On-load tap changer control method, excitation control system carrying out said control method and power excitation chain

The present invention relates to an on-load tap changer control method for a power transformer (5) in a power system, where the power transformer has a primary side for a connection to a first grid in which electric power is generated (3), and a secondary side for connection to a second grid (9) in which electrical power is consumed, the power transformer (5) being equipped with an on-load tap changer (7), said method comprising the following steps: - measuring the voltage and current at least on the primary side ( u 1 , i 1 ) or on the secondary side (u 2 i 2 ) of the power transformer, - processing said measured voltages (u 1 ; u 2 ) and currents (i 1 ; i 2 ) in order to derive prospective reactive power at the output of the power transformer (5) after prospective tap-change, - compare prospective reactive power to a predefined set-point, - initiate tap-change of on-load tap changer (7) if prospective reactive power is closer to said predefined set-point than actual reactive power.