Analysis of the effects from constant random and wear-out failures of sub-modules within a modular multi-level converter with varying maintenance periods

Constant random and wear-out failures of sub-modules (SM) within a Modular Multi-level Converter (MMC) are examined. Constant random failures are calculated using the binomial distribution formula, using a constant failure rate. Wear-out failures are calculated using discrete Markov chain modelling and Monte Carlo estimation, with the failure rate of a SM increasing with time. The results are compared with various maintenance intervals and different levels of availability to show the trends and characteristics. It is shown that for large gains in availability, relatively small increases in the number of redundant SMs are required. Additionally, the number of redundant SMs required to support longer maintenance intervals increase at less than a linear rate, making increasing maintenance intervals for applications such as offshore converter stations attractive.