A multiphase inverter for connection to an energy supply network, comprising a control device with an evaluation unit, the multiphase inverter comprising an output current filter on an alternating current side thereof that is connected to the energy supply network via a switching element and at least one intermediate circuit capacitance on the direct current side thereof, wherein the control device, in conjunction with the evaluation unit, is configured to:
- disconnect the photovoltaic system from the energy supply network by opening the switching element;
- operate the inverter to set up an island network after the disconnecting, wherein an in-phase AC voltage is applied to at least two outputs of an inverter bridge of the multiphase inverter and a flow of current is produced between the at least one intermediate circuit capacitance and at least one filter capacitance of the output current filter;
- measure currents (Ia, Ib, Ic) flowing at the outputs of the inverter bridge and at least one voltage present at one of the capacitances, and
- determine a capacitance value of at least one of the capacitances using the determined voltage and the measured currents (Ia, Ib, Ic).