Those of us who’ve experienced unexpected failure of variable speed drives may be interested in an analysis of the following problem:
Problem: A plant operating a large number of VFD’s is facing the problem of spikes resulting in damaging of the drives. Can you please suggest some means to overcome this serious problem? The plant has its own generation through its gas-operated engines and during gas shutdown the plant is supplied power through furnance oil engines. Power is generated at 11Kv and is stepped down to 400 volts through step-down transformers.
Analysis: It’s almost always advantageous in cases like this to take the “top down” approach; that is, start by investigating system elements common to all of the components being affected. If you find a problem, it may just be that correcting it will resolve all of your downstream issues. Power supply stability/quality should be checked first. If incoming sources are surging/sagging, why? Are the power supplies stable under conditions of varying load? If not, it pays to understand whether the generators are sized properly and the controls/monitoring equipment are in good working condition. Keep in mind also that it is possible that the problem is originating outside of your facility – especially in fringe areas, there are cases where the delivering utility has recently had to absorb new loads on an existing, possibly under-capacity, system. So check with your outside power supplier as well, if you are using one. If you find an incoming power quality problem, can any upstream corrective measures be implemented? These may be as straightforward as correcting generator control problems or as complex as investigating and improving system impedance.
If you can’t do anything about the spikes, then you are basically left with two solutions: isolation transformers at each of the drives, or AC line reactors on each drive. Isolation transformers will provide the greatest protection, although at a larger initial capital cost. They can also be specified to reduce line-side harmonics generated by the drive electronics. Where reactors are concerned, although many drives are equipped with factory-installed DC bus reactors these will not damp line surges seen by the converter SCR’s; only AC line reactors will do this. Check manufacturers’ literature for the drives to determine what type/size of reactors to obtain. Third party units are also made; in particular, we supply units from Trans-Coil and MTE sized to match impedance and current requirements. Impedance needs to be calculated based on the source impedance and the cable distances from the source to the drives; current is taken from the drives’ rated outputs. Keep in mind that there is some voltage drop associated with AC reactors, so if you are operating at the lower end of the drive’s range you may get sporadic under-voltage faults.
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