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Transformer Inrush Tripping

04/13/2017 3:40 AM

Dear all,

There is 2nos. 11kV, 2750kVA generator connected to a sync panel with 2 incomers and 6nos. outgoing VCBs, 1600kVA, 11kV/415V transformers. The generator incomers in the switchgear is provided with the siemens 7UM62 protection relay. While switching the transformer by the outgoing breaker, the protection relay of the incomer trips on over current and differential protection. We hope the reason is due to the inrush current drawn by the transformer for a short time. Please advise

1) how to prevent the tripping, the relay supplier is also investigating I hope there might be a provision of inrush blocking.

2) The generator is a back up for a hospital and supply must be restored withing 10seconds when there is power outage. So we are afraid that if we connect all 3 transformers which is already connected to the loads in 10seconds it may trip the generator also.

3) Presently there is not much load is available since the project is not yet commissioned, so we need to consider the possibility of the switching of the transformers which will be in fully loaded condition also. because nobody will switch off any loads in a power outage, will wait for the supply restoration. hence it will lead to the full loading of transformer in zero time which is inevitable

Please share your valuable knowledge in this subject, Thanks for spending your time and help.

Thanks

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#1

Re: Transformer inrush tripping

04/13/2017 5:07 AM

2 x 2750 = 5500 << 9600 = 6 x 1600. See the problem?

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#2
In reply to #1

Re: Transformer inrush tripping

04/13/2017 5:32 AM

On that basis, the overcurrent protection arrangement is doing its job well.

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#3
In reply to #1

Re: Transformer inrush tripping

04/13/2017 7:01 AM

generator power is (2750x.8) x 2 = 4400kW and its prime rated

transformer design is 1600x 0.8 x 0.8 = 1024kw

4transformers only connected to load 2nos. are spare only provided as per contract requiremet for future purpose. effectively generator design is only for 4nos. transformers. the tripping happened since there was no any load

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#4
In reply to #3

Re: Transformer inrush tripping

04/13/2017 7:08 AM

So, go back to the Designer and discuss it there. No-one else on CR4 can see the design documents, after all.

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#5

Re: Transformer Inrush Tripping

04/13/2017 11:00 PM

Instead of switching on all Transformer feeders at a time, switch on every transformer sequentially with certain time delay.

The inrush current will be 10 times for 5 cycles.

If all the 6 transformers switched on simultaneously, the Generator will not be in a position to feed.

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#6
In reply to #5

Re: Transformer Inrush Tripping

04/14/2017 2:44 AM

Thank you sir,

That means the inrush last for 5/50 times of 1seconds? but we noticed it last for around 3 to 4 seconds. even if it lasts for 1seconds we dont have any problem because with an interval of 1seconds we can switch on all 4 transformers within 3seconds. our target is to switch on all 6 transformers in 10seconds.

thanks

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#7
In reply to #6

Re: Transformer Inrush Tripping

04/14/2017 2:54 AM

Huh? Earlier you said only four, with two spares. Even then, your generators will be lucky to support even just four of these transformers.

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#8
In reply to #6

Re: Transformer Inrush Tripping

04/14/2017 4:18 AM

The full load current of a Transformer is 83 Amps.

Inrush current of a Transformer will be 830 Amps for 100 milli sec

If you switch on all the transformers at a time the current drawn by the system would be 4980 Amps.

The full load current of the Generators 291.5 Amps.

Any Generator can feed momentary overload of 200% only.

Therefore switch on Transformers with time delay of 1 sec.

Within 7 sec your complete system will work without any problem.

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#9
In reply to #8

Re: Transformer Inrush Tripping

04/14/2017 6:20 AM

Yes , I totally agree with Mr. Ramvinod comments, for more safety a timer (through a small PLC or built in timer) in series to ensure the safe switching .

regards

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#10
In reply to #9

Re: Transformer Inrush Tripping

04/14/2017 8:50 AM

I agree with Ramvinod.

A project of this nature ought to have had a sequential restore, most vital load to least, designed in at the beginning. You will have to decide priority of loads with help of users.

Unfortunately, the distribution of loads between transformers and circuits has to be thought about and optimised for "boot-up" switching before build or fix may take a lot of time and money.

You may find that some of the loads have air-conditioning (not immediately vital and heavy start) or similar thrown on with everything else. You might have to resort to no-volt cutoff relays, with manual or time-delayed restore on some LV load circuits, if no-one thought about re-boot at design stage.

You do not want to get early loads restored, then get the system "blacked-out" by a later load connection.

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#12
In reply to #8

Re: Transformer Inrush Tripping

04/14/2017 11:41 AM

Thank you sir, its really good solution.

Thanks to all

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Power-User

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#13
In reply to #5

Re: Transformer Inrush Tripping

04/14/2017 12:05 PM

The magnetizing current can be even higher than typical when energizing a transformer designed for step-down service using the secondary winding, or a step up design transformer, energized from the primary side.

I have always set the relays assuming 12x full load current, 8-12x is a typical range.

http://apps.geindustrial.com/publibrary/checkout/Transformer2?TNR=White%20Papers|Transformer2|generic

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#11

Re: Transformer Inrush Tripping

04/14/2017 8:52 AM

Ramvinod is right. It also would be helpful to know the settings of the Siemens protective relay, it has 5 or 10 settings.

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#14

Re: Transformer Inrush Tripping

04/19/2017 10:40 PM

Dear all,

Thanks for your support, we solved the issue by providing an interval of 1second between each swtching as engr. Ramvinod said. We faced another problem also during the simultaneous switching that there will be huge voltage dip which will cause an undervoltage trip also so an interval is mandatory. thanks

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