Research On Plug And Play Robust Control Strategy For Micro Sources Of Distributed Microgrid

With the rapid development of distributed generation technology,the proportion of distributed power sources with power electronic inverters as interfaces is increasing,the output of conventional units is decreasing.The grid presents low inertia and underdamped operating characteristics,posing risks to the security and stability of power system.In order to improve the operation characteristics of distributed generation in power system and realize friendly grid connection,virtual synchronous generator（VSG）technology has been recognized widely.VSG control automatically adjusts the active power and reactive power output of inverter based on the grid frequency and voltage by simulating the external characteristics of synchronous generator set,so that the distributed generator has good autonomy and plug and play characteristics.Taking the distributed generators based on VSG control as the research object,the paper studies the pre-synchronization control and parameter design method of micro-source synchronverter,the accurate power distribution control strategy of distributed generators,and the grid connected operation control strategy of photovoltaic and energy storage integrated generator（PV/ESIG）.Firstly,the paper introduces the basic principle of micro-source synchronverter.Aiming at the problem of protection tripping caused by excessive frequency deviation（?f）and rate of change of frequency（Ro Co F）in the process of off/grid dual-mode dynamic regulation,the pre-synchronization control and parameter design method of micro-source synchronverter based on virtual power are proposed.By constructing the small signal model of pre-synchronization control and using the time-domain dynamic response analysis method,the pre-synchronization control parameters with?f,Ro Co F and pre-synchronization adjustment time as performance indicators are designed.The simulation and experimental results show that the micro-source synchronverter with this parameter design scheme meets the requirements of IEEE standard 154 power grid synchronization process standard and Ro Co F limit in the process of pre-synchronization and smooth grid connection.Secondly,aiming at the problems of high degree power coupling,low distribution accuracy and poor reactive power-voltage droop accuracy among multiple distributed generators caused by line impedance difference,a droop control strategy for accurate distribution of reactive power by VSG considering line impedance is proposed.According to the local information control,the impedance voltage drop of the line is compensated to ensure the consistency of the control voltage in parallel operation of distributed generators,so as to realize the accurate allocation of reactive power and the precise droop control of the voltage of PCC.Meanwhile,virtual negative impedance control is added to change the equivalent output impedance characteristics of the system to achieve power decoupling.On this basis,the decoupling control effect of the improved control strategy is quantified and analyzed using the relative gain matrix.The accuracy of reactive power distribution and droop control is theoretically derived and analyzed by establishing a small-signal model of a two-machine parallel system.The simulation and experimental results are consistent with the theoretical analysis,which verifies the effectiveness of the proposed control strategy.Finally,to address the problem that the existing control strategy is difficult to ensure that the PV/ESIG can participate in the primary frequency modulation of the power grid while realizing the seamless switching of off/grid operation mode and taking into account the high reliability of load,a grid connected operation control strategy of PV/ESIG based on VSG technology is proposed.The PV/ESIG presents the characteristics of voltage source in both off-grid and grid connected modes,and avoids the transient impact caused by the switching of control mode.At the same time,in order to improve the primary frequency modulation capability of PV/ESIG grid-connected operation,the traditional VSG active power-frequency control loop is improved by using the disturbance characteristics of DC side voltage(u_dc)to build a PI feedback control loop.It better realizes the power interaction with power grid while maintaining the u_dcconstant.The working mode of energy storage unit and inverter is adaptively selected based on the energy storage charge state and load.On this basis,the improved active power-frequency small signal model and simulation model of PV/ESIG are constructed,the dynamic regulation performance generated by different system parameters is analyzed,and the effectiveness of the proposed control strategy is verified.