A High-Efficiency 5-kW Soft-Switched Power Conditioning System for Low-Voltage Solid Oxide Fuel Cells

This paper presents a high-efficiency power conditioning system design that employs a soft-switched dc-dc converter and a soft-switched dc-ac inverter for a low-voltage 26-V, 5-kW solid oxide fuel cell. The dc-dc converter converts fuel cell voltage to a 400-V dc bus voltage using a 3-phase 6-leg phase-shift modulated converter to achieve higher than 96% peak efficiency with zero-voltage zero-current switching. The dc-ac inverter then produces 120/240V ac using a CoolMOS based soft-switching inverter to achieve 98% peak efficiency. The inverter output is universal for both standalone and grid-tie modes operation. The standalone load comes out of two sets of LC filter with a neutral line for equal voltage splits. For grid-tie mode, a circuit breaker needs to turn on, and an additional inductor is included for grid current ripple reduction. Test results indicated that peak efficiency of 94% is achieved for the two-stage power conversion PCS. In addition to high efficiency design, this paper also introduces advanced controls including fuel cell current ripple reduction using a current loop in the dc-dc converter and ac output current steady-state error reduction using an admittance compensation technique in the dc-ac inverter.