Analysis and Optimization for Voltage Overload Resilience of Hybrid Transformers Under Grid Voltage Fluctuations

The magnetic coupling between windings in hybrid transformers (HTs) leads to a voltage overload phenomenon for converters during voltage fluctuations, diverging from those conventional converters. Parameter design principles of hybrid transformers considering this characteristic are proposed. A comprehensive optimization approach is then introduced to enhance voltage overload resilience of HTs, encompassing both control strategy and hardware performance. For one thing, an optimized active-DC-voltage-boost control strategy is proposed to mitigate DC voltage oscillation and overload risks during the high-voltage ride-through events. For another, a gap-MOV-based protection circuit is implemented in the submodules to achieve direct clamp of the IGBTs voltage. The submodule with this protection circuit can operate safely under 1.3 times voltage overload verified by double-pulse experiments. Validation of the proposed optimization method is conducted through comparative system-level simulations of a wind power delivery system with a hybrid transformer, employing HVRT testing waveforms specified by the national standard of China. Results show a 70% reduction in DC voltage oscillation under 1.3 times grid voltage step-up conditions and reliable voltage clamp by the protection circuit inside submodules are achieved. Overall, the proposed approach enhances overvoltage resilience of HTs in power system applications.