Research On Self-Synchronous Control Technology In Offshore Wind Power Integration Via High-Voltage DC Transmission

The use of high voltage DC transmission is the main way of connecting distant offshore wind power to the onshore grid.The phase locked loop-based current control strategy used in the normal operation of wind turbine grid-side converter(GSC)and voltage source converter-based high voltage DC transmission(VSC-HVDC)inverter is not capable of forming AC voltage.This control strategy may fail when the converter is connected to a weak grid or a passive system.The self-synchronous control technology does not depend on the grid-side AC voltage phase signal and ensures that the converter can be controlled under the special operating conditions,thus contributing to the safety and stability of offshore wind power integration via highvoltage DC transmission.Therefore,this paper focuses on the self-synchronous control technology for fully rated converter-based wind turbine(FRCWT)GSC and VSCHVDC inverter for two types of HVDC: the widely adopted VSC-HVDC technology and the hybrid HVDC topology with uncontrolled diode rectifier unit(DRU)and VSCbased inverter.The content of the paper mainly includes:(1)The principles and control architectures of existing voltage-source converter self-synchronous control technologies are reviewed.They are divided into active power-based,DC voltage-based,reactive power-based and integrated types according to the self-synchronous principle,and into droop control-based,virtual synchronous control-based,PID control-based and modern control technology-based types according to the self-synchronous controller structure.(2)Due to the high cost of additional equipment on offshore platforms,reactive power compensation for VSC-HVDC integrated offshore wind farms needs to be reasonably configured to improve power quality and restraining overvoltage.For the FRCWT-based offshore wind farms integration via VSC-HVDC transmission,the reactive power characteristics of the offshore AC system during steady-state operation are analyzed.The mechanism of overvoltage generation in the event of the fault in the offshore AC system is studied.To solve the problem of overvoltage in the offshore AC system caused by the disconnection of the high-voltage AC cable connecting the wind farm to the VSC-HCDC rectifier station,a overvoltage restraining strategy using selfsynchronous control in FRCWT-GSC is proposed,which is suitable for grid voltage imbalance and distortion conditions and gives GSC the ability to absorb reactive power controllably and actively restrain overvoltage.(3)To improve the robustness of the control strategy of the VSC-HVDC inverter station with DC voltage-base self-synchronous control when connected to the weak grid,the DC voltage-base self-synchronous control strategy with active disturbance rejection control(ADRC)is designed.This control strategy improves the stable operating range of the VSC-HVDC inverter station from the short circuit ratio(SCR)of less than 1.5 to less than 3 when compared to the PI controller.The fault on the AC side of the VSC-HVDC inverter causes the DC energy dissipation equipment to be put into operation,resulting in the failure of the DC voltage-base self-synchronous control.For this reason,a fault ride-through strategy using active power synchronous control during this fault is designed.This strategy can reduce the amount of active power overshoot by at least about 41% during the fault recovery period compared to relying on the dissipation equipment alone.(4)In order to make wind turbines connected to the grid via DRU-VSC hybrid HVDC capable of forming offshore AC voltages,the self-synchronous control strategy is used for FRCWT-GSC.A method for calculating the reactive power reference values of wind turbines to guarantee the reactive power balance of the offshore AC system is proposed.For onshore grid faults,a fault ride-through strategy combining power reduction in offshore wind farms and the energy dissipation equipment in hybrid HVDC is designed.For offshore AC system faults,a fault ride-through strategy combining adjustment of the power reference value with the energy dissipation equipment in the wind turbine is designed.In general,in order to guarantee controllability in the FRCWT-GSC and the VSCHVDC inverter when they lose AC power support or are connected to passive networks or to weak grids,this paper investigates the self-synchronous control for restraining AC overvoltage of the FRCWT-GSC,the self-synchronous control and fault ridethrough techniques for VSC-HVDC inverter stations connected to weak grids,and the self-synchronous control and fault ride-through techniques for FRCWT integration via the DRU-VSC hybrid HVDC Transmission.