A problem exists with a medium-volt capacitor bank switches out a bank, 600-kVAR down to 300-kVAR, and it trips the VSDs off. The drives need to be reset after this occurs. The Power Co. owns the capacitor bank and they say we should install line reactors just ahead of our drives. I believe there is a problem in the capacitor bank, possibly switching noise on the bank contactor... but I honestly do not know for certain what the cause and solution is.

I adding reactors was only necessary when the drives were generating too many harmonics and damaging the capacitors, not the other way around. I would appreciate hearing from anyone who has had a problem like this and what the cause and solution is.

 

A friend of mine worked on a project where the VFDs were failing to operate correctly due to some line characteristic. When line reactors were installed, the VFDs worked normally, so this is not outside the realm of possibility. Are the VFDs small enough that you can try a line reactor on one or two motors and see if the utility is correct?
--
Steve Myres, PE
Automation Solutions
(480) 813-1145

 

Responding to Joel's Wed, Nov 10, 5:13pm query:

Could be a voltage regulation problem, and not a harmonic one. Here are some questions which will shed some light (excuse the pun) on the problem:

1) What indications are given for the VFD tripping? Undervoltage? Overvoltage? Overcurrent? Phase voltage or current unbalance?

2) Is tripping random? All machines? Some machines? The same machines?

3) Voltage recording/monitoring at VFD location or bus could provide a clue!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

It is quite possible for the PFC bank to cause the VSDs to trip. I had a similar experience with a can body-blank welder using IGBTs. If the PF got to over .99 or above, the welder would fault with an "phase assymetry" fault. Measuring this proved to be very hard though but the welder had protection to detect this condition and protect the IGBTs.

I suggest the power company make sure all of their capacitors are still at rated levels and none of the capacitor fuse protection has activated, causing resonance. Also see what their PF setting is. If it's around .99 or 1 there is a good chance it's that. .98 is a better setpoint.

Regards

Mike