Stability Analysis And Optimized Design Of Multiple Parallel-connected Powerelectronic-based Converters In Weak Grid

As interface units for distributed generations such as wind energy and solar energy and power conversion interface for DC powered load,power-electronic-based converters have high penetration rate in modern power system,which poses a huge threat to the stability of the system.The power-electronic-based converter include current-controlled voltage-source converters(VSCs)based on traditional double closed-loop control and voltage-controlled VSCs based on droop control.In order to ensure the stability of the system with multiple converters connected in parallel,this dissertation analyzes the stability of the system under different application backgrounds and control strategies,and proposes the corresponding design method of the key-parameters for control system of the converter and the method for improving the stability of the system.The research contents include the design of the keyparameters for the traditional current controlled VSCs,power transmission limit of the cascaded converter system and methods for improving the stability of the cascaded system,the mechanism and suppression of the high-frequency oscillation caused by the virtual inductance in droop-controlled converters,and the power-distribution streategy when connecting multiple converters in parallel and its stability.In order to ensure the stability of the rectifiers connecting to weak grids,considering the influence of the inductive grid impedance,a stability-margin-oriented parameter-design method for the dc voltage control loop is proposed.Firstly,the proportional gain of the dc voltage controller is designed based on Routh criterion to obtain the required gain margin.Secondly,a method for designing the grid voltage feedforward gain based on the singular value of the impedance ratio is proposed to improve the phase margin of the system.Then,the stability margin of the weak-grid system with multiple rectifiers connected is studied,and the serious shortage of phase margin is found.The efficiency of the proposed parameter-design method is analyzed when connecting multiple rectifiers to the weak current grid with different short circuit ratios.In order to facilitate the use in practice and eliminate the influence of the number of rectifiers on the design parameters,a conservative range for the proportional gain of the dc voltage controller is proposed.Finally,the dynamic response of dc voltage of rectifier with the designed parameters under the grid voltage disturbances and dc load disturbance is discussed and the stability of the load power feedforward algorithm for improving the dynamic response of the dc voltage when applied to rectifier connected to weak grids is verified.In order to suppress the harmonic-oscillation caused by the phase-locked-loop(PLL)of the inverters in the weak grid,considering the influence of the inductive grid impedance,a stability-margin-oriented key-parameters design-method for inverters for is proposed.Firstly,the output impedance model of the inverter considering the influence of the grid voltage feed-forward gain and the PLL is established.Focusing on the system with multiple inverters connected to weak grids,the proportional gain of PLL controller is designed based on Routh criterion to ensure that the system has the required gain margin.Secondly,the optimal grid voltage feedforward gain is designed by using the singular value of the impedance matrix ratio.Then,the effectiveness of the designed parameter on guarantee the stabiltity margin of the weak-grid system with multiple inverters connected is verified.In order to facilitate the use in practice and eliminate the influence of the number of inverters on the designed parameters,a conservative range for the proportional gain of the PLL controller is proposed based on the use of the designed grid voltage feedforward gain.Finally,a virtual capacitance algorithm is proposed to improve the power quality of the output current in the transient process for thee inverter in the very weak grid.The superiority of the proposed method is verified by comparing with the traditional method that uses the improved PLL.Stability of the cascaded voltage-controlled VSCs and the current-controlled VSCs is studied considering the impacts of the PLL of the inverter and the dc voltage control loop of the rectifier.Firstly,by using the complex torque coefficient method,it is found that when the voltage-controlled VSCs adopts the traditional voltage-current double-loop control strategy,the PLL of the inverter and the DC voltage closed-loop of the rectifier will limit the power transmission capacity of the system and the upper limit value is derived theoretically.Secondly,based on the impedance analysis method and Nyquist criterion,the correctness of the upper limit value of the power transmission derived in this paper is verified and the influence of other control parameters on the upper limit value is analyzed.Finally,in order to break through the maximum power transmission limit of the cascaded system and comprehensively,considering the fault-current-limiting ability of the voltage-controlled VSCs,an improved voltage control method for the voltage-controlled VSCs is proposed.The effectiveness of the proposed voltage control method in enhancing the stability of the cascaded system and improving the power transmission ability of the system are verified with the experimental and simulation results.The influence of virtual inductance on the high-frequency stability of the LCL filtered droop-controlled converter is studied.Firstly,the application of virtual inductance in improving the active power transient response of the droop-controlled converter and limiting the fault current to enhance the system fault-ride-through capability are studied.It is found that the interaction between virtual inductance and LCL filter can cause high-frequency oscillations.Then,the mathematical model of droop-controlled converter including LCL filter,virtual reactance and voltage controller is established.Based on the model,the simplified analysis method of system stability is studied when the grid-side current or inverter-side current of the LCL filter is used as the current feedback of the virtual inductance.The mechanism of high-frequency oscillation caused by virtual inductance is revealed and the application of the virtual inductance is pointed out.The stability of systems with these two currents as the feedback is different.Finally,it is found that the stability of the system will not be affected if the virtual inductive reactance value is within a certain range.Therefore,the feasible range of virtual inductance is derived and the validity of the derived virtual inductance range is verified.The reactive power sharing method and its stability for the system with multiple droop-controlled converters are studied.Firstly,the power sharing problem of droop control in traditional network structure is analyzed.It is pointed out that the change of line impedance and local load can lead to the uneven distribution of reactive power.Secondly,in order to achieve the reactive power sharing among the converters and avoid the use of complex control algorithm and communication equipments,a converter network structure and the corresponding power allocation strategy are proposed by dividing the converters into energy routers and energy servers.Power control strategies are designed for the energy routers and energy servers respectively.The influence of controller parameters on system stability is studied.Finally,the reliability of the proposed converter-based power grid structure and the corresponding control strategy under different operating conditions is verified by the simulation and experimental results.There are 134 pictures,12 tables and 210 references in this dissertation.