The control of NPC grid connected converter for interfacing to unbalanced DC link loads

A novel control scheme for neutral point clamped converters is presented, which allows interfacing of two unbalanced DC sources or loads to the AC grid. Multivariable control provides power matching for each DC link, under unbalanced DC loading conditions. Due to operation with unequal DC link capacitor voltages feedforward compensation is used to minimise the even harmonics caused in the supply current. An analytical method is derived which uses system parameters and loading to deduce the limits of control over the capacitor voltages. Experimental and simulation results are given showing the control performance.