Transformer Protection
Transformer Protection
(OP)
Hi,
Your answers/response to the following would be appreciated.
Consider a typical transformer 13kV / 0.415kV rated at 750kVA.
The HV side protection device is operated by the LV protection relay that is monitoring Over-current and un-restricted Earth Fault current.
The loading is a mixture of motors, heating and lighting - general loads.
Would the over-current be set for Normal Inv, Very Inv or Extremely Inv? .
Is there a relationship to the transformer withstand / thermal damage (decrement) curve or are the above settings purely related to the load and its' characteristics?.
The relay uses IEC 255-4 or BS 142 curves.
Kind regards.






RE: Transformer Protection
Once all this information in known, a suitable overcurrent curve will become apparent.
RE: Transformer Protection
As DiscoP has suggested the LV protection relay should be set so that it is coordinated with upstream and downstream protection and provide thermal protection for everything from the the transformer bushings through to the downstream protective devices. Short circuit protection won't cover anything upstream of the relay CT's.
RE: Transformer Protection
RE: Transformer Protection
raghunath_n00@rediffmail.com
RE: Transformer Protection
Dont forget about the magnetizing current when you energize this xformer after being off for a while.
RE: Transformer Protection
Thanks for the replies, sorry for delay, have been working away.
vtpower and buzzp,
There are other HV devices upsteam, however, untill I return to this site and check it out , then I am unsure of the exact details.
rraghunath,
Yes, old site with HRC fuses on nearly all downstream loads.Thanks.
Regards,
mrbj.
RE: Transformer Protection
I would use the following procedure:
1) You have to calculate three things for the transformer: Magnetization Curve, Damage Curve and the Maximum Short Circuit Current.
2) The magnetization curve is:
25 times the rated current (High or Low Side, depending
what are you protecting) at 0.01 sec
12 times the rated current at 0.1 sec
6 times the rated current at 1 sec
3 times the rated current at 10 sec
3) You will plot this on a chart with the Y's being time and the X's being the current (plot the current on log scale)
4) The damage curve is:
25 times the rated current at 2 sec
20 times the rated current at 3 sec
16.6 times the rated current at 4 sec
14.3 times the rated current at 5 sec
5) Plot this curve too.
6) Calculate the Maximum Short Circuit current by dividing the Rated Voltage (High or Low) over the transformer's impedance. Plot this point too.
7) Finally plot the time-current curve of your protective device.
8) You have to make sure that your device's curve is between the Magnetization Curve and the Damage Curve. That means that the device will allow the transformer to be energized without triping the protection and also that the protection will operate before damaging the transformer.
9) Finally, make sure that the maximum fault current that the protection device can handle is greater that the calculated maximum fault current.
This steps should provide a good protection coordination for the transformer.
Hope this helps.