Line PT Primary Side Flashover in 6.6 KV Siemens Panel

Interesting problem, you haven't told us what type of PTs you're using so I'm going to guess they're oil filled. Is there are any type of PFC (Power Factor Correction/Controller/Capacitor) on this particular feeder? Are these leads significantly different in length than the others? Have you done a physical and electrical inspection of the PTs between failures, if not why not?

Assuming that there is nothing unusual/different about the wiring on this particular feeder, then it sounds like it's time to scrutinize everything about the PT's, the wiring, clearances, insulation resistance, etc., etc.; because everything is pointing to a loss of insulating media leading to an internal failure to ground upon energization.

A really wild guess would be that there is some sort of resonant condition between the cables (capacitance) and motor windings (inductance) made worse by any PFC equipment.

One thing for sure is to do a complete cleanup and inspection before you energize this circuit again, with a complete monitoring of the voltage and current using a Transient Data Recorder (TDR) to capture what's happening here.

Please let us know what you find during these inspections and tests.

sir, the PT's are not oil filled but made of some epoxy based compound. Further, one thing which we are unable to trace out is the fact that there ate many such pt' s (same make, same type)in the circuit which is leading to the power circuit of our motor. Further, there is no problem when the bus is energised through the breaker which consists of these transformers. It is only when we are starting the BFP that this flashover is occuring. We doubted a resonance but agai, if its resonance.. Then why is it happening only in the above said breaker??

Ferro resonance seems most likely as the learned member menntioned. This requires detailed study. Is the 6.6kV neutral resiatnce grounded! More details will help. The switchgear comprises fuse backed MV vacuum contactors, I guess. Have you got surge suppression devices in the cable chamber!

Many questions come to mind, it is not going to be easy.

Sir, we have found out the reason for PT failures. The open delta resistance was found open in the panel where there was repeated PT failure and this happened because of a problem in one of the terminal blocks of the panel.we have rectified the same and started the motor again. we faced a problem again. The PT's were intact but there was a heavy ionization in the bus duct chamber of the same breaker. The Icomer ultimately tripped clearing the fault. Upon inspection, we have found out porions of bus bar where there was pitting. The entire chamber was intensely ionized. The station transformer neutral is grounded through a resistance.

So you found the failed resistor connection and assumed that it somehow caused ionization in the bus duct, then re-energized a fourth time without any tests, analysis, or inspections to enable you to come up with a good engineering reason why any of this keeps happening? Time for you to lock-out that breaker and hire some competent talent to steer you away from this mode of operation before you do some really significant damage or worse, kill someone.

Sir, I agree to the fact that there should have been a detailed investigation after the first failure. But if you expect a guy with some 16 month experience in Indian industry to become an expert in power systems, I am sorry for that. I am not that intelligent enough and I learn from my mistakes. We have traced out the problem. There was a crack in the Y phase busbar near the breaker sprout of that particular breaker. There was heavy arcing because of that gap. We have replaced that particular bus bar and the system is healthy now. Thank you all for your valuable suggestions and critical criticism.

No one expects anyone to become an expert in anything in 16 months, so shame on your management for putting you in a position that you acknowledge you are not qualified for, but worse yet, you hid this fact and proceeded anyway. No experienced engineer would have told you to keep energizing an obviously faulty piece of equipment until the cause of the failure finally revealed itself. (Management might but that's a whole 'nother story, you'd be fired anyway for not "alerting" them to the danger)

What is expected is that you will use your knowledge and training to protect lives first and company assets second. If you do not have either than get them before you find yourself regretting a decision you shouldn't have made that will haunt you for the rest of your life. This probably took place before you were born, don't let something similar happen on your watch.

Fine sir, I respect your comments and criticism.My management was beside me and they knew how severe the situation was and they knew if it was correct to charge the bus or not and they clearly knew why they needed to do that even when they knew that they should not do it.

Dear MSCS,

I think, the 6.6 kV panel is feeding power supply to run BFP motor and the motor is also run thro' VFD. If this is so, how much is the cable lengths from the panel to the motor? Whether the harmonic filters,reacters, etc. installed in the system are in working condition or not. For malfunctioning of the above, harmonics generated by VFD may feed back to the source( in this case 6.6kV panel). Due to malfunctioning of filters,reacters,etc. also wavefornt may generate(depending on cable lengths,type of cable etc.) and hit back to the power system with higher voltage and this type of electrical insulation damage may occur.

Regards,

Manindra