In transformer-less grid-connected renewable energy systems, interfacing of photovoltaic (PV) generators is typically implemented by means of DC-DC boost-power-stage converter, mainly because of its voltage-boosting capability. In order to track the maximum power point of a PV generator, input voltage of the converter is usually feedback controlled, forcing the converter to operate as a current-sourced rather than voltage-sourced converter. Nevertheless, PV generator interfacing power stage is commonly assumed to possess the same dynamic properties as corresponding voltage-sourced power stage. Investigations presented in this study reveal explicitly that the dynamics of PV generator interfacing DC-DC boost power stage resembles conventional buck power stage behaviour with duty-cycle independent resonance and additional right-halfplane zeros. In addition, the duty cycle has to be decreased for increasing the corresponding output variables (i.e. input voltage and output current). Extended experimental results are given to support the theoretical findings.
Abstract:The application of constant power control and inclusion of energy storage in grid-connected photovoltaic (PV) energy systems may increase the use of two-stage system structures composed of DC-DC-converter-interfaced PV generator and grid-connected inverter connected in cascade. A typical PV-generator-interfacing DC-DC converter is a boost-power-stage converter. The renewable energy system may operate in three different operation modes-grid-forming, grid-feeding, and grid-supporting modes. In the last two operation modes, the outmost feedback loops are taken from the input terminal of the associated power electronic converters, which usually does not pose stability problems in terms of their input sources. In the grid-forming operation mode, the outmost feedback loops have to be connected to the output terminal of the associated power electronic converters, and hence the input terminal will behave as a negative incremental resistor at low frequencies. This property will limit the operation of the PV interfacing converter in either the constant voltage or constant current region of the PV generator for ensuring stable operation. The boost-power-stage converter can be applied as a voltage or current-fed converter limiting the stable operation region accordingly. The investigations of this paper show explicitly that only the voltage-fed mode would provide feasible dynamic and stability properties as a viable interfacing converter.
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