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.

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.

This article proposes a novel inertia emulation and fast frequency-droop (IEFF) control strategy for a point-to-point voltage source converter-based high voltage direct current (VSC-HVdc) transmission system. It allows asynchronous ac grid synchronization in forms of inertial and fast frequency-droop responses provided to “both” sides of the VSC-HVdc interconnected grids, different from the historically reported schemes only providing these responses to one grid. Under the IEFF scheme, the power-regulating VSC (PR-VSC) station emulates a synchronous generator directly through its active power control, whereas the dc-voltage-regulating VSC (DR-VSC) station employs a dc voltage/frequency droop control to convey its connected grid frequency communication-free to the remote PR-VSC for inertial and fast frequency-droop oriented power variations. To avoid the clash of the PR-VSC IEFF and DR-VSC IEFF control under simultaneous frequency events in the asynchronous grids, a novel frequency support mode selection scheme is designed to effectively prioritize the IEFF support with a higher frequency gradient. The key parameters of the IEFF controller are optimized through small signal stability analysis. The effectiveness of the proposed IEFF scheme is verified through controller hardware-in-the-loop experiments. The results show that the proposed IEFF scheme effectively promotes synchronization and enhances overall frequency stability for the two interconnected asynchronous grids.

This paper investigates the stability of diode rectifier based HVDC (DR-HVDC) connected offshore wind farms. The impedance model of the DR-HVDC connected offshore wind farms in dq frame is analytically developed and is validated by comparing its impedances to those obtained from frequency scanning of time domain EMT model in PSCAD/EMTDC. Based on the impedance model, the impacts of the sizes of DR-HVDC DC smoothing reactors and AC filters on offshore system stability are analyzed. It shows reduced DC smoothing reactance degrades the d-axis stability margin and can potentially lead to system instability. Higher bandwidth of offshore AC voltage control of wind turbine line side converter is proposed to minimize such instability potential. The analysis also shows that reduced AC filters size helps to increase q-axis stability margin and leads to a more stable system. Moreover, stability analysis considering the interaction between WTs is conducted and it reveals that the WT P-V control has a significant impact on the stability of the DR-HVDC connected WTs.