Transformer Magnetic balance test results: Interpretation 1

[RRaghunath]

We had a 5MVA trafo tripping on Buchholz protection operation.
Field checks revealed that the trafo is healthy (electrically speaking) and also nothing wrong with the Buchholz relay / circuit.
The magnetic balance test results do not conform to the pattern that we normally see in a healthy trafo.
Results are as attached.
Can someone help me interpret the results!
Thanks in anticipation.

 

[prc]

From the results, it looks like there is an interturn fault in U phase.U phase is not inducing voltage due to shorted turn there.

Frankly magnetic balance test is not a good diagnostic test and used only in India. It will give correct results only when you can excite near to rated flux density ie in distribution trfs. Universally accepted test for detecting turn faults ( introduced by Doble around 1964) is low volatge excitation current measurement . Excite each phase by 230 V and note the excitation current and compare with earlier measurements.In this case you will find A in U phase and mA in other phases.

 

[RRaghunath]

prc,
Thanks for sharing your thoughts. The exciting current measurement by energising 11kV windings (Y-connected) phase-by-phase, we will try and come back.
We had earlier energised 33kV windings (Delta) and observed that with the power supply voltage of 440V, the exciting current is immeasurably low except in U&W-phases where it is ~4mA.
Thanks once again.

 

[prc]

raghun, If there is a turn to turn fault, it is not wise to energise at rated voltage as it will aggravate the fault.Apply 230 V to 11 kV line to neutral and measure excitaion current. Normally this will reveal any fault by showing a relatively higher current in u phase. Magnetic balance test results also clearly show a fault in U phase.

 

[RRaghunath]

prc,
Star to you! You are on dot with your expectation of high exciation current in U-phase.
The magnetising current measurement test revealed that when excited from 11kV side (250V), the current in U-phase wasn ~100-times more than that in other phases. The same is about 59 times when excited from 33kV side (440V ph-to-ph). See the attachment.
Is there any way, to know whether the interturn short is on HV side or LV side (lookig at the results)!!

 

[prc]

raghun,I did not get your star!!Only jocking,apart from stars, let us learn from each other through our experience.I dont think we can conclude the location of fault from these results.Please do short circuit test also from both sides ie short secondary and apply 440V 3 phase on primary and measure currents.

When you finally locate fault please share with us.When there is a shorted turn or turns and you apply voltage to that winding or to the other winding, the shorted portion acts as a short circuited secondary winding.The circulating current in the loop,makes the primary to draw more current to compensate the AT in the shorted winding section.