Harmonic Suppression Method For VSG Connected To Weak Grid And Device Development

Renewable energy connected to the power grid by electronic equipments is an important means to solve the global energy crisis and environmental problems.But,those electronic equipments have no similar inertia to traditional generators and can not provide damping for the power grid.Large-scale renewable energy will lead to the frequency and voltage stability problems in the power grid.Virtual synchronous generator(VSG)technology can actively participate in frequency/voltage regulation of syatem,which is of great significance to promote the friendliness and sustainable development of renewable energy.However,in our country,most of the renewable energy is located in the weak power grid areas such as the northwest,where the harmonic content of the power grid voltage is large.As the development of industrial society,the harmonic current introduced by power electronic devices is more larger.The above factors will lead to the deterioration of the grid-connected current quality of VSG and the stability of the system.Therefore,it is urgent to study the harmonic current suppression technology under the weak power grid connected by VSG.The main work of this paper is as follows:Firstly,this paper introduces the research background and significance,discusses the research status of VSG,expounds the power quality problems of VSG connected to the weak power grid,and analyzes the harmonic suppression method of the traditional grid-connected inverters connected to the weak power grid.Secondly,According to the rotor motion equation,primary frequency modulation principle and excitation control system of traditional synchronous machine,this paper constructes the active power-frequency and the reactive power-voltage control strategy of VSG,and establishes the small signal model of active and reactive power loops.Based on this model,the moment of inertia and damping coefficients of this control strategy are designed.The simulation results verify the effectiveness of VSG control performance and the parameter design method.Thirdly,this paper establishes the harmonic domain model of VSG connected to weak current network.And it points out that harmonic voltage and harmonic current of nonlinear load will seriously reduce the quality of grid-connected current and voltage stability at PCC.And then,the principle of conventional harmonic current suppression method and its failure in VSG are analyzed.On this basis,an impedance remolded harmonic current suppression strategy is proposed to improve the friendliness of VSGconnected to weak current network,which mainly includes passive branch and active branch.The passive branch refers to place a small inductance at PCC point and the grid side to introduce an additional voltage variable.The active branch refers to the appropriate negative and positive feed-forward of PCC point voltage and additional voltage.This control strategy not only increases the harmonic impedance from the grid side to VSG,but also reduces the output harmonic impedance from PCC to VSG,which makes most of the harmonic current of the nonlinear load absorbed by VSG,and weakens the harmonic current brought by the harmonic voltage of the grid,thus solving the problem of serious distortion of the grid-connected current under the weak current network connected to VSG.Finally,a 10 kW virtual synchronous machine system platform are built.The main circuit design of the prototype,control system hardware design and software flow chart design are given.The experimental results verify the effectiveness of the harmonic suppression method proposed in this paper.