Grid-forming-mode operation of boost-power-stage converter in PV-generator-interfacing applications

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 composing of DC-DC-converter-interfaced PV generator and grid-connected inverter connected in cascade. 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 either in 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 shows explicitly that only the voltage-fed mode would provide feasible dynamic and stability properties as a viable interfacing converter.