A control system for controlling an operation of a hydroelectric turbine. The control system comprises a converter system to convert AC power, supplied by a generator connected to the turbine, and having a voltage and frequency that is a function of a rotational speed of the turbine, to AC power hav
A control system for controlling an operation of a hydroelectric turbine. The control system comprises a converter system to convert AC power, supplied by a generator connected to the turbine, and having a voltage and frequency that is a function of a rotational speed of the turbine, to AC power having a voltage and frequency of a transmission system for transmitting the AC power to a receiving station. The system further comprises a control unit that is arranged to co-operate with the converter system to adjust the AC voltage supplied by the generator in response to a water flow speed through the turbine to thereby control rotation of the turbine. The converter system comprises a first-stage converter and a second-stage converter, with a DC link provided between the first and second-stage converters. The first-stage converter is arranged to convert the AC power supplied by the generator to DC power. The second-stage converter is arranged to convert the DC power to the AC Power for transmission to the receiving station and the second-stage converter is of a voltage-source inverter type.
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1. A control system for controlling an operation of a hydroelectric turbine, the control system comprising: a converter system, arranged to convert AC power supplied by a generator connected to the turbine, and having a voltage and frequency that is a function of a rotational speed of the turbine, t
1. A control system for controlling an operation of a hydroelectric turbine, the control system comprising: a converter system, arranged to convert AC power supplied by a generator connected to the turbine, and having a voltage and frequency that is a function of a rotational speed of the turbine, to AC power having a voltage and frequency of a transmission system for transmitting the AC power to a receiving station, the converter system comprising a first-stage converter and a second-stage converter, with a fixed-voltage DC link provided between the first and second-stage converters, the first-stage converter being arranged to convert the AC power supplied by the generator to DC power, the first stage converter being of a voltage-source type, and the second-stage converter being arranged to convert the DC power to the AC power for transmission to the receiving station;a control unit, the control unit being co-operable with the converter system to adjust an AC voltage supplied by the generator in response to the water flow speed through the turbine to thereby control rotation of the turbine,wherein, in response to determining that the water flow speed is less than a first threshold value, the control unit is arranged to determine a DC link current flowing through the DC link, to determine an optimum DC power value associated with the water flow speed for the turbine, and to adjust the operation of the first stage converter to adjust the frequency of an AC voltage at AC input terminals of the first stage converter to a value that provides the determined optimum DC power value depending on the determined DC link current,the control unit further being arranged to determine, responsive to the water flow speed exceeding a second threshold value, a maximum DC power value and to adjust the operation of the first stage converter to adjust the frequency of the AC voltage at the input terminals to a value that restricts the DC power to the maximum DC power value. 2. The control system of claim 1, wherein the DC link includes at least one sensor for sensing a DC current and is arranged to provide signals associated with the sensed DC current to the control unit. 3. The control system of claim 2, wherein the DC link includes at least one sensor for sensing a DC voltage and is arranged to provide signals associated with the sensed DC voltage to the control unit. 4. The control system of claim 1, wherein the first second stage converter is of a voltage-source inverter type. 5. The control system of claim 1, wherein the first-stage converter and the second-stage converter are six-device, three-phase bridges, each device comprising a switch and free-wheel diode. 6. The control system of claim 5, wherein the switches are selected from any of semiconductor switches, such as Insulated-Gate Bipolar Transistors (IGBT), Integrated Gate Commutated Thyristors (IGCT), or Gate Turn Off (GTO) thyristors. 7. The control system of claim 6, wherein the switches are arranged to receive and operate in accordance with signals received from the control unit. 8. The control system of claim 1, wherein a capacitor is connected in parallel with DC terminals and is arranged to maintain a substantially constant DC link voltage over a period of a switching cycle of the first and second-stage converters. 9. The control system of claim 1, wherein the first-stage converter comprises a plurality of switching devices, and the control unit is arranged to control the switching devices of the first-stage converter to thereby control the AC voltage provided at the input terminals of the first-stage converter. 10. The control system of claim 9, wherein the first-stage converter is controlled to provide to the generator the AC voltage in a manner that changes according to the electrical frequency in such a way that the resulting AC current is in phase with an electro-magnetic force induced in windings of the generator. 11. The control system of claim 9, wherein the control unit is arranged to control an amplitude and frequency of the AC output voltage of the second stage converter by means of switching signal transmitted to switching devices of the second stage converter. 12. The control system of claim 9, wherein the control unit is arranged to modify an operation of the switching devices of the first-stage converter to adjust the frequency of the AC voltage at the input terminals of the first-stage converter-to control the rotation of the turbine. 13. The control system of claim 9, wherein the control unit is arranged to adjust the operation of the first-stage converter by modifying a switching sequence of the switching devices of the first-stage converter to adjust the frequency of the AC voltage at the AC input terminals to a value to provide the optimum DC power value for the determined DC link current. 14. The control system of claim 1, wherein the control unit is arranged to control the switching devices of the first-stage converter to set an amplitude and frequency of the voltage at terminals of the generator and the corresponding real and reactive power flows. 15. The control system of claim 1, wherein the first threshold value is a normal operating flow speed or rated speed. 16. The control system of claim 1, wherein the control system is arranged to cooperate with a supervisory controller to determine the first threshold value for the turbine. 17. The control system of claim 16, wherein the first threshold value is based on any of a performance level of each turbine within an array of turbine systems, a pattern of water flow across an array of turbines, and grid operator preferences. 18. The control system of claim 1, wherein the AC power supplied by the generator has a voltage and frequency proportional to the rotational speed of the turbine. 19. The control system of claim 1, wherein the control system is arranged to cooperate with a supervisory controller to determine the first threshold value for the turbine. 20. The control system of claim 1, wherein said first and second threshold values are the same threshold value. 21. A control system for controlling an operation of a hydroelectric turbine, the control system comprising: a converter system, arranged to convert AC power supplied by a generator connected to the turbine, and having a voltage and frequency that is a function of a rotational speed of the turbine, to AC power having a voltage and frequency of a transmission system for transmitting the AC power to a receiving station, the converter system comprising a first-stage converter and a second-stage converter, with a fixed-voltage DC link provided between the first and second-stage converters, the first-stage converter being arranged to convert the AC power supplied by the generator to DC power, the first stage converter being of a voltage-source type, and the second-stage converter being arranged to convert the DC power to the AC power for transmission to the receiving station;a control unit, the control unit being configured to:upon determining that the water flow speed is less than a first threshold value, determine an optimum DC power value associated with the water flow speed, and to adjust the operation of the first stage converter to adjust the frequency of the AC voltage at the input terminals of the first stage converter to a value that provides the optimum DC power value; andupon determining that the water flow speed exceeds a second threshold value, determine a maximum DC power value, and to adjust the operation of the first stage converter to adjust the frequency of the AC voltage at the input terminals of the first stage converter to a value that prevents the DC power exceeding the maximum DC power value.
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