Reliability analysis of modular multi-level converters for high and medium voltage applications
Creators: name: Wylie, James
Publisher: name: Imperial College London
Year: 2019

description: Modular Multi-level Converters (MMC) are currently the favoured voltage source converter for High Voltage Direct Current (HVDC) systems because they feature a good combination of low power losses and high quality waveforms, with an increasing interest in applying multi-level converters for Medium Voltage Direct Current (MVDC) systems, where those same benefits are expected to be seen but perhaps not to quite the same extent. A further claimed benefit of the MMC is that inclusion of redundant modules yields high reliability, however, there is little formal analysis available concerning how to choose the degree of redundancy in the light of various assumed failure functions of individual components and the choices over the maintenance regime to be used. This thesis considers two different reliability techniques to determine the required number of redundant Sub-Modules (SM) within an MMC for the cases of both constant-random and wear-out failure functions of SM components. Analysis is performed for both HVDC and MVDC since they differ markedly in the number of SMs present and the scale of redundancy that might be expected. The availability of an MMC is calculated using the \textit{k-out-of-n} model for constant random failures and with a discrete-time Monte Carlo simulation performed on Markov Chains to calculate the availability of an arm due to wear-out failures. The reduced number of SMs within an arm designed for MVDC results in the number of redundant SMs required to maintain a given level of availability being a large relative addition to the basic arm. This lowers the efficiency of the converter due to greater number of SMs in the conduction path of each arm. This initial finding was for redundancy designed for use with periodic preventative maintenance, which was the approach taken in HVDC. To improve efficiency at MVDC, it is proposed to change to reactive maintenance which will allow a smaller number of redundant SMs within each arm while maintaining comparable availability. To improve further the efficiency of an MMC at MVDC, an MMC topology is proposed that features a mixture of SM voltages (and combinations of full- and half-bridges) within each arm. High voltage-rated IGBT modules, which have low-loss conduction characteristics, are used to generate the majority of the arm voltage and a small number of lower voltage-rated IGBT modules, which have low-loss switching characteristics, are used to refine the arm voltage through smaller voltage steps and pulse-width modulation. Power loss and efficiency, as assessed through circuit simulation, are compared for the proposed mixed-SM-voltage MMC and conventional MMCs operated at the same power rating and the same total harmonic distortion. All the converters included redundant modules, based on reactive maintenance, to achieve the same availability. The mixed-SM-voltage MMC has been found to have the highest efficiency of the case-study designs. The control and operation of the mixed-SM-voltage MMC was validated through experimental testing of a lab-scale prototype.