Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) connected with offshore wind farms could suffer serious challenge: surplus power of MMC-HVDC caused by AC fault in onshore power grid. This paper proposes an active energy control (AEC) scheme for converter stations utilizing MMC sub-module capacitance. It can be used to actively absorb the surplus power of the MMC-HVDC in the stage of grid-side AC fault and enhance the AC fault ride-through (ACFRT) ability. First, the energy decoupling principle of MMC is analyzed, and a steady-state control scheme is proposed for MMC-HVDC with energy decoupling capability. Based on this strategy, an AEC scheme between onshore and offshore converter stations is proposed for ACFRT. It is divided into the following four stages: passive energy recovery, active energy recovery, active energy maintenance, and active energy release. Then, a set of coordinated control schemes for AEC and active power reduction of offshore wind farms are designed. This is used as an example to illustrate that the proposed AEC can effectively enhance the fault ride-through ability of the existing strategies, such as active power reduction of offshore wind farms. Finally, the correctness and effectiveness of the proposed control schemes are verified via the PSCAD/EMTDC platform.

The valve-side ground fault in the converter station of modular multilevel converter based high voltage direct current (MMC-HVDC) is a serious threat to the security of power systems. Hence, this paper analyzes the most common single-phase-to-ground (SPG) fault characteristics of valve-side faults in onshore converter stations of MMC-HVDC integrated offshore wind farms. The propagation and impact mechanism of onshore SPG fault to offshore HVDC transmission lines, offshore converter station, and wind farms are deduced based on the MMC control principle. Furthermore, an overvoltage suppression strategy based on zero-sequence voltage modulation is proposed. The effect mechanism of this strategy on the transient stability of the system is analyzed. The key parameter of the proposed strategy is also designed considering both the practical engineering requirements and the impact of the system operation. Finally, the theoretical analysis of fault characteristics and the proposed overvoltage suppression strategy are verified by the PSCAD/EMTDC simulation platform. Simulation results not only show good agreement with the theoretical analysis of the fault characteristics, but also prove the effectiveness of the proposed overvoltage suppression strategy.

Onshore AC faults may cause the modular multilevel converter based high voltage direct current integrated offshore wind farms (MMC-HVDC-OWF) to develop surplus power and trigger overvoltage problems. This paper proposes a dynamic reactive current optimization (DRO) strategy that combines MMC energy control capability with transient overload capability to achieve onshore AC fault ride-through (ACFRT). Firstly, the decoupling control mechanism and constraints of the active and reactive currents of the onshore gird-side MMC (GSMMC) are analyzed. Furthermore, a dynamic reactive current optimization based ACFRT strategy is proposed based on the current constraints and the energy control capability of MMCs. The main parameters of DRO are set according to the fault characteristic analysis results. Once a fault occurs, the proposed DRO based ACFRT strategy can control the offshore surplus power by controlling the MMC internal sub-module energy. The DRO strategy quickly optimizes the active and reactive power output commands of the GSMMC by collecting and comparing the local power signals. It enables to increase the onshore AC grid voltage and the transient active power output of the MMC-HVDC-OWF. Finally, The effectiveness of the proposed control strategy is verified through comprehensive case studies conducted on the PSCAD/EMTDC.

The frequency support capability of each wind turbine (WT) within a wind farm varies based on factors such as location, wind direction, operating conditions, and wake effect. To fully utilize the frequency support capability of a wind farm, output power should be allocated to each WT based on its individual states. This article proposes a distributed adaptive leaderless consensus control (DALC) scheme for wind farms to achieve fast frequency support. The leaderless consensus algorithm is utilized among WTs to exchange operating information with neighbors and ensure reasonable power allocation. Additionally, frequency regulation coefficients adaptively change to ensure safe WT operation. After the frequency support stage, a preset recovery control is proposed to alleviate the second frequency drop (SFD) during the restoration stage. Case studies are conducted based on a two-area power system using MATLAB/Simulink and opal-RT real-time simulation platforms. Simulation results demonstrate the effectiveness and universality of the proposed DALC control scheme in both over-frequency and under-frequency events. Furthermore, the proposed control is found to be more reliable than leader-follower consensus-based control in the event of communication failure.

To cope with onshore AC faults in modular multilevel converter based high voltage direct current integrated offshore wind farms (OWF-MMC-HVDC), this paper proposes a multi-mode matching control (MMMC) approach. Mode I is active energy control. It is activated at high fault levels, when the reactive power output capability of the MMC is weak. Mode I uses the MMC sub-module energy to maximise the absorption of offshore surplus power to mitigate DC overvoltage problems. Mode II is dynamic reactive current optimization (DRO) control. It is activated at medium fault levels. It dynamically optimises the MMC active and reactive current command values within the permitted current margins. This simultaneously enables voltage support to the onshore grid and maximises offshore active power output, reducing offshore surplus power. Mode III is energy coordinated synchronous (ECS) control. It is activated under more mild faults. It allows simultaneous adjustment of the energy absorption rate of the wind farm-side MMC (WFMMC) and the grid-side MMC (GSMMC). This allows the energy margin of the MMCs to be utilised more fully to improve support to the onshore grid while maintaining fault ride-through capability. Finally the correctness and validity of the proposed method was verified by the PSCAD/EMTDC.