Transformer Protection

[mrbj]

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.

 

[DiscoP]

The thermal damage curve, the load characteristic and all other protection/fuses need to be considered.
Once all this information in known, a suitable overcurrent curve will become apparent.

 

[PowerfulStuff]

My understanding was that none of the standard curves are adequate for transformer protection due to the extremely long thermal time constant of the transformer. The best protection is the use of temperature monitoring of the transformer itself with a back up instantaneous setting (HV side) in case of short circuit.

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.

 

[vtpower]

mrbj, What are you using for the HV protection. I am curious if you are operating HV breaker with the LV relay, what are you doing for transformer faults, i.e. winding shorts or anything else? I currently use a software called CYME TCC to plot damage curves along with fuse and relay curves which allows you to adjust settings easily.

 

[RRaghunath]

If I guess right, your 415V loads are all fuse protected. If true, the "Extremely Inverse" characteristic, as defined by IEC, selected in the OC protection on the LV side of the transformer will give you better coordination with downstream fuses.

raghunath_n00@rediffmail.com

 

[buzzp]

What about the high side? I am with vtpower when he asks his questions concerning grounds, differentials, etc. What about the high side protection?
Dont forget about the magnetizing current when you energize this xformer after being off for a while.

 

[mrbj]

Hi guys,
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.

 

[GamezBeCJ]

mrbj:

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.