This paper investigates the harmonic interactions of current controllers in multiparalleled grid-connected inverters. Potential harmonic instability phenomenon, which features oscillations above the fundamental frequency, is evaluated by the impedance-based stability criterion. The possible reasons of the destabilized inverter with different types of grid impedances are explained by the passivity-based analysis and impedance-based stability criterion. Thereafter, case studies are provided, which shows how the nonpassive grid connected inverters interact with the varying grid impedance due to load changes or by the new connections of other grid-connected inverters. Time domain simulations in the PSCAD/EMTDC environment and experimental results show the corresponding harmonic interaction problems may exist in nowadays power system and the impedance-based stability analysis can be an effective tool to assess those problems.
In this paper, a isolated soft-switching DC-DC converter topology with high-step-down conversion is proposed. Comparing with the conventional two-switch forward converter, it uses two series inductors L1 and L2 and a clamp capacitor C to achieve the lower output voltage and soft-switching operation. In order to prove above results, detailed steady-state analysis is presented, and an input voltage 380V, output voltage 12V, output current 75A prototype converter was built. Finally, simulation results verify the theoretical analysis and show the significant advantages.
For a constant on time controller to be stable with all ceramic output capacitance, a virtual ESR (Equivalent Series Resistance) circuit is needed to inject the required ripple voltage into the FB (Feedback) node. This paper proposes a new stability criterion, which is that the FB node ripple voltage need to have the same phase as the inductor current and have certain amplitude to filter out noise. Based on this stability criterion, the virtual ESR circuit is analyzed. The contributions from the phase node voltage, the inductor current and the output voltage are derived with superposition. Two methods to design the virtual ESR circuit, the In Phase Dominance Method and the Phase Cancellation Method, are proposed and experimentally verified. Comparisons between real ESR and the virtual ESR are made. The effects of virtual ESR circuit on load transient responses, power saving mode waveforms and output voltage setting are also discussed.
This paper presents an analysis and comparison of isolated topologies for bidirectional fuel cell systems. The analyzed topologies are the dual active bridge (DAB) and the isolated full bridge boost converter (IFBBC). The analysis is performed based on the component stress factor (CSF). Results highlight that the DAB has lower CSF than the IFBBC for narrow converter operating points. On the other hand the IFBBC presents a more homogeneous CSF over the entire converter operating range. Finally, experimental results obtained from a 30-80 V 80 A 6 kW 40 kHz IFBBC are presented. The converter achieves efficiencies up to 98.2% and 97.45% depending on the converter power flow.
