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Line commutated converter (LCC) has been widely applied in hybrid high voltage direct current (HVDC) transmission system, where the modular multilevel converter (MMC) is applied as receiving end and may induce low order harmonics in DC-link voltage. These low order harmonics will introduce the low frequency fluctuation into the control loop of LCC and complex the harmonics analysis. The traditional method to calculate LCC harmonics is derived by the switching function and modulation theory, which only obtains characteristic harmonics. The low-frequency fluctuation superimposed on the firing angle in the control system is ignored in the traditional method. In this paper, the interharmonics, which is introduced by firing angle fluctuation, is revealed by the double Fourier method. A fitting function method is applied for depicting the varied commutation overlap caused by firing angle fluctuation. Compared to the traditional method, the proposed method can effectively analyze both characteristic harmonics and interharmonics in the condition of low-frequency fluctuation of the firing angle. The accuracy and effectiveness of the theoretical analysis are verified with a three-phase LCC rectifier built in PSCAD/EMTDC and experimental test.

Accurate Analysis of Harmonic Transmission of Line Commutated Converter Considering Firing Angle Fluctuation