Advanced 2<i>N</i>+1 Submodule Unified PWM With Reduced DC-Link Current Ripple for Modular Multilevel Converters

The 2 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> +1 submodule unified pulsewidth modulation (SUPWM) is attractive for modular multilevel converters (MMCs). To generate 2 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> +1 voltage levels, the SUPWM results in voltage pulses imposed on the arm inductors, which produces large high-frequency current ripple in the dc link of the MMC and deteriorates the dc-link performance. In this article, the dc-link high-frequency current ripple under 2 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> +1 SUPWM is analyzed and a reduced dc-link current ripple control method is proposed for advanced 2 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> +1 SUPWM. By regulating the phase angles of the three-phase carriers in each carrier period, the resulted voltage pulses on the arm inductors of three phases can be counteracted, and therefore the dc-link high-frequency current ripple is suppressed. With the proposed method, the MMC has much reduced dc-link current ripple, which effectively improves the performance of the MMC under 2 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> +1 SUPWM. Simulations and experiments are conducted to verify the validity and effectiveness of the proposed control.