Abstract:As a basic building block in power systems, the three-phase voltage-source inverter (VSI) connects the distributed energy to the grid. For the inductor-capacitor-inductor (LCL)-filter three-phase VSI, according to different current sampling position and different reference frame, there mainly exist four control schemes. Different control schemes present different impedance characteristics in their corresponding determined frequency range. To analyze the existing resonance phenomena due to the variation of grid impedances, the sequence impedance models of LCL-type grid-connected three-phase inverters under different control schemes are presented using the harmonic linearization method. The impedance-based stability analysis approach is then applied to compare the relative stability issues due to the impedance differences at some frequencies and to choose the best control scheme and the better controller parameters regulating method for the LCL-type three-phase VSI. The simulation and experiments both validate the resonance analysis results.
During the dead-time interval for a phase-shifted full-bridge (PSFB) converter, switches can achieve zero-voltage switching (ZVS) operation by using the energy stored in the transformer leakage inductance to discharge or charge the output capacitance of the switches. The value of dead-time has a great effect on ZVS range so it is a key parameter that needs to be optimised during designing the converter. This study develops theoretical analysis to estimate the dead-time as a function of load current for PSFB converter. The equivalent circuits of PSFB converter during the deadtime interval with considering magnetising inductance are mainly analysed in this study. Compared with previous analyses, the influences of dead-time on ZVS range and efficiency are discussed in detail. The analysis results show that the magnetising inductance of transformer can ensure ZVS over a wide range of load conditions, especially under discontinued current mode operation, without a penalty on the heavy-load efficiency. By adjusting the dead-time dynamically, both conduction and switching losses can be reduced. A 1 kW prototype is implemented and experimental results are consistent with the theoretical analysis.
Magnetic components are important parts of the phase shifted full bridge (PSFB) converter. During the dead-time of switches located in the same leg, the converter can achieve zero-voltage-switching (ZVS) by using the energies stored in magnetic components to discharge or charge the output capacitances of switches. Dead-time is usually calculated under a given set of pre-defined load condition which results in that the available energies are insufficient and ZVS capability is lost at light loads. In this paper, the PSFB converter is controlled by variable dead-time method and thus full advantage can be taken of the energies stored in magnetic components. Considering that dead-time has a great effect on ZVS, the relationship between available energies and magnetic component values is formulated by analyzing the equivalent circuits during dead-time intervals. Magnetic component values are chosen based on such relationship. The proposed choice procedure can make the available energies greater than the required energies for ZVS operation over a wide range of load conditions. Moreover, the burst mode control is adopted in order to reduce the standby power loss. Experimental results coincide with the theoretical analysis. The proposed method is a simple and practical solution to extend the ZVS range.
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