Research On The Converter Of The Receiving End Station Of The Flexible Straight System Of The Offshore Wind Farm

China has a vast sea area and abundant offshore wind energy resources.Offshore wind power has become one of the key areas of wind power development in the future.HVDC-Flexible system would be the main transmission method for the offshore wind power,and HVDC-Flexible technology is also an important research direction at present.With the increasing scale of offshore wind farms,the capacity of the receiving end station converters(RESC)will inevitably increase,so it has become an inevitable choice of RESC that multiple DC/AC grid-connected converter modules operates in parallel to improve the system capacity and reliability.However,the parallel connection of multiple converter modules is easy to cause the circulating current problem,which could result in the distortion of the output current of the system,and even the system operation failure.Therefore,restraining the circulating current is an important prerequisite to ensure the stable operation of the parallel system.In addition,the grid voltage imbalance is also a challenge to the grid-connected control of RESC.To this end,this paper focuses on the grid-connected current control of high-power RESC under unbalanced grid voltage and the circulation current suppression in the parallel system of multiple grid-connected converter modules.First,for the control target of the RESC,i.e.,ensuring the voltage stability of DC grid and realizing the AC grid-connection,the topology and mathematical model of the converter module of RESC under the condition of ideal grid are studied.According to the current closed-loop control strategy based on the grid voltage orientation,the simulation software MATLAB/Simulink is used to build a simulation model of the converter module of RESC,which realizes the control target of RESC mentioned above.Then an experimental prototype is developed and an experimental platform is built.The results of off-grid and grid-connected experiments show that the control goal is achieved,and the effectiveness of the control strategy is proved.Second,the definition of the circulating current is given according to the topology structure of the parallel system with multiple converter modules and Kirchhoff's law,and the generation mechanism of the circulating current is analyzed and studied.Taking the parallel connection of two converter modules of RESC as an example,the voltage equation of zero-sequence circulating current is derived,and the influence of the output filter impedance on the circulating current is theoretically deduced and analyzed;then the circulating current control strategy based on the zero vector adjustment factor is proposed to suppress the circulating current.In order to verify the effectiveness of the proposed circulation control strategy,MATLAB/Simulink is used to build a parallel simulation model of two 500 k W high-power converter modules of RESC.The simulation analyses without circulation control and with circulation control are carried out respectively for the four working conditions with different output filter impedances.The results show that the suppression effect of the zero-sequence circulating current is obvious,and the fluctuation range of the zero-sequence circulating current is significantly reduced,which verifies the feasibility of the proposed control strategy.Finally,an experimental platform with two1.5k W converter modules in parallel is built to verify the effectiveness of the proposed zero-sequence circulating current control strategy.Finally,the output voltage vector and current vector of RESC under the unbalanced grid voltage condition are analyzed.According to the control principle of proportional reduced order resonance(P-ROR),a current control strategy based on P-ROR is proposed to construct the control branch circuit for the fundamental wave and harmonics in the output current,and realize the tracking control of the fundamental wave and the 3rd,5th,and 7th harmonics of the output current of RESC.The simulation analysis based on MATLAB/Simulink shows that the control strategy can ensure the stable grid-connected current output by RESC under the unbalanced grid voltage condition.