Unipolar clocked h4-bridges pv one-phase inverter with bipolar clocking close to the zero crossings to suppress common-mode oscillations
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- SMA SOLAR TECH AG
- Filing Date
- 2024-08-07
- Publication Date
- 2026-06-17
Smart Images
Figure EP2024072328_13022025_PF_FP_ABST
Abstract
Description
[0001] Unipolar switching H4 bridge PV single-phase inverter with bipolar switching near zero crossings for suppression of common-mode oscillations
[0002] TECHNICAL FIELD
[0003] The application relates to a method for operating an inverter with an H4 bridge circuit, as well as to an inverter with an H4 bridge circuit. The H4 bridge circuit comprises an arrangement of semiconductor switches in two half-bridges, each with two switches, and is configured to convert a direct voltage into an alternating voltage by clocked switching of the semiconductor switches.
[0004] STATE OF THE ART
[0005] When connecting a photovoltaic generator (PV generator) to an AC grid via a power electronic inverter, it is important to note that both the individual phase conductors of the AC grid and the PV generator have a potential relative to earth potential. If there is no galvanic isolation, and in particular no transformer, between the PV generator and the grid, the pulsed switching of the inverter bridge circuit will directly or indirectly influence the potential of the PV generator. The dynamic conversion of the DC voltage of the PV generator into an AC voltage for the grid by dynamic clocking of the inverter generally leads to a periodically changing potential of the PV generator relative to earth potential.
[0006] A PV generator is structurally connected to earth potential via a leakage capacitance and a leakage resistance. The specific values of the leakage capacitance, in particular, are largely determined by the specific electromechanical design of the PV generator and other environmental conditions such as humidity. Depending on the specific leakage capacitance and the specific timing scheme used for the inverter's switches, the periodic changes in the potential level of the PV generator during operation can lead to unfavorable or even unacceptably high leakage currents.
[0007] EP 2 136 465 B1 describes an inverter for feeding power from a DC voltage source, in particular a photovoltaic generator, into an AC voltage grid. The inverter has an asymmetrically clocked bridge circuit, with at least two switches clocked at the grid frequency and at least two additional switches clocked at a higher clock frequency.
[0008] JP 3 316 735 B2 describes an inverter for feeding the power of a photovoltaic generator into an AC grid, wherein the inverter is operated with unipolar switching. CN 202565189 II describes a method for operating an inverter in which the inverter is generally switched unipolarly and, in the range around the zero crossings of the AC voltage, is switched bipolarly.
[0009] TASK
[0010] The application is based on the task of further improving the power conversion by the inverter and, in particular, reducing the leakage currents between the PV generator and earth potential.
[0011] SOLUTION
[0012] The object is achieved by a method having the features of claim 1, an inverter having the features of claim 10, and a computer program product having the features of claim 11. Embodiments are specified in the dependent claims.
[0013] DESCRIPTION
[0014] An inverter comprises an H4 bridge circuit with a first half-bridge and a second half-bridge for converting an input-side DC voltage into an output-side AC voltage. The inverter is configured to exchange electrical power between a DC voltage source with a variable potential reference to ground potential and an AC voltage grid. The inverter can be configured, in particular, to feed electrical power from a photovoltaic generator with a leakage capacitance relative to ground potential into an AC voltage grid.
[0015] A method for operating the inverter comprises:
[0016] Unipolar pulse width modulated clocking of each of the two half-bridges for half-wave generation of essentially sinusoidal half-bridge voltages.
[0017] Modification of the timing of the half-bridges in a transition area around a zero crossing of the alternating voltage to dampen a leakage current flowing through the leakage capacitance to earth.
[0018] The method is particularly suitable for damping oscillations of the leakage current flowing in a resonant circuit consisting of the leakage capacitance of the DC voltage source and common-mode inductances of the inverter.
[0019] In the unipolar pulse-width modulated clocking of each of the two half-bridges for the half-wave generation of the essentially sinusoidal half-bridge voltages, one half-bridge is used to generate the positive half-wave of the output AC voltage, and the other half-bridge is used to generate the negative half-wave of the output AC voltage. The pulse width of a clock, i.e., the relative width of a switch-on or switch-off phase of the bridge switches within a clock, sets the respective half-bridge voltage for the respective clock. The respective half-bridge voltage of the respective clocked half-bridge is thus directly related to the current pulse width. The clocking of the half-bridges occurs with the pulse width, which indicates the switched, i.e., conductive, state of the respective switches of the respective half-bridge. The two switches of the respective half-bridge can be switched in opposite directions, i.e.If one of the switches is open, the other is closed. The pulse width of the respective switching follows the desired half-bridge voltage, which in the case of unipolar switching, in turn follows the curve of the desired output voltage of the inverter.
[0020] In a conventional, exclusively unipolar switching system, one half-bridge is used to generate the output voltage during each half-cycle. This means that one half-bridge generates one half-cycle of the output voltage, and the other half-bridge is inactive. This means that the other half-bridge is not switched and generates a half-bridge voltage of zero. During the other half-cycle, the first half-bridge is inactive, and the other half-bridge generates the second half-cycle of the inverter's output voltage.
[0021] In the transition region around the zero crossing of the output voltage, the clocking pattern deviates from the exclusively unipolar clocking with only one clocked half-bridge, as claimed in the application. The clocking patterns of the half-bridges are modified there so that the leakage current flowing through the leakage capacitance of the PV generator is dampened. Damping can mean, for example, that the leakage current is reduced in magnitude and / or oscillations are reduced, so that, in particular, current peaks in the leakage current are reduced. The clocking is modified in such a way that the modified clocking in the transition region generates modified half-bridge voltages compared to the exclusively unipolar clocking, which excite a reduced leakage current to ground.In particular, effects in the course of the potential position of the PV generator relative to the earth potential are reduced, which occur abruptly, particularly around the zero crossing, and can generate current peaks in the leakage current.
[0022] The transition region can, for example, surround the zero crossing of the output-side AC voltage symmetrically in time, so that the zero crossing is located in the middle of the transition region. Using the described method, excitations and oscillations of the leakage current at or after the zero crossing of the output-side AC voltage can be dampened. In particular, by modifying the timing in the transition region, a leakage current can be reduced that can be caused by a sudden change in the gradient of the half-bridge voltages and thus the potential position of the PV generator at the zero crossing. This dampens the sudden recharging of the leakage capacitance at the zero crossing, reducing the excitation of a resonance and oscillation of the leakage current.
[0023] In one embodiment of the method, the first half-bridge and the second half-bridge can be pulse-width modulated simultaneously in the transition region before and after the zero crossing. This can reduce the change in the gradient of the half-bridge voltages and thus the potential level of the PV generator at the zero crossing.
[0024] In one embodiment of the method, the half-bridge voltages for the first and second half-bridges deviate from the otherwise half-wave sinusoidal shape due to the simultaneous clocking of the half-bridges during the transition region. The half-bridge voltages of the half-bridges can run in opposite directions to each other in the transition region and, in particular, mirror-symmetrically to each other with respect to the time of the zero crossing. This can reduce the change in the gradient of the half-bridge voltages and thus the potential position of the PV generator, while simultaneously maintaining the desired sinusoidal shape of the inverter's output voltage.
[0025] In one embodiment of the method, the transition region comprises a time span of + / - 0.02-1.0 milliseconds, preferably + / - 0.1-0.5 milliseconds around the zero crossing of the alternating voltage. In this time span around the zero crossing, the highest gradients of the half-bridge voltages and thus the highest leakage currents occur with conventional leakage capacitances, whereby the method achieves both a reduction in the amplitude of the leakage current and a damping of the oscillations of the leakage current. The transition region can comprise up to one tenth of the period of the alternating voltage in the AC voltage network. For such a transition region, depending on the leakage capacitance, good damping of the oscillations of the leakage current can be achieved.
[0026] In one embodiment of the method, the unipolar pulse-width modulated timing of a respective half-wave in the transition region is modified such that the respective half-bridge voltage has a profile whose gradient is smaller than the gradient of the ideal sinusoidal profile and which is, in particular, linear. The flatter gradient also allows the rate of change of the potential position of the PV generator relative to ground potential to be reduced, thus reducing the excitation of the leakage current and achieving good damping.
[0027] In embodiments, the sum of the half-bridge voltages in the transition region can have a constant value. This allows the modified clocking to be particularly well integrated into the inverter control system, as oscillation of a specific resonant circuit consisting of a leakage capacitance of a given DC voltage source and the common-mode inductance of the inverter's filters can be prevented by appropriately adjusting the width of the transition region. The ideal width of the transition region is approximately half the period of a resonant oscillation of this resonant circuit.
[0028] In embodiments, the sum of the magnitudes of the half-bridge voltages in the transition region can have a form approximated by a polynomial. This can further reduce the rate of change of the potential position of the PV generator relative to ground potential, thus further improving damping.
[0029] The inverter for exchanging power between the DC voltage source, in particular the photovoltaic generator, and the AC voltage grid comprises a control unit and an H4 bridge circuit with a first and a second half-bridge. The control unit is configured to carry out the described method.
[0030] The inverter thus changes the timing in the transition region around the zero crossing of the output AC voltage compared to an exclusively unipolar timing, so that, compared to an inverter with exclusively unipolar timing, a smoother curve of the potential position of the PV generator relative to ground potential is achieved at the zero crossing. This serves to reduce the leakage current peak that occurs at the zero crossing. The modification of the timing can, in particular, involve the timing pattern, i.e., the pulse width modulation, following a different function in the transition region than outside the transition region.
[0031] A computer program product contains instructions which, when executed by the control unit, cause the control unit to carry out the described method.
[0032] BRIEF DESCRIPTION OF THE CHARACTERS
[0033] The invention is further explained and described below with reference to exemplary embodiments illustrated in the figures. Figure 1 schematically shows an inverter with an H4 bridge circuit.
[0034] Fig. 2 shows possible curves of half-bridge voltages, duty cycles and switching signals for the half-bridges of an H4 bridge circuit.
[0035] Fig. 3 shows examples of possible sums of the half-bridge voltages.
[0036] Fig. 4 shows an example of an undamped and a damped leakage current.
[0037] Fig. 5 shows an example of a comparison of conventional and modified clocking according to the application with respective possible associated leakage currents.
[0038] Fig. 6 shows schematically another embodiment of an inverter.
[0039] The same reference numerals are used throughout the figures for identical or similar elements. The illustrations in the figures may not be to scale.
[0040] FIGURE DESCRIPTION
[0041] Fig. 1 shows an inverter 10 with an H4 bridge circuit comprising a first half-bridge 20 and a second half-bridge 22. The inverter 10 converts electrical power from a DC voltage source, e.g., a photovoltaic generator 12, into an AC voltage that is exchangeable with an AC voltage grid 14, and / or vice versa. In the illustrated embodiment, an optional DC-DC converter 26 is arranged between the PV generator 12 and the H4 bridge circuit, and single-phase AC voltage is generated, so that a single-phase AC current is exchanged with the AC voltage grid 14 via a feed-in network 16. Details of the feed-in network 16 are explained in connection with Fig. 6.
[0042] The connection to the AC voltage network 14 is established via a phase conductor L and a neutral conductor N. The AC voltage network has a reference to the earth potential 30, in particular via the neutral conductor N. The reference of the neutral conductor N to the earth potential can be specified directly, for example by grounding the neutral conductor, or indirectly, for example in a split-phase network or other network types (e.g., delta-corner-ground, stinger-ground).
[0043] On its DC side, the inverter 10 has an intermediate circuit 24. For the exchange of alternating current with an alternating voltage network 14 with, for example, a 230 V nominal voltage and Ü = 325 V peak voltage, the intermediate circuit 24 can be designed, for example, as a 600 V intermediate circuit. The DC voltage source has, in particular, a photovoltaic generator 12, which in turn can comprise several PV modules connected in series and / or parallel. The DC voltage source has a reference to earth potential 30. The reference to earth potential 30 is largely predetermined for the photovoltaic generator 12 by its structural design and can be represented, in particular, by a leakage capacitance 18 with a leakage resistor 19 connected in parallel. In this case, the leakage capacitance 18, in particular, can vary over time and can fundamentally change, for example, depending on climatic conditions during operation of the DC voltage source.
[0044] The first half-bridge 20 has a first switch S1 and a second switch S2. The second half-bridge 22 has a third switch S3 and a fourth switch S4. To convert the DC-side direct voltage into the AC-side alternating voltage and / or vice versa, the inverter 10 is generally clocked unipolarly. During a half-cycle of the output-side alternating voltage, only one of the two half-bridges 20, 22 provides the AC-side alternating voltage by controlling their respective switches complementarily at a clock frequency of a few kilohertz with a sinusoidal pulse-width modulated duty cycle. In the non-clocked half-bridge, one of the two switches is permanently switched on during the respective half-cycle.
[0045] The output-side alternating voltage for exchanging an alternating current with the alternating voltage network 14 is formed from the two half-bridge voltages UL, UN, whereby the first half-bridge voltage u L the positive half-wave and the second half-bridge voltage u N represents the negative half-wave of the output alternating voltage. The voltage curve of the first half-bridge voltage u L at the first half-bridge 20 is shown as an example in Fig. 1. The first half-bridge voltage UL corresponds in magnitude to the profile of one half-wave of a sinusoidal voltage waveform. The voltage waveform of the second half-bridge voltage UN at the second half-bridge 22 is shown as an example in Fig. 1. The second half-bridge voltage UN corresponds in magnitude to the profile of the other half-wave of a sinusoidal voltage waveform.
[0046] Due to the respective reference of the AC voltage network 14 and the PV generator 12 to ground potential 30, a leakage current IA results, which is exchanged between the DC voltage source and ground potential 30, i.e., flows from the PV generator 12 to ground via the leakage capacitance 18. The leakage current lA is driven by the leakage voltage UA, the characteristic of which is determined by the specific timing of the inverter 10.
[0047] The course of the leakage voltage UA is shown in Fig. 1 as an example for a purely unipolar clocking system, in which each half-wave is generated completely and exclusively by clocking a respective half-bridge. In this configuration, the magnitude of the leakage voltage UA follows the course of the half-bridge voltage UN, which is generated on the AC side by the pulse-width modulated clocking of the second half-bridge. Accordingly, a leakage current IA results, the course of which is shown as an example in Fig. 1. The leakage current IA has pronounced maxima, particularly at the zero crossings of the output-side AC voltage, which are mainly caused by charge reversals of the leakage capacitance 18 when the AC voltage changes sign.
[0048] In Fig. 2 above is an example of the curve of the half-bridge voltages u L , UN when applying a method according to the application. The middle graph in Fig. 2 shows the duty cycles dL , dN for the respective half-bridges 20, 22. Here, dI is the duty cycle for the first half-bridge 20, which results in the first half-bridge voltage UL, and dN is the duty cycle for the second half-bridge 22, which results in the first half-bridge voltage UL. The lower graphic in Fig. 2 shows the switching signals used for the switches S1, S2 of the first half-bridge 20 and the switches S3, S4 of the second half-bridge 22, where the value "1" means that the respective switch is conductive, and the value "0" means that the respective switch is open and thus non-conductive.
[0049] During unipolar clocking outside the transition region B, the switches S1, S2 of the first half-bridge 20 are clocked in the first half-wave, and the switches S3, S4 of the second half-bridge are clocked in the second half-wave. The first half-bridge 20 generates the first half-bridge voltage u L, which has a sinusoidal shape during the first half-wave of the output AC voltage and is largely zero in the second half-wave. The second half-bridge 22 generates the second half-bridge voltage u N , which is largely zero during the first half-wave of the output AC voltage and has a sinusoidal shape in the second half-wave. The shape of the duty cycles d L , dN corresponds to the curve of the respective half-bridge voltage UL, UN.
[0050] In a transition region B between the half-waves, the clocking is modified compared to the otherwise exclusively unipolar clocking. In the transition region B, both half-bridges 20, 22 are clocked and thus each generate a half-bridge voltage u L, UN not equal to zero. The modified clocking modifies the waveforms of the half-bridge voltages UL, UN in the transition region B. In particular, the gradient of the half-bridge voltages UL, UN in the transition region B can be adjusted. A reduction in the gradient of the half-bridge voltages UL, UN compared to the gradient of the half-bridge voltages UL, UN at the zero crossing with purely unipolar clocking is particularly advantageous for damping high leakage currents IA. The half-bridge voltages UL, UN for the first and second half-bridges are therefore changed by modifying the clocking in the transition region B compared to the otherwise half-wave sinusoidal shape of conventional exclusively unipolar clocking.
[0051] In the exemplary embodiment illustrated in Fig. 2, the half-bridge voltages UL, UN in the transition region B run in opposite directions to one another and are mirror-symmetrical with respect to the time of the zero crossing. The shape of the curve of the half-bridge voltages UL, UN in the transition region B can be linear, for example. Alternatively, the curve of the half-bridge voltages UL, UN can also correspond, for example, to the shape of the respective sine half-wave, which, however, has been stretched over time, so that in the example illustrated, it reaches the value zero not in the middle but at the edge of the transition region B.
[0052] In the illustrated embodiment, transition region B comprises a time span of + / - 1 ms around the zero crossing of the alternating voltage. The timing is modified in particular so that the characteristic of the discharge voltage UA at the zero crossing of the alternating voltage is as flat and continuous as possible, i.e., with the smallest possible gradient and continuously.
[0053] The mains-side alternating voltage ULN, which is to result between phase conductor L and neutral conductor N, has a sinusoidal shape with the parameters of the alternating voltage network: u LN = [ / •sin(cot), with Ü= 325 V and f=50 Hz=co / 2n:. Half-bridge voltages UL and UN suitable for generating this output voltage ULN can be determined as follows by decomposing the output-side alternating voltage ULN into positive-sequence system components ui, U2 and a zero-sequence system component uo:
[0054] The respective duty cycle dL and dN results from this, taking into account the intermediate circuit voltage UDC ZU:
[0055] Fig. 3 shows examples of possible sums of the half-bridge voltages UL, UN with clocking in the transition region B as per the application. Due to the basic relationship u0= u L + u NThe curve of this sum voltage corresponds to the curve of the zero-system uo. In the embodiment of Fig. 3 above, the sum of the magnitudes of the half-bridge voltages UL, UN in the transition region B has a constant value. The constant value can, for example, correspond to a threshold value TH, which corresponds to the sum of the magnitudes of the half-bridge voltages UL, UN at the edges of the transition region B. Within the transition region B, both half-bridges 20, 22 are clocked.
[0056] In the embodiment shown in Fig. 3 below, the sum of the magnitudes of the half-bridge voltages UL, UN in the transition region B has a form that is approximated by a polynomial. For this approximation, the sum of the magnitudes of the half-bridge voltages u L , UN within the transition region B can be described, for example, by the following polynomial: / c3(cot) 3 + / c2(cot) 2+ / cicot + ko for TH > cot > 0. TH is defined as the threshold which the sum of the magnitudes of the half-bridge voltages UL, UN at the edges of the transition region B assumes.
[0057] Fig. 4 shows exemplary resulting leakage currents IA for an embodiment of a system with inverter 10, PV generator 12 as DC voltage source and AC voltage network 14, as shown in Fig. 1.
[0058] The top of Fig. 4 shows a leakage current IA resulting from operation with conventional, purely unipolar switching. Oscillations of the leakage current IA can be seen, which occur in the areas around the zero crossing of the AC voltage of the AC voltage network 14 and exhibit maxima with considerable amplitudes. Particularly in the area of the negative leakage current IA, fluctuations can occur that can trigger safety mechanisms, in particular a residual current monitor, and thus shut down the inverter. These oscillations also negatively impact the EMC behavior of the inverter. The oscillations are caused by the sudden change in the gradient of the leakage voltage UA – corresponding to a large jump in its derivative – at the zero crossing of the AC voltage.This change in slope causes a resonant circuit comprising the PV leakage capacitance and common-mode impedances of the inverter filters to oscillate.
[0059] The bottom of Fig. 4 shows a damped leakage current IA, which results when the inverter is operated using the method according to the application. Compared to the conventional situation, both the amplitudes and oscillations of the leakage current IA are significantly reduced.
[0060] Fig. 5 shows, as an example, a comparison of conventional, purely unipolar clocking (left) and modified clocking (right) with the resulting leakage currents IA. The leakage capacitance 18 forms a resonant circuit via the ground potential 30 with filter inductances of the inverter 10, which is oscillated by the second half-bridge voltage u N the second half-bridge 22. The sudden change in the slope of u NWith purely unipolar switching, as shown in the left part of Fig. 5, an equally sudden increase in the leakage voltage UA is generated. This triggers a rapid charge reversal of the leakage capacitance 18 and, on the other hand, excites a resonance of the oscillating circuit consisting of the leakage capacitance and the common-mode inductance of the inverter 10. This, in combination, leads to an oscillating leakage current IA, which can also be called a common-mode current. In particular, the high magnitude of the leakage current IA at the maximum of the oscillation can also lead to saturation effects in the filter inductances of the inverter 10, which can have a particularly adverse effect on electromagnetic compatibility.
[0061] Using the described method, the clocking in transition region B around the zero crossing of the output-side AC voltage is modified compared to the exclusively unipolar clocking in such a way that the leakage current IA is attenuated. The modified clocking and the resulting leakage current IA are shown in the right-hand part of Fig. 5. In particular, in transition region B, before and after the zero crossing, the first half-bridge 20 and the second half-bridge 22 are simultaneously clocked in a pulse-width modulated manner. As a result, the clocking profile of the two half-bridges within the transition region deviates from the otherwise half-wave sinusoidal profile and, in particular, has a lower gradient there than the underlying sinusoidal shape. Associated with this is a profile of the zero-system component uo corresponding to the upper graph in Fig. 3. As a result, the amplitude of the leakage current is significantly reduced.
[0062] Fig. 6 shows the inverter 10 according to Fig. 1 , in which the feed-in network 16 specifically comprises the inductors L1a, L1b arranged in the phase conductors L and N, and in which the phase conductors L and N are connected to the negative DC potential of the intermediate circuit 24 and the PV generator 12 via capacitors C1a, C1b. Alternatively, the capacitors C1a, C1b can be arranged between the phase conductors L, N and the positive DC potential of the intermediate circuit 24 or between the phase conductors L, N and a center point of the intermediate circuit. The remaining feed-in network 16' can in particular comprise further filter elements for common-mode and / or differential-mode interference.
[0063] The transition region B is optimally selected to be exactly as wide as half the period of a resonant oscillation of the resonant circuit, consisting of the leakage capacitance of the DC voltage source and the common-mode inductance of the inverter 10, in a worst-case scenario. The worst-case scenario is defined by the maximum expected leakage capacitance of the DC voltage sources that are to be connected to the inverter. In such a worst-case scenario, in which both the amplitude and the period of the resonant oscillation are at a maximum, the resonances in the leakage current IA are suppressed as best as possible by ensuring that the width of the transition region corresponds to half the resonant period of the resulting resonant circuit.With smaller leakage capacitances and an unchanged width of the transition region, the oscillation is no longer suppressed as optimally as possible, but the leakage current through the leakage capacitance is then also lower overall, so that the peak value of the leakage current is generally lower than in the worst case with smaller leakage capacitances. Alternatively, the width of the transition region can also be adjusted depending on the actual, possibly currently determined leakage capacitance. The width of the transition region is then set to half the period of the resonant oscillation of the specific resulting resonant circuit. In the case of a PV generator as a DC voltage source with leakage capacitance that varies, particularly due to weather conditions, it can therefore be advantageous to adjust the width of the transition region in order to optimally dampen the oscillations of the leakage current.
[0064] LIST OF REFERENCE SYMBOLS
[0065] 10 inverters
[0066] 12 photovoltaic generators
[0067] 14 AC network
[0068] 16, 16' feed-in network
[0069] 18 Leakage capacity
[0070] 19 Leakage resistance
[0071] 20 first half bridge of the H4 bridge
[0072] 22 second half bridge of the H4 bridge
[0073] 24 intermediate circuit
[0074] 26 DC-DC converters
[0075] 30 Earth potential
[0076] L phase conductor
[0077] N neutral conductor
[0078] S1, S2, S3, S4 switches of the H4 bridge
[0079] L1a, L1a filter inductance
[0080] C1a,C1b filter capacity
[0081] IA leakage current
[0082] UA leakage voltage
[0083] B Transition area
[0084] UL first half-bridge voltage
[0085] UN second half-bridge voltage d duty cycle first half-bridge
[0086] CIN duty cycle second half bridge
[0087] TH threshold
Claims
PATENT CLAIMS 1. A method for operating an inverter (10), wherein the inverter (10) has an H4 bridge circuit with a first half-bridge (20) and a second half-bridge (22) for converting an input-side DC voltage into an output-side AC voltage and is configured to exchange electrical power between a DC voltage source with a variable potential reference to an earth potential (30), in particular a photovoltaic generator (12) with a discharge capacitance (18) relative to the earth potential (30), and an AC voltage network (14), the method comprising: - Unipolar pulse-width modulated clocking of each of the two half-bridges (20, 22) for half-wave generation of essentially sinusoidal half-bridge voltages (UL, UN), - Modification of the timing of the half-bridges (20, 22) in a transition region (B) around a zero crossing of the output-side alternating voltage for damping a leakage current (IA) flowing through the leakage capacitance (18) to earth.
2. Method according to claim 1, wherein in the transition region (B) before and after the zero crossing, the first half-bridge (20) and the second half-bridge (22) are simultaneously clocked in a pulse-width modulated manner.
3. Method according to claim 1 or 2, wherein the half-bridge voltages (UL, UN) for the first and the second half-bridge (20, 22) are changed by the simultaneous clocking in the course of the transition region (B) compared to the basic half-wave sinusoidal shape.
4. Method according to claim 3, wherein the half-bridge voltages (UL, UN) in the transition region (B) run in opposite directions to one another and in particular mirror-symmetrically with respect to the time of the zero crossing.
5. Method according to one of the preceding claims, wherein the transition region (B) comprises a time period of + / - 0.02-1.0 ms, in particular + / - 0.1-0.5 ms, around the zero crossing of the alternating voltage.
6. Method according to one of the preceding claims, wherein the transition region (B) comprises up to one tenth of the period of the alternating voltage in the alternating voltage network.
7. Method according to one of the preceding claims, wherein the unipolar pulse width modulated timing of a respective half-wave in the transition region (B) is is modified so that the respective half-bridge voltage (UL, UN) has a curve whose gradient is smaller than the gradient of the ideal sinusoidal curve with exclusively unipolar clocking and which in particular has a time-stretched sinusoidal shape or is linear.
8. The method according to claim 7, wherein the sum of the magnitudes of the half-bridge voltages (UL, UN) in the transition region (B) has a constant value.
9. The method according to claim 7, wherein the sum of the magnitudes of the half-bridge voltages (UL, UN) in the transition region (B) has a form that is approximated by a polynomial.
10. Inverter for feeding a power of a DC voltage source, in particular a photovoltaic generator (12), into an AC voltage network (14), wherein the inverter (10) has a control unit and an H4 bridge circuit with a first and a second half-bridge (20, 22), wherein the control unit is set up to carry out the method according to one of the preceding claims.
11. A computer program product comprising instructions that can be executed by a control unit and cause the control unit to carry out a method according to any one of claims 1 to 10.