Research On Additional Frequency Adaptive Droop Control Of VSC-MTDC System Considering AC Side Operation State And DC Voltage Stability

The reverse distribution of the energy and the load in China can be effectively solved by HVDC power transmission.Compared with the two terminal HVDC,the modular multilevel converter multi terminal DC(MMC-MTDC)is more cost-effective and can flexibly control the power flow.In the HVDC system,the voltage source converter can adjust the output power quickly when the AC power grid is disturbed.Therefore,with the increase of new energy generation and the decrease of the overall inertia of the grid year by year,MMC-MTDC can provide strong frequency support for the AC system.Initially,in order to solve the problems of large frequency fluctuation and high load rate of the AC system in the process of DC system participating in AC system frequency regulation,an additional frequency adaptive droop control strategy of MMC-MTDC considering AC side operation state is proposed and the stability of the control system is analyzed.The droop coefficient of the converter station is adjusted according to the frequency fluctuation and the load rate of each AC system,so that the unbalanced power can be distributed more flexibly and reasonably in each interconnected AC system without changing the structure of the controller,and the excessive deviation of the frequency of AC power system can be restrained to prevent the AC system from fully loaded.Furthermore,in view of the problem that the DC voltage fluctuates greatly when DC system participates in frequency regulation of AC system,an additional frequency adaptive droop control strategy of VSC-MTDC considering DC voltage stability is proposed.Firstly,the active power of DC side and the system frequency of AC side are coupled by virtual inertia technology and the expression of adaptive frequency droop coefficient is derived.Then,the coupling relationship between the DC voltage and the active power is established,and the adaptive voltage droop control strategy is obtained.Finally,the stability of the control system is analyzed.This method has no communication between the converter stations and reduces the overshoot of DC voltage and improves the voltage quality and the stability of the interconnected system.Finally,the simulation models of three-terminal HVDC system and four-terminal HVDC system are built based on PSCAD/EMTDC software.In the three-terminal HVDC transmission system,the additional frequency constant coefficient droop control and the additional frequency adaptive droop control proposed in this paper are compared and simulated.In the four terminal HVDC transmission system,the additional frequency constant frequency droop control and the adaptive frequency droop control strategy considering DC voltage stability are compared and verified by the simulation.The simulation results verify the feasibility and effectiveness of the proposed control strategy.