Research On Frequency Support And DC Voltage Regulation Schemes For VSC-MTDC System

The total electricity installation and power generation of renewable energy in China have expanded year by year as the "dual carbon" goal has been gradually promoted.A number of high voltage direct current(HVDC)projects have been put into service to address the need for electricity transmission.And the modular multilevel converter(MMC)or voltage source converter based multi terminal high voltage direct current(VSC-MTDC)has been widely employed in the field of sustainable energy transmission due to its advantages of eliminating the need for auxiliary commutation of the power grid and providing variable operation modes.Renewable energy units,HVDC,and other asynchronous power sources,on the other hand,are all connected to the power grid via power electronic converters and cannot participate in the electromechanical coupling dominated by the synchronous machine rotor automatically.Traditional control systems are incapable of responding to variations in grid frequency.The replacement of synchronous power sources on a large scale with renewable energy resources and HVDC reduces the system’s inertia level and primary frequency regulation capability,resulting in a sharp deterioration in the frequency-inertia security status of the high-proportion new energy power system.To enhance the frequency security of new-type power systems,the development of the inertia control capability of sustainable energy and MTDC systems to offer active frequency support has become the top priority.The subject of active frequency support for VSC-MTDC systems is investigated in this work.Firstly,a general frequency response model of the MTDC system is established to serve as a foundation for analyzing the system’s frequency operation state and stability characteristics.Then a coordinated frequency control strategy for the VSC-MTDC system connected to conventional AC power sources is designed.Moreover,the application scenario is extended to the VSC-MTDC system connected to the AC grids and wind farms at the same time,and a zoning and hierarchical frequency support approach is provided based on the aforementioned frequency control.Finally,a DC voltage control approach is investigated in order to address the DC voltage deviation issue associated with the VSC-MTDC system that participates in frequency regulation.The specific research contents of this paper are summarized as follows:A general frequency response model of MMC-MTDC system integrated with wind farms is established to address the problem that existing small signal models of wind farms,AC grids,and MTDC system cannot intuitively and effectively analyze the dynamic characteristics of system frequency-active power-DC voltage.Hence,taking advantage of the decoupling properties of the AC and DC sides of the MMC average model,the AC side of the grid side voltage sourced converter(GSVSC)and the generators are individually modeled.Considering the losses of back-to-back converters,AC lines,and wind farm voltage sourced converters(WFVSC),the intermediate links of permanent magnet direct drive wind turbines(PMSG)and WFVSC are simplified according to the power transfer law.Based on the equivalent models of GSVSC and WFVSC,the DC lines are employed as the interactive ports,and the small signal model of active power components of the MMC-MTDC system integrated with wind farms is established by the modular modeling approach.The simulation results show that the general frequency response model can accurately simulate the dynamic process after a small frequency disturbance,making it a useful tool for developing frequency control strategies and studying the stability of VSC-MTDC systems.In order to address the issue of frequency control’s insufficient flexibility,the key influencing factors of DC voltage deviation and unbalanced power distribution ratio caused by frequency disturbance are quantitatively investigated,and a frequency support strategy for the VSC-MTDC system oriented to conventional AC power sources is proposed.A dynamic additional frequency control strategy for the VSC-MTDC system is developed.The AC frequency coefficient and DC power coefficient are intended to define the different frequency support needs of the disturbed AC system and DC grid,respectively,and the transition factor is utilized to actualize the combination of the two.A dynamic frequency droop coefficient is designed to adaptively adjust the support power of the DC grid,balancing the frequency regulation of the disturbed AC system with the voltage stability of the DC grid.Besides,an adaptive active power distribution method for the VSC-MTDC system is proposed.The frequency deviation of AC systems and power margin of VSC are introduced into the DC voltage droop coefficient to dynamically adjust the unbalanced power distribution ratio and improve the frequency stability of weak AC systems.The simulation research shows that the collaborative frequency support strategy can automatically adjust the frequency support intensity of VSC based on the severity of power disturbances,achieve a reasonable allocation of unbalanced power in combination with system operating conditions,and provide a control basis for the construction of the frequency regulation framework of the VSC-MTDC system integrated with wind farms.A coordinated frequency control strategy for VSC-MTDC systems integrated with wind farms is proposed to address the lack of coordination between wind farms,AC grids and the VSC-MTDC system during frequency regulation,as well as the lack of means for wind turbines to cope with wind speed changes during frequency support and restoration stages.A novel approach for evaluating the frequency regulation capability of wind turbines is proposed that takes into account both the kinetic energy margin of the wind turbine and the power margin of the converters.And an adaptive frequency regulation strategy for PMSG is designed,which considers system frequency evolution and the real-time frequency regulation capability of the wind turbines.A power adaptation method for variable wind speed is proposed to smoothly transition the power reference during the frequency support stage and restoration stage.A zoning and hierarchical frequency regulation framework for wind farms and VSCs is constructed,and the access principles of each link are designed to weaken multiple frequency dips while ensuring frequency support performance.The simulation results show that the novel frequency responsive strategy can maintain frequency stability after the wind speed changes and avoid multiple frequency dips.Amid the DC voltage deviation caused by the unbalanced active power distribution,a cooperative optimal droop control strategy of the VSC-MTDC system with DC voltage "quasi non-error" regulation ability is proposed based on research on the infeasibility of DC voltage non-error recovery under droop control.An additional DC voltage controller is designed for analog synchronous machine damping and inertia and automatically adjusts the active power reference value to improve the DC voltage during dynamic processes.In order to minimize the variation of VSC active power and bring the DC voltage closest to the initial value,the reference value increment of droop control is optimized,and the steady-state DC voltage recovery is performed in conjunction with power smoothing technique.The control action sequence is proposed to coordinate the control logic in the dynamic and steady processes to deal with complex situations such as continuous power fluctuation and secondary power drop during DC voltage regulation.The simulation results show that the modified strategy can smoothly perform the global optimal regulation of DC voltage at the cost of the minimum power variation of multiple VSCs and complete the ’quasi non-error’ recovery of DC voltage.