Generator Differential Trip

[emviana]

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.

 

[slavag]

Hi.
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

 

[davidbeach]

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.

 

[alehman]

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.

 

[emviana]

Thanks a lot for the ansers. Now I am uploading graphical oscilography of phase currents, differential currents ans restriction currents.
Only by information, relay is a SIEMENS 7UM62.

 

[emviana]

Now, the differential currents.

 

[alehman]

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.

 

[davidbeach]

I concur with Alan's assessment.

 

[rbulsara]

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

 

[rbulsara]

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

 

[davidbeach]

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.

 

[rbulsara]

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

 

[RRaghunath]

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.

 

[alehman]

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.

 

[davidbeach]

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.

 

[RRaghunath]

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.

 

[electricpete]

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

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[alehman]

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.

 

[RRaghunath]

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.