Research On Multi-port DC Power Flow Controller

Multi-terminal high voltage direct current(MTDC)transmission is developed from the traditional high-voltage direct current transmission.It has more economical and flexible operation mode benefits from its multi-power supply and multi-drop-point power reception.Therefore,the MTDC offers a huge opportunity in solving large-capacity energy long-distance transmission and clean energy grid connection.Due to the relatively large number of converter stations and transmission lines in the MTDC transmission system,only relying on the coordinated control between converter stations cannot achieve the comprehensive system power flow control.This will cause unnecessary line loss or even line overload,which may endanger the safe and stable operation of the entire system.The direct current power flow controller(DCPFC)has become the most attractive method to solve the power flow control of DC transmission system.To this end,this study takes M-DCPFC as the research object,and conducts from three different levels of its basic topology design,control function extension and application technology extension.(1)For applications where no power flow reversal is required,a half-bridge module based inter-line M-DCPDC is proposed.It has fewer number of switching devices,and the half-bridge-based modular structure makes the port redundancy and expansion easier and more economical.In order to solve the problem of insufficient control freedom of this shared capacitor type M-DCPFC,a control strategy with phase-shift modulation technology is designed,which realizes completely independent power flow control of different transmission lines.In addition,the relationship between the state variables of the system is discussed in detail and the application characteristics of the proposed topology are further analyzed.Finally,simulations and experiments are carried out under different test conditions.The results show the effectiveness of the proposed topology.(2)For applications where power flow reversal is required,an output decoupling type M-DCPFC is proposed,which retains the advantages of modular design,easy port expansion and no auxiliary power supply.What is more,it fundamentally solves the problem of control coupling between different transmission lines by adding a partial power auxiliary converter,thereby achieving a higher degree of power control freedom.The corresponding control strategy is designed,and by establishing the equivalent circuit and related equations of the M-DCPFC connected transmission system,the power flow regulation characteristics of M-DCPFC and its influence on the overall transmission system operation are analyzed in detail.In addition,combining the power flow regulation requirements of the system,the device capacity estimation method is given.Finally,the effectiveness of the proposed topology is verified in the simulation and experment under different conditions.(3)Considering the influence of abnormal operating conditions of the transmissio n system,a M-DCPFC control technology with two active damping compensation mechanisms in series and parallel is proposed.While ensuring the normal power flow control,it can also assist in the realization of power transmission system stability control.The basic principle of the M-DCPFC active damping control is analyzed in detail,and its general control principle is promoted on this basis.Then,for different damping control mechanisms,corresponding control strategies are designed.Finally,combining the two typical abnormal operating conditions of asymmetric AC grid voltage and sub-synchronous oscillation in the transmission system,the application method and control stability of the proposed active damping control are analyzed in detail.The detailed simulation mode is developed in a real time test system based on RT-LAB,and the effectiveness of the proposed active damping control is validated.(4)Considering the application advantages of M-DCPFC in solving the power flow control and stability control of ring-type MTDC transmission systems,the generalization of the proposed M-DCPFC for other special type transmission systems is further discussed.Combining the low-cost advantages of diode-rectifier unit based MTDC(DRU-MTDC)for offshore wind power grid connection,an application solution of M-DCPFC interconnection DRU-MTDC offshore wind power transmission system is presented.This solution can not only easily and effectively realize the power transmission management of different wind farms,reducing the difficulty of power coordinated control between various wind farms,but also assist in suppressing the current ripple on the DRU ouput side.With the basic control analysis of the DRU-MTDC offshore wind power transmission system,the existing technical challenges are pointed out.Then,the topology of the M-DCPFC interconnection DRU-MTDC offshore wind power transmission system and the participating functions of M-DCPFC are analyzed in detail.Meanwhile,the related system control strategies are designed.Finally,simulation mode is developed in a real time test system based on RT-LAB,and the proposed scheme is validated.The results show its technical feasibility.