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
RE: Inrush current Loading
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
RE: Inrush current Loading
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
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
These are net work transformers and hence applied voltage cannot increased gradually.