Dynamic electric brake for a variable speed wind turbine having an exciter machine and a power converter not connected to the grid
A variable speed wind turbine having a doubly fed induction generator (DFIG), includes an exciter machine mechanically coupled to the DFIG and a power converter placed between a rotor of the DFIG and the exciter machine. Thus, the power converter is not directly connected to the grid avoiding the introduction of undesired harmonic distortion and achieving a better power quality fed into the utility grid. Moreover, the variable speed wind turbine includes a power control and a pitch regulation.
1. A variable speed wind turbine comprising:
a rotor including at least one blade;
a drive train coupled to the rotor, the drive train including at least a doubly fed induction generator (DFIG), said DFIG having at least a stator connectable to a power grid,
at least an exciter machine coupled to the drive train; and
at least a power conversion device isolated from the grid and electrically coupled to a rotor of the doubly fed induction generator and to the exciter machine to transfer electrical power between the rotor and the exciter machine; and
an electric braking circuit placed in the power conversion device between the rotor and the exciter machine.
2. The variable speed wind turbine system according to claim 1 , wherein the electric braking circuit is connected to a direct current (DC) bus in the power conversion device.
3. The variable speed wind turbine system according to claim 2 , wherein the power conversion device includes a first electronic power converter and a second electronic power converter,
wherein the direct current bus couples the first electronic power converter to the second electronic power converter,
wherein the first electronic power converter is connected to the doubly fed induction generator and the second electronic power converter is connected to the exciter machine.
4. The variable speed wind turbine system according to claim 1 , wherein the electric braking circuit dissipates power from at least one of the exciter machine and the rotor circuit of the DFIG.
5. The variable speed wind turbine system according to claim 1 , wherein the electric braking circuit includes at least a dissipative element activated by a controllable switch device.
6. The variable speed wind turbine system according to claim 5 , wherein the dissipative element is a resistor.
7. The variable speed wind turbine system according to claim 1 , wherein the electric braking circuit dissipates the power from at least one of the exciter machine and the rotor circuit of the DFIG, if electric braking is activated.
8. The variable speed wind turbine system according to claim 1 , wherein the electric brake is activated in response to a sensing of at least one of double fed induction generator shaft speed and doubly fed induction generator shaft acceleration.
9. The variable speed wind turbine system according to claim 7 , wherein the electric brake is activated in response to a sensing of at least one of exciter shaft speed and exciter shaft acceleration.
10. The variable speed wind turbine system according to claim 7 , wherein the electric brake is activated in response to a sensing of at least one of the DFIG rotor speed and the DFIG rotor acceleration.
11. The variable speed wind turbine system according to claim 7 , wherein the electric brake is activated in response to a sensing of at least one of a DFIG rotor frequency and sequence, and a DFIG rotor frequency change rate and sequence.
12. The variable speed wind turbine system according to claim 7 , wherein the electric brake is activated in response to a sensing of at least one of an exciter machine frequency and an exciter machine frequency change rate.
13. The variable speed wind turbine system according to claim 7 , wherein the electric brake is activated in response to a sensing of at least one of an exciter machine voltage and an exciter machine voltage change rate.
14. A variable speed wind turbine system comprising:
a first rotor shaft including at least one blade;
a doubly fed induction generator coupled to the first rotor shaft and having a stator connectable to a power grid and having a shaft;
an exciter machine coupled to the doubly fed induction generator shaft; a power conversion system isolated from the grid and electrically connected to a rotor of the doubly fed induction generator and to the exciter machine to control the doubly fed induction generator; and
an electric braking means for braking at least one of the doubly fed induction generator shaft and the first rotor shaft.