A methodology for even-power-distribution within single time-blocks of power-frequency in CHB MLIs for PV systems

Photovoltaic systems for ac-load/grid invariably use dc-ac conversion. Multilevel inverters (MLIs) have shown a high techno-economic feasibility for such conversion. Cascaded H-bridge (CHB) MLI has been found to be extremely suitable for integration of renewable energy sources due to its modularity and simplicity. Multiple “electrically isolated dc sources” (such as PV panels/modules/strings, after suitable conditioning) are used as input. These sources undergo periodic charging and discharging as power is drawn from them. For equal lifetimes of multiple sources, undistorted multilevel waveform and better performance of power switches, power drawn has to be balanced. This is called “charge-balance-control”. For this, CHB-MLI is modulated using either space vector pulse width modulation (SV-PWM) or phase-shifted multi-carrier sine-triangle PWM (PS-PWM). For number of levels more than five, SV-PWM becomes cumbersome. PS-PWM, on the other hand, is less preferred as compared to level-shifted carriers based PWM (LS-PWM) as the harmonic profile of the line voltage is better in the latter case. State selection flexibility for charge balance control is not offered by LS-PWM and hence, carrier rotation with LS-PWM (CR LS-PWM) is used. This paper proposes a scheme to overcome an important limitation of the CR LS-PWM scheme. In CR LS-PWM, for ‘n’ number H-bridges and an output line frequency “f_L”, charge-balance-control is achieved in periodic time-blocks of “n/f_L”. An improvised LS-PWM scheme is proposed in which these time-blocks are reduced to “1/f_L” irrespective of the number of H-bridges, using the state selection flexibility of the scheme. Experimental results are discussed for validation.