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This paper proposes an adaptive fault current limiting control (AFCLC) for modular multilevel converters (MMC). Without introducing extra current limiting devices, this control scheme enables fast fault current suppression during DC faults. The AFCLC will be triggered automatically once DC faults occur. By adaptively reducing the output DC voltages of MMCs, the fault current can be suppressed. Compared with the existing current limiting methods, the proposed AFCLC has a better performance on fault current limiting, since it only depends on the real-time operating condition and no fault detection delay is imposed. Firstly, the principle of the proposed AFCLC together with the mathematical analysis is disclosed. Then, the sensitivity analysis of the impact of key control parameters on the current limiting effect is investigated. Finally, the effectiveness of AFCLC is demonstrated in a four-terminal HVDC grid test model. The simulation results show that the proposed AFCLC can reduce the interrupted current and energy absorption of a DCCB from 10.39 kA and 38.24 MJ to 4.62 kA and 8.32 MJ, respectively. The simulation results also prove that the AFCLC will not affect the accuracy of the DC fault detection algorithms under DC faults. [ABSTRACT FROM AUTHOR]

The hybrid energy storage system (HESS) plays an essential role in the shipboard medium-voltage dc (MVdc) system to provide backup power, buffer large load change, as well as improve power quality. In this paper, a novel virtual resistor and capacitor droop (VRCD) control is first proposed for HESS including battery, supercapacitor, and flywheel. It enables the generator and multiple types of energy storages (ESs) with different characteristics to respond to multiple-frequency unbalanced power instead of simple high-frequency and low-frequency power sharing by the conventional droop control method. A detailed design procedure for the VRCD control is presented. Also, the secondary state of charge recovery control is proposed to maintain high energy reserve of ESs for further power demand. Simulations are conducted to demonstrate the effectiveness of the control scheme proposed in a shipboard MVdc under particular loads fluctuation, such as the propulsion motor and the pulsed load. [ABSTRACT FROM AUTHOR]