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.