Medium frequency shaft voltages in large frequency converter driven electrical machines

摘要

The operation of large electrical machines like turbo-generators with ratings of several hundred MVAs by static frequency converters can cause shaft voltages of several hundred volts. The shaft voltages consist of peaks with a time constant of some μs. They occur mainly during the rectifier switching at the grid-linked section of the converter. The voltages are the result of a coupled capacitive and inductive phenomenon. The root cause of these voltages is shown and a machine model of turbo generators for several 10 kHz is developed taking parasitic capacities and eddy currents into account. It is not sufficient to limit the model just to the generator. It is the combination of transformator, cables, frequency converter and generator, which allow for such high voltages. Comparisons with measurements on a 300 MVA generator are made to verify the model. A high level of agreement between measurement and calculation is reached for this recently discovered capacitive inductive shaft voltage phenomenon. Especially the high power class of the equipment involved reveals the underlying physics fairly clearly, but enforces a deep investigation and high predictability of the right counter measures in order to avoid substantial risk exposures.