| The increasingly prominent energy and environmental issues and the establishment of the "double carbon" goal have accelerated the development of renewable energy power generation.In the coming decades,large-scale photovoltaic,wind power and other new energy power stations will be connected to the power system every year.When renewable energy power generation is connected to the grid in multiple places and multiple points on a large scale,its inherent intermittency and uncertainty may lead to power mismatch between the generation side and the load side,resulting in power quality problems,such as voltage fluctuation,voltage flicker,frequency change and harmonic problems,which will bring great challenges to the stable operation of the grid.On the power load side,voltage fluctuation and flicker will cause flicker of lighting facilities,instability of motor speed,failure of precision electronic instruments and automatic control equipment in some occasions requiring high-precision voltage,and even cause major safety accidents in serious cases.Electric Spring(ES)is a kind of power electronic device used in power system and a new control method to solve power quality problems.Its biggest feature is that the power consumption on the load side changes with the change of power generation on the grid side,thus ensuring the stability of power quality and voltage of key loads on the user side,and compensating reactive and active power according to the actual situation,which can effectively overcome the unpredictable shortage of renewable energy power generation.At present,there are still some problems in ES research,such as slow regulation speed,large overshoot and weak robustness of the controller,and the ES topology does not consider the battery power,and the function is relatively simple,and so on.In view of the above problems,this dissertation proposes and designs a new multi-functional electric spring(MF-ES)topology based on the actual situation of the user side ES,focusing on its voltage compensation control strategy and the automatic switching of multi-function operation mode.The main work of the dissertation includes the following aspects:(1)Aiming at the problems of bus side voltage fluctuation,short time interruption and battery charging,a new ES topology(MF-ES)that can realize four functions and safely switch is designed based on the analysis of the characteristics and scope of application of ES topology and the requirements of distributed energy storage;Secondly,the bidirectional DC/DC/AC two-stage converter is applied to ES for the first time to reduce the series voltage of the DC side battery pack and realize the constant current charging function of the battery pack;Finally,according to the multiple functional requirements of MF-ES,four operation modes,namely,compensation mode,power supply mode,energy storage mode and standby mode,are proposed and their working mechanisms are analyzed.Furthermore,the control strategy for automatic switching of different operation modes of MF-ES is proposed,and the feasibility and effectiveness of this strategy are verified by simulation.(2)Aiming at the problems of bus side voltage fluctuation,short time interruption and battery charging,a new ES topology(MF-ES)that can realize four functions and safely switch is designed based on the analysis of the characteristics and scope of application of ES topology and the requirements of distributed energy storage;Secondly,the bidirectional DC/DC/AC two-stage converter is applied to ES for the first time to reduce the series voltage of the DC side battery pack and realize the constant current charging function of the battery pack;Finally,according to the multiple functional requirements of MF-ES,four operation modes,namely,compensation mode,power supply mode,energy storage mode and standby mode,are proposed and their working mechanisms are analyzed.Furthermore,the control strategy for automatic switching of different operation modes of MF-ES is proposed,and the feasibility and effectiveness of this strategy are verified by simulation.(3)Aiming at the problems of weak robustness and poor adaptability of traditional controllers and control methods in voltage compensation,the proposed phase locking method firstly proposes the control strategy of voltage compensation using terminal sliding mode control(T-SMC)and finite control set model predictive control(FCS-MPC),establishes the corresponding control system model and theoretically deduces the control algorithm;Secondly,the traditional quasi proportional resonant control(QPRC),T-SMC and FCS-MPC control strategies are simulated for voltage sag and voltage rise,and the dynamic and steady-state performances of the three controllers are compared and analyzed.It is concluded that T-SMC has strong robustness and less harmonic components when the voltage is unstable at the initial stage of power spring input,and is the most effective voltage compensation control mode.(4)In view of the single function of the traditional ES and the lack of battery power is not considered,this dissertation adds two new functions in the ES function design: energy storage and standby power supply,corresponding to the energy storage mode and power supply mode.Firstly,the mathematical model of AC/DC rectifier control and its rectification control strategy are analyzed,and then a three zone multi strategy adaptive fruit fly optimization algorithm is proposed to optimize the PI parameters of the double closed-loop control,and the effectiveness of the improved fruit fly optimization algorithm is verified by simulation.Secondly,the control strategy of bi-directional DC/DC converter is studied.The bi-directional energy flow of bi-directional DC/DC converter is used to control the charge and discharge of the battery pack,which solves the problems of constant current and constant voltage control during the charge and discharge process,and realizes the three-stage stable charge of the battery pack.(5)In order to solve the problem that the critical load voltage is too high or too low when the grid side power supply fails or the bus fails in an emergency,the topology of MF-ES power supply mode is analyzed and modeled,and then a finite control set model predictive controller(FCS-MPC)is designed to control the power supply,complete the independent power supply of the critical load,and ensure that the critical load can continue to work normally when a sudden failure occurs;Finally,the feasibility of MF-ES energy storage mode and power supply mode control strategy is verified by simulation.(6)The experiment platform of MF-ES system based on d SPACE is designed and built,mainly including the operation mode automatic switching experiment of grid side voltage change,voltage rise and drop experiment under voltage compensation mode,battery charging experiment under energy storage mode and key load power supply experiment under power supply mode,which further verifies the effectiveness of the MF-ES topology designed in this dissertation and the feasibility of control strategies under different operation modes,It provides technical support for the power spring to enter the practical stage. |
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