Modeling And Small-Signal Stability Of DC Current Flow Controller Integrated Multi-Terminal HVDC System

In a multi-terminal high-voltage direct current(HVDC)transmission system,the problem of insufficient control degree of the power flows may emerge and it is necessary to introduce a DC current flow controller(CFC)to achieve complete control and optimal distribution of the power flow on the DC lines.In this dissertation,research is conducted in three aspects: control strategy design of the CFC,dynamic modeling of the CFC integrated multi-terminal HVDC system,and the system small-signal stability analysis.The main research contents are as follows:(1)Control strategy design of the CFC.Firstly,regarding the problem that the common capacitor voltage of the CFC cannot meet the actual control requirements,a selection method of the common capacitor voltage based on various specific control requirements with explicit control margins of the DC line currents is proposed with analysis of the network topology and limits of the control domain.Secondly,for the problem that the CFC cannot achieve the control function when the power flow direction at the DC terminal is not consistent with that of the DC lines,the original control strategy is improved to make it generic for any initial directions through in-depth analysis of its control mechanism.Thirdly,for the redundancy of the bypass state during the operation of the CFC,a simplified control strategy is proposed.Finally,the effectiveness of the theoretical analysis is verified by case studies in PSCAD/EMTDC.(2)Dynamic modeling of the CFC integrated multi-terminal HVDC system.Regarding the problem that the existing model cannot be quantitatively analyzed,a detailed dynamic model of the system is established step-by-step.Firstly,divide the overall system into the CFC module,DC transmission lines module,AC system module,and intermediate module,according to the topological structure of the system.Secondly,establish the state-space model of each module and form the state-space model of the overall system based on the interconnection relationship and the intermediate module.Thirdly,conduct the Taylor expansion at the steady-state operating point and select the fundamental frequency proportions to obtain the linearized small-signal model.Finally,the effectiveness of the established small-signal model is verified by comparison with the electromagnetic transient simulation results in the PSCAD/EMTDC.(3)Small-signal stability analysis of the CFC integrated multi-terminal HVDC system.Firstly,the impact of the CFC on the multi-terminal HVDC transmission system is analyzed by comparing the difference of oscillation mode.Secondly,the selection principle of the parameters is proposed by investigating the impact of the CFC control system parameters,CFC structure parameters,DC line parameters and AC system parameters on the small-signal stability of the entire system.Finally,the effectiveness of the theoretical analysis is verified by electromagnetic transient simulation results in the PSCAD/EMTDC.