METHOD FOR FEEDING ELECTRICAL POWER INTO AN ELECTRICAL SUPPLY NETWORK

Abstract

A method for feeding electrical power into an electrical, three-phase supply network by means of an inverter device, wherein the electrical supply network has a three-phase line voltage with a first, second and third line voltage phase, comprising the steps: feeding the electrical power during normal operation if a fault-free operation has been identified for the electrical supply network, wherein during normal operation a positive sequence voltage and optionally a negative sequence voltage is recorded from the line voltage and a reactive current is specified at least depending on the positive sequence voltage and optionally depending on the negative sequence voltage, and changing to a fault operation if a voltage change in the line voltage meets a predetermined fault criterion, in particular if the voltage change exceeds a predeterminable minimum amount of change or a minimum amount of change gradient, wherein during the fault operation, at least directly after the change, the reactive current is specified depending on a space vector voltage.

Claims

1. A method for feeding electrical power into an electrical, three-phase supply network by an inverter device, wherein the electrical supply network has a three-phase line voltage with first, second, and third line voltage phases, the method comprising: feeding the electrical power during normal operation if a fault-free operation has been identified for the electrical supply network, wherein during normal operation: a positive sequence voltage is recorded from the line voltage, and a reactive current is specified at least depending on the positive sequence voltage; and changing to a fault operation if a voltage change in the line voltage meets a predetermined fault criterion, wherein during the fault operation, at least directly after the change, the reactive current is specified depending on a space vector voltage.

2. The method as claimed in claim 1, wherein the space vector voltage is defined by the equation
{right arrow over (v)}=[v.sub.1+v.sub.2 exp(j⅔π)+v.sub.3 exp(j 4/3π)], wherein with v.sub.1, v.sub.2, and v.sub.3 each have an instantaneous value.

3. The method as claimed in claim 2, wherein the instantaneous values of v.sub.1, v.sub.2, and v.sub.3 are instantaneous measured values of the first, second, and third line voltage phases, respectively.

4. The method as claimed in claim 1, wherein the space vector voltage is determined during normal operation and, depending on the recorded space vector voltage, a switch is made to the fault operation.

5. The method as claimed in claim 1, wherein: specifying the reactive current depending on the space vector voltage transitions back to specifying the reactive current depending on the positive sequence voltage, if the positive sequence voltage has essentially assumed a stationary value or if changes in the positive sequence voltage are below a predeterminable limit gradient in terms of magnitude, if the space vector voltage has reached a minimum value in the event of a voltage drop of one or a plurality of line voltage phases, and/or if a curve of the space vector voltage has reached a turning point, and/or if a predetermined transition period after identifying the fault operation has elapsed.

6. The method as claimed in claim 1, wherein: after changing to the fault operation, specifying the reactive current takes place depending on the space vector voltage, until a switchback criterion has been identified, when the switchback criterion is identified, the reactive current value, which has been calculated depending on the space vector voltage, is held as a space vector reactive current value, after identifying the switchback criterion, a positive sequence reactive current value is continuously calculated depending on the positive sequence voltage, and the predetermined reactive current is calculated by a held space vector reactive current value transitioning to the positive sequence reactive current value over a predeterminable transition curve.

7. The method as claimed in claim 1, wherein: at least one of changing to the fault operation or specifying the reactive current is carried out depending on a space vector voltage for a fault period, wherein the fault period is less than a line period.

8. The method as claimed in claim 7, wherein the fault period is 5% to 50% less than the line period.

9. The method as claimed in claim 1, wherein specifying the reactive current depending on a space vector voltage transitions to specifying the reactive current depending on a positive sequence voltage in a crossfade period.

10. The method as claimed in claim 9, wherein the crossfade period is less than a line period.

11. The method as claimed in claim 9, wherein the crossfade period is between 20% to 90% the line period.

12. The method as claimed in claim 1, wherein when changing to the fault operation, the method comprises: specifying the reactive current depending on the positive sequence voltage changes to specifying the reactive current depending on the space vector voltage by the fact that the reactive current is specified depending on a reference value and the reference value transitions from the positive sequence voltage to the space vector voltage over a predeterminable transition curve, wherein: the predeterminable transition curve is linear, and the predeterminable transition curve is realized by the fact that the reference value is made up of a positive sequence voltage with a first weighting and a space vector voltage with a second weighting, and wherein the first weighting decreases with time, while the second weighting increases, wherein: specifying the reactive current depending on the space vector voltage transitions back to specifying the reactive current depending on the positive sequence voltage by the fact that the reactive current is specified depending on a reference value and the reference value transitions from the space vector voltage to the positive sequence voltage over a predeterminable transition curve, wherein: the predeterminable transition curve is in particular linear, and the predeterminable transition curve is realized by the fact that the reference value is made up of a positive sequence voltage with a first weighting and a space vector voltage with a second weighting, and wherein the first weighting increases with time, while the second weighting decreases.

13. The method as claimed in claim 1, wherein when changing to the fault operation, the method comprises: specifying the reactive current depending on the positive sequence voltage transitions to specifying the reactive current depending on the space vector voltage according to a predeterminable transition curve, wherein: a positive sequence reactive current is calculated as a reactive current depending on the positive sequence voltage, a space vector reactive current is calculated as a reactive current depending on the space vector voltage, and a predetermined reactive current transitions from the positive sequence reactive current according to a predeterminable transition curve to the space vector reactive current, and wherein: the predeterminable transition curve is linear, and the predeterminable transition curve is realized by the fact that the predetermined reactive current is made up additively of the positive sequence reactive current with a first weighting and the space vector reactive current with a second weighting and the first weighting decreases with time, while the second weighting increases, wherein: specifying the reactive current depending on the space vector voltage transitions back to specifying the reactive current depending on the positive sequence voltage by the fact that the predetermined reactive current transitions from the space vector reactive current according to a predeterminable transition curve back to the positive sequence reactive current, and wherein: the predeterminable transition curve is linear, and the predeterminable transition curve is realized by the fact that the predetermined reactive current is made up additively of the positive sequence reactive current with a first weighting and the space vector reactive current with a second weighting and the first weighting increases with time, while the second weighting decreases.

14. The method as claimed in claim 1, comprising: interrupting a measurement of the line voltage and estimating the space vector voltage by a rotating voltage vector, and wherein the rotating voltage vector continues to be calculated depending on a value of the space vector voltage before the interruption of the measurement of the line voltage and depending on a nominal frequency of the line voltage.

15. The method as claimed in claim 1, wherein during normal operation a negative sequence voltage is recorded from the line voltage, and the reactive current is specified depending on the negative sequence voltage.

16. The method as claimed in claim 1, wherein the predetermined fault criterion is met when the voltage change exceeds a predeterminable minimum amount of change or a minimum amount of change gradient.

17. A wind power installation for feeding electrical power into an electrical, three-phase supply network by an inverter device, wherein the electrical supply network has a three-phase line voltage with first, second, and third line voltage phases, the wind power installation comprising: a controller configured to control the feeding of electrical power into the electrical, three phase supply network, wherein the feeding comprises: feeding the electrical power during normal operation if a fault-free operation has been identified for the electrical supply network, wherein during normal operation: a positive sequence voltage is recorded from the line voltage, and a reactive current is specified at least depending on the positive sequence voltage, changing to a fault operation if a voltage change in the line voltage meets a predetermined fault criterion, in particular if the voltage change exceeds a predeterminable minimum amount of change or a minimum amount of change gradient, wherein during the fault operation, at least directly after the change, and the reactive current is specified depending on a space vector voltage.

18. The wind power installation in claim 17, wherein during normal operation a negative sequence voltage is recorded from the line voltage, and wherein the reactive current is specified depending on the negative sequence voltage.

19. The wind power installation in claim 17, wherein the predetermined fault criterion is met when the voltage change exceeds a predeterminable minimum amount of change or a minimum amount of change gradient.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0049] The disclosure is explained in greater detail hereinafter by way of example using embodiments with reference to the attached figures.

[0050] FIG. 1 shows a perspective representation of a wind power installation.

[0051] FIG. 2 shows a schematic control structure.

[0052] FIG. 3 shows an element of the control structure from FIG. 2.

[0053] FIGS. 4 and 5 each show diagrams of voltage curves and in each case a reactive current curve calculated therefrom in different manners.

DETAILED DESCRIPTION

[0054] FIG. 1 shows a wind power installation 100 having a tower 102 and a nacelle 104. A rotor 106 with three rotor blades 108 and a spinner 110 is arranged on the nacelle 104. The rotor 106 is transferred into a rotational movement by the wind during operation and thus drives a generator in the nacelle 104.

[0055] The wind power installation 100 has an electrical generator 101 which is indicated in the nacelle 104. Electrical power can be generated by means of the generator 101. A feed-in unit 105 is provided for feeding electrical power, which feed-in unit can in particular be designed as an inverter. This makes it possible to generate a three-phase feed-in current and/or a three-phase feed-in voltage according to amplitude, frequency and phase, for feeding in at a network connection point PCC. This can take place directly or also together with other wind power installations in a wind park. A system control 103 is provided for controlling the wind power installation 100 and also the feed-in unit 105. The system control 103 can also include external default values, in particular from a central park computer.

[0056] FIG. 2 shows a schematic control structure 202 which controls an inverter 204 of a wind power installation 200. The wind power installation 200 can correspond to the wind power installation 100 from FIG. 1. The wind power installation 200 is to be understood particularly schematically and it can include the controller structure 202 or its elements. The controller structure 202 can be included or implemented in a controller 206. The inverter 204 can feed active power P and reactive power Q into an electrical supply network 208.

[0057] According to an embodiment, the proposed method works in such a way that the three voltage phases v.sub.1, v.sub.2 and v.sub.3 of the electrical supply network 208 are measured. This can take place by means of the measuring sensor 210 in the electrical supply network 208 or, as represented in FIG. 2, at a connecting line 212 between the inverter 204 and the electrical supply network 208.

[0058] During normal operation, these 3 phase voltages are disassembled or transformed into a positive sequence voltage v.sup.+ and a negative sequence voltage v.sup.− in a disassembly block 214 according to the method of the symmetrical components. This positive sequence voltage v.sup.+ is fed into the changing block 216. The negative sequence voltage v.sup.− can optionally also be fed into this changing block 216, which is indicated by a correspondingly dotted arrow.

[0059] In the changing block 216, this positive sequence voltage v.sup.+ can be selected as a voltage which is to be used for determining the reactive power. This is indicated in the changing block 216 by a corresponding switch position. The changing block 216 then outputs the voltage which is to be used for controlling the reactive power as a reference value or reference voltage v.sub.ref.

[0060] The switch position of the changing block 216 indicated in FIG. 2 thus relates to normal operation in which the reactive power control depends on the positive sequence voltage v.sup.+. The reference voltage v.sub.ref therefore corresponds to the positive sequence voltage v.sup.+.

[0061] For further implementation, this reference voltage v.sub.ref is fed into the reactive power block 218 as an input variable. In the reactive power block 218, a desired reactive power is calculated as a function depending on the input variable of the reactive power block 218, i.e., depending on the reference voltage v.sub.ref, and is output as a nominal reactive power Q.sub.s. This nominal reactive power Q.sub.s then forms an input value for the inverter 204. In this respect, this reactive power nominal value Q.sub.s forms a reference variable for controlling the inverter.

[0062] In order to control the inverter 204, even more variables are required which, however, are not represented here for the sake of simplicity. The inverter 204 can be supplied with energy by way of a direct voltage on the input side. It can obtain a direct voltage of this type from a generator of the wind power installation 200, for example, which generates an alternating current which is rectified.

[0063] In the event that normal operation must or should be left and a fault operation is to be used, the three phase voltages v.sub.1,v.sub.2,v.sub.3 are transformed into a space vector voltage {right arrow over (v)} in the transformation block 220. This space vector voltage {right arrow over (v)} is also fed into the changing block 216. The changing block 216 can change to this space vector voltage {right arrow over (v)} as an input variable for the reactive power control if required, i.e., in particular during fault operation, by way of the indicated switch. However, in this respect, the switch indicated in the changing block 216 only serves to illustrate. In fact, it is proposed not to switch rigidly between the positive sequence voltage v.sup.+ and the space vector voltage {right arrow over (v)}, but instead to change by means of a transition function. This is explained in greater detail further on in an exemplary manner and any preceding or subsequent explanations for changing can be implemented in this changing block 216.

[0064] One possible realization of the changing block 216 is represented in FIG. 3. A linear change of the reference voltage v.sub.ref from the positive sequence voltage v.sup.+ to the space vector voltage {right arrow over (v)} is implemented there by way of the weighting functions g.sub.1(t) and g.sub.2(t) as a mathematical function. The first weighting function g.sub.1(t) thus drops linearly from 1 to 0 over the transition period T, so that the portion of the positive sequence voltage v.sup.+ drops from the maximum value to 0 over this transition period. At the same time, the weighting function g.sub.2(t) increases from 0 to 1 over the transition period T, so that the portion of the space vector voltage increases from 0 to the maximum value over the transition period.

[0065] Correspondingly, the same mathematical context can be implemented even if the reference voltage v.sub.ref is to change back from the space vector voltage {right arrow over (v)} to the positive sequence voltage v.sup.+, wherein the weighting functions g.sub.1(t) and g.sub.2(t) would have to be exchanged. However, a shorter or longer transition period can also be used for changing back, to point out an example of a variation.

[0066] FIG. 4 shows a feed-in of the reactive current in the positive and negative sequence. Since it is not possible to determine the positive and negative sequence voltages immediately, but rather over the duration of a period of the line voltage, the increase in the reactive current feed-in derived therefrom can only take place “slowly”. This is illustrated in FIG. 4.

[0067] FIG. 4 includes three individual diagrams, of which the top diagram shows the curve of the three measured phase voltages v.sub.1, v.sub.2 and v.sub.3, namely the curve of the instantaneous values. At the time t.sub.1, the two phase voltages v.sub.2 and v.sub.3 drop to a low value.

[0068] The middle diagram shows the positive sequence voltage v.sup.+ and the space vector voltage {right arrow over (v)}, which have been calculated from the three phase voltages v.sub.1, v.sub.2 and v.sub.3 and in the diagram are represented standardized to the nominal voltage V.sub.N. It can be recognized that the positive sequence voltage v.sup.+ and the space vector voltage {right arrow over (v)} are approximately the same before the voltage drop at the time t.sub.1, in any case they cannot be distinguished in the diagram. The three phase voltages v.sub.1, v.sub.2 and v.sub.3 are thus still approximately symmetrical to one another. After the voltage drop, the space vector voltage {right arrow over (v)} changes more quickly and the voltage drop can be identified therefrom more quickly as a result. In principle, the space vector voltage {right arrow over (v)} nevertheless maintains a vibration.

[0069] The positive sequence voltage v.sup.+ reacts more slowly to the voltage drop and reaches a new value after a period T, namely at the time t.sub.2.

[0070] The bottom diagram from FIG. 4 shows a reactive current determined from the positive sequence voltage v.sup.+ for feeding a reactive power, which is referred to as a nominal reactive current I.sub.QS. The nominal reactive current I.sub.QS thus also only reaches a new value after a period T, i.e., at the time t.sub.2. The bottom diagram from FIG. 4, the same applies to FIG. 5, assumes by way of illustration that the nominal reactive current reaches a maximum reactive current km to which the diagram is standardized.

[0071] In order to be able to provide the reactive current more quickly, a variant is proposed which is illustrated in FIG. 5. FIG. 5 shows three diagrams and the top and middle diagram correspond to the top and middle diagram from FIG. 4, apart from deviations in the temporal resolution, which are not important here. The embodiments from FIG. 4 are thus referred to for explaining the top and middle diagram from FIG. 5.

[0072] The calculation of the reactive current nevertheless differs between FIGS. 4 and 5. The bottom diagram therefore shows a calculated nominal reactive current I.sub.QS, as in the bottom diagram from FIG. 4, but with the nominal reactive current I.sub.QS being calculated in a different manner.

[0073] It is proposed that providing the initial reactive current, namely initially from a detected fault criterion, is carried out using the measured space vector voltage {right arrow over (v)}. The fault criterion in the example shown in FIG. 5 is the voltage drop of the two phase voltages v.sub.2 and v.sub.3 to a low value. This initial reactive current provision therefore takes place from the time t.sub.1.

[0074] The initial reactive current provision, i.e., determining the nominal reactive current, therefore takes place from the time t.sub.1 using the measured space vector voltage {right arrow over (v)}. As soon as the first minimum in the space vector voltage {right arrow over (v)} has been identified, the nominal value thus obtained is frozen. This is somewhat the case at the time t.sub.E. It is then possible to crossfade from the space-vector-based nominal value to the positive-sequence-based nominal value over a period of 15 ms. The period of 15 ms is somewhat smaller than a period T, which is 20 ms here, since it is based on a 50 Hz network.

[0075] It has been recognized that it can be useful to carry out a quicker feed-in of reactive current in the case of an error, which can also be referred to as a fault, namely within the first 10 ms if possible.

[0076] One or more embodiments makes it possible to provide a reactive current for network support more quickly than is known to date where the reactive power is fed exclusively based on the positive sequence voltage. A higher network stability can also be achieved as a result.

[0077] It is thus proposed to identify an error with a space vector voltage and then correspondingly switch to a space vector voltage for referencing. This is quicker than referencing the positive sequence. If the sequence is levelled out, it is possible to switch back to referencing the positive sequence.

[0078] One known problem involves finding a good transition from referencing the positive sequence to referencing the space vector voltage and back. A crossfade is proposed for this purpose.

[0079] Crossfading from referencing the space vector voltage to referencing the positive sequence, i.e., the positive sequence voltage, can start at the minimum of the space vector voltage. Other possibilities are also considered, such as only referencing the space vector voltage for a predetermined time, for example. A predetermined time of this type can be a quarter of a line period, for example. It is also possible to evaluate the space vector voltage and to reference the space vector voltage until it has a turning point, and then to change to referencing the positive sequence voltage. 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.