Tap changing for a power transformer

A control system configured to control a switching of taps of an electronic on-load tap changer provided on an electric power transformer of a wind turbine electrical power system, wherein the electronic on-load tap changer comprises switches that are controllable to switch between transformer taps, wherein the transformer taps are provided on a primary side of the electric power transformer, wherein the electric power transformer is configured to provide electric power transformation between a second voltage on a secondary side of the electric power transformer and a first voltage on the primary side, the first voltage being higher than the second voltage, wherein the control system comprises:

• a detector configured to detect an indication of a zero crossing of a transformer voltage of the electric power transformer based on a measurement of a voltage on the secondary side of the electric power transformer;
• and
• a control signal generator configured to generate a control signal that controls a switching of the switches for performing a tap change to a new transformer tap;
• wherein the control signal generator is configured to determine a timing of the control signal for changing the transformer tap based on the detected indication of the zero crossing of the transformer voltage such that the switching of at least one of the switches for performing the tap change occurs a predetermined time prior to a zero crossing of a tap voltage at the new transformer tap, the predetermined time being less than 40% of a duration of an electrical period of the tap voltage.