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In a line-commutated converter based high-voltage direct-current (LCC-HVDC) system, the inverter control system usually includes constant extinction angle (CEA) controller, constant current (CC) controller, current error controller (CEC), and voltage-dependent current order limiter (VDCOL). However, the interaction influence of multiple controllers may cause repetitive commutation failure (CF). For this purpose, this paper deeply investigates the mechanism and the influencing factors of repetitive CF caused by controller interaction. It is found that the lower limit value of CEA controller can affect the control switching process and thus the probability of repetitive CF. Besides, the minimum gamma measurement (MGM) unit prevents the system from acquiring the real extinction angle in time, which results in the DC current fluctuation during the recovery process. Moreover, the probability of control-induced repetitive CF is much higher when the HVDC inverter is connected to a weak AC system or the AC fault is not serious but not cleared in time. Based on the above analyses, an improved control method is proposed for recovery performance improvement, which is mainly realized by modifying the lower and upper limits of the CEA controller during recovery. Finally, the simulation and experimental results verify the theoretical analyses and the proposed control method.

This letter analyzes the mechanism of commutation failures (CFs) caused by sending end AC faults. It is founded that the dc impulse current after fault clearance is the direct reason of CF occurring under nonserious sending end AC faults, and the controller interaction is the main reason of CF occurring under severe sending end AC faults. Moreover, a prevention strategy is proposed based on the revealed mechanism, and the validity is verified by the simulation results.

Medium-voltage DC (MVDC) distribution system is considered as the promising power architecture of future shipboard power system. Fault-tolerance ability is of great importance for power system on ships. In this paper, zonal DC-DC conversion system based on modular multilevel converter (MMC) and three-phase bridge rectifier are proposed with its hierarchical fault-tolerant scheme. The topology configurations of the front-end MMC with multi-winding medium frequency transformer (MFT) under normal and post-fault circumstances are presented. The recon-figuration scheme and fault-tolerant control strategy are also proposed to ride-through sub-module (SM) level fault, phase level fault and bus level fault. Based on the hierarchical fault-tolerant scheme, redundant configuration of the MMC sub-module is further analyzed with the hierarchical reliability modelling. The effectiveness of the proposed control strategy is verified by the experimental results.

In this paper, a three-phase modular multilevel converter (MMC) with four-winding medium frequency transformer (MFT) is proposed to improve the power density and fault-tolerant capability of shipboard DC power system. First, a MMC-based MVDC on-ship sub-system with double independent LVDC buses is presented. Then the modeling and analysis of the MMC integrated with four-winding MFT under normal and fault-tolerant conditions are derived. Further, the corresponding multi-layer fault-tolerant control scheme is designed to ride-though MMC sub-module (SM) fault and leg fault. With the proposed individual-phase circulating current control, the MMC can maintain normal operation with the faulty SM bypassed. With system structure and modulation reconfiguration, the MMC is able to work with the faulty leg bypassed. Finally, the effectiveness of the proposed architecture and fault-tolerant control scheme is verified by the experimental results.