METHOD FOR ISOLATION RESISTANCE MEASUREMENT IN MULTI-POINT TOPOLOGY INVERTERS AND MULTI-POINT TOPOLOGY INVERTERS

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to a method for measuring insulation resistance in an inverter and to an inverter for carrying out such a method. In particular, the invention relates to a method with the features of the preamble of independent claim 1 and to an inverter with the features of the preamble of independent claim 9.

[0002] The insulation resistance measurement in the inverter is not only about the insulation resistance of the inverter itself, but also about the insulation resistance of a generator connected to an input-side DC link, in particular a photovoltaic generator. STATE OF THE ART

[0003] From WO 2013 / 178654 A1, a method for measuring insulation resistance and an inverter with a device for measuring insulation resistance are known, which have the features of the preamble of claim 1 and the preamble of claim 8. In the known method and the known inverter, a bridge center located between switching elements of a half-bridge of the inverter is connected to an earthing point by closing an earthing switch. The bridge center connected to the earthing point is successively connected by means of the switching elements of the half-bridge to the two ungrounded poles of an intermediate circuit voltage of the inverter applied to the half-bridge, and the current flowing to the earthing point via this connection is measured.In this way, the voltages of the poles of the intermediate circuit voltage relative to earth are used as test voltages for measuring the insulation resistance and are switched on via the switching elements of the half-bridge.

[0004] A method with the features of the preamble of claim 1 and a device with the features of the preamble of independent claim 9 are also known from US 2012 / 0026631 A1. In this method, the current flowing through the connection to the grounding point is measured with a current sensor. Additionally, the potentials of the ungrounded poles of the intermediate circuit voltage are measured.

[0005] While the switching elements of the inverter known from WO 2013 / 178654 A1 are only subjected to half the DC link voltage during normal operation, this load can rise to the DC link voltage when performing the insulation resistance measurement procedure known from WO 2013 / 178654 A1, even if neither of the two DC link voltage poles is grounded. Therefore, with DC link voltages that exceed the nominal dielectric strength of the switching elements of the bridge circuit and, in particular, of hardware components of the measuring circuit, there is a risk that this voltage limit will be exceeded during the insulation resistance measurement.

[0006] For inverters with higher power ratings, and especially those with higher DC link voltages, bridge circuits with a multi-point topology are known, for example in so-called 3-level inverters. A bridge circuit with a multi-point topology can, for example, reduce circuit losses at high voltages applied across the switching elements. In a 3-level inverter, the bridge circuit connects its bridge center not only alternately to the two endpoints of the DC link, between which the DC link voltage is applied, but also to a voltage center point of the DC link, which is provided by means of a split DC link.

[0007] In practice, 3-level inverters are used, for example, in photovoltaic systems, whose photovoltaic generators charge the DC intermediate circuits of the inverters to up to 1,500 V, whereby switching elements with a nominal voltage rating of, for example, 1000 V are used in the bridge circuit.

[0008] WO 2013 / 178654 A1 discloses a method for measuring the insulation resistance for an inverter. TASK OF INVENTION

[0009] The invention is based on the object of providing a method having the features of the preamble of independent patent claim 1 and a corresponding inverter having the features of the preamble of independent patent claim 9, which allow a reliable insulation resistance measurement even when the DC link is charged to an intermediate circuit voltage that significantly exceeds a nominal dielectric strength of switching elements of its bridge circuit and in particular of hardware components of the measuring circuit, so that the voltage of one of the end points of the ungrounded DC link with respect to earth can also exceed this voltage limit. SOLUTION

[0010] The object of the invention is achieved by a method for measuring insulation resistance with the features of independent claim 1 and by an inverter with the features of independent claim 9. Preferred embodiments of the method and inverter according to the invention are defined in the dependent claims. DESCRIPTION OF THE INVENTION

[0011] In a method for measuring insulation resistance in an inverter comprising a DC link and a bridge circuit connected to the DC link for driving an AC current via a bridge center point, comprising the steps of connecting the bridge center point to an earthing point, connecting the bridge center point connected to the earthing point successively to two voltage-different points of the unearthed DC link by means of the bridge circuit, and measuring the currents flowing from the two voltage-different points to the earthing points, the two voltage-different points of the unearthed DC link are selected from a group of points which, in addition to the two endpoints of the DC link, includes at least one intermediate voltage point of the DC link, such thatthat the electrical voltages between the two points with different voltages and earth do not exceed a specified voltage limit.

[0012] The electrical potentials of the two endpoints of the DC link correspond to the electrical potentials of its two poles, between which the DC link is charged to an intermediate circuit voltage. The intermediate voltage point has an electrical potential between the electrical potentials of the two poles of the DC link and thus also an electrical voltage relative to ground that lies between the voltages of the two endpoints of the DC link relative to ground.

[0013] By selecting the two points with different voltages for the insulation resistance measurement from the group of points that also includes the intermediate voltage point of the DC link, it is possible to avoid stressing the switching elements of the bridge circuit and, in particular, the hardware components of the measuring circuit during the insulation resistance measurement with voltages above the voltage limit. This is achieved when the intermediate voltage exceeds the specified voltage limit and there is a risk that one of the two endpoints of the DC link will have a voltage to ground that also exceeds the voltage limit. This allows the hardware components of the measuring circuit to be designed for a lower voltage and power load than would result from the maximum system voltage of the inverter to ground.This allows for cost savings, robustness advantages, lower power loss and reduced space requirements in the hardware components of the measurement circuit, for example.

[0014] The method according to the invention can therefore be carried out in any inverter whose bridge circuit can connect the bridge center point not only to the two endpoints of the DC link but also to at least one intermediate voltage point of the DC link. This is the case with inverters with a multi-point topology, in particular so-called 3-level inverters, which are frequently used anyway with high intermediate voltages. Measuring devices for detecting the electrical voltages of all points with different voltages from the group of possible points with respect to ground are also present in many inverters or can be retrofitted with minimal effort.

[0015] When selecting the two voltage-different points of the ungrounded DC link from the group of points that also includes at least one intermediate voltage point of the DC link, it can be checked whether either of the two voltages between the two endpoints of the DC link and ground exceeds the specified voltage limit. If this is not the case, the two endpoints of the DC link are selected for the insulation resistance measurement. However, if one of the two endpoints exceeds the voltage limit with its voltage to ground, the other endpoint of the DC link, where the voltage to ground does not exceed the specified voltage limit, and the intermediate voltage point of the DC link are selected as the two voltage-different points.

[0016] Specifically, the selection of the two voltage-different points of the ungrounded DC link can be implemented as follows: First, it is checked whether the voltage between the first of the two endpoints of the DC link and ground exceeds the specified voltage limit. Second, it is checked whether the voltage between the second of the two endpoints of the DC link and ground exceeds the specified voltage limit. Then, based on the first result of the first check and the second result of the second check, a truth table can be used to determine which two voltage-different points of the ungrounded DC link are selected from the group of points. This truth table could look like this: Test results Resulting selection Voltage 1. Endpoint to PE > Limit value Voltage 2nd endpoint to PE > limit value 1. Endpoint 2. Endpoint Intermediate voltage point No No Yes Yes No Yes No No Yes Yes No Yes Yes No Yes

[0017] In the preceding truth table, the entry in the first row—that is, selecting the first and second endpoints as the two points with different voltages—leads to a maximum difference between the two voltages driving the current flowing to the ground point, provided that both voltages of the first and second endpoints do not exceed the specified voltage limit. This is advantageous for accurately measuring the insulation resistance. However, selecting one of the two endpoints and the intermediate voltage point as the two points with different voltages of the ungrounded DC intermediate circuit would also be permissible.

[0018] Alternatively, the selection of the two voltage-different points of the ungrounded DC link can be implemented by first checking whether the DC link voltage between the two endpoints actually exceeds the specified voltage limit. If this is not the case, it can generally be assumed that the two voltages between the two endpoints of the DC link and ground also do not exceed the limit, because while the two voltages between the two endpoints of the DC link and ground do not have to be equal in magnitude, and indeed will not be equal in magnitude, they generally do not have the same sign. Accordingly, the two endpoints of the DC link can be selected as the two voltage-different points.

[0019] In a second step of selecting the two points with different voltages, which is only carried out if the DC link voltage exceeds the specified voltage limit, it can then be checked whether the voltage between one of the two endpoints of the DC link, i.e. between one of the two poles of the DC link, and earth exceeds the given limit.If this is the case, the second of the two end points and the intermediate voltage point of the DC link are selected as the two voltage-differential points of the unearthed DC link because it can be assumed that the two voltages of these two voltage-differential points with respect to earth do not exceed the voltage limit, provided that the intermediate voltage point is a voltage midpoint of the DC link, provided that the DC link voltage is within the permissible range of less than twice the voltage limit and provided that the rule applies that the two voltages between the two end points of the DC link and earth do not have the same sign.

[0020] In a third step of selecting the two voltage-differential points, which is only performed if the voltage between the first of the two endpoints of the DC link and ground does not exceed the specified voltage limit, it can be checked whether the voltage between the second of the two endpoints of the DC link and ground exceeds the voltage limit. If this is not the case, the two endpoints of the DC link can be selected as the two voltage-differential points of the ungrounded DC link, even though the DC link voltage between them exceeds the voltage limit. Otherwise, the first of the two endpoints and the intermediate voltage point of the DC link are selected because no voltage exceeding the voltage limit is present at these points relative to ground.This is true at least if the intermediate voltage point is a voltage center point of the DC link and the intermediate link voltage remains within the permissible range of less than twice the voltage limit. Due to the inverter's topology, the available intermediate voltage point of the DC link will generally be the voltage center point of the DC link anyway.

[0021] It is understood that for each of the aforementioned tests to determine whether a voltage exceeds the specified voltage limit, the respective voltage does not need to be measured directly, but can also be derived from other measured values, for example, from other measured voltages that are linked to the voltage of interest via a voltage loop. Thus, the voltage between the second of the two endpoints of the DC link and ground can be determined by subtracting the voltage between the first of the two endpoints of the DC link and ground.

[0022] Specifically, the specified voltage limit can be between 800 and 1,500 V or between 900 and 1,200 V, i.e., at 1,000 V.

[0023] In addition to its special features described here regarding the selection of the voltage-different points of the ungrounded DC link, the method according to the invention can be carried out with regard to all details as is known from WO 2013 / 178654 A1.

[0024] In particular, when measuring the currents flowing to the earthing point, the bridge center point will regularly be disconnected from an AC network connected during normal operation of the inverter, ie when driving the AC current across the bridge center point.

[0025] The current flowing to the grounding point can be measured by measuring the voltage drop across a measuring resistor connected between the bridge center and the grounding point. Specifically, the voltage drop across the measuring resistor can be measured using a measuring device on the inverter, which, during normal operation, measures the voltage drop across a lower-resistance measuring resistor to measure the alternating current.

[0026] The switching elements of the bridge circuit can be closed sequentially to allow two direct currents to flow successively from the two voltage-differential points of the DC link to the ground point. The insulation resistance can then be calculated in the usual way from the currents of the two direct currents and the corresponding voltages at the voltage-differential points relative to ground.

[0027] Alternatively, the switching elements of the bridge circuit can be controlled to allow a low-frequency alternating current to flow from the two voltage-different points through a line choke of the inverter to the grounding point. This low-frequency alternating current then has a period that is at least twice as long, preferably at least five times longer, more preferably at least ten times longer, even more preferably at least 50 times longer, and most preferably at least 100 times longer than the alternating current flowing during normal operation of the inverter.

[0028] The switching elements of the bridge circuit can also be controlled to set at least one driving voltage at a filter output of a filter downstream of the bridge center for the current flowing through a line choke of the inverter to the grounding point, which lies between the voltages of the two voltage-different points of the DC link with respect to ground.

[0029] The voltage driving the current to the grounding point can be measured using measuring devices that measure the voltages between the two points of the ungrounded DC link and ground when selecting them. Furthermore, to measure the current flowing to the grounding point, a voltage can be determined between a point located between a protective resistor and the grounding point and one of the points of the ungrounded DC link. Specifically, the voltage at this point can be measured relative to the center point of the DC link.

[0030] In an inverter according to the invention, comprising a DC link, a bridge circuit connected to the DC link for driving an alternating current via a bridge center point, and a device for measuring an insulation resistance, which includes an earth switch with which the bridge center point can be connected to an earthing point, and which is configured to successively connect the bridge center point connected to the earthing point to two different points of the unearthed DC link by means of the bridge circuit and to measure the current flowing from the two points with different voltages to the earthing point, the bridge circuit is a multi-point circuit configured to connect the bridge center point, when driving the alternating current, next to the two endpoints of the DC link between which the DC link voltage is applied.also to be connected to at least one intermediate voltage point of the DC link, and the device for measuring the insulation resistance is designed to carry out the method according to the invention.

[0031] The inverter according to the invention is therefore one with a multi-point topology. In particular, it can be a 3-level inverter. In this case, the bridge circuit is a three-point circuit designed to connect the bridge center point, when driving the alternating current, not only to the two endpoints but also to a voltage center point, which is at least one intermediate voltage point of the DC link.

[0032] The nominal dielectric strength of switching elements of the bridge circuit and / or of components of the device specifically designed for measuring insulation resistance can be equal to the specified voltage limit in the inverter according to the invention. However, the maximum DC link voltage to which the DC link is charged during operation of the inverter can significantly exceed the voltage limit, for example by 50%.

[0033] The inverter according to the invention can also have all the features and combinations of features disclosed in WO 2013 / 178654 A1 with regard to its detailed design. For example, the device can be specifically designed to open a mains switch of the inverter in order to disconnect the bridge center point from an AC mains connected during normal operation of the inverter for measuring the current flowing to the grounding point.

[0034] Furthermore, the device can include a measuring resistor arranged between the bridge center and the grounding point and be configured to measure the current flowing to the grounding point as a voltage drop across this measuring resistor. The measuring resistor can be connected in series with the earth switch and a protective resistor to prevent high earth currents between the bridge center and the grounding point.

[0035] A connection from the bridge center to the grounding point can branch off from a current path through which the alternating current flows during normal operation of the inverter, wherein measuring devices of the inverter, which during normal operation of the inverter measure a voltage drop across a further measuring resistor that has a lower resistance than the measuring resistor for the current flowing to the grounding point, can be connected across the measuring resistor and the further measuring resistor so that they can be used to measure both the alternating current and the current to the grounding point.

[0036] The device can also be configured to connect the grounding point to a filter output of a filter located downstream of the bridge center.

[0037] The device may include additional measuring devices to measure the voltage driving the respective current to the grounding point. These measuring devices can simultaneously be used to select the two voltage-differentiated points of the DC link according to the method according to the invention.

[0038] A filter capacitor can be connected in parallel to the measuring resistor for measuring the current flowing to the grounding point.

[0039] Measuring devices for measuring the current flowing to the respective grounding point can be designed to measure a voltage present between a point located between a protective resistor and the grounding point and one of the voltage-different points of the ungrounded DC link. The respective point can be located between the protective resistor and the grounding switch, whereby the protective resistor can be located between the grounding switch and the grounding point. The one of the voltage-different points of the ungrounded DC link, against which the voltage is measured, can be the voltage midpoint of the DC link.

[0040] Furthermore, a filter capacitor can be connected at a filter output between a line choke of the inverter and the protective resistor, between the connection to the grounding point and one of the voltage-differential points of the ungrounded DC link. This voltage-differential point can also be the voltage center point of the ungrounded DC link.

[0041] The invention is as defined in the claims. Advantageous embodiments of the invention are described in the claims, the description, and the drawings. The advantages of features and combinations of features mentioned in the description are merely examples and can have an effect alternatively or cumulatively, without necessarily requiring that the advantages be achieved by embodiments of the invention. Without altering the subject matter of the attached claims, the following applies with regard to the disclosure content of the original application documents and the patent: further features can be seen in the drawings – in particular, the relative arrangement and functional connection of components.The combination of features from different embodiments of the invention or from features of different claims is also possible, deviating from the chosen cross-references of the claims, and is hereby encouraged. This also applies to features that are illustrated in separate drawings or mentioned in their description. These features can also be combined with features from different claims.

[0042] The features mentioned in the claims and the description are to be understood as meaning that exactly that number or a greater number than stated is present, without the need for the explicit use of the adverb "at least." Thus, for example, if a switching element is mentioned, this is to be understood as meaning that exactly one switching element, two switching elements, or more switching elements are present. The features listed in the claims may be supplemented by other features or may be the only features that the respective method or product possesses. The reference numerals contained in the claims do not constitute a limitation of the scope of the subject matter protected by the claims. They serve only to make the claims easier to understand. BRIEF DESCRIPTION OF THE CHARACTERS

[0043] In the following, the invention is further explained and described with reference to preferred embodiments shown in the figures. Fig. 1 is a schematic diagram of an inverter according to the invention with a photovoltaic generator connected to its input-side DC voltage intermediate circuit and Fig. 2 is a flowchart for selecting two voltage-different points of the ungrounded DC link of the inverter according to Fig. 1 for measuring the insulation resistance in the inverter. FIGURE DESCRIPTION

[0044] In Fig. 1An inverter 1 is shown, to whose input-side DC intermediate circuit 2 a photovoltaic generator 3 is connected. The inverter 1 has a bridge circuit 4 connected to the DC intermediate circuit 2, which is designed as a multi-point circuit, specifically as a three-point circuit. A bridge center point 5 can be connected not only to end points 8 and 9 of the DC intermediate circuit 2 using two switching elements 6 and 7, but also, using an additional switching element 10, to an intermediate voltage point 11, specifically a voltage center point 12 between two equally sized intermediate circuit capacitances 13 and 14 of the DC intermediate circuit 2. The DC intermediate circuit 2 is charged to an intermediate circuit voltage by the photovoltaic generator 3.In a photovoltaic generator 3 with a center point that is neither directly nor indirectly grounded, a significant voltage can develop between the voltage center 12 and ground 15, and this voltage can vary considerably during the operation of the inverter 1. Thus, the voltages between the endpoints 8 and 9 of the DC link 2 and ground 15 can reach values ​​that are significantly more than half the DC link voltage.

[0045] The insulation resistance 16 of the photovoltaic generator 3 and the connected inverter 1 to earth 15 must be checked at least before the photovoltaic generator 3 and the inverter 1 are put into operation. Currents flowing from the photovoltaic generator 3 to earth 15 are also affected by parasitic capacitances 17. The inverter 1 has a device 18 for measuring the ohmic insulation resistance 16. The device 18 includes an earth switch 19 to connect the bridge center 5 to an earthing point 21 via a measuring resistor 20.When the bridge center 5 is connected to one of the switching elements 6, 7, or 10 with one of the endpoints 8 and 9 or the voltage center 12 of the DC link 2, a current flows to ground 15 via the measuring resistor 20. Because the DC link 2 is only electrically connected to ground 15 via the insulation resistance 16 of the photovoltaic generator 3, the DC component of this current is determined by this insulation resistance 16. To fully measure the insulation resistance 16, at least two voltages, as different as possible, must be applied to the bridge center 5 using the switching elements 6, 7, and 10. These voltages drive the current to ground 15 through the measuring resistor 20 with different magnitudes and / or directions. The influence of the insulation resistance 16 on the current to ground 15 depends on how the insulation resistance 16 is distributed across the photovoltaic generator 3.At which voltage relative to earth 15 in the photovoltaic generator 3 the insulation resistance 16 has its lowest value. Different voltages that drive the current to the grounding point 21 through the measuring resistor 20 can also be set in the form of an alternating voltage, which should then be low frequency so that the resulting alternating current through the measuring resistor 20 is not dominated by the parasitic capacitances 17 and 18.

[0046] Measuring devices 22 to 24 are provided for measuring the voltage driving the current through the measuring resistor 20. Measuring device 22 measures the DC link voltage across the DC link 2. Measuring device 23 measures the voltage at endpoint 8 of the DC link 2 relative to ground 15. From this, in conjunction with the voltage measured by measuring device 22, the voltage at the second endpoint 9 of the DC link 2 relative to ground 15 can also be determined. Measuring device 24 determines the voltage between a point 25 and the voltage center point 12. Point 25 is located between the measuring resistor 20 and the ground switch 19. With the ground switch 19 closed and the switching element 10 closed, measuring device 24 measures the voltage drop across the measuring resistor 20.With the earth switch 19 closed and the switching element 10 open, the measuring device 24 measures the voltage between the voltage center 12 and earth 15. Using Kirchhoff's voltage law, all other voltages of interest that are present at certain points relative to earth 15 or that drop across the measuring resistor 20 can be determined from the voltages measured by the measuring devices 22 to 24.

[0047] A choke 26 is part of a filter in an output of the inverter 1, through which an alternating current is output during normal operation of the bridge circuit 4. This output would then branch off between the choke 26 and the measuring resistor 20, which in Fig. 1 is not shown.

[0048] To ensure that the voltages driving the current through the measuring resistor 20 to earth 15 do not exceed a predetermined voltage limit for which the components of the inverter 1 are designed, two points of the DC intermediate circuit 2 with different voltages, which are successively connected to the bridge center 5 by the device 18 in order to measure the insulation resistance 16, are connected according to the Fig. 2The illustrated procedure is selected. After a start 27 of the selection process, a comparison 28 of the DC link voltage Uzwk, measured by the measuring device 22 between the endpoints 8 and 9 of the DC link 2, is performed with the predetermined voltage limit of 1,000 V. If the DC link voltage Uzwk is not greater than 1,000 V, the two poles Udc+ and Udc- of the DC link 2 are selected for driving the current through the measuring resistor 20 to ground 15 31, i.e., the device 18 connects the corresponding endpoints 8 and 9 of the DC link 2 successively to the bridge center 5 using the switching elements 6 and 7. If the DC link voltage Uzwk exceeds the predetermined voltage limit of 1,000 V, a further comparison 29 checks whether the voltage DC+ measured by the measuring device 23 is within the specified range relative to PE, i.e.,The voltage between endpoint 8 of the DC link 2 and earth 15 is greater than or equal to the specified voltage limit of 1,000 V. If this is the case, in addition to the potential of the other pole Udc- of the DC link 2 present at the second endpoint 9 of the DC link 2, the potential M at the voltage center 12 is selected for measuring the insulation resistance 16 32. If the voltage measured with the measuring device 23 is less than 1,000 V, a comparison 30 is performed to check whether the voltage PE relative to DC-, i.e., the voltage between earth 15 and the second endpoint 9 of the DC link 2, is greater than or equal to the voltage limit of 1,000 V. This voltage is obtained by subtracting the voltage measured by the measuring device 23 from the DC link voltage measured by the measuring device 22.If the result of comparison 30 is no, despite the intermediate circuit voltage Uzwk exceeding the voltage limit, the two poles Udc+ and Udc- of the DC link 2 are selected to drive the current through the measuring resistor 20 for measuring the insulation resistance 16 31. However, if the result of comparison 30 is positive, in addition to the potential of the positive pole Udc+ of the DC link 2, the potential M at the voltage midpoint 12 relative to ground is selected for measuring the insulation resistance 16 33, because at least neither of these two voltages exceeds the voltage limit of 1,000 V. Overall, by selecting the voltage-differentiated points of the DC link 2 for measuring the insulation resistance according to . Fig. 2This ensures that the two selected points with different voltages differ as much as possible in their voltage relative to earth 15 and, as a rule, also have different signs of this voltage, but without one of these voltages exceeding the voltage limit value. LIST OF REFERENCE SYMBOLS

[0049] 1 Inverter 2 DC link 3 Photovoltaic generator 4 Bridge circuit 5 Bridge center point 6 Switching element 7 Switching element 8 (First) DC link endpoint 2 9 (Second) DC link endpoint 2 10 Switching element 11 DC link point 12 DC link center point 13, 14 DC link partial capacitance 15 Ground 16 Insulation resistance 17 Parasitic capacitance 18 Device 19 Ground switch 20 Measuring resistor 21 Grounding point 22-24 Measuring device 25 Point 26 Choke 27 Start 28-30 Comparison 31-33 Selection

Claims

1. A method for measuring insulation resistance in an inverter (1) that has a DC link circuit (2) and a bridge circuit (4), connected to the DC link circuit (2), for driving an AC current via a bridge center tap (5), having the steps of - connecting the bridge center tap (5) to a grounding point (21), - successively connecting, by way of the bridge circuit (4), the bridge center tap (5) connected to the grounding point (21) to two points of the ungrounded DC link circuit (2) that differ in terms of voltage and - measuring the current flowing from the two points that differ in terms of voltage to the grounding point (21), characterized by a selection of the two points of the ungrounded DC link circuit (2) that differ in terms of voltage are selected from a group of points that comprises at least one intermediate voltage point (11) of the DC link circuit (2) in addition to two end points (8, 9) of the DC link circuit (2), such that the voltages present between the two points that differ in terms of voltage and ground (15) do not exceed a predefined voltage limit value.

2. The method as claimed in claim 1, characterized in that the selection of the two points of the ungrounded DC link circuit (2) that differ in terms of voltage comprises a test whether one of the two voltages present between the two end points (8, 9) of the DC link circuit (2) and ground (15) exceeds the predefined voltage limit value, and in that, if this is the case, the end point (8, 9) of the DC link circuit (2) at which the voltage does not exceed the predefined voltage limit value and the intermediate voltage point (11) of the DC link circuit (2) are selected.

3. The method as claimed in claim 2, characterized in that the test comprises a first test, in which it is checked whether the voltage present between a first of the two end points (8) of the DC link circuit (2) and ground (15) exceeds the predefined voltage limit value, and that the test comprises a second test, in which it is checked whether the voltage present between the second of the two end points (9) of the DC link circuit (2) and ground (15) exceeds the predefined voltage limit value, and in that, based on a first result of the first test and a second result of the second test, a truth table is used to determine which two points of the ungrounded DC link circuit (2) that differ in terms of voltage are selected from the group of points.

4. The method as claimed in claim 2, characterized in that a first sub-step of the selection of the two points of the ungrounded DC link circuit (2) that differ in terms of voltage comprises a third test, in which it is checked whether a link circuit voltage present between the two end points (8, 9) of the DC link circuit (2) exceeds the predefined voltage limit value, and in that, if this is not the case, the two end points (8, 9) of the DC link circuit (2) are selected.

5. The method as claimed in claim 4, characterized in that a second sub-step of the selection of the two points of the ungrounded DC link circuit (2) that differ in terms of voltage, when the link circuit voltage exceeds the predefined voltage limit value comprises a fourth test, in which it is checked whether the voltage present between a first of the two end points (8) of the DC link circuit (2) and ground (15) exceeds the predefined voltage limit value, and in that, if this is the case, the second of the two end points (9) and the at least one intermediate voltage point (11) of the DC link circuit (2) are selected.

6. The method as claimed in claim 5, characterized in that a third sub-step of the selection of the two points of the ungrounded DC link circuit (2) that differ in terms of voltage, when the voltage present between the first of the two end points (8) of the DC link circuit (2) and ground (15) does not exceed the predefined voltage limit value comprises a fifth test, in which it is checked whether the voltage present between the second of the two end points (9) of the DC link circuit (2) and ground (15) exceeds the predefined voltage limit value, and in that, if this is the case, the first of the two end points (8) and the at least one intermediate voltage point (11) of the DC link circuit (2) are selected, whereas, if this is not the case, the two end points (8, 9) of the DC link circuit (2) are selected.

7. The method as claimed in one of the preceding claims, characterized in that the at least one intermediate voltage point (11) is a voltage center tap (12) of the DC link circuit (2).

8. The method as claimed in one of the preceding claims, characterized in that the predefined voltage limit value is between 800 and 1500 V or between 900 and 1200 V.

9. An inverter (1) having - a DC link circuit (2), - a bridge circuit (4), connected to the DC link circuit (2), for driving an AC current through a bridge center tap (5) and - a device (18) for measuring an insulation resistance (16) that has a grounding switch (19) by way of which the bridge center tap (5) is able to be connected to a grounding point (21) and that is designed to successively connect, by way of the bridge circuit (4), the bridge center tap (5) connected to the grounding point (21) to two points of the ungrounded DC link circuit (2) that differ in terms of voltage and to measure the current flowing from the two points that differ in terms of voltage to the grounding point (21), characterized - in that the bridge circuit (4) is a multi-level circuit that is designed, when driving the AC current, to also connect the bridge center tap (5) to at least one intermediate voltage point (11) of the DC link circuit (2) in addition to two end points (8, 9) of the DC link circuit (2), and - in that the device (18) is designed to perform the method as claimed in one of the preceding claims.

10. The inverter (1) as claimed in claim 9, characterized in that the bridge circuit (4) is a three-level circuit that is designed, when driving the AC current, to also connect the bridge center tap (5) to a voltage center tap (12) as the at least one intermediate voltage point (11) of the DC link circuit (2) in addition to the two end points (8, 9).

11. The inverter (1) as claimed in claim 9 or 10, characterized in that a nominal voltage rating of switching elements (6, 7) of the bridge circuit (4) and / or of components of the device (18) that are specifically provided for measuring the insulation resistance (16) is equal to the predefined voltage limit value.