Space Spread-Spectrum Strategy for MMC to Reduce the Conducted EMI

A modular multilevel converter (MMC) is widely used due to its various advantages. However, it causes serious electromagnetic interference (EMI) problems. Therefore, it is necessary to suppress the conducted EMI of MMC. A novel spread-spectrum (SS) strategy, called space SS (SSS), is proposed in this article. The core idea of the SSS strategy is to make the carrier frequency of each submodule different. The principle and formula derivation of the SSS strategy are elaborated. SSS is a highly free strategy because carrier frequency can be distributed arbitrarily with space position. Several methods for determining carrier frequency distribution are proposed in this article. These methods not only achieve EMI suppression, but also take into account less circulating current, higher switching frequency, and lower switching loss. Total harmonic distortion (THD) and switching loss variation under the SSS strategy are also studied in the article. Experiments are carried out to validate the effectiveness of the SSS strategy. The SSS strategy can reduce the EMI by more than 12 dB and all obvious spikes in the spectrum are eliminated effectively. Besides, it is easy to implement and suitable for practical systems with quantities of submodules.