A Time-Domain Piecewise Analytical AC Harmonic Current Calculation Method Suitable for Capacitor Commutated Converter

The Capacitor Commutated Converter (CCC) is constructed as a conventional Line Commutated Converter (LCC) modified with a series capacitor between the converter transformer and thyristor valves in each phase. This article proposes a time-domain piecewise analytical AC harmonic current calculation method for the CCC system. Compared to the existing calculation methods, the proposed method can calculate both characteristic and non-characteristic harmonics simultaneously when various asymmetric factors are considered. Firstly, the overlap angles are calculated considering the asymmetric factors after partitioning the piecewise intervals in a fundamental period. Secondly, the AC harmonic currents are calculated considering the DC side harmonic currents based on the Thevenin equivalent model of the DC circuit. Then, the effectiveness of the proposed calculation method is validated by simulation results in PSCAD/EMTDC. The comparison result shows that the proposed method is high-efficiency and high-accuracy for the AC harmonic current calculation of the CCC system and can meet the requirement of CCC filter design in practical projects. Finally, the influence of the asymmetric factors on the overlap angle calculation and the influence of the commutation capacitor variation on the commutation parameters are studied.