Inrush Restraint in Differential Protection

What is the voltage level during these conditions?

More than likely the electrical system is suffering a severe voltage drop due to being overloaded by the equipment inrush current.

1. I suggest that you look closely at harmonic filter configuration, inductors, capacitors, and whatever other power factor correction devices present in your system.

2. I suggest that you install low-voltage tripping protection on all equipment & transformers so that all of the equipment/transformers trip off line from the grid when a failure occurs.

3. I also suggest you selectively energize your plant equipment in an orderly sequence so that all of the transformers are not being energized at the same time. This can be accomplished manually by adopting a switching program or by installing a PLC or DCS system with proper programming that prioritizes equipment start sequence position and timing.

Did it energize on Load or without Load?

All the 300 odd TRFs are always energized without load only.

More clarity is required.

Which transformer trips? 5MVA or 300 transformers. The inrush current will be 8 to 10 times full load current. How many differential relays are there and how many tripping ?Are these tripping due to differential relay operation or for some other fault.

Please clarify so that appropriate solution can be suggested.

During mass energization of all the 300 odd TRFs, some or all (14nos) the transformers with differential protection trip on differential protection only. It has been observed (from relay disturbance recordings) that on these instances (mass energization) TRF energization current is not exceeding 0.2 to 0.3 FLC with very low 2nd harmonic component whereas in normal energization TRF energization current is >2X FLC and the 2nd harmonic component is also much higher than 20% of fundamental.

All the 300 odd TRFs are always energized in OFF LOADed condition.

some link is missing.

What will be rating of each of the transformer.

How is that on 5 MVA system you have connected 300 and Odd Transformers. Even if you take 14 transformers (as given by you) into consideration of 1 MVA rating , the total capacity will be 14 MVA.Generally Differential relays are provided for rating 2.5 MVA and above.

Another point is for any transformer the inrush current will not be less than 100%.

I have a feeling most of the information furnished by you are not in order, some where some important information is missing. This is my opinion

Sorry I misunderstood as 5 MVA total plant Capacity. My apologies to you.

Please check the voltage monitoring of these relays. Can you please share the SLDand relay setting details.

1)We have a 132KV GRID incomer connected to 132 KV bus to which 4nos (2X26.5MVA, 2X37.5MVA, 132/11KV) power TRFs connected. To the 11KV system along with these power TRFs 4 nos of TGs (18.5MW each) are also connected.

2) The total plant load is 80MW out of which 45-50 MW is provided by the TGs and the rest by GRID.

3) The total plant load is catered by 300 odd distribution TRFs of various ratings (12.5MVA/5MVA/3.15MVA/2MVA/1.6MVA/1.25MVA etc). The TRF of more than 5MVA capacity (14 nos out of 300 odd) are equipped with DIFF protection.

4) In case of Grid failure the TGs also collapse in some cases before Islanding & load shedding takes place. In such cases the 11KV CBs of distribution TRFs remains ON & on resumption of GRID supply all the TRFs are charged simoultaneously. In such cases we are facing the typical problem.

5) In case the Islanding & load shedding is successful the 11KV CBs of the TRFs (with DIFF protection) trip and on power resumption they are closed one by one. In such cases the TRFs close successfully.

5) I am also aware that TRF magnetizing current can't be below 100% of FLC. The same also occurs in case of charging of individual TRFs ( observed from relay disturbance recordings) . But in case of DIFF tripping (in 14 nos of TRFs) during GRID resumption the relay disturbance records show magetizing current less than FLC and very low 2nd harmonic magnitude due to which harmonic restraint of DIFF protection fails to act.

6) The relays have been tested to be working perfectly ok with OMICRON relay test kit CMC356.

7) Although the DIFF relays installed (P632) are having 2 Phase voltage input (used for overfluxing protection & for voltage display purpose) the is no provision of any voltage based protection in them.

Differential current setting of 20% seems to be slightly less. You can try to set it at 30-35% and try .Similarly slightly increase the setting of trip time. Different manufacturer of relays recommend different settings,ABB recommends min settings of 35%