Life-time investigation of mica-based insulation for high voltage machines subjected to converter-like voltages

The industrial use of converter fed high-voltage machines is steadily growing. The driver behind the growth is the possibility to control the output power from the machine to be adjusted to the actual needed power of the load. This leads to a more cost- and energy efficient use of the assets. For some applications the development of the converter topology has gone from 2-level to multi-level, which has changed the voltage waveshape from a square-wave looking voltage to a sinusoidal looking voltage superimposed with a ripple of pulses, characterized by their discrete pulse voltage amplitudes, the pulse repetition frequency, and the pulse rise-time. The question is how this type of converter-like voltage will affect the aging of the machine insulation in comparison to power frequency ac voltage. This paper presents the test set-up for generating voltage wave-shapes similar to multi-level converter output voltages. The flexibility of the test set-up to combine ac and pulsed voltages into converter-like voltages with different ripple contents, has enabled a study of the influence of the ripple content on the lifetime of mica-based insulation. We define the ripple content as the ratio between the pulse voltage amplitude and the total peak-to-peak voltage. It is shown that the peak-to-peak voltage and the fundamental frequency, and not the pulse repetition frequency, influence the insulation aging rate up to a ripple content of at least 43 %.