To acquire a multi-level converter with reduced device count applied in the three-phase mid-voltage system

two half-bridge power electronic building blocks(PEBB)are inserted into the DC side of a typical three-phase H-bridge converter

a novel seven-level converter is demonstrated. By using the H-bridge as a level doubling network

the natural point voltage imbalance of the conventional natural-point clamped converter(NPC)is bypassed. Moreover

because of the symmetrical structure

a rational control scheme for the DC voltage balancing is adopted to facilitate active current superposition and switching states selection. The operation principle is analyzed in detail and the structure characteristics of the topology are deeply revealed while a two-layer compound control strategy for DC voltage balancing is designed. A novel multi-level pulse-width modulation(PWM)algorithm based on objective wave-state vector function is proposed for use in drive pulse generation in the novel topology. The MATLAB/Simulink model of 3. 3 kV /5 MW grid-connected converter is built to verify the validity of the operation principle

modulation algorithm and voltage balancing control strategy. This work is supported by National Natural Science Foundation of China(No. 51177147)and Zhejiang Key Science and Technology Innovation Group Program(No. 2010R50021).