Protecting a permanent magnet generator
A method for protecting a permanent magnet generator of a wind turbine with a multiphase generator, and n number of isolated converters, the multiphase generator including a rotor carrying permanent magnets and n number of independent multiphase sub-stators comprising a plurality of windings, each converter being connected to an independent multiphase sub-stator and configured to control the plurality of windings of the multiphase sub-stator comprises determining an asymmetrical short circuit current in one of the sub-stators, which generates a first oscillating torque, disconnecting the converter linked to the sub-stator with an asymmetrically short circuited group of windings, and injecting an asymmetrical current with the remaining connected converters, wherein said injected asymmetrical current generates a second oscillating torque that is substantially opposed in phase to the first oscillating torque so that the first oscillating torque is at least partly compensated is disclosed. Permanent magnet generators are also disclosed.
1. A method for protecting a permanent magnet generator with a multiphase generator, and n-number of electrically isolated converters, wherein the multiphase generator has a rotor carrying permanent magnets and n-number of independent multiphase sub-stators each having a plurality of windings, and each converter is connected to an independent multiphase sub-stator and configured to control the plurality of windings of the multiphase sub-stator,
the method comprising:
determining an asymmetrical short circuit current in one of the sub-stators that results in generation of a first oscillating torque;
disconnecting the converter linked to the sub-stator with an asymmetrically short circuited group of windings; and
injecting an asymmetrical current with the remaining connected converters, wherein the injected asymmetrical current generates a second oscillating torque that is opposed in phase to the first oscillating torque to an extent that the first oscillating torque is at least partly compensated.
2. The method according to claim 1 , wherein the asymmetrical short circuit comprises a two-phase short circuit.
3. The method according to claim 1 , wherein the asymmetrical short circuit comprises a phase-to-neutral short circuit.
4. The method according to claim 1 , wherein determining the asymmetrical short circuit current comprises measuring currents in between a plurality of converters and windings.
5. The method according to claim 1 , wherein the permanent magnet generator comprises a plurality of stator windings constituting a plurality of 3-phase systems, each 3-phase system connected to an electrically isolated one of the converters.
6. The method according to claim 1 , wherein determining the short circuit current further comprises mathematically expressing the short circuit current in terms of a positive and a negative current sequence, the positive current sequence being the direct component of N symmetrical sets of phasors that express N asymmetrical sets of phasors as a linear combination, and the negative current sequence being the inverse component of N symmetrical sets of phasors that express N asymmetrical sets of phasors as a linear combination.
7. The method according to claim 6 , wherein the converters are configured to control the positive current sequence in the windings according to a standard operational control.
8. The method according to claim 7 , wherein injecting a symmetrical current by the remaining connected converters comprises generating a current wherein the negative sequence current of the asymmetrical current generates the second oscillating torque.
9. A permanent magnet multiphase generator, comprising:
at least two electrically isolated converters;
a rotor carrying permanent magnets and n-number of independent multiphase sub-stators, each sub-stator comprising a plurality of windings;
each converter connected to an independent multiphase sub-stator and configured to control the plurality of windings of the multiphase sub-stator;
a plurality of sensors disposed to measure parameters indicative of currents in one or more of the stator windings; and
a control configured to balance a first oscillating torque generated by an asymmetrical short circuit current in accordance with the method of claim 1 .
10. The generator according to claim 9 , wherein the sensors are current sensors.
11. The generator according to claim 10 , wherein each of the sensors is arranged at or near an end of each of the stator windings located between the stator winding and the converter.
12. The generator according to claim 9 , wherein the independent multiphase sub-stators comprise a plurality of stator windings constituting a plurality of 3-phase systems, each 3-phase system connected to an electrically isolated one of the converters.
13. The generator according to claim 12 , wherein the generator comprises three converters, each converter connected with a set of 3-phase stator windings, the sets of stator winding 40° electrically phase shifted in relation to each other.
14. A wind turbine, comprising a permanent magnet generator according to claim 9 .