An inverter capable of switching between single-phase and three-phase operation

Abstract

In order to propose an improved single-phase operation (EB) of an inverter (1) having a first input pole (A) and a second input pole (B) and comprising at least three phase legs (U, V, W) each having at least one upper power switch (SU1+, SV1+, SW1+) and at least one lower power switch (SU1−, SV1−, SW1−) connected in series with the at least one upper power switch (SU1+, SV1+, SW1+) via an output pole (u, v, w), a switching unit (5) is provided which is designed for switching the inverter (1) from a multi-phase operation (MB) to a single-phase operation (EB) in such a way that a center point connection switch (S2) is closed in order to connect a third output pole (w) with a center point (M) of an intermediate circuit, and the switching unit (5) is further designed for controlling the upper and lower power switches (SW1+, SW1−) of the third phase leg (W) in the single-phase operation (EB) in such a way that the intermediate circuit voltages (UC1, UC2) at the first and second intermediate circuit capacitors (C1, C2) are symmetrical.

Description

TECHNICAL FIELD

[0001] The invention relates to an inverter having a first input pole and a second input pole and comprising a first phase leg having one upper power switch and one lower power switch connected in series with the upper power switch via a first output pole, a second phase leg having one upper power switch and one second lower power switch connected in series with the upper power switch via a second output pole, and a switching unit designed to switch the inverter from a multiphase operation to a single-phase operation in order to output a single-phase output alternating voltage. The invention also relates to a method for operating an inverter. BACKGROUND

[0002] An inverter, also called inverter, constitutes a DC / AC converter and thus converts an input DC voltage on the input side to one or more output AC voltages on the output side. The input of the inverter is connected to a DC voltage source, such as a photovoltaic device in generator mode, which provides the input DC voltage. The inverter can comprise one or more phase legs, each of which generates an output AC voltage. The output side of the inverter can be connected to an energy supply network in order to feed energy into the energy supply network. For this purpose, for example, the three output poles of the three phase legs of a three-phase inverter can be connected to three grid phases of the energy supply network. Furthermore, the input side of the inverter is provided with a capacitive intermediate circuit. The input DC voltage is applied to this intermediate circuit. If the intermediate circuit comprises a first intermediate circuit capacitor, which is connected to a second intermediate circuit capacitor via an intermediate circuit center point, the input DC voltage is divided into a first intermediate circuit voltage at the first intermediate circuit capacitor and a second intermediate circuit voltage at the second intermediate circuit capacitor.

[0003] Each phase leg is provided with at least one first upper power switch and at least one first lower power switch, the output pole of the respective phase leg being arranged at the connection point between the upper power switch and the associated lower power switch. In a multiphase operation, the at least one first upper power switch in each phase leg serves to generate the upper half-wave of the output AC voltage of the respective phase leg and the at least one first lower power switch serves to generate the lower half-wave of the output AC voltage of the respective phase leg, the power switches being controlled accordingly by a control unit. The power switches can be implemented, for example, as IGBTs or MOSFETs.

[0004] If the inverter is configured as a multi-level inverter, each phase leg can be provided with at least one second upper power switch, which is connected with the first upper power switch via an upper center point. Furthermore, each phase leg can be provided with at least one second lower power switch, which can be connected with the second upper power switch via a lower center point. In a multi-phase NPC (neutral point clamped) multi-level inverter, in each phase leg, the first upper power switch and the second upper power switch connected via the respective upper center point form an upper half-bridge, and the first lower power switch and the second lower power switch connected via the respective lower center point form a lower half-bridge. In each phase leg, the upper half-bridge is connected with the lower half-bridge via a matching output pole. Furthermore, the upper center point and the lower center point can each be connected with the intermediate circuit center point via a diode.

[0005] A multi-phase inverter, i.e. an inverter having a plurality of phase legs, can also be operated in a single-phase operation. The single-phase operation can be carried out, for example, as an island operation, i.e. when the output poles of the inverter are separated from an energy supply network. In order to realize the single-phase operation, for example, only two of the three phase legs can be operated, in which a single-phase output alternating voltage is output between two of the three output poles, as disclosed in DE 10 2014 104 216 B3. The single-phase output alternating voltage is thus generated by a potential difference of the potentials of the two output poles. The first phase leg and the second phase leg thus jointly generate the output alternating voltage and generate an upper half-wave of the two phase legs and a lower half-wave of the two phase legs. The third phase leg is not included in the single-phase operation. SUMMARY

[0006] It is the task of the present application to propose an inverter which enables an improved single-phase operation.

[0007] According to the application, the task is solved in that the inverter comprises a third phase leg having one upper power switch and one lower power switch connected in series with the upper power switch via a third output pole, and further comprises a control unit designed to control the upper power switches and the lower power switches of the phase legs in a multi-phase operation such that an input direct voltage applied between a first input pole and a second input pole is converted into an output alternating voltage applied to the respective output poles, a center point connection switch is provided between the third output pole and the intermediate circuit center point, the switching unit is designed to close the center point connection switch in a single-phase operation in order to connect the third output pole with the intermediate circuit center point, and the switching unit is designed to control the upper and lower power switches of the third phase leg in a single-phase operation such that the intermediate circuit voltages at the first and second intermediate circuit capacitors are symmetrical.

[0008] The task is also solved by a method for operating an inverter, the inverter comprising a first phase branch with an upper power switch and a lower power switch connected in series with the upper power switch via a first output pole, a second phase branch with an upper power switch and a lower power switch connected in series with the upper power switch via a second output pole, a third phase branch with an upper power switch and a lower power switch connected in series with the upper power switch via a third output pole, in a multiphase mode, the respective upper power switches and the respective lower power switches of the phase branches being controlled in such a way that an input DC voltage applied to an intermediate circuit is converted into an output AC voltage applied to the output poles, the intermediate circuit having a first intermediate circuit capacitor and a second intermediate circuit capacitor connected via an intermediate circuit center point, the inverter being switched from the multiphase mode into a single-phase mode by a switching unit in order to output a single-phase output AC voltage, the third output pole being connected to the intermediate circuit center point and the upper and lower power switches of the third phase branch being controlled in such a way that the intermediate circuit voltages at the first and second intermediate circuit capacitors are symmetrical.

[0009] The enumeration of the first, second and third phase branches is of course not to be regarded as exhaustive, rather the inverter according to the application comprises at least three phase branches.

[0010] In an inverter with a split intermediate circuit it is provided that the intermediate circuit voltages of the intermediate circuit capacitors of the intermediate circuit are equal. However, it can occur that the intermediate circuit voltages at the intermediate circuit capacitors deviate from one another, for example because of a load at the output poles which is half-wave unsymmetrical. An unsymmetry of the intermediate circuit voltages means a reduction of one of the intermediate circuit voltages and an increase of the other intermediate circuit voltage, so that the associated intermediate circuit capacitor with the increased intermediate circuit voltage can be overloaded. This can lead to a damage of the associated intermediate circuit capacitor. If a suitable protection mechanism is provided, an emergency shutdown can be implemented before the damage. A further effect of the unsymmetrical loading of the intermediate circuit capacitors is a loss of the true sine wave of the output AC voltage output at the output poles. In order to prevent the described and other disadvantages, the intermediate circuit voltages are made symmetrical according to the application. This is achieved in that in the single-phase mode, i.e. when only one output AC voltage is output (for this only the first and / or the second output pole is used; see below), the third output pole is not used for outputting an output AC voltage, but is connected to the intermediate circuit center point. The power switches of the third phase branch are controlled in such a way that the intermediate circuit voltages at the first and second intermediate circuit capacitors are symmetrical. This is particularly advantageous in an inverter with a small construction size, because no additional symmetrization unit is required for symmetrizing the intermediate circuit voltages.

[0011] The switching unit can be designed to take over the control of the upper and lower power switches of the third phase branch itself in the single-phase operation in order to symmetrize the intermediate circuit voltages at the first and second intermediate circuit capacitors. It thus independently performs this control.

[0012] But the switching unit can also be designed to instruct the control unit to control the upper and lower power switches of the third phase branch in the single-phase operation in order to symmetrize the intermediate circuit voltages at the first and second intermediate circuit capacitors. The control unit thus takes over the clocking of the power switches of the third phase branch in order to symmetrize the intermediate circuit voltages. The switching unit thus performs the control by instructing the control unit.

[0013] The power switches of the first and / or second phase branch are controlled in the single-phase operation for outputting a single-phase output alternating voltage. This can also be performed by the switching unit itself or by the switching unit instructing the control unit for this purpose.

[0014] The switching unit can be designed to control the respective upper and respective lower power switches of the phase branches in order to output the output alternating voltage at the respective output poles when switching from the single-phase operation to the multi-phase operation. The switching from the multi-phase operation to the single-phase operation and thus also the switching from the single-phase operation to the multi-phase operation can thus be performed completely by the switching unit.

[0015] The switching unit is preferably designed to close a phase connection switch arranged between the first and second output poles in the single-phase operation in order to connect the first and second output poles into a common output pole and to control the respective upper and respective lower power switches of the first and second phase branches in order to output a single-phase output alternating voltage between the common output pole and the intermediate circuit center point. The inverter can thus output twice as much power at one output pole in the single-phase operation compared to operating only one phase branch.

[0016] The single-phase output alternating voltage is thus generated by the potential difference between the common output pole and the intermediate circuit center point. By connecting two (or more) phase branches together, the current-carrying capacity in the single-phase operation, i.e. the maximum output alternating current at the common output pole, can be increased by a factor of two (or more) compared to the multi-phase normal operation.

[0017] If a common output pole is provided in the single-phase operation, the switching unit can be designed to instruct the control unit to control the respective upper and respective lower power switches of the first and second phase branches in the single-phase operation in order to output a single-phase output alternating voltage at the common output pole. The control unit is thus put into a single-phase control mode by the switching unit.

[0018] If a common output pole is provided in the single-phase mode of operation, the inverter can preferably be switched from the single-phase mode of operation to the multi-phase mode of operation by the switching unit in such a way that the first output pole and the second output pole are separated.

[0019] The switching unit can be designed to open the phase connection switch when switching from the single-phase mode of operation to the multi-phase mode of operation in order to separate the first output pole and the second output pole. The switching unit can thus also be used to switch from the single-phase mode of operation to the multi-phase mode of operation.

[0020] In the phase branches, one second upper power switch each can be connected to the first upper power switch via an upper center point each and one second lower power switch each can be connected to the first lower power switch via a lower center point each, the upper center points and the lower center points being connected to the intermediate circuit center point in the phase branches. The inverter thus corresponds to an NPC multi-level converter.

[0021] An inverter can also be provided which has a first input pole and a second input pole and comprises at least three phase branches, each phase branch having at least one upper power switch and at least one lower power switch connected in series with the at least one upper power switch via an output pole, comprises an intermediate circuit provided between the first input pole and the second input pole, the intermediate circuit having a first intermediate circuit capacitor and a second intermediate circuit capacitor connected to the first intermediate circuit capacitor via an intermediate circuit center point, and comprises a control unit designed to control the at least one upper power switch and the at least one lower power switch of each of the at least three phase branches in the multi-phase mode of operation in such a way that an input direct voltage applied between the first input pole and the second input pole is converted into an output alternating voltage applied to the respective output pole, and a switching unit is provided, the switching unit being designed to close a phase connection switch provided between the first and the second output pole in order to connect the first and the second output pole into a common output pole and to switch the inverter from the multi-phase mode of operation to the single-phase mode of operation, and the switching unit is designed to control the power switches of the first and the second phase branch in the single-phase mode of operation in order to output a single-phase output alternating voltage between the common output pole and the intermediate circuit center point, and a symmetrization unit can be provided in order to symmetrize the intermediate circuit voltage at the first and the second intermediate circuit capacitor, or a center point connection switch is provided between the third output pole and the intermediate circuit center point, the switching unit being designed to close the center point connection switch in the single-phase mode of operation in order to connect the third output pole to the intermediate circuit center point, preferably the switching unit is designed to control the power switches in the single-phase mode of operation in order to symmetrize the intermediate circuit voltage at the first and the second intermediate circuit capacitor. BRIEF DESCRIPTION OF DRAWINGS

[0022] In the following reference is made to Figures 1 to 7The application is explained in detail, the drawing shows by way of example, schematically and not limitatively advantageous embodiments of the application. The drawing is as follows:

[0023] Figure 1 An inverter according to the prior art is shown;

[0024] Figure 2 An inverter with phase connection switches and a center point connection switch in a multiphase operation is shown;

[0025] Figure 3 An inverter in a single phase operation is shown; Figure 2

[0026] Figure 4 An inverter with closed center point connection switch is shown;

[0027] Figure 5a Switching pulses of power switches in a multiphase operation are shown;

[0028] Figure 5b Output voltage and output current in a multiphase operation are shown;

[0029] Figure 6 Switching pulses of power switches of the third phase branch in a single phase operation, and output voltage and output current, are shown when symmetrical;

[0030] Figure 7 A possible embodiment of a symmetrical unit is shown. DETAILED DESCRIPTION

[0031] Figure 1 An inverter 1 according to the prior art is shown, which has a DC voltage source 2 at its input side, such as a photovoltaic device in a generator-like operation. The DC voltage source 2 thus provides an input DC voltage Ue to the inverter 1. Furthermore, a capacitive intermediate circuit ZK is provided at the input side of the inverter 1, to which the input DC voltage Ue is applied. The intermediate circuit ZK here comprises one upper intermediate circuit capacitor C+ and one lower intermediate circuit capacitor C- connected in series via an intermediate circuit center point M, it being possible for other series and / or parallel connected capacitors to be provided in the intermediate circuit ZK as well. The input DC voltage Ue is applied between a first input pole A and a second input pole B, the series circuit of the two intermediate circuit capacitors C+, C- being arranged between the first input pole A and the second input pole B. The input DC voltage Ue is thus distributed to the two intermediate circuit capacitors C+ and C-, an upper intermediate circuit voltage UC1 being applied to the upper intermediate circuit capacitor C+ and a lower intermediate circuit voltage UC2 being applied to the lower intermediate circuit capacitor C-.

[0032] ​The inverter comprises at least three, preferably exactly three, phase legs U, V, W. Each phase leg U, V, W connects a first input stage A with a second input stage B by means of a first upper power switch SU1+, SV1+, SW1+ and a first lower power switch SU1-, SV1-, SW1- connected in series with the first upper power switch SU1+, SV1+, SW1+ via an output pole u, v, w. In a multiphase operation MB, an output alternating voltage ua1, ua2, ua3 is output for each output pole u, v, w, respectively, on the output pole u, v, w. A self-oscillating diode D is arranged in parallel with the upper power switch SU1+, SV1+, SW1+ and the lower power switch SU1-, SV1-, SW1+, respectively, said self-oscillating diode being polarized in the direction of the first pole A.

[0033] In the shown embodiment, a second upper power switch SU2+, SV2+, SW2+ is connected to the first upper power switch SU1+, SV1+, SW1+ via an upper center point MU+, MV+, MW+, respectively, in each of the phase legs U, V, W. Similarly, a second lower power switch SU2-, SV2-, SW2- is connected to the first lower power switch SU1-, SV1-, SW1- via a lower center point MU-, MV-, MW-, respectively. A self-oscillating diode D is arranged in parallel with the second upper power switch SU2+, SV2+, SW2+ and the second lower power switch SU2-, SV2-, SW2-, respectively, said self-oscillating diode being polarized in the direction of the first pole A. Of course, further power switches can be provided in the phase legs U, V, W, for example to increase the power of the inverter 1. The intermediate circuit center point M is connected to the upper center point MU+, MV+, MW+, respectively, via an upper diode D+ in the respective phase leg U, V, W and is polarized in the direction of the upper center point MU+, MV+, MW+. The lower center point MU-, MV-, MW- is connected to the intermediate circuit center point M by means of a lower diode D- in the respective phase leg U, V, W, the lower diode D- being polarized in the direction of the intermediate circuit center point M. Thus, the shown inverter 1 constitutes, for example, a three-phase NPC (neutral point clamped) multilevel inverter with phase legs U, V, W, wherein each phase leg U, V, W comprises an upper half-bridge (with the associated first and second upper power switches SU1+ and SU2+, SV1+ and SV2+, SW1+ and SW2+, respectively) and a lower half-bridge (with the associated first and second lower power switches SU1- and SU2-, SV1- and SV2-, SW1- and SW2-, respectively). The upper and lower half-bridges can together be regarded as a direct voltage / alternating voltage-bridge.

[0034] The power switches SU1+, SV1+, SW1+, SU1-, SV1-, SW1-, SU2+, SV2+, SW2+, SU2-, SV2-, SW2- of the at least three phase branches U, V, W are controlled by the control unit 4 in the multi-phase operation MB in such a way that an output alternating voltage ua1, ua2, ua3 is output at each output pole U, V and W, which output alternating voltage ua1, ua2, ua3 is preferably phase-shifted. The input direct voltage Ue is thus converted by the inverter 1 into one output alternating voltage ua1, ua2, ua3 per phase branch U, V, W. The output alternating voltage ua1, ua2, ua3 can be applied at the output of the inverter 1 to the grid phases of the energy supply network 3, respectively, so that an output alternating current ia1, ia2, ia3 is fed into the energy supply network 3. This is shown in Figure 1 by the closed power switch SN. The energy supply network 3 comprises several grid phases, each of which comprises a phase-shifted network voltage uL1, uL2, uL3 with a network frequency, for example 50 Hz, for example 230 volts. The output alternating voltage ua1, ua2, ua3 is preferably synchronized with the respective network voltage uL1, uL2, uL3 in order to be able to be simply fed into the energy supply network 3. Furthermore, the output poles u, v, w are optionally provided with a series filter coil X and a parallel filter capacitor C. The filter capacitor C is here star-connected, the star point being connected to the intermediate circuit center point M. The principle of action of the multi-phase inverter 1 in the multi-phase operation MB is known and is therefore not described in detail here.

[0035] Figure 2 A preferred embodiment of the inverter 1 according to the application is shown, in which the first output pole u is connected to the second output pole v by means of a phase connection switch S1. Of course, instead, the second output pole v can also be connected to the third output pole w by means of a phase connection switch S or the first output pole u can be connected to the third output pole w by means of a phase connection switch S1. Furthermore, a switching unit 5 is provided, which is designed for closing the phase connection switch S1 in order to switch into the single-phase operation EB and preferably also for opening it in order to switch into the multi-phase operation MB. The switching unit 5 is preferably an integrated component of the control unit 4.

[0036] In the multi-phase operation MB, the energy supply network 3 is connected to the output poles u, v, w, as is shown in Figure 2 by the closed power switch SN. Furthermore, the phase connection switch S1 is open, as a result of which the inverter 1 can be operated in the multi-phase operation MB in a known manner.

[0037] If the switch is specified to change from multiphase operation (MB) to single-phase operation (EB), the phase connection switch S1 can be closed by the switching unit 5, and thus the first and second output poles u and v can be connected to a common output pole uv. However, only the first or second output poles u and v can be used in single-phase operation EB, in which case the output poles u and v are not connected to a common output pole uv. This embodiment of the inverter 1 according to the invention is not shown in the accompanying drawings.

[0038] In single-phase operation EB, inverter 1 is preferably separated from the energy supply network 3, such as Figure 3 The image shows the power switch SN being turned on. Furthermore, in... Figure 3 The load Z between the common output pole u and the intermediate circuit center point M is represented by a dashed line. If only the first or second output pole u, v is used in single-phase operation EB, then the load Z is set between the first output pole u and the intermediate circuit center point M or between the second output pole v and the intermediate circuit center point M. Here, the power switches SU1+, SV1+, SU1-, SV1- of the first or second phase branch U, V can continue to be controlled as in multi-phase operation MB, so that a (now only single-phase) output AC voltage ua1, ua2 is output between the first output pole u and the intermediate circuit center point M or the second output pole v and the intermediate circuit center point M, i.e., at the load Z, as in multi-phase operation MB. Therefore, only one of the phase branches U, V operates here to output a single-phase output AC voltage.

[0039] However, if a common output pole uv is provided as shown in the figure, then in single-phase operation, the switching unit 5 EB controls the power switches SU1+, SV1+, SU1-, and SV1- of the first and second phase branches U and V to output a single-phase output voltage u12 between the common output pole uv and the intermediate circuit center point M, i.e., at the load Z. The control of the power switches SU1+, SV1+, SU1-, and SV1- of the first and second phase branches U and V to output the single-phase output voltage u12 can be performed, for example, by the switching unit 5 itself taking over the control of the relevant power switches SU1+, SV1+, SU1-, and SV1-, or by the switching unit 5 instructing the control unit 4 to correspondingly control the relevant power switches SU1+, SV1+, SU1-, and SV1-, as shown in the figure. Figure 3 The connection arrow between the switching unit 5 and the control unit 4 is shown in the diagram.

[0040] If a common output pole uv and a corresponding output voltage u12 are provided, the inverter 1 preferably switches from single-phase operation EB to multi-phase operation MB in the following manner: that is, the phase connection switch S1 is preferably opened again by the switching unit 5 in order to separate the first output pole u and the second output pole v.

[0041] Furthermore, the switching unit 5 can control the power switches SU1+, SU1-, SV1+, SV1- of the at least three phase branches U, V, W in order to output the at least three output alternating voltages ua1, ua2, ua3 at the respective output poles u, v, w when switching from the single-phase operation EB to the multi-phase operation MB. This can be done in such a way that the switching unit 5 instructs the control unit 4 to control the relevant power switches SU1+, SU1-, SV1+, SV1- accordingly, as shown in Figure 3 by the connection arrows.

[0042] In order to symmetrize the intermediate circuit voltages UC1, UC2 at the first and second intermediate circuit capacitors C1, C2, especially at irregular loads of the upper and lower half waves of the single-phase output voltage u12, a symmetrization unit 6 (not shown in Figure 3 for reasons of space) can also be provided. The symmetrization unit 6 is connected with the first and second input poles A, B and with the intermediate circuit center point M. One possible embodiment of the symmetrization unit 6 is shown in Figure 7 and comprises one upper symmetrization switch SS1 and one lower symmetrization switch SS2, which are connected in series and connect the first and second input poles. The connection point between the upper symmetrization switch SS1 and the lower symmetrization switch SS2 is connected with the intermediate circuit center point M directly or via a symmetrization choke L. The upper symmetrization switch SS1 and the lower symmetrization switch SS2 are controlled by symmetrization control means (which can be an integrated component of the control unit 4) in such a way that the intermediate circuit voltages UC1, UC2 at the first and second intermediate circuit capacitors C1, C2 are symmetrized.

[0043] However, according to the application, a center point connection switch S2 is provided between the intermediate circuit center point M and one output pole, here the third output pole w, which is not connected with the other output stage via a phase connection switch S1, as is also shown in Figure 3 . The switching unit 5 is designed to close the center point connection switch S2 when switching from the multi-phase operation MB to the single-phase operation EB and preferably also to open the center point connection switch S2 when switching from the single-phase operation EB to the multi-phase operation MB. Therefore, no additional symmetrization unit 6 is required.

[0044] If the phase connection switch S1 is not provided, in single-phase operation EB, as described above, one of the output poles (such as the first output pole u) is used to output the single-phase output AC voltage ua1. Therefore, one of the output poles not used for this purpose (i.e., for example, the second or third output poles v, w, if the first output pole u is used to output the single-phase output AC voltage ua1) is connected to the intermediate circuit center point M via the center point connection switch S2 when switching from multi-phase operation MB to single-phase operation EB, and is also preferably opened via the center point connection switch S2 when switching from single-phase operation EB to multi-phase operation MB, so as to disconnect the output pole (such as the first or second output pole v, w) from the intermediate circuit center point M again.

[0045] If the third output terminal w is connected to the intermediate circuit center point M in single-phase operation EB, the switching unit 5 can control the power switches SW1+, SW1-, SW2+, and SW2- of the third phase branch to make the intermediate circuit voltages UC1 and UC2 at the first and second intermediate circuit capacitors C+ and C- symmetrical. This can be done by the switching unit 5 instructing the control unit 4 to control the relevant power switches SW1+, SW1-, SW2+, and SW2- to make the intermediate circuit voltages UC1 and UC2 at the first and second intermediate circuit capacitors C1 and C2 symmetrical.

[0046] On the half-wave, the control unit 4 selects the duty cycle of the clock switch pair in such a way as to output the desired output voltage u1 and preferably follow the first network voltage uL1. Figure 5a This illustrates the control of the power switch for the first phase branch U in a multiphase operation MB. Figure 5a The topmost diagram shows the control of the first upper power switch SU1+, and... Figure 5a The second diagram shows the control of the second power switch SU2+, the diagram below it shows the control of the first lower power switch SU1-, and the diagram at the very bottom shows the control of the second lower power switch SU2-. Figure 5b The first graph shows the first output voltage u1 at the first output terminal u, and Figure 5b The second diagram shows the output current ia1 output at the first output terminal. For the upper half-wave of the output voltage u1 of the phase branch U, the control unit 4 switches the first upper power switch SU1+ (of the upper half-bridge) to the first lower power switch SU1- (of the lower half-bridge) at a fixed predetermined or variable clock frequency. During the upper half-wave, the duty cycle of the first upper power switch SU1+ changes from approximately 0% to approximately 100% and then back to approximately 0%, while the duty cycle of the first lower power switch SU1- changes from approximately 100% to approximately 0% and then back to approximately 100%. During the upper half-wave, the second upper power switch SU2+ is closed and the second lower power switch SU2- is open.

[0047] For the lower half-wave, the first lower power switch SU2+ (of the upper half-bridge) is switched inversely to the second lower power switch SU2- (of the lower half-bridge) with a fixed predetermined or variable clock frequency. During the lower half-wave, the duty cycle of the second upper power switch SU2+ varies from about 100% to about 0% and again to about 100%, and the duty cycle of the second lower power switch SU2- varies from about 0% to about 100% and again to about 0%. During the lower half-wave, the first lower power switch SU1- (of the lower half-bridge) is closed and the first upper power switch SU1+ (of the upper half-bridge) is open.

[0048] The power switches SV1+, SV1-, SV2+, SV2-, SW1+, SW1-, SW2+, SW2- of the second and third phase branches V, W are likewise controlled in the multi-phase operation MB, usually with a 120° phase shift between the respective output voltages u1, u2, u3. Of course, the control of the power switches SU1+, SU1-, SU2+, SU2-, SV1+, SV1-, SV2+, SV2-, SW1+, SW1-, SW2+, SW2- of the respective phase branch (except for the phase shift) need not be exactly precise. In particular in the presence of an asymmetric network voltage uL1, uL2, uL3, the control of the power switches SU1+, SU1-, SU2+, SU2-, SV1+, SV1-, SV2+, SV2-, SW1+, SW1-, SW2+, SW2- of the respective phase branch can deviate slightly by the control unit 4.

[0049] If only one output pole, like the first or the second output pole u, v, is used for outputting the single-phase output alternating voltage in the single-phase operation EB, the power switches of the relevant phase branch (SU1+, SU1-, SU2+, SU2- when the first output pole U is used or SV1+, SV1-, SV2+, SV2- when the second output pole V is used) are preferably controlled as in the multi-phase operation MB.

[0050] If the first and the second output pole u, v are connected to one common output pole uv in the single-phase operation EB, the power switches SU1+, SU1-, SU2+, SU2-, SV1+, SV1-, SV2+, SV2- of the first and the second phase branch are likewise preferably controlled as in the multi-phase operation MB. However, a phase shift is preferably not provided. If one common output pole uv is provided, double the power can be output to the load Z.

[0051] It is also conceivable that the common output pole uv in the single-phase operation EB is also connected with the third output pole and that the power switches SW1+, SW1-, SW2+, SW2- of the third phase branch are controlled essentially as in the multi-phase operation MB. In this case, it is preferred that no phase shift is provided between the output ac voltages generated at the output poles u, v, w, so that triple power can be output to the load Z at the common, triple output pole uv. However, if all phase branches U, V, W are used to output single-phase output ac voltages, no phase branch remains idle for symmetrization. Therefore, symmetrization can be achieved by means of an external symmetrization unit 6.

[0052] In the single-phase operation EB, when using the first or the second output pole, but also when using the common output pole uv, as described above, a situation can arise in which the intermediate circuit voltages UC1, UC2 at the intermediate circuit capacitors C1, C2 are not identical. In order to generate a symmetrization current via the third output pole w, which counteracts this asymmetry, a symmetrization unit 6 can be used in the single-phase operation EB as described above. If not all output poles u, v, w are connected to a common output pole uv, but at least one output pole remains idle (in this case the third output pole w remains idle, since the first and the second output pole u, v are connected to the common output pole uv), in the single-phase operation EB, the idle output pole (in this case the third output pole w) can be connected by the switching unit 5 via the center point connection switch S2 to the intermediate circuit center point M. Thus, the symmetrization of the intermediate circuit voltages UC1, UC2 at the intermediate circuit capacitors C1, C2 can be achieved by a suitable clocking of the power switches SW1+, SW1-, SW2+, SW2- of the third phase branch.

[0053] Figure 6 An exemplary pulse pattern of the power switches SW1+, SW2+, SW1-, SW2- of the third phase branch W for symmetrizing the intermediate circuit voltages UC1, UC2 is shown. Thus, a symmetrical voltage uaM is generated at the third output pole w, which generates a symmetrical current iaM in the case of an asymmetry of the intermediate circuit voltages UC1, UC2, which suppresses and compensates for this asymmetry of the intermediate circuit voltages UC1, UC2. In order to achieve this compensation, the first upper power switch SW1+ and the first lower power switch SW1- are clocked inversely, and in addition, the second upper power switch SW2+ and the second lower power switch SW2- are clocked inversely.

[0054] The symmetrical voltage uaM is commutated between a positive phase and a negative phase, which preferably have the same length. The symmetrical voltage uaM is positive if the first and second upper power switches SW1+, SW2+ are closed and the first and second lower power switches SW1-, SW2- are open. In the positive phase, the symmetrical voltage uaM corresponds to the upper intermediate circuit voltage UC1. The symmetrical voltage uaM is negative if the first and second lower power switches SW1-, SW2- are closed and the first and second upper power switches SW1+, SW2+ are open. In the negative phase, the symmetrical voltage uaM corresponds to the negative lower intermediate circuit voltage UC2.

[0055] If the positive and negative phases of the intermediate circuit voltages UC1, UC2 and thus of the symmetrical voltage uaM are symmetrical to each other, the symmetrical current ima has a zero average value, as shown in Figure 6 But if the positive and negative phases of the intermediate circuit voltages UC1, UC2 and thus of the symmetrical voltage uaM are not symmetrical (for example based on an asymmetrical load Z), the curve of the symmetrical current ima is shifted such that its average value is positive or negative depending on the asymmetry of the intermediate circuit voltages UC1, UC2, and thus, the intermediate circuit voltages UC1, UC2 are made symmetrical again by the symmetrical current ima provided, i.e. compensated. The symmetrical current ima thus causes a power flow from the intermediate circuit capacitors C1, C2 having a lower intermediate circuit voltage UC1, UC2 to the intermediate circuit capacitors C1, C2 having a higher intermediate circuit voltage UC1, UC2. During this compensation, the symmetrical current ima is again shifted such that its average value becomes smaller until the positive and negative phases of the intermediate circuit voltages UC1, UC2 and thus of the symmetrical voltage uaM are symmetrical again and thus the symmetrical current ima is zero.

[0056] Advantageously, the positive phase of the symmetrical voltage uaM does not directly change into the negative phase of the symmetrical voltage uaM and vice versa. Thus, it is possible to prevent the power switches SW1+, SW2+, SW1-, SW2- from being in an unpermitted switching state, which can lead to a damage of the inverter 1. This means that, when switching from the positive phase of the symmetrical voltage uaM (at the level of the intermediate circuit voltage UC1) to the negative phase of the symmetrical voltage uaM (at the level of the intermediate circuit voltage UC2), the symmetrical voltage uaM remains zero (zero phase) for a certain delay time. Thus, the first and second upper power switches SW1+, SW2+ are first closed and the first and second lower power switches SW1-, SW2- are opened (positive phase of the symmetrical voltage uaM). Then, the symmetrical voltage uaM remains zero for the delay time by the second upper power switch SW2+ and the first lower power switch SW1- being closed and the first upper power switch SW1+ and the second lower power switch SW2- being opened (zero phase of the symmetrical voltage uaM). After the delay time, the first and second lower power switches SW1-, SW2- are closed and the first and second upper power switches SW1+, SW2+ are opened (negative phase of the symmetrical voltage uaM). Similarly, the negative phase of the symmetrical voltage uaM is switched via the zero phase to the positive phase of the symmetrical voltage uaM, where advantageously the same delay time is provided for the zero phase as for the zero phase when switching from the positive phase to the negative phase. By a suitable selection of the zero phase, it is possible to reduce the ripple current through the filter coil L of the third phase branch W in the idle mode, i.e. when the intermediate circuit voltages UC1, UC2 are symmetrical.

[0057] In order to further prevent the occurrence of unpermitted switching states or switching state transitions of the power switches SW1+, SW2+, SW1-, SW2- which can lead to a damage of the inverter 1, it is possible to first open the first lower power switch SW1- and then close the first upper power switch SW1+ only after a dead time when transitioning from the zero phase (first lower power switch SW1- and second upper power switch SW2+ closed and second lower power switch SW2- and first upper power switch SW1+ opened) to the positive phase (first and second upper power switches SW1+, SW2+ closed and first and second lower power switches SW1-, SW2- opened). Likewise, when starting from the positive phase into the zero phase, it is possible to first open the first upper power switch SW1+ and then close the first lower power switch SW1- only after a dead time.

[0058] Likewise, starting from the zero phase (first lower power switch SW1- and second upper power switch SW2+ are closed, second lower power switch SW2- and first upper power switch SW1+ are open), upon transition to the negative phase (first and second lower power switches SW1-, SW2- are closed and first and second upper power switches SW1+, SW2+ are open), the second upper power switch SW2+ can first be opened and the second lower power switch SW2- can then be closed only after a dead time. Likewise, upon entering the zero phase from the negative phase, the second upper power switch SW2+ can be closed again only after a dead time after opening the second lower power switch SW2-. For the sake of clarity, the dead times are not shown in Figure 6 the following figures.

Claims

1. An inverter (1) comprising a first input pole (A) and a second input pole (B) and comprising: a first phase branch (U) having an upper power switch (SU1+) and a lower power switch (SU1-) connected in series with the upper power switch (SU1+) via a first output pole (u), a second phase branch (V) having an upper power switch (SV1+) and a second lower power switch (SV1-) connected in series with the upper power switch (SV1+) via a second output pole (v), an intermediate circuit (ZK) arranged between a first input pole (A) and a second input pole (B) and having a first intermediate circuit capacitor (C1) and a second intermediate circuit capacitor (C2) connected with the first intermediate circuit capacitor (C1) via an intermediate circuit center point (M), and a switching unit designed to switch the inverter (1) from a multiphase operation (MB) to a single-phase operation (EB) in order to output a single-phase output alternating voltage, characterized in that the inverter (1) comprises a third phase branch (W) having an upper power switch (SW1+) and a lower power switch (SW1-) connected in series with the upper power switch (SW1+) via a third output pole (w), and a control unit (4) designed to control the upper power switches (SU1+, SV1+, SW1+) and the lower power switches (SU1-, SV1-, SW1-) of the phase branches (U, V, W) in the multiphase operation (MB) respectively in such a way that an input direct voltage (Ue) applied between the first input pole (A) and the second input pole (B) is converted into output alternating voltages (ua1, ua2, ua3) applied on the respective output poles (u, v, w), a center point connection switch (S2) is arranged between the third output pole (w) and the intermediate circuit center point (M), the switching unit (5) is designed to close the center point connection switch (S2) in the single-phase operation (EB) in order to connect the third output pole (w) with the intermediate circuit center point (M), and the switching unit (5) is designed to control the upper and lower power switches (SW1+, SW1-) of the third phase branch (W) in the single-phase operation (EB) in such a way that the intermediate circuit voltages (UC1, UC2) at the first and second intermediate circuit capacitors (C1, C2) are symmetrical.

2. The inverter (1) according to claim 1, characterized in that the switching unit (5) is designed to instruct the control unit (4) to control the upper and lower power switches (SW1+, SW1-) of the third phase branch (W) in the single-phase operation (EB) in such a way that the intermediate circuit voltages (UC1, UC2) at the first and second intermediate circuit capacitors (C1, C2) are symmetrical.

3. The inverter (1) according to claim 1 or 2, characterized in that The switching unit (5) is designed for controlling the respective upper and the respective lower power switch (SU1+, SV1+, SW1+, SU1-, SV1-, SW1-) of the respective phase branch (U, V, W) for outputting the output alternating voltage (ua1, ua2, ua3) at the respective output pole (u, v, w) when switching from single-phase operation (EB) to multi-phase operation (MB).

4. The inverter (1) according to claim 1 or 2, characterized in that In the first phase branch (U), a further upper power switch (SU2+) is connected to the upper power switch (SU1+) of the first phase branch (U) via a first upper center point (MU+), and a further lower power switch (SU2-) is connected to the lower power switch (SU1-) of the first phase branch (U) via a first lower center point (MU-), In the second phase branch (V), a further upper power switch (SV2+) is connected to the upper power switch (SV1+) of the second phase branch (V) via a second upper center point (MV+), and a further lower power switch (SV2-) is connected to the lower power switch (SV1-) of the second phase branch (V) via a second lower center point (MV-), In the third phase branch (W), a further upper power switch (SW2+) is connected to the upper power switch (SW1+) in the third phase branch (W) via a third upper center point (MW+), and a further lower power switch (SW2-) is connected to the lower power switch (SW1-) of the third phase branch (W) via a third lower center point (MW-), and The first, second and third upper center points (MU+, MV+, MW+) and the first, second and third lower center points (MU-, MV-, MW-) are connected to the intermediate circuit center point (M) in the first, second and third phase branch (U, V, W), respectively.

5. The inverter (1) according to claim 1 or 2, characterized in that The switching unit (5) is designed for closing a phase connection switch (S1) arranged between the first and the second output pole (u, v) in single-phase operation (EB) in order to connect the first and the second output pole (u, v) to a common output pole (uv) and for controlling the respective upper power switches (SU1+, SV1+) and the respective lower power switches (SU1-, SV1-) of the first and the second phase branch (U, V) for outputting a single-phase output alternating voltage (u12) between the common output pole (uv) and the intermediate circuit center point (M).

6. The inverter (1) according to claim 5, characterized in that The switching unit (5) is designed for instructing the control unit (4) to control the respective upper power switches (SU1+, SV1+) and the lower power switches (SU1-, SV1-) of the first and the second phase branch (U, V) in single-phase operation (EB) for outputting a single-phase output alternating voltage (u12) at the common output pole (uv).

7. The inverter (1) according to claim 5, characterized in that The switching unit (5) is designed for opening the phase connection switch (S1) when switching from single-phase operation (EB) to multi-phase operation (MB) in order to separate the first output pole (u) from the second output pole (v).

8. The inverter (1) according to claim 6, characterized in that The switching unit (5) is designed to open the phase connection switch (S1) when switching from single-phase operation (EB) to multi-phase operation (MB) in order to separate the first output pole (u) and the second output pole (v).

9. A method for operating an inverter (1), characterized in that The inverter comprises a first phase branch (U) having an upper power switch (SU1+) and a lower power switch (SU1-) connected in series with the upper power switch via the first output pole (u), a second phase branch (V) having an upper power switch (SV1+) and a lower power switch (SV1-) connected in series with the upper power switch via the second output pole (v), and a third phase branch (W) having an upper power switch (SW1+) and a lower power switch (SW1-) connected in series with the upper power switch via the third output pole (w), the respective upper power switches (SU1+, SV1+, SW1+) and the respective lower power switches (SU1-, SV1-, SW1-) of the phase branches (U, V, W) being controlled in multi-phase operation (MB) in such a way that an input DC voltage (Ue) applied to an intermediate circuit (ZK) having a first intermediate circuit capacitor (C1) and a second intermediate circuit capacitor (C2) connected to the first intermediate circuit capacitor (C1) via an intermediate circuit center point (M) is converted into an output AC voltage (ua1, ua2, ua3) applied to the output poles (u, v, w), The inverter (1) is switched from multi-phase operation (MB) to single-phase operation (EB) by the switching unit (5) in order to output a single-phase output AC voltage, the third output pole (w) is connected to the intermediate circuit center point (M), and the upper and lower power switches (SW1+, SW1-, SW2+, SW2-) of the third phase branch (W) are controlled in such a way that the intermediate circuit voltages (UC1, UC2) at the first and second intermediate circuit capacitors (C1, C2) are symmetrical.

10. The method of claim 9, wherein, In single-phase operation (EB), the first and second output poles (u, v) are connected to form a common output pole (uv), and a single-phase output AC voltage (u12) is output between the first and / or second output pole (u, v) and the intermediate circuit center point (M).

11. The method of claim 10, wherein, The respective upper and lower power switches (SU1+, SV1+, SU1-, SV1-) of the first and second phase branches (U, V) are controlled in single-phase operation (EB) by the switching unit (5) in such a way that a single-phase output AC voltage (u12) is output at the common output pole (uv).

12. The method of claim 10, wherein, The switching unit (5) instructs the control unit (4) in single-phase operation (EB) to control the respective upper and lower power switches (SU1+, SU1-) of the first and second phase branches (U, V) in such a way that a single-phase output AC voltage (u12) is output at the common output pole (uv).

13. The method according to any one of claims 10 to 12, characterized in that, When the inverter is switched from single-phase operation (EB) to multi-phase operation (MB), the first output pole (u) and the second output pole (v) are separated by the switching unit (5).

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