LalitKumar
Member
- Location
- Qatar
In our operation at factory, we have total six rings of load(heating elements) fed by a 5MVA transformer, DY5 33kv/500-2500V (5tapes). Each secondary phase is connected to equal load of heatingelement rings through thyristor and we recently carried out a thyristor firing program change that allows even one ring in operation and supporting the production. Earlier one ring stops due to any genuine interlock/reason caused all the rings down - loosing production.
Now after program change, sometimes one ring trips and loses power and then we have to re-start it within15-20minute.
Recently we have undergone one phase failure of one such transformer under operation. I believe this type of switching program change which allows other phase ON, may have caused neutral shift and increasing high current in remaining phases for say 10-15 minutes and thus this transformer burnt. Even after re-startup we graduallyincrease the current at faster rate to reach up to the last ampere value beforetripping. So unbalance situation lies roughly 3-4 hours.
How much unbalance typically a transformer is allowed which supposed to run at equal loading ? our relay at upstream 33kV side is set at 2times of FLC for 51N.
Now after program change, sometimes one ring trips and loses power and then we have to re-start it within15-20minute.
Recently we have undergone one phase failure of one such transformer under operation. I believe this type of switching program change which allows other phase ON, may have caused neutral shift and increasing high current in remaining phases for say 10-15 minutes and thus this transformer burnt. Even after re-startup we graduallyincrease the current at faster rate to reach up to the last ampere value beforetripping. So unbalance situation lies roughly 3-4 hours.
How much unbalance typically a transformer is allowed which supposed to run at equal loading ? our relay at upstream 33kV side is set at 2times of FLC for 51N.