Method for feeding electrical power into an electrical supply network

Abstract

A method for feeding electrical power into an electrical supply grid by means of at least one wind power installation at a grid connection point, wherein the at least one wind power installation has an aerodynamic rotor with rotor blades and the rotor has a moment of inertia and can be operated with variable rotor speed, the at least one wind power installation has a generator for generating a generator power, multiple energy generators feed power into the electrical supply grid and multiple consumers take power from the electrical supply grid, so that a power balance in the electrical supply grid between the power fed in and the power taken is produced and is positive if more power is fed in than is taken, and the method comprises the steps of: feeding in a basic electrical power in dependence on available wind power, specifying a supporting power to be additionally fed in and additionally feeding in the specified supporting power to be additionally fed in for supporting the electrical supply grid, an amount of supporting energy available for the supporting power to be fed in being determined and the specifying of the supporting power to be additionally fed in taking place in dependence on the available amount of supporting energy determined.

Claims

1. A method for feeding electrical power into an electrical supply grid by at least one wind power installation at a grid connection point, wherein: the at least one wind power installation has an aerodynamic rotor with rotor blades and the rotor has a moment of inertia and is configured to be operated with a variable rotor speed, the at least one wind power installation has a generator for generating a generator power, a plurality of energy generators feed power into the electrical supply grid, and a plurality of consumers take power from the electrical supply grid so that a power balance in the electrical supply grid between the power fed in and the power taken is produced and is positive when more power is fed in than is taken, wherein the method comprises: feeding in a basic electrical power in dependence on available wind power; determining an amount of supporting energy available for a supporting power to be fed; specifying the supporting power to be additionally fed in, wherein the specifying the supporting power depends on the amount of supporting energy available and on a grid state of the electrical supply grid; and feeding in the specified supporting power for supporting the electrical supply grid, wherein an upper power limit of the supporting power is determined in dependence on the amount of supporting energy and maintained; and wherein the specified supporting power is reduced with a progression falling over time in response to the available amount of supporting energy reaching a specified limit value.

2. The method as claimed in claim 1, wherein the amount of supporting energy available comprises at least one of: the moment of inertia of the rotor; at least one operating parameter of an operating point of the at least one wind power installation; or an admissible lower rotor speed.

3. The method as claimed in claim 2, wherein the at least one operating parameter of the operating point at a particular time comprises at least one of: rotor speed at the particular time; generator power at the particular time; or prevailing wind speed.

4. The method as claimed in claim 1, wherein at least one inverter is used for feeding in the electrical power, and wherein the supporting power to be additionally fed in is determined depending on a capacity utilization of the at least one inverter and depending on a reactive power fed in.

5. The method as claimed in claim 1, wherein for determining the available amount of supporting energy, a power loss due to a reduction of the rotor speed is taken into consideration.

6. The method as claimed in claim 1, wherein the at least one wind power installation is coupled to at least one electrical energy store, and wherein a rotational energy of the rotor and in each case a storage content of the at least one electrical energy store are used to determine the available amount of supporting energy, the at least one electrical energy store comprising at least one of: a battery or a high-capacity capacitor.

7. The method as claimed in claim 1, wherein when feeding in the specified supporting power, wherein the available amount of supporting energy is updated: in dependence on the supporting power fed in; and while taking into consideration a behavior of change of the at least one wind power installation as a reaction to the feeding in of the supporting power.

8. The method as claimed in claim 1, wherein the at least one wind power installation is coupled to at least one consumer that can be controllably reduced in its consumption and the determination of the available amount of supporting energy takes into consideration an amount of energy that is available due to a reduction of the at least one controllably reducible consumer.

9. The method as claimed in claim 1, wherein specifying the supporting power comprises specifying a progression of the supporting power in dependence on the amount of supporting energy.

10. The method as claimed in claim 1, wherein the at least one wind power installation is operated at an operating point that is initially improved when delivering rotational energy.

11. A wind power installation for feeding electrical power into an electrical supply grid at a grid connection point, the wind power installation comprising: an aerodynamic rotor with rotor blades, wherein the rotor has a moment of inertia and is configured to be operated with variable rotor speed; a generator for generating a generator power; a control unit for controlling feeding in of electrical power in dependence on wind power available; a specifying means for specifying a supporting power to be additionally fed in; a feeding device for feeding in the specified supporting power for supporting the electrical supply grid; and a calculating unit for determining an available amount of supporting energy for the supporting power to be fed in, wherein the supporting power is specified in dependence on the amount of supporting energy determined, wherein the supporting power is specified in dependence on the amount of supporting energy available and a grid state of the electrical supply grid; wherein an upper power limit of the supporting power is determined in dependence on the available amount of supporting energy and maintained; and wherein the specified supporting power is reduced with a progression falling over time in response to the available amount of supporting energy reaching a specified limit value.

12. A wind power installation configured to carry out the method as claimed in claim 1.

13. A wind farm comprising: a plurality of wind power installations comprising the wind power installation as claimed in claim 11.

14. The method as claimed in claim 2, wherein the admissible lower rotor speed is determined depending on a rotational speed at a particular moment in time.

15. The method as claimed in claim 9, wherein specifying the supporting power depending on the amount of supporting energy takes place by way of a time-dependent progression.

16. The method as claimed in claim 10, wherein the at least one wind power installation is operated at such a high rotor speed that the rotor speed is temporarily reduced to an optimum rotor speed in terms of power by delivering centrifugal energy.

17. The method as claimed in claim 1, the supporting power is specified in dependence on the power balance of the electrical supply grid.

18. The method as claimed in claim 1, wherein the specified supporting power is reduced with a flank falling over time.

19. The method as claimed in claim 1, wherein the specified limit value is a minimum amount of supporting energy.

20. The method as claimed in claim 7, wherein while taking into consideration the behavior of change of the at least one wind power installation as the reaction to the feeding in of the supporting power, particularly while taking into consideration a decrease in the rotor speed as a reaction to delivering rotational energy of the rotor.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The invention is now explained in more detail below on the basis of exemplary embodiments with reference to the accompanying figures.

(2) FIG. 1 shows a wind power installation in a perspective representation.

(3) FIG. 2 shows a wind farm in a schematic representation.

(4) FIG. 3 schematically shows a control structure of a wind power installation.

(5) FIG. 4 shows a power- and energy-time diagram.

(6) FIG. 5 schematically shows in a diagram the progression of a rotational energy in dependence on a rotational speed.

DETAILED DESCRIPTION

(7) FIG. 1 shows a wind power installation 100 with a tower 102 and a nacelle 104. Arranged on the nacelle 104 is a rotor 106 with three rotor blades 108 and a spinner 110. During operation, the rotor 106 is set in a rotary motion by the wind, and thereby drives a generator in the nacelle 104.

(8) FIG. 2 shows a wind farm 112 with, by way of example, three wind power installations 100, which may be the same or different. The three wind power installations 100 are consequently representative of essentially any number of wind power installations of a wind farm 112. The wind power installations 100 provide their power, to be specific in particular the electricity generated, by way of an electrical farm grid 114. In this case, the electricity or power respectively generated by the individual wind power installations 100 is added together and there is usually a transformer 116, which steps up the voltage in the farm in order then to feed into the supply grid 120 at the feed-in point 118, which is also referred to generally as the PCC. FIG. 2 is just a simplified representation of a wind farm 112, which for example does not show any controller, although there is of course a controller. It is also possible for example for the farm grid 114 to be differently designed, in that for example there is also a transformer at the output of each wind power installation 100, to name just one other exemplary embodiment.

(9) FIG. 3 shows in a schematic and illustrative structure a nacelle 2 of a wind power installation with a control structure 4 including an inverter 6, which forms a feeding device. There is also a battery store 8 as a further energy source. With the feeding device 6, specifically the inverter, feeding takes place via a transformer 10 into a schematically indicated electrical grid 12. Further energy generators also feed into this electrical grid 12 and electrical consumers are connected to it, neither of which is shown here.

(10) There follows first a general description of the operating principle of the wind power installation, of which the nacelle 2 and the end piece of the tower 14 are indicated. Arranged on the nacelle 2 are likewise schematically represented rotor blades 16, to be specific in the region of a hub 18 of a spinner 20. These rotor blades 16, of which there are preferably three, though only two are shown in FIG. 3, are driven by the wind and set the rotor 22, which consists substantially of the rotor blade 16, the hub 18 and the spinner 20, in a rotary motion. From this, electrical power is generated with the aid of the generator 24, to be specific generator power. The generator 24 has for this purpose a generator rotor 26 and a stator 28. The term generator rotor is used here irrespective of the type of generator 24, to avoid confusion with the aerodynamic rotor 22. The generator rotor 26 is in this case fixedly connected to the hub 18, and consequently to the rotor 22. In principle, however, a gear mechanism may also be interposed.

(11) The preferred generator 24 is a synchronous generator. In particular, an externally excited synchronous generator is proposed. For providing exciter power for the externally excited synchronous generator, the current adjuster 30 is provided, feeding exciter current to the generator rotor 26. A slip ring used for this is not shown here.

(12) The generator power generated by the generator 24 is passed by the stator 28 to the rectifier 32, which rectifies it and provides a correspondingly rectified direct current with a corresponding direct voltage at a bus bar 34. When operation is in progress, the current adjuster 30 can also draw its energy from the bus bar 34. For the following observations of the feeding in and grid support, the power component that the current adjuster 30 requires is ignored.

(13) The bus bar 34, which does not necessarily have to be configured as a bar but may also be realized by lines or bundles of lines, is coupled to an input of the inverter 6.

(14) The inverter 6 consequently converts the direct current or the direct voltage of the bus bar 34 into an alternating current or an alternating voltage, which is transformed via the transformer 10 and fed into the electrical grid 12.

(15) If there is then a more or less sudden power demand, because for example the power balance in the electrical supply grid 12 has become negative, it may be envisaged to feed additional supporting power in through the inverter 6. In the embodiment shown in FIG. 3, it is particularly considered in this respect to use power from the battery store 8 or from the rotational energy of the rotor 22. For this purpose, the control unit 36 may specify a corresponding preset power P.sub.S for the inverter 6. Otherwise, it is suggested that the control unit 36 can also specify a reactive power Q.sub.S for the inverter 6. Furthermore, the inverter 6 may give the apparent power S to the control unit 36 as information.

(16) This power P.sub.S to be fed in, which to this extent represents a preset power, is made up of a basic electrical power and the supporting power to be fed in. In a normal case, if there is no demand for supporting power, the preset power P.sub.S substantially corresponds to the basic power. It is consequently also used in normal operation to control the power delivery of the wind power installation. If a supporting power is then additionally to be fed in, because the value of this preset power P.sub.S is therefore to be additionally increased, the control unit 36 must first have a reason for this. Often, the reason will arise from the sensing of the grid frequency f. Particularly if the grid frequency f falls, such a demand for feeding in additional power for grid support may arise. Provided for this is the voltage measuring device 38, which apart from the output voltage U of the inverter 6 also senses the grid frequency f and feeds it to the control unit 36. Alternatively, the measurement of the voltage and frequency may also be performed elsewhere, such as for example between the transformer 10 and the electrical supply grid 12, that is to say at the grid connection point 40 there.

(17) Also or alternatively, a demand for grid support may be detected or initiated by way of an external signal, which in FIG. 3 is indicated as “ext”. The signal “ext” may be provided by a grid operator.

(18) Consequently, the control unit 36 can detect the demand for feeding in additional supporting power for grid support on the basis of at least one of these variables mentioned.

(19) It is thus proposed that, in addition to the calculation of the supporting power to be fed in, depending on these variables, which indicate such a demand, an amount of supporting energy that is present altogether is first sensed and then taken into consideration.

(20) For the determination of the available supporting energy E.sub.V, the calculating unit 42 is provided. The calculating unit 42, which can also be combined with the control unit 36 in one device or in a process controller, takes into consideration in the embodiment of FIG. 3 both available energy from rotational energy of the rotor 22 and storage energy of the battery store 8. The available amount of supporting energy E.sub.V thereby determined altogether is then taken into consideration for specifying the supporting power and for this, according to the structure of FIG. 3, is transmitted to the control unit 36.

(21) The available amount of supporting energy E.sub.V is in this case additively made up of the individual available amounts of supporting energy of the rotor 22 and of the battery store 8. It is therefore made up of the available rotational energy of the rotor 22 and the available storage energy of the battery store 8.

(22) The available storage energy of the battery store 8 may be determined for example depending on the battery voltage U.sub.B. For this, the battery store may transmit this value of the battery voltage U.sub.B to the calculating unit 42. Alternatively, particularly whenever it is a complex bank of stores comprising many units, the battery store 8 could itself determine the available storage energy and transmit it to the calculating unit 42 or for example transmit a state of charge to the calculating unit 42, from which the calculating unit 42 then determines the available amount of storage energy.

(23) For the determination or calculation of the available rotational energy of the rotor 22, the calculating unit 42 obtains inter alia the rotational speed n, which can be sensed by the speed sensor 44. The mass moment of inertia of the rotor 22 is available to the calculating unit 42, and consequently the rotational energy can be calculated in dependence on the rotational speed n.

(24) The rotational energy stored by the rotor 22 on the basis of its rotational speed n at the time is however not in fact available completely, because, in particular at a moment in which the grid is to be supported, the rotor should not or must not be reduced to 0, or another rotational speed that is too low.

(25) Correspondingly, the calculating unit 42 also takes into overall consideration the operating point of the wind power installation, which can also be referred to as the working point. In this respect, it is particularly conceivable also to take into consideration the power P at the time, to be specific the installation power at the time, that is to say the power that the wind power installation is delivering at the particular time. The calculating unit 42 obtains this currently applicable power P from the control unit 36. The control unit 36 usually knows this currently applicable power P because it uses this power P for controlling the wind power installation itself. In particular, the control unit 36 sets the working point of the wind power installation.

(26) As a precaution, it is pointed out that such control of the wind power installation may also be carried out in a further control unit or that a common central control unit that combines performing the tasks of the control unit 36 and the calculating unit 42 may also be provided.

(27) On the basis of this operating point, which is consequently known to the calculating unit 42, at least with regard to the rotational speed n and the power P, it can thus be estimated down to which speed the rotor 22 could be braked. This can then not only be used for determining the rotational energy that is present, but also the rotational energy that is available, which is part of the rotational energy that is present altogether.

(28) Moreover, the calculating unit 42 may give the control unit 36 a proposal or setpoint value for a speed increase, to which the speed can be increased in order in this way to create greater rotational energy, to then achieve a greater available rotational energy after all. However, this is not shown any further in FIG. 3.

(29) In addition, the blade angle of the rotor blades may also be taken into consideration for the evaluation of the operating point. This blade angle α may for example be measured at the rotor blades 16, which is indicated in FIG. 3. Alternatively, the control unit 36 may specify such a blade angle and at the same time transmit it to the calculating unit 42. If for example in part-load operation, when the installation is therefore not running at rated power, the blade angle is greater than a part-load angle typical for this operating mode, this indicates that a stronger wind is blowing than would be assumed just on the basis of the rotational speed and the power. This may mean for example that the operating point at the time is at a flank of a speed/power characteristic curve, and therefore the rotational speed cannot be reduced greatly, because that could lead to a great power loss. This is taken into consideration in the determination of the supporting energy present. If this available amount of supporting energy EV has been determined, the control unit 36 can consequently determine the supporting power to be additionally fed in, and can correspondingly increase the preset power PS and give a corresponding setpoint value to the inverter 6.

(30) In addition, the control unit 36 can for this purpose increase the exciter current I.sub.E and for this purpose pass a corresponding signal to the current adjuster 30.

(31) It is also conceivable in principle to use instead of a passive rectifier 32 a controlled rectifier and in addition to change the stator currents, and thereby change the generator power. By changing the generator power, particularly by increasing the exciter current I.sub.E, the rotor 22 is braked and the rotational energy is thereby taken.

(32) It is thus possible to already preplan the supporting power feeding in for grid support in light of the available amount of supporting energy, and thereby achieve a support of the electrical supply grid and at the same time ensure that the wind power installation has a stable operating point at every point in time. It is particularly prevented in this way that the wind power installation suddenly collapses in its power delivery while it is feeding additional supporting power into the electrical supply grid.

(33) One possible way of planning supporting power P.sub.A in dependence on the available supporting energy E.sub.V is illustrated in FIG. 4. There, the available amount of supporting energy or the available supporting energy E.sub.V is plotted in dependence on the time t and the supporting power P.sub.A is likewise plotted in the same diagram. The embodiment illustrated there provides that the supporting power P.sub.A is first specified in dependence on a sensed underfrequency in the electrical supply grid, and for the amount of supporting energy E.sub.V it is checked whether this lies above or below a specified limit value, to be specific whether it lies below the minimum amount of supporting energy E.sub.min. For this, it is assumed as a simplification that the criterion that initiates the supporting power P.sub.A in principle, for example a frequency that is too low, would lead to a constant supporting power P.sub.A. A constant value is correspondingly initially assumed for the supporting power P.sub.A to be fed in. That could however also vary, for example depending on the deviation of the grid frequency from a grid frequency setpoint value, or be determined in dependence on a frequency gradient of the grid frequency.

(34) Consequently, this supporting power P.sub.A initially has the value P.sub.A0 and retains this value from the point in time t.sub.0 to the point in time t.sub.1. The available amount of supporting energy E.sub.V begins at the point in time t.sub.0 with the maximum value for the amount of supporting energy E.sub.max and falls linearly as a result of the constantly fed-in supporting power P.sub.A up to the point in time t.sub.1.

(35) At the point in time t.sub.1, it reaches and goes below the value of the minimum amount of supporting energy E.sub.min and that leads to the supporting power P.sub.A being reduced.

(36) The embodiment shown there proposes in this case that, from the point in time t.sub.1, the supporting power P.sub.A is reduced linearly to 0, to be precise such that exactly the amount of supporting energy still available at the point in time t.sub.1 is used up. For the special case where a power is reduced linearly from an initial value, here specifically the constant supporting power value P.sub.A0, to 0, the energy E required thereby can be calculated with the formula E=½P*t. Since the energy to be consumed corresponds here to the energy still present at the point in time t.sub.1, to be specific the minimum amount of supporting energy E.sub.min, and the initial power corresponds to the power P.sub.A0, the time in which power is reduced linearly to 0 is calculated with the formula t=2*E/P.sub.A0. The time thus calculated is the time period according to the diagram of FIG. 4 from the point in time t.sub.1 to the point in time t.sub.2. This progression is illustrated in FIG. 4 and, with this variant, at first a great supporting power P.sub.A can be fed in, and only later is reduced, and even with the reduced values can initially still provide a supporting contribution. At the same time, the available amount of supporting energy E.sub.V is optimally utilized.

(37) FIG. 5 illustrates relationships between the rotational speed n and the available rotational energy. The diagram shows on the x axis the rotational speed n from the value 0 to beyond an extended rotational speed n.sub.e. In addition, the rotational energy E.sub.R is plotted in dependence on the rotational speed n. It is evident that the rotational energy E.sub.R at the rotational speed with the value 0 is likewise 0 and, from there, increases quadratically with the speed n.

(38) In the case of the diagram of FIG. 5, it is assumed that the wind power installation is operated at the rotational speed n.sub.0. That represents the normal rotational speed for it, which would usually be used, in particular with the prevailing wind speed in this case. This rotational speed n.sub.0 is consequently assigned a rotational energy E.sub.R0. This rotational energy E.sub.R0 is however not the available amount of rotational energy, because the rotational speed may only be reduced down to the minimum speed n.sub.min. Correspondingly, here the available amount of rotational energy is only the normally available amount of rotational energy, indicated by E.sub.V0. Precisely this normally available amount of rotational energy E.sub.V0 is calculated here as the available amount of supporting energy, assuming that no further energy sources are present.

(39) FIG. 5 also illustrates that an increase of the available amount of rotational energy is possible if the wind power installation is operated at a higher rotational speed. For illustrative purposes, the increased or extended rotational speed n.sub.e is proposed for this. This leads to an energy value E.sub.Re as the extended rotational energy of the rotor. As a result of this increase in the rotational speed to this extended speed n.sub.e, the available amount of rotational energy can consequently be increased to the value of the extended available amount of rotational energy E.sub.Ve. On account of the quadratic relationship between the rotational energy E.sub.R and the rotational speed n, this proposed increase in speed has a very great effect. As a precaution, it is pointed out that FIG. 5 is illustrative and, depending on the operating point, smaller speed increases are also conceivable, and to this extent FIG. 5 is only illustrative.