In order to optimize characteristics

cost and volume of modular multilevel converter（MMC） through expanding linear modulation region

the influence of third-order harmonic voltage injection and negative voltage level utilization of full-bridge submodules on the linear modulation region of MMC is analyzed comprehensively. A unified analysis model of the linear modulation region of MMC is established

and an analytical expression of the maximum attainable modulation index of MMC considering both the factors of third-order harmonic voltage injection and negative voltage level utilization is obtained. Moreover

an analytical analysis model of the power fluctuation of the arm with the variation of modulation index is established

which reveals that there is a minimal point of the fundamental frequency power fluctuation with the increase of the modulation index under the active power condition. Based on a 400 kV/1 250 MW MMC example

the influences of several schemes for expanding linear modulation region on the MMC parameter design and arm power fluctuation and capacitor voltage ripples are compared and analyzed. Through the coordination of the third-harmonic injection and negative voltage level utilization

the modulation index can be operated near the optimal value

and the submodule capacitor voltage fluctuation can be greatly reduced. Digital simulation results verify the correctness of the proposed analysis model

and show the effectiveness of the feasibility of the third-harmonic injection and negative voltage level utilization.