I am observing different current (at EM) in each phase of a 11KV feeder, while it is feeding to a DYn11 Transformer.

I believe that whatever is the load on LT side, the load on HT side must be balanced & ammeter must show equal current in all phase.

I am not able to calibrate the EM, Kindly help..

Thanks
Ashish.sharma [at] maruti.co.in

 

Ashish, the CT connections must be opposite to those of the power transformer's winding connections.

1) The D-side CTs must be connected in Wye, and the y-side CTs must be connected in Delta.

2) Typically there may also be a slight error because the CT ratios can not be exactly matched.

3) The protective relay should have taps to account for the slight error.

Regards, Phil Corso (cepsicon [at] aol.com)

 

I do not think that's possible. If the loads on secondary side (wye) are un-balanced, then the line currents on primary side (delta) will also be un-balanced.

Remember, a delta connection is analogous to a wye connection with provision of a return neutral.

Best regards,
Shahvir

 

Responding to Ashish's 14-Jul-09 (13:00)... you are dead wrong! As proof that unbalanced currents on the y-side of a Dyn transformer do not result in balanced currents on the D-side consider the extreme cases, i.e. fault currents on the LV or y-side:

Case 1, three-phase fault.
LV windings a, b, and c each carry 1.000 per-unit. The corresponding per-unit current magnitudes in the HV-windings A-B, B-C, and C-A are 0.577, 0.577, and 0.577, respectively. In addition, the per-unit current in each conductor in the HV feeder is 1.000. (This is the only case when primary and secondary currents are balanced!)

Case 2, phase b-to-c fault.
LV windings b and c each carry 0.866 per-unit, but winding a current is zero. The corresponding per-unit primary winding currents A-B, B-C, and C-A are 0.500, 0.500 and zero, respectively. Furthermore, the per-unit currents in the HV feeder conductors A, B, and C are 0.500, 1,000, and 0.500, respectively.

Case 3, phase a-to-earth fault.
LV winding a carries 1.000 (*) per-unit, but b and c winding currents are zero. The corresponding per-unit primary winding currents A-B, B-C, and C-A are zero, zero, and 0.577, respectively. Furthermore, the per-unit currents in the HV feeder conductors A, B, and C are 0.577, zero, and 0.577, respectively.

The asterisk (*) in Case 3 indicates that the maximum phase-to-earth current could be greater than the three-phase fault current because the transformer’s zero-sequence impedance might be less than its positive-sequence impedance. This condition was explained in previous topics about "Symmetrical Components."

Regarding the reason the D-side currents are not balanced is due to the 30deg phase-shift in a Dy or Yd transformer. As above, detail is provided in "Symmetrical Component" topics.

Regards, Phil Corso (cepsicon [at] aol.com)