VFD intermittent fault

[GTstartup]

I have a 2MW LCI VFD that is tripping intermittently on converter overcurrent. I know it’s not a protection circuit problem because there are 2 protection circuits and both circuits pick up the fault. The circuits are independent except that they are sourced by the same CTs (located on the line side of the converter section) and a current to voltage converter. The strange thing is that it does not happen every time, maybe 1 out of 3 times and it trips approximatly when the drive switches from pulse mode to LCI mode (this may be a coincidence).

I suspect failing thyristor(s). Is it possible for a thyristor to be "on the edge" of shorting and possibly only short intermittently? Any other ideas of what might be causing this?

 

[electricuwe]

Maybe the converter tires to change from pulse-mode to load- commutated operation witn insufficient load voltage ?

The load-side converter depends on on voltage supplied by the load for commutation.

Check if the thyristors in the load side converter are really all of the type the manufacturer specified. Suffixes in the type designation may be important (turn-off time and test conditions for it)

 

[GTstartup]

Thanks, electricuwe for your response.

One thing I should have mentioned is that this drive has been in service for 20 yrs, so all possiblities involving incorrect supplied equipment can be ruled out.

Also, I understand that "the load-side converter depends on voltage supplied by the load for commutation". That's why its called an LCI (Load Commutated Inverter)

My primary question still stands:

Is it possible for a thyristor to be "on the edge" of shorting and possibly only short intermittently?

 

[Marke]

Hello GTstartup

At that age, it is possible that there is a Thyristor that is beginning to fail, but there is also potential problems in the firing circuits and commutation circuits. I would check the values of all the capacitors in the power circuit. You may find that one or more capacitors have changed in value.
Power film capacitors are often "self healing" and over a period of time can loose capacitance due to damaged sections. This can cause problems with thyristor based inverters.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[laundry]

hi

as you know a thyristor is a semiconductor,and yes a thyristor can develop a fauly on its gate. if you have
the means you can look at the wave pulses you should see
if you have a pulse missing.or simpily check the thyristor
with your multi meter on diode test this will show up any
leakage. check gate anode to kathod.
I hade a fault like this on a mentor dc drive some years ago.