| With the increasing proportion of wind power/photovoltaic power,high-voltage direct current(HVDC)transmission,and power-electronic-based loads in the power system,the utility grid trends to become a power electronics dominated power systems.The power electronic based grid-connected equipment has typical multi-timescale control dynamics,that is,the dynamic timescales of rotor speed,DC voltage and AC current control.This characteristic of the equipment determines the multi-timescale dynamic characteristics of the power electronic power system and brings new challenges to the dynamic and stable operation of the system.The dynamic stability of the AC current control timescale has not been taken seriously in the analysis of traditional power systems.However,with the deepening of the power electronicity of power system,the harm caused by the dynamic of AC current control timescale to the safe and stable operation of the system is increasing.Therefore,this article takes the voltage source converter(VSC),a typical power electronic equipment,as the research focus.In this paper,the traditional mathematic modeling methods of grid-connected equipment are abandoned.According to the physical process of equipment control under AC current control timescale,a modeling method based on the physical relationship between the motion of VSC internal voltage and input / output current is proposed.Based on the proposed equipment model,this paper further analyzes the dynamic characteristics of the AC current control timescale of the grid-connected VSC and its impact on the dynamic stability of power systems.The specific research content of this article is divided into several aspects as follows:(1)According to the basic control structure of the grid-connected converter,the physical process of the converter control under the AC current control time scale is analyzed,and the concepts of current balancing and internal voltage of grid-connected equipment are presented.Then,the idea of grid-connected equipment modeling based on the motion of internal voltage is proposed in AC current control timescale.Under the condition of ignoring and considering the dynamics of phase-locked loop,the current balancing-internal voltage motion model of grid-connected VSC in stationary coordinate system is established respectively,and the relations between the proposed model and the existing harmonic linearization based impedance are discussed.In the application scenario of single-converter infinite-bus system,the accuracy and effectiveness of the proposed model are verified by simulation analysis.(2)The dynamic characteristics of grid-connected VSC under AC current control timescale and their effects on the stability of the system with a VSC attchs to weak grid are studied.Based on the equipment current balancing-internal voltage motion model,a physical discreption of system energy exchange during the VSC dynamics of AC current control timescale is presented from a perspective of circuitization,which is based on the concepts of analog variable capacitors,variable inductors.Through the frequency response of each part of the converter current balancing-internal voltage motion model,the effects of the PLL bandwidth,current loop bandwidth,terminal voltage feedforward,and digital control delay on the dynamic characteristics of each frequency band of the converter are analyzed.Based on the analyses of the factors that impact VSC dynamic characteristics,a corresponding optimal control scheme is proposed on the classical vector current control,which improves the stability of the system with VSC injects rated power into the weak grid.Detailed simulation and rigorous experiments verify the correctness of the theoretical analysis and the effectiveness of the optimized control scheme.(3)Based on the VSC current balancing-internal voltage motion model,the idea of multi-variable feedback control is introduced for the modelling of converter dominated power system and its dynamic anlysis in AC current control timescale.Futher more,according to the back ground wind power/photovoltaic generation station with complex collection system,the influence of the type of collector feeder and topology of the feeder on the stability of the multi-converter grid-connected system is specifically analyzed based on the proposed model.The correctness of the theoretical analysis is verified by the simulation of a detailed electromagnetic transient model.(4)The control interaction between grid-connected VSCs under AC current control timescale is studied.Based on the multi-variable feedback control modeling theory and VSC current balancing-interal voltage motion model proposed in this paper,a frequency domain model which is suitable for analyzing control interaction between grid-connected VSCs is proposed.Under the proposed modeling approach,the effects of control loop bandwidth,operating point and other factors on the control interaction between the grid-connected VSCs under different network structures in the AC current control timescale are analyzed. |
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