Unit transformer connected motors - Differential protection 1

[RRaghunath]

Differential protection for Unit transformer connected motor includes transformer as well in its zone and for that reason the biased type differential relay instead of conventional high impedance type is used.
I have come across a case where PF correction capacitor is connected in a tap off at the motor terminals whereas, the CTs for differential protection are on the primary of transformer and the other on the neutral end of the motor windings.
With the CT locations as above, the capacitor current is not accounted in the differential protection scheme.
Can any one tell me how sould this affect the protection settings.

Appreciate any help / experiences in this regard.

Thanks in anticipation.

 

[alehman]

Can you put another set of CT's on the capacitor leads and sum that current with the neutral lead CTs?

 

[bigamp]

An interesting problem that I have not come across before so a bit of thinking out loud!

(1) Check with the relay manufacturer and see if they have any suggestions.

(2) I dont think a set of CT's to measure the capacitative current is going to be the answer because the capacitor current will be out of phase with the motor current so how do you summate the capacitor current with the supply current?

(3) Due to the capacitors, the current in the motor star point will be displaced in phase from the current in the transformer primary CT's by something a little different than the transformer vector group.

(4) If the capacitors are correctly sized, the capacitor current should be a bit less than the motor no load current and well less than motor full load current. The capacitor current will be essentially constant, and will only go high if there is a fault in the capacitors themselves. The affect of the capacitor current will be most noticeable at no load, progressively less as the motor is loaded and very little during starting, which is what your differential protection must be "stable" for. Do some maths and work out what your currents will be for the various conditions described. Then, see if you can come up with relay settings that allow for the capacitor current by a higher than normal relay differential setting and maybe higher than normal bias (slope) settings. Remember that a differential relay does not need to be overly sensitive, it is there to detect a gross fault and trip quick.

(5) Can you get rid of the capacitors?

Good luck, and please let us know what you end up doing as the problem is of interest.

 

[jbartos]

Suggestion: Please, double check that you do not missed one set of CTs.
Else, the protection scheme is incomplete, meaning incorrect.

 

[RRaghunath]

Thanks bigamp. I am following your suggestion in setting the differential protection. I need to set the relay at 30% instead of usual 10%.
Parallely I am also trying to get a set of CTs fitted in the capacitor tapoff, so that the current can be added to the transformer HV side CT current and the setting made sensitive.

Thanks for your mail. I find the same really useful.

 

[bigamp]

Faulty,

I think that a scheme that sums transformer primary current and capacitor current could work OK, but the capacitor CT ratio and connection would need to account for the unit transformer ratio and vector group.

As an alternative, would it be possible to "sum" the motor current with the capacitor current on the LV side of the unit transformer via a summation CT (being very careful with the phasing) and then the "summated" current as the "motor" current input to the differential relay?

If you could fit CT's to the capacitors, it would be desirable for the CT's to located in such a way that the capacitors remained within the differential zone. You would achieve this with star connected capacitors by locating the CT's on the starpoint side of each phase capacitor.