Wind farm with increased revolution speed

p0001The invention relates to a wind power plant with a wind rotor, a so driven doubly fed induction generator and a converter for feeding electrical power into a network, a machine-side unit of the inverter are connected to a rotor and a line-side portion of a stator of the double-fed asynchronous, and a control device is provided with a converter control, which controls the inverter on the basis of predetermined parameters in a normal operating mode.

p0002Modern wind turbines, especially those of higher power ratings in the megawatt range are running at variable speed. This means that the speed of the wind rotor can be adjusted by adjusting the angle of attack of the rotor blades to prevailing wind conditions. While at low wind speeds, a low speed is set, a large rotational speed is adjusted accordingly at high wind speeds. At constant torque in the rotor shaft between the wind rotor and generator here, applies that the higher the speed, the higher the transmitted power and hence the yield of the wind turbine. One difficulty is that certain minimum and maximum speeds are observed due to boundary parameters in the wind power plant. With respect to the rotor of the generator voltage limitation is that even when it reaches the maximum (or minimum) speed, the voltage must not be higher than the maximum producible by the inverter AC voltage. Conventionally, therefore, the electric gear ratio is selected according to of the double-fed asynchronous machines. Thus can be achieved that the rotor voltage during operation is found within the limits of the converter. It has been found that the required se ratio is no longer feasible with very powerful generators. This is especially true in the retrofitting of wind turbines. To be able to transmit the high power, it is either necessary to replace the inverter against a higher voltage limit, which is expensive, or to provide a restriction of the speed range of the wind turbine, which the usability and thus the yield diminishes the wind turbine. Particularly troublesome is in the advantageous from a cost perspective restriction of the speed range, the remaining speed reserve for wind speed fluctuations, particularly gusts, lost.

p0003It is known to limit an undesirable increase in rotor voltage for deviating mains frequency by switching to a different torque / speed curve (<patcit id="pcit0001" dnum="US2007069522A1"><text>US 2007/069522 A1</text></patcit>). At low load, ie a rotational speed below the synchronous speed, the characteristic curve is shifted to a lower torque, so that a new operating point at a somewhat higher speed, ie closer to the synchronous speed setting. The slip of the generator is so reduced, whereby the rotor-voltage is reduced. Accordingly, at high load, that is, a rotation speed over the synchronous speed, the torque characteristic to a higher displaced, so that a new operating point is set at a somewhat lower speed. The slip and thus the rotor voltage are also reduced. A disadvantage of this known approach is that the torque / speed characteristic is shifted to medium speeds, which include the intended maximum speed is reduced. The gusts reserve is reduced by further increasing the load torque of the powertrain.

p0004The invention has the object to improve a wind turbine of the type mentioned to the effect that needs to be not limited even at high power ratings, the speed range without changing the inverter.

p0005The inventive solution lies in the features of the independent claims. Advantageous further developments are subject of the dependent claims.

p0006In a wind power plant with a wind rotor, a so driven doubly fed induction generator and a converter for feeding electrical power into a network, a generator-side unit of the inverter are connected to a rotor and a line-side part of the inverter to a stator of the double-fed asynchronous, and a control device having a converter control, a mode selector according to the invention is provided, which cooperates with the converter control such that it is switchable between two modes of operation, normal mode and a Spannungssenkbetriebsart, wherein the excitation of the generator is reduced compared to the normal mode.

p0007With the mode selector, the invention acts purposefully to the inverter control, so as to force a particular mode of the inverter, more specifically one or two inverters of the inverter. When operating in the super-synchronous; ie, with high load, it may be provided that at least one of the inverters of the inverter is brought into the under-excited operation. This produces the additional inverter (inductive) reactive power, the tension acts downward with respect to the rotor voltage. Thus, the limit values ​​are exceeded, the rotor voltage can be prevented. The wind turbine can be operated thus, even at high power and thus high-speed in-synchronous operation, yet a sufficient reserve is maintained for the rotor voltage. A speed limitation to protect the generator or inverter against excessive rotor voltages thanks to the invention no longer necessary.

p0008Below its some terms:<ul><li>Under-synchronous operation operating the wind turbine at speeds is understood to be higher than the synchronous speed. Accordingly, a synchronous operation is an operation of the wind energy plant at synchronous speed and a synchronous operation under a operating at speeds that are lower than the synchronous speed. The latter is done at low wind speeds, while the synchronous operation and then the super-synchronous operation is selected with increasing wind. As can be seen from the above, takes place operating with high load in the super-synchronous range.</li></ul>

p0009The invention achieves a number of advantages. Firstly, an extension of the useful speed range is achieved while limiting the rotor voltage to certain maximum values. Since the speed limits do not need to be lowered, the speed reserve is the same as for plants lower performance classes, so that the wind power plant according to the invention also comprises a corresponding high strength gusts. The invention provides thus in particular also for the upgrading of existing wind turbines, the generator is replaced by a higher power; thanks to the inventive-βen design is an appropriate and costly replacement of the inverter to a higher power is not required. The invention thus makes it possible to operate a more powerful generator with a designed for a lower power class inverter, while still obtaining the useable torque.

p0010Preferably, a tension sensor is provided which determines the voltage applied to the stator of the generator voltage and cooperates with the mode selector such that the excitation of the generator is reduced depending on the stator. Thus, a voltage drop (difference between rotor and stator voltage) are set with the generator so as to produce a desired rotor voltage. The rotor voltage can so from an unwanted rise, for example due to the standard value different voltage or frequency, be preserved.

p0011There may be a system for the rotor voltage can be provided that is expediently designed such that in the Spannungssenkbetriebsart regulation takes place the rotor voltage to an adjustable value below the value in the normal mode. This value is referred to as a low value. Here, a target value determination means may be provided, which determines the low value depending on the stator and / or the mains frequency. The invention has recognized that the risk of excessive rotor voltages especially in certain constellations with deep grid frequency of eg. 47.5 Hz (in a 50 Hz network) and / or increased mains voltage is (from about 110% of the target value), and a particularly requires high reactive power generation for the rotor.

p0012For the inventive generation of reactive power to lower the rotor voltage can be used in principle, both inverters, generator-side as well as the line-side inverter, the inverter. Advantageously, the generator-side inverter is used since it can use the transmission ratio of the double-fed asynchronous than gain, and also the voltage drop can be utilized to the generator impedance. However, it should not be excluded that additionally or alternatively the line converter is used.

p0013In particular, for the grid-side inverter an additional switch module is advantageously provided, which brings the grid-side inverter in an underweight, normal or over-excited mode. It cooperates with the mode selector together as described below. Basically, the neutral mode is selected, and that is when the line converter has not to generate reactive current. This mode is particularly suitable for cases of high peak real power on, if in any case little or no power reserve is available. To support the desired effect of voltage reduction, the underexcitables mode is appropriately selected. This is especially true when the voltage drop of the rotor voltage should be as high as possible. But it may also be that while the greatest possible reduction of the rotor voltage is to be achieved with the aid of the grid-side inverter, but that the impact on the grid to which the wind turbine is connected, should remain as low as possible. Then the over-excited mode would be chosen. In this mode, the generator-side inverters for the desired rotor voltage reduction, while the line converter, the impact of the performed by the generator-side inverter reactive power feed low holds in relation to the behavior of the network provides.

p0014Preferably, the additional switch module operates with a compensation system together such that it supports the grid-side inverter in the overexcited operation. This ensures that existing compensation systems can in particular be anyway used to provide the desired inductive reactive power. Moreover, the converter control be adapted to measure the rotor voltage and monitored. Exceeds the rotor voltage a predetermined limit value which the inverter enabled protective devices, such as a crowbar, to reduce the rotor voltage and thus protects the generator from damage caused by excessive rotor voltage.

p0015The invention further relates to a method for operating a wind power plant, wherein a converter control the inverter so switching between a normal mode and a Spannungssenkbetriebsart that the excitation of the rotor of the doubly fed induction generator in the super-synchronous operation at high power (corresponding to high winds) compared to the normal mode is reduced. For further explanation, reference is made to the above statements.

p0016The invention will be explained with reference to the accompanying drawing, in which an advantageous embodiment is illustrated. Show it:<dl id="dl0001"><dt>Fig. 1</dt><dd>a schematic view of a wind turbine according to an embodiment of the invention;</dd><dt>FIG. 2</dt><dd>a detailed view of an inverter and a converter control of the wind turbine in accordance with <figref idrefs="f0001">Fig. 1</figref>;</dd><dt>Fig. 3</dt><dd>an equivalent circuit diagram of the generator and its connection to a network;</dd><dt>Fig. 4</dt><dd>Characteristics of the wind turbine according to <figref idrefs="f0001">Fig. 1</figref>; and</dd><dt>Fig. 5</dt><dd>Diagrams for effects of the invention at different operating conditions.</dd></dl>

p0017An executed according to an embodiment of the invention, wind turbine comprises a tower 1, at the upper end of a nacelle 2 is rotatably disposed in the azimuth direction. At a front end of the nacelle 2, a wind rotor 3 with a plurality (in the example shown 3) with respect to their angle of incidence adjustable blades 31st About a shaft 32 drives the wind rotor 3 a arranged in the nacelle 2 generator 4 at. The generator is a double-fed asynchronous generator with a stator and a rotor. Next, an inverter 5 and an operation controller 6 are arranged for the wind turbine, which is connected to a converter regulation 7 in the nacelle. The latter controls the inverter and the inverter and directly sets the electrical parameters of the inverter. 5 The provided by the generator 4 in connection with the inverter 5, electric power is discharged via a line 15 and a normally disposed in the tower base medium-voltage transformer 19 to a network. 9 The network 9 may be a public communications network or a wind park internal connection piping.

p0018Subsequently, the electrical connection of the generator 4 in conjunction with the converter 5 and its interaction are described in more detail with the converter regulation. 7 The double-fed asynchronous generator 4 comprises a rotor and a stator. At the stator, the connecting line 15 is connected directly. The voltage on the stator is thus directly determined by the mains voltage. The rotor of the generator 4, however, is connected with the inverter fifth The inverter 5 is divided into three parts, a generator-side inverter 51, an intermediate circuit 52 and a grid-side inverter 53. The inverters 51, 53 are preferably fully controllable switching elements equipped as GTO or IGBT. The generator-side inverter 51 is connected to the rotor of the doubly-fed generator 4, and fed through the DC link 52 (which may be implemented as a DC or circuit) the grid-side inverter 53, which is in turn connected via a throttle 54 to the connecting line 15th

p0019The converter 7 has a control core 70 which controls the switching elements of the generator-side and grid-side inverter 51, 53rd This is done in accordance with performance indicators, which are created from the parent operation control. 6 In normal operation, the inverters are driven so that power from the power supply to the rotor of the generator 4 flows (under synchronous operation), no power by the inverter 5 flows (synchronous operation) or in case of high power generation by the generator 4 electric power from the rotor winding of the generator 4 taken and is fed via the inverter 5 in the network (via synchronous operation). This mode of operation of the inverter control and the inverter is known per se and need not be explained in detail. It is implemented in the control core 70th The control core 70 provides, which are applied to the generator-side and grid-side inverter 51, 53 at its output terminals control signals. According to the invention, the converter control determines the appropriate control signals for the inverter in a conventional manner in a normal mode, but in addition also for Spannungssenkbetriebsart. By means of a mode selector 72, the mode is selected, and transmits the respective associated signals to the inverter 51, 53rd

p0020The Spannungssenkbetriebsart is provided for this purpose that the generator-side inverter 51 is controlled such that the double-fed asynchronous generator 4 is operated under-excited. This means that additional inductive reactive power is fed into the rotor of the generator. 4 This reactive current acts downward on the rotor voltage, it may be that securely prevents the rotor voltage (minimum or maximum) Maximum limits exceeds even with a high load with extreme speeds, that is in particular not greater than the maximum producible by the inverter 5 voltage. This ensures that even in such extreme cases, the inverter 5 may regulate as desired and thus memorize the desired operating points.

p0021To explain the operation of the rotor voltage reduction according to the invention by feeding inductive reactive current is on <figref idrefs="f0002">Fig. 3</figref> Referring. Shown are lumped element active and reactive resistances of the generator 4, the line reactor 54, the medium-voltage transformer 19 and the impedance of the assumed to be rigid (ie, a constant voltage having) network 9. The additional feeding reactive power Q₀ in the rotor of the generator 4, the voltage drop across the impedance of the generator 4 can be enhanced. The rotor voltage U_R decreases so against the stator voltage U_S, This means that by under-excited operation according to the invention the voltage drop across the generator 4 increases in the Spannungssenkbetriebsart and so the rotor-voltage U_R can be lowered as desired.

p0022This effect can be enhanced by that the reactive power Q₀ and optionally further (unsigned same) reactive power Q₁, The grid-side inverter 53 in a corresponding manner, the voltage drop .DELTA.U_T on the impedance of the medium-voltage transformer 19 and the voltage drop .DELTA.U_N can be increased through the impedance of the network. Also in this case, a higher voltage drop is achieved by additionally supplying reactive power, whereby based on the line voltage U_N the stator voltage U_S is correspondingly reduced, and thus taking into account the additional voltage drop .DELTA.U_D via the throttle 54 and the internal electrical translation of the generator 4, the rotor voltage is further reduced. Altogether this results in a significant reduction of the rotor voltage by an additional feeding reactive power Q. Conversely, by over-excited operation of the line-side converter 53 reactive power Q₁ are generated with the opposite sign so that they related to the network 9, the reactive power Q₀ (Wholly or partially) compensated.

p0023The effect of Underexcitation invention in Spannungssenkbetriebsart is in the characteristics in <figref idrefs="f0002">Fig. 4</figref> shown. <figref idrefs="f0002">FIG. 4a</figref> shows the characteristic curve generator as a function of active power P over the speed of the wind rotor 3. The point at which the rated output is given, is located at 1170 1 / min. In weaker wind a speed provides a lower, and it shall consist of less active power; with stronger wind is regulated to said nominal output point at 1170 1 / min, and reached the above it speed range with a correspondingly higher effective power output only dynamic at gusts. Accordingly, in<figref idrefs="f0002">Fig. 4b</figref> the supply of reactive power shown on the speed. One can see that is generated by Underexcitation additional reactive power at high speeds. This acts, as stated above in connection with<figref idrefs="f0002">Fig. 3</figref> explains spannungsabsenkend with respect to the rotor voltage. In this case, the operating point is fed from the additional reactive power can be varied, for example. Depending on the condition relating to over- or under-voltage in the grid, grid frequency or requested by the operation controller 6 reactive power.

p0024The control of Spannungssenkbetriebsart works as follows. The device connected to the control core 70 mode selector 72 cooperates with a target value determination module 71st On this are each an input to the voltage to the stator winding of the generator 4, which is measured directly by a first voltage sensor 44, and the voltage to the rotor winding, which is also determined directly by means of a second voltage sensor 45 or indirectly from a drive signal for a pulse width modulator of the inverter 5 is determined, as well as an input signal for the mains frequency f is connected, which is determined by a sensor, not shown, or is applied from the operation control 6th It can also be provided the use of alternative or additional parameters. The target value determination module 71 determines based on definable limits, when a normal mode is switched to the Spannungssenkbetriebsart, and operated in accordance with the mode selector 72. The mode selector 72 of the generator-side inverter 51 is driven such in Spannungssenkbetriebsart that the rotor of the generator 4 is underexcited. Thus reactive power is additionally fed into the rotor of the generator 4, whereby the rotor-voltage is lowered. There may be formed a system of rotor voltage to the excitation of the generator-side inverter 51 as the manipulated variable and the network frequency or voltage as a reference variable.

p0025In addition it can be provided that acts on the grid-side inverter 53 by means of an additional switching module 73 of the mode selector. It can also be operated under energized or operated overexcited, or it can remain in the neutral mode. The underexcitables operation is selected when the voltage drop should be as high and an effect on the power plays no or only a minor role; This mode is especially appropriate when the rotor voltage should be reduced with the highest priority. The overexcitation would be selected if the impact on the network should be kept as low as possible. Ideally, the over-excitation is adjusted so that the grid (see<figref idrefs="f0002">Fig. 3</figref>) Provides compensation to the generator under excited about the rotor. The used for voltage reduction inductive reactive power Q₀ the generator 4 does not or at least not completely then flows into the net, but flows as reactive power Q₁ So that no or only little reactive power Q enters the grid-side inverter 53 into the net. 9 Thus, although only a slightly lower reduction of the rotor voltage is reached, but the wind power plant behaves neutral towards the net.

p0026Finally, a neutral setting can be selected if the inverter 5 has no more power reserve for generation of reactive power, ie in particular at high load, for example. Due to gusts.

p0027Alternatively or additionally, the additional switch module module be 73 configured to interface with a compensation system 90th This can be done in such a way that the compensation system is used specifically with 90 for generation of reactive power. As a compensation system is usually present in any case, can be increased in this way with minimal additional effort, the reactive power supply. This is especially true for the mode in which the line-side inverter is operated over 53 energized.

p0028The achieved in the operating characteristics are for an exemplary embodiment of the invention in <figref idrefs="f0003">Fig. 5</figref> shown. <figref idrefs="f0003">Fig. 5a</figref> shows the achieved by feeding reactive power change in the rotor voltage, namely for different line frequencies. The abscissa shows the reactive power generated is removed, with negative values ​​for inductive reactive power are so untererregtem operation. On the ordinate is reached with each injected reactive power rotor voltage U_R removed, namely for line frequencies of 50.5 Hz and 47.5 Hz. The threshold voltage of the rotor voltage is 750 V (dashed line). It is evident that with the inventive supply of reactive power in the under-excited operation, the rotor voltage even under unfavorable conditions (mains frequency 47.5 Hz) can be kept below the limit.

p0029In <figref idrefs="f0003">Fig. 5b</figref> is in accordance with the rotor current I_R shown on the reactive power Q. It is evident that at a moderate under-excited operation (reactive power supply from -500 to -3000 kVA) the rotor current reaches the lowest values ​​while increases significantly with greater supply of reactive power of the rotor current. In<figref idrefs="f0003">Fig. 5c and 5d</figref> accordingly, the stator voltage U_S and the stator current I_S shown with the dotted line represents the rated current.

p0030The diagrams in <figref idrefs="f0003">Figures 5a to 5d</figref> refer to a high-load case under adverse conditions, with a rotor speed above the rated speed (1330 1 / min compared to 1170 1 / min) and maximum permissible voltage at the connection of the medium voltage transformer 19th to the network 9 in the amount of 110% of the mains voltage It is apparent that even under these extremely unfavorable conditions through the supply of reactive power, the voltages below the limit values ​​can be maintained. Thus, this speed range can be used even at high supply voltages, without causing harmful over-voltage on the rotor of the generator. 4 Since this speed range is typically achieved only in the dynamic short-time operation (especially in gusts), charged the additional reactive current for the generation of reactive power, the system 5 formed from generator 4 and inverter hardly so that short term high currents can be tolerated. The invention achieves the fact that even under unfavorable operating conditions such as a power surge, the upper speed range for the operation of the system is maintained. The usable speed range is thus extended upwards without requiring complex, expensive hardware through an enhanced inverter is required.