Reserved-Energy-Aided Control in Solid-State Transformers

Solid-state transformers (SSTs) operate within the overvoltage and undervoltage limits to ensure safety and power quality (prevent overmodulation). This constrained region of operation creates a reserve of available energy in the system, which is primarily stored in the multiple dc-link capacitors. The energy reserves in the system are essential during load transients as the initial energy demand is compensated by the dc-link capacitors. This article presents a detailed analysis of the energy distribution during load transients for different SST control strategies and the impact of control gains. Subsequently, a reserved-energy-aided control parameter design approach is proposed that effectively distributes the stored energy of the capacitors within prescribed operating limits for a safe and controllable operation. The proposed design approach expands the operational range of SSTs, allowing them to handle larger variations in load power compared to conventional control design strategies. The results are validated on a single-phase, two-stage, two-string 230-V/250-V 1-kVA 50-kHz SST system.