As per IEC 60076-5 , abnormal operation of frequent faults should be informed to transformer manufacturer in advance.IEC does not prescribe the no of faults for abnormal operation.
Is there any paper or research done to estimate how many full level short circuit currents(dead short circuit on terminals) can be withstood by the transformer in its life time?
Hello Virendra Lakhiani ,
Usually, it is impossible to get a pure dead short, i.e., a true zero ohm short. What usually happens is that the fault occurs for a certain time period at a multiple of the transformer's rated current: the higher the multiple of the rated current drawn by the fault, the shorter the duration of the fault that is allowed without compromising the life expectancy of the transformer. So it is not the number of faults, per se, that is important, as much as it is the character of the faults. Thermal relays are usually used to enforced limits on the time and multiple of rated current to which the transformer is exposed.
Regards,
Tom Cuff
Hello Mr Thomas Cuff
Thanks for your quick response, but,I don't quite agree!
i agree that the fault current shall vary. The duration of the fault will depend upon the response of the protective circuit (100-200 ms). This time is sufficient to cause deterioration in the winding because the highest magnitude of the force is at the first peak of the wave. Most of the time the fault currents are less than 40-60% of the dead short circuit current. But too many faults even of this range perhaps may cause a kind of fatigue/deterioration in the winding strength to sustain longer. the System Regulators should impose some limiting no of faults.
Regards
Virendra
Hello Virendra Lakhiani ,
From your response, it appears that you are more worried about the mechanical forces the windings will experience due to the primary and secondary currents flowing in the same direction (attraction/compression), or in the reverse direction (replusion/tension). Are you also worried about the thermal effects such as hot spots, due to excess Joule dissipation, that may cause a deterioration of the winding insulation?
I suppose it depends on how you, or the IEC standard defines a fault. I would think that mutliples of the rated current would be more reasonable in practice than assuming each and every fault is a dead short somehow moderated by the vagaries of the connections to the transformer, or some other unspecified limiting factor. For example, suppose that the load on the transformer draws twice the rated current due to an unexpected change in the load. The transformer might be able to withstand this condition for 30 minutes, say, without affecting its life expectancy, but isn't this condition still a fault?
Regards,
Tom Cuff
Thanks Mr Thomas Cuff,
In case of large power Transformers,as designers,we are more worried about damage due to mechanical forces.
A failure in Transformer industry is understood as one that causes forced outage of the transformer from service.
For most fault currents remaining below 40-60 % ,the hot spot is not reached and is not of concern.
Thanks.My question is : is any study done on the no of faults/month/year declaring a system as abnormal considering s/c
damages to transformers?
Regards,
Virendra Lakhiani