CLAIMS :

1. Method for controlling a dual three-phase machine (14), in particular a fractional slot concentrated winding electrical machine (14), having a first three-phase winding set (21) being connected to a first AC-DC converter (33) and having a second three-phase winding set (23) being connected to a second AC-DC converter (41), the method comprising:

controlling the first (33) and the second AC-DC con-verter (41) such that first electric currents carried in the first three-phase winding set (21) are essentially 30° phase shifted relative to respective second electric currents carried in the second three-phase winding set (23) .

2. Method according to the preceding claim,

wherein a first current of an A phase of the first three-phase winding set is generated to be essentially 30° phase shifted relative to a second current of an A phase of the second three-phase winding set,

wherein a first current of a B phase of the first three-phase winding set is generated to be essentially 30° phase shifted relative to a second current of a B phase of the second three-phase winding set,

wherein a first current of a C phase of the first three-phase winding set is generated to be essentially 30° phase shifted relative to a second current of a C phase of the second three-phase winding set.

3. Method according to one of the preceding claims,

wherein the first converter (33) and second converter (41) are controlled by PWM such as space vector modulation or third harmonic injection modulation, across one fundamental period, for six separated time intervals (47a, 47f) in overmodulation, intercalated with six separated time inter-vals (49a, 49f) in linear range, wherein another number of separated time intervals, e.g. 12, is present depending on the way of overmodulation.

4. Method according to one of the preceding claims,

wherein the first converter (33) is controlled by PWM such as space vector pulse width modulation based on a first control vector (CV1, CV2) defining intended AC voltages, in particu-lar in a dq-coordinate system, at first terminals (32) of the first three-phase winding set (21), the first control vector having a first magnitude varying across a fundamental period of the AC voltage at the first terminals, in particular with a frequency six times a fundamental frequency being the in-verse of the fundamental period.

5. Method according to one of the preceding claims,

wherein the first magnitude is defined with reference to a circle (51) and to a hexagon (3) having a common central point (5) ,

wherein the first magnitude corresponds to a length of a straight line from the central point (5) to an edge of the hexagon (3), if the radius (r, M) of the circle (51) is equal to or larger than the length,

wherein the first magnitude corresponds to the radius (r, M) of the circle (51), if the radius (r) of the circle (51) is smaller than the length of a straight line from the central point (5) to a corner (CI, C6) or an edge of the hexagon

(3) ,

wherein in particular sections of the circle (51) lie outside the hexagon (3) .

6. Method according to one of the preceding claims,

wherein the radius (r, M) of the circle (51) is greater than 1, in particular between 1 and 1.1, of a radius (ri) of a maximal inner circle (7) fitting within the hexagon (3) .

7. Method according to one of the preceding claims,

the hexagon (3) having a size such that the corners (CI, C6) are away from the central point (5) by a distance corresponding to the maximal voltage achievable at atleast one of the first terminals.

8. Method according to one of the preceding claims,

wherein the maximal voltage is achieved, when controllable switches in the first or second converter (33, 41) are switched according to an active vector in the space vector modulation scheme.

9. Method according to one of the preceding claims,

the method further comprising:

measuring the first and/or second currents;

filtering the first and/or second currents to reduce an amplitude of a 6f harmonic;

supplying the filtered first and second currents to respective first and second current controllers (71a, 71b) ;

the current controllers outputting preliminary first and second control vectors (Vdl, Vql, Vd2, Vq2), in particular having a magnitude smaller than the radius (r) of the circle.

10. Method according to one of the preceding claims, the method further comprising:

supplying the preliminary first and second control vectors (Vdl, Vql, Vd2, Vq2 ) to respective first and second pre-modulation modules (75a, 75b) ;

the first and second pre-modulation modules deriving the first and second control vectors (CVdl, CVql, CVd2, CVq2) .

11. Method according to one of the preceding claims, wherein the first and second pre-modulation modules (75a, 75b) are adapted to:

increase the amplitude of the preliminary first and sec-ond control vectors (Vdl, Vql, Vd2, Vq2), in particular from the radius (ri) of the maximal inner circle (7), to the radius (r, M) of the circle (51);

derive the first and second control vectors (CVdl, CVql, CVd2, CVq2) such that their length is within the hexagon (3) .

12. Method according to one of the preceding claims, the wherein the second converter (41) is controlled by space vector pulse width modulation and/or third harmonic injection modulation based on a second control vector (CV2) defining intended AC voltages at second terminals of the second three-phase winding set, the second control vector (CV2) essentially being equal to the first control vector (CV1) phase shifted by 30°,

wherein in particular the electrical machine (14) is a generator, in particular of a wind turbine (87), wherein the first AC-DC converter and second AC-DC converter are generator connected converters,

wherein further in particular a ratio between a number of slots and a number of poles of the electrical machine is not an integer.

13. Arrangement (69) for controlling a dual three-phase ma-chine, in particular a fractional slot concentrated winding electrical machine (14), having a first three-phase winding set (21) being connected to a first AC-DC converter (33) and having a second three-phase winding set (23) being connected to a second AC-DC converter (41), the arrangement being adapted:

to control the first AC-DC converter (33) and the second AC-DC converter (41) such that first electric currents carried in the first three-phase winding set are essentially 30° phase shifted relative to respective second electric currents carried in the second three-phase winding set,

wherein the arrangement is in particular adapted to perform a method according to one of the preceding claims.

14. Wind turbine (87) having a generator system (15) compris-ing:

a dual three-phase machine, in particular a fractional slot concentrated winding electrical machine (14), configured as a generator;

a first converter (33) ;

a second converter (41); and

an arrangement according to the preceding claim, connected for controlling the electrical machine.