Method for the voltage-impressing feed of electrical power into an electrical supply grid by means of a wind power installation

Abstract

Provided is a method for feeding electrical power into an electrical supply grid having a grid voltage by a wind power installation. The installation comprises a generator for generating a generator current, an active rectifier for rectifying the generator current into a rectified current, a direct voltage intermediate circuit having an intermediate circuit voltage for receiving the rectified current, a chopper circuit for diverting excess energy out of the direct voltage intermediate circuit, and an inverter for generating an infeed current for feeding into the electrical supply grid. The feed takes place in a voltage-impressing manner, so that the inverter counteracts a deviation of the grid voltage from a voltage setpoint value through an adjustment of the fed current. The active rectifier has a lower current limit to limit a fall of the rectified current to protect the generator during a change of the grid voltage amplitude or phase.

Claims

1. A method for feeding electrical power into an electrical supply grid having a grid voltage by a wind power installation, comprising: generating, by a generator, a generator current; rectifying, by an active rectifier coupled to the generator, the generator current into a rectified current; receiving, by a direct voltage intermediate circuit coupled to the active rectifier and having an intermediate circuit voltage, the rectified current; diverting, by a chopper circuit coupled to the direct voltage intermediate circuit, excess energy out of the direct voltage intermediate circuit; generating, by an inverter coupled to the direct voltage intermediate circuit, an infeed current; and feeding the infeed current into the electrical supply grid in a voltage-impressing manner, wherein the inverter is configured to adjust the infeed current to counteract a deviation of the grid voltage from a voltage setpoint value, wherein: the active rectifier has a lower current limit operative to, in response to a change in an amplitude or phase of the grid voltage, limit a drop in the rectified current to protect the generator, the lower current limit is set and changed based on an operating point of the wind power installation, and the chopper circuit is controlled to divert the excess energy arising in the direct voltage intermediate circuit or a portion of the excess energy arising in the direct voltage intermediate circuit in response to current limiting of the active rectifier.

2. The method as claimed in claim 1, wherein the operating point of the wind power installation is an operating point of the active rectifier.

3. The method as claimed in claim 1, wherein: the chopper circuit is configured to divert the excess energy from the direct voltage intermediate circuit in response to the intermediate circuit voltage reaching a threshold voltage, the active rectifier is controlled by the rectified current and the active rectifier regulates the intermediate circuit voltage, and in response to the rectified current reaching the lower current limit, the rectified current is limited, the intermediate circuit voltage rises and reaches the threshold voltage, and the chopper circuit responds and diverts the excess energy away from the direct voltage intermediate circuit.

4. The method as claimed in claim 3, wherein the active rectifier regulates the intermediate circuit voltage to a voltage value or a range of voltage values below the threshold voltage based on a rectifier droop function.

5. The method as claimed in claim 1, wherein the chopper circuit diverts the excess energy based on a chopper droop function that specifies a relationship between a chopper power to be diverted from the direct voltage intermediate circuit and a difference between the intermediate circuit voltage and a threshold voltage.

6. The method as claimed in claim 5, wherein the chopper droop function specifies a linear relationship.

7. The method as claimed in claim 1, wherein: using a regulation specification to regulate the intermediate circuit voltage, wherein the regulation specification includes a rectifier regulation and a chopper regulation, wherein: the rectifier regulation specifies a relationship between the intermediate circuit voltage and the rectified current, wherein the rectified current is set by the active rectifier such that: the rectified current falls with a rising intermediate circuit voltage until the rectified current reaches the lower current limit, and the rectified current is held at the lower current limit as the intermediate circuit voltage continues to rise, and the chopper regulation specifies a relationship between the intermediate circuit voltage and chopper power to be diverted from the direct voltage intermediate circuit by the chopper circuit such that: in response to the intermediate circuit voltage exceeding a threshold voltage, the chopper power increases with the rising intermediate circuit voltage, wherein the lower current limit is changed depending on the threshold voltage or the threshold voltage is changed depending on the lower current limit.

8. The method as claimed in claim 7, wherein the regulation specification is a regulation droop function, the rectifier regulation is a rectifier droop function and the chopper regulation is a chopper droop function.

9. The method as claimed in claim 7, wherein the rectified current falls linearly with the rising intermediate circuit voltage.

10. The method as claimed in claim 7, wherein the threshold voltage is changeable based on the operating point of the wind power installation, and wherein: the threshold voltage is set depending on the lower current limit and is changed in response to a change in the lower current limit, the threshold voltage is set to an intermediate circuit reference value that the intermediate circuit voltage reaches when the rectified current has fallen to the lower current limit, or the threshold voltage is set depending on the intermediate circuit reference value and to a value that differs from the intermediate circuit reference value by less than 5%.

11. The method as claimed in claim 1, wherein: the lower current limit of the active rectifier is a dynamic function that depends on an instantaneous rectified current.

12. The method as claimed in claim 11, wherein: in a stationary case, the lower current limit is less than the rectified current by an undershoot difference, and in an event of a fall in the rectified current, the lower current limit tracks, using the dynamic function, the rectified current less the undershoot difference, wherein the dynamic function has a low-pass behavior.

13. The method as claimed in claim 1, wherein: the inverter has an upper inverter current limit, and in response to a voltage drop in the grid voltage, the upper inverter current limit is operative to place an upper limit on a rise of the infeed current to protect the generator.

14. The method as claimed in claim 13, wherein the upper inverter current limit is set and changed depending on the operating point of the wind power installation.

15. The method as claimed in claim 14, wherein the operating point of the wind power installation is an operating point of the inverter or an operating point of the intermediate circuit voltage.

16. The method as claimed in claim 13, wherein: the upper inverter current limit is a dynamic function that depends on an instantaneous infeed current, in a stationary case, the upper inverter current limit is above the instantaneous infeed current by an overshoot difference, and in an event of a rise in the infeed current, the upper inverter current limit tracks a sum of the instantaneous infeed current and the overshoot difference, wherein the dynamic function has a low-pass behavior.

17. The method as claimed in claim 16, wherein the overshoot difference is based on an overshoot value by which the intermediate circuit voltage exceeds a threshold voltage.

18. The method as claimed in claim 1, wherein: an increase in a real power fed in is limited to 5% to 20% above the real power fed in or in relation to a rated power of the wind power installation, and a reduction of the real power fed in of up to a value of −100% in relation to the real power fed in or in relation to the rated power of the wind power installation is permitted.

19. A wind power installation for feeding electrical power into an electrical supply grid having a grid voltage, the wind power installation comprising: a generator configured to generate a generator current; an active rectifier coupled to the generator configured to rectify the generator current into a rectified current; a direct voltage intermediate circuit coupled to the active rectifier and having an intermediate circuit voltage for receiving the rectified current; a chopper circuit coupled to the direct voltage intermediate circuit configured to divert excess energy out of the direct voltage intermediate circuit; an inverter coupled to the direct voltage intermediate circuit configured to generate an infeed current for feeding into the electrical supply grid; and an installation controller configured to control the feeding of the infeed current by at least: feeding the infeed current in a voltage-impressing manner, wherein the inverter is configured to adjust the infeed current to counteract a deviation of the grid voltage from a voltage setpoint value, wherein the active rectifier has a lower current limit operative to, in response to a change in an amplitude or phase of the grid voltage, limit a drop in the rectified current to protect the generator, wherein the lower current limit is set and changed based on an operating point of the wind power installation, and wherein the chopper circuit is controlled to divert excess energy arising in the direct voltage intermediate circuit or a portion of the excess energy arising in the direct voltage intermediate circuit in response to current limiting the active rectifier.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The invention is explained in more detail below on the basis of forms of embodiment and with reference to the accompanying figures.

(2) FIG. 1 shows a perspective view of a wind power installation.

(3) FIG. 2 shows schematically an electrical power path of a wind power installation from the generator through to the electrical supply grid.

(4) FIG. 3 shows a diagram with two partial diagrams for the explanation of the proposed method for an increase in the infeed current.

(5) FIG. 4 shows a diagram with two partial diagrams for the explanation of the proposed method for a reduction in the infeed current.

DETAILED DESCRIPTION

(6) FIG. 1 shows a wind power installation 100 with a tower 102 and a nacelle 104. A rotor 106 with three rotor blades 108 and a spinner 110 is arranged at the nacelle 104. The rotor 106 when operating is set into rotary movement by the wind, thereby driving a generator in the nacelle 104.

(7) The wind power installation 100 here comprises an electric generator 101 that is suggested in the nacelle 104. Electrical power can be generated by means of the generator 101. A feed unit 105 is provided for feeding electrical power, and can in particular be designed as an inverter. With this, a three-phase infeed current and/or a three-phase infeed voltage can be generated according to amplitude, frequency and phase for feeding to a grid connection point PCC. This can be done directly, or together with further wind power installations in a wind farm. An installation controller 103 is provided for controlling the wind power installation 100 as well as the feed unit 105. The installation controller 103 can also receive specified values from outside, in particular from a central farm computer.

(8) FIG. 2 shows the power path of the electrical power. It starts at the generator 201, which can be designed as a synchronous generator, and which, as the generator current I.sub.G, generates an alternating electric current, in particular a stator current. The generator current I.sub.G is rectified by the active rectifier 202, and passed as rectified current I.sub.DC to a direct voltage intermediate circuit 204. The generator current I.sub.G, in particular the stator current, is thereby also controlled. A generator current I.sub.G can also be controlled thereby, which puts power into the generator when electrical power is drawn from the electrical supply grid.

(9) The direct voltage intermediate circuit 204 comprises an intermediate circuit capacitor 206 that can also be referred to as a smoothing capacitor. A chopper circuit 208, which can divert electrical power and thereby electrical energy out of the direct voltage intermediate circuit, is also connected to the direct voltage intermediate circuit 204.

(10) An infeed current is generated as alternating current from the direct voltage intermediate circuit 204 with a grid-side inverter 210, in order thereby to feed electrical power, namely real power, into the electrical supply grid 212. A transformer 214 is also arranged between the inverter 210 and the electrical supply grid 212.

(11) The wind power installation, and in particular the grid-side inverter 210, here operate in a voltage-impressing, in particular grid-forming, manner.

(12) The rectifier 202 together with the chopper circuit 208 here performs regulation of the direct voltage intermediate circuit, i.e., of the intermediate circuit voltage. This regulation can be referred to as intermediate circuit regulation 216. The rectifier 202 performs regulation of the intermediate circuit voltage that includes a lower current limit for the rectified current. If the lower current limit is reached, the intermediate circuit voltage can no longer be maintained by the rectifier 202, and the chopper circuit 208 can then take over the regulation of the intermediate circuit voltage. For that reason, the rectifier 202 and the chopper circuit 208 perform the regulation of the intermediate circuit voltage together. This thus distinguishes it from a variant in which the rectifier, without a lower current limit, or with a lower current limit that is very much higher, carries out the regulation of the intermediate circuit voltage alone.

(13) A dynamic current limit 218 is, moreover, implemented in the inverter 210, which prevents excessively large jumps in the infeed current in order thereby to avoid large jumps in the torque at the generator.

(14) FIG. 3 shows two partial diagrams that explain at least a part of the mode of operation of the proposed method.

(15) A curve against time of an infeed current I.sub.Net fed into the electrical supply grid and of an associated torque T.sub.Gen is shown for an exemplary situation in the upper diagram.

(16) It is assumed in the exemplary situation that a large current pulse, above an infeed current at that moment, arises in the grid. For this purpose a lower current limit 302 is provided, which is reached by the infeed current and which can, for example, lie above the infeed current at that moment by 10% of the rated current IN. In this example, the maximum current, power and torque pulse in the positive direction is thus 10%. A torque pulse corresponding to the limit minus the current at that moment is thus permitted by the dynamic current limit. This current limit 302 is thus movable, as is suggested by the double arrow. The impulse begins at time point t.sub.2, and ends at time point t.sub.3; and between them, the result is an adjustment power P.sub.A that acts for the intermediate period of time, as suggested by a double arrow, even though its height is limited. It can thus be regulated without, however, overloading the wind power installation.

(17) The limitation can also be done using a droop function of the intermediate circuit regulation. The grid-side voltage-impressing inverter would then limit the infeed current as the intermediate circuit voltage collapses. It would then recognize its current limitation by way of the intermediate circuit voltage.

(18) Following the jump in the torque, the limits are brought toward the working point at that moment following a PT1 characteristic. The delay characteristic results from the permitted mechanical loading.

(19) The lower partial diagram of FIG. 3 shows how a realization of the method can be carried out by means of appropriately set droop functions. Droop functions specify a relationship, in particular a linear relationship, between an input magnitude and an output magnitude. A rectifier droop function 320 has been taken as a basis here, giving a relationship between the intermediate circuit voltage V.sub.ZK and the generator power P.sub.Gen. It is illustrated as solid lines. For the sake of simplicity, the rectifier droop function here is then shown at a level somewhat above the coordinate axis which marks a normal or mean value. At low intermediate circuit voltages, the generator power P.sub.Gen accordingly increased in accordance with the rectifier droop function 320, up to a maximum value.

(20) In the example shown, there is still no current pulse or current jump at time point t.sub.1. The time points from the upper partial diagram are drawn, for better orientation, in the lower partial diagram as working points of the direct voltage regulation. At time point t.sub.1, i.e., during normal operation, the corresponding working point is located at an edge of the droop function of the rectifier, i.e., of the rectifier droop function 320. The rectifier thus performs a regulation of the intermediate circuit voltage.

(21) The current pulse occurs at time point t.sub.2; this can be triggered by a phase jump in the grid voltage, as a result of which the intermediate circuit voltage falls, which changes the working point. The generator power P.sub.Gen consequently rises up to a maximum value in accordance with the rectifier droop function 320. This is adjusted to the current limit of the grid-side inverter.

(22) The chopper droop function 330, which is also drawn and illustrated as dashed lines, is not used in this example, since the intermediate circuit voltage falls below its rated value, and the chopper circuit is thus not triggered. The rectifier droop function 320, as well as the chopper droop function 330, are dynamically changeable, and are here representative of a rectifier regulation or a chopper regulation. It is particularly proposed for the rectifier characteristic 320 that its end values can be changed for this purpose. This is suggested by the first and second double change arrows 322 and 324.

(23) The lower end value of the rectifier characteristic 320 here forms a lower power limit 325 which can be realized by a lower current limit, or vice versa. This lower power limit 325 is dynamically changeable, as is suggested by the double change arrow 324. The edge of the rectifier characteristic 320 reaches as far as the intermediate circuit reference value 326 of the intermediate circuit voltage V.sub.ZK. The intermediate circuit reference value 326 also changes, depending on what type of power limit 325 has been set. The chopper characteristic 330 can be correspondingly adjusted, in that the trigger voltage 332 is placed at the intermediate circuit reference value 326. This can be recognized in FIG. 3 in that the chopper droop function 330 reaches the abscissa, i.e., the zero line at the intermediate circuit reference value 326.

(24) A fixed trigger voltage 332′ can alternatively be provided, located at a fixed value above the intermediate circuit reference value 326, and also above a rated voltage of the intermediate circuit voltage V.sub.ZK.

(25) The explanations of the lower diagram are identical to those for FIG. 4, except for the explanations of the operating points. The chopper droop function 330, which was explained above in relation to FIG. 3, does not have any effect on the operating situation of FIG. 3. An operating situation is therefore shown below in connection with FIG. 4, in which the chopper droop function 330 does have its effect, in particular in cooperation with the rectifier droop function 320.

(26) FIG. 4 thus shows an operating situation in which the fed power, i.e., the real power, falls suddenly. This is referred to here as a negative load jump. Normal operation is still present at time t1 and thus, as in FIG. 3, the time point t1 is shown at an edge of the rectifier droop function 320, which is represented here by three solid lines. The rectifier droop function 320, as well as the chopper droop function 330, are here identical to FIG. 3, since the present FIG. 4 only shows a changed operating situation, but not a changed control.

(27) The sudden collapse in the fed power, and thereby of the infeed current I.sub.Net, i.e., the negative load jump, occurs at time t.sub.2. The intermediate circuit voltage V.sub.ZK rises as a result. The working point consequently changes along the rectifier droop function 320. The rectifier droop function 320 however falls with rising intermediate circuit voltage only as far as the power limit 325, in order thus to reach a limit value for the associated generator power. Here again, the power limit 325 is shown above the coordinate axis that identifies the power value of zero.

(28) The rectifier thus only steers the rectified current, and thereby finally the generator power, after the fall in the intermediate circuit voltage to the point at which the right-hand horizontal branch of the rectifier droop function is reached, i.e., until the power limit 325 is reached. The generator torque does therefore fall somewhat, but not completely.

(29) The working point, or a working point, does however lie on the chopper droop function 330, and falls further with it as the intermediate circuit voltage increases. The trigger voltage 332 is placed here at the intermediate circuit reference value 326.

(30) The chopper circuit accordingly draws power out of the intermediate circuit. As a result of this, the generator torque could be limited, but a further fall in the power fed does not have to be restricted. This is clarified by the illustration of the infeed current I.sub.Net. The generator torque T.sub.Gen and the infeed current I.sub.Net are each shown normalized with respect to their rated value in the upper partial diagram, so that without the limitation of the torque described they would have to lie approximately on top of one another.

(31) In the event of negative load jumps, the torque pulse is thus limited through the dynamic limits in the droop function of the intermediate circuit regulation.

(32) The chopper droop function 330 can also be shifted dynamically, in order to take over the further power. It is particularly proposed that for this purpose the trigger voltage 332 is made to track the intermediate circuit reference value 326, whereby a change in the power limit 325 is tracked, i.e., a change in the lower current limit. The chopper droop function 330 is of course correspondingly adjusted, in that its edges are shifted, as is suggested by the double arrow 338. A shift in the lower power limit according to the change arrow 324 thus leads to a change in the chopper droop function according to the double arrow 338.

(33) In the FIG. 4, as corrected the beginning of the chopper droop function 330, which can also be referred to as the characteristic chopper curve, thus of course only starts at the zero point which is located on the abscissa of the intermediate circuit voltage. Two possibilities are proposed for the placement of the characteristic chopper curve. Either the characteristic chopper curve is to the right of the entire droop function of the intermediate circuit regulation, i.e., according to FIGS. 3 and 4, to the right of the edge of the rectifier droop function 320, i.e., to the right of the intermediate circuit reference value 326. If the rectified current is limited, i.e., if the generator power is limited, the intermediate circuit voltage must first achieve a certain level before the chopper circuit becomes active. This is drawn with the alternative trigger voltage 332′ as an alternative possibility.

(34) The second possibility is a shift to the left. The beginning is then the intermediate circuit voltage at which the limitation of the current starts, i.e., at the intermediate circuit reference value 326, which is drawn in FIGS. 3 and 4 as the main variant. Together with the dynamic limit of the power limit 325, a continuous regulation thus results.

(35) The maximum current, power and torque pulse for the generator in the negative direction is 10%. The infeed current can, however, drop much more quickly yet without risking an overload of the generator.

(36) The chopper circuit, which can be identified simply as the chopper, takes up the additional power (approximately 15% in this case). Two working points form at t.sub.2.

(37) Following the jump in the torque, the limits are brought to approach the working point at that moment following a PT1 characteristic. The delay characteristic results from the permitted mechanical loading. The rectifier furthermore takes over the power, with a delay, that is converted in the chopper.

(38) Thus with the proposed method, the following situation is avoided or improved:

(39) Without the proposed limits, a grid-side current pulse that can arise in the presence of the voltage change from a voltage-impressing feed, flows almost unhindered into the generator torque. The generator torque can thus rise equally sharply. A chopper circuit would, however, not become active even in the case of negative power gradients, since the change in the voltage of the intermediate circuit voltage resulting from the negative power gradients would be compensated for by the active rectifier.

(40) The chopper circuit only takes over when the generator-side rectifier reaches its current limit. The current, power and torque jumps would thus not be limited, and jumps of this sort can reach up to 100% of the rated value.

(41) A solution is thus created that enables a provision of grid-forming properties with a wind power installation, without (or without large) over-dimensioning of the mechanical system, and without the integration of an electrical store going beyond a usual intermediate circuit capacitor.

(42) It is considered that the mechanical system of a wind power installation, which can also be labelled WEA, can sustain torque jumps of, for example, a maximum of 10%. Grid-forming properties at the installation terminals however require intermediate circuit regulation on the generator side, which can lead to high transients in the torque. It acts like a direct coupling of the generator to the grid.

(43) The idea is to limit load pulses through impressing a voltage in a positive and a negative direction. In the positive direction, i.e., when the real infeed power or the associated real power I.sub.ist of the infeed current I.sub.Net is increased, this can in particular result from a limitation of the output current to, for example, I.sub.ist+10% I.sub.N. Alternatively or in addition, the limit in the droop function of the intermediate circuit regulation can be adjusted dynamically.

(44) In the negative direction, i.e., when the fed power or the associated real power I.sub.Net or I.sub.ist is reduced, the limitation preferably takes place through a dynamic adjustment of a chopper droop function and a limitation of the droop function of the intermediate circuit regulation of the generator-side converter, which can also be referred to as generator-side inverter or active rectifier.

(45) The tracking after a load pulse takes place by means of a time delay, for example in accordance with a PT1 behavior.

(46) In the case of an increase of the fed power, a current limitation can take place directly at the grid-side, voltage-impressing inverter, or a sharply collapsing intermediate circuit voltage is reacted to with a current limitation. Such a collapsing intermediate circuit voltage can result if the active rectifier, i.e., the generator-side rectifier, limits the torque jump.

(47) Classically, a wind power installation is constructed such that the generator is decoupled from the grid. Regulation of the intermediate circuit voltage takes place through the grid-side inverter.

(48) A wind power installation with a grid-forming converter or inverter can be constructed such that the generator is decoupled from the grid in that an intermediate circuit regulation, i.e., a regulation of the intermediate circuit voltage, takes place by means of a battery store with a direct current chopper. The battery store is coupled for this purpose via the chopper, which can thus also be referred to as the storage-side DC/DC chopper. The battery store can perform the regulation of the intermediate circuit voltage.

(49) To avoid this complexity with the battery store, a wind power installation with a grid-forming converter or grid-forming inverter is proposed, in which the generator is fundamentally coupled to the grid, i.e., to the electrical supply grid, namely through an intermediate circuit regulation through a generator-side inverter that can also be referred to as the generator-side rectifier or the active rectifier.

(50) It is now proposed that the generator is decoupled from the grid for the wind power installation with grid-forming converter or inverter. An intermediate circuit regulation by a chopper circuit, which can also synonymously be referred to simply as the chopper, with generator-side active rectifier is proposed for this purpose. A torque regulation is here realized through a dynamic, grid-side current limit.

(51) The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.