The following is a history of a 50 HP, 480 Volt Variable Frequency Drive (VFD) that had failed. The drive was the alternate for a small two pump sewage station. The operation was that one and only one pump would be operating at any time.

Loss of incoming power caused the emergency generator to start and provide power to one of the two pumps. When the normal power resumed, the mechanical interlock device failed to disconnect the generator from the line, consequently, both the generator and the normal power were connected in parallel and to the VFD at the same time.

The generator was wired to provide the rotational phase sequence of the normal power, but, since it was never designed to operate in parallel with the incoming power, there was no provision for the generator phase angles to be in sequence with the incoming power phase angles. The resulting current surge that ensued from the “battle” that took place between the power company and the generator, in addition to any contact bounce that may have resulted from the mechanical interlock attempting to open, created transient surges. The surges then damaged the VFD input section before the breakers or fuses began to clear either the generator, distribution system, or the VFD.

The drive manufacturer was called to arrange for repair. The manufacturer then advised to apply power to the drive in order to read the fault indicator so that the service personnel might know what replacement parts to bring. Upon application of power, the output devices immediately failed. The manufacturer was called once again and the manufacturer advised that, since everything that could be damaged was already damaged, another attempt to read the fault indicator should be made.

Upon application of power it was discovered that the bus capacitors had not yet blown. After the smoke cleared, they re-entered the building. Discretion being the better part of valor, the drive manufacturer elected to perform the repairs under warranty since the manufacturer’s recommended advice accounted for most of the damage.

Notice the sequence of events: The initial surge damaged the input devices, and the second surge which occurred at the re-application of power damaged the output devices. A similar sequence of failure is discussed in Transients: Incoming Power Disturbance.