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Member

Join Date: Nov 2008
Posts: 6

Back Charge of Transformer from Diesel Generator

01/31/2014 2:06 AM

I came across a situation where i need to charge 2 different transformer in series with capacity of 630 kVA(20kV/0.4kV) and 800 kVA (20 kV/0.4kV) from 0.4 kV side by using 400 kVA Diesel Generator set.

Is there any calculation which shall confirm that the the Diesel Generator set is suitable to back charge the transformer or Any factors are required to ensure during ordering of the Diesel Generator.

Please help.

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Power-User

Join Date: Nov 2011
Posts: 286
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#1

Re: Back charge of transformer from Diesel Generator

01/31/2014 2:34 AM

If your sole source of supply to these transformers is the 400kVA gen set then regardless of the size of your transformers the maximum you will be able to get out of 630kVA and 800kVA is 400kVA. You are likely to have all sorts if interesting issues with in rush when you turn this lot on.

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Anonymous Poster #1
#2

Re: Back charge of transformer from Diesel Generator

01/31/2014 5:21 AM

Strange; I was just reading about this very same question only a minute ago....

The transformers must surely be in parallel rather than series...?

I don't know if this would work, but how about emulating a soft starter? Begin with the voltage regulator adjusted as low as possible and ramp up over several seconds.

Self-marked OT because it's a WAG.

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Guru
Engineering Fields - Power Engineering - New Member

Join Date: May 2007
Location: NYC metropolitan area.
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#3
In reply to #2

Re: Back charge of transformer from Diesel Generator

01/31/2014 8:59 AM

AP is correct, this may be the only way to minimize the inrush current, but it does take some measure of knowledge about the overall system and protective relaying (if any) to do properly. There are multiple previous threads on CR4 that address this very question.

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Member

Join Date: Nov 2008
Posts: 6
#4
In reply to #3

Re: Back charge of transformer from Diesel Generator

01/31/2014 10:10 PM

The requirment is related to the hydro project. Where to start generation unit axillary load to be fed from diesel generators if the grid power is not availbe. Now in this case the unit auxiliary load is approximately of 250 kw which can be easily be ytaken care by DG. My questions was releted to the dealing of inrush current which DG has to supply to charge the transformer. Is there any mean by which same can be ensured.

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Power-User

Join Date: Jul 2013
Location: Faridabad Near New Delhi India
Posts: 240
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#5
In reply to #4

Re: Back charge of transformer from Diesel Generator

02/01/2014 12:07 AM

Mr. Pathak thumb rule is that Rating of transformer should not exceed rating of 3 phase alternator coupled to Diesel Engine. Reasons, effect, alternatives, solutions have been earlier discussed on this forum twice.

Obviously your alternator rating is less than that of Transformer. Safer way in Emergency is to

1.First close breaker between DG set and transformers to be charged with DG at standstill.

2. Start Diesel Gen set. This way inrush shall be minimum as the voltage to transformer shall be slowly applied with voltage build up of DG Set.

Alternatively you can install a switch in supply to exciter field of DG Set. Line up system as in 1 above, Start DG Set, Set AVR voltage set point to minimum using its potentiometer for this on AVR, switch on in exciter field and same results as above shall be achieved. Normalize AVR set point.

Pl note you may have to bypass some protections (under voltage etc.). Once the transformers are charged, these shall be normalized.

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