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.
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
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
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
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RE: Generator Differential Trip
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
Muthu
www.edison.co.in
RE: Generator Differential Trip
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
S1 = side of generator's terminal.
S2 = side of generator's grounding.
The CT's are of equal specefication on two sides (4000-5A).
Thanks.
RE: Generator Differential Trip
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RE: Generator Differential Trip
Slava - reversed polarity ?
Muthu
www.edison.co.in
RE: Generator Differential Trip
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 ?
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RE: Generator Differential Trip
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
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RE: Generator Differential Trip
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RE: Generator Differential Trip
RE: Generator Differential Trip
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.
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RE: Generator Differential Trip
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RE: Generator Differential Trip
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
RE: Generator Differential Trip
RE: Generator Differential Trip
Not seems good.
Yep, it's generator differential protective relay, usually not needed 2-nd harmonic restraint ( OK , I think need, but ..
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
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
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
Only by information, relay is a SIEMENS 7UM62.
RE: Generator Differential Trip
RE: Generator Differential Trip
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: Generator Differential Trip
RE: Generator Differential Trip
Waveform signature will tell you if it is a transformer inrush or a fault.
Rafiq Bulsara
http://www.srengineersct.com
RE: Generator Differential Trip
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
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 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://
Also see page 3 of h
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
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
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
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
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
http://
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RE: Generator Differential Trip
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: Generator Differential Trip
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.