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Inrush current Loading

Inrush current Loading

Inrush current Loading

(OP)
In case  a 20 MVA 33/220 KV  transformer is used for back charging a 188 MVA 220/33 KV transformer, how the inrush current of 188 MVA will affect 20 MVA unit? Rated  current of 20 MVA is 52 A at 220 KV while inrush current demand of 188 MVA will be 2500 A at 220 KV.Will the voltage dip and cause  tripping of relay?

RE: Inrush current Loading

A good engineer can complete an analysis of the conditions given the impedances involved.

RE: Inrush current Loading

It's quite possible, depending on the type of relay and the relay settings.

Unless this is the normal configuration, the low side relays will not be set to ride through transformer inrush and the back-to-back transformers will likely extend the inrush period.  CT saturation might also be a concern if the inrush period is long.  It  might depend how lucky you are on the voltage phase angle at closing.

If this is a one-off situation, you might need some temporary relay settings applied or some elements turned off.  I'd expect a lot of harmonic currents that could create some issues with the differential relays as well.   

RE: Inrush current Loading

(OP)
dpc,This is only a  one off situation as the client wants to test charge the 188 MVA  from HV side.It is true everybody will be lucky in case the switch closes when voltage  is at its peak.But we,electrical engineers have to be pessimistics and hence I will think switch will close when voltage is crossing zero, momentarily putting 50 times full load current on 20 MVA.( Full short circuit current will be only 10times FL). I thought it can damage the 20 MVA transformer. Does any one tried similar loadings?

RE: Inrush current Loading

It's depend on the protective terminals.
As I know, problem will be occurred with 87N, restricted earth fault.

From other hand, few types of protective terminals is not included a second harmonic continues detector, it will be problem too.

I would recommended use 87T protective terminals with waveform blocking option.

Next point a grounding fault protection with calculated ( internal or external ) grounding current.
For avoid maloperation possible use in-rush detector or, if possible, 2-nd harmonic block signal from 87T protection.

If you use a high class of protective terminals, I'm not see problems with this issue    

RE: Inrush current Loading

prc,
The voltage drop "across" the 20 MVA transformer will naturaly limitate the inrush current in the 188 MVA transformer
As this configuration looks like that of a generation plant, I would suggest to energize both transformers gradually and concomitantly from 0 to nominal voltage (if possible).

Best Regards,

Herivelto S. Bronzeado
Ministério da Integração Nacional - MI
Brasília, Brazil

RE: Inrush current Loading

(OP)
Bronzeado,I also felt that voltage drop may limit the current.But the 20 MVA may see the first peak before voltage drop brings down current. Transformer may not stand the forces from that one peak.Finally the utility refrained from the excercise based on my advice.

These are net work transformers and hence applied voltage cannot increased gradually.

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