Patent Description:
In order to insure a grid stability and/or to prevent a network blackout, a grid balancing between the electrical power production and the electrical power consumption must be achieved.

Hydroelectric power plants have an electrical power reserve, via water reserves contained in a reservoir, that can be provided upon demand by starting a hydroelectric turbine in order to compensate any variation of the consumption and/or the production of the electrical power.

To this regard, the time response for providing such an electrical power reserve is thus a critical factor, on the basis of which the electricity producer can expect a more or less advantageous remuneration.

Prior art methods are known, for example from <CIT>, to start a turbine with help of a first control loop and second control loop, one of said loops comprising a variable frequency drive connected to the grid.

However, a variable frequency drive is a costly device.

Furthermore the prior art does not provide any solution to start a hydroelectric power plant comprising at least <NUM> generators. It is therefore an object of the invention to propose a method for coupling a hydroelectric power plant to the grid, in particular a hydroelectric power plant of the type which comprises at least <NUM> hydroelectric units, in a faster way than known prior art method.

It is also an object of the invention to propose a method for coupling the hydroelectric power plant to the grid, in particular a hydroelectric power plant of the type which comprises at least <NUM> hydroelectric units, that does not require any additional investments.

The aforementioned objects are, at least partly, achieved by a method for coupling a hydroelectric power plant in a turbine mode to a grid, in order to produce power to be injected into said grid, said hydroelectric power plant comprising at least a first hydroelectric unit and a second hydroelectric unit, each provided with a runner mechanically coupled to a shaft line and to a generator, a distributor comprising guide vanes to control a flow of water to said runner, said hydroelectric power plant further comprising a variable frequency drive, the method successively comprising, after starting the rotation of both hydroelectric units :.

If the hydroelectric power plant has more than <NUM> hydroelectric units, said method further comprises, after the above steps:.

A more specific embodiment at least comprises, in the following order:.

In an embodiment of a method according to the invention, step a), includes partially opening the guide vanes of the distributor of the first hydroelectric unit and of the second hydroelectric unit.

The variable frequency drive is common to both, or to all, hydroelectric units.

The guide vanes of the distributor of the first hydroelectric unit, respectively the second hydroelectric unit, can be further opened after step c), respectively after step e).

In a particular embodiment, the guide vanes of the distributor of the first hydroelectric unit are more open than the guide vanes of the distributor of the second hydroelectric unit during part of the time span between the beginning of step a) and the beginning of step c).

Preferably the second hydroelectric unit is connected to the variable frequency drive less than <NUM> after connecting the first hydroelectric unit to the grid.

The invention also concerns a hydroelectric power plant comprising at least a first hydroelectric unit and a second hydroelectric unit , each provided with a runner mechanically coupled to a shaft line and to a generator and comprising a distributor comprising guide vanes to control a flow of water to said runner, said power plant further comprising a variable frequency drive and a controller to couple said hydroelectric power plant to the grid so as to implement a method according to the invention, for example as recited above.

The hydroelectric power plant can comprise at least a third hydroelectric unit, also provided with a runner mechanically coupled to a shaft line and to a generator, a distributor comprising guide vanes to control a flow of water to said runner of said third hydroelectric unit, the method further comprising:.

The guide vanes of the distributor of the third hydroelectric unit can be further opened after step c').

The variable frequency drive can be controlled during step b') through a second control loop which includes said variable frequency drive.

The invention also concerns a hydroelectric power plant comprising at least a first hydroelectric unit and a second hydroelectric unit, each provided with a runner mechanically coupled to a shaft line and to a generator and comprising a distributor comprising guide vanes to control a flow of water to said runner, said power plant further comprising a variable frequency drive and a controller.

Said controller can be configured, and said hydroelectric power plant can be used, to implement a method according to the invention.

In a hydroelectric power plant or in a method according to the invention, each hydroelectric unit comprises a turbine which can be of the Francis or Kaplan or bulb or Pelton or reversible Francis or pump turbine type.

According to a particular embodiment of a method or of a hydroelectric power plant according to the invention:.

In a hydroelectric power plant according to the invention, each of the hydroelectric units can comprise:.

An embodiment of a hydroelectric power plant according to the invention can comprise at least a third hydroelectric unit, also provided with a runner mechanically coupled to a shaft line and to a generator, a distributor comprising guide vanes to control a flow of water to said runner of said third hydroelectric unit, said variable frequency drive and said controller being configured to couple said hydroelectric power plant to a grid so as to implement a method according to the invention for coupling a hydroelectric power plant to the grid, comprising at least a third hydroelectric unit, in a turbine mode.

The invention also concerns a hydroelectric power plant comprising a plurality of hydroelectric units, each provided with a runner mechanically coupled to a shaft line and to a generator and comprising a distributor comprising guide vanes to control a flow of water to said runner, said power plant further comprising a variable frequency drive and a controller, each of the turbines further comprising.

Said hydroelectric power plant can further comprise the above features of a power plant according to the invention.

The invention further concerns a computer program comprising instructions for implementing a method according to the invention, for example as recited above.

The invention allows a reduction of the time response defined as the time between receiving the order to provide a given level of power to the grid and the moment when this level of power is provided to the grid.

Other characteristics and advantages shall appear in the following description of embodiments of the method for coupling a hydroelectric turbine to the grid according to the invention, given by way of non-limiting examples, in reference to the annexed drawings wherein:.

An example of a hydroelectric unit (pump-turbine) <NUM> which can be used in the frame of the present invention is illustrated in <FIG>. Said hydroelectric unit can be implemented in a hydroelectric power plant with one reservoir upstream of the plant and one reservoir downstream of the plant. It can be used as a pump, to pump water from the downstream side of the plant to the upstream side. Alternatively, it is operated in a turbine mode, to produce electricity to the grid from the water head difference between the upstream and the downstream reservoirs.

Hydroelectric unit <NUM> comprises a runner <NUM>, a distributor <NUM>, a draft tube <NUM> and a shaft line <NUM>. A spiral case <NUM> guides a flow of water from a duct <NUM> connected to a main inlet valve <NUM> to the distributor, downstream of a penstock.

Via the shaft line <NUM> the runner <NUM> is mechanically coupled to the rotor of a generator; when rotating, the runner drives the rotor into rotation inside the stator windings. The stator windings are themselves intended to be connected to a grid via a circuit breaker and a transformer.

The distributor <NUM> comprises guide vanes and is water-tight in closed position.

The main inlet valve <NUM> may be for example a spherical valve or a butterfly valve. Both need a certain time to be opened, for example <NUM>, which can comprise <NUM> to <NUM> to equilibrate the pressure between the upstream side and the downstream side of the valve, for example by opening one or more bypass pipes <NUM>. After this pressure balancing, the valve <NUM> can be opened.

As disclosed in <CIT>, a variable frequency drive can be used to assist a start-up mode of pump-turbine <NUM> in the turbine mode.

The electric torque can be provided through a variable frequency drive connected to the grid and to an alternator of the hydraulic machine <NUM>; it comprises for example a static frequency converter, which can be a voltage source inverter or a current source inverter. An example of a variable frequency drive <NUM> is given on <FIG>: it is here a static frequency converter, comprising a rectifier and an inverter; said static frequency converter comprises networks of thyristors to convert a current from a grid at a fixed frequency (<NUM> in this example) into a current at a variable frequency; it forms a controlled electrical torque provider, working at a variable frequency.

As can be understood from <FIG>, the static frequency converter <NUM> can be connected to the generator of the turbine and can be connected to the grid at a fixed frequency (<NUM> in this example) through first connection means or switch <NUM>. The generator of the turbine can also be connected to the grid through second connection means or switch <NUM> (or generator circuit breaker). A third switch <NUM> has the same function as switches <NUM> and <NUM> (<FIG>) which are used to connect/disconnect each hydroelectric unit to the variable frequency drive <NUM>.

In a hydroelectric power plant comprising <NUM> (or more) hydro-electric units <NUM>, <NUM> (<FIG>), a single common variable frequency drive <NUM> can be used for both (or for all) generators, each generator being for example connected to the variable frequency drive <NUM> through corresponding connection means or switches <NUM>, <NUM> (so-called starting disconnector switches) and to the grid through two other connection means or switches <NUM>, <NUM> (so-called generator circuit breakers) and two transformers <NUM>, <NUM>. Variable frequency drive <NUM> can itself be connected to the grid through connection means or switch <NUM>. Each connection means or switch can comprise one or more IGBT(s) (Insulated Gate Bipolar Transistor). An example of a hydro-electric unit <NUM> was described in connection with <FIG>; Hydroelectric unit <NUM> (and any further hydro-electric unit if the plant has more than <NUM> hydro-electric units) is identical to hydro-electric unit <NUM>.

The rotation speed of hydroelectric unit <NUM>, respectively <NUM>, is controlled through a first control loop <NUM>, respectively <NUM>, controlling the guide vanes orientation of the corresponding machine based on the difference between a target speed N10_sp, respectively N100_sp, and a rotation speed N10, respectively N100, of said corresponding machine. Rotation speed can be measured with a speed sensor, for example using an inductive sensor placed opposite a toothed wheel; alternatively the speed measurement may be obtained through conversion from the frequency signal (the signal being taken from the secondary of a voltage transformer of the main generator). Said first control loop <NUM>, respectively <NUM>, comprises a guide vanes controller <NUM>, respectively <NUM>, and a guide vanes actuator <NUM>, respectively <NUM>, which provides a guide vane orientation y10, respectively y100, for the guide vanes of the hydroelectric unit <NUM>, respectively <NUM>. Said first control loop <NUM>, respectively <NUM>, provides a coarse regulation. If the plant has more than <NUM> hydroelectric units any further hydroelectric unit also has a first control loop similar to control loop <NUM> or <NUM>.

A second control loop <NUM>, <NUM> controls the electric torque of variable frequency drive <NUM> by a variable frequency drive controller <NUM> (which is common to both loops <NUM>, <NUM>). The input of this second control loop is the difference between the measured rotation speed N10, N100 and the target rotation speed N10_sp and N100_sp. Said second control loop <NUM>, respectively <NUM>, provides a fine regulation.

The control loops <NUM>, <NUM>, <NUM>, <NUM> will not be represented on <FIG> but are included in the hydroelectric plant represented thereon.

An example of a method according to the invention, in particular for coupling a hydroelectric power plant to the grid as described above, is now described.

In this example this method is for coupling the hydroelectric power plant or each of the two hydroelectric units <NUM>, <NUM> of the hydroelectric power plant to the grid, so that the power plant produces the maximum power in the shortest possible time.

Each hydroelectric unit <NUM>, <NUM> is driven into rotation in a turbine mode, with water flowing from the upstream to the downstream reservoir. Preferably, both hydroelectric units <NUM>, <NUM> are simultaneously driven into rotation. The guide vanes of each turbine are controlled by each of the first control loops <NUM>, <NUM> and are partly opened and the speed of each turbine progressively increases.

The speed of the first hydroelectric unit <NUM> is stabilized with help of the variable frequency drive <NUM> (through second control loop <NUM>), so that the first hydroelectric unit can be directly connected to the grid. In other words the speed of the first hydroelectric unit becomes equal to the speed required to produce power at frequency of the grid (for example <NUM>).

Then the speed of the second hydroelectric unit <NUM> is stabilized with help of the variable frequency drive <NUM> (through second control loop <NUM>), so that the second hydroelectric unit can be directly connected to the grid. In other words the speed of the second hydroelectric unit becomes equal to the speed required to produce power at frequency of the grid (for example <NUM>).

The inventors have noted that connecting the first hydroelectric unit directly to the grid creates disturbances of the speed of the second hydroelectric unit <NUM> (which, as explained above, was driven into rotation simultaneously to the first hydroelectric unit). For this reason, the speed of the second hydroelectric unit is stabilized by the variable frequency drive as soon as possible after the first hydroelectric unit is connected to the grid.

The different steps of this example are now set out in more detail in connection with <FIG> and <FIG>.

Both hydroelectric units <NUM>, <NUM> are simultaneously driven into rotation, the guide vanes of each of them being controlled through the control loop <NUM>, respectively <NUM>.

As illustrated on <FIG>, one of the connection means <NUM>, <NUM>, for example <NUM>, is first switched on in a conducting state in order to connect the corresponding hydroelectric unit <NUM> to the variable frequency drive <NUM> which is itself connected to the grid through connection means <NUM>. Variable frequency drive <NUM> can thus reduce the rotation speed of hydroelectric unit <NUM> (through loop <NUM>) when its speed is exceeding the upper limit of a coupling range. When its speed has reached a prescribed speed target, corresponding to the grid frequency, connection means <NUM> can be switched on in a conducting state, thus connecting hydroelectric unit <NUM> to the grid (<FIG>). Connection means <NUM> and <NUM> can then be switched off (<FIG>). It has to be noted that after connection means <NUM> are switched on, the guide vanes of the first hydroelectric unit <NUM> (like those of the second hydroelectric unit <NUM>) are only partly open and are then progressively opened until the power produced by hydroelectric unit <NUM> is at the desired level (or setpoint), for example at maximum power.

The connection means <NUM> can then be switched on in order to connect the corresponding hydroelectric unit <NUM> to the variable frequency drive <NUM> which is itself connected to the grid through connection means <NUM> (<FIG>). It has to be noted that switching off connection means <NUM> and switching on connection means <NUM> can take some time, for example between <NUM> and <NUM>, for example <NUM>. Variable frequency drive <NUM> reduces the rotation speed of hydroelectric unit <NUM> (through loop <NUM>) when its speed is exceeding the upper limit of a coupling range. When said speed has reached a prescribed speed target, corresponding to the grid frequency, connection means <NUM> can be switched on in a conducting state, thus connecting hydroelectric unit <NUM> to the grid (<FIG>). Connection means <NUM> and <NUM> can then be switched off (<FIG>). It has to be noted that after connection means <NUM> are switched on, the guide vanes of the second hydroelectric unit <NUM> are only partly open and are progressively opened until the power produced by generator <NUM> is maximum.

As already explained above, switching on connection means <NUM> (<FIG>) for connecting the first hydroelectric unit <NUM> to the grid creates disturbances of the speed of the second hydroelectric unit <NUM>. For this reason, connection means <NUM> are switched off and connection means <NUM> are switched on as soon as possible after connection means <NUM> are switched on to couple hydroelectric unit <NUM> to the grid. Due to switching times, the second hydroelectric unit <NUM> is coupled to the Variable frequency drive <NUM> after only <NUM> to <NUM>.

<FIG> is a diagram showing several parameters of the first hydroelectric unit <NUM> and of the second hydroelectric unit <NUM> when implementing a method according to the invention:.

Both hydroelectric units are driven into rotation at the same time t1, after receipt of the start order, the guide vanes of both being rapidly partially opened as illustrated by curves GV1 and GV2. The speed of both hydroelectric units thus increases from t1. Coupling the first hydroelectric unit to the variable frequency drive, for example some seconds after receipt of the start order allows an early stabilization of the speed and an early coupling of the first hydroelectric unit to the grid (« U1 coupling » at about <NUM> on <FIG>).

As can be understood from this diagram, according to a particular embodiment, GV1 can be more open than GV2 shortly after t1, between t1 (starting of both turbines) and the connection of the first hydroelectric unit to the grid ("U1 coupling"), or shortly after starting the rotation of both hydroelectric units. This dynamic opening of the guide vanes of the first hydroelectric unit can disturb its speed, the variable frequency drive <NUM> absorbing the hydraulic fluctuations during its coupling. Alternatively it is possible to have a same opening of both GV1 and GV2.

The first hydroelectric unit produces power P1 which is injected to the grid through the variable frequency drive <NUM>. Alternatively said first hydroelectric unit absorbs power from the grid. In both cases, the power produced by the first hydroelectric unit increases, together with the further opening of the guide vanes GV1, after coupling of the hydroelectric unit turbine to the grid (« U1 coupling »).

As can be seen on <FIG>, the load ramp up of the first generator to the grid creates a pressure dip for both machines, and in particular disturbances of the speed S2 of the second hydroelectric unit: curve S2 shows a dip D shortly after the « U1 coupling ». S2 is however stabilized after coupling of the second hydroelectric unit to the variable frequency drive <NUM>, which then allows a coupling of said second hydroelectric unit to the grid (« U2 coupling »). The second hydroelectric unit produces power P2 injected to the grid through the variable frequency drive. Alternatively said second hydroelectric unit absorbs power from the grid. In both cases, the power produced by the second hydroelectric unit increases, together with the further opening of the guide vanes GV2, after coupling of the second hydroelectric unit to the grid.

The total power produced by both hydroelectric units together amounts to P1 + P2. A full power, with P1 + P2 close to its maximum, is produced at about <NUM> after t1.

<FIG> is a diagram showing the same parameters S1, S2, GV1, GV2, P1 and P2 of the first hydroelectric unit <NUM> and of the second hydroelectric unit <NUM> when implementing a start-up method according to the prior art, without variable frequency drive.

The initial opening of the guide vanes GV1 of the first hydroelectric unit turbine <NUM> is less than on <FIG> because of the lack of variable frequency drive.

The coupling of the first hydroelectric unit <NUM> (« U1 coupling ») also occurs later than on <FIG>.

Like on <FIG>, the coupling of the first hydroelectric unit to the grid creates disturbances of the speed S2 of the second hydroelectric unit. However, due to the lack of variable frequency drive, S2 stabilizes later than on <FIG> and the coupling of the second hydroelectric unit to the grid (« U2 coupling ») also occurs later (at about <NUM>).

In the above examples the plant system comprising two hydroelectric units. However, the invention also applies to a plant comprising for example <NUM> or <NUM> hydroelectric units connected to a common variable frequency drive. A third (respectively a fourth) hydroelectric unit can be started at the same time as the first and second hydroelectric units (and possibly a third), but it will be connected to the variable frequency drive after the second (respectively the third) hydroelectric unit is connected to the grid (« U2 coupling » on <FIG>) and while the guide vanes of the third hydroelectric unit (respectively the fourth) are in the process of being further opened. In other words the same sequence of steps described above for the second generator with respect to the first hydroelectric unit can apply to the third (respectively the fourth) hydroelectric unit with respect to the second hydroelectric unit (respectively this third), etc..

The system of <FIG>, and in particular the switching on/off of the connection means, the variable frequency drive <NUM>, the opening and the closing of the main inlet valve (<FIG>, ref <NUM>) and of the guide vanes of both hydroelectric units, is controlled by one or more processor(s) or computer(s) <NUM>, or by a computer system, configured or programed so as to implement a method according to the invention, in particular in order:.

For example said processor(s) or computer(s) <NUM> or said computer system implements a computer program comprising instructions for implementing a method according to the invention.

In a particular embodiment a computer system implementing a method according to the invention comprises a central control system which supervises one or more controllers, each of said controllers controlling part of the hydroelectric power plant comprising two or more hydroelectric units.

Claim 1:
Method for coupling a hydroelectric power plant in a turbine mode to a grid, in order to generate power for said grid, said hydroelectric power plant comprising at least a first hydroelectric unit (<NUM>) and a second hydroelectric unit (<NUM>), each provided with a runner (<NUM>) mechanically coupled to a shaft line (<NUM>) and to a generator, a distributor (<NUM>) comprising guide vanes to control a flow of water to said runner, said hydroelectric power plant further comprising a variable frequency drive (<NUM>), the method comprising:
a) starting the rotation of at least said first hydroelectric unit (<NUM>) and said second hydroelectric unit (<NUM>);
b) connecting the variable frequency drive (<NUM>) to the generator of the first hydroelectric unit (<NUM>) and to the grid and stabilizing the speed of the first hydroelectric unit;
c) connecting the first hydroelectric unit (<NUM>) to the grid and disconnecting the generator of the first hydroelectric unit from the variable frequency drive (<NUM>);
d) connecting said variable frequency drive (<NUM>) to the generator of the second hydroelectric unit (<NUM>) and to the grid and stabilizing the speed of the second hydroelectric unit;
e) connecting the second hydroelectric unit (<NUM>) to the grid and disconnecting the generator of the second hydroelectric unit from said variable frequency drive (<NUM>).