Improvements to Hydraulic Machines During Grid Disconnections

Abstract

Installation for converting hydraulic energy into electrical energy, provided with a fixed speed hydraulic machine comprising a pump-turbine (2) linked to a generator (20) by a shaft.

The installation further comprises: an electrical torque actuator (34) to establish a circuit with the generator (20), electrical torque actuator connection means (35) for connecting the generator (20) the electrical torque actuator, and command means (29) for detecting a disconnection of the generator (20) from the grid, and for commanding the electrical torque actuator connection means (35) to connect the generator (20) to the electrical torque actuator (34).

Claims

1. A method for limiting the rotational speed of a fixed speed hydraulic machine comprising a generator following a disconnection of the hydraulic machine from an electricity grid, the method comprising the steps of: a) detecting a disconnection of the generator (20) from the grid (30); and b) connecting the generator (20) to an electrical torque actuator (34) by switching electrical torque actuator connection means (35) to a conducting state.

2-12. (canceled)

Description

[0032] 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:

[0033] FIG. 1 is a schematic section of an installation for converting hydraulic energy into electrical energy comprising a pump-turbine; and

[0034] FIG. 2 is a schematic drawing of a hydraulic installation for limiting the rotational speed of the hydraulic machine included therein.

[0035] A pump-turbine 2 as illustrated by FIG. 1 is arranged to start in turbine mode without any external device. The hydraulic flow F provides a motor torque that enables the speed of the pump-turbine 2 to reach the synchronous speed, required to match the frequency of an electricity grid 30, as shown in FIG. 2, without external power. In pump mode, the flow cannot provide this motor torque and a variable frequency drive is used to power the pump-turbine 2 and ramp up its speed up to the synchronous speed. The grid 30 can then be directly connected to the pump-turbine.

[0036] The invention is arranged to restore the electrical braking torque in the rotor so as to limit the overspeed during transition of the operating mode, such as during disconnection from the grid 30, and before the guide vanes 9 and/or the water flow regulation devices are closed. The limitation of rotor speed directly impacts the overpressure upstream of the pump-turbine 2 and the pressure drop downstream.

[0037] As described above, the disconnection of the generator from the grid 30 leads to an open circuit, wherein no current flows and no voltage drop is present across the generator. Restoring an electrical braking torque requires the restoration of a voltage drop and a current in the stator of the generator linked to the pump-turbine 2. This is achieved by establishing a circuit with an electrical torque actuator able to withstand the current and voltage drop generated by connection to said hydraulic machine 2 operated as a fixed speed turbine, or any fixed speed hydraulic turbine. Circuit breakers are open and/or closed to switch between the regular connection to the grid 30 and the connection to the electrical torque actuator in emergency mode.

[0038] In a first embodiment, the electrical torque actuator is the variable frequency drive used to start the pump-turbine 2 in pump mode. Variable frequency drives are used to start a hydraulic machine in pump mode, and are linked to several generators to allow sequential startup of the hydraulic machines connected thereto and to lower the overall cost of the plant. The variable frequency drives are therefore already present in pump-turbine power plants and already connected to the generators. Using the variable frequency drive is advantageous because no additional part or component is required. It can also open up a secondary route to the grid for electrical energy produced by the pumped storage power plant (PSPP) in the event that the grid is still available despite the grid disconnection of the pump-turbine.

[0039] In a second embodiment, the electrical torque actuator can be a battery or an adequately sized electrical resistance, such as a resistor. Alternatively, or in combination with the electrical resistance, heat generated during operation of the electrical torque actuator may be absorbed by cooling apparatus. The cooling apparatus may use a heat-transfer fluid, such as water, to cool the electrical torque actuator.

[0040] Reference is now made to FIG. 2 which illustrates a hydraulic installation 1 comprising means for limiting the rotational speed of the hydraulic machine 2 included therein.

[0041] For brevity's sake, FIG. 2 does not comprise any drawing relative to the flow of water. However, the person skilled in the art would refer to FIG. 1 and the related specification for matters relative to the flow of water and its control. Parts in common between FIG. 1 and FIG. 2 bear the same references.

[0042] The installation 1 comprises control means 29 for controlling the pump-turbine 2 through a control loop 22, for determining the state of the connection between the generator 20 and the grid 30 and for controlling the grid connection means 31.

[0043] The control loop 22 comprises a guide vane controller 23 that takes as input a speed difference between the rotational speed N_sp of the pump-turbine and the target rotational speed Nc. The control loop 22 also comprises a guide vane actuator 24. The guide vane controller 23 outputs an orientation control signal _sp to the guide vane actuator 24 to affect accordingly the orientation of the guide vanes 9. The control loop 22 may also command an additional waterflow regulation device.

[0044] The guide vane controller 23 may, for example, be a Proportional Integrative Derivative controller (PID).

[0045] The control loop 22 is arranged to command the opening of the guide vanes 9 so that the rotational speed N of the pump-turbine 2 matches the target rotational speed Nc.

[0046] The generator 20 linked to the pump-turbine 2 is connected to the grid 30 through grid connection means 31.

[0047] A resistor 32 and a current sensor 33 are connected in parallel to the generator 20 for determining the voltage drop at the terminals of the generator 20. Alternatively, the resistor 32 and current sensor 33 may be replaced by a voltage sensor.

[0048] Generator 20 is connected to an electrical torque actuator 34 through electrical torque actuator connection means 35.

[0049] An electrical torque actuator 34 is connected to electrical torque actuator connection means 35 by one terminal, the second terminal being connected between grid connection means 31 and the generator 20.

[0050] Grid connection means 31 and electrical torque actuator connection means 35 are switchable between a conducting state and a non-conducting state depending on a command signal received from control means 29.

[0051] The control means 29 is able to control the installation according to the following method for limiting the rotational speed of a hydraulic machine.

[0052] A method for limiting the rotational speed of the hydraulic machine 2 comprises the following steps.

[0053] At a first step, the command means 29 determines if the generator 20 is disconnected from the grid 30 by measuring the voltage drop at the terminals of the generator 20. Voltage measurement can be achieved directly by a voltage sensor or by a resistor 32 connected in series with a current sensor 33 as illustrated on FIG. 2. Alternatively, command means 29 can receive a signal linked to a default device in the connection between the generator 20 and the grid 30, for example a signal from a tripped differential protection device.

[0054] Once it has been determined that the generator 20 is disconnected from the grid 30, the method continues at a second step during which electrical torque actuator connection means 35 is switched to a conducting state, closing a circuit comprising the electrical torque actuator 34 and the generator 20. This allows a voltage drop to be restored at the terminals of the generator and restores the electromotive braking force in the generator 20, slowing down the pump-turbine 2. Optionally, grid connection means 31 is switched to a non-conducting state, safely insulating the electrical torque actuator 34 and the generator 20 from the grid 30.

[0055] The present method and installation apply to any hydraulic machine, including hydraulic machines with S-characteristics.

[0056] 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.