Method and apparatus for computer-implemented controlling of a doubly-fed electric machine

An apparatus for computer-implemented controlling a doubly-fed electric machine, where stator windings of a stator are directly connected to an electrical grid and where rotor windings of a rotor are connected to the electrical grid via a power conversion system, comprising an AC-to-DC converter and a DC-to-AC converter and being adapted to control a rotor current, wherein the apparatus comprises:

• a processor configured to:
• a) obtain a rotational speed of the doubly-fed electric machine;
• b) determine, whether the rotational speed is within a predetermined operational speed range around a synchronous speed;
• wherein, if it is determined that the rotational speed is within the predetermined operational speed range, the processor is further configured to:
• c) control the AC-to-DC converter of the power conversion system to force injection of a stator reactive power to create a harmonic at a frequency different than a rated frequency of the doubly-fed electric machine;
• and
• d) control the DC-to-AC converter of the power conversion system to compensate the stator reactive power and the harmonic,
• wherein
• the processor is configured to control the AC-to-DC converter in step c) such that the forced injection of the stator reactive power is maximized at the synchronous speed and decreasing to borders of the predetermined operational speed range.