Mission Profile-Based Preventive Maintenance for Modular Multilevel Converters in MVDC Systems

Preventive maintenance of modular multilevel converters (MMCs) improves reliability by minimising unexpected breakdowns and replacing faulty components in converters. This paper evaluates the converter reliability and preventive maintenance of MMCs with multiple sub-module (SM) topologies under different operating conditions in medium-voltage DC (MVDC) systems. It demonstrates that for converters integrating solar farms, the semi-full-bridge SM (SFBSM) can provide the greatest reliability, while the full-bridge SM (FB-SM) has the lowest reliability. Depending on the objectives, the maintenance strategies for MVDC MMCs have been optimised based on cost-efficiency and availability-prioritised criteria. It proves that greater converter reliability allows for longer optimal maintenance intervals, resulting in higher cost efficiency and converter availability. Additionally, the assessment provided in this paper reveals that mission profiles in MVDC systems have significant effects on converter reliability, with systems connected to wind farms typically exhibiting greater reliability than systems that operated at the grid side of photovoltaic (PV) systems. Therefore, preventive maintenance of MVDC MMCs needs to be considered according to anticipated operating scenarios and conditions, even within the same MVDC system.