We have distribution transformer 11KV/433V, 1250KVA. Primary is delta, secondary is star. Suddenly it tripped and while checking we found the secondary voltages are unbalanced. Checked neutral earthing found OK. This transformer has On load tap changer also. We checked up the primary winding which is unbalanced. R phase has more value say R- 9ohm, Y-4.5 ohm, B-4.6ohm. Will the problem may be in OLTC contacts or winding itself??

 

BD... please provide the following measurements from which you concluded "secondary voltages are unbalanced":

o primary phase-to-phase voltages.

o secondary line-to-phase voltages.

o secondary phase-to-neutral voltages.

o secondary phase-to-earth voltages

Regards, Phil Corso

 

I have seen this condition in a factory a few years ago. The voltages were seemly close to being balanced but the current in the motors powered from this transformer were reading about 30% higher than the other (2) phases. The power transformer also ran close to the maximum rated temperature with one third it’s rated load.

The cause was found by physical inspection of the primary tap changer. After replacement everything was back to normal.

We hired a substation service company to drain the oil, replacement of the tap changer, filtering the oil, refilling the tank (you need to allow ALL the bubbles to come out of the oil before restoring power) and running a filter press for about (2) weeks to remove moisture from the oil.

I hope this helps,

William Hinton

 

William... an accepted Rule-Of-Thumb (ROT) is that the Ampere-Unbalance-Factor (AUF) will approximate the square of the Voltage-Unbalance-Factor (VUF)!

For the value you observed, the VUF is about 5.5%, thus requiring maximum de-rating of the motor, per NEMA standards!

Regards, Phil Corso