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A device, a system, and a method for preventing power-producing wind turbine generators from tripping due to the detection of a low-voltage condition on a power grid are disclosed. The disclosed device includes a resistor bank that absorbs real power and a control system that maintains the collector

A device, a system, and a method for preventing power-producing wind turbine generators from tripping due to the detection of a low-voltage condition on a power grid are disclosed. The disclosed device includes a resistor bank that absorbs real power and a control system that maintains the collector bus voltage above a threshold voltage level during the duration of low-voltage condition on the power grid. Collector bus voltage is maintained using gating signals that include phase delay angles to adjust the opening/closing of the switching devices.

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What I claim is: 1. A device for controlling voltage on a collector bus in operational association with at least one wind turbine generator having an output with at least one phase, the device comprising: a controllable resistance that is disposed between the collector bus and a load of each phase

What I claim is: 1. A device for controlling voltage on a collector bus in operational association with at least one wind turbine generator having an output with at least one phase, the device comprising: a controllable resistance that is disposed between the collector bus and a load of each phase of the output, the controllable resistance including: a controllable resistor bank to provide a variable resistance, wherein the resistor bank is structured and arranged to absorb real power generated by the at least one wind turbine generator when a voltage drop or a voltage sag occurs on the load and to maintain the voltage on the collector bus at or above a predetermined threshold voltage level, and at least one switching device in parallel with the controllable resistor bank, that passes collector bus current to the load when closed and that diverts collector bus current to the resistor bank when open; and a controller that is structured and arranged to receive current level and voltage level data and, based on current level and voltage level data, to provide gating signals to open or close the at least one switching device, to adjust the variable resistance of the resistor bank. 2. The device as recited in claim 1, wherein each of the at least one switching devices is an alternating current (AC) switch. 3. The device as recited in claim 2, wherein the AC switch is rated to carry a full, collector bus current for each phase continuously and to block some portion of a full, collector bus voltage for each phase. 4. The device as recited in claim 2, wherein the AC switch is selected from the group comprising silicon-controlled rectifiers, break-over diodes, and one or more levels of thyristors pairs. 5. The device as recited in claim 1, wherein the resistor bank includes a plurality of resistors in parallel, which, in combination, provides an effective total resistance to provide the full collector bus voltage during the low-voltage, grid fault occurrence. 6. The device as recited in claim 5, wherein a switching device is structured and arranged in series with each of the plurality of resistors. 7. The device as recited in claim 6, wherein the switching device associated with each of the plurality of resistors can be open or closed for all or some portion of each half-cycle based on the gating signal received from the controller. 8. The device as recited in claim 7, wherein the controller is structured and arrange to provide gating signals to the switching device associated with each of the plurality of resistors, and the gating signals include phase delay angles that open or close the switching device. 9. The device as recited in claim 8, wherein the controller is structured and arranged to open and close the switching device associated with each of the plurality of resistors to provide a resistance to maintain the collector bus voltage at or above a threshold voltage level. 10. The device as recited in claim 9, wherein the controller is structured and arranged to transmit gating signals to N+1 switching device associated with each of the plurality of resistors (where n is a positive integer), n switching devices of which are either open or closed for the entire half-cycle and an N+1th switching device is open for a first portion of the half-cycle and closed for a second portion of the half-cycle. 11. The device as recited in claim 9, wherein the threshold voltage level is a minimum voltage that prevents the at least one wind turbine generator from tripping. 12. The device as recited in claim 1, wherein the controller provides gating signals at half-cycle intervals. 13. The device as recited in claim 1, wherein the controller is structured and arranged to provide balanced voltages on the collector bus in each phase during an unbalanced low-voltage, grid fault occurrence on the load. 14. The device as recited in claim 1, the device further comprising a transient voltage protection device to protect the at least one switching device and the resistor bank from damage from high voltages. 15. The device as recited in claim 14, wherein the transient voltage protection device includes a plurality of RC snubber circuits and/or a plurality of arrestor devices. 16. The device as recited in claim 1, wherein the device further includes a cooling system for cooling the device during periods of peak heat generation. 17. The device as recited in claims 16, wherein the cooling system comprises a plurality of cooling blowers. 18. The device as recited in claim 1, wherein the device further includes a plurality of potential transformers to measure voltage levels in the device continuously and to provide voltage level data to the controller. 19. The device as recited in claim 1, wherein the device further comprises at least one current sensor to measure current levels in the device or on the collector bus continuously and to provide current level data to the controller. 20. The device as recited in claim 1, wherein the device further comprises a harmonic filter to minimize harmonic disturbances. 21. The device as recited in claim 1, wherein the controllable resistance is structured and arranged as a plurality of modules cascaded in series, each of the plurality of modules comprising a resistive element from the controllable resistor bank in parallel with one of the at least one switching device to provide a variable resistance. 22. A method of controlling collector bus voltage for at least one wind turbine generator when a low-voltage, grid fault occurrence on a utility power grid causes a voltage drop, the method comprising the steps of: providing a low-voltage fault ride-through device as recited in claim 1; measuring voltage levels on the collector bus continuously; and adjusting an operating state of the at least one switching device between an open state and a closed state, to pass collector bus current to the load, respectively, through the controllable resistor bank or through the at least one switching device to maintain the collector bus voltage levels above a predetermined threshold voltage level, to prevent one or more of the at least one wind turbine generator from tripping. 23. The method as recited in claim 22, the method further comprising measuring current levels in the low-voltage fault ride-through device continuously; and re-adjusting the opening state of the at least one switching device between the open state arid the closed state, to maintain the current level in the low-voltage fault ride-through device below a predetermined maximum allowable current. 24. A device for controlling voltage on a collector bus, the device being disposed between at least one wind turbine generator having an output with at least one phase and a utility grid point of common coupling (PCC) and the collector bus being in operational association with said at least one wind turbine generator, the device comprising: a controllable resistance that is disposed between the collector bus and the utility grid PCC of each phase of the output, the controllable resistance including: a controllable resistor bank to provide a variable resistance, wherein the resistor bank is structured and arranged to absorb real power generated by the at least one wind turbine generator when a low-voltage fault event occurs at the utility grid PCC and to maintain the voltage of the collector bus at or above a predetermined threshold voltage level, and at least one switching device in parallel with the controllable resistor bank, that passes collector bus current to the utility grid PCC when closed and that diverts collector bus current to the resistor bank when open; and a controller that is structured and arranged to receive current level and voltage level data and, based on current level and voltage level data, to provide gating signals to open or close the at least one switching device, to adjust the variable resistance of the resistor bank. 25. A device for controlling voltage on a collector bus, the device being disposed between two power sources and the collector bus being in operational association with one of the power sources, the device comprising: a controllable resistance that is disposed between the collector bus and the utility grid PCC of each phase of the output, the controllable resistance including: a controllable resistor bank to provide a variable resistance, wherein the resistor bank is structured and arranged to absorb real power generated by the at least one wind turbine generator when a low-voltage fault event occurs at the utility grid PCC and to maintain the voltage of the collector bus at or above a predetermined threshold voltage level, and at least one switching device in parallel with the controllable resistor bank, that passes collector bus current to the utility grid PCC when closed and that diverts collector bus current to the resistor bank when open; and a controller that is structured and arranged to receive current level and voltage level data and, based on current level and voltage level data, to provide dating signals to open or close the at least one switching device, to adjust the variable resistance of the resistor bank. 26. The device as recited in claim 25, wherein one of the two power sources is at least one wind turbine generator having an output with at least one phase; another of the two power sources is a utility grid point of common coupling (PCC); and the collector bus is in operational association with said at least one wind turbine generator.