Lightning Overvoltage Response Modeling and Protection Design for Solid State Transformer

The rapid development of the AI and ICT industry has driven massive data center construction. Compared to conventional LFT (line-frequency transformer)-isolated conversion system, SST-based uninterruptible power supply solution offers high efficiency, high power density and control flexibility, making it a competitive power supply solution. Eliminating the LFT and adopting the cascaded modular design enables CHB-based SST to better meet the demands of large-scale and rapid deployment. However, without the isolation provided by LFT, power converter in SST is directly exposed to the MV (Medium Voltage) grid side surges, leading to failure with insufficient overvoltage and overcurrent withstand capabilities of the power electronic devices. The lightning overvoltage, one of the most severe grid-side surges, poses a significant threat to the internal power electronic devices and the insulation of the SST. Yet, existing research lacks a quantitative description of lightning overvoltage response of cascaded SST under three-phase steady-state operation, and is insufficient to evaluate the impact of filter design, floating DC bus design, operating states and grounding scheme. Lightning overvoltage response model for three-phase steady-state operating SST based on the configuration of a 2 MVA 10 kV CHB-based SST is established to assess and quantify the effects of the aforementioned parameters design. A comparison with the LFT-isolated conversion system is made to clarify the isolation effect of LFT, providing design guidelines for the SST lightning protection design.