Determining parameters of a filter circuit

A method for determining at least one parameter (b) of a filter circuit of a converter device, the converter device comprising the filter circuit, a DC link and an active rectifier interconnected between the filter circuit and the DC link, the method comprising:

• switching the active rectifier into an active state, in which a first active rectifier leg of the active rectifier connects a first phase of the filter circuit with a positive side of the DC link and a second active rectifier leg of the active rectifier connects a second phase of the filter circuit with a negative side of the DC link;
• after that, switching the active rectifier into a zero state, in which two or more active rectifier legs of the active rectifier connect two or more phases of the filter circuit to the same voltage potential of the DC link;
• determining a time series (y) of current values by measuring a current through the filter circuit at a plurality of consecutive time steps;
• and
• determining the at least one parameter (b) of the filter circuit by calculating a best fit of the at least one parameter (b) from a difference equation of the filter circuit into which the time series of current values is input, wherein the difference equation is a function of current values at different time steps and of the at least one parameter (b).