A method and arrangement for determining a capacitor degradation parameter associated with the degree of degradation of a cell capacitor in a converter cell of a power converter including a plurality of identical converter cells, including redundant converter cells, which converter cell, prior to determining the capacitor degradation parameter, has been bypassed. The method includes a) obtaining a measurement parameter value associated with discharging of the cell capacitor, forming a dc-link, from a measurement start voltage level at a measurement start time to a measurement end voltage level at a measurement end time, and b) determining a value of the capacitor degradation parameter based on the measurement parameter value.