Abstract

With the rapid development of flexible DC transmission systems, the application of modular multilevel converter MMC is becoming increasingly widespread. Meanwhile, the nearest-level modulation is most applied. This paper establishes a simple and efficient harmonic model of MMC based on nearest-level modulation with the equidistant controlled firing scheme. First, based on the analysis of the working principle of MMC and the modulation principle, the Fourier decomposition of the stair wave is obtained. Then, according to the equivalent circuit diagram of MMC, the stair wave is exactly the differential-mode voltage. Moreover, the amplitude and phase of each differential-mode voltage harmonic content are obtained. So far, MMC can be regarded as a harmonic voltage source. Finally, simulations were performed on PSCAD to verify the accuracy of the MMC harmonic model proposed in this paper. Furthermore, the MMC harmonic model under the equidistant controlled firing scheme and real-time trigger control were also compared. The final results show that the model proposed in the paper can accurately reflect the harmonic characteristics of MMC in the existing power system after considering the equally-spaced trigger control.