TECHNICAL FIELD

The present invention relates to a wind turbine for generating and feeding electric energy to an electric power grid.

More specifically, the present invention relates to a wind turbine, for generating and feeding electric energy to an electric power grid, comprising :

a blade assembly;

an electric machine comprising a stator; and a rotor connected to the blade assembly to generate electric energy;

a ^" first switch converter connected to the electric machine to control stator electric quantities;

a second switch converter connected to the electric power grid; and

a DC link circuit for connecting the first switch converter to the second switch converter.

BACKGROUND ART

A wind turbine of the above type is known from US 5083039.

The wind turbine supplies energy to the grid at a given voltage, which must be the same as the grid voltage, and supplies a given current. The grid voltage is determined by the power server to which the wind turbine is connected, whereas current supply depends on wind conditions and server power demand.

The grid voltage is not perfectly constant, but has a reference value about which it can vary by roughly 10%; and wind turbine voltage must equal and follow variations in grid voltage.

To maximize conversion of kinetic wind energy to electric energy, modern wind turbines can adapt the speed of the rotor to wind strength, so the voltage and/or current of the electric machine vary in amplitude and frequency, depending on the speed of the rotor.

For the wind turbine to function properly, adjustments are therefore needed, and which are made by the first and second switch converter.

The first switch converter comprises switches, and acts on the electric machine current and/or voltage to control the electric machine and electric energy flow from the electric machine to the DC link circuit.

The second switch converter also comprises switches, and is designed to connect the DC link circuit and the grid, and to control electric energy transfer from the DC link circuit to the grid. More specifically, the second converter acts on respective switches to couple the direct voltage of the DC link circuit to the grid voltage, or vice versa.

The direct voltage of the DC link circuit is fixed, and is set at the design stage to a value of V2 times whichever is higher : the maximum possible voltage of the electric machine, or the maximum possible grid voltage .

So designed, the electric machine can function over a wide range of wind speeds, and the direct voltage always being higher than the electric machine and grid voltages prevents undesired turn-on of the diodes connected to the converter switches. Known turbines pose problems caused by inevitable switching losses, which normally depend on the voltage and current of the switch involved and the time taken for it to switch. Since these values are normally fairly high, the amount of power dissipated by switching on known turbines is significant and has a noticeable effect on performance.

Moreover, when using certain control techniques, such as discontinuous pulse-width modulation, power dissipation also depends on the total number of switching operations, which varies. Discontinuous pulse- width modulation, in fact, acts on the number and duration of the switching operations per period to adjust the output voltage of the converter, so the amount of power dissipated increases in direct proportion to the number of switching operations per period .

DISCLOSURE OF INVENTION It is an object of the present invention to provide a wind turbine of the above type for producing electric energy, and designed to eliminate the drawbacks of the known art .

More specifically, it is an object of the present invention to provide a wind turbine of the above type for producing electric energy, and designed to reduce power dissipation.

According to the present invention, there is provided a wind turbine for generating and feeding electric energy to an electric power grid, the wind turbine comprising:

a blade assembly;

at least one electric machine comprising a stator, and a rotor connected to the blade assembly to generate electric energy;

a first switch converter connected to the electric machine to control stator electric quantities;

a second switch converter connected to the electric power grid; and

a DC link circuit for connecting the first switch converter to the second switch converter;

the wind turbine being characterized by comprising a control device which, by means of at least one of the first and second switch converters, controls a direct voltage in the DC link circuit on the basis of an operating parameter of the electric machine indicating the stator voltage of the electric machine, and on the basis of a quantity indicating the line voltage of the electric power grid.

By virtue of the present invention, the direct voltage adapts to the variable stator voltage of the electric machine, and to the variable line voltage of the electric power grid, thus making it possible to maintain practically ideal ratios between the direct voltage and the stator voltage of the electric machine, and between the direct voltage and the line voltage of the electric power grid - which, as stated, determine the switching losses. In known turbines, on the other hand, the direct voltage is not controlled, and is set once and for all on the basis of the maximum predicted stator and line voltages, which, in actual operating conditions, however, inevitably vary, with negative effects on switching losses.

The turbine according to the invention therefore provides on average for reducing power dissipation caused by switching of the converters.

Moreover, when using the discontinuous pulse-width modulation technique, the favourable ratios between the direct voltage and the stator and line voltages also reduce the number of switching operations, thus further reducing power dissipation caused by switching losses, at least for one of the first and second switch converters .

A further object of the present invention is to provide a method of controlling a wind turbine for generating electric energy.

According to the present invention, there is provided a method of controlling a wind turbine for generating electric energy, the wind turbine comprising: a blade assembly;

at least one electric machine comprising a stator; and a rotor connected to the blade assembly to generate electric energy;

a first switch converter connected to the electric machine to control stator electric quantities;

a second switch converter connected to an electric power grid; and

a DC link circuit for connecting the first switch converter to the second switch converter;

the method comprising controlling, by means of at least one of the first and second switch converters, a direct voltage in the DC link circuit on the basis of an operating parameter of the electric machine indicating the stator voltage of the electric machine, and on the basis of a quantity indicating the line voltage of the electric power grid.

BRIEF DESCRIPTION OF THE DRAWINGS A non- limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which :

Figure 1 shows a partly sectioned side view, with parts removed for clarity, of a wind turbine in accordance with one embodiment of the present invention;

Figure 2 shows an operating block diagram of the Figure 1 embodiment of the wind turbine;

Figure 3 shows an operating block diagram of an alternative embodiment of the wind turbine to the one in Figure 2.

BEST MODE FOR CARRYING OUT THE INVENTION

Number 1 in Figure 1 indicates a wind turbine - in the example shown, a direct-drive, variable-angular- speed wind turbine - for generating electric energy.

Wind turbine 1 comprises a supporting structure 2 ; a nacelle 3 fitted to supporting structure 2 to rotate about an axis Al ; a hub 4 connected to nacelle 3 to rotate about an axis A2 ; a number of blades 5 fitted to hub 4 and adjustable about respective axes A3; an electric machine 6; an electric transmission 7 (Figure 2) ; and a control device 8 for controlling wind turbine 1.

With reference to Figure 2, wind turbine 1 is designed to generate and feed electric energy to an electric power grid 9. With reference to Figures 1 and 2, electric machine 6 comprises an annular stator 10; and an annular rotor 11 coupled magnetically and mechanically to stator 10 to rotate about axis A2 by means of a bearing assembly (not shown) . In other words, electric machine 6 is an annular electric generator.

Electric machine 6 is connected to electric power grid 9 by electric transmission 7.

Hub 4 is fitted directly to rotor 11 to transfer wind- induced rotation to rotor 11.

Nacelle 3 is fixed to supporting structure 2 to rotate about axis Al and position hub 4 and blades 5 facing into the wind.

With reference to Figure 1, hub 4, blades 5, and rotor 11 define a rotary assembly 12 housed partly inside nacelle 3. In the example shown, rotor 11 is housed inside nacelle 3 and supported solely by the bearing assembly at hub 4.

Stator 10 comprises a number of multiphase - in the preferred embodiment of the invention, three-phase stator windings (not shown) arranged in stator segments.

Rotor 11 is hollow, and comprises a number of magnetized modules, in particular permanent magnets, arranged in rotor segments .

In the example shown, electric machine 6 is a synchronous, preferably three-phase type; it being understood, however, that the present invention applies to any type of rotating electric machine, e.g. asynchronous, preferably three-phase electric generators with a squirrel-cage rotor, or synchronous electric generators with a rotor with rotor windings instead of permanent magnets .

Electric transmission 7 comprises a multiphase, in particular three-phase, electric transmission line 18; a switch converter 19 connected to electric machine 6 by multiphase electric transmission line 18; a DC link circuit 20; a switch converter 21 connected to switch converter 19 by DC link circuit 20; and a multiphase electric transmission line 22 for connecting switch converter 21 to electric power grid 9 at a switch point 23.

Switch converter 19 may comprise a bridge of controlled switches, such as IGBTs , power MOSFETs or others .

Switch converter 21 may also comprise a bridge of controlled switches, such as IGBTs, power MOSFETs or others .

Control device 8 comprises a control unit 30 connected to and for controlling switch converter 19; and a control unit 31 connected to and for controlling switch converter 21. More specifically, control unit 30 is connected to electric machine 6 to control said stator electric quantities .

Wind turbine 1 comprises a measuring block 35 connected to electric machine 6 - more specifically, to multiphase transmission line 18 - to determine said stator electric quantities.

Control unit 30 is connected to measuring block 35 to receive the stator electric quantities.

More specifically, the stator electric quantities are stator currents I _s flowing along multiphase transmission line 18.

Measuring block 35 comprises a speed sensor 40, e.g. an encoder, coupled to the rotor 11 of the electric machine 6 for determining the angular speed of rotor 11.

The speed sensor 40 is configured for providing the position of rotor 11.

Control unit 30 is connected to measuring block 35 to receive stator currents I _s and the speed and position of rotor 11. Further, the control unit 30 is supplied by control device 8 with a reference target torque C calculated on the basis of the various parameters of wind turbine 1 and defined to maximize efficiency of wind turbine 1.

On the basis of stator currents I _s , the speed and position of rotor 11, and reference target torque Cj, control unit 30 acts on switch converter 19 so that electric machine 6 exhibits a resisting torque C _r equal to reference target torque C _d . In other words, control unit 30 effects a so-called current control.

In an alternative embodiment of the present invention, stator currents I _s and/or the speed of rotor 11 and/or the position of rotor 11 are calculated, as opposed to being detected.

Wind turbine 1 comprises a measuring block 36 connected to and for measuring electric quantities of electric power grid 9.

More specifically, the electric quantities of electric power grid 9 are line currents Ii _in flowing along multiphase transmission line 22, and of which measuring block 36 determines amplitude and phase.

Control unit 31 is connected to measuring block 36 to receive the amplitude and phase of line currents Iii _n .

Wind turbine 1 comprises a measuring block 37 connected to electric machine 6 to measure a stator voltage V _s , in particular a linked stator voltage V _s , of electric machine 6; a measuring block 38 connected to electric power grid 9 to measure a line voltage Vi _in , in particular a linked line voltage Vi _in , of electric power grid 9; and a measuring block 39 connected to and for measuring a direct voltage V _DC of DC link circuit 20. Wind turbine 1 comprises a control unit 32 connected to measuring block 37 to receive stator voltage V _s of electric machine 6, to measuring block 38 to receive line voltage Vi _in of electric power grid 9, and to measuring block 39 to receive direct voltage V _DC of DC link circuit 20.

Control unit 32 is connected to and supplies control unit 31 with a command string S _c defined on the basis of line voltage Vi _in of electric power grid 9 and stator voltage V _s of electric machine 6.

More specifically, command string S _c comprises a logic value indicating the higher of stator voltage V _s and line voltage Vi _in , the value of stator voltage V _s , and the value of line voltage V _lin .

Control unit 32 sends command string S _c to control unit 31, which acts on switch converter 21 on the basis of the electric quantities, i.e. line currents I _lin , of electric power grid 9, and on the basis of command string S _c in turn determined on the basis of line voltage V _lin of electric power grid 9 and stator voltage V _s of electric machine 6.

More specifically, control unit 31 operates so that the voltage V _dc of DC link circuit 20 is always proportional to the higher of line voltage Vi _in of electric power grid 9 and stator voltage V _s of electric machine 6. In other words, when stator voltage V _s of electric machine 6 is higher than line voltage Vi _in of electric power grid 9 (V _s > V _n ) , control unit 32 supplies control unit 31 with command string S _c indicating the higher, and the respective values, of stator voltage V _s of electric machine 6 and line voltage Vi _in of electric power grid 9. And control unit 31 acts on the basis of the electric quantities, i.e. line currents In _n , of electric power grid 9, stator voltage V _s of electric machine 6, and line voltage Vn _n of electric power grid 9. The control unit 31 acts on switch converter 21 so that the direct voltage V _DC of DC link circuit 20 is proportional to stator voltage V _s of electric machine 6 according to a predetermined coefficient. In this case (V _s > V n) switch converter 21 is controlled to convert the alternating line voltage V _Un of electric power grid 9 to direct voltage V _DC and vice versa, and operates so that direct voltage V _DC is proportional to stator voltage V _s of electric machine 6, and preferably V2 times the peak stator voltage V _s of electric machine 6. In other words, switch converter 21 operates as an AC/DC converter and voltage booster.

Conversely, when line voltage Vn _n of electric power grid 9 is higher than stator voltage V _s of electric machine 6, control unit 32 supplies control unit 31 with command string S _c indicating Vn _n > V _s and the value of line voltage V _lin of electric power grid 9. And control unit 31, on the basis of the electric quantities, i.e. line currents Iii _n , and line voltage Vi _in of electric power grid 9, operates so that direct voltage V _DC of DC link circuit 20 is proportional to, and preferably V2 times, the line voltage Vi _in of electric power grid 9.

In a variation of the present invention, measuring block 37 for measuring stator voltage V _s of electric machine 6 is eliminated, and control unit 32 is connected to speed sensor 40 of rotor 11, and operates on the basis of the speed of rotor 11 of electric machine 6, i.e. determines stator voltage V _s of electric machine 6 from the speed of rotor 11 and operates as described previously.

In the Figure 3 variation of the present invention, control unit 32 is connected to and controls control unit 30 by a control signal S _d , so that stator voltage V _s of electric machine 6 is less than or equal to line voltage Vi _in . More specifically, control unit 30 current- controls switch converter 19 on the basis of reference target torque Ca and control signal Sa- More specifically, control unit 30 modifies the current control described with reference to Figure 2, to lower stator voltage V _s to the value indicated by control signal S _d , by reducing a magnetic flux of electric machine 6. In a variation not shown of the present invention, the electric machine stator voltage measuring block is eliminated, and the wind turbine comprises an estimating unit connected to the control unit to provide an estimate of the electric machine stator voltage. The control unit operates as described previously.

In a variation of the present invention, the stator electric quantities are stator voltages V _s , and the electric quantities of electric power grid 9 are line voltages V ^lin , so control units 30 and 31 operate respectively on the basis of stator voltages V _s and line voltages Vi _in instead of stator currents I _s and line currents Ii _in .

In a variation not shown of the present invention, the wind turbine comprises a further electric machine connected to the wind turbine hub and switch converter 19.

In a variation not shown of the present invention, the wind turbine comprises a further electric machine connected to the wind turbine hub; and a further switch converter connected to the further electric machine and the DC link circuit.

In a variation not shown of the present invention, the wind turbine comprises a further electric machine connected to the wind turbine hub; and a further electric transmission connected to the further electric machine and the electric power grid.

According to the present invention, direct voltage V _DC is constantly proportional to stator voltage V _s of electric machine 6 or line voltage Vi _in of electric power grid 9, depending on the operating conditions, thus making it possible to maintain practically ideal ratios between direct voltage V _DC and stator voltage V _s , and between direct voltage V _DC and line voltage Vi _in , and so reduce switching losses caused by lower voltages on component parts of switch converters 19 and 21, regardless of the pulse-width modulation technique employed. In known turbines, on the other hand, direct voltage V _DC is not controlled, and is set once and for all on the basis of the maximum predicted stator voltage V _s and line voltage Vi _in , which, in actual operating conditions, however, inevitably vary, with negative effects on switching losses.

Wind turbine 1 therefore provides on average for reducing power dissipation of at least one of switch converters 9 and 21.

Moreover, when using the discontinuous pulse-width modulation technique, the above ratios determine the number of switching operations and the switching losses of switch converters 19 and 21. So the favourable ratios between direct voltage V _DC and stator voltage V _s and between direct voltage V _DC and line voltage Vi _in also reduce the number of switching operations, thus further reducing power dissipation caused by switching losses, at least for one of switch converters 19 and 21.

Clearly, changes may be made to the wind turbine and method described herein without, however, departing from the scope of the accompanying Claims.