Transformer Inrush on Offshore Generators

[KenAlmon]

My question relates to the pick up of a transformer. The transformer would be an unloaded, un-energized dry-type transformer with the following characteristics, (as supplied to me by the TX vendor):

- 3750/5000 kVA
- 4.16 kV - 600 V
- 7.5% impedance (on base rating) (0.71 + j7.47%)
- quoted as 1.71% inrush

I did some research to see whether our generators would have any sort of a problem picking up this transformer without any resultant problems and came across the following paper: Analysis of transformer inrush transients in offshore electrical systems
- Smith, K.S.; Ran, L.; Leyman, B.; Dept. of Comput. & Electr. Eng., Heriot-Watt Univ., Edinburgh, UK
This paper appears in: Generation, Transmission and Distribution, IEE Proceedings-On page(s): 89 - 95

The fact that there appears to be a paper written about it leads me to believe it is important to look at.

Although my first impressions is that this should not be a problem, I have a hard time trying to find a method to prove it. What I am looking for is approximate length of time that the voltage will drop and the depth of the drop. Any other complicating issues that should be addressed would also be good.

TIA

Ken

 

[busbar]

For lack of any more detailed information, one rule-of-thumb for power-transformer inrush is 25x for 0.01 second, and 12x for 0.1 second.

 

[jbartos]

Suggestion: Please, would you provide:
1. Length of a run between the transformer and generator.
2. Generator size and parameters
3. Static transfer switches data, if any
4. Anything else which may be considered special or irregular from the normal industry standard power distribution.

 

[KenAlmon]

jbartos:

Gen and TX distance expected to be about 50 m.
Gen is 5.9 MVA, 0.85, X''d = 0.2
No STS, but there are MV drives (non-regen), one size about 2000 hp and the other 500 hp.
We have two 3750 kVA transformers in a double-ended sub with secondary tie open. I suppose that there could be a scenario where we are operating with one transformer and the plant operational (each TX is 100% rated). To pick up the TX under these circumstances, with about 2000 hp of running induction motors, and 2500 hp of MV drive online, the pickup of the TX could result in a pretty good drop. Albeit for how long, and how much I am not sure.

Appreciate any comments.

 

[Modula2]

Forget about the rules of thumbs, etc. Same for "software for the masses". This requires Captain EMTP to the rescue. In short, the inrush can be massive, exceeding expectation, full of harmonics, and last a longer time than thought. There are no typicals. As a min, the magnetizing curve is needed because that is time domain and the inrush is exciting current and a time domain problem.

 

[bigamp]

Modula2, an excellent answer. What is Captain EMTP?

In the situation of generator energising transformer, where the generator is not much bigger than the transformer, the generator vendor must be consulted. He should be able to provide the answer. We had a situation a couple of years ago with a 50MVA 11kV generator possibly having to energise a 25MVA 11kV/33kV grid connection transformer. The generator manufacturer was not happy for the generator to energise the transformer so we did not attempt to do so.

Regards

 

[KenAlmon]

bigamp

Thanks for your post.

Did the generator manufacturer advise you of any work arounds? What about cold pick up of the TX where no other loads are involved i.e. no concern about depth of voltage drop nor length of time of drop? Did the Gen mfgr hint that it was actually bad for the generator manufacturer or was it strictly a performance issue?

Thanks

Ken

 

[bigamp]

Ken,

Sorry but I never took the subject further with the generator vendor so am unable to provide much more specific information.

For that plant, the 25MVA transformers (there were two of them operating in parallel) would always be energised from the grid.

We obtained inrush current data from the transformer manufacturer which we forwarded to the generator manufacturer. We asked if the generator could be used to energise such transformers (one at a time) and they said no. From memory they did not give reasons. Since we did not really need to be able to energise the transformers from the generator we did not ask further questions so I can not say what their actual concerns were. I suspect we would have had a bit of a voltage dip on the LV side (11kV) but I do not thing energising the transformer would have hurt the generator.

Regards