Research On Combined Frequency Regulation Strategy Of Wind Turbines And Storage System Based On Virtual Synchronous Generator

In the national "14th Five-Year" new energy plan,it is clearly pointed out that the goal of carbon neutrality will be achieved by 2060,the goal and vision of carbon neutrality have put forward higher requirements for the low-carbon transformation of energy and power,and vigorously developing new energy has become an inevitable choice to help achieve the goal of low-carbon emission reduction.Major power companies are accelerating the pace of energy transformation and continuously increasing their deployment and investment in new energy businesses such as wind and solar power.The power system based on the high permeability of new energy will have the characteristics of low inertia and low damping of the converter,which reduces the system's ability to adjust the frequency.The intermittency and randomness of new energy increases the probability of frequency disturbance and threatens the system frequency stability.In addition,when emergency situations such as DC block,machine cutting or load mutation occur,the response speed of the system based on the high permeability of new energy is too fast and the frequency fluctuates greatly,which further reduces the safe and stable operation level of the system.Therefore,it is of great significance and application value to study the new energy wind farms that can participate in the frequency regulation function of the power grid to improve the frequency stability of the power grid and the capacity of wind power to accept.Virtual Synchronous Generator(VSG)technology uses the flexibility and rapidity of power electronic converter control to introduce the electromagnetic and mechanical characteristics of the synchronous generator into the inverter control strategy,thereby improving the output characteristics of the new energy grid-connected converter,so that the new energy has the similar reactive-voltage and active-frequency adjustment capabilities of synchronous generators.Aiming at the key technical problems,such as how to design reasonable control strategy and parameter matching,realize reasonable power distribution and cooperative operation among multiple VSGs,this dissertation focuses on the frequency characteristics of doubly-fed induction generator(DFIG),the frequency modulation strategy of wind-storage system based on virtual synchronous generator technology,and wind farm coordinated frequency modulation method based on multiple virtual synchronous generators.Then,through theoretical research and experimental verification,the theoretical and technical foundation is laid for the frequency stability of the wind farm support system.The research contents of this dissertation mainly include the following aspects:(1)Multi-time scale frequency modulation characteristics of doubly-fed induction generator is studied.The similarities and differences between the frequency modulation characteristics of doubly-fed induction generators and traditional synchronous generators are compared,and the participation of doubly-fed induction generators in the primary frequency modulation process of the power grid is discussed.Combined with the implementation principle,advantages and disadvantages of different frequency modulation strategies of doubly-fed induction generator,the influence of frequency variation on the transient and steady-state processes of doubly-fed induction generator is studied,and the relevant laws are revealed.Finally,a combined frequency modulation strategy that takes into account the different response time scales is proposed based on the different response time scales of the virtual inertia and droop control of wind turbines,which provides a theoretical basis for subsequent research.(2)The frequency modulation scheme based on energy storage virtual synchronous machine technology is studied.Combining the mechanical torque equation and electromagnetic equation of the synchronous generator,the virtual speed governor and virtual exciter model controlled by the energy storage inverter are built to realize the primary frequency and voltage regulation functions of the energy storage VSG,and analyze the virtual through the small signal equation The main parameters of mechanical torque,virtual inertia,virtual impedance,and the influence of unit power adjustment system on system frequency stability,which provides some theoretical basis for parameter selection.In order to avoid the adverse effects of the phase-locked loop on the VSG frequency modulation and grid connection and off-grid process,a virtual impedance-based virtual synchronous generator grid-connected control method is proposed,which realizes the seamless switching of the VSG with load off-grid and stable operation.(3)The frequency modulation control strategy of wind-storage system based on virtual synchronous machine technology is studied.Based on the frequency response function of wind-storage VSG,the frequency modulation characteristics of wind-storage VSG are analyzed,and the output characteristics of wind farm and energy storage system are comprehensively considered.Considering the output characteristics of wind farm and energy storage system,a coordinated control strategy of wind storage is proposed based on the release of wind power inertia and the steady state support of energy storage.Through the parallel output of wind farm and energy storage system,the control strategy in this dissertation can reduce the capacity demand of energy storage system and give full play to the role of short-term power support of wind turbine,which can significantly reduce the capacity configuration of energy storage system on the basis of stabilizing the system frequency and improve the efficiency of wind farm frequency modulation.(4)A coordinated control method of primary and secondary frequency modulation for wind farm with multi-VSG is studied.To solve the problems of overshoot and reverse modulation in the coordinated control of the primary and secondary frequency modulation of wind farms,the grid-connected transfer function of multi-virtual synchronous generators was established to analyze the power-frequency oscillation characteristics and influence laws of the multi-virtual synchronous generators grid-connected system when the number of grid-connected VSGs,inertia constant and equivalent impedance of the grid changed,and the simulation results were verified.Aiming at the problem that the existing centralized frequency-modulated wind farm cannot provide inertia support,the virtual inertia adaptive coordination control strategy of primary and secondary frequency modulation for multi-VSG wind farm is proposed,which can adjust the virtual inertia coefficient adaptively,effectively reduce the oscillation phenomenon when the frequency and power are given change,and verify the effectiveness of the proposed coordination strategy of primary and secondary frequency modulation.(5)Build a hardware-in-the-loop platform,and verify relevant conclusions in a variety of scenarios.The hardware-in-the-loop simulation platform of doubly fed wind turbine based on virtual inertia and frequency droop control and the hardware-in-the-loop simulation platform of isolated grid and off-grid seamless switching based on energy storage virtual synchronous generator are built.Experiments are carried out to verify the correctness and effectiveness of some of the key technologies proposed in this dissertation.