Minimum DC-Link Capacitance Requirement of a Two-Stage Photovoltaic Inverter

Photovoltaic generator is often interfaced with the utility grid using a two-stage inverter. The two-stage inverter comprises of a dc-dc boost power-stage converter and a three-phase inverter. The duty ratio of the dc-dc converter can be given directly by a MPPT-algorithm or, alternatively, by a controller which regulates the voltage of the PV generator. It is shown in this paper that the two-stage inverter has a RHP-pole in its dc-link-voltage control dynamics with the first control option but the RHP-pole is effectively removed when the voltage of the PV generator is controlled. The RHP-pole introduces a minimum value for the required dc-link capacitance. The minimum value of the dc-link capacitance can be solved when the maximum power of the PV generator is known. Too small capacitance values in the dc-link make the dc-link-voltage control of the inverter unstable leading to subharmonic ac current components. The existence of the RHP-pole is inherent to the power stage and, thus, does not depend on the power or voltage level. The results are verified using a low-power prototype operating at reduced voltages and a switching model operating at grid voltage levels.