Appearance of a RHP-zero in VSI-based photovoltaic converter control dynamics

A typical grid-connected voltage source inverter (VSI) in its simplest form consists of an input voltage source, switch matrix, output inductor and grid, which acts as a voltage type load. This arrangement yields a converter with first-order control dynamics, a buck-type converter behavior and discontinuous input current. A photovoltaic (PV) array can provide a specific maximum current depending on the irradiation conditions. This implies that if the peak value of the pulsating input current of the converter tries to exceed the PV array current, the array voltage will collapse to a value dictated by the output voltage of the converter while the output current is determined by the array current. Furthermore, when the discontinuous input current of the converter is zero, the PV array operates in an open circuit. As a consequence, it may be obvious that an energy storage element, i.e. an input capacitor, is needed in parallel with the PV array in order to filter the pulsating input current drawn by the VSI. A vast majority of VSI analysis in PV applications assumes a constant voltage input source and current control dynamics depending only on the filtering elements at the output, e.g. first order dynamics with a mere inductor. However, this assumption is not well justified as will be proven in this paper.