A distributed PLL-based frequency control is proposed in this paper for offshore wind turbine converters connected with diode-rectifier based high-voltage-direct-current (HVDC) systems. The proposed control enables a large number of wind turbines to work autonomously to contribute to the offshore AC frequency and voltage regulation. The proposed control also provides automatic synchronization of the offline wind turbines to the offshore AC grid. Stability of the proposed frequency control is analyzed using root locus method. Moreover, an active DC voltage control of the onshore modular multilevel converter (MMC) is proposed to ride-through onshore AC fault, where the onshore MMC converter quickly increases the DC voltage by adding additional submodules in each phase, in order to rapidly reduce wind farm active power generation so as to achieve quick active power re-balance between the offshore and onshore sides. Thus the overvoltage of the submodule capacitor is alleviated during the onshore fault, reducing the possibility of system disconnection. Simulation results in PSCAD verify the proposed control strategy during start-up, synchronization and under onshore fault conditions.

This paper presents a complete review of MVDC applications and their required technologies. Four main MVDC applications were investigated: rail, shipboard systems, distribution grids, and offshore collection systems. For each application, the voltage and power levels, grid structures, converter topologies, and protection and control structure were reviewed. Case studies of the varying applications as well as the literature were analyzed to ascertain the common trends and to review suggested future topologies. For rail, ship, and distribution systems, the technology and ability to implement MVDC grids is available, and there are already a number of case studies. Offshore wind collection systems, however, are yet able to be implemented. Across the four applications, the MVDC voltages ranged from 5–50 kV DC and tens of MW, with some papers suggesting an upper limit of 100 kV DC and hundreds of MV for distribution networks and offshore wind farm applications. This enables the use of varying technologies at both the lower and high voltage ranges, giving flexibility in the choice of topology that is required required.

Offshore AC fault protection of wind turbines (WTs) connecting with diode rectifier unit based HVDC (DRU-HVDC) system is investigated in this paper. A voltage-error-dependent fault current injection is proposed to regulate the WT current during offshore AC fault transients and quickly provide fault current for fault detection. Considering different fault locations, the fault characteristics during symmetrical and asymmetrical faults are presented and the requirements for fault detection are addressed. A simple and effective offshore AC fault protection solution, combining both overcurrent protection and differential protection, is proposed by utilizing the developed fast fault current providing control. To improve system availability, reduced DC voltage of the DRU-HVDC system is investigated, where one of the series-connected DRUs is disconnected and the onshore modular multilevel converter (MMC) actively reduces DC voltage to resume wind power transmission. The proposed scheme is robust to various offshore AC faults and can automatically restore normal operation. Simulation results confirm the proposed fault protection strategy.

The adoptions of medium-voltage in AC collection networks of large DC connected wind farms significantly increase the AC current magnitudes during normal and fault conditions. Controlling fault currents at zero during asymmetric AC faults is possible, but it has several drawbacks such as increased risk of protection mal-operation due to absence of fault currents, which also tends to prevent the recovery of AC voltage in post-fault. Therefore, this paper presents an enhanced control strategy that exploits the induced negative sequence voltages to facilitate controlled injection of negative sequence currents during asymmetric AC faults. The proposed control not only defines a safe level of fault current in the offshore AC network but also instigates immediate recovery of the AC voltage following clearance of AC faults, which avoid protection mal-operation. In addition, the positive sequence voltage set-point of the offshore modular multilevel converter (MMC) is actively controlled by considering the negative and zero sequence voltages, which effectively avoids the excessive overvoltage in the healthy phases during asymmetrical AC faults. The theoretical basis of the proposed control scheme is described, and its technical viability is assessed using simulations.

Whilst third harmonic injection is extensively used in modular multilevel converter (MMC) control, its significant advantages over sinusoidal modulation have not been fully explored. This study evaluates the influence of third harmonic injection on system power losses, submodule (SM) capacitance, circulating current, and fault current and mathematical models are derived. Station conduction losses are reduced by 11%, yielding higher efficiency and lowering cooling system capacity. The SM capacitance is reduced by 24%, which significantly lowers the capital cost, weight, and volume of the station converter. Additionally, the phase energy variation is reduced by around 18%, which benefits circulating current control. Due to the lower AC currents, the semiconductor current stresses are correspondingly reduced. In addition to the performance improvement in normal operation, the third harmonic injection reduces the DC fault currents by 13.4% and thus the fault current stresses on semiconductors and DC circuit breakers are lowered. Simulation of a point‐to‐point high‐voltage direct current system demonstrates the effectiveness of the above analysis.