Model predictive control of a transformerless CSI7 photovoltaic system for leakage current and THD reduction

Current Source Inverters (CSIs) have gained significant attention for grid-tied photovoltaic applications due to their inherent current-limiting capabilities and superior reliability. Among these, modifying the seven-switch CSI (CSI7) with an internal current-return path has emerged as a promising transformerless solution for isolating leakage currents. However, controlling this hardware is challenging. Conventional Space Vector PWM (SVPWM) assumes a constant DC-link current, causing it to fail under the highly variable DC dynamics introduced by the frequent operation of the seventh switch. Furthermore, directly applying standard predictive control to this hardware excites severe high-frequency resonances within the return path due to variable switching frequencies. This paper bridges these gaps by proposing an advanced, hardware-optimized Finite-Control-Set Model Predictive Control (FCS-MPC) strategy. First, the proposed framework integrates dynamic DC-current tracking directly into the predictive model. This adapts to time-varying fluctuations, actively suppressing ripple and enabling the use of a significantly smaller DC inductor. Second, to resolve high-frequency resonances, a novel overlap-time commutation strategy is introduced; by routing current through the bypass switch (S7) during active vector transitions, it prevents resonance while ensuring zero-current switching (ZCS) for the main switches (S1–S6). Finally, a three-objective cost function is formulated to directly penalize temporal deviations in the predicted Common-Mode Voltage (CMV), suppressing the root cause of ground leakage current. Comprehensive Hardware-in-the-Loop (HIL) tests and physical validations under dynamically varying working conditions demonstrate that the proposed FCS-MPC provides superior robustness, substantial reductions in Total Harmonic Distortion (THD), and effective leakage current mitigation compared to traditional CSI7 controllers.