Harmonics mitigation in multiphase generator-conversion systems

1. A multiphase generator-conversion system comprising:

a multiphase generator having one rotor and m+1 number of electromagnetically coupled stators, each stator comprising a plurality of phase legs,

a converter having m+1 conversion lines, each conversion line connected to the plurality of phase legs of one of the m+1 stators, each conversion line comprising a rectification module, wherein

at most m of the m+1 rectification modules comprise respective active filtering converters, and at least one of the m+1 rectification modules comprises a passive rectifier,

whereby at least one of the active filtering converters is configured to directly control its current to vary the magnetic flux of the stator to which it is connected and to indirectly affect the magnetic flux of the rest of the stators through the electromagnetic coupling between them, to mitigate the harmonic content.

2. The multiphase generator conversion system according to claim 1 , wherein each active filtering converter is configured to receive the values of the currents of the phase legs of all stators and regulate its current to reduce the harmonic content from all stators when applied to the stator to which it is connected.

3. The multiphase generator conversion system according to claim 1 , wherein each of the m+1 conversion lines comprises either one of the active filtering converter or the passive rectifier, the passive rectifier comprising at least one diode rectifier, each of the active filtering converters or diode rectifiers being connected to a different one of the m+1 stators.

4. The multiphase generator conversion system according to claim 1 , wherein each rectification module comprises a diode rectifier, each connected to one of the m+1 stators, and wherein the active filtering converters are each connected to the conversion line between one of the m+1 stators and its respective diode rectifier, in a shunt configuration.

5. The multiphase generator conversion system according to claim 1 , wherein each rectification module comprises a diode rectifier, each connected to one of the m+1 stators, and wherein the converter further comprises

a switching circuit, arranged between at least one of the active filtering converters and a plurality of the m+1 stators, wherein the switching circuit is configured to selectively connect the at least one of the active filtering converters to the conversion line between one of the plurality of m+1 stators and its respective diode rectifier, in a shunt configuration, so that at any moment in time only one of the rectification modules connected to the plurality of the m+1 stators comprises the at least one of the active filtering converters and the respective diode rectifier.

6. The multiphase generator conversion system according to claim 5 , wherein the switching circuit comprises

a plurality of branched lines, each branched line corresponding to one of the phase legs of the plurality of m+1 stators and being connected at the root to one of the active filtering converters, wherein each branched line comprises at most m+1 number of branches,

at most m+1 switches each receiving one branch of each of the plurality of branched lines, each switch being arranged between the active filtering converter of an associated one of the branched lines and one of the plurality of m+1 stators.

7. The multiphase generator conversion system according to claim 1 , wherein the multiphase generator is a permanent magnet generator.

8. The multiphase generator conversion system according to claim 1 , wherein the multiphase generator is a nine-phase generator and m equals 2.

9. The multiphase generator conversion system according to claim 1 , wherein the rectification module comprises exactly one active filtering converter.

10. The multiphase generator conversion system according to claim 1 , wherein the active filtering converter is a voltage source converter (VSC).

11. The multiphase generator conversion system according to claim 1 , wherein each conversion line further comprises:

a dc/ac inverter connected to an output of the respective rectification module of the conversion line,

a transformer, connected to an output of the respective dc/ac inverter, and

an output diode rectifier connected to an output of the respective transformer.

12. The multiphase generator conversion system according to claim 1 , wherein each phase leg comprises a single winding.

13. A wind turbine system comprising a multiphase generator conversion system according to claim 1 .

14. A method of mitigating harmonics in a multi-phase generator-conversion system according to claim 1 , comprising:

measuring the currents of the phase legs of the m+1 stators of the multi-phase generator;

identifying a plurality of harmonic contents present in the phases of the m+1 stators of the multi-phase generator;

calculating a mitigation current based on said current values;

with the corresponding active filter, applying the calculated current to directly control its current to vary the magnetic flux of the stator where the active filter is connected and to indirectly affect the magnetic flux of the rest of the stators through the electromagnetic coupling between them, to mitigate the harmonic content.