When a single-line-to-ground (SLG) fault occurs on the ac side of the modular multilevel converter (MMC) in an high-voltage direct-current transmission (HVDC) system, it results in the ac-side voltage sag and leads to an instantaneous reduction of the MMC power capacity. Thus, it calls for the fault ride through (FRT) strategy to coordinate two MMC stations in the HVDC system to protect the MMCs against the submodule (SM) capacitor overvoltage in case of the SLG fault. In the meantime, the HVDC system is expected to track the prefault active power as much as possible during the FRT to secure the power system stability. In this paper, a comprehensive FRT strategy is proposed, which is free of interstation communication. The proposed FRT strategy presents fast dynamic response, and it can prevent effectively the MMC SM-capacitor overvoltage and HVDC transmission line overvoltage. Moreover, the second-order voltage and current fluctuations in the HVDC transmission line caused by the grid unbalance are inherently avoided. Validity of the proposed strategy and its superiority over existing methods are demonstrated by simulation of a 200-kV, 400-MW cable-based HVDC system.

A hybrid modular multilevel converter (MMC) consists of both Half-Bridge Submodule(HBSM)s and Full-Bridge Submodule(FBSM)s in each arm. It has several advantages over other MMC topologies, such as low system loss, reduced cost of the system, but still maintains fault ride-through capability against DC side fault. However, the hybrid MMC may have a difficulty of submodule (SM) voltage balancing under variation of its DC bus voltage. Since the voltage unbalance of SMs results in SM capacitor overvoltage and low reliability of the system, the voltage of SMs, which can be also represented as energy of SMs, should be balanced at all time. In this paper, to extend the narrow adjustable range of DC bus voltage due to the voltage/energy unbalance, new feedback and feedforward energy balancing controller with revised sorting algorithm is proposed. The feedback control ensures the zero steady state error, while the feedforward control provides fast dynamic response in the energy balancing control. By applying the proposed control scheme, the hybrid MMC can keep the voltage balance between HBSMs and FBSMs in wider range of DC bus voltage. The full-scale computer simulation results and down-scale experimental results demonstrate the validity of the proposed control scheme.

This letter presents a novel method to enhance the transient stability and grid forming (GFM) capability under current limiting conditions. The proposed method employs dual-loop vector voltage control with a back-calculation anti-windup strategy, using adjustable gain to modify the reactance-to-resistance ratio of the equivalent virtual impedance. The method ensures seamless operation during transitions between normal and abnormal grid conditions while significantly improving transient stability and GFM capability. Theoretical analysis has been validated through 2-kVA-scale experiments, demonstrating the effectiveness of the proposed method.

With the strengthening of international environmental regulations, many studies on the integrated electric propulsion systems applicable to eco-friendly ship are being conducted. However, few studies have been performed to establish a guide line for the overall pure electric propulsion ship design. Therefore, this paper introduces the comprehensive design of DC shipboard power system for pure electric propulsion ship based on battery energy storage system (BESS). To design and configure the pure electric propulsion ship, 2 MW propulsion car ferry was assumed and adopted to be the target vessel in this paper. In order to design the overall system, a series of design processes, such as the decision of the ship operation profile, BESS capacity selection, configuration of the power conversion systems for propulsion, battery charging/discharging procedures, classification of system operation modes, and analysis of the efficiency, were considered. The proposed efficient design and analysis of the pure electric propulsion ship was qualitatively and quantitatively validated by MATLAB Simulink tool. The methodology presented in this paper can help design real ships before the system commissioning.

This letter presents a new phase-locked loop (PLL) control method for enhancing the synchronization stability of grid-following voltage-source converters in very weak grid conditions. This can be conveniently implemented without additional efforts on hardware and retains the existing PLL structure and current controller parameters. The proposed PLL method is competitive and effective for stability in emerging power systems with a high penetration of inverter-based resources. The effectiveness of the proposed method is confirmed through mathematical proofs and down-scaled experiments.

Leg capacitor energy balancing control is one of the crucial issues for stable operation of a cascaded H-bridge (CHB) converter. Because this topology inherently consists of numerous submodule cells with DC capacitors, the cell voltages and leg capacitor energy instantaneously fluctuate depending on operation sequence of the CHB converter. In general, a wye-connected CHB-converter-based static synchronous compensator (STATCOM) utilizes a zero-sequence voltage component for leg capacitor energy balancing. In this paper, to improve the dynamics of leg energy balancing control, a feedforward calculation method of the zero-sequence voltage injection is proposed. The feedforward term can be instantaneously calculated by using the information from the measured leg voltages and leg currents, and the method ensures successful regulation of the leg energy balance even under unbalanced grid and load conditions. Moreover, the verification of the proposed method is supported by the mathematical vector theorems. A 50MVA full-scale wye-connected CHB–STATCOM system simulation was performed to verify the proposed feedforward calculation method considering unbalanced grid as well as unbalanced load conditions.