Modeling And Interaction Analysis Of VSCs In HVDC Grids In DC Voltage Control Timescale

Large-capacity and long-distance high voltage direct current(HVDC)transmission system based on voltage source converters(VSCs)is an important part of the modern power systems.In HVDC grids,VSCs closely interconnect AC and DC networks,as well as different AC systems.As a result,the dynamic stability of HVDC grids has become one of the important issues for the safety and stability of the power systems.The dynamic behavior of traditional AC power systems is dominated only by the dynamic characteristics of its grid-connected devices,e.g.synchronous generators,in AC side.While the dynamic behavior of HVDC grids(including interconnected AC and DC systems)is dominated by the dynamic characteristics of VSCs with multiple timescale control structures and interconnection to both AC and DC systems.It means,on one hand,VSCs are connected to the DC and AC networks at the same time,thus forming the unique dual-port feature of the equipment.One the other hand,VSCs have different types of energy storage elements such as DC capacitors and AC inductors,as well as regulators corresponding to the DC voltage control scale and AC current control scale.Multiple timescale response characteristics of the potentials on the AC and DC sides of the VSCs are formed by the disturbance of different scales and the action of energy storage elements and their controllers.The dynamic behavior of HVDC grids is closely related to the energy exchange among DC capacitors.Therefore,the characterization of the dynamics of VSCs to AC and DC sides in DC voltage control timescale,as well as interaction analysis of the same timescale formed by the AC and DC networks,form the basis of dynamic stability problems of HVDC grids.This paper conducts research on the above issues.The research of modeling and stability analysis for the dynamic stability problems in DC voltage control(DVC)timescale based on classical methods still have some deficiencies.(1)Frequency domain models considering the dynamics of active power only in DC sides or AC sides cannot be applied to the stability analysis of HVDC grids with different DC and AC networks.(2)The stability analysis based on timedomain models and the existing frequency domain models is not enough to explain the influence of interactions on the dynamic stability of DC power grids,and it is difficult to provide a reliable theoretical basis for its stability control synthesis.To solve the above problems,this paper conducts research from the aspects of modeling and interaction analysis of VSCs in HVDC grids.Based on the concept of the motion equation,a small signal model of VSCs in HVDC grids with multiple inputs and outputs in DVC timescale is established under different control strategies.An analytic quantification method for the interaction analysis based on the concepts of self-stabilizing and en-stabilizing are proposed.The concrete contents are as follows.(1)The connotation of the dynamic stability of HVDC grids in DC voltage control timescale is expound.Basic structures and control strategies of VSCs in HVDC grids are introduced.The multiple timescale characteristics of VSCs and DC grids are summarized then.Moreover,the connotation of the dynamic stability of DC grids and the challenges to solve the dynamic stability problem in DVC timescale are explained.(2)A small signal model of VSCs in HVDC grids in DC voltage control timescale is established.The related concepts and modeling ideas based on the motion equation are introduced.Based on the concept of the motion equation,a small signal model of VSCs in a DC grid with three pairs of input and output in DVC timescale is established.The model separates the characteristics of VSCs independent from the characteristics of AC and DC networks to makes it easy to extend and analyze the interactions among VSCs in a HVDC grid regardless of the topologies AC and DC networks.In addition,the characteristics of this proposed model under different control strategies are summarized compared with the models of other AC devices.(3)A method for analytic quantification of interactions in a DC grid is proposed.Two important problems of interaction analysis are refined compared with the dynamic stability analysis methods in the existing body of knowledge.In addition,the characteristics of interactions among VSCs in DC grid compared to the interactions of devices in AC systems are extracted based on the understanding of the dynamic process of interactions.Furthermore,self-/en-stabilizing coefficients are defined and principles to classify en-stabilizing coefficients into different terms through different paths are proposed to quantify the interactions among different terminals in DC grids.Moreover,the relationships between the self-/en-stabilizing coefficients and the stability of the system are built.(4)Interactions among VSCs in a typical HVDC grid without considering reactive power branch control are analyzed.Analytical quantification method of interactions is applied on a typical three-terminal HVDC system.The self-stabilizing coefficient and enstabilizing coefficients through 6 different paths are calculated.The damping generated by enstabilizing through different paths are evaluated.Moreover,how controller parameters of active power control branches,including DC voltage control,active power control,I-U and U-P DC voltage-droop control,affect different paths of interactions and further make the system unstable are analyzed.(5)Interactions among VSCs in a typical HVDC grid when considering reactive power branch control are analyzed.The self-stabilizing coefficient and en-stabilizing coefficients through 6 different paths are calculated by considering the effect of reactive power branches on the active power branches.Furthermore,how controller parameters of reactive power control branches,including phase-locked loop control and reactive power control,affect the stability of the system are analyzed.