Abstract: An active filter (2) is connected to a dc circuit (L1, L2) in a converter (SR1) in an installation for transmission of high-voltage direct current. Control equipment (5) generates, in dependence on a control signal, a control order (SCF) for the active filter which, in dependence thereon, generates and supplies to the dc connection one of a filter voltage (UF) and a filter current (IF) for reducing at least one tone of a chosen tone frequency in a current (Id) in the dc connection. The control equipment comprises a controlling unit which forms a controller signal in dependence on a supplied current measurement value (IM) of the current in the dc connection, and an identification unit. The identification unit forms a first compensating parameter in dependence on the current measurement value and on an excitation signal generated by the control equipment and added to the control order.