Analysis and Protection Research on Switching Overvoltage Characteristics in GWh-Level Cascaded High-Voltage Battery Energy Storage Systems

This study presents the first comprehensive investigation of switching overvoltage characteristics in transformerless 35 kV cascaded battery energy storage systems (BESS) with direct grid integration. In contrast to conventional transformer-coupled systems, the proposed 30-level cascaded architecture eliminates power conversion losses while introducing unique overvoltage distribution challenges caused by extended cascaded chains and distributed electromagnetic coupling effects. Electromagnetic transient modeling of 90 storage submodules (SSMs) using PSCAD systematically quantifies port-to-ground overvoltage under critical operational scenarios, including no-load transformer switching, transmission line operations, and internal/external short-circuit faults. Key findings demonstrate: (1) Bus-proximate SSMs exhibit overvoltage amplitudes reaching 50.28 kV, exceeding those near neutral points by a factor of ten (5.14 kV), necessitating prioritized insulation reinforcement for the first five SSM tiers; (2) Transformer energization generates severe electromagnetic oscillations producing overvoltage exceeding 50 kV, whereas other operational conditions limit amplitudes below 41 kV. A dual protection strategy is proposed, combining graded insulation design for bus-side modules with system lockout protocols during transformer charging sequences. These findings establish critical guidelines for insulation coordination and overvoltage mitigation in GWh-class cascaded BESS.