An Energy Differential Protection Scheme for Three-Stage SST AC-Side Outgoing Lines with Submodule Capacitor Energy Compensation

Protection of AC-side outgoing lines in three-stage solid-state transformers (SSTs) is challenging because control driven port currents undermine conventional current differential criteria, while overcurrent schemes are sensitive to transition resistance. This paper focuses on the phase-to-phase and three phase short-circuit faults, which are typically severe and require fast, reliable discrimination. Energy-differential protection offers a promising alternative; however, its direct application to SSTs is affected by substantial, control-dependent internal energy variations—mainly associated with submodule capacitors—which may delay internal-fault operation or trigger maloperation under external faults. This paper proposes an energy-differential protection scheme for three-stage SST AC-side outgoing lines with submodule-capacitor energy compensation. The proposed method suppresses the influence of internal energy dynamics without increasing setting margins or requiring additional measurement points. Its effectiveness is verified via MATLAB/Simulink simulations and RT-LAB real-time tests, demonstrating improved tolerance to transition resistance and enhanced noise immunity compared with existing methods.