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Influence of magnetic field of AC transmission lines on parallel DC transmission systems

Based on magnetic induction and Fourier series theory, a method is presented to calculate the direct current flowing into the converting equipments induced by proximate AC transmission lines in parallel. The generated direct currents due to magnetic induction from AC lines are calculated and analyzed in the condition of steady state and fault state for an actual engineering case. Discussions are performed to illustrate the effects of various factors upon the direct currents flowing into the converting equipments. It provides insight on how to control the generated direct current by induction of AC lines. I. INTRODUCTION In recent years, HVDC power transmission system has been developed rapidly because of its advantages over HVAC power transmission system for long distance and bulk power transmission in China. However, new right-of-ways for the transmission lines are difficult to obtain due to public attention to the electromagnetic environment generated by transmission lines. Consequently, more and more parallel AC/DC transmission lines systems have appeared. When AC lines approach to DC lines, fundamental frequency voltage and current would be induced on the DC lines due to the magnetic induction between AC and DC lines. After flowing into the converting equipments, the induced fundamental frequency current on DC lines would be converted to direct current component and harmonic current components. The direct current component may cause saturation, noise and irregular working of the converting transformer. Thus, it is necessary to analyze the direct current component due to magnetic induction from AC lines. The interference between AC and DC lines was studied in (1-8). However, the detailed study of the direct current flowing into the converting equipments induced by proximate AC lines in parallel was not found in literatures. II. NUMERICAL METHOD A. Numerical Method Description A method based on magnetic induction and Fourier series theory is used in this paper. Firstly, the per-unit-length (PUL) electric parameters of the transmission lines are computed by the electromagnetic field method. Then using these obtained electric parameters, an equivalent circuit model is established and the induced longitudinal electromotive force on the DC lines is computed. With the total impedance of the DC lines, the fundamental frequency current induced on the DC lines can be calculated. Finally, by means of the Fourier series theory, the direct current flowing into the converting transformer is obtained. B. Calculation of PUL Parameters