3PHASE STEP-DOWN Transformer in Check

[dennis_elx33]

Hello All,

i would like to post an interesting topic regarding 3-phase stepdown transformer which we suspect to cause an overcurrent (138A) which resulted to power tripped off of the HV power supply (80KV output).


Problem statement: Power tripped Off

Initial Problem: Found 2 IGBT and 2 rectifier diodes shorted (burnt).

Action Done:
1) Replaced the faulty parts with good spare.
2) Measured impedance of the 3phase input lines and found that L2 to L3 has 23.3ohm while other pairs are 51Kohm. This pins will go to MCB and directly to Transformer.
3) We ignore this low ohm reading and instead power up using the 208VAC 3phase input since we dont have any reference to compare with.
4) performed power up testing but power supply tripped off. (inrush current shows 138A).

Result: Failed. 1 IGBT and 1 Diode shorted this time at same location

Earlier Question: is this a normal resistance reading for the transformer at L2 to L3 reading?

Latest Update: Inductance readings..

Questions:
1) Do you think the inductance from pin 2 to pin 3 at primary side a normal reading?
2) I am Off today so our team tried simulating by injecting voltage using a single phase instead of 3phase to each pair, do you think the result will be same with the 3phase voltage input? They said that the secondary voltages were good.


Any other testing method that you could suggest in order to confirm the functionality? thank you so much!

Note: We could not find any datasheet for this transformer.

[R_J]

Like I said, I don't think the transformer is the problem, one of the lines from three-phase input should connect to primary 0, One of the other lines from the three-phase input should connect to one of the voltage taps, likely 240v the third line from the three-phase will not connect to this transformer, it operates off a single phase.

Initial Problem: Found 2 IGBT and 2 rectifier diodes shorted (burnt).

These shorted components may have also blown a fuse on one of the lines that supplies the low voltage transformer primary

[R_J]

Like I said, I don't think the transformer is the problem, one of the lines from three-phase input should connect to primary 0, One of the other lines from the three-phase input should connect to one of the voltage taps, likely 240v the third line from the three-phase will not connect to this transformer, it operates off a single phase.

Initial Problem: Found 2 IGBT and 2 rectifier diodes shorted (burnt).

These shorted components may have also blown a fuse on one of the lines that supplies the low voltage transformer primary

[dennis_elx33]

Thank you so much R_J, indeed the transformer is not the issue. But we have worse problem now as the IGBT and rectifier diodes keeps blowing when the setpoint of the HV output is around 52KV. Maximum voltage is supposed to be 80KV.

I have here some sketched circuit diagram for you to refer to. Maybe someone can help us solve this problem. We have tried to replaced the affected parts including the PWM IC and OP-amps but still encountered the same burnt parts.

Is there any chance that the big capacitors are the issue?