Analysis and Optimization for Current Overload Capability of Hybrid Transformers Under Grid Voltage Sag

The integration of extensive power electronic equipment rises a demand for flexible and rapid voltage regulation in future grids. Hybrid transformers (HTs) are considered highly competitive solutions, combining electromagnetic transformers with partially rated capacity converters. However, the tight electromagnetic coupling of HTs results in current overloads when compensating for voltage sags, limiting the full utilization of their rated compensation capacity. Furthermore, this overload increases as voltage sag deepens. These unique current overload aspects are crucial for HTs to fully utilize their voltage regulation capability in practical applications. In this article, the current overload characteristics and influencing factors of HTs are discussed. Based on this, a coordinated optimization of hardware and software components is proposed in light of their electromagnetic coupling feature. The performances of optimized topology selection and operation mode are verified by simulation cases separately and evaluated by a current overload index quantitatively, decreased by 59% and 66%, respectively. The enhanced current overload method is verified with an 800-ms three-times-module-rated-current overcurrent ride-through experiment. These improvement help meet the HT current overload requirements including those in the LVRT process.