Direct Amplitude-frequency Strategy In Current Control-time Scale Of Voltage Source Grid-connected Converter

With the widespread use of renewable energy,the problem of energy crisis and environmental pollution has been gradually mitigated.However,more and more photovoltaic power plants and wind turbines being connected to the power grid through power electronic equipment,and the use of flexible direct current transmission in the field of power transmission,which are exacerbating the trend of power-electronics-enabled power systems.There is a significant difference between the power-electronic-enabled power system and the traditional power system,which gives rise to new challenges of stability and mechanism analysis of the power-electronic-enaled power system.The grid-connected converter is the bridge connecting generatortor to the grid,which has a significant effect on the stability of the power system.The grid-connected converter generally adopts vector current control,in which the terminal voltage is phase-locked through a phaselocked loop,and the phase is used to perform coordinate transformation so the control can be realized in dq coordinate system.In vector current control,the phase-locked loop is usually a high-order nonlinear loop,and its structure is closed-loop,so its dynamic characteristics are fairly complex,which pose a major obstacle to understanding the phase-locked loop,in addition,the phase-locked loop may bring about adverse effects in weak power grids.Therefore,this paper proposes a converter control strategy without phase-locked loop,namely direct amplitude-frequency control.The direct amplitude-frequency control strategy is also based on voltage orientation,regarding the position of converter output voltage vector as the position of d-axis of the synchronous rotating coordinate,and the current of grid is then decomposed and controlled in this coordinate axis.Under the direct amplitude-frequency control,The amplitude and frequency of internal potential are directly generated by the controller.To further compare the working principle of the grid-connected converter under the two control strategies,it can be seen that direct amplitude-frequency control strategy and voltage-oriented vector control strategy have two main differences in structure,in which direct amplitude-frequency control strategy lacks phase-locked loop and has different controller.It can be easily seen that the direct amplitude-frequency control strategy describes the ac signal with amplitude-frequency signals,which makes the physical meaning of the control strategy relatively intuitive.In addition,the output signal of controller under the direct amplitude-frequency control strategy is regarded as signal of internal potential,which makes the physical characteristics of VSC are relatively simple.The direct amplitude-frequency control strategy has different ideas of the description form of ac signal and formation mechanism of internal potential,which in turn provides a new solution to the circumstances where VSC is connected to the grid.Under the current time scale,a VSC motion model based on the direct amplitudefrequency control strategy in which response of internal potential is driven by active and reactive power(current)and a network model in which response of current is driven by amplitude and frequency of internal potential are established,and then complete model of system of VSC connected to the grid is composed.In order to verify the accuracy of the amplitude-frequency model,a scenario where the grid-connected converter is connected to the infinite grid is set in Matlab/Simulink.Finally,the response process of the internal potential and the feedback current to the disturbance of reference of current show that the dynamic response of the time-domain model and the amplitude-frequency model to the disturbance is completely consistent,which means the amplitude-frequency model is correct.Based on the current balance-internal potential motion model,this paper simplifies the motion model through the analysis of self-stabilizing and en-stabilizing,and on this basis,the effect of network strength and controller parameters on system stability are studied.Finally,in order to deepen the understanding of the direct amplitude-frequency control strategy,the dynamics of converter under the current time scale is explained physically by using the concepts of capacitance and rotor.