Probabilistic aspects and flexible thresholds of waveform distortions

The probabilistic aspects of time-varying waveform distortions present an interesting and valuable research avenue, especially applied to the design of a tightly coupled integrated power systems (IPS) such as onboard an electric ship. The power quality thresholds that are currently practiced may not hold appropriate for the efficient operation of the IPS. One school of thought defers to an equipment based flexible threshold of power quality rather than the current practice of a fixed threshold of power quality for each voltage class. This is augmented by the lack of utility-customer like structure and points of common coupling in the IPS. Of particular interest to this research is the cumulative effect of time-varying waveform distortions on the equipments onboard. In this presentation, a case study on a reduced IPS consisting of the AC ring, propulsion systems and the DC-ZEDS is performed with artificially induced time-varying waveform distortions. The statistics (probability density function PDF) of a power quality parameter (total voltage harmonic distortion V_THD in this case) at the terminals of a power conversion module (PCM2) that feeds other AC loads are quantified. Further, discussion of the probabilistic aspects of the time-varying waveform distortion with respect to additional thermal heating in an example load (3-phase induction machine) and the corresponding reduction in life expectancy will be presented. The basis for seeking flexible/relaxed thresholds for power quality at the equipment terminals based on the time-varying nature of waveforms will be established. Also a brief description into the appropriate tools required to quantify/visualize the time-varying waveform distortions and effects will be made