Improved Nearest-Level Modulation for a Modular Multilevel Converter With a Lower Submodule Number

A Hybrid Modular Multilevel Converter Topology with an Improved Nearest Level Modulation Method

“…In addition, the level numbers of the arm voltages and output voltage all are increased to 2N+1. In addition, compared to the methods mentioned in [21] and [22], the sum of the upper and lower arm voltages with the proposed method is always equal to the bus voltage, which means that there is no pulse voltage across the arm inductors, and SM capacitor voltage is not decreased. Therefore, the arm inductor can be designed as a conventional MMC.…”

“…1, the arm of which consists of N series connected half-bridge submodules (HBSMs), one full-bridge submodule (FBSM) and an arm inductor. Compared with the conventional MMC topology with the NLM methods proposed in [21] and [22], the improved MMC topology with the proposed method is also able to increase the level number of the output voltage to 2N+1. Moreover, it keeps the total number of inserted SMs unchanged.…”

“…Moreover, a close-loop voltage balancing controller needs to be designed. Therefore, some techniques are proposed to improve the performance of NLM methods in case of a small number of SMs [19]- [22]. By combining NLM and PWM, the submodule unified PWM (SUPWM) [19] and the improved SUPWM [20] can generate an output voltage with a high quality.…”

“…A modified NLM method proposed in [21] is used to shift the moments of the step changing in the upper and lower arm voltages. Another improved NLM method is presented in [22], which introduces an alternating offset into the reference signals of the upper and lower voltages. Both methods are able to increase the level number of the output voltage from N+1 to 2N+1, where N is the number of SMs per arm, and the switching frequency remains unchanged.…”

“…Moreover, it keeps the total number of inserted SMs unchanged. As a result, the high inductor voltage pulses caused in [21] and [22] can be eliminated, and the SM capacitor voltage and output voltage are not decreased. In addition, the voltage balancing of the HBSM and FBSM capacitors are realized independently.…”