METHOD FOR GENERATING ELECTRIC ACTING CURRENT

Description

[0001] The present invention relates to a method for generating alternating current for feeding into an electrical supply network. The present invention further relates to a corresponding feed-in device. The invention also relates to a wind turbine with such a feed-in device.

[0002] It is particularly well known for wind turbines to generate alternating current for feeding into an electrical supply network by having several inverters each generate a partial current, superimposing these partial currents to form a total current, and feeding this superimposed total current into the network.

[0003] Such partial currents are generated using a modulation method also known as pulse-width modulation. A fundamental method for generating such an alternating current is triangular modulation. In simplified terms, this involves superimposing a sawtooth signal onto a desired sinusoidal waveform, and then, at each intersection point of the sawtooth signal with the desired sinusoidal waveform, a corresponding semiconductor switch is closed or opened to trigger or terminate a voltage pulse. Such a method, which is not the subject of the present invention, can also be referred to simply as a control method, because neither the predetermined sinusoidal signal nor the superimposed sawtooth waveform is based on the generated result.

[0004] Another method is a tolerance band method. Here, a tolerance band is defined around a sinusoidal function corresponding to the desired current, i.e., a lower and an upper deviation limit. The generated output current is then measured and compared to this tolerance band. If the current reaches the lower tolerance band limit, a switching pulse is triggered; if the measured current reaches the upper tolerance band limit, this pulse is terminated. As a result, the current varies within this tolerance band around the predefined, idealized sinusoidal waveform.

[0005] Improving the quality of the generated current can be achieved particularly by narrowing the tolerance band. Making this band narrower reduces the current's variation around the ideal sinusoidal waveform, which regularly leads to an increase in the switching frequency. This is because the generated current reaches the required frequency more quickly due to the tighter limits, thus triggering a switching operation sooner.

[0006] This method is therefore well-known and can also be used for individual partial currents, which then combine to form a total current. This total current generated in this way can then be fed into the electrical supply network.

[0007] Through this superposition, which essentially results in the summation of these partial currents, the currents at any given time are added together. In other words, the instantaneous values ​​of the currents at each moment are added. This can also lead to a certain smoothing of the superimposed total current. This can occur because the respective positive and negative deviations from the ideal sine wave of the many individual partial currents cancel each other out, either completely or partially. This is especially true if these individual positive and negative deviations are statistically evenly distributed. However, it can also happen that many positive or many negative deviations of the individual partial currents combine, resulting in a correspondingly high overall deviation.

[0008] To prevent this, the individual tolerance bands can each be made so narrow that even a theoretical addition of a positive deviation of each of the partial currents does not exceed a desired maximum value for the total current.

[0009] This would mean specifying particularly narrow tolerance bands, which would consequently lead to exceptionally high frequencies. Therefore, if one wanted to implement such a reliable limit in this way for 10 inverters, each generating a partial current, each tolerance band would have to be set to one-tenth the width corresponding to the maximum permissible deviation of the total current. The result could be a tenfold increase in the switching frequency when modulating the currents. The effort required to ensure that the generated total current does not exceed a certain value is therefore enormous.

[0010] The German Patent and Trademark Office has stated in the priority application for the present The application researched the following state of the art: DE 40 23 207 C1 and an excerpt from Power Electronics and Variable Speed ​​Drives by M. López et al. "Control design for parallel-connected DC-AC inverters using sliding mode control". Further relevant prior art for the present invention is also MILOSEVIC M ET AL: "Interaction between hysteresis controlled inverters used in distributed generation systems", POWER ENGINEERING SOCIETY GENERAL MEETING, 2004. IEEE, IEEE, PISCATAWAY, NJ, USA, June 6, 2004 (2004-06-06), pages 2188-2193, BOSE BK: "AN ADAPTIVE HYSTERESIS-BAND CURRENT CONTROL TECHNIQUE OF A VOLTAGE-FED PWM INVERTER FOR MACHINE DRIVE SYSTEM", IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, IEEE SERVICE CENTER, PISCATAWAY, NJ, USA, Vol. 37, No. 5, October 1, 1990 (1990-10-01), pages 402-408, SAE-SUE S ET AL: "Comparative performance evaluation of fixed and adaptive hysteresis band delta modulation techniques for UPS", POWER ELECTRONICS AND DRIVE SYSTEMS, 1999.PEDS '99. PROCEEDINGS OF THE IEEE 1999 INTERNATIONAL CONFERENCE ON HONG KONG, JULY 27-29, 1999, PISCATAWAY, NJ, USA, IEEE, US, Vol. 2, July 27, 1999 (1999-07-27), pages 956-96 and JP 2 906616 B2. These publications also address the topic of modulation methods in frequency converters using tolerance band methods.

[0011] The object of the present invention is therefore to address at least one of the aforementioned problems. In particular, a solution is to be proposed that prevents the total output current from deviating too significantly from the specified sinusoidal waveform in the simplest and most efficient way possible. At the very least, an alternative solution compared to previously known solutions is to be proposed.

[0012] According to the invention, a method for generating an alternating electric current according to claim 1 is proposed. This method comprises the steps of generating several partial currents and superimposing the partial currents to form a total current. Each partial current is generated using a modulation method that employs a tolerance band method with tolerance limits. It is further proposed that the tolerance limits are variable, wherein the tolerance limits of the modulation methods for the partial currents are selected such that the total current lies within a predetermined tolerance limit.

[0013] Therefore, standard-width tolerance bands can initially be used. If, for example, the total current exceeds the desired optimal sinusoidal waveform, this can be counteracted by lowering the tolerance limit for one, several, or all of the partial currents. These reductions can also vary. The lower limit can also be lowered, so that a higher frequency is not necessarily achieved.

[0014] Preferably, these tolerance limits are thus changed depending on the total generated current. Indirect feedback in the sense of control can therefore be achieved without directly regulating the individual currents. Rather, this feedback of the total current is achieved via the change in the tolerance limits. According to one embodiment, it is proposed that the tolerance limits of each of the modulation methods form a tolerance band with an upper and a lower tolerance limit, and that the upper and lower tolerance limits are changed independently of each other, or that the tolerance band is shifted while maintaining a constant distance between the lower and upper tolerance limits.

[0015] The tolerance band of each modulation method, i.e., for generating each of the partial currents, has an upper and a lower tolerance limit. It is proposed that these upper and lower tolerance limits be adjusted independently. For example, the upper tolerance limit can be lowered as needed without changing the lower limit, or vice versa. Alternatively, it is proposed that the entire tolerance band be shifted. Shifting the tolerance band allows the amplitude of each partial current to be adjusted without changing the switching frequency.

[0016] Preferably, the tolerance limits of the individual modulation methods for the partial currents are selected or modified such that the total current remains within a predefined tolerance limit. Accordingly, a tolerance limit or tolerance band is specified for the total current. Compliance with this tolerance limit is then achieved by adjusting the individual tolerance limits of the partial currents. Thus, instead of triggering immediate switching operations when the total current reaches its tolerance limit, as is the case with the tolerance band method for each individual current, control is achieved indirectly by changing the tolerance limits of the individual partial currents.

[0017] However, changing these individual tolerance limits does not require waiting for the total current to reach its tolerance limit or one of the two tolerance limits of the tolerance band. Instead, or preferably, the deviations of the total current from its tolerance limit can already lead to a change, in particular a shift, of the tolerance limits of the modulation methods of the individual partial currents. Alternatively, or in addition, the deviation of the total current from the optimal achievable waveform, i.e., in particular the optimal sinusoidal waveform, can be evaluated, and the tolerance limits of the individual modulation methods of the partial currents can be changed accordingly. For example, if the total current rises above its optimal value, the upper limits of the modulation methods for the partial currents can be lowered. Should the total current rise even further above its optimal value, the tolerance limits can be adjusted accordingly.In this example, the upper tolerance limit of each modulation method for each partial stream is further lowered. The same can, of course, be done analogously for a drop below the optimal value.

[0018] Preferably, the partial currents and the total current are measured to set the tolerance limits. Each individual modulation method thus receives the measured value of the total current as an input for measuring its respective partial current. This also shows that the total current is used as a measured variable for several, sometimes very many, modulation methods simultaneously. The proposed method also prevents an overreaction that could occur, for example, if all modulation methods were to suddenly react and terminate every pulse that is currently applied due to an excessively high total current. Preferably, the tolerance limits of the individual modulation methods are shifted to the maximum value of the underlying curve, i.e., to the optimal sinusoidal waveform. This optimal waveform thus represents the minimum value for the upper limit of the tolerance band and the maximum value for the lower limit.This allows for a quick response to excessively deviating total currents, while the individual partial currents still move around their optimal value, i.e., close to the sinusoidal waveform to be set.

[0019] According to one embodiment, it is proposed that the partial currents and the total current be transformed into a common coordinate system in which limits to be observed are specified, such that the total current lies within a specified tolerance limit. Preferably, such a transformation can be into a rotating coordinate system. This allows measured values, optimal values, and limit values ​​to be specified, particularly with respect to magnitude and phase. For the optimal value, only the phase would change, not the magnitude. The limit values ​​could be defined more easily in the rotating coordinate system. However, the measured value would have to be converted each time.

[0020] Such a transformation also includes weighting sub-streams with different values. This weighting thus constitutes a transformation, and this can mean, for example, that the tolerance limits for different sub-streams are changed differently depending on the total stream.

[0021] Such weighting of the partial currents is particularly advantageous when equalizing currents arise between the individual partial currents, i.e., between the individual inverters. Such equalizing currents can also occur between individual inverters if they are galvanically connected to the same DC input. Insofar as such equalizing currents are known, they are included as a component in the respective partial current but do not contribute to the total current. The corresponding measured partial current, which is also fed back for the modulation process, therefore does not correspond to the partial current that actually contributes to the total current. This can be taken into account by appropriately adjusting the tolerance limits through weighting. In this case, the corresponding partial current is no longer considered; instead, a transformation is performed using this weighting.

[0022] The invention also proposes a feed-in device for feeding electrical current into an electrical supply network according to claim 6. Such a feed-in device has several inverters, each with a partial current output, wherein a partial current is generated or provided at the partial current output.

[0023] Furthermore, a summing current output is provided, which sums the partial currents to a total current, with the partial current outputs being connected to the summing current output at a summing node. A method for generating the current according to one of the preceding embodiments is now proposed. Preferably, the inverters are connected in parallel and each has a line choke at its partial current output. Preferably, only the line choke is provided without additional output filters. In particular, no conventional LCL filter is used, but only the single line inductor or line choke. This method eliminates the need for such a filter. Due to the total current-dependent change in the tolerance bands, such a conventional LCL filter can be rendered unnecessary. No special smoothing or...Filtering the individual streams so that they combine on average to form the most favorable overall stream, which in particular is as close as possible to the optimal value, can therefore be omitted.

[0024] Preferably, only one inductor or line inductor, which can also be called an L-filter, is present between each partial current output and the summing node. Typically, the partial current outputs are also three-phase, and a three-phase line choke is preferably proposed as the inductor, in which the phases are magnetically coupled, for example by using a choke with a 5-legged core.

[0025] Preferably, an additional line choke at the total current output can also be dispensed with, because the proposed method already leads to individual currents that advantageously complement each other to form the total current.

[0026] Furthermore, it is proposed that, for setting the tolerance limits, a measuring device be provided at each partial current output to measure the respective partial current, and that a measuring device be provided at the total current output to measure the total current. One measuring device is sufficient here, but it must feed its measured values ​​back to various inverters.

[0027] According to a further embodiment, it is proposed that the inverters, or some of them, are galvanically isolated on the input side and also, or alternatively, on the output side. Input-side decoupling can, for example, mean that the input-side busbars or DC feeders are galvanically isolated. For this purpose, the generation of the DC current can, for example, be carried out galvanically isolated in several systems at the generator, especially when a wind turbine is used, and the DC current can be routed separately to the individual inverters.

[0028] On the output side, galvanic decoupling can also be achieved at a common transformer. One possibility is for the transformer to have different taps. Galvanically isolated partial windings of the common transformer are then supplied. Instead of a galvanic summing node, a magnetic summation occurs. The transformer can then act as the summing node. Such decoupling can be particularly well integrated with the proposed method of overall current-dependent tolerance band matching. Here, the individual currents are preferentially generated and can be superimposed to the total current effectively. This avoids compensating currents.

[0029] The invention also proposes a wind turbine that is prepared for generating and feeding in electrical current and includes a feed-in device according to one of the embodiments described above. The wind turbine thus has several inverters that together generate the total current for the wind turbine to feed into the power grid.

[0030] The invention is explained in more detail below by way of example with reference to embodiments and the accompanying figures. Fig. 1 shows a wind turbine in a perspective view. Fig. 2 schematically shows a connection of several inverters for generating a total current. Fig. 3 illustrates a tolerance band method. Fig. 4 shows a schematic structure to clarify part of a control method according to one embodiment.

[0031] Fig. 1 Figure 1 shows a wind turbine 100 with a tower 102 and a nacelle 104. A rotor 106 with three rotor blades 108 and a spinner 110 is mounted on the nacelle 104. During operation, the wind sets the rotor 106 into rotation, thereby driving a generator in the nacelle 104.

[0032] The circuit arrangement according to Fig. 2 Figure 1 illustrates a feed-in device 1 and shows three inverters 2, which are representative of other inverters. These three inverters 2 also generate the partial currents i1, i2, and i3. Each inverter 2 has a DC input 4. The inverters 2 receive their input power via this DC input 4. These DC inputs 4 of the inverters 2 are coupled via a DC bus 6. However, according to one embodiment, it is also proposed that these DC inputs 4 are not coupled, but each is connected to its own DC source 8. Fig. 2 Figure 1 shows both of these possibilities. A separation of the DC inputs 4, so that each DC input 4 can have its own DC source 8, can be implemented, for example, by having a generator, particularly of a wind turbine, feed separate DC sources 8.

[0033] The inverters 2 now generate the respective output currents i1, i2, and i3 at their outputs, which are each designated as partial current outputs 10. The output of each inverter also has an output inductance 12. The notation following each output inductance indicates that each inverter 2 generates a three-phase current. Fig. 2 It can therefore also be deduced that this output inductance 12 at each partial current output 10 can be sufficient in the proposed method. A conventional filter, in particular an LCL filter, is not required. The output partial currents i1, i2, and i1 are superimposed at a summing node 14, i.e., added together, and fed to the summing current output 16 as a total current iG. The summing current output has a common network inductance 18, which, however, can also be dispensed with. The total current iG can then be fed into the electrical supply network 22 via a transformer 20.

[0034] The further operation will now be explained by considering the currents. It should be noted that both the partial currents at the output of each inverter 2 and the total output current at the sum current output 16 are three-phase. However, the following explanations will only address one phase of these three-phase currents at a time. Therefore, only one phase will be considered, and the other phases function in the same way.

[0035] It is now in Fig. 2 It can be seen that there is a current sensor 24 for each partial current i1, i2, and i3. A current sensor 26 is also provided for the total current iG.

[0036] Each inverter 2 now uses a measured value of its partial current, i.e., i1, i2, or in, and also the measured value of the total current iG. The total current iG thus flows into each of the inverters 2. Depending on the total current iG, each inverter then sets the corresponding tolerance band or tolerance limits of the tolerance band and then controls the corresponding semiconductor switches, depending on its partial current, to modulate a corresponding current.

[0037] The currents i1, i2, and i1 are then generated, which already exhibit a favorable, low-oscillation state due to the nature of their circuit and the output inductance 12, and are then superimposed at the summing node 14. The result is the total current iG, the measured value of which is fed back to each of the inverters 2 as described.

[0038] Fig. 3 Figure 30 illustrates an optimal sine curve 30 for a tolerance band method, around which a tolerance band with an upper tolerance limit T1 and a lower tolerance limit T2 is placed. For illustrative purposes, this tolerance band is drawn very wide and would of course be much narrower in reality.

[0039] The generated current i1, which is used here as an example, lies within this tolerance band between the limits T1 and T2.

[0040] The current is generated by closing a switch to create a positive pulse. As long as this positive pulse is present, the current increases, and once it reaches the upper limit T1, the corresponding switch opens again, ending the pulse. The current then decreases until it reaches the lower limit T2, at which point the switch closes again—this is a simplified and clear explanation of the process.

[0041] Fig. 3 The graph now shows a tolerance band where the optimal sine wave 30° lies in the middle, meaning it has equal distances to the upper and lower limits T1 and T2, respectively. To account for or counteract a high total current, for example, the upper limit T1 can be shifted downwards so that it is closer to the optimal sine wave 30°. The lower limit T2 can also be shifted further downwards, or it can remain unchanged.

[0042] Following such a shift of the tolerance band, i.e., the shift of the upper limit T1 described by way of example, the basic tolerance band procedure for the [variable / variable] is carried out. Fig. 3 The partial current i1 shown as an example continues unchanged. The method therefore continues to check whether the current has reached the upper tolerance limit T1 with a rising edge, which is now located elsewhere, or whether it has reached the lower tolerance limit T2 with a falling edge.

[0043] The procedure is in Fig. 4 The diagram is presented in a schematic structure illustrating or simplifying a feed-in device 41. The actual generation of the partial current i1 takes place in the inverter 42, which here schematically represents a DC link 44. The two switches S1 and S2 are arranged between the positive and negative nodes, generating a voltage pulse pattern so that the partial current i1 is also generated at the partial current output 50 due to the output inductance 52. This partial current i1 sums with various other partial currents i2 to form the total current iG. A network inductance 58 may be provided for the total current iG, but it may also be unnecessary.

[0044] This total current iG is measured with a total current meter 66 and entered into a tolerance block 70. Depending on the total current and depending on tolerance limits TG1 and TG2 for the total current, the tolerance block 70 can then define the specific upper tolerance limit T1 and lower tolerance limit T2, which are specified in Fig. 3 These upper and lower tolerance limits T1 and T2, as illustrated, are specified or modified. These upper and lower tolerance limits are then entered into the control unit 72. The control unit 72 also receives the current partial current i1 and then operates as shown. Fig. 3 As explained below, depending on the position of the partial current i1 within the tolerance band defined by the upper and lower tolerance limits T1 and T2, switching signals S are generated and sent to the inverter 42. The inverter 42 then switches switches S1 and S2 accordingly. Specifically, for a positive pulse, switch S1 is closed and switch S2 is open; conversely, for the end of a positive pulse (i.e., a negative pulse), switch S2 is closed and switch S1 is opened.

[0045] A partial current i1 is then established, which is fed back for the next calculation. A new value for the total current iG is also established, namely together with the further currents i2 to i, and this value of the total current iG is also fed back as described.

[0046] In addition to this basic schematic description, especially in connection with the Figuren 3 and 4 It may also be provided for to define the tolerance range, in particular a defined tolerance range for the total current i G, i.e., the one in Fig. 4 The illustrated tolerance limits TG1 and TG2 are to be transformed into suitable coordinates in order to better verify compliance by the total current and / or to better derive reactions, in particular changes to the upper and lower tolerance limits T1 and T2. Accordingly, a method is proposed that can achieve compliance with this tolerance range for the total current.

[0047] This section considers the case where several power electronic systems are operated together, i.e., in series and / or parallel connection, and controlled independently of each other using approximate sliding regime controllers, which can also be referred to as tolerance band controllers or may include such controllers. These sliding regime controllers can, for example, be implemented as hysteresis controllers. It can then usually be ensured that the control deviation of the sliding function remains within certain tolerance bands for each subsystem.

[0048] However, since there is no synchronization of the switching operations in the individual subsystems, it can happen that the control deviation of interconnected systems deviates simultaneously in the same direction, resulting in an unfavorable superposition. A solution to this problem is proposed as described above.

[0049] To selectively influence the superposition of current or voltage ripples, methods using pulse-width modulation or space vector modulation are typically employed in practice. With these methods, the switching frequency is usually fixed, and the switching points of interconnected systems are deliberately offset to achieve a desired superposition of the current or voltage ripple.

[0050] The disadvantage of this solution is that one has to forgo the advantages inherent in sliding regime control, namely in particular the property that certain disturbances are strongly suppressed.

[0051] It is now also proposed to operate interconnected power electronic systems in an approximate sliding regime in such a way as to ensure compliance with a defined tolerance range as far as possible. By appropriately selecting the tolerance range, an "unfavorable superposition" of harmonics, as described above, can be avoided or significantly reduced.

Claims

1. Method for generating an alternating electric current (iG), comprising the steps of - generating multiple partial currents (i1, i2, in) and - superimposing the partial currents (i1, i2, in) into a total current (iG), wherein each of the partial currents (i1, i2, in) is generated using a modulation method, the modulation method uses a tolerance band method having tolerance limits (T1, T2) and the tolerance limits (T1, T2) are changeable wherein the tolerance limits (T1, T2) of the modulation methods of the partial currents (i1, i2, in) are selected in such a way that the total current (iG) lies within a predefined tolerance limit (TG1, TG2) characterized in that that the tolerance limits (T1, T2) of each of the modulation methods form a tolerance band having an upper and a lower tolerance limit (T1, T2), and the upper and lower tolerance limits (T1, T2) are changed independently of each other, or the tolerance band is shifted while retaining a constant distance between the lower and upper tolerance limits (T1, T2) wherein the feed-in device further comprises a control unit (72), wherein the control unit (72) is arranged to execute a method for generating the current according to one of the preceding claims.

2. Method according to claim 1, characterized in that the tolerance limits are changed as a function of the generated total current (iG).

3. Method according to one of the preceding claims, characterized in that for setting the tolerance limits (T1, T2), the partial currents (i1, i2, in) and the total current (iG) are measured.

4. Method according to one of the preceding claims, characterized in that the partial currents (i1, i2, in) and the total current (iG) are transformed into a shared coordinate system in which limits to be complied with are predefined, so that the total current (iG) lies within a, or the, predefined tolerance limit (TG1, TG2).

5. Method according to claim 4, characterized in that the shared coordinate system is a rotating coordinate system.

6. Feed-in device (1, 41) for feeding in electric current into an electric power supply network (22), comprising - multiple inverters (2), each having a partial current output (10), each for generating a partial current (i1, i2, in) at the partial current output (10), - a sum current output (16) for summing up the partial currents (i1, i2, in) to a total current (iG), wherein the partial current outputs (10) are connected to the sum current output (16) at a summing node (14), wherein the feed-in device (1, 41) further comprising a control unit (72), wherein the control unit (72) is arranged to perform a method for generating a current according to one of the preceding claims.

7. Feed-in device (1, 41) according to claim 6, characterized in that the inverters (2) are connected in parallel and include a line reactor (12) at each of their partial current outputs (10).

8. Feed-in device (1, 41) according to claim 6 or 7, characterized in that the inverters (2) operate using a line reactor (12) at each of their current outputs (10) without an additional output filter and / or without an additional line reactor (18) at the sum current output (16).

9. Feed-in device (1, 41) according to one of claims 6 to 8, characterized in that for setting the tolerance limits (T1, T2), a measuring means is provided at each partial current output (10) for measuring the particular partial current (i1, i2, in), and a measuring means is provided at the sum current output (16) for measuring the total current (iG).

10. Feed-in device (1, 41) according to one of claims 6 to 9, characterized in that the inverters (2), or some of them, are galvanically decoupled on the input side and / or on the output side.

11. Wind turbine (100) for generating and feeding electric current into an electric power supply network (22) including a feed-in device (1, 41) according to one of Claims 6 to 10.