A Novel Voltage-Clamped Submodule Scheme for Enhancing Overvoltage Withstand Capability of High Voltage Hybrid Transformers

The hybrid transformer (HT), which integrates the power electronic converters and the electromagnetic transformer, has the ability to adjust the transformation ratio rapidly, making it a crucial component in the development of future flexible power grids. However, one big challenge is the mismatch of overvoltage withstand capability of electromagnetic transformer and converters in the HTs. The limited overvoltage capacity of the converter limits the overall performance of the HT in overvoltage events in the power grid. To address this issue, a novel voltage-clamped submodule (VCS) scheme is proposed for enhancing overvoltage withstand capability of high-voltage HT. The voltage-clamped circuit, which consists of a gap and a metal oxide varistor, ensures the safe operation of the VCS without the risks of breakdown during overvoltage period. Thus, this scheme allows the high-voltage HT to take advantage of the surge resistance of electromagnetic transformers and keep the enhanced controllability normally during transient surges. An experimental platform was built to verify the effectiveness of the proposed VCS, and simulation results demonstrated the usefulness of the high voltage HT with proposed scheme in power delivery systems subjected to different transient surges.