Calculation Method For Flexible DC Supply Capacity Based On Impedance Analysis

In recent years,with the development of power electronic technology,flexible DC transmission technology is becoming increasingly mature.Flexible DC transmission not only improves the shortcomings of the traditional DC transmission,but also improves the transmission capacity and efficiency.However,flexible HVDC technology is still a new field,the actual operating experience is not very rich.Therefore,there are still many problems to be solved when the flexible DC transmission line is put into operation..The capacity of the flexible HVDC transmission will affect the stability and economy of the receiving terminal.Therefore,it is of great significance to determine the reasonable limit value of DC feed capacity to ensure the safe and stable operation of the receiving network.After analyzing the basic principle of impedance analysis,it is proposed to determine whether the system is stable by the ratio of load impedance to equivalent impedance of grid.This paper,basing on the Thevenin principle,puts forward the calculation method of impedance of grid and VSC-HVDC.After introducing the impedance analysis into grid-connected VSC-HVDC,this paper deeply analyses the Thevenin equivalent circuit and Norton equivalent circuit of grid-connected VSC-HVDC.Moreover,by judging whether the ratio of the equivalent impedance of VSC-HVDC to the equivalent impedance of grid satisfies the Nyquist criterion as a criterion for measuring the stability of the system.Simutaneously,the limit value of VSC-HVDC feed capacity can be determined.Based on the control principle of the converter of flexible HVDC transmission system,the calculation formula of equivalent impedance of a single grid-connected HVDC is derived.And,according to the node admittance array of the power system,the equivalent impedance matrix of multiple grid-connected HVDC transmission systems is determined by the calculation formula.Building on the Thevenin principle,this paper deduces the calculation formula of the equivalent impedance of grid,which depends on the line impedance and the transformer impedance.This impedance based analysis method does not need to be analyzed inside the converter to make this method simpler.Finally,the effectiveness of the proposed method is verified by simulation analysis on the IEEE-39 node system,and the factors that affect the DC side feed capacity at the DC side of the HVDC system are analyzed.