VFD issues on generator
VFD issues on generator
A customer has two sites where the loads consist mostly of VFD driven 3 phase 480 volt motors, driving rock cutting machines. One site is powered by utility, the other by a 100 kw diesel generator.
The customer says that at the site with the generator, they have experienced numerous VFD failures. These failures are often several months apart, but there have been many more occurrences than the site on utility. They have experienced these issues with four different generators in place, at different times.
Customer says when a VFD "fails", there are no fault codes on the drive--the symptom is that the motor output is "jerky". The drive is replaced and the problem goes away. I haven't spent any time looking at how the drives are programmed and there was never any failure analysis done on the "failed" drives. The rock cutting machines utilize one TECO N3 drive and one Mitsubishi FR-D740. Apparently both have had this problem.
I performed a power analysis of the entire load over the course of two days. Looking over the data, voltage is maintained within about 5% and frequency within about 2%, the only exception being when one large load is started, the voltage and frequency each drop close to 10%. Imbalance between phases is minimal. As expected, the sinewave is horribly distorted (see attachment showing voltage and current sine waves). For whatever reason, phase A current is much more distorted than B and C. This phase A distortion was present even before any load was put on the generator, therefore I wonder if this is just a problem with my instrument. THDI is about 25% on phase A and 18% on phases B & C. The 5th harmonic is greatest, around 11% of fundamental. Peak load is around 60 kW/65 kVA. I understand that this percentage of THD may shorten generator life, but will this in itself cause the VFD issues I described? Most of the time, the plant electrical loads function normally. I did not see any harmonic filters present.
One issue I see is that the generator main circuit breaker is not equipped with a shunt trip to open the breaker on a generator shut down fault. If the generator engine overheats for example, the load will experience an under voltage condition for a few seconds as the generator winds down. A shunt trip coil is going to be installed on the breaker.
Any thoughts would be appreciated.