Analysis and control design for grid-tied transformerless inverters

The addition of renewable energy sources to the grid poses challenges related to control schemes and the stability of inverters. For this reason, this research proposes a hardware modification for grid-tied h-bridge neutral point clamped (HB-NPC) and cascade h-bridge (CHB) inverters to address the issue of leakage ground current (LGC). The proposed modification is evaluated under different modulation schemes, and it is found to be effective in mitigating LGC. The approach used involves a system model that separates the differential-mode (DM) and common-mode (CM) parts, making the control design simpler by breaking it down into two independent processes. Furthermore, this work presents a control design for balancing the DC-link capacitor voltages of a transformerless HB-NPC inverter. The control law is designed based on the DM and CM models, where the DM model is used for grid current injection control, and the CM model is used for balancing the DC-link capacitors' voltages. Finally, this work introduces a prescriptive control approach for grid-tied inverters, which is different from the common feedback actions such as proportional (P), integral (I), derivative (D), and resonant (R) controls. The prescriptive approach begins by prescribing desired dynamics and then synthesizes the exact feedback to achieve these dynamics. The approach is found to be compatible with popular P+R control but offers a wider range of dynamic responses and improved performance metrics. The hardware solution and control methods are tested through simulation and experimental evaluation to demonstrate their effectiveness.

https://orcid.org/0000-0003-0223-2748