Method for identifying filter inductors in grid inverters

Abstract

This disclosure relates to a method for operating an inverter (1), wherein the method comprises the following steps: - applying an AC voltage to a phase line (3) by means of a switching unit (4) of the inverter (1), wherein a filter inductor (2) is arranged in the phase line; - determining the coil current (i) of the filter inductor (2). L And calculate the coil voltage (u) of the filter inductor (2). L ), - for the coil current (I x The first value of the filter inductor (2) is obtained by taking the first value of (L(I)). x - If necessary, obtain at least one additional value of the filter inductor (2) for at least one additional value of the coil current; - If using the first value obtained by the filter inductor and if necessary, if using the at least one additional value obtained by the filter inductor, obtain the inductance curve (6) of the filter inductor (2) related to the coil current; - Adjust the switching unit (4) of the inverter (1) by means of the adjustment unit (5) to generate an alternating current in the phase line (3), wherein, according to the obtained inductance curve (6) related to the current, at least one parameter of the adjustment is continuously adapted to the current coil current.

Description

Technical Field

[0001] This disclosure relates to methods for operating an inverter and an inverter. Background Technology

[0002] To filter the AC current generated by the inverter's switching unit, filter coils are installed in each phase conductor on the AC side. For good regulation characteristics, the filter coils must possess sufficient linearity, which requires relatively expensive filter coils of a sufficiently large size. Filter coils are increasingly operating in the region of nonlinear inductance to enable smaller structural forms, or due to the nonlinear characteristics of the inductance caused by the core material. Nonlinearity is also used selectively to better meet different requirements through different operating ranges. If the assumed characteristics or the coil's inductance curve differ from the actual characteristics, this can have a problematic effect on the sinusoidal waveform of the current.

[0003] As is known from US 2015 / 0016162 A1, when adjusting the pulse width modulation used in a photovoltaic inverter, the value assigned to the filter inductor is determined based on the current current intensity and taken into account during the adjustment. Here, the assigned value of the filter inductor is obtained from a configuration table or approximate formula. In both cases, the inductance curve is assumed to be known.

[0004] Under laboratory conditions, the inductance curve of a filter coil or inductor can be obtained relatively easily using current techniques, and this curve can be stored for control or regulation. However, laboratory conditions typically differ from the installation and operating conditions of inductors. Stored inductance curves often involve averaging random samples from multiple inductors and do not reflect the actual inductors installed. Once an inductor is installed in a device, especially when the device is in operation, obtaining a usable inductance curve becomes very difficult. Even more challenging is the fact that the current flowing through the inductor in the device can include high-frequency distortion.

[0005] Therefore, a common problem in practice is that the stored inductance curve for an inductor or filter coil deviates significantly from the actual inductance curve (that is, the curve showing the actual change in inductance value with respect to current intensity) for the reasons just mentioned. Furthermore, the inductance curve is not uniform or constant over time due to aging, changing environmental conditions, and manufacturing tolerances. This deviation from the stored inductance curve can be problematic, especially when the inductor is operating in a non-linear region. Finally, a series of current variations caused by the inductor can also affect the actual value of the inductance at a given current. In other words, for a given current value, there can be not just one possible inductance value, but multiple possible values. This is due to the hysteresis in the magnetization of the core material of the inductor or filter inductor. Summary of the Invention

[0006] This disclosure relates to methods and apparatus that can overcome these and other problems of the prior art.

[0007] In a first aspect, this disclosure relates to a method for operating an inverter, the method comprising the following steps:

[0008] - AC voltage is applied to the phase lines by means of the switching unit of the inverter, and filter inductors are arranged in the phase lines.

[0009] - Determine the characteristic values ​​of the coil current and coil voltage of the filter inductor in a region of multiple measurement points, wherein at least two measurement points are located on at least one edge of the current ripple.

[0010] - The first value of the filter inductor's inductance is obtained from the first value of the coil current based on the aforementioned characteristic value.

[0011] - If necessary, determine at least one additional value for the inductance of the filter inductor for at least one additional value for the coil current.

[0012] - Using the first value of the inductance of the filter inductor, and if necessary, using at least one other value of the filter inductor, obtain at least one inductance curve of the filter inductor related to the coil current.

[0013] - The switching unit of the inverter is adjusted by means of an adjustment unit to generate alternating current in the phase line, wherein at least one parameter of the adjustment is continuously adapted to the current value of the inductance according to the obtained current-related inductance curve.

[0014] Therefore, the inverter's regulation always corresponds very precisely to the actual inductance curve of the filter inductor, thereby improving the regulation quality.

[0015] In an advantageous implementation of this method, determining the first value of the filter inductor may include the following steps:

[0016] - Calculate the first edge slope m of the current ripple for the first value of the coil current. x ,

[0017] - Determine the first value U of the coil voltage with the first edge slope. x ,

[0018] - The first value L(I) of the filter inductor is obtained from the first edge slope and the first value of the coil voltage according to the following formula. Ly )

[0019] .

[0020] The evaluation of the current ripple edge slope allows for a very accurate determination of the filter inductor value given a specific current intensity. Here, the current ripple edge slope corresponds to the derivative of the current with respect to time di / dt, thus the filter inductor value can be directly obtained using the formula for inductance.

[0021] or

[0022]

[0023] Come and ask.

[0024] Determining the edge slope typically requires evaluating at least two measurement points on the same edge of the current ripple. Each edge of the current ripple lies between two switching points of the switching unit, thus the duration of the edge is known in the adjustment. This knowledge can be used to ensure that measurements taken only on a single edge of the same current ripple are used to determine the edge slope. It is also known whether there is exactly a rising or falling edge of the current ripple. When the inductance should be determined using only two measurement points on one edge, then a measurement point is set after the first switching point and another measurement point is set before the subsequent switching point. Here, the two measurement points are as far apart from each other as possible.

[0025] To improve accuracy or to account for edge curvature, more than two measurement points are set on a single edge. If necessary, more than two measurement points can be considered to determine the edge slope and the average value to calculate the filter inductor. Here, the average values ​​of the coil current and coil voltage are formed on the edge of the current ripple, from which the average coil current I is obtained. Ly The corresponding inductance value. Thus, for example, the effect of the curvature of the ripple current edge can be considered in a first approximation. In the case of strongly nonlinear inductors or correspondingly extended ripple edges, as may occur, for example, when using an H-bridge, that is, the edge slope at the beginning of the edge differs from the edge slope at the end of the edge. Improved regulation quality can be achieved by using an average value. If a current ripple edge occurs during regulation, and this edge is too short for measuring the edge slope (that is, especially if only one measurement point is within it), then these edges are not considered in the evaluation.

[0026] The characteristics or inductance curve of a filter coil can be detected using eigenvalues. Eigenvalues ​​are essentially derived from the correlation between the coil current and coil voltage and their rate of change. As an example in this case, the slope of the edge of the current ripple is called the eigenvalue. Instead of current, magnetic flux can also be used to express the eigenvalues. Similarly, changes in the characteristics or inductance curve of the filter coil, such as those occurring according to its temperature, the amplitude of the alternating current, the switching frequency, or the switching unit / topology used, can be determined or depicted by appropriately detecting these correlations and their temporal relationships as eigenvalues.

[0027] In any case, the characteristic values ​​include values ​​from which at least one value of the coil's inductance can be determined. Multiple inductance values ​​related to the coil current and coil voltage are considered for determining at least one inductance profile.

[0028] Advantageously, the coil current can be determined by measuring at a sampling frequency at least twice, and preferably more than twice, the clock frequency of the pulse width modulation implemented by the switching unit. This "oversampling" allows for very accurate evaluation of the current ripple variation curve. The current ripple is generated, for example, by the switching process used for pulse width modulation. This method can be implemented using different types of pulse width modulation, such as based on a fixed frequency or a variable frequency.

[0029] Advantageously, the edge slope can be obtained by forming a current difference between at least two sampling steps and taking the sampling frequency into account. Therefore, the value of the edge slope can be obtained well and simply as the difference between the measurements of two consecutive current measurements divided by the length of the measurement interval.

[0030] In a favorable manner, the slope and / or curvature of the inductance curve can be determined after obtaining it.

[0031] When adjusting the switching unit of the inverter to generate alternating current in the phase line using the adjustment unit, at least one parameter of the adjustment can be adapted in an advantageous manner to the current value of the slope and / or curvature of the inductance curve.

[0032] When an inverter has multiple phase lines, each with a filter inductor, in another advantageous embodiment, at least one additional inductance curve of another filter inductor in another phase line is obtained and taken into account during regulation. Therefore, the method disclosed herein can be advantageously applied to both single-phase and multi-phase inverters. The term "filter inductor" can also be understood here as meaning that a "filter inductor" can consist of multiple interconnected filter inductors. For simplicity, the filter inductor whose phase, in summation, affects regulation is considered a "filter inductor".

[0033] Advantageously, the methods disclosed herein can be performed before the inverter's first startup, and / or at planned intervals before one or every startup of the inverter, and / or regularly or continuously during the inverter's operation. The chosen strategy also depends on available resources. For example, the coil current and coil voltage are obtained at a sufficiently high sampling frequency (“oversampling”), as are the values ​​of the filter inductor and, if necessary, the inductance curve, which consumes computational power in the regulation unit. To free up this computational power, the calculation can therefore be performed, for example, over one (or more) cycles, wherein the obtained inductance curve is stored incrementally in memory if necessary. The regulation can then access the stored values ​​in a different order without using additional computational power. The next calculation cycle can then be triggered by a specific event (e.g., when the regulation unit determines a reduced regulation quality), or the calculation can be performed at regular intervals during operation.

[0034] In another aspect, this disclosure relates to an inverter having a switching unit by means of which an AC voltage can be applied to at least one phase line, wherein a filter inductor is arranged in the phase line, and wherein the switching unit is adjusted by means of an adjustment unit, wherein the coil current and coil voltage of the filter inductor can be determined by the adjustment unit, and wherein the adjustment unit is configured to perform the following steps:

[0035] - Operate the switching unit to apply AC voltage to the phase lines of the inverter.

[0036] - Determine the first value of the filter inductor for the first value of the coil current.

[0037] - If necessary, determine at least one additional value for the filter inductor for another value of the coil current.

[0038] - Using the first value obtained from the filter inductor, and if necessary, using at least one other value obtained from the filter inductor, obtain the inductance curve of the filter inductor related to the coil current.

[0039] - Adjust the switching unit of the inverter to generate alternating current in the phase line, wherein, according to the obtained current-related inductance curve, at least one parameter of the adjustment is continuously adapted to the current coil current.

[0040] This inverter achieves improved regulation performance and consequently a more ideal sinusoidal shape for the AC variable at the inverter output. Furthermore, it allows the use of cheaper components, a challenge often insurmountable in conventional regulation. In particular, smaller and lighter inductors enable miniaturization and cost optimization. However, smaller and lighter inductors exhibit a nonlinear relationship between inductance and current due to earlier magnetic saturation, a nonlinearity addressed by the solution according to the invention.

[0041] Advantageously, the regulation unit can be configured to calculate the inductance curve of the filter inductor before the inverter's initial startup, and / or at planned intervals before the inverter's initial startup or each startup, and / or regularly or continuously calculate the inductance curve of the filter inductor during the inverter's operation. This ensures optimal regulation performance at all times.

[0042] In a favorable design, the inverter can have multiple phases, each with its own filter inductor, where the inductance curve can be determined for each filter inductor. This allows the current teachings to be advantageously applied to multiphase inverters as well. Attached Figure Description

[0043] The following reference Figures 1 to 4 The invention will be described in more detail below. These figures exemplify, schematically and not restrictively, advantageous designs of the invention.

[0044] Here it is shown:

[0045] Figure 1 An exemplary schematic circuit layout of an inverter is shown.

[0046] Figure 2 A graph showing the qualitative inductance curve

[0047] Figure 3a A graph showing the current change curve

[0048] Figure 3b A graph showing the current change curve with coil voltage.

[0049] Figure 3c A graph showing the current change curves at two measurement points, and

[0050] Figure 4 A graph showing the fit of the inductance curve. Detailed Implementation

[0051] exist Figure 1 The inverter 1 shown herein uses a switching device that, by means of a known switching unit 4, switches the DC potential U of the DC power supply 7.DC Converted into a clock-controlled, rectangular AC voltage u with a rectangular orientation. R The rectangular AC voltage is applied to phase line 3. The rectangular AC voltage u has a rectangular orientation. R Then, the voltage is converted by filter unit 8 into an approximately sinusoidal AC voltage (filter capacitor voltage u). ac Or the corresponding approximately sinusoidal alternating current i ac .

[0052] Inverter 1 can be connected to the power grid 10 via network relay 9, wherein, in the simplified case shown, the inverter is connected to phase P and neutral conductor N of the power grid 10. AC voltage u ac With network relay 9 closed, the phase current i ac It is fed into the power grid 10.

[0053] To determine the inductance profile, the inverter is typically connected to a phase of the power grid. However, the inductance profile can also be determined by the inverter without phase connection to the power grid, through appropriate switching procedures and connection to the neutral conductor of the power grid before feed-in operation. Within the scope of inverter manufacturing, tests are typically performed on the inverter, within which currents equivalent to those typically encountered during operation are flowed. Therefore, an accurate inductance profile of the inverter itself can be determined during manufacturing without additional time and intermediate costs. This ensures that defect-free current is achieved during initial startup or initial feed-in operation.

[0054] The filter unit 8 includes a filter coil 2 and a filter capacitor 11 arranged in the phase line 3. The filter coil 2 is directly connected to the switching unit 4 at its first end, and the phase line 3 is further guided to the network relay 9 from the second end of the filter coil 2. The filter capacitor 11 is arranged in the region between the phase line 3 and the neutral conductor 12, between the second end of the filter coil 2 and the network relay 9.

[0055] Switching unit 4 in Figure 1 The diagram only schematically illustrates a large number of known topologies for switching unit 4, which is capable of generating AC voltage. Figure 1 The switching unit 4 shown corresponds to a three-point circuit, wherein the phase line 3 can be loaded with three DC potentials: a positive DC potential DC+, a negative DC potential DC-, and a neutral DC potential 0 at the center point of the intermediate circuit, which is connected to the neutral conductor 12. However, the teachings of this disclosure can also be applied to topologies with only two DC inputs (e.g., DC+ and DC-) or with only one DC input (whose potential differs from that of the neutral conductor). The switching unit 4 can be used in... Figure 1The topology shown is, for example, the so-called NPC topology; however, any other topology can be used, and adapting the circuit to other topologies is a skill of those skilled in the art. The circuit may also have, for example, two or three phase outputs, each with a filter coil, as is known in the art. The topology of the inverter's switching device 4 can be, for example, selected from H-bridge, H5, HERIC, REFR, FB-DCBP, FB-ZVR, NPC, Conergy-NPC, and related topologies. The topologies described in this way are well known in the art and therefore need not be explained in further detail here.

[0056] Switching unit 4 is connected to regulating unit 5, which regulates the opening and closing of the semiconductor switches arranged in switching unit 4. Here, pulse width modulation (PWM) is typically used. Depending on the regulation scheme, regulating unit 5 has multiple measured values, which are used as input parameters for the regulation. Figure 1 In the diagram, the positive and negative DC potentials relative to the intermediate circuit center point (i.e., the voltage across the intermediate circuit capacitor) are exemplarily and schematically shown by dashed lines as input parameters for adjustment. dc + and u dc - The AC voltage u after passing through filter capacitor 11 ac and the coil current i after filtering coil 2 L The measuring devices and circuits required to determine these input parameters are known to those skilled in the art and do not need to be described in detail here. Depending on the topology, other or different measured values ​​may also be set as input parameters for adjustment. If necessary, other parameters may also be measured if the input parameters required for adjustment can be obtained directly or indirectly from these parameters.

[0057] Therefore, in Figure 1 In the examples shown, for example, the coil voltage u L It can be used as a rectangular AC voltage u generated by the switching unit 4 R and the voltage u of the filter capacitor ac The value is obtained by subtraction. Rectangular AC voltage u R The coil current i can also be determined based on the corresponding switch position of switch unit 4 and the value of the intermediate circuit voltage. L The current can be determined directly, for example, by using an ammeter 13 positioned before or after the filter coil 2 in phase line 3. If necessary, current measurements can also be performed at other locations, such as measuring the phase current i. ac and the current through filter capacitor 11, wherein the coil current i is derived from these values. LHowever, the coil current can also be obtained indirectly, for example, by measuring the magnetic flux of the filter coil. For the method disclosed herein, it is important that there exists a way to obtain the coil current i in any manner. L and coil voltage u L The possibility.

[0058] Filter coil 2 can operate in a current range where a nonlinear inductance curve exists, such as when its curves vary according to the different variations of inductance curves 6a, 6b, and 6c. Figure 2 As illustrated both exemplarily and qualitatively, the inductance L of the filter coil 2 is defined by inductance curves (6a, 6b, 6c) that are values ​​related to the coil current.

[0059] The inverter's regulation unit 5 also uses the inductance value of the filter coil 2 as a parameter. However, using an incorrect value results in poor regulation quality, which can lead to undesirable voltage or current fluctuations. This may undesirably alter reactive power or cause current fluctuations. iac The sinusoidal shape is distorted, which may overload the components or even cause the inverter to shut down incorrectly.

[0060] In order to enable the use of a (favorable and small) filter coil 2 with a nonlinear inductance curve 6 exhibiting strong performance, the adjusted inductance-related parameters can be based on the coil current i. L The current value is continuously adapted. In the inverter according to the invention, the measurement or determination of the coil current is performed very accurately and at a high sampling rate, so that the adjustment can be adapted very precisely to the current value of the coil current across the entire phase change curve. Therefore, high regulation quality can be achieved, provided that the inductance curve 6 is correct.

[0061] However, the inductance curve 6 of filter coil 2 may change significantly due to environmental influences (e.g., temperature, magnetic field of adjacent choke coils, etc.) and / or aging. Furthermore, there are manufacturing-determined tolerances and deviations in the inductance of the filter coil. Although the adjustment is (assuming) optimally fitted to only the assumed correct inductance curve 6, adjustment errors may therefore occur in practice.

[0062] exist Figure 3a The diagram shows the coil current i. L The curve showing the change over time t. (Except for...) Figure 3a Besides the current ripple shown, there are methods for generating alternating current, in which the current ripple can be much larger. For example, a method is known from the prior art in which each current ripple has a "nullurchgang", and thus the inductance curve of a filter inductor having one or more of these large ripples can be obtained, particularly within the range of the fundamental current amplitude. Figure 3aThe current in the circuit generally follows a sinusoidal curve (the so-called fundamental current 14), however, it exhibits a noticeable current ripple, which... Figure 3b and Figure 3c The chart shown is enlarged in the cropped portion. Figure 3b and Figure 3c The variation curves shown in the chart segment can be located in different regions of the fundamental wave 14; however, the chart can be arbitrarily selected and should not be designed restrictively.

[0063] The occurrence of this current ripple is technologically determined and to some extent unavoidable. However, especially within the current amplitude range, excessive current ripple can cause overcurrent, which can lead to erroneous shutdown (or even component damage). Therefore, another objective of this invention is to identify the magnitude of the current ripple before an overcurrent occurs using a precise inductance curve and, if necessary, limit the magnitude of the current ripple by adapting the modulation method or adjusting the switching unit to avoid erroneous shutdown. Here, accurate understanding of the filter circuit parameters is crucial for adjustment quality. In the case of a nonlinear filter coil 2, the most accurate possible understanding of the inductance curve 6 is particularly necessary.

[0064] Each current ripple has a rising edge and a falling edge, wherein the edges intersect the fundamental current 14, respectively. Coil current i L Measurement sampling is performed at a high frequency, thus providing multiple measurements for each edge of the current ripple, such as ten or more measurements, but at least two. Measurement sampling is performed at... Figure 3b The example is shown through measurement points 15, 16, and 17. Figure 3b As can be seen, each edge has curvature. The curvature of this edge is determined by the nonlinear characteristics of filter coil 2. It is also possible that the curvature of the edge can be better accounted for by taking at least two values ​​of the inductance at different locations on the curved edge using three or more measurement points on one edge.

[0065] The slope m of the current ripple edge can be determined between two measurement samples on the edge of the current ripple. An excessively low sampling rate, an inappropriate location, or a poorly chosen measurement point for determining the inductance value can lead to undesirable results, as shown by lines 18 and 19. Figure 3b As shown in the diagram, line 18 is formed through measurement points 15 and 16 and has a slope m. x Through the curvature of the edge of the coil current or ripple current 20, the straight line 18 gradually deviates from the edge with measurement points 15 and 16. In the coil current I... max In this case, the adjustment of the falling edge should be introduced at time point t1. This can be simplified according to the inductance curve, plotted with the slope m of straight line 18 at time point (t1 + t).e The falling edge is introduced only at time point (t1 + t). e The coil current is already I max +I e Because the coil current is delayed by t e The introduction of I further increased e If we consider two measurement points that are not on the edge when calculating the slope m, for example, at... Figure 3b The measurement points 15 and 17 shown will produce even greater deviations. Line 19, drawn between measurement points 15 and 17, illustrates that I is reached at a very late time point. max The expected adjustment would thus be even more pronounced than the example using straight line 18. In this case, adjustment might no longer be possible. To avoid erroneous shutdown and improve adjustment quality, in order to obtain the inductance curve according to the invention, the correlation between the measurement point and the corresponding edge, and in particular the existing curvature, is considered.

[0066] exist Figure 3a In the chart screenshot, in Figure 3c The diagram exemplarily illustrates two measurement points on the edge of the current ripple. Here, for example, at the two measurement points M... x and M x+1 Find the edge slope M between x Each measurement point M x Including time point t x Current values ​​I for (x = 1, 2, ..., n) Lx Based on the current difference ΔI Lx Divide by the time interval Δt between the two measurement samples x The quotient yields the edge slope m. x , where ΔI Lx =(I L（x+1） -I Lx ) and Δt x =(t x+1 -t x ).

[0067] In addition, the coil voltage u at each or more measurement points is calculated. L The value U x For example, in Figure 3b As shown, the coil voltage u is indicated by the dashed line 21. L For the sake of simplicity, the duration of the edge is assumed to be constant.

[0068] Using the edge slope m x and the coil voltage U x The inductor L (I Ly ) and coil current I Ly The relevant values ​​can be obtained from the formula

[0069]

[0070] Come and ask.

[0071] The value of the inductance obtained in this way is related to the coil current I. Ly Together, they form a pair of values ​​that create the inductance curve. For example, the coil current I. Ly It can correspond to I Ly =I Lx Average value I Ly =(I L（x+1） +I Lx ) / 2, or by more than two current values ​​I Lx The value is obtained. The slope is especially meaningful when derived from more than two current values. Additionally, other characteristic values, such as temperature, current amplitude, and slope m, can be added to the inductance curve. x The sign of the value determines the inductance curves, which can be divided into different curves based on these eigenvalues. These additional eigenvalues ​​are either already included by the measurement points or are determined only with respect to the value pairs.

[0072] The edge slope m x and the coil voltage U x The above processing method can be used to adjust the coil current i L Multiple different values ​​I Lx To obtain, the value obtained can basically cover the coil current i of the fundamental frequency 14 that forms the current. L The entire variation curve. Therefore, for the coil current i L With enough values, the inductance L (I) of filter coil 2 can be calculated separately. Ly This allows for a sufficiently accurate determination of at least one inductance curve 6 as a function of the entire range of the relevant current. This can be achieved, for example, through a suitable regression method, such as linear regression. In practical attempts, a fit with a second-order polynomial function has proven to be a sufficiently good approximation for some filter coils 2, but depending on the application, higher-order polynomial functions or even a first-order polynomial function (i.e., represented by a straight line) can be used, if advantageous. Using either a fit or regression, the inductance curve 6 can be represented by a simple formula that expresses the correlation between the inductance of the filter coil 2 and the coil current.

[0073] For example, the inductance curve can be used as a polynomial.

[0074]

[0075] Let represent , where n > 0, and where the coefficient a i The value is obtained through regression analysis.

[0076] If the qualitative change curve of inductance curve 6 is known (or assumed to be known), then inductance curve 6 can be obtained, if necessary, by uniquely determining the inductance under a given coil current. Figure 2 In the diagram, lines 6a, 6b, and 6c show possible qualitative variations in the inductance curve of the coil, which can be quantitatively adapted in terms of its position given a determined coil current using the obtained inductance value. Figure 4 This is illustrated by inductance curve 6d, which shows that the inductance curve from Figure 2 6a in the middle is achieved by means of the previous current (I) Ly =30A) The inductance value (L(I) obtained Ly The inductance (I) = 230μH is quantitatively adapted in terms of position. The inductance curve 6d utilizes I Ly and L (I Ly The value pair 22, composed of , is determined in its position. Therefore, in Figure 4 Then, the regulating unit can adjust any current I. Ly Determine the inductance value. Figure 4 In the example, for I Ly Defined in I x and I x+1 The current value is between [value] and assumed to be 30 A. This roughly corresponds to the current value of the fundamental frequency 14 during the measurement in the case shown. As mentioned before, I Ly You can also use I Ly =I Lx Or may be defined in any other reasonable manner.

[0077] However, there are usually enough measurements available to perform a more accurate fit based on multiple calculated inductances under different coil currents.

[0078] Given the already determined inductance curve 6, its slope and curvature can then be determined. If the inductance curve 6 exists, for example, in polynomial form, this is particularly possible by means of the first and second derivatives with respect to the current.

[0079] Using inductance curve 6, the adjusted parameters can be continuously adapted to the current coil current. For example, a variable gain factor K can be defined. p (L(I)) proportional regulator. Gain factor K p The value of the inductor is always matched to the current value, thus ensuring that the exact value of the inductor is always used across the entire (sinusoidal) current variation curve. Therefore, the dynamic characteristics of the regulator are independent of the amplitude of the current alternating current.

[0080] In an extended implementation, the adjusted parameters can be continuously adapted to the current coil current by means of the slope of the inductance curve 6, so that the expected changes in the coil current can be better estimated.

[0081] In an extended implementation for an operating range where the inductance curve is particularly nonlinear, the adjusted parameters can be continuously adapted to the current coil current by means of the curvature of the inductance curve 6, so that the expected changes in the coil current can be better estimated.

[0082] The method described above for obtaining the inductance curve 6 can, for example, be performed before the first startup of the inverter 1. This may be sufficient for the filter coil 2, whose inductance curve undergoes only very minor changes over its lifespan. If necessary, the method can also be performed at planned intervals according to any scheme before a single startup or each startup of the inverter 1. This scheme can, for example, be condition-dependent. For instance, if the parameters characterizing regulation quality fall below or exceed the range during the final operation, the determination can always be performed before startup. If necessary, the determination of the inductance curve 6 can also be performed regularly, irregularly, or continuously during the operation of the inverter 1. This is entirely possible because the determination of the inductance curve 6 can be performed without problems during the actual operation of the inverter 1. For example, the regulation unit 5 can randomly sample and verify the consistency between the measured inductance value and the value determined according to the inductance curve 6. If an excessively large deviation is found, the determination and updating of the inductance curve 6 can be performed during operation or planned before the next startup.

[0083] List of reference numerals

[0084] 1 Inverter

[0085] 2 filter coils

[0086] 3-phase line

[0087] 4 Switching Units

[0088] 5 adjustment units

[0089] 6 Inductance curve

[0090] 7 DC power supply

[0091] 8 filter units

[0092] 9 Network Relays

[0093] 10 power grids

[0094] 11 Filter Capacitors

[0095] 13 Ammeter

[0096] 14 fundamental

[0097] Measurement points 15, 16, and 17

[0098] Lines 18 and 19

[0099] 20 coil current i L

[0100] 21 coil voltage u L

[0101] 22 value pairs

[0102] i L Coil current

[0103] i ac Phase current

[0104] u L Coil voltage

[0105] u R Rectangular AC voltage

[0106] u ac Filter capacitor voltage

[0107] u R Rectangular AC voltage

[0108] m-edge slope

[0109] L inductor

Claims

1. A method for operating an inverter (1), wherein The method has the following steps: - applying an alternating voltage to a phase line (3) by means of a switching unit (4) of an inverter (1), in which phase line a filter inductor (2) is arranged, - determining eigenvalues for the coil current (i L ) and the coil voltage (u L ) of the filter inductor (2) in the region of a plurality of measurement points, wherein at least two of the measurement points are on at least one edge of the current ripple, - from said characteristic value, for a first value of said coil current (I Ly ), a first value of the inductance of said filter inductor (2) (L(I Ly )), characterized in that the method also has the following steps: - determining at least one further value of the inductance of the filter inductor (2) for at least one further value of the coil current, - determining at least one coil current dependent inductance curve (6) of the filter inductor (2) using the determined first value (L(I Ly ) ) of the inductance of the filter inductor and using the determined at least one further value of the inductance of the filter inductor, wherein the inductance curve (6) describes the dependency of the value (L(I Ly ) ) of the inductance of the filter inductor from the coil current, - adjusting the switching unit (4) of the inverter (1) by means of an adjusting unit (5) for generating an alternating current in the phase line (3), wherein at least one parameter of the adjustment is continuously adapted to the current value of the inductance in accordance with the determined current-dependent inductance curve (6), wherein the inductance curve (6) is determined before the first start of the inverter (1) and / or at regular intervals before or before each start of the inverter (1).

2. The method of claim 1, wherein, The determination of the first value of the filter inductor comprises the following steps: - for a first value of the coil current (I Ly ), a first edge slope (m x ) of the current ripple is determined, - a first value (U x ) of the coil voltage is taken at the first edge slope, - a first value (L(I Ly )) of the filter inductor (2) is determined from a first edge slope (m x ) and a first value (U x ) of the coil voltage according to the following formula L(I) = U(I) / m(I) 。 3. The method of claim 2, wherein, The coil current is determined by measuring at a sampling frequency which is at least twice the clock frequency of the pulse width modulation implemented by the switching unit (4).

4. The method of claim 2, wherein, The coil current is determined by measuring at a sampling frequency which is a multiple of the clock frequency of the pulse width modulation implemented by the switching unit (4).

5. The method according to claim 3 or 4, characterized in that, The first edge slope is determined by forming the current difference between at least two sampling steps and taking into account the sampling frequency.

6. The method according to any one of claims 1 to 4, characterized in that, The slope and / or the curvature of the inductance curve (6) is determined after the inductance curve (6) has been determined.

7. The method of claim 6, wherein, When adjusting the switching unit (4) of the inverter (1) by means of an adjusting unit (5) for generating an alternating current in the phase line (3), at least one parameter of the adjustment is continuously adapted to the current value of the slope and / or the curvature of the inductance curve (6).

8. The method according to any one of claims 1 to 4, characterized in that, At least one further inductance curve of a further filter inductor in a further phase line is determined and taken into account in the adjustment.

9. The method according to any one of claims 1 to 4, characterized in that, The determination of the inductance curve (6) is carried out regularly, irregularly or continuously during operation of the inverter (1).

10. The method according to any one of claims 1 to 4, characterized in that, A plurality of values of the filter inductor is determined in the current ripple and an average value is formed from the plurality of values of the filter inductor, wherein the inductance curve (6) of the filter inductor (2) is determined using the average value.

11. The method according to any one of claims 1 to 4, characterized in that, A first inductance curve is determined from edges having a positive slope and a second inductance curve is determined from edges having a negative slope.

12. The method according to any one of claims 1 to 4, characterized in that, For a plurality of current amplitudes of the alternating current, at least one inductance curve is determined respectively.

13. An inverter (1) having a switching unit (4) with which an alternating voltage can be applied to at least one phase line (3), wherein A filter inductor (2) is arranged in the phase line (3), and the switching unit (4) is regulated by means of a regulating unit (5), wherein the coil current (i L ) of the filter inductor (2) and the coil voltage (u L ) of the filter inductor (2) can be determined by the regulating unit (5), characterized in that the regulating unit (5) or a further computing unit of the inverter is configured to carry out the following steps: - manipulating the switching unit (4) for applying an alternating voltage to a phase line (3) of an inverter (1), - for a first value of the coil current (I y ), a first value (L(I y ) of the filter inductor (2) is determined, - determining at least one further value of the filter inductor for a further value of the coil current, - determining an inductance curve (6) of the filter inductor (2) in relation to the coil current, wherein the inductance curve (6) describes the dependency of the value (L(I Ly )) of the inductance of the filter inductor (2) on the coil current, using the determined first value of the filter inductor and using the determined at least one further value of the filter inductor, and - determining the filter inductor (2) in such a way that the inductance curve (6) of the filter inductor (2) in relation to the coil current is substantially linear. - adjusting the switching unit (4) of the inverter (1) for generating an alternating current in the phase line (3), wherein at least one parameter of the adjustment is continuously adapted to the current coil current in accordance with the determined current-dependent inductance curve, - manipulating the switching unit (4) for applying an alternating voltage to a phase line (3) of an inverter (1), - determining at least one further value of the filter inductor for a further value of the coil current, - adjusting the switching unit (4) of the inverter (1) for generating an alternating current in the phase line (3), wherein at least one parameter of the adjustment is continuously adapted to the current coil current in accordance with the determined current-dependent inductance curve, The regulating unit (5) or a further computing unit of the inverter is configured to determine the inductance curve of the filter inductor (2) before the first start of the inverter (1) and / or at regular intervals before or each start of the inverter (1).

14. The inverter (1) according to claim 13, characterized in that The regulating unit (5) or a further computing unit of the inverter is configured to determine the inductance curve (6) of the filter inductor (2) regularly, irregularly or continuously during operation of the inverter (1).

15. The inverter (1) according to claim 13 or 14, characterized in that The inverter (1) has a plurality of phases (3) each having a filter inductor, wherein for each filter inductor an inductance curve can be determined.

16. The inverter of claim 13 or 14, wherein, The determination of the inductance curve is performed by a dedicated microcontroller, control device or further computing unit outside the regulating unit.

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