Transformer Differential Settings

You need to provide more information for us to be able to make an informed guess. Has this transformer and its protection been in operation for a long period of time (years?), or is it a new installation? Is the differential relay new or has it been in operation for some time? When you closed the downstream breaker (I assume by "reset" you mean close), was that the only load on the transformer, or were there other feeders off the 11kV? How much of the total 11kV load was this added load?

Current differential relays are generally designed to operate when what comes in doesn't match what goes out. Even though the diff protection tripped your transformer breakers, I wouldn't expect the feeder breaker to trip, since that probably is protected with only overcurrent relays.

If the reclosed breaker was the only load (or a major percentage of the load) on the transformer, I would suspect that you have the harmonic restraint improperly set. That function is supposed to distinguish between the inrush current on a suddenly-loaded transformer from a high fault current, based on harmonic content (usually 2nd, and often also 4th). Another possibility is that the slope of your restraint function is improperly set.

The position of the OLTC could possibly cause the trip if the range of its ratio change wasn't properly taken into account in the relay settings. What is a good setting (including CT ratios) at one end of the OLTC range could be marginal at the other end. What position was the OLTC in? Usually it is good to have it in Neutral or as close as possible when such switching is done.

Those are some of the main considerations. You have to do some detective work to determine tha actual cause of your trip. You probably should also have an experienced professional look at your relay settings to see if you need any adjustments.

Many thanks for the reply. Its quite useful. We commissioned our substation on 6th June 2011. We are using Heag Hyp600 programmable relays on both 33kV & 11kV panels.

tx rating=10MVA 33/11kV, X=2.87pu, Ct ratio=400/1A, Peak load=109.7A(Lv side), transformer tap position = 5(nominal tap)

The following are the current differential settings on our 33kV transformer protection relay.

E87R E87U

1. E2HB=1 1. E87U=1

2. E5HB=1 2. U87P=0.600 Ie

3. E87R=1

4. ECTB=0

5. PCT2=15%

6. PCT5=15%

7. O87P=0.500 Ie

8. IRS1=0.200 Ie

9. SLP1=0.500

10. SLP2=1.00

After the first tripping, the display was showing; Id3=0.700 Ie. The second tripping after two weeks it showed;

Id3=0.510 Ie and Ir3=0.260 Ie

We have only one loaded 11kV outgoing feeder with 18 transformers on this circuit. There is an 11kV circuit breaker after our substation (downstream). 14 of these transformers are rated at 500kVA and the rest are 200kVA. Both trippings occured as a result of closing the downstream circuit breaker.

Ilukenak,

Your guess could be right. Bias setting may be a problem. In many cases, we also see problem with zero sequence filter settings (or vector group matching settings). OLTC range is a factor considered while deciding the setting for minimum pickup of the relay. This is rarely a problem unless your trafo is made to operate at a tap that is far away from the nominal tap.

Tell us more so that the discussion can be more fruitful (like relay type, trafo name plate details, relay settings, type of fault and current magnitude etc.).