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Ageing behaviour and temperature rise in end corona protection layer for high voltage machines subjected to converter-like voltages

The concern of energy consumption regarding the usage of rotating electrical machines, has in recent years made a major impact on the market for converter driven high voltage machines. Converter driven machines may better adjust the output power to the need of the load, in comparison to line-fed machines, and thereby save energy. Throughout the years the corona protection system of high voltage rotating electrical machines has been developed to have good field grading properties and low temperature rise at operations under normal power frequency voltages. Today, knowledge on how the corona protection system is affected by voltage stresses generated by the converter/motor set-up is advancing. Over the years, this topic has attracted many researchers and it is a common understanding that a converter voltage containing pulses with short rise time and high repetition frequency will affect both the temperature and voltage distribution along the corona protection system [1-4]. For some applications, the development of high voltage converters has gone from 2-level and 3-level converter topology to multi-level topology, which has changed the voltage wave-shape from a more square-wave voltage to a sinusoidal looking voltage superimposed with a ripple of pulses, see Figure 1. In order to test the performance of the corona protection system under more realistic conditions, testing with a converter-like voltage is preferable.