Four-Transistor Four-Diode (4T4D) Series/Parallel Chopper Module for Auto-Balancing STATCOM and Low Control and Development Complexity

Abstract: Static synchronous compensators (STATCOMs) manage reactive power compensation in modern power grids and have become essential for the integration of renewable energy sources such as wind farms. Cascaded H bridges have become the preferred topology for high-power STATCOMs, but balancing module capacitor voltages remains a persistent challenge. Conventional solutions equip every module with a voltage sensor -- a component that is costly, temperature-sensitive, and prone to aging-related failures. Recent parallel-capable module topologies can balance voltage through switched-capacitor operation. The latest developments reduced the sensor requirement from one per module to one per arm. However, these implementations require twice as many individual transistors compared to series-only topologies. We present a STATCOM solution based on the four-transistor four-diode (4T4D) series\,/\,parallel chopper cell. This topology achieves bidirectional parallelization with only four transistors per module -- exactly as many as a conventional full bridge. Furthermore, we propose a dual-loop control strategy that fully eliminates module voltage sensors by inferring voltage levels from the modulation index. This scheme also improves output quality by regulating the modulation depth. We validated our proposal through simulation and experiments. We built a prototype to interface the grid. The prototype further passed robustness tests with step change, current direction reversal, and grid disturbance. This work demonstrates the first modular STATCOM implementation that combines minimum transistor count with complete elimination of module voltage sensors.