Multi-converter Cooperative Control Based On Virtual Synchronous Generator

In order to alleviate fossil energy shortages and environmental problems,Distributed Generations(DGs)of renewable energy is taking an increasingly important role in traditional power grid.Microgrid is an important form of DGs to work cooperatively,which can realize seamless switching,active regulation and flexible operation of DGs in the power grid.The power electronic converters are important interfaces for the DGs in the microgrid and play an important role in power transmission and power quality control.Compared with traditional synchronous generators,DGs have almost no rotating inertia and can`t provide the necessary damping to the grid.The Virtual Synchronous Generator(VSG)utilizes the fast response characteristics of the power electronic converter to introduces the electromagnetic and mechanical characteristics of the synchronous generator into the control strategy of the converter,thereby improving the dynamic characteristics of microgrid and providing it with voltage and frequency support capabilities.At the same time,by designing reasonable control strategies and parameter matching methods,VSG can achieve reasonable power distribution and cooperative operation of multiple DGs,which is significant to the stability of the microgrid and the development of renewable energy DGs.Firstly,by establishing the electromagnetic model of an ideal steam turbine synchronous generator,a novel VSG control strategy is given,which enables the converter to provide the inertia and damping effects to grid like a synchronous generator,as well as power-frequency and reactive power-voltage support function.VSG realizes the capability of multi-converter cooperating and can actively participates in the voltage and frequency adjustment process of the power grid.The frequency regulation loop is designed in combination with the rotor equation which can realize active power-frequency dynamic adjustment.The excitation loop is designed to realize dynamic adjustment of reactive power and voltage,by combining with the electromagnetic model which is based on the stator windings of the synchronous generator.The virtual stator winding concept is discussed to adjust the output impedance of converters which can satisfies the decoupling condition of the output power,and can achieves accurate load distribution among multiple converters.Simultaneously,the virtual excitation compensation and secondary frequency modulation strategies are discussed to eliminate the static voltage and frequency adjustment error in island mode.Secondly,to research the output characteristic of VSC,the role of the bottom control loop in the control strategy is analyzed.Combined with the small signal model of the output side of the converter,the output model of VSG is established.A power loop parameters design method was proposed to provide sufficient stability margin and improve the performance of the converter.Subsequently,in order to analyze the cooperative operation mechanism of multi-converter,the impedance characteristics of the VSG and the role of the virtual winding were studied.According to the power and voltage characteristics of the parallel converter,combined with the active power-frequency and reactive power-voltage relationships in the control strategy,the load distribution method of the parallel converter in the islanded microgrid is analyzed.The matching principle of parameters including power loop,virtual stator impedance and line impedance of parallel converters is discussed.Through the output impedance adjustment of the converter and the power loop parameters regulation,the cooperative operation of multi-converter in island mode is realized.Finally,the correctness and effectiveness of the above cooperative control strategy and parameters matching principle under different operating modes of the converter are verified through experiments and simulations results.