Current imbalance on VFD output

[thinker]

We develop a test stand for Hybrid Electric Vehicle e-motor durability testing. The e-motor is fed from the vehicle PWM inverter. Customer requires to continuously monitor a current imbalance between phases in order to use the imbalance (if occurs) as indication of a problem which would require a more detailed investigation. Whould it be possible to use for this purpose a earth leakage relay with the core balance CT (all theree phase conductors running through CBCT). The current waveform is very close to sinusoidal (typical for PWM). We expect that for the balanced system sum of three phase currents (fundamental) is equal zero, so we might be able to set the imbalance threshold above the noise level. We actually need quite coarse indication of the imbalance event. We do realize that the standard earth leakage relay (we plan to use Howard Butler/Hobut system RCR-V30) is designed for sinewave, however, could it work for our application? If this is a wrong way, can anybody recommend a commercially available system for current imbalance monitoring on the VFD output? [The vehicle inverter measures phase currents, but this information is not available, the task should be done by external system].

 

[ScottyUK]

Unbalanced phase currents can still produce a vector sum of zero. The CBCT will detect leakage to ground but not imbalance among the phases. There are a number of monitoring and protection relays which measure phase imbalance or negative sequence current which might be a solution to your problem.

Is this a variable frequency system? That makes things a little trickier.



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If we learn from our mistakes I'm getting a great education!

 

[thinker]

Thanks for your reply, Scotty. Yes, this is a variable frequency system. Could you please give any example of imbalance measuring relay?

 

[DickDV]

Most imbalance detectors will not work correctly on drive output leads because there is no sine wave, only high frequency pulses.

Check out the specs for the drive. Most drives are not very tolerant of imbalance and will fault under those conditions.