Enhanced Protection of Grid-Connected Solid-State Transformers Against Sympathetic Inrush Current

The transition from line frequency transformers (LFT) to solid-state transformers (SST) in modern distribution systems introduces new challenges, particularly in managing sympathetic inrush current (SIC). This current, often ranging from 1.5 to 10 times the rated current, induces overcurrent stresses on the SST's semiconductor switches, potentially compromising the system's reliability. However, the existing body of research offers limited insight into the effects of SIC on SST performance, leaving a critical gap in the understanding of these phenomena during grid events. This paper addresses this gap by analyzing the impact of SIC on SSTs and developing a mathematical model to simulate SIC events in grid-connected SSTs. Through this model, the relationship between the peak value of the SST input current and the phase shift angle (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\phi$</tex-math></inline-formula>) of high-frequency transformer (HFT) voltages is established, providing key insights into the root causes of overcurrent stresses. To mitigate these stresses, a novel Phase Shift Angle Control (PSAC) algorithm is proposed. This technique dynamically calculates the second harmonic during SIC events on a per-cycle basis, enabling precise control and effective mitigation of the current surge. The PSAC algorithm's effectiveness is demonstrated through both MATLAB simulations and experimental validation. Results show a significant reduction in SIC from 12.96 A to 8.1 A in simulations, and from 25 A to 2.5 A in experimental tests, aligning the current with rated levels. The findings suggest that the proposed PSAC technique is a robust solution for protecting SSTs from the adverse effects of SIC, thereby enhancing their operational resilience and reliability in modern power distribution systems.