Research On Virtual Inertia Cooperative Control Strategy Of Multi-type Renewable Energy Units

As the process of building new-type power system accelerates,the share of renewable energy generation and the penetration of power electronics continues to increase,the equivalent inertia level of the system continues to decrease and frequency stability operation is being challenged more severely.Wind power and photovoltaic units can have a certain inertia response capability without configuring energy storage by relying on their own power generation structure and control strategy,but for wind power units,the existing traditional inertia control methods that involve the wind turbine in the frequency response process may produce problems such as reduced economic efficiency and secondary frequency dips,while for photovoltaic units,they may produce problems such as insufficient inertia support capability and grid-connected photovoltaic instability.With the in-depth study of energy storage inertia mechanism and the enrichment of energy storage control means,energy storage shows a broader use prospect in assisting the operation of renewable energy unit and improving the frequency response performance of the system.Therefore,it is of great significance to study the virtual inertia control method of multiple types of renewable energy unit and their cooperative control strategy to enhance the inertia response of renewable energy unit and maintain the frequency stability of high percentage renewable energy grid.Based on the above background,this paper focuses on improving the virtual inertia control method for renewable energy unit and proposing a multi-type and multi-time scale inertia cooperative control strategy.(1)A comprehensive control strategy for wind turbine frequency based on the improved virtual inertia method is proposed for the problem of secondary dips in system frequency caused by the virtual inertia response of wind turbines.Firstly,the control strategies for wind turbine inertia response are sorted out,and the principles and advantages and disadvantages of each control strategy are compared and analysed.Then the traditional virtual inertia method based on rotor kinetic energy is improved by attaching a differential control link based on wind turbine speed feedback after the wind turbine exits the inertia response process,reducing the speed of speed recovery and avoiding a sudden drop in wind turbine output to suppress frequency fluctuations.Over-speed control is also integrated to give better performance of the wind turbine inertia response.Finally,the proposed control strategy is validated by simulation examples to provide better inertia response of the wind turbine.(2)The photovoltaic virtual inertia control strategy is proposed to address the problem of insufficient active reserve in the photovoltaic virtual inertia response,considering multiple inertia sources.Firstly,the traditional photovoltaic virtual inertia control technology is summarized,the mechanism of each control method is analyzed,the conductivity increment method is improved based on PI controller,so that the photovoltaic power generation system is in the reduced load state,in order to simulate the inertia response characteristics of synchronous generator and frequency regulation characteristics,the virtual inertia control strategy of photovoltaic power generation system is proposed based on sag control and virtual inertia control.In addition,the control methods of energy storage for inertia response are sorted out,the mechanism of each control method is analyzed,the control method of energy storage for inertia response of renewable energy unit is studied,and an inertia cooperative control strategy is proposed based on photovoltaic capacitor energy storage and battery energy storage.Finally,the proposed inertia control method is verified to improve the inertia response capability of photovoltaic unit through simulation examples.(3)In response to the lack of effective cooperative control of each inertia support resource,a wind-photovoltaic-energy virtual inertia cooperative control strategy based on two-stage control of energy storage is proposed.First,the principles of inertia cooperative control in high proportional renewable energy grid are studied and the problems of inertia cooperative control are summarised.Then,based on the inertia cooperative control principles proposed in this paper,the wind turbine is made to participate in the inertia response based on the rotor kinetic energy method and the photovoltaic unit is made to participate in the inertia response based on the capacitive energy storage method.In the inertia response stage,energy storage is used to support the inertia performance of the wind turbine and the photovoltaic unit based on droop control,and in the frequency recovery stage,energy storage is used to enhance the frequency response performance of the cooperative control strategy based on fuzzy control.Fuzzy control rules are formulated and the wind-photovoltaic-energy virtual inertia cooperative control method is proposed based on the two-stage control of energy storage.Finally,the proposed inertia cooperative control strategy is verified in a simulation example to enable the energy storage system to assist the wind turbine and photovoltaic unit for better inertia support and improve the frequency response performance of the system from multiple time scales.