Fredrik Sahlen

Investigation of mica based insulation for high voltage machines subjected to repetitive pulsed voltage

The interest in the prediction and qualification of insulation life performance in high voltage machines fed by converters has steadily grown, since the use of these machines is gradually increasing. The voltage stress on the insulation system is different for converter fed machines compared to machines fed online with power frequency voltage. The common interest in this subject has lead to the release of a Technical Specification 60034-18-42, which gives recommendations on partial discharge resistant insulation system testing at voltage converter conditions. In this paper the set-up of test equipment for testing lifetime of mica based insulation exposed to converter generated repetitive pulsed voltage as well as to sinusoidal voltage is presented. The influence of repetition frequency as well as pulse rise time on the lifetime is compared to 50 Hz ac conditions. Partial discharges are believed to be the dominant ageing factor for voltage stresses above the partial discharge inception voltage. Increased repetition frequency shortens the lifetime as a function of the increase in frequency. However, indications are given that the pulse rise time may somewhat influence the ageing of mica based insulation in the case of pulsed voltage stress.

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 %.

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.