Improved MCVDC Breaker Operation by Existing Power Converters in Shipboard Applications

The medium voltage dc (MVDC) power system is gaining increasing attention in applications such as renewable energy and shipboard power systems due to its advantages in reliability, efficiency, power quality and power density. However, the short-circuit fault management in a MVDC system is a key issue because of the lack of natural zero-crossing point and conventional mechanical circuit breaker (CB) design challenge. So solid state CBs (SSCB) or hybrid CBs (HCB) are under development to enable the breaker-based fault protection approach. Another breaker-less approach utilizing the inherent current-limiting capability of power semiconductor devices is also promising in a MVDC system. The state-of-the-art of the two fault management approaches are reviewed. This thesis is focused on developing a technology originated from the breaker-less method to improve the performance of breaker-based fault protection approach. In the shipboard breaker-based MVDC system, converters are normally shut down to react to the fault. The diodes freewheeling phenomenon is a concern but a quantitative analysis is still not available. Moreover, the potential of converter during fault has not been exploited completely. Considering large number of converters already existing in the shipboard MVDC system, the overall benefits at no extra hardware cost may be significant. Therefore, a converter fault ride through (FRT) strategy aiming at improving CB operation is proposed in this thesis. The proposed research found that there are some converters located in parallel with activated CBs during the fault and thus, active fault current sharing (FCS) by these converters are possible. In addition, this peak fault current reduction effect on CBs can be amplified from the systematic perspective because multiple CBs in various fault scenarios can be benefited from a single converter. As a first step, the short-circuit fault scenarios of a shipboard breaker-based MVDC system are analyzed comprehensively and fault equivalent circuits including power converters adjacent to CBs are developed. The equivalent circuits are analyzed mathematically and the reaction of passive mode converters to the fault are discussed, which can be a benchmark to evaluate the proposed active FCS strategy. Next, the proposed converter FCS strategy to reduce the CB fault current is illustrated in detail, together with the device stress analysis. Finally, the experimental verifications on a down-scaled test setup are provided.