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Generator Feeding Loads Via Transformer Backfeed

Generator Feeding Loads Via Transformer Backfeed

Generator Feeding Loads Via Transformer Backfeed

(OP)
Hi all,

Please find attached system arrangement for reference. During normal conditions, our plant COGEN will be in parallel with the utility line.

Now if I loss my utility line due to fault, the assocoiated utility breaker opens and isolate our plant from the utility system. With the plant COGEN running at the time of isolation, is it an acceptable practice to backfeed the transformer T2 and power will flow from the transformer T2 primary into the transformer T1 primary to feed the transformer T1 loads?

Are there any limitations or repercussions to this operating condition?

RE: Generator Feeding Loads Via Transformer Backfeed

There isn't a simple answer. In addition to grounding, there would be major implications for the protection schemes of the various items of plant and also the relay settings as the fault level will change dramatically, and you are changing the direction of the fault infeed.
Regards
Marmite

RE: Generator Feeding Loads Via Transformer Backfeed

(OP)
In addition, T1 and T2 have automatic on-load tap changers on their respective primary windings. If I backfeed through the secondaries from my generator, is this going to create havoc on voltage stability when I get load rejections on the T1 and T2 loads feeders?

Thank you Marmite for your initial comments. Can you elaborate further or perhaps site an example as to why there might be some protection implications on the other parts of the system? Is this because the fault levels now changes (especially to downstream of T1) due to the series impedances of T1 and T2? Is there any implication of the direction fault in-feed if I am not using any directional power or current relays?

I would appreciate any guidance or links where I can get more information and cautions as I do not support this idea (considering that T1 and t2 is not designed for backfeed operation).

RE: Generator Feeding Loads Via Transformer Backfeed

Depending on the protection you have, different settings may apply. You have correctly identified that fault levels will (likely) change. If you have overcurrent relays as most of your distribution protection, I'd expect different settings to apply. There are many workable solutions for this, especially if you are in the design stage. I would recommend relays (IED's) where you can change the settings to accommodate the change in your system. This could apply to overcurrent pick up settings or could also be an entirely new protection scheme logic. The change in settings can be initiated by the utility breaker opening (52b contact). You should also be able to control the tap changers in a similar manner (52U trips - return to neutral - hold taps - whatever you want).

This could be a good application for a communications based scheme which handles these changes. I have worked on systems like this that utilized Schweitzer (SEL) Mirrored Bits at a university having a similar paralleling scheme with a utility. The mirrored bits scheme was simple to implement, easy to understand (and train the plant operators) and straightforward. You could just as easily implement a transfer scheme back to the utility feed, when stable voltage is available. I believe all relay OEM's have the ability to implement this. IEC61850 would be another communication protocol to achieve this.

RE: Generator Feeding Loads Via Transformer Backfeed

(OP)
I agree with you smallgreek. We are looking into the fault calculations at the moment.

The overall system design is conceived to have future power export option to utility. Hence, the transformer should have been manufactured to have such backfeed capability. However, we have sent a query to the transformer manufacturer to verify the suitability of this operation to their transformers.

My concern is more of the overall system with the kind of operation shown in my attachment hoping some folks who did the same approach can give their insights.

RE: Generator Feeding Loads Via Transformer Backfeed

ThePunisher,
I had observed something unusal in your SLD.In my opinion whenever there is a fault in the Utility, the 72 kV side bkr of GSU should be tripped and de-couple the generator from the Utility.Say if there is a ground fault on one of the 72 kV feeders going out from trafo T1,since all your trafos are WYEGROUNDED/WYEGROUNDED,the generator continues to feed to the ground fault.Is it allowed? Any thoughts?

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