Converter with intermediate circuit center point and method for insulation resistance measurement by means of controlled asymmetry

Abstract

The invention relates to a method for operating a converter (10) which is designed to exchange electrical power between a DC side and an AC side. According to the invention, the converter (10) has a bridge circuit (12) and a split intermediate circuit (14) which is arranged between the DC side and the bridge circuit (12) and which has at least two sub-capacitances. The method comprises:-creating a first asymmetry of the sub-capacitances with respect to one another in order to produce a first potential position of the DC potential of the sub-capacitance of the intermediate circuit (14) with respect to the ground potential,-setting the first potential position of the DC potential of the sub-capacitance of the intermediate circuit (14) to a first setpoint by changing the asymmetry, the first setpoint remains constant for a first duration or is modulated at a frequency at least 100 times less than an AC frequency of the switching power. The invention further relates to the use of the method and to a converter.

Description

Technical Field

[0001] The present application relates to a method for operating a converter, which is configured to exchange electric power between a DC side (DC: direct current, direct current / direct voltage in English) and an AC side (AC: alternating current, alternating current / alternating voltage in English). In addition, the present application relates to the application of this method for insulation resistance measurement and the converter. Background Art

[0002] A converter is a power electronic device that can convert electric power from direct current to alternating current or vice versa from alternating current to direct current. In the case of converting from alternating current to direct current, the converter serves as a rectifier. When operating a DC source attached to the converter on the DC side, it may be desirable and / or necessary for safety reasons to measure the insulation resistance of the DC source, especially when the converter can be attached to an alternating voltage network on the AC side.

[0003] To measure the insulation resistance of a DC source attached to a converter relative to the ground potential, different devices and methods are known. In the present case, a DC source generally refers to an electrical device that can be operated with a direct voltage, and the electrical device can output and / or receive electric power in the form of direct current, that is, can be used as a source and / or as a load. For example, a DC source can be a PV generator, a battery (especially a vehicle battery), a DC bus, an electrolysis device, a fuel cell, etc. The core feature of such a method for insulation resistance measurement is to purposefully shift the potential position of the DC source relative to the ground potential and detect the resulting ground current at different offset values of the potential position. The detection of the ground current can be carried out directly on the DC line between the converter and the DC source on the DC side by means of a current sensor, or can be carried out at the output end of the AC side of the attached converter, or can be carried out indirectly on a suitably arranged resistor by means of a voltage sensor.

[0004] The DC source can be connected to an alternating voltage network through the converter and output electric power to the alternating voltage network or receive electric power from the alternating voltage network. If the converter has electrical isolation between its DC side and its AC side (for example, through a transformer), then the offset of the potential on the DC side can in principle be achieved in a manner independent of the AC side. However, if the converter does not have electrical isolation between its DC side and its AC side, that is, is especially configured as a transformerless converter, then the potentials on the DC side and on the AC side are usually also electrically connected to each other, so that it is not easy to achieve the offset of the potential on the DC side without electrically isolating the converter from the AC network on the AC side.

[0005] In order to well distinguish the ohmic ground current from the capacitive leakage current in terms of measurement technology, it is advantageous to design the potential offset on the DC side as low-frequency as possible or even quasi-steadily, i.e., for example, to raise the potential of the DC side relative to the ground and hold it for a few seconds and then lower it relative to the ground and hold it for a few seconds. Wherein, in the case where the DC voltage of the DC source is, for example, several hundred volts, the change in the potential position of the DC source is respectively, for example, several V to several tens of V.

[0006] It is proposed in WO 2014 079 775A1 to apply a zero-sequence system (Nullsystem) to the AC output voltage provided by the half-bridge of the converter on the phases of the AC network. For the so-called two-level topology (in which the intermediate circuit on the DC side of the converter is implemented bipolar, i.e., has only two voltage levels and in particular does not have an intermediate tap with a connection to the converter bridge), the value of this zero-sequence system directly corresponds to the potential offset of the DC source attached to the converter relative to the ground on the DC side. However, this method cannot be applied to a multilevel converter with a split intermediate circuit, especially when the intermediate tap of the intermediate circuit is briefly switched to the switching node of the half-bridge, because the subsequent application of the zero-sequence system will unevenly load the sub-capacitors of the split intermediate circuit. In addition, this method cannot be applied to a converter that connects the intermediate tap of the split intermediate circuit to the neutral line, because it is essentially impossible to apply the zero-sequence system thereto.

[0007] A circuit component for symmetrizing a split DC voltage intermediate circuit arranged between two DC attachment ends is described in DE102020103839A1. Summary of the Invention

[0008] Task

[0009] The task on which the present application is based is to clarify a method for operating a multilevel converter and a multilevel converter with a split intermediate circuit, wherein the measurement of the insulation resistance on the DC side can be achieved by means of the potential offset on the DC side.

[0010] Solution

[0011] The task is solved by a method having the features of claim 1, the application of the method according to claim 11, and a converter having the features of claim 12. Embodiments are given in the dependent claims.

[0012] Description

[0013] The converter is set up to exchange electrical power between the DC side and the AC side. The converter has a bridge circuit and a split intermediate circuit, which is arranged between the DC side and the bridge circuit and has at least two sub-capacitors. The bridge circuit has semiconductor switches and can convert electrical DC power into AC power and / or vice versa. The method for operating the converter includes:

[0014] - creating a first asymmetry of the sub-capacitors relative to each other to generate a first potential position of the DC potential of the sub-capacitors of the intermediate circuit relative to the ground potential, and

[0015] - setting the first potential position of the DC potential of the sub-capacitors of the intermediate circuit to a first setpoint (Sollwert) by changing the asymmetry, where the first setpoint is constant within a first period of time or is modulated at a frequency that is at least 100 times lower than the AC frequency of the exchanged power.

[0016] The resulting asymmetry causes a shift in the potential of the DC attachment end of the converter relative to ground and thus mainly causes a change in the ground current, which can be used to measure the insulation resistance of the DC source relative to ground. By setting the first potential position to the first setpoint, a quasi-steady state is set up to ensure correct measurement with regard to the capacitors involved. Here, it is advantageous to keep the voltage of the sub-capacitors still greater than the network peak voltage (Netzkuppenspannung) of the AC network that may be attached to the AC side of the converter, so that the generation of the AC voltage and the exchange of electrical power with the AC network can still be achieved in the desired form and quality through the converter.

[0017] By modulating the first setpoint at a low frequency, sufficient time is provided to measure the value characterizing the insulation resistance of the converter and / or of the system in which the converter is located. In particular, the ground current can be measured and thus the insulation resistance relative to ground can be measured.

[0018] The intermediate circuit having two sub-capacitors has a center point, which can be connected to the ground potential, in particular in such a way that the center point is led out of the appliance as the neutral line of the converter and connected to the AC network. One sub-capacitor is arranged between the center point and one DC attachment end on the DC side. The other sub-capacitor is arranged between the center point and the other DC attachment end on the DC side. In the case of an intermediate circuit having more than two series-connected sub-capacitors, the total capacitance of the intermediate circuit between the two DC attachment ends is divided among the sub-capacitors accordingly. An intermediate circuit having more than two sub-capacitors may have more intermediate taps between its two DC attachment ends and the center point, and the center point can be connected to the ground potential. In an intermediate circuit having a center point connected to the ground potential, half of the sub-capacitors are connected in series between the center point and one of the DC attachment ends, and the other half of the sub-capacitors are connected in series between the center point and the other DC attachment end. Accordingly, such an intermediate circuit - if it is symmetric - has a DC attachment end with a positive voltage relative to the ground potential and a DC attachment end with a correspondingly equal negative voltage relative to the ground potential.

[0019] The DC attachment ends of the intermediate circuit can be connected to the DC attachment ends of the DC side of the converter and can thus be connected to a DC source attached to the converter on the DC side. Here, one of the two attachment ends on the DC side of the converter or both attachment ends on the DC side of the converter can be directly or via a DC / DC converter connected to the corresponding DC attachment ends of the intermediate circuit such that there is a fixed potential reference between the attachment end at the DC input and the attachment end of the intermediate circuit. If the split intermediate circuit has a center point between its DC attachment ends and the center point is connected to the ground potential, then in the case of a symmetric intermediate circuit, the two DC attachment ends are at corresponding potentials that are symmetric with respect to the ground potential, one having a positive sign and one having a negative sign.

[0020] By the method described, it is possible to create an offset in the potential position of the DC attachment ends of the intermediate circuit by means of the asymmetry between the sub-capacitors of the intermediate circuit. If the two sub-capacitors of the intermediate circuit are charged asymmetrically and if the potential position of the center point is kept at the ground potential, then the total potential of the intermediate circuit is offset relative to the ground potential, and the potentials of the two DC attachment ends of the intermediate circuit are also offset relative to an intermediate circuit having sub-capacitors charged symmetrically. In the case of more than two sub-capacitors, the corresponding situation applies. Here, due to the asymmetry of the intermediate circuit, the total potential of the intermediate circuit and thus also the potentials of the DC attachment ends of the intermediate circuit are offset relative to the ground potential.

[0021] Shift the split intermediate circuit to a first potential position by adjusting the first asymmetry of the intermediate circuit. By changing the asymmetry, the first potential position is adjusted to a first setpoint, and the first potential position is either kept constant by maintaining the first setpoint for a first duration or modulated at a frequency. Here, the frequency is small, at least 100 times smaller than the AC frequency on the AC side of the converter.

[0022] In one embodiment, the method further has:

[0023] - Create a second asymmetry or symmetry of the sub-capacitors relative to each other to generate a second potential position of the DC potential of the sub-capacitors of the intermediate circuit relative to the ground potential.

[0024] Here, the second asymmetry of the intermediate circuit can have a sign opposite to the first asymmetry and the second potential position can be adjusted to a second setpoint within a second duration. Alternatively to the second asymmetry, the symmetry of the intermediate circuit can be adjusted within the second duration by setting the asymmetry setpoint to zero.

[0025] By such targeted generation of the first and / or second potential positions of the split intermediate circuit, targeted measurements can be carried out, in particular current measurements and / or voltage measurements on the converter, which measurements are suitable for determining the insulation resistance by the calculation (Verrechnung) of the adjusted voltage and the measured current, which is known per se.

[0026] In one embodiment of the method with modulation of the first potential position, the modulated first setpoint for the potential position is adjusted by modulation of an asymmetry with alternating signs. For example, the asymmetry can have different half-waves with different signs. Here, different forms of modulation of the first setpoint can be considered, such as a sinusoidal form.

[0027] Thus, the method enables the creation of a potential offset on the DC side of the converter and thus of the DC source, solely or mainly by the quasi-steady-state asymmetry of the split intermediate circuit of the converter, which potential offset can be used to determine the insulation resistance. For this purpose, the split intermediate circuit is deliberately tilted (schiefgezogen) into an asymmetric state and the thus adjusted potential position is maintained quasi-steadily, i.e., the potential position is kept constant for at least approximately one second or several seconds or modulated at a very low frequency (i.e., at a frequency significantly lower than the network frequency, in particular less than 1 Hz), such that the time curve of the potential position has, in particular at the extrema of the modulation function used, sufficiently long time segments with quasi-steady-state values.

[0028] For better detection of the complex insulation resistance, i.e., the insulation resistance composed of a network of resistors, capacitors, and inductors and thus having different real and imaginary parts for different frequencies, the setpoint for the potential position is preferably modulated sinusoidally. Particularly preferably, the setpoint for the potential position can be modulated successively with sine functions of different frequencies in order to determine not only the real and imaginary parts of the insulation resistance at a determined frequency but also the underlying component values of the resistor, capacitor, and inductor. Alternatively, when modulating the setpoint and thus the potential position, sine functions of different frequencies can also be superimposed.

[0029] This method can be achieved by means of symmetry adjustment of the sub-capacitors of the intermediate circuit, in such a way that the setpoint for symmetry adjustment is set to a desired first or second setpoint, the so-called asymmetry setpoint, in a way different from the common value of zero ( = symmetric intermediate circuit), and this asymmetry setpoint is, for example, 50 volts.

[0030] For example, the first and / or second asymmetry can be created by directly redistributing charges within a split intermediate circuit, and the first and / or second asymmetry can be changed in order to set the first and / or second potential position to a setpoint. This can be achieved, for example, by means of a symmetrizer circuit, in particular by means of a DC / DC converter attached to the intermediate circuit. Here, the symmetrizer circuit transfers charges between the sub-capacitors of the intermediate circuit.

[0031] In the case of creating an asymmetry by directly redistributing charges, the potential position of the attachment end of the intermediate circuit is determined only by the asymmetry of the intermediate circuit. In this case, instead of adjusting the potential position to a setpoint, the asymmetry can also be directly adjusted to an equivalent setpoint, i.e., the setpoint for the asymmetry is pre-given and modulated in time such that the potential position is adjusted accordingly.

[0032] Generally, a symmetrizer circuit suitable for direct redistribution already exists in the converter. In addition to the original function of making the intermediate circuit symmetric, according to the present application, this symmetrizer circuit can also be used to deliberately create an asymmetry of the intermediate circuit. In particular, here, the asymmetry is set by pre-giving a first or second setpoint different from zero for the symmetrizer circuit. In particular, the first or second setpoint can also be modulated. By this modulation, the setpoint can adopt different values in its time profile.

[0033] For example, such a method can be implemented by means of a converter with an intermediate circuit balancer circuit by setting the setpoint for the balancer circuit to a value different from zero. Here, the center point of the intermediate circuit can be connected to the N wire and is thus held at ground potential, such that depending on the set asymmetry, the potential of the DC connection terminals of the intermediate circuit and thus of the DC source is offset with respect to the situation with an intermediate circuit charged symmetrically. Here, the balancer circuit extracts a current from one intermediate circuit half and supplies another current to the other intermediate circuit half. These two currents are superimposed on the currents exchanged via the DC side and / or the converter bridge starting from the intermediate circuit to form a total current, which respectively, in a time-integrated manner and by dividing by the intermediate circuit capacitance, gives the voltage of the respective intermediate circuit half. By changing the current of the balancer circuit, the total current can be influenced and thus the asymmetry of the intermediate circuit halves can be maintained or changed.

[0034] Alternatively or additionally, the asymmetry of the intermediate circuit and the first and / or second potential positions of the DC potential can be generated or maintained by means of interaction with the AC side of the converter.

[0035] In the case of a multilevel converter in which the center point of the intermediate circuit is temporarily directly connected to the switching node of a half-bridge, the application of the zero-sequence system leads to an uneven loading of the intermediate circuit halves and thus, in a time-accumulated manner, to an asymmetry of the intermediate circuit.

[0036] In one embodiment of the method, the first and / or second asymmetry is initially created by means of a bridge circuit in such a way that the bridge circuit generates an initialization zero-sequence system voltage, wherein the initialization zero-sequence system voltage has a curve obtained according to regulation and creates an initial asymmetry of the intermediate circuit. Then, the desired potential position is set by means of a stabilizing zero-sequence system voltage, which generates the desired potential position in a manner coordinated with the initial asymmetry caused by the initialization zero-sequence system voltage and also has a curve obtained according to regulation. Specifically, for this purpose, a regulator can be used, which adjusts the setpoint for the potential position in such a way that the regulator changes the zero-sequence system voltage as the setting parameter. Then, the first and / or second potential position is directly set to the first or second setpoint on the regulator. Here, the initialization zero-sequence system voltage and the stabilizing zero-sequence system voltage have different signs. Here, the converter bridge can generate the initialization zero-sequence system and / or the stabilizing zero-sequence system in such a way that the converter bridge generates a constant voltage offset relative to the ground potential on all phase lines and possibly on the N line. If a specific converter with a separate intermediate circuit already has a symmetric regulator, which can create a suitable asymmetric loading of the intermediate circuit half by means of a zero-sequence system applied by a single voltage provided by a bridge branch to reduce the asymmetry, then the symmetric regulator can be superimposed with the zero-sequence system according to the present application in such a way that, instead of the common zero setpoint, a finite value is obtained for the setpoint of the symmetric regulator. Since the zero-sequence system itself is a disturbance parameter for such a superimposed regulation, the zero-sequence system needs to be adjusted, and simply adjusting the asymmetry is not sufficient.

[0037] Alternatively or additionally, the initial asymmetry can also be created by initially loading the intermediate circuit asymmetrically, i.e., by exchanging different electrical powers with the sub-capacitors. However, especially in the case of supplying electrical power from a DC source to the intermediate circuit and / or in the case of extracting electrical power from the intermediate circuit for feeding into an AC network, the initial asymmetry of the intermediate circuit created either by the initialization zero-sequence system or by the asymmetric loading will be strengthened by the converter itself and thus stabilized by the stabilizing zero-sequence system in order to either keep the desired asymmetry constant for a first period of time or modulate the desired asymmetry at a low frequency.

[0038] In rectifier operation, i.e., when transferring electrical power from the AC side of the converter to the DC side, the initially created asymmetry of the intermediate circuit decreases during operation. Therefore, in rectifier operation, it is also necessary to stabilize the desired asymmetry.

[0039] Preferably, the method in the case of using the zero-sequence system voltage is applied in a converter in which the center point of the intermediate circuit does not have a connection to the N wire of the AC network that can be attached to the AC side, and in this regard there is no fixed grounding reference for the center point of the intermediate circuit.

[0040] In one embodiment of the method, the zero-sequence system voltage is respectively modulated at the following frequencies to set the first potential position to a modulated set value: the frequency is at least a factor of 100 less than the frequency of the AC exchange power.

[0041] In an alternative embodiment, especially if a calibrated equilibrium state is achieved by the power flowing into the intermediate circuit asymmetrically and the power extracted from the intermediate circuit asymmetrically, the zero-sequence system voltage can also be temporarily and especially within the first and / or second durations largely constant to set the first and / or second constant potential positions. In the current case, "largely constant" is understood to mean that the value is constant except for the fluctuations used to adjust the first or second potential position. "Largely constant" can also be understood to mean that the zero-sequence system voltage does not have a sign change at least during the first and / or second durations. This facilitates the corresponding partial measurements in the case of a determined quasi-steady-state potential position.

[0042] For example, such an embodiment of the method can also be implemented by a converter without an intermediate circuit symmetry circuit in such a way that the initial asymmetry is stabilized by generating a stabilizing zero-sequence system, so that a first or second potential position pre-given by a set value appears. Here, a separate half-bridge can be attached to each output terminal L1, L2, L3 or N of the converter. In addition, the asymmetry can be "initialized" by generating an initializing zero-sequence system, where the stabilizing zero-sequence system for stabilizing the potential position - at least in inverter operation - has a sign opposite to that of the initializing zero-sequence system for initializing the asymmetry.

[0043] In one embodiment, the first and / or second duration includes at least one second, especially several seconds.

[0044] In one embodiment, the setting of the first and / or second potential position is repeated periodically, especially at a low frequency. In the current case, low frequency means that the frequency is at least 100 times less than the AC frequency of the exchange power of the converter.

[0045] The method can be used to measure the insulation resistance of a DC source attached to a converter on the DC side. For this purpose, first, the first potential position of the intermediate circuit is set, and the first grounding current is set at a time point within the first duration or in the first half-wave. Then, the second potential position is set and the second grounding current is detected within the second duration, or the second grounding current is detected at a time point in the second half-wave of the modulated first potential position. Then, the insulation resistance of the DC side of the converter is obtained from the set potential positions and the measured grounding currents by means of a calculation operation known per se.

[0046] By means of this method, the following DC attachment ends of the intermediate circuit are brought to their respective first or second potential positions: the DC attachment ends correspond to the DC attachment ends of the DC side of the converter. The DC attachment ends can be connected to a DC source. Then, if the DC source is connected to the converter on the DC side, the potential position of the DC attachment end corresponds to the potential position of the DC attachment end of the DC source. Therefore, during the conversion of electrical power between the AC side and the DC side of the converter, for example, in order to feed the electrical power of the attached PV generator into the attached AC network, the method enables continuous determination of the insulation resistance of the DC source, which may be a DC energy source, in particular a PV generator on the DC side of the converter. Thereby, the safety of the PV installation during operation can be improved.

[0047] The converter is configured to convert electrical power between the DC side and the AC side. The converter has a bridge circuit and a separate intermediate circuit arranged between the DC side and the bridge circuit, the separate intermediate circuit having at least two sub-capacitors. The converter is configured to generate a first potential position of the DC potential of the sub-capacitors of the intermediate circuit relative to the ground potential. This first potential position is generated by creating a first asymmetry between the sub-capacitors relative to each other. The converter is further configured to set the first potential position to a first set value. The first set value is kept constant for a first duration or is modulated at a low frequency. Low frequency means a frequency that is at least 100 times smaller than the AC frequency at which the converter exchanges power on the AC side.

[0048] The converter can further be configured to generate a second potential position of the DC potential of the sub-capacitors of the intermediate circuit relative to the ground potential by creating a second asymmetry or symmetry between the sub-capacitors relative to each other, the second asymmetry having a sign opposite to that of the first asymmetry. The converter can further be configured to set the second potential position to a second set value within a second duration, or to set the symmetry of the intermediate circuit to an asymmetry set value of zero within the second duration.

[0049] In one embodiment, the converter is configured to set the potential position to a modulated first setpoint by means of a modulation with an asymmetry having alternating signs in different half - waves.

[0050] In one embodiment, the converter has a device for redistributing charge in a split intermediate circuit. For example, the device for redistributing charge can be configured as a balancer circuit and can be arranged between the sub - capacitors of the intermediate circuit. For example, the balancer circuit is configured as a DC / DC converter for transferring charge between the sub - capacitors of the intermediate circuit.

[0051] Thus, an asymmetry can be generated by means of a device on the DC side of the converter, and the potential position of the DC potential of the sub - capacitors of the intermediate circuit relative to the ground potential can be set according to a setpoint by exchanging charge, in particular by means of a balancer circuit, between the intermediate - circuit halves. It is possible that the center point of the intermediate circuit has a fixed reference with respect to the ground potential, for example by being directly or indirectly connected to the neutral line of the AC network attached to the AC side. In the case where there is such a fixed potential reference between the center point of the intermediate circuit and the N - line of the AC network, especially in the case of such a direct connection, it is particularly advantageous to use the intermediate - circuit balancer circuit for creating the asymmetry of the intermediate circuit and for setting the required potential position.

[0052] In one embodiment, it is provided that the zero - line or neutral line (N - line) of the AC network that can be attached to the AC side is connected via the converter bridge circuit to the center point between the first and second sub - capacitors of the intermediate circuit.

[0053] In one embodiment, the converter bridge circuit is configured to generate an initializing zero - sequence system voltage that offsets the potential position of the center point between the first and second sub - capacitors of the intermediate circuit relative to the ground potential and asymmetrically loads the intermediate circuit. By means of such an initializing zero - sequence system voltage, an initial asymmetry of the intermediate circuit can be created, but this initial asymmetry will always continue to increase, especially if the sign of the initializing zero - sequence system remains unchanged.

[0054] Thus, preferably, the bridge circuit is configured to generate a stabilized zero-sequence system voltage, which also offsets the potential position of the center point of the intermediate circuit relative to the ground potential and asymmetrically loads the intermediate circuit, but with a sign opposite to that of the initialized zero-sequence system. By means of the stabilized zero-sequence system voltage, the first and / or second asymmetry of the intermediate circuit can be influenced, and in particular the potential position can be set to the required setpoint and stabilized. Here, the initialized zero-sequence system voltage and the stabilized zero-sequence system voltage have different signs, which results in the stabilization of the asymmetry of the intermediate circuit during the inverter operation of the converter.

[0055] In the case of a converter having more than two sub-capacitors in series, i.e., in the case of a converter having more than three DC potentials in the intermediate circuit, for example, in the case of a five-level converter having a four-part intermediate circuit with five voltage taps, the method can be implemented accordingly, either by having the existing symmetrizer circuit for generating asymmetry and for setting the potential position act on more than two sub-capacitors of the intermediate circuit or by generating a zero-sequence system.

[0056] The described method and the described converter offer the advantage that embodiments with at least charge transfer between the sub-capacitors can also be applied to converters in which the center point of the intermediate circuit is connected to the N wire of the AC network. The method enables the continuous determination and thus permanent monitoring of the insulation resistance of the DC source attached to the converter. In particular, in the case of a transformerless converter, this monitoring can be implemented in a simple, cost-effective, and reliable manner. Embodiments without charge transfer between the sub-capacitors of the intermediate circuit have the additional advantage that no special symmetrizer circuit is required. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Hereinafter, the present application will be further elaborated and described based on the embodiments shown in the drawings.

[0058] Figure 1 A method for operating a converter is shown,

[0059] Figure 2 An embodiment of a converter with an attached DC source and an attached AC network is schematically shown,

[0060] Figure 3 Another embodiment of a converter with an attached DC source and an attached AC network is schematically shown,

[0061] Figure 4 An equivalent circuit diagram of an embodiment of a split intermediate circuit with a current source is schematically shown, and

[0062] Figure 5 Shows an exemplary time curve of the voltage on the DC side of the converter and an exemplary time curve of the zero-sequence system voltage when applying the method according to the present application.

[0063] In the drawings, the same or similar elements are labeled with the same reference numerals. The illustrations in the drawings may be not to scale. Detailed description of the invention

[0064] Figure 1 Schematically shows a method for operating a converter 10 (see Figure 2 and Figure 3 ), as it can be applied in a method for measuring insulation resistance. The converter 10 is configured to exchange electrical power between the DC side and the AC side. The converter 10 has a bridge circuit 12 and a split intermediate circuit 14, which is arranged between the DC side and the bridge circuit 12 and has at least two sub-capacitors.

[0065] In S1, by creating a first asymmetry of the sub-capacitors relative to each other, a first potential position of the DC potential of the sub-capacitors of the intermediate circuit 14 relative to the ground potential is generated. This can be achieved, for example, by charge transfer between the sub-capacitors or by adjusting the zero-sequence system voltage set on the AC side of the converter 10 (see Figure 2 , Figure 3 ).

[0066] The split intermediate circuit has a center point M, and the respective sub-capacitors are arranged between the center point M of the intermediate circuit and the respective DC attachment ends. Here, the DC attachment ends of the intermediate circuit can correspond to the DC attachment ends of the DC side of the converter 10. Here, in the case of a DC source 18 attached to the DC side, such as a photovoltaic generator, each DC potential of the split intermediate circuit corresponds to the potential at the DC attachment ends of the DC side of the converter 10 and thus of the DC source 18. Now, if the center point M is kept at the ground potential, the asymmetry of the intermediate circuit 14, i.e., the different charges and thus different voltages of the sub-capacitors of the split intermediate circuit 14, results in an offset of the DC potential of the DC attachment ends of the intermediate circuit 14 relative to the ground potential.

[0067] In S2, the first potential position is adjusted to a first set value. Here, the first set value is kept constant for a first duration or is modulated at a frequency that is at least 100 times smaller than the AC frequency of the exchange power of the converter 10.

[0068] In the case of a separated intermediate circuit 14 that is asymmetrically charged, where the midpoint M is connected to or held at ground potential, the voltage distribution between the midpoint M of the intermediate circuit 14 and the DC attachment end is asymmetric. This means that the midpoint M is no longer located at the center of the two DC potentials of the DC attachment end of the intermediate circuit. Hence, the term "asymmetry of the intermediate circuit 14".

[0069] After setting the first potential position in S2, in S3, the first ground current is detected at a time point within the first duration while maintaining the first set value or in the first half-wave in the case of a modulated first set value.

[0070] In S4, by creating a second asymmetry of the intermediate circuit, i.e., by creating an asymmetry of the sub-capacitors relative to each other, a second potential position of the DC potentials of the sub-capacitors of the intermediate circuit 14 relative to ground potential is generated. Alternatively, in S4, the symmetry of the intermediate circuit is created. Both can be achieved, for example, by charge transfer between intermediate circuit sections or by setting the zero-sequence system voltage on the AC side of the converter 10 (see Figure 2 , Figure 3 ).

[0071] In S5, the second potential position is set to a second set value having a sign opposite to that of the first set value within the second duration, or the symmetry of the intermediate circuit is set to a second set value, which is zero, within the second duration.

[0072] After setting the second potential position in S5, in S6, the ground current is detected again within the second duration. Alternatively, after detecting the first ground current at a time point in the first half-wave of the modulated first set value in (S3), then in S6, the second ground current is detected at a time point in the second half-wave of the modulated first set value. That is, for the modulated first set value of the asymmetry of the intermediate circuit 14, the separate setting of the second potential position in steps S4 and S5 can be omitted, and S6 can be directly implemented after S3.

[0073] Then, after S6, in S7, the insulation resistance is calculated from the set potential positions and the detected ground current.

[0074] In an embodiment, it may be possible to repeatedly implement the method for measuring the insulation resistance during normal operation of the converter 10. This is shown by the dashed arrow between S7 and S1 in Figure 1 .

[0075] In Figure 2Converter 10 is schematically shown, which has a DC source 18 attached to the DC side and an AC network 16 attached to the AC side. The AC network 16 is configured as a three-phase alternating voltage network with a neutral line or neutral wire N, and has three alternating voltage phases L1, L2, and L3. The converter 10 has a bridge circuit 12, which causes the conversion from direct current to alternating current and / or vice versa. For this purpose, preferably, the bridge circuit has rhythmically controlled semiconductor switches. In Figure 2 it, the bridge circuit is shown as an equivalent circuit diagram, and the bridge circuit has phase-related equivalent current sources, Q.N, Q.1, Q.2, and Q.3. The electrical characteristics of the rhythmically controlled bridge circuit are simulated in a suitable manner by this equivalent circuit diagram. In the example, each phase L1, L2, L3, N is assigned an equivalent current source. The neutral line N is assigned an equivalent current source Q.N. The phase line L1 is assigned an equivalent current source Q.1, the phase line L2 is assigned an equivalent current source Q.2, and the phase line L3 is assigned an equivalent current source Q.3. In one embodiment, each of the equivalent current sources Q.N, Q.1, Q.2, and Q.3 can have a bridge circuit, in particular a half-bridge with at least two rhythmically controlled semiconductor switches.

[0076] In addition, the converter 10 has a split intermediate circuit 14. The split intermediate circuit 14 has two sub-capacitors and a center point M. Optionally, the center point M can be connected to the neutral line N of the AC network 16 on the AC side ( Figure 2 the dashed line in). Thus, the center point M can be held at the ground potential.

[0077] A DC / DC converter 20 is arranged between the two sub-capacitors of the intermediate circuit 14. The DC / DC converter is set up to transfer charge between the two sub-capacitors. Therefore, the DC / DC converter 20 can be used as a balancer between the two sub-capacitors, that is, if desired, the sub-capacitors can be adjusted to have as identical charges as possible.

[0078] Through the DC / DC converter 20, the charge can also be offset between the sub-capacitors to create an asymmetry in the intermediate circuit. Asymmetry means that the halves of the intermediate circuit 14 have different charges respectively. Therefore, in the case of an asymmetric intermediate circuit, the voltages of the sub-capacitors are different. Therefore, the DC / DC converter 20 can not only be used as a balancer circuit, but also as an unbalancer circuit, and can create an asymmetry in the intermediate circuit through the charge transfer between the sub-capacitors of the intermediate circuit, and can adjust the required potential positions. In particular, the DC / DC converter 20 can adjust the potential position to a first and / or second set value. By suitable manipulation of the DC / DC converter, the first set value can also be modulated.

[0079] The asymmetry that has occurred is automatically strengthened (inverter operation, power transfer from DC to AC) or reduced (rectifier operation, power transfer from AC to DC) depending on the operating mode of the converter 10, and should therefore be maintained by the symmetrizer circuit (asymmetrizer circuit) such that the potential position is held at the desired value. To this end, a first or second setpoint different from the zero value of the intermediate circuit for symmetry can be set to the desired asymmetry setpoint for the symmetrizer circuit, for example, set to 50 volts when the given total voltage of the intermediate circuit is 1000V, for example.

[0080] The insulation resistance of the attached DC source 18 can be determined by correspondingly detecting the ground current in the case of the first potential position, in the case of the second potential position, and / or in the case of the symmetry of the intermediate circuit.

[0081] In Figure 3 Another embodiment of the converter 10 is shown. In the example, the intermediate circuit 14 of the converter 10 does not have a symmetrizer circuit. In accordance with Figure 3 the example of the separate intermediate circuit 14, the center point is not connected to the neutral line N of the AC network 16 and, in this respect, does not have a fixed ground reference. In Figure 3 the converter 10 is also connected to the DC source 18 on the DC side and to the AC network 16 on the AC side. For example, the DC source 18 is configured as a photovoltaic generator. For example, the AC network 16 is a three-phase AC voltage network with a neutral line N. The first phase of the AC voltage network is labeled L1, the second phase is labeled L2, and the third phase is labeled L3.

[0082] The bridge circuit 12 is shown as an electrical equivalent circuit diagram with sources Q.1, Q.2, Q.3, and Q.N and is set up to construct a zero-sequence system 30 in addition to the common phase voltage or outer conductor voltage. In Figure 3In the example shown, the zero-sequence system is shown as a voltage source 30 in the electrical equivalent circuit diagram. By setting the initialized zero-sequence system voltage, the intermediate circuit 14 can be loaded asymmetrically, and thus the intermediate circuit can be brought into an asymmetric state. If the converter 10 is operated during inverter operation, i.e., if the converter is operated in the following operating mode: in this operating mode, electrical power is transferred from the DC side (e.g., DC source 18) to the AC side (e.g., AC network 16), then the asymmetry generated, for example, by the zero-sequence system 30 is self-reinforcing during operation, especially by unevenly distributing the DC power flowing from the DC source into the intermediate circuit onto the sub-capacitors. That is, after setting the initialized zero-sequence system voltage, in a second step, a stabilizing zero-sequence system voltage is generated, which has a sign opposite to that of the initialized zero-sequence system voltage. Thereby, the intermediate circuit is loaded asymmetrically in a manner opposite to the power flowing asymmetrically into the DC side, and the asymmetry of the intermediate circuit is stabilized such that the potential position is set to a first set value. Then, by setting another initialized and subsequently stabilized zero-sequence system voltage, the second potential position can be set accordingly. Alternatively or additionally, the first asymmetry of the intermediate circuit 14 and thus the potential position can be modulated at a low frequency in time, especially by corresponding modulation of the zero-sequence system.

[0083] Therefore, the asymmetry generated respectively by the initialized zero-sequence system 30 can be stabilized by the generation of the stabilizing zero-sequence system 30 on the AC side, and the potential position can be set thereby. The asymmetry in the intermediate circuit 14 causes the incoming DC power to be unevenly distributed onto the sub-capacitors. Therefore, the stabilizing zero-sequence system 30 is generated and adjusted such that the uneven power inflow into the sub-capacitors (when using the DC source 18, e.g., a photovoltaic generator) is mirror-compensated by the uneven power extraction from the intermediate circuit half, such that the asymmetry and thus the potential position remain constant. This can also be referred to as an asymmetry regulator.

[0084] It should be noted that the zero-sequence system 30 can only be generated when the center point M is not directly connected to the N wire N.

[0085] In Figure 4Two equivalent circuit diagrams of an intermediate circuit 14 with an attached three-level half-bridge are shown in . On the left, a possible switch (here an IGBT) is shown by way of example, with the help of which the switching node on the left side of the choke can be connected to the three taps of the intermediate circuit. By suitable control of the semiconductor switches of the bridge circuit, the following characteristics of the bridge circuit can be generated: this characteristic corresponds to the characteristics of the voltage sources Q, Q.1, Q.2, Q.3, QN on the conductors of the AC network on the output side. Here, the voltage value of the voltage source is obtained by multiplying the voltage of the intermediate circuit half by the duty cycle of the associated switch. The loading of the intermediate circuit half can be shown by a current source, the value of which is obtained by multiplying the choke current by the duty cycle of the switch associated with the intermediate circuit half. By correspondingly controlling the bridge circuit 12, a zero-sequence system voltage 30 can also be generated. In Figure 4 In FIG. 1 , the schematic diagrams in the left and right halves are equivalent and respectively show electrical equivalent circuit diagrams of the characteristics of the bridge branches of the bridge circuit 12 .

[0086] The generation of the zero-sequence system 30 results in an undesired shift of the potential of the center point M relative to the ground potential, which offsets the potential shift induced by the asymmetry. Therefore, the resulting potential shift on the DC side is often smaller than the potential shift caused by the asymmetry of the intermediate circuit alone. That is, the initial asymmetry caused by the zero-sequence system feed 30 for initialization should be greater than the potential shift of the center point M induced by the zero-sequence system feed 30 for stabilization, which is necessary for stabilizing the asymmetry.

[0087] An electrical three-phase system can be described by so-called phasors. In this case, the phasors are divided into a symmetrical positive sequence system, whose pointer moves with the rotating field, a negative sequence system with a reverse rotating field, and a zero sequence system. In the zero sequence system, the phasors of different phases have the same direction and the same length. The zero sequence system occurs in asymmetrical three-phase systems.

[0088] For example, refer to Figure 3 , Figure 4 The described embodiment can be used for a converter 10 in which the outlay for a symmetrizer circuit 20 is too great and in which all output attachment terminals, the phase lines L1, L2, L3 and the neutral line N are each provided via their own half-bridge of a bridge circuit 12, so that the desired zero-sequence system can be fed into the conductors of the AC network.

[0089] Figure 5Exemplarily shown is the time profile of the potential position of the DC potential of the sub - capacitors of the intermediate circuit relative to the ground potential when performing the method for measuring the insulation resistance by means of the zero - sequence system voltage according to the present application. Here, the profile of the negative DC potential of the intermediate circuit is labeled with reference numeral 51, and the profile of the positive DC potential is labeled with reference numeral 52. Additionally, the profile of the virtual center potential of the intermediate circuit, i.e., the profile of the arithmetic mean between the DC potentials 51 and 52, is labeled with reference numeral 50. Further, in Figure 5 the profile of the zero - sequence system voltage is labeled with reference numeral 53, and the profile of the resulting asymmetry of the sub - capacitors of the intermediate circuit is labeled with reference numeral 54.

[0090] In the initial situation at time point t = 0, the intermediate circuit is in a symmetric state, i.e., the DC potentials 51 and 52 have the same magnitude with different signs, and the virtual center potential 50 corresponds to the reference potential of the system, in particular the ground potential, and by definition has the value zero.

[0091] The method starts at time point t1 from step S1 (see Figure 1 ), in which, by creating a first asymmetry of the sub - capacitors relative to each other, a first potential position of the DC potentials 51 and 52 of the sub - capacitors of the intermediate circuit relative to the ground potential is generated. For this purpose, in the time period between t1 and t2, an initializing zero - sequence system is generated in such a way that the zero - sequence system voltage 53 assumes a positive value. Thereby, first, the potential position of the intermediate circuit is generally increased, such that the DC potentials 51 and 52 and the virtual center potential 50 initially increase proportionally to the zero - sequence system voltage 53. In the case of a given power exchange from the intermediate circuit to the AC side of the converter, due to this increase in the potential position, the intermediate circuit is loaded asymmetrically, and an asymmetry, i.e., a difference between the magnitudes of the DC potentials 51 and 52, is generated, such that the asymmetry 54 increases; this can also be recognized from the fact that the virtual center potential 50 exceeds the zero - position system voltage 53 by the magnitude of the asymmetry 54.

[0092] At time point t2, the asymmetry 54 reaches the target value, and the zero - sequence system voltage 53 returns to the value zero, because further asymmetric loading by means of the initializing zero - sequence system is no longer required. Thus, at time point t2, there is a first asymmetry, and the DC potentials 51 and 52 are offset at time point t2 relative to the initial state by the magnitude of the asymmetry 54.

[0093] Next, according to Figure 1In step S2 thereof, the potential position is set to a first set value. If no zero-sequence system is generated after time point t2, due to the current asymmetry caused by the inflow of electric power from the DC source, the intermediate circuit is charged asymmetrically, and the asymmetry increases. Therefore, a stable zero-sequence system is generated by setting the zero-sequence system voltage 53 to a value having a sign opposite to that of the zero-sequence system initialized between t1 and t2. Thereby, the asymmetric inflow of electric power into the intermediate circuit is balanced by the asymmetric extraction of electric power from the intermediate circuit mirroringly, so that the asymmetry 54 remains constant. Since the stable zero-sequence system in turn causes an offset of the potential position of the intermediate circuit as a whole, which is opposite to the offset caused by the asymmetry, the resulting potential positions of the DC potentials 51, 52 are obtained, and the resulting potential positions are offset from the symmetric initial position by the sum of the asymmetry 54 and the zero-sequence system voltage 53, which can be clearly identified according to the position of the virtual center potential 50.

[0094] In the time period between t2 and t3, according to Figure 1 step S3 thereof, a first measurement of the grounding current is performed, and the first measurement is assigned to the specific values of the DC potentials 51, 52 in the case of the stabilized first potential position.

[0095] At time point t3, according to Figure 1 step S4 thereof, a second potential position of the DC potentials 51, 52 is generated by setting the asymmetry 54 to a value having an opposite sign. For this purpose, an initialized zero-sequence system is generated again, in which the zero-sequence system voltage 53 takes a negative value, so that the potential position of the intermediate circuit is reduced as a whole, and the DC potentials 51, 52 and the virtual center potential 50 initially decrease proportionally to the zero-sequence system voltage 53. Due to this reduction of the potential position, the intermediate circuit is loaded asymmetrically, and the asymmetry 54 decreases.

[0096] At time point t4, the asymmetry 54 reaches the target value, and the zero-sequence system voltage 53 first returns to the value zero, so that there is a second asymmetry at time point t4, and the DC potentials 51, 52 are offset from the initial state by the magnitude of the asymmetry 54 at time point t4. Next, according to Figure 1In step S5 therein, the potential position is set to a second set value, in such a way that a stabilized zero-sequence system is generated, i.e., in such a way that the zero-sequence system voltage 53 is set to a value having a sign opposite to that of the initialized zero-sequence system between t3 and t4. Based on the zero-sequence system voltage 53, the stabilized zero-sequence system is adjusted in such a way that the asymmetry 54 remains constant. This results in a second potential position of the DC potentials 51, 52, which is offset from the symmetric initial position by the sum of the asymmetry 54 and the zero-sequence system voltage 53, wherein the offset of the second potential position has a sign opposite to that of the offset of the first potential position.

[0097] In the time period between t4 and t5, based on Figure 1 step S6 therein, a second measurement of the ground current is carried out, and the second measurement is assigned to the specific values of the DC potentials 51, 52 in the case of the stabilized second potential position.

[0098] Then, based on Figure 1 step S7 therein, the insulation resistance of the DC side of the converter is determined based on the measured ground currents in the case of the first and second potential positions.

[0099] After the measurement process is completed at time point t5, the intermediate circuit can be brought into a symmetric state, and the potential position of the intermediate circuit can be restored to the initial state reached at time point t6. Alternatively, the method can be repeated by returning to step S1 according to Figure 1 at time point t5 or t6.

[0100] List of Reference Numerals

[0101] 10 Converter

[0102] 12 Bridge circuit

[0103] 14 Intermediate circuit

[0104] 16 AC network

[0105] 18 DC source

[0106] 20 DC / DC converter

[0107] 30 Zero-sequence system equivalent voltage source

[0108] L1, L2, L3 AC phases

[0109] M Intermediate circuit center point

[0110] N Neutral line

[0111] Q.1, Q.2, Q.3, Q.N, Q network equivalent voltage source

[0112] Method steps S1, …, S7

Claims

1. A method for operating a converter (10), the converter being configured to exchange electrical power between a DC side and an AC side, wherein, The converter (10) has a bridge circuit (12) and a split intermediate circuit (14) arranged between the DC side and the bridge circuit (12), the split intermediate circuit having at least two sub-capacitors. Wherein, the method includes: - Creating a first asymmetry of the sub-capacitors relative to each other to generate a first potential position of the DC potential of the sub-capacitors of the intermediate circuit (14) relative to the ground potential. - By changing the asymmetry, setting the first potential position of the DC potential of the sub-capacitors of the intermediate circuit (14) to a first set value, where the first set value remains constant within a first duration or is modulated at a frequency at least 100 times smaller than the AC frequency of the exchange power.

2. The method according to claim 1, wherein the method further has: - Creating a second asymmetry or symmetry of the sub-capacitors relative to each other to generate a second potential position of the DC potential of the sub-capacitors of the intermediate circuit (14) relative to the ground potential. - Among them, The second asymmetry has a sign opposite to that of the first asymmetry, and within a second duration, setting the second potential position to a second set value, or within a second duration, setting the symmetry of the intermediate circuit (14) to an asymmetry set value, the asymmetry set value being zero.

3. The method according to claim 1, wherein, In the time profile, the modulated first set value is set by modulating asymmetries with alternating signs in different half-waves.

4. The method according to claim 1 or 3, wherein The time profile of the modulated first set value successively or superimposedly has multiple different frequencies.

5. The method according to any one of claims 1 to 4, wherein the method has: - Creating and changing the first and / or second asymmetry by redistributing charges within the split intermediate circuit (14), in particular by means of a balancer circuit, in particular by means of a DC / DC converter (20), the balancer circuit or the DC / DC converter transferring charges between the sub-capacitors.

6. The method according to claim 5, wherein the method has: - Setting the first potential position by pre-giving a non-zero and optionally modulated asymmetry set value for the balancer circuit.

7. The method according to any one of claims 1 to 4, wherein the method has: - Creating the first and / or second asymmetry of the intermediate circuit (14) by means of a zero-sequence system voltage (30) initialized by the bridge circuit (12). - Setting the first and / or second potential position to the first or the second set value by means of a zero-sequence system voltage (30) stabilized by the bridge circuit (12). - Among them, The initialized zero-sequence system voltage and the stabilized zero-sequence system voltage have different signs.

8. The method according to claim 7, wherein The zero-sequence system voltage (30) has a modulation respectively at a frequency which is at least a factor of 100 less than the frequency of the AC power, wherein the zero-sequence system voltage is in particular sinusoidally modulated or is temporally and in particular within the first and / or second duration substantially constant.

9. The method according to any one of the preceding claims, wherein, The first and / or second duration includes at least one second, in particular several seconds.

10. The method according to any one of the preceding claims, wherein, The setting of the first and / or second potential position is periodically repeated, in particular at a low frequency.

11. Application for insulation resistance measurement of the method according to any one of the preceding claims, the application having - setting the first potential position and detecting a first ground current at a time point within the first duration or within the first half-wave, - setting the second potential position and detecting a second ground current within the second duration, or - in the first potential position with a modulated first setpoint: detecting a second ground current at a time point within the second half-wave, - calculating the insulation resistance from the set potential position and the measured ground current.

12. Converter, which is configured to exchange electric power between the DC side and the AC side, wherein, The converter (10) has a bridge circuit (12) and a split intermediate circuit (14), the split intermediate circuit being arranged between the DC side and the bridge circuit (12), the split intermediate circuit having at least two sub-capacitors, wherein the converter (10) is configured for - generating a first potential position of the DC potential of the sub-capacitors of the intermediate circuit (14) relative to the ground potential by creating a first asymmetry of the sub-capacitors relative to each other, and - setting the first potential position to a first setpoint, wherein the first setpoint remains constant within a first duration or is modulated at a frequency which is at least 100 times less than the AC frequency of the exchange power.

13. The converter according to claim 12, wherein, The converter (10) is furthermore configured for generating a second potential position of the intermediate circuit (14) relative to the ground potential by creating a second asymmetry or symmetry of the sub-capacitors relative to each other, and setting the second potential position to a second setpoint within a second duration or setting the symmetry of the intermediate circuit (14) to an asymmetry setpoint within the second duration, the asymmetry setpoint being zero.

14. The current converter according to claim 12, wherein, The converter (10) is furthermore configured for setting the first potential position to a modulated first setpoint by modulating an asymmetry having alternating signs in different half-waves.

15. The current converter according to claim 12 or 13, wherein The converter (10) has a device for redistributing charge within the split intermediate circuit (14), wherein the device for redistributing charge is in particular configured as a balancer circuit between the sub-capacitors of the intermediate circuit (14).

16. The current converter according to claim 15, wherein The balancer circuit is configured as a DC / DC converter (20) for transferring charge between the sub-capacitors.

17. The current converter according to claim 15 or 16, wherein A setting is provided to connect the neutral line (N) of the AC network (16) that can be attached to the AC side through the bridge circuit (12) to the center point (M) between the first sub-capacitor and the second sub-capacitor of the intermediate circuit (14) of the intermediate circuit (14).

18. The current converter according to any one of claims 12 to 14, wherein, The bridge circuit (12) is configured to generate an initializing zero-sequence system voltage (30), and the initializing zero-sequence system voltage shifts the potential position of the center point (M) between the first and second sub-capacitors of the intermediate circuit (14) of the intermediate circuit (14) relative to the ground potential.

19. The current converter according to claim 18, wherein, The bridge circuit (12) is configured to generate a stabilizing zero-sequence system voltage (30), and the stabilizing zero-sequence system voltage shifts the potential position of the center point (M) of the intermediate circuit (14) relative to the ground potential, wherein the initializing zero-sequence system voltage (30) and the stabilizing zero-sequence system voltage (30) have different signs.

20. The current converter according to claim 18 or 19, wherein The bridge circuit (12) is configured to adjust the potential position to a temporarily constant or modulated first or second set value for the potential position by means of the stabilizing zero-sequence system voltage (30).

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