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A turbine generator system includes a doubly-fed alternating-current (AC) generator having a first poly-phase circuit (e.g., a stator circuit) and a second poly-phase circuit (e.g., a rotor circuit), a poly-phase AC-to-AC converter circuit coupled between the first and second poly-phase circuits, a

A turbine generator system includes a doubly-fed alternating-current (AC) generator having a first poly-phase circuit (e.g., a stator circuit) and a second poly-phase circuit (e.g., a rotor circuit), a poly-phase AC-to-AC converter circuit coupled between the first and second poly-phase circuits, a poly-phase transformer having input windings coupled to the first poly-phase circuit and having output windings, and a uni-directional rectifier circuit coupled to the output windings of the poly-phase transformer and configured to convert poly-phase AC from the transformer output windings to direct current (DC).

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1. A turbine generator system, comprising: a doubly-fed alternating-current (AC) generator having a first poly-phase circuit and a second poly-phase circuit;a poly-phase AC-to-AC converter circuit coupled between the first poly-phase circuit and the second poly-phase circuit configured to produce a

1. A turbine generator system, comprising: a doubly-fed alternating-current (AC) generator having a first poly-phase circuit and a second poly-phase circuit;a poly-phase AC-to-AC converter circuit coupled between the first poly-phase circuit and the second poly-phase circuit configured to produce a substantially fixed frequency voltage output;a poly-phase transformer having input windings coupled to the first poly-phase circuit and having output windings, wherein the poly-phase transformer is configured to increase, a voltage level of the substantially fixed-frequency voltage output from the AC-to-AC converter circuit for rectification;a uni-directional rectifier circuit coupled to the output windings of the poly-phase transformer and configured to convert poly-phase AC from the transformer output windings to direct current (DC);a grid-fault protection circuit configured to de-energize the second poly-phase circuit in the event of a fault on the DC side of the unidirectional rectifier circuit, wherein the grid-fault protection circuit includes a rectifier circuit coupled to the second poly-phase circuit, the rectifier circuit being combined with a thyristor and a crowbar resistor, wherein the second poly-phase circuit is de-energized by firing the thyristor and dissipating energy in the crowbar resistor; anda backup power source coupled to the AC generator and configured to power the turbine generator system during startup, wherein the backup power source includes a battery coupled to the doubly-fed AC generator with a battery inverter circuit, wherein the backup power source further comprises a flywheel mechanically coupled to the AC generator, wherein the battery is arranged to power the flywheel. 2. The turbine generator system of claim 1, wherein the doubly-fed AC generator is one of the following: a wound-rotor induction generator with slip rings;a dual-stator winding, squirrel-cage induction generator;a dual-stator winding, synchronous reluctance generator; anda wound-rotor induction generator with brushless excitation. 3. The turbine generator system of claim 1, wherein the AC-to-AC converter circuit is an indirect converter comprising a poly-phase AC-to-DC converter, a poly-phase DC-to-AC converter, and a DC link between the poly-phase AC-to-DC converter and the poly-phase DC to-AC converter. 4. The turbine generator system of claim 1, wherein the AC-to-AC converter is configured to convert a variable-voltage, variable-frequency, output of the second poly-phase circuit to a substantially fixed frequency output at the input windings of the poly-phase transformer. 5. The turbine generator system of claim 4, wherein the substantially fixed frequency output is at a frequency between about 40 Hz and about 600 Hz. 6. The turbine generator system of claim 5, wherein the substantially fixed frequency output is at a frequency greater than about 100 Hz. 7. The turbine generator system of claim 1, wherein the AC-to-AC converter circuit has a power rating of less than about 40% of the power rating of the doubly-fed AC generator. 8. The turbine generator system of claim 1, wherein the uni-directional rectifier circuit is connected to the output windings of the poly-phase transformer via a protection device. 9. The turbine generator system of claim 1, wherein the uni-directional rectifier circuit is a multi-pulse rectifier circuit. 10. The turbine generator system of claim 1, wherein at least the converter circuit, the poly-phase transformer, and the uni-directional rectifier circuit are enclosed within a single housing or within a tower structure. 11. A power generation system comprising: a plurality of turbine generator systems configured to provide direct current (DC) power via respective DC buses; anda collection circuit connected to each of the DC buses and configured to combine the provided DC power;wherein each of one or more of the turbine generator systems comprisesa doubly-fed alternating-current (AC) generator having a first poly-phase circuit and a second poly-phase circuit;a poly-phase AC-to-AC converter circuit coupled between the first poly-phase circuit and the second poly-phase circuit configured to produce a substantially fixed-frequency voltage output;a poly-phase transformer having input windings coupled to the first poly-phase circuit and having output windings, wherein the poly-phase transformer is configured to increase a voltage lever of the substantially fixed-frequency voltage output from the AC-to-AC converter circuit for rectification;a uni-directional rectifier circuit coupled to the output windings of the poly-phase transformer and configured to convert poly-phase AC from the transformer output windings to direct current (DC); anda backup power source coupled to the doubly-fed AC generator and configured to power the turbine generator system during startup, wherein the backup power source includes at battery coupled to the doubly-fed AC generator via a battery inverter circuit, wherein the backup power source further comprises a flywheel mechanically coupled to the AC generator, wherein the battery is arranged to power the flywheel. 12. The power generation system of claim 11, wherein the collection circuit further comprises a converter circuit configured to transform the combined DC power to AC or to a DC voltage different from the DC voltage of the combined DC power. 13. The power generation system of claim 11, wherein each of the one or more of the turbine generator systems comprises: a grid-fault protection circuit configured to de-energize the second poly-phase circuit in the event of a fault on the DC side of the uni-directional rectifier circuit, wherein the grid-fault protection circuit includes a rectifier circuit coupled to the second poly-phase circuit, the rectifier circuit being combined with a thyristor and a crowbar resistor, wherein the second poly-phase circuit is de-energized by firing the thyristor and dissipating energy in the crowbar resistor. 14. A method for direct-current (DC) power generation, the method comprising: providing a doubly-fed alternating-current (AC) generator having a first poly-phase circuit and a second poly-phase circuit and having as poly-phase AC-to-AC converter circuit coupled between the first poly-phase circuit and the second polyphase circuit, such that the doubly-fed AC generator and the converter circuit are configured to provide a substantially fixed-frequency voltage output;powering the doubly-fed AC generator with a backup power source that includes a battery coupled to the doubly-fed AC generator with a battery inverter circuit, wherein the backup power source further comprises a flywheel mechanically coupled to the AC generator, wherein the battery is arranged to power the flywheel;increasing a voltage level from the substantially fixed-frequency voltage output sufficiently for rectification to provide power to a DC transmission line;converting the increased voltage level to a direct-current (DC) voltage, using a rectifier circuit that is co-located with the doubly-fed AC generator and the converter circuit; andapplying the DC voltage to is DC transmission line. 15. The method of claim 14, wherein the doubly-fed AC generator is one of the following: a wound rotor induction generator with slip rings;a dual-stator winding, squirrel-cage induction generator;a dual-stator, synchronous reluctance generator; anda wound-rotor induction generator with brushless excitation. 16. The method of claim 14, wherein increasing the voltage level from the substantially fixed-frequency voltage output comprises providing a poly-phase transformer having input windings coupled to the first poly-phase circuit of the AC generator and having output windings coupled to the uni-directional rectifier circuit. 17. The method of claim 16, wherein converting the increased voltage to a direct-current (DC) voltage comprises providing a uni-directional rectifier circuit co-located with the AC-to-AC converter circuit and the transformer, coupled to the output windings of the polyphase transformer, and configured to convert AC from the transformer output windings to direct current (DC). 18. The method of claim 14, wherein the AC-to-AC converter circuit is an Indirect converter comprising a poly-phase AC-to-DC converter, a poly-phase DC-to-AC converter, and a DC link between the poly-phase AC-to-DC converter and the poly-phase DC-to-AC converter. 19. The method of claim 14, wherein the AC-to-AC converter circuit is configured to convert a variable voltage, variable frequency, output of the rotor windings to a substantially fixed frequency output at the input windings of the three-phase transformer. 20. The method of claim 19, wherein the substantially fixed frequency output is at a frequency between about 100 Hz and About 600 Hz. 21. The method of claim 14, wherein the AC-to-AC converter circuit has a power rating of less than about 40% of the power rating of the doubly-fed AC generator. 22. The method of claim 14, further comprising de-energizing the second poly-phase circuit via a grid fault protection circuit connected thereto in response to a fault on the DC side of the uni-directional rectifier circuit, wherein the grid-fault protection circuit includes a rectifier circuit combined with a thyristor and a crowbar resistor, wherein the second poly-phase circuit is de-energized by firing the thyristor and dissipating energy in the crowbar resistor.