Control of a doubly-fed induction machine

A control system for controlling the operation of a doubly fed induction machine (DFIM), wherein the DFIM includes a stator configured to be electrically coupled to a power grid and a rotor configured to rotate with a rotational speed, and wherein the DFIM has a synchronous rotational speed of the rotor and a rated rotational speed of the rotor, wherein rotation of the rotor at the synchronous rotational speed generates slot harmonic distortions having a harmonic order,

• wherein the control system is configured to perform the step of operating the DFIM at the rated rotational speed of the rotor, wherein the rated rotational speed of the rotor is set to a value selected such that a shift of the harmonic order of one or more of the slot harmonic distortions at the rated rotational speed of the rotor is an integer number or is within a predefined limit of an integer number;
• wherein a slip of the rotor is defined in dependence on a difference between the synchronous rotational speed (nsync) of the rotor and the rotational speed (nrot) of the rotor, wherein the rated rotational speed of the rotor is set by setting the slip at the rated rotational speed of the rotor to a predetermined value;
• wherein the rated rotational speed of the rotor is set to a value nr in units of rpm determined by the equation nr=60*(1+k*F)*f/p, wherein k is an integer, F is a predetermined fraction determined by a symmetry of the rotor, f is the grid frequency and p is the pole pair number of the rotor.