Stabilization of hydraulic machines with S-zone characteristics

Abstract

This method for stabilizing the rotation speed of a hydraulic machine having S-characteristic and comprising a distributor (9) is adapted to modify a water flow, so that the machine can be coupled to a grid. The method comprises the steps of calculating an orientation of the distributor (9); and orienting the distributor according to the calculated orientation. The method further comprises the steps of providing an electric torque to the machine so as to reach a target speed.

Claims

1. A method for stabilizing the rotational speed of a hydraulic machine in turbine mode for coupling the hydraulic machine to a grid, the hydraulic machine comprising a distributor adapted to modify a water flow, wherein the method comprises the steps of: calculating a speed difference (ε) between the rotational speed (N) of the hydraulic machine and a target rotational speed (N_sp); using the speed difference (ε) as an input, calculating an orientation (γ) of the distributor by a control loop feedback system having a first control loop; orienting the distributor according to the calculated orientation (γ); using the speed difference (a) as an input to a second control loop having an electric torque controller, outputting an electric torque set-point (Telec_sp); applying an electric torque (Telec) to the hydraulic machine corresponding to the electric torque set-point (Telec_sp).

2. The method according to claim 1, wherein the method comprises coupling the hydraulic machine to a grid.

3. The method according to claim 2, wherein the step of coupling the hydraulic machine to the grid comprises at least one of the following steps: opening an electric torque circuit breaker between the source of the electric torque and the machine; closing a grid circuit breaker between the hydraulic machine and the grid.

4. The method according to claim 1, wherein the method comprises processing the speed difference (ε) to output at least one of: an orientation control set-point (γ_sp).

5. The method according to claim 4, wherein the orientation control set-point (γ_sp) is received by an actuator to orientate the distributor.

6. The method according to claim 1, wherein the electric torque (Telec) is provided by at least one of: an electric power source to reduce the speed difference (ε); a variable-frequency drive; a battery.

7. The method according to claim 6, wherein the electric torque (Telec) is provided by the electric power source that is connected to the grid.

8. The method according to claim 1, wherein orienting the distributor according to orientation (γ) provides a coarse regulation, and applying the electric torque to the hydraulic machine provides a fine regulation.

9. A system for converting hydraulic energy into electrical energy, the system comprising: a hydraulic machine comprising a distributor adapted to modify a water flow, and a rotor that is rotated by operation of the hydraulic energy; a first control loop comprising a controller, the first control loop configured to use a speed difference (ε) between a rotational speed (N) of the hydraulic machine and a target rotational speed (N_sp) as an input to calculate an orientation of the distributor and orientate the distributor according to the calculated orientation; and a second control loop comprising an electric torque controller that uses the speed difference (ε) as an input, the second control loop configured to output an electric torque set-point (Telec_sp) and provide an electric torque (Telec) to the rotor corresponding to the electric torque set-point (Telec_sp) so as to reach a target rotational speed (N_sp) of the hydraulic machine.

10. The system according to claim 9, wherein the first control loop is configured to output an orientation set point (γ_sp) for the distributor.

11. The system according to claim 9, wherein the first control loop comprises an actuator arranged to orientate the distributor.

12. The system according to claim 9, wherein the second control loop comprises an electric power source arranged to provide the electric torque (Telec) to the hydraulic machine to reduce the speed difference (ε).

13. The system according to claim 12, further comprising an electric source circuit breaker between the electric power source and the machine.

14. The system according to claim 12, wherein the electric power source comprises at least one of a variable-frequency drive or comprises a battery.

15. The system according to claim 12, wherein the electric power source is connected to an electricity grid.

16. The system according to claim 9, further comprising a grid circuit breaker between the hydraulic machine and an electricity grid.

17. The system according to claim 9, wherein the electric torque set-point (Telec_sp) has a value above a threshold (Telec_threshold).

Description

(1) Other features and advantages will become apparent from the following description, given only by way of example, in view of the following drawings in which:

(2) FIG. 1 is a schematic section of an installation for converting hydraulic energy into electrical energy comprising a pump-turbine;

(3) FIG. 2 is a graph representing characteristics, in turbine mode, of the pump-turbine of the installation of FIG. 1;

(4) FIG. 3 is a graph representing the rotation speed of the machine, plotted over time, in a turbine mode of the pump-turbine of the installation of FIG. 1;

(5) FIG. 4 is a control scheme illustrating a method according to the invention that aims at stabilizing the rotation speed of a pump-turbine belonging to the installation of FIG. 1;

(6) FIG. 5 is a schematic drawing illustrating the variable-frequency drive unit and the power connections existing between it, the grid and the pump-turbine; and

(7) FIG. 6 is a schematic drawing illustrating another embodiment of the invention which includes a battery, a DC/AC power conversion unit and the connections existing between them, the grid and the pump-turbine.

(8) Reference is first made to FIG. 1 which represents an installation 1 for converting hydraulic energy into electrical energy. The installation 1 includes a hydraulic machine which is subject to S-characteristics. In the example described herein, the hydraulic machine is a pump-turbine 2 that uses, in a turbine mode, hydraulic energy to set a shaft 3 in rotation. The shaft 3 is coupled to the rotor of a generator having an alternator that converts mechanical energy of the rotating rotor into electrical energy.

(9) The functioning of the pump-turbine 2 is described below in turbine mode. The pump-turbine 2 includes a volute 4 that is supported by concrete blocks 5, 6. For example, a non-represented penstock extends between a non-represented upstream reservoir and the volute 4. This penstock generates a forced water flow F to power the machine 2.

(10) The machine 2 includes a runner 7 coupled to the shaft 3 that is surrounded by the volute 4 and that includes blades 8 between which water flows in operating conditions. As a result, the runner 7 rotates around an axis X-X′ of the shaft 3.

(11) A distributor is arranged around the runner 7. It includes a plurality of movable guide vanes 9 that are evenly distributed around the runner 7. A pre-distributor is disposed upstream of and around the distributor. The pre-distributor is formed by a plurality of fixed vanes 10 evenly distributed around the axis of rotation X-X′ of the runner 7.

(12) A suction pipe 11 is disposed below the runner 7 and is adapted to evacuate water downstream.

(13) The guide vanes 9 of the distributor have each an adjustable pitch around an axis parallel to the axis of rotation X-X′ of the runner 7. Consequently, the guide vanes 9 may be swiveled to regulate the water flow rate. The guide vanes 9 are all oriented with the same angle relative to a closed position.

(14) Reference is now made to FIGS. 2 and 3 which illustrate curves representing a parameter T11 that corresponds to the hydraulic torque applied to the runner 7, plotted over a parameter N11 that corresponds to the rotation speed of the machine 2 at a given opening of the guide vanes 9 (FIG. 2) and the rotation speed N plotted over time (FIG. 3).

(15) Referring to FIG. 2, the iso-opening curve of the parameter T11 depending on the hydraulic torque, plotted over parameter N11 depending on the rotation speed, exhibits an S-portion in which the curve has a positive slope for which a slight increase of the parameter N11 results in a significant increase in the parameter T11.

(16) In other words, a slight variation of the rotation speed results in a significant increase of the torque applied to the machine 2. As will be appreciated, stabilization of the machine rotation speed is difficult to achieve (FIG. 3) under these conditions.

(17) A method for stabilizing the rotation speed of the hydraulic machine according to the invention is implemented by means of a control loop feedback system 20, as shown in FIG. 4. The control loop feedback system 20 comprises a first control loop 22 comprising a guide vane controller 23 that takes as an input a speed difference ε between the rotational speed N of the hydraulic machine and a target rotation speed N_sp. The first control loop 22 also comprises a guide vane actuator 24.

(18) The first controller 23 processes the speed difference ε and outputs an orientation control set-point γ_sp to the guide vane actuator 24. The orientation control set-point γ_sp corresponds to the optimum guide vane orientation γ to stabilize the hydraulic machine. The guide vane actuator 24 orientates the guide vanes according to the optimum orientation γ.

(19) For example, the rotation speed of the turbine 2 can be determined by measuring the frequency of the generator coupled to the shaft 3.

(20) The guide vane controller 23 may, for example, be a Proportional Integral Derivative controller (PID).

(21) In addition, the control loop feedback system 20 comprises a second control loop 25 comprising an electric torque controller 26 that takes as an input the speed difference ε between the rotation speed N of the hydraulic machine and the target rotation speed N_sp to output an electric torque set-point Telec_sp. The control loop feedback system 20 also comprises an electric power source 27 that accordingly affects the electric torque Telec provided to the rotor.

(22) In a further embodiment of the invention, a single main controller is configured to perform the functions of one or both of the guide vane controller 23 and the electric torque controller 26.

(23) The electric torque set-point is calculated by the controller 26 to accelerate or decelerate the machine rotation speed to reduce or eliminate the speed difference ε.

(24) In one embodiment, the second controller 26 is a variable-frequency drive controller, for example a static frequency converter (SFC) controller.

(25) The second control loop 25 thus comprises a variable-frequency drive for example a static frequency converter. The static frequency converter may be a voltage source inverter or a current source inverter. The variable-frequency drive is connected to an electricity distribution grid and controlled by the variable-frequency drive controller 26 to provide either a positive or a negative electric torque to the generator.

(26) As previously indicated, the variable-frequency drive may comprise a static frequency converter (SFC) and comprise a rectifier stage connected to the grid to produce a direct current and an inverter stage for voltage and frequency conversion.

(27) FIG. 5 illustrates an embodiment of the invention in which the variable frequency drive 27 provides an electrical torque to the pump-turbine machine 2. A circuit breaker 28 links the variable-frequency drive 27 with the machine 2. When the circuit breaker 28 is in a closed position the variable frequency drive 27 provides the electrical torque to the machine 2. At the same time a main circuit breaker 30 located between the machine 2 and the grid 34 is in an open position. The variable frequency drive (VFD) 27 is powered by the grid 34 via an AC transformer 33 and VFD cable 32.

(28) Once the rotational speed of the machine 2 is stabilized, the frequencies on each side of the main circuit breaker 30 are equalized. Once equalization has been achieved, connection of the machine 2 to the grid 34 is performed by closing the main circuit breaker 30 and opening circuit breaker 28. Power is then provided directly to the grid 34 through a grid line 31 in generation operation.

(29) The variable-frequency drive 27 may comprise switching cells using diodes and transistors or thyristors working as switches which may be controlled by the VFD controller 26 to produce the desired electric torque.

(30) FIG. 6 illustrates an alternative way to provide an electrical torque to the machine 2. Identical features found in FIG. 6 retain the reference numerals of FIG. 5.

(31) In this alternative embodiment a battery 35 is used instead of the variable frequency drive 27. A DC/AC conversion unit 36 provides the electrical power of the battery 35 to the machine 2 via the circuit breaker 28. When the battery 35 provides the electrical power of the battery 35 to the machine 2, the circuit breaker 28 is in closed position and a main circuit breaker 30 is in an open position.

(32) Once the speed of the pump-turbine 2 is stabilized, the frequencies on each side of the main circuit breaker 30 are equal. This enables connection of the machine 2 to the grid 34 which is achieved by closing main circuit breaker 30 and opening circuit breaker 28. The power is then provided directly to the grid 34 through the grid line 31 and AC transformer 33 in generation operation.

(33) According to another embodiment of the invention, the electric power source comprises a battery 35 connected to the generator of the machine 2, wherein the battery 35 is arranged to be charged by the grid 34.

(34) For example, the battery 35 may comprise an internal control stage connected to the controller 26 in order to provide the generator of the machine 2 with a positive or a negative electric torque to adjust the rotation speed of the machine 2 to the target speed value.

(35) According to another embodiment of the invention, the second control loop 25 comprises a variable-frequency drive governed by a VFD controller and a battery governed by a battery controller. The battery and battery controller are connected in parallel to the variable-frequency drive to provide the generator of the machine 2 with an electric torque to adjust the rotation speed of the machine 2.

(36) It should be appreciated that the invention, which comprises a control loop feedback system comprising (i) a first loop with, in some embodiments, a turbine speed load governor (TSLG) controller used to output an opening value to affect the guide vanes; and (ii) a second control loop having an electric power source providing the generator of the machine with an electric torque, may provide a first coarse regulation in which the speed difference is reduced by the first control loop and a fine regulation in which the speed difference ε is reduced by the second control loop 25.

(37) For example, the first control loop may be used to regulate the rotation speed of the machine 2 around a desired value and the second loop is used to dynamically compensate for the speed error.

(38) For example, 100% of the power may be provided by the hydraulic torque, while 10% of the power, corresponding to the error range may be provided by the additional electric torque source.

(39) According to a further embodiment of the invention, the controller 26 provides an electrical setpoint Telec_sp which is maintained above a setpoint threshold Telec_threshold. This results in an electrical counter-torque applied to the machine 2 which exceeds a set counter-torque threshold.

(40) To counteract the setpoint threshold Telec_threshold, a positive hydraulic torque is provided to the turbine 7 by the guide vane actuator 24. In order for the speed difference ε to become negligible and stabilization of the speed to occur, the controller 23 may be a classical PID which orientates the guide vanes 9 via the guide vane actuator 24. Accordingly, the controller 23 provides a corresponding command γ_sp to stabilize the speed of the machine.

(41) By applying an electrical setpoint Telec_sp which is above a setpoint threshold Telec_threshold and a corresponding counter torque to the machine 2, the coupling operating point is located in a naturally stable zone of the hydraulic turbine 7 outside of the “S-zone” of the turbine characteristic.

(42) As can be seen in FIG. 2, the slope of the T11/N11 curve becomes positive above a certain threshold of T11. The threshold Telec_threshold is chosen in order to move the hydraulic point outside the “S-zone”.

(43) The aforementioned embodiments are not intended to be limiting with respect to the scope of the appended claims. Furthermore, features of one or more of the above embodiments may be readily combined with one or more features of another embodiment. It is also contemplated by the inventors that various substitutions, alterations, and modifications may be made to the invention without departing from the scope of the invention as defined by the claims.