show why the relay trips.

RE: 87 Differential tripping main breaker???

Is it possible to disconnect the (added "small" load) and eliminate that possibility? May save some time and give you the necessary information to replace/recal your relaying.

As Iron sharpens Iron so does a man sharpen the countenance of his friends.

RE: 87 Differential tripping main breaker???

David,Thansks a lot for the explanation.
Could you please add for our education.:
Are possible watch diff. current on-line in thoses relays at the both sides?
Are protection curve include restraint curve?
Best Regards.
Slava

RE: 87 Differential tripping main breaker???

Intresting thread, intresting relay.
I see short manual of relay ( for full instruction manual registration needed). Nothing about commissioning. No data on the screen about diff currents and angles between phase currents on the both side of line.
What I do for stability test, from my point of view, only DR possible use ( if raley have this option)

RE: 87 Differential tripping main breaker???

I think that you can get a snap shot of the currents using the meter command, you can always trigger an event and get the full picture.

If I recall the principle of operation properly:

Each relay of the pair compares its current with the remote (other end of line) current, dividing one by the other.  In an ideal world the result of that division is -1 as one current is in and one current is out.  So, -1 is clearly a point of restraint.  Then they expand that to the range of (if I remember right) -1/6 to -6 to account for ratio errors and minor tapped load.  Then to account for angle errors, asymmetric channel delay, and various other errors, that band from -1/6 to -6 is rotated around the origin to cover the area from about 85° to 275°; forming a large "C".  No other restraints and unlike any "historic" or "conventional" differential relay that I am aware of.  The 311L uses the same principle and allows adjusting the settings, but the 387L is billed as the "Zero-Setting" relay (it has a total of 4 settings - Relay Id, Terminal Id, Transmit Address, and Receive Address, but no protection settings) and you get what you get.

Because of the range from -1/6 to -6 on the ratio of currents, you can have connection errors that won't cause a trip until load gets large enough.  The OP is at the point that his connection error/load combination is just below the trip point, but during inrush it goes above the trip point, and there is next to no delay on the trip.

That's why an event report would provide so much information and allow us to figure out what the connection problem is.

RE: 87 Differential tripping main breaker???

David,thanks a lot again.
Now it's clear.
It's other principle, that I know.
Rockman, you have provide all needed tests again.
I fill, your additional loads it's this additional line on the right side of SLD.
Best Regards.
Slava

RE: 87 Differential tripping main breaker???

(OP)
Thanks for all of your responses.  Here is a better SLD that shows a more complete picture of the relaying scheme.  Unfortunately I do not have the software to connect to the relay to obtain an event report.  It is the utility's relay, so they are coming out first thing this morning to look at the relay.  Hopefully they will be able to address the isssue, however I will try to obtain an event report to post as well.

RE: 87 Differential tripping main breaker???


Hi rockman

Maybe i'm missing something, but there's a 25 kVA Txfo (station service) into the 387L differential protection zone!  The current into this txfo get in the diferential zone but not get out.  Can this be a cause of tripping?

RE: 87 Differential tripping main breaker???

The station service transformer seems to be in the transformer differential zone in the new SLD, so this would not be a cause of the 387L trip.

The new SLD shows a 311L on one end and a 387L on the other end.  I'm not familiar with the 387L, but if as David says, it uses the same principle as the 311L, I suppose that it might be compatible.  I'd certainly get the advice of Schweitzer before assuming this.  This is not likely to be the source of the problem; it would most likely either not work at all or not be a problem.

You can download software from the Schweitzer site to access the event reports.  Or you can use any terminal program and relay commands.  You do need a special cable to connect to the relay.  A standard serial cable or null modem cable doesn't work.
 

RE: 87 Differential tripping main breaker???

No problem with a 311L-387L combination.

RE: 87 Differential tripping main breaker???

According to short manual, it's standard application 381L and 311L

RE: 87 Differential tripping main breaker???

(OP)

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??

RE: 87 Differential tripping main breaker???

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

 
 

RE: 87 Differential tripping main breaker???

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.

RE: 87 Differential tripping main breaker???

2
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.

RE: 87 Differential tripping main breaker???

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

RE: 87 Differential tripping main breaker???

(OP)

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.

RE: 87 Differential tripping main breaker???

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.

RE: 87 Differential tripping main breaker???

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  
 

RE: 87 Differential tripping main breaker???

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.
 

RE: 87 Differential tripping main breaker???

(OP)



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?

RE: 87 Differential tripping main breaker???

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.  

RE: 87 Differential tripping main breaker???

We agree that saturation is not a problem.  

Quote:

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).

RE: 87 Differential tripping main breaker???

(OP)

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?

RE: 87 Differential tripping main breaker???

"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.

RE: 87 Differential tripping main breaker???

(OP)

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.

RE: 87 Differential tripping main breaker???

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

RE: 87 Differential tripping main breaker???

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.

RE: 87 Differential tripping main breaker???

(OP)

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.

RE: 87 Differential tripping main breaker???

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

RE: 87 Differential tripping main breaker???

I don't see the half cycle shift looking at the L currents (local) and the X currents (remote end).  The IA/IB/IC currents in stevenal's image would be the actual measured currents, no time shift, and the L currents would be what is used in the differential calculation, time shifted by half the round trip channel delay.  But it is clearly the case that the CTs at one end, probably the 387L end, are connected backwards.

Not an inrush problem, but a CT connection problem.  You will trip for load with this CT connection.

RE: 87 Differential tripping main breaker???

Also, in SEL's free (registration required) AcSELerator program, there is a fairly complete subset of the 5601 software that will allow anyone to look at the event file as posted.  The L and X currents are perfectly in phase and plot directly on top of each other.  The relay is calculating a perfect operate current, both ends of equal magnitude and phase angle, both into the line.  The only thing that kept it from tripping sooner was that the minimum trip threshold had never been reached before.

This system was put into service without ANY commissioning.  Somebody did not do their job.

RE: 87 Differential tripping main breaker???

(OP)

david beach

I will go out and check the CT connections.  If it is the CT connections why is it tripping with a transient inrush current but not with a steady state current?

I agree that both currents appear to be in phase on top of each other.  Shouldn't the relay trip with them both being in phase and not 180deg out of phase.  I'm assuming they get 180deg out of phase by the arrangment/placement of the CT at the 387 end.  

Are you saying that the IAX, IBX, and ICX currents are the currents from the 387L remote relay, and the time shift is due to a data transmission time between relays?

RE: 87 Differential tripping main breaker???

Yep.  There is a minimum trip setting in the relays, below which it won't trip on differential.  Your load is still below that minimum setting, but the inrush is above it.  Steady state load equal to your inrush current would cause a trip.

Differential schemes work best when currents can be compared at the same instant.  With data being communicated between the relays at both ends of the line there is some channel delay time.  To minimize the errors associated with that delay, the relays at both ends delay their own current data by half the round trip channel delay, so that they are comparing data gathered at the time the remote data was gathered.  If the channel delay is symmetrical this works well; part of the angular spread of the "C" shaped restraint region is to account for channel asymmetry.

RE: 87 Differential tripping main breaker???

Per David's suggestion, here is a replot. Read IAL as local A phase current, and IAX as remote end A phase current (over channel X) and IAT as the summation of the two (or A phase differential current).

Agree with David. Techs and or engineers involved may need some training.

RE: 87 Differential tripping main breaker???

David, Stevanal some Q to you.
Normal situation ( I don't know this relay, is depend on the relay algorit.)
local and remote current in same phase angle or 180deg between them.

RE: 87 Differential tripping main breaker???

Read my post of 23 Jun 08 0:13.  Ideal restraint is same magnitude, 180° phase angle difference.  Both ends should be measuring current into the line.

In this case it appears one end is measuring current in and the other is measuring current out.  No settings available for phase angle compensation.

RE: 87 Differential tripping main breaker???

slavag,

While the SEL alpha plane algorithm may be unique, the connection requirement is not. Differential relays should more properly be called summation relays, since they will trip on a non-zero sum of the two currents. Through current will always show as 180 degrees out of phase and will sum to zero.

RE: 87 Differential tripping main breaker???

Great discussion especially stevenal and davidbeach (lps for you both).

So, it was the CT's after all ?

RE: 87 Differential tripping main breaker???

OK.
David , Stevenal, thanks a lot again.
Yes, it's standard solution : summation.
First suggestion of Edison was right, start to you too.
As usually 99% of this faults, CT's and non stability test by load.
Best Regards.
Slava
Now, Q's, what Rochman have do with utilities

RE: 87 Differential tripping main breaker???

slavag

you are too generous.

Mine was just a drive-by shooting, theirs was a planned assassination (of nailing the right root-cause for this problem).

 

RE: 87 Differential tripping main breaker???

(OP)

Thanks for all of the responses.  I am going to confront the utility with the findings of this thread.

Just so I can explain it to the utility correctly I am going to present them with the following:  The reason that the relay is tripping is because there are a set of CT's that are installed incorrectly most likely at the 387L relay.  Because they are installed correctly, both sets of currents are in phase with each other instead of being 180deg out of phase.  There is a minimum trip setting on the relay, and during steady state we are below this trip setting so the relay does not trip.  During inrush the current is above this setting (500A + 500A =1000A) and is causing the relay to trip.  The CT's will need to be looked at.  Am I explaining this correctly?

I find it hard to believe that the utility technicians and engineers did not notice that the phase currents were in phase.  I agree that they may not have adequate training.  

To answer Slavag's question I am an engineer at a cement plant and the utility services our substation which we own and is on-site.  They are in charge of all the relaying and settings in the substation as well as relay's and settings for the differential relay on our main cables.  This is a new system and we are in the commisioning phases.  

RE: 87 Differential tripping main breaker???

You are correct, except for one typo.

Quote:

Because they are installed correctly,
should be

Quote:

Because they are not installed correctly,

RE: 87 Differential tripping main breaker???

Another small clarification.

The way the CTs are physically installed is fine, it is just the CT secondary wiring that needs to be corrected. Even if it is the primary polarity that doesn't match the drawings, it can be adjusted for on the secondary side. Usually easier to change the drawings than to change the primary polarity.  

RE: 87 Differential tripping main breaker???

(OP)

One last question regarding cable differential relaying in this application.

Should the remote set of CTs be placed on the line side (before) or load side (after) of the main breaker on the switchgear that is being fed?

RE: 87 Differential tripping main breaker???

The CTs should be placed on the load side so that the breaker is in the differential zone.  If there is bus differential on the 4 kV switchgear, it should use CTs on the source side so that the zones overlap and the breaker is in both the line and bus differential zones.
  

RE: 87 Differential tripping main breaker???

(OP)

I was finally able to download the SEL software necessary to view the event files that I had.  I noticed when looking at these files that although the the waveform plot appeared as if the two sets of currents were in phase, the phasor plot showed the two sets of currents (IA & IAX etc..)were 180deg out of phase.  Any thoughts on why this is?

I'm not sure if these are from the same event file as I posted before, however they are still from the same events that was tripping our main.

RE: 87 Differential tripping main breaker???

When looking at IAX from the remote end you need to look at IAL from the local end.  IA is time shifted to match the timing for IAX to produce IAL.  The relay uses IAL and IAX for the differential calculation, not IA and IAX.

Look at the phasor plots and you will see that IAX and IAL are in phase, as are IBL/IBX and ICL/ICX.

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