Reverse energising the stepdown transformer for Step up purpose ???? 1

[shersan]

Is it possible to use a stepdown transformer (11/.4kV ) ,1MVA transformer can be used for step up transformer from .4 to 11kV. If yes ,What are the precaution to be consider such as changing the CT polarity of the primary side etc ?????

 

[RRaghunath]

The bigger issue, I think, is lack of earth reference when you feed 11kV system through 0.4/11kV transformer (11kV is delta connected, as is the normal practice with 11/0.4kV distribution transformers).

 

[HiSet]

Shresan,
Tecnically speaking there is no issue in back charging a transformer except that it's inrush current will be now be quite high in term of amperage as the inrush current could be in order of upto 6 times of the full load current.
HiSet

 

[RalphChristie]

I agree with the comment of rraghunath. The transformer will most probably have a delta primary.

Regarding the inrush issue:
Inrush currents can be quite high if energized from any side, (primary side or secondary side) but the inrush multiplier tends to be higher if the transformer is energized from the secondary side. See also the comments of cuky2000 in thread238-101496
Depending on the kind of protection you use - especially if you have elements that operate in a few cycles - you might find a trip. What kind of protective devices do you use on the transformer?

Regarding the CT-issue:
Nothing I am aware of, do not see any problem.

 

[habor]

You can use your transformer as a step up transformer. I assume that low voltage side of your transformer is star and high voltage side is delta connected. Than rraghunath's comment should be considered. Depending on your design criterias, earth reference may be required on the 11 kV side. You may use an additional zigzag connected earthing transformer on the 11 kV bus.

 

[shersan]

Thanks rrghunath ,

What kind of problem would encounter by not having earth reference on the 11kV Side .I have only O/C protection and Eearth fualt indicator on the Ring IN / Ring Out feeder ofthe RMU which is feeding the 11/.4 kV transformer. Any kind ofadditional protection we should consider??? Relay polarity would n't be an issue as the RMU is having self powered MICOM P124 O/C relay ???

High inrush current would be an issue ???

 

[habor]

You will probably not face with inrush current. How will you make the synchronization with the grid? If from LV side, than transformer will be already live with grid voltage from HV side.

If you will make the synchronization from the HV side, than you may start the generator when the genset breaker is already closed to the transformer.

 

[RRaghunath]

Shersan,

With no earth reference, the system is prone to over voltages and resultant damage and also it will be a safety hazard considering that the 11kV OHLs run in the city (as earth faults in the system go undetected). I think this will not be permitted by the statutory authorities in your area.

If the 11kV network is a private one, then you may go ahead with the plan but you need to introduce residual voltage sensing type earth fault protection on 11kV side.

High inrush currents when the transformer is energised from low voltage side can cause unacceptably high voltage dips in the low voltage system. This, however depends on the sources available (or how strong is the system) in low voltage system.

If the 11kV system is a ring network (as mentioned) and has some more sources in addition to the subject one, you will have to consider synchronisation requirements as suggested by habor.

 

[Colinjohnson]

Hi,

I read about this very application at an airfield where 415V at a high kVA was required about a mile from the main substation. The designers installed two distribution transformers back-to-back and thus avoided all the volt drop problems associated with LV distribution.
A neat solution.

 

[wfowfo]

Surge protection should be considered since an arrester operation on one phase will apply phase to phase voltages across the other arresters.