Hamiltonian Control of a 4-Leg Converter for Mitigation of Common Mode-Induced Current Offset

In this paper, a control strategy for the mitigation of common mode (CM) current is examined. A 4-leg solid-state transformer (SST) architecture is utilized. Hamiltonian surface shaping and power flow control (HSSPFC) is used to design a nonlinear controller for the output inverter of the SST. The goals of the controller are to mitigate common mode-induced AC current offsets and to drive AC current to desired set points. Mathematical analysis of the nonlinear systems is used establish a controller which meets these goals. Two cases are tested: (a) proposed 4-leg inverter, (b) standard 3-leg inverter. The proposed controller response was found to rapidly mitigate CM insults while also maintaining the quality of the AC current waveforms for the 4-leg case. However, for higher CM voltage levels the 3-leg was not able to successfully track the current setpoints. That is, the standard 3-leg inverter had degraded performance compared to the 4-leg inverter. In a broader sense, the proposed controller can be utilized to protect the SST itself, standard transformers, and other equipment on the grid.