| The energy and power scene of our country is experiencing the transformation from the traditional power system with mainly fossil energy to the new electric power system with mainly clean energy to achieve the peak carbon emissions and carbon neutrality.Due to the dependence of new energy generation technologies like wind power and photovoltaic on power electronic equipment,whose dynamic behavior is different from synchronous generator(SG)and dominated by static physical components and corresponding control strategies,the power system is undergoing profound changes in dynamic characteristics of grid-connected equipment,which leads to various dynamic stability problems with unknown mechanism in the early stage of the new power system construction.Among them,the dynamic behavior of the voltage source converter(VSC)dominated by the voltage control timescale and current control timescale is closely related to the new dynamic problems of power system.Meanwhile,with the transformation of types of grid-connected equipment,the dynamic stability of the system voltage amplitude/frequency level and the transmission of active/reactive power remain the general objectives of AC power system.Therefore,to serve the goal of new power system construction as well as to understand the influence of power electronic equipment on power system dynamics,it is necessary to focus on the independent excitation-response characteristics of grid-connected power electronic equipment facing the closed-loop dynamic process between active/reactive power and voltage amplitude/frequency of AC power system,including the mathematical relationship characterization and physical mechanism understanding of the characteristics.Since the phase-locked loop(PLL)appears to introduce direct response for the formation of internal voltage from terminal voltage of the grid,existing studies generally lack the understanding of independent power excitation and internal voltage response characteristics of PLL-based VSC.In addition,the time-varying mechanism of voltage amplitude/frequency has not been widely recognized.Therefore,the mainstream studies either directly characterize VSC based on the physical connection relationship,or establish the mathematical model of VSC based on the harmonic perspective of system dynamic process.These works fail to reflect the time-varying mechanism of active/reactive power and voltage amplitude/frequency during AC power system operation,and the corresponding recognition of VSC’s characteristics fails to reflect the role of equipment in the dynamic process of the system.Therefore,this paper takes the current control timescale of VSC as an example,and innovatively proposes the mathematical characterization and physical understanding method of the independent characteristics of active/reactive current excitation and internal voltage amplitude/frequency response oriented to the dynamic process of system amplitude/frequency modulation.The specific research contents are as follows:(1)The research idea of independent characteristics of current control timescale of VSC facing the amplitude/frequency modulation dynamic process of power system is proposed.The cyclic regulation mechanism between power and internal voltage and the time-varying amplitude/frequency modulation mechanism of power system operation are elaborated.The foundamental function of power transmission by establishing internal voltage amplitude/frequency of VSC is revealed.The sequential operation process of multi-timescale control of VSC is explained,and further the independent characterization idea of active/reactive current excitation and internal voltage amplitude/frequency response is proposed for the current control timescale of VSC.(2)The independent characteristics of active/reactive current excitation and internal voltage amplitude/frequency response of current control timescale of PLL-based VSC is established.The system causal dynamic process connected by current control timescale of PLL-based VSC is summarized.The solely dependent mechanism of internal voltage vector on current vector of current control time scale of terminal voltage detection-based VSC is clarified.Based on the understanding of the connotation of active/reactive current,the independent excitation-response model is proposed.The corresponding linear model under small perturbation is further established by identifying the small perturbation linearization method,which provides the basis for the characteristic cognition study.(3)The dynamic mechanism understanding of current control timescale of VSC based on the excitation-response characteristics is formed.The relationship between the dynamic of internal voltage amplitude/frequency and that of VSC’s energy level is expounded.The unbalanced power storage mechanism of current control timescale of VSC forced by unbalanced active/reactive current is formed.Therefore,the rotational motion dynamic mechanism of internal voltage vector with amplitude/frequency as motion state driven by unbalanced active/reactive current is established.(4)The influence factor understanding of the dynamic characteristics of VSC’s current control timescale is established.The influence mechanism of PLL and dynamics of active/reactive current reference on the dynamic characteristics of current control timescale is elaborated.The influence of controller parameters on dynamic characteristics is summarized.Furthermore,the role of current control timescale during system frequency modulation dynamic process is preliminary explained,thereby demonstrating the significance of dynamic characteristics understandings for the understanding of system frequency modulation dynamic process. |
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