×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Generator Differential Trip

Generator Differential Trip

Generator Differential Trip

(OP)
Hello. I need some help.

In my area we have a 13.8kV, 74.3 MVA generator (wye connection) connected to a 80 MVA 13.8/69 kV (delta/ground-wye) transformer. The 69 kV side of this transformer is connected to a bus fed by a 100MVA 230/69 kV (grounded wye/grounded wye). The 230 kV side of this transformer is connected into a bus that feed other four 230/69 kV transformers. The industry is fed by this 230 kV bus.

The problem is: when a 100MVA is energized on 230 kV bus, the generator is tripped by the differential relay.

Does anyone have an idea? Thanks a lot.

RE: Generator Differential Trip

Yes, any one or more of the following:

Improper settings - (Most likely culprit- check for harmonic restraint function and/or sufficient delay to allow transformer transients to die down.)
Improper wiring
Faulty relay, CT
Actual fault

Under what condition the 230 kV bus is energized? Dead bus and transformers or with synchronization on? What is in the protected zone? Gen alone?, gen and one transformer?

You may want to minimize transformer inrush if possible by staggered energization of the transformers.

At the end of the day, you need an experienced professional at the site, working with the project team.

(PS:  I am not an expert on 230 kV systems, but just thoughts based on experiences on medium voltage systems)
 

Rafiq Bulsara
http://www.srengineersct.com

RE: Generator Differential Trip

(OP)
We have two differential relays:

1) One protecting only generator (this one trips on energization of 100 MVA tranformer);

2) One protecting generator + 80 MVA transformer (this one doesn't trip).

230 kV bus is always energized. The problem occurs when we energize one 100 MVA transformer at no load.

I´ve studied relays parametrization. Until now, I didn't find any discrepance.

Thanks.

RE: Generator Differential Trip

If the CT's approach saturation during the transient, then small deviations of magnetic and burden parameters can cause bigger differences on CT output.      

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Generator Differential Trip

This is very akin to the problem I had with smaller size set up. See thread thread238-125456: Traces of 2MW transformer inrush

Along with what electricpete said and the lack of harmonic restraint are the prime suspect. In our case we implemented (GE has a application bulletin on it as well) dual settings. The one with longer delay was used during energization and then quickly reverted to the proper settings with a reasonable delay.

You may want to review the traces of waveforms if available and consult the relay mfr.

I would also expect folks more experienced than I, to chime in. Letting them know the relay mfr and model may also help.

I am wondering if the starting the gen & transformer as one unit would help as it will energizing the transformer on reduced voltage as the gen voltage builds up.



 

Rafiq Bulsara
http://www.srengineersct.com

RE: Generator Differential Trip

Have you checked for proper CT polarities ?

Muthu
www.edison.co.in

RE: Generator Differential Trip

Hi.
Usually you have some level of 2-nd harmonics for avoid maloperation of differential relay in energazing mode.
Posible..
1. level of second harmonic in your setting is high >15%.
2. different saturation curve of CT on the geneerator side and on the 230kV transformer side.
3. low setting of differntial protection base current (<25-30%).

Please attach SLD with CT disposition and CT data ( include wiring size of CT connection and lenght), please attach a setting of differential protection ( type of differential relay will be help too).

What is a log data of fault, disturbance recorder data?

Best Regards.
Slava
 

RE: Generator Differential Trip

It looks like phases close to 180 apart?  Seems to point toward connection / polarity problem.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Generator Differential Trip

The phase angles seem screwy.

Slava - reversed polarity ?

Muthu
www.edison.co.in

RE: Generator Differential Trip

"phases close to 180 apart"
I meant close to 180 difference between the two locations for each phase.
(180 difference for A, 211 difference for B, 157 for C)

Was this data recorded by the relay itself or some other device fed from same CT's ?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Generator Differential Trip

No, phase angles seem just about right.

Phase A - 1370A of restraint current and 9.6A of operate current.
Phase B - 1090A of restraint current and 787.6A of operate current.
Phase C - 1215A of restraint current and 504.4A of operate current.

Where restraint current = (|It|+|In|)/2 and operate current = |It+In|

Probably was the B-phase differential that tripped.

RE: Generator Differential Trip

I guess there is a polarity convention on how the device reports polarity of the current.  I can see the logic you're assuming currents which are same polarity in the system are reported as opposite polarity by the device, since we would connect so that normal same-polarity currents create opposing flux in an electromechanical relay.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Generator Differential Trip

Also, if there was reversal one would assume the differential would trip more than just during energization of transformer... (provided loading level is high enough)

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Generator Differential Trip

Differential is always the sum of the currents into (or occasionally out of) the protected zone.  Kirchhoff says that sum has to be zero and it doesn't matter how many inputs (restraints) you have.  If you want to try to set up a differential that subtracts one current from another to get to zero, that's not too difficult for two inputs, but gets complicated fast as the number of inputs goes up.  Add all the currents and get to zero works for any number.

RE: Generator Differential Trip

I rather thought that polarity convention and Kirchoff's law were two different things.  But thanks for explaining Kirchoff's law.

As far as I remember, 3-phase differential relays are usually three single phase element, unless there is some kind of pilot wire scheme. I might be wrong... it's been awhile.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Generator Differential Trip

Sorry if my comments was in any way sarcastic.  David you were after all only replying to my comment /question in a logical manner (and I hope you will continue to do so anytime I say something that sounds like an uncertainty or a question).  I will shut up now since as I said my memory of differentials is faded and David and others can contribute much more.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Generator Differential Trip

Good analysis, David.  I'm guessing that Phase C tripped and that the relay is set up to inhibit only individual phases when 2nd harmonic exceeds the setting.

Phase C had 41%  operate current (100% x 504/1215) but only 13.3% 2nd harmonic if I read the data correctly.  Phase B had 35.1% 2nd harmonic and was probably inhibited if the setting was the typical 20%.

The solution is to change programming to inhibit all phases if any one phase sees 2nd harmonic or to drop the inhibit level to 10-12%.

 

RE: Generator Differential Trip

Since it was the generator only differential it might not have the harmonic restraint/blocking typical of differential relays intended to include transformers in the zone of protection.  Probably need emviana to tells us what kind (make and model) of relays he has in use, and which one tripped.

RE: Generator Differential Trip

Can't get a clear picture with just limited data. David is correct. OP should provide more info: relay specs, specific element tripping, and probably more.

RE: Generator Differential Trip

Hi.
Not seems good.
Yep, it's generator differential protective relay, usually not needed 2-nd harmonic restraint ( OK , I think need, but ..smile ).

Im not like this current recorder. Please pay attention, 1.7kA with 4kA size of CT.
I hade this problem before few years with 87G of big synch. motors. NOT PROPER CHOICE of CT and CT wiring size.

Could you please add data in additional to all asks, about CT data and wiring data.
From my point of view, you need increase setting of protection.

Best Regards.
Slava

RE: Generator Differential Trip

Hang on guys, this was a running generator, no transformer in the zone that tripped.  (At least from what emivana has said to date.)  If it wasn't a running generator just consider it an highly advantageous inadvertent energization trip and move on. Given that:

1. Transformer inrush outside the generator zone would have produced identical currents at the terminal and neutral ends of the generator.

2. The currents provided are not identical (and opposite) at both ends.

3. No load condition or transient can cause the generator currents shown.

4. Differential faults do not have to exceed the full load current of the generator, one of the very nice features of a differential relay - immunity to load.

5. Therefore, in absence of clarifying information, I'm going to say it was a legitimate Phase B differential trip, possibly also a Phase C trip.

6. Trip on A would require a slope of 0.7% or less and an excessively low minimum operate current.

7. Trip on B would require a slope of 72.3% or less, way above any two winding differential I'd set.

8. Trip on C would require a slope of 41.5% or less.  I'd probably be somewhere between 30% and 40%, but don't know enough about this installation to know.  To need to be above 40% would be an indication of a sub-optimal installation.

RE: Generator Differential Trip

emviana,
Are the burdens in the CT circuits on the neutral side and hot side the same?

Can you tell us the relay settings?

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: Generator Differential Trip

Looks like an internal fault involving phase B and subsequently, phase C.

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: Generator Differential Trip

I concur with Alan's assessment.

RE: Generator Differential Trip

Not that fast...are there waveform captures?

Waveform signature will tell you if it is a transformer inrush or a fault.


 

Rafiq Bulsara
http://www.srengineersct.com

RE: Generator Differential Trip

More importantly, is this setup new or existing?

If it happens every time the transformer is energized, it may be an internal fault. If intermittent, then probably the transformer inrush.

Rafiq Bulsara
http://www.srengineersct.com

RE: Generator Differential Trip

Quote (emviana 18 Dec 09 19:23):


We have two differential relays:

1) One protecting only generator (this one trips on energization of 100 MVA tranformer);

2) One protecting generator + 80 MVA transformer (this one doesn't trip).

I fail to see how a transformer inrush can trip a generator only differential unless the CTs at the two ends of the generator are so mismatched as to be worthless in a differential application.  Inrush to a transformer out of zone would have to show up as the same current in the CTs at both ends of the generator, can't have transformer inrush at the terminal end and not at the neutral end.

RE: Generator Differential Trip

A few more thoughts:

A fault in the generator should trip the gen diff even without energizing the transformer!

If it only happens when the transformer breaker is closed, it could not be the fault internal to the gen. (or much less likely).

It should not be very difficult to read any current in the gen diff CTs, before 230kv xfmr is energized.


Relays without harmonic restraint can trip on transformer energization or if the CTs saturate unequally for some reason, at high inrush say more than 10 pu.  GE has a bulletin, http://pm.geindustrial.com/faq/documents/489/get-8403.pdf , to address this issue on their SR-489 gen relays, which I had the first hand experience of. In this case the transformer is even larger than the gen rating so it is not impossible to see the xfmr inrush to exceed gen's 10 pu. The GE bulletin also shows typical signature of transformer inrush current waveforms.

Also see page 3 of http://search.woodward.com/PDF/IC/DOK-TD-XD1-GE.pdf

Consulting Siemens on this relay would shed some light too, if it is determined that it is not the gen's internal fault.

Rafiq Bulsara
http://www.srengineersct.com

RE: Generator Differential Trip

Many a time, the generator differential protection (87G)will not have 2nd harmonic restraint feature. Relay type DGP of GE-Multilin for example. In such a case, there is no option but to delay the protection operation.
Using the CB auxiliary contact and to trigger alternate settings as suggested by 'rbulsara' is ofcourse a better option that can be considered on case-to-case basis.
Bulletin from GE on relay type DGP (attached) shows that in revision F2, the relay processing delay has been increased from 6cycles to 12cycles by GE to provide stability for 87G.

RE: Generator Differential Trip

Let's back up. Was this a one-time event or something that happens repeatedly?

The OP still has not provided the relay settings.

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: Generator Differential Trip

Why the concern about 2nd harmonic?  Because the transformer being energized is not in the differential zone, there is no difference in what is seen at the terminals from what is seen at the neutral.  Unless the relay does things differently at the two ends, certainly not a good thing to do in a differential, it doesn't matter what the relay does or doesn't do with/about 2nd harmonics.

For a differential that includes both the generator and the transformer, then absolutely you need to be concerned about what the relay does with the harmonics associated with transformer inrush, but not for a differential that includes no transformer.

RE: Generator Differential Trip

davidbeach,
Exactly, we should n't be concerned with second harmonics when the transformer is not within the differential protection zone. I too think so.
The attached paper analyses similar maltripping of generators and recommends including 2nd harmonic restraint even for generator protection.
It seems the protection maloperation by itself is due to DC component in the currents and associated unequal CT saturation, the recommendation in the paper is to achieve protection stability by sensing 2nd harmonics and setting the relay to stabilise.  

RE: Generator Differential Trip

I haven't followed the thread or the last attachment. But here is a document that shows concern for generator differential trip upon energization of transformer outside of the zone. The reason they cite is related to saturation of the CT's
http://pm.geindustrial.com/faq/documents/489/get-8403.pdf

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Generator Differential Trip

The SR489 doesn't have a 2nd harmonic restraint, so their solution was to add a time delay, which I don't like. Some GPR's do include 2nd harmonic restraint, primarily for use with unit transformers in the zone.
 

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: Generator Differential Trip

alehman,

I was just now looking in to P34x of AREVA and 7UM of Siemens. Both are generator protection relays.
7UM has 2nd harmonic restraint element associated with Differentiantial function. P34x doesn't.
Just to share.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources