This study provides a novel control method for voltage source converter high-voltage direct current (VSC-HVDC) systems with two salient features: autonomous grid-synchronisation without phase-locked loop and real-time grid frequency mirroring capability. The first feature is realised using a newly proposed control method for receiving end converter (REC) called inertial synchronising control (ISC). This method innovatively adopts the physical inertia of DC capacitors to directly synchronise with the grid, without emulating the motional equation of a synchronous generator (SG) as virtual SG control. Hence, less control loops and easy parameter tuning are achieved. The control robustness to grid impedance variation is also improved. Moreover, the second feature is realised with the merit of ISC. Owing to the fast grid-synchronisation capability of ISC, grid frequency dynamic is bound with DC voltage dynamic intrinsically; hence, the instantaneous grid frequency information can be obtained by measuring the local DC voltage of sending end converter, whereby wind turbines can proceed with inertial and frequency response. Consequently, REC is similar to SG in terms of electromechanical characteristics. The effectiveness of the proposed method is evaluated by comparing it to an existing method in PSCAD/EMTDC, and its performance on the robustness to grid impedance variation is addressed.

Recent analyses have shown that the grid-integration of offshore wind farms through MTDC systems has brought low inertia and small-signal stability issues, in which the dynamics of phase-locked-loop (PLL) play a crucial role. To address this issue, this paper proposes a control strategy for the multi-terminal VSCs aiming at PLL-less synchronization and autonomous frequency response of the MTDC system. One of the significant features of the proposed control is that the deviation of the grid frequency can be instantaneously reflected on the deviation of the DC voltage without ancillary control. Based on this feature, a fast inertia response and primary frequency regulation among wind farms and AC systems interconnected by the MTDC system can be achieved. A small-signal model is established to evaluate the overall system stability using the proposed control. Finally, comparative studies of this proposed control with the conventional PLL-based vector control are conducted in PSCAD/EMTDC based on a practical MTDC system in China, the Zhangbei fourterminal HVDC transmission system. The analysis shows that the proposed control exhibits advantages in weak grid operation and autonomous frequency response.Index Terms-frequency response, weak grid, wind farms, inertia, MMC MTDC, small signal stability analysis NOMENCLATURE Ceq Equivalent DC capacitance Udc DC voltage Udc_nom Nominal DC voltage Pdc DC side active power of receiving end converter (REC) Pac AC side active power of the REC Qac AC side reactive power of the REC Urec AC voltage of REC ωrec AC frequency of REC ωnom Nominal frequency J Moment of inertia ωm Rotor speed of synchronous generator (SG) Pm Mechanical power input of SG ωe AC frequency of SG Pe Active power output of SG

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Modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission is becoming a trend in offshore wind-farm integration. However, the large DC-side energy dissipation equipment, which is utilized to dissipate surplus wind power under grid fault conditions, will largely increase the cost of MMC-HVDC systems. To reduce the cost, a novel low voltage ride-through (LVRT) strategy is proposed in this paper. When a grid fault occurs and the DC voltage exceeds the limit, the sending end converter is controlled to reduce the AC voltage of the wind farm. The LVRT of the wind generators will be activated, and the output active power of the wind farm is reduced. With this coordination, the DC-side energy dissipation equipment only needs to dissipate surplus power in the early stage of the grid fault before the output active power of the wind farm drops. Therefore, the heat generated by braking resistors can be significantly reduced. On this basis, the braking resistors can be distributed into the submodules of the receiving end converter (REC) station. The LVRT problem can be solved without building an individual energy dissipation station. Using the proposed coordination strategy, the construction cost of the MMC-HVDC system with offshore wind farm integration can be significantly reduced.INDEX TERMS HVDC transmission, low voltage ride-through, modular multilevel converters, offshore wind farm.

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