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.

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 multiwinding medium frequency transformer under normal and postfault circumstances are presented. The reconfiguration scheme and fault-tolerant control strategy are also proposed to ride-through submodule (SM) level fault, phase level fault and bus level fault. Based on the hierarchical fault-tolerant scheme, redundant configuration of the MMC SM is further analyzed with the hierarchical reliability modeling. The effectiveness of the proposed control strategy is verified by the experimental results.

Load converter operating in the weak grid potentially suffers harmonic oscillation problem due to the impedance interaction. In this paper, considering DC-link voltage dynamics and frequency-coupling effects, the precise sequence impedance models are derived for the traditional load converter (TLC) and the load virtual synchronous machine (LVSM). Based on the derived models, their sequence impedance characteristics are analyzed and compared. The analysis shows that the positive-sequence impedance of the LVSM is generally inductive in the middle- and low-frequency bands, which is basically consistent with the grid impedance characteristics. In contrast, the positive-sequence impedance of the TLC is negative-resistive-capacitive (i.e., phase angle is between -180° and -90°) in the middle- and low-frequency bands, and its amplitude-frequency curve is easy to intersect with the amplitude-frequency curve of the grid impedance, which may lead to harmonic oscillation in the weak grid. Furthermore, based on the derived model and Nyquist stability criterion, the effects of the grid impedance and load power on the stability are analyzed for TLC and LVSM, which indicates that the LVSM has better stability in the weak grid than the TLC. Finally, the experimental results validate the correctness of the theoretical analyses.

During grid faults, the grid-forming inverter (GFMI) needs to suppress overcurrent and provide grid support. However, the grid-supporting capability is commonly overlooked while designing the current-limiting method. Therefore, a two-stage adaptive virtual impedance-based current limitation for improved grid-supporting capability is proposed. Theoretical analysis shows that the better response performance of reactive power can be achieved by setting a larger impedance ratio (n = ωL_v/R_v), but this will increase the peak magnitude of fault current. Hence, during the fault transient stage, the proposed method uses the fault voltage drop to quickly calculate the large magnitude of virtual impedance to suppress the peak magnitude of fault current. However, during the fault steady stage, the large magnitude of virtual impedance limits the capacity utilization, thus the amplitude of the fault current is used to build a state machine to further optimize the magnitude of virtual impedance, to improve the capacity utilization. Then, the fault current can be suppressed under various fault conditions and the grid-support capability of GFMI can be improved from two aspects: the response performance of reactive power and the capacity utilization. Finally, simulations and experimental results are used to verify the effectiveness of the proposed method.

A damping-controllable-commutated converter (DCCC) is proposed to enhance the commutation failure (CF) mitigation capability of line-commutated converter-based high-voltage direct current (LCC-HVDC) systems. The DCCC comprises a main branch and an auxiliary branch, with the main branch consisting of thyristors and a few high-current capacity integrated gate-commutated thyristors ( i -IGCTs) and the auxiliary branch composed of damping modules and a few high-voltage tolerance IGCTs ( v -IGCTs). In normal operating conditions, the DCCC operates like the LCC for commutation. However, under fault conditions, the commutation process in the DCCC is decomposed into a current-limiting process and a forced commutation process. During the current-limiting process, the damping module transfers the valve current to the auxiliary branch, and the i -IGCTs actively block the main branch in the zero-current state. When the current reaches the turn-off value of the v -IGCTs, the forced commutation process is initiated, and the v -IGCTs actively turn off the current to complete the commutation. Operation strategy for DCCC and parameters for damping and snubber circuits are devised based on the preceding work modes. Simulations show that the DCCC can effectively enhance the CF mitigation capacity of the system, with the switching device maintaining reasonable electrical stress.