Reduction of disturbances in a power network

A device in an electric power network (1), comprising at least one input line (21, 22, 23), at least one output line (31, 32), an active filter (4) controllable via a control input (41) as well as a plurality of interconnected components (5), for reducing on the output line (31, 32) disturbances generated by a disturbance source (6) located in, or connected to, the power network, wherein of the components (5) a first component group (101), a second component group (102) and a third component group (103) are formed such that the input line (21, 22, 23) and the disturbance source (6) are connected to the first component group (101), at least the second component group (102) comprises at least approximately linear components and comprises the first active filter (4), and that the output line (31, 32) is connected to the third component group (103), that a first section (A--A) and a second section (B--B) through the power network (1) are defined such that the input line (21, 22, 23) and the disturbance source (6) are located on one side of the first section (A--A) and the output line (31, 32) and the control input (41) are located on the other side of the first section (A--A) and that the control input (41) and the output line (31, 32) are located on different sides of the second section (B--B) and the second component group (102) is located between the first section (A--A) and the second section (B--B), and that the second section (B--B) doe snot intersect the first section (A--A), which device comprises a control member (8) which is supplied with at least on first state variable (Y1i) sensed on the power network (1) and generates and supplies the control input (41) with a control signal (C1), which control member (8) comprises,

• a first calculating member (81) which, based on the first state variable (Y1i) and a model of the power network (1), determines an equivalent impedance (61) for the disturbance source (6), as viewed from the first section (A--A), and an equivalent disturbance voltage generator or an equivalent disturbance current generator (62), such that the voltage generator or current generator (62) respectively at the first section (A--A), would generate at least substantially the same disturbances (X1) as the disturbance source (6),
• a second calculating member (82) which, based on the equivalent disturbance voltage generator or equivalent disturbance current generator (62) respectively, on the equivalent impedance (61) and a model of the second component group (102) and the third component group (103), calculates those disturbances (X2) which occur in the second section (B--B) without any action from the active filter (4), and
• a fourth calculating member (84) which calculates the control signal (C1) such that, supplied to the control input (41), it would generate in the second section (B--B) the disturbance (X2), calculated by means of the equivalent disturbance voltage generator or equivalent disturbance current generator (62) respectively, with reversed polarity.