Power converter and method for operating a power converter

Abstract

For a power converter that is simple in terms of circuit technology and using which it is possible to supply energy itself, to balance the DC link and to precharge the DC link from the AC side, a DC / DC supply converter (16) is provided in the power converter (1), wherein a first DC converter terminal (17) of the DC / DC supply converter (16) is connected to the DC self-supply bus (15) and a second DC converter terminal (18) of the DC / DC supply converter (16) is connected to the DC link (8) and, during operation of the power converter (1), the DC link voltage (UZK) or at least a capacitor voltage (UC1, UC2) is applied to the second DC converter terminal (18) of the DC / DC supply converter (16), and a DC / DC charging converter (20) is also provided in the power converter (1), wherein a first DC converter terminal (22) of the DC / DC charging converter (20) is connected to the DC self-supply bus (15) and a second DC converter terminal (21) of the DC / DC charging converter (20) is configured having a number of DC charging terminals (21a, 21b) corresponding to the number of DC link capacitors (CZK), wherein each DC charging terminal (21a, 21b) is connected to a respective DC link capacitor (CZK).

Description

[0001] Power converter and method for operating a power converter

[0002] The present invention relates to a power converter with a DC connection to which an electrical direct voltage can be applied during operation, and an AC connection to which an electrical alternating voltage can be applied during operation, wherein a DC / AC main converter with a DC converter connection and an AC converter connection is provided in the power converter, and the AC converter connection is connected to the AC connection of the power converter, and the DC converter connection is connected to a DC intermediate circuit to which an intermediate circuit voltage is applied during operation of the power converter, wherein the DC intermediate circuit comprises at least two intermediate circuit capacitors connected in series, and a capacitor voltage is applied to each of the intermediate circuit capacitors during operation of the power converter, wherein the DC connection of the power converter is connected to the DC intermediate circuit, wherein an AC / DC supply converter is provided in the power converter,which is connected to an AC converter terminal of the AC / DC supply converter with the AC terminal of the power converter, and is connected to a DC converter terminal of the AC / DC supply converter with a DC internal supply bus of the power converter. The invention also relates to a method for operating such a power converter.

[0003] Power converters are well-known power electronic devices that operate as inverters (DC / AC converters) to convert a direct current (DC) voltage (electrical current or voltage) into an alternating current (AC) voltage (electrical current or voltage), or vice versa (AC / DC converters). Bidirectional power converters are also known, which allow energy to flow in both directions. A DC voltage is applied to one terminal of the power converter, and an AC voltage is applied to the other terminal.

[0004] The DC voltage is provided by a DC voltage source, such as a battery, photovoltaic system, etc., or is absorbed by a DC voltage sink, such as a battery, inverter, etc. The AC voltage is absorbed by an AC voltage sink, such as an AC grid, etc., or is provided by an AC voltage source, such as an AC grid, wind turbine, etc.

[0005] A DC link consisting of one or more series-connected DC link capacitors is typically provided between the DC and AC sections of the power converter. A DC / AC converter, at least unidirectional, is connected to the DC link. This converter converts the DC link voltage into an AC voltage of the power converter, or converts an AC voltage of the power converter into an DC link voltage. As a power electronic device, a power converter comprises at least one semiconductor switch, such as a transistor, usually a plurality of semiconductor switches, which are controlled by a control unit of the power converter to convert energy from DC to AC, or vice versa. This results in certain requirements for the power converter.

[0006] The control unit is typically a microprocessor-based component or an integrated circuit (such as an application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA)) that requires an electrical power supply. The power converter therefore requires its own electrical power supply, for example, to operate the control unit or to cool the power converter or for other peripherals. The power converter includes a self-powered power supply unit for the electrical power supply, which can be supplied on the AC side, for example, from an AC network (AC voltage source or AC voltage sink) or an AC output of the power converter. An example of such a power supply for the power converter can be found in WO 2013 / 178546 A1.An AC-side supply is particularly necessary when no DC voltage source is available, for example at night in a PV system or when the battery is empty. If a DC voltage source is available, a DC-side power supply can also be provided, for example by an inverter (DC / DC converter) as a self-power supply power supply. A combination of an AC-side supply with a DC-side supply is also known, for example from DE 10 2008 032 317 A1. In particular, it is also known to provide a small AC power supply for the standby mode of the converter with a more powerful DC power supply for feeding power from the DC voltage source into an AC voltage sink for the purpose of reducing losses.

[0007] The DC link of the power converter is often designed as a split DC link with at least two DC link capacitors in series. A DC voltage is applied to each DC link capacitor, which together result in the DC link voltage. With two DC link capacitors, the DC link voltage across the DC link capacitors would be halved, provided identical DC link capacitors are used. However, as is well known, asymmetrical DC voltages can occur across the DC link capacitors during operation of the power converter for various reasons. Such asymmetries can lead to improper operation of the power converter or even to damage to parts of the power converter. During operation of the power converter, balancing of the DC voltages across the DC link capacitors is therefore necessary. Various approaches are known for this.In a simple design, balancing resistors are simply connected in parallel with the DC link capacitors. However, electrical current flows continuously through these balancing resistors during operation, resulting in losses and reduced efficiency of the converter. There are also known approaches that utilize the distribution of the DC link between several DC link capacitors to supply the converter with its own power. However, this requires switching converters connected to the DC link capacitors. Examples of these can be found in EP 2 826 126 B1.

[0008] Another balancing circuit can be found in EP 2 760 103 A1. It describes a balancing circuit consisting of a transformer whose primary side is connected to the intermediate circuit voltage via a switching element. The secondary side of the transformer consists of two secondary windings, each connected in series with a diode. Each secondary winding with a diode is connected in parallel with an intermediate circuit capacitor. The diodes ensure that the intermediate circuit capacitor is charged with the lower voltage via the transformer. However, balancing cannot be ensured by charging via the transformer alone, because this could also overload an intermediate circuit capacitor (charging beyond a rated capacitor voltage).

[0009] When the converter is switched off, the DC link is usually also discharged. To start up the converter, the DC link must therefore first be precharged. If no DC voltage is available on the DC side, the DC link must be precharged on the AC side, for example, from an AC grid. An example of precharging the DC link capacitors on the AC side can be found in WO 2019 / 206910 A1.

[0010] However, these requirements—self-power supply, balancing, and precharging of the intermediate circuit from the AC side—require a wide variety of circuits, which should be implemented in a power converter with as little loss as possible. This makes a power converter, apart from the power electronics components for energy conversion, also complex in terms of circuitry, because a number of additional circuits are required.

[0011] It is therefore an object of the present invention to provide a power converter with a simple circuit design, with which the self-power supply, the balancing of the intermediate circuit and the pre-charging of the intermediate circuit from the AC side are possible.

[0012] This object is achieved with a power converter as mentioned above with the features of claim 1, as well as with a method for operating such a power converter as mentioned above according to claim 5. By providing an additional DC / DC charging converter, which is powered by the DC bus, in combination with the DC / DC supply converter, the various operating modes can be easily implemented, so that, in particular, the balancing of the intermediate circuit and the precharging of the intermediate circuit from the AC side can be easily implemented. The internal power supply of a control unit can also be ensured, if required, by providing the AC / DC and DC / DC supply converters.

[0013] In particular, balancing is achieved automatically and in an energy-efficient manner by recharging the intermediate circuit capacitors via the DC / DC charging converter from the DC internal power supply bus and simultaneously discharging the DC intermediate circuit via the DC / DC supply converter into the DC internal power supply bus. The energy extracted from the intermediate circuit capacitors is at least partially reused to recharge the intermediate circuit capacitors until symmetry is restored.

[0014] The DC internal power supply bus, to which the DC supply voltage is applied during operation, can also ensure the internal power supply of certain components of the power converter. If the power converter is equipped with at least one control unit with a supply input, with the supply input connected to the DC internal power supply bus, the internal power supply of the control unit can also be ensured.

[0015] In an advantageous embodiment, at least one DC / DC main converter is provided between the DC connection and the DC intermediate circuit, with a first DC converter connection of the DC / DC main converter and a second DC converter connection of the DC / DC main converter. The DC connection of the power converter is connected to the DC intermediate circuit via the at least one DC / DC main converter, in that the first DC converter connection of the DC / DC main converter is connected to the DC connection of the power converter, and the second DC converter connection of the DC / DC main converter is connected to the DC intermediate circuit. This allows different DC sources or DC sinks to be connected. In the case of multiple DC / DC main converters, multiple DC sources or DC sinks can also be connected.

[0016] In an advantageous embodiment, the DC / DC charging converter comprises a transformer with a primary winding on a transformer core, wherein the primary winding is connected to the DC internal supply bus via at least one clock switching element, that on the secondary side of the transformer on the transformer core a plurality of secondary windings corresponding to the number of intermediate circuit capacitors are provided, wherein a turns ratio between the primary winding and the respective secondary winding is selected such that during operation of the power converter a DC charging voltage is induced at a secondary winding which substantially corresponds to the capacitor voltage desired at the respectively assigned intermediate circuit capacitor, and that a diode is connected in series with each secondary winding such that during operation of the power converter the diode conducts when the capacitor voltage is lower than the respective DC charging voltage.With this circuit, no active control is required on the secondary side of the transformer, since the diodes regulate the energy flow independently.

[0017] With the power converter according to the invention, different operating modes can be realized in a simple manner.

[0018] The DC / DC supply converter connected to the DC link can be switched off to precharge the DC link capacitors of the DC link via the AC / DC supply converter, the DC self-supply bus and the DC / DC charging converter in the absence of DC voltage at the DC terminal.

[0019] To symmetrize the DC link voltage via the DC / DC supply converter, the DC internal power supply bus, and the DC / DC charging converter when a DC voltage is applied to the DC connection, the DC / DC supply converter can be switched on. This converts the DC link voltage or at least a capacitor voltage of a DC link capacitor in the DC link into the DC supply voltage via the DC / DC supply converter and feeds it into the DC internal power supply bus. The DC link voltage is thus discharged into the DC internal power supply bus via the DC / DC supply converter and recharged from the DC internal power supply bus via the DC / DC charging converter to establish symmetry of the capacitor voltages. This symmetrization can also be used for precharging.

[0020] To prevent voltage overload of the DC link capacitors of the DC link, both precharging and balancing can be configured to detect the capacitor voltages and reduce the output power of the DC / DC charging converter if one of the capacitor voltages exceeds a specified nominal voltage. This reduces the charging current for the DC link capacitors and prevents overvoltage on the DC link capacitors.

[0021] The present invention will be explained in more detail below with reference to Figures 1 to 3, which show exemplary, schematic, and non-limiting advantageous embodiments of the invention. Figure 1 shows a power converter according to the invention,

[0022] Fig.2 shows a further embodiment of a power converter according to the invention and

[0023] Fig.3 an advantageous embodiment of a DC / DC charging converter of the power converter.

[0024] Fig. 1 shows a power converter 1 according to the invention (in the form of a voltage source converter) with a DC connection 2 and an AC connection 3. During operation of the power converter 1, an electrical direct voltage DCG, such as a DC voltage or a DC current, can be applied to the DC connection 2 via two connection contacts DC+ and DC-. A direct voltage source providing the direct voltage DCG, or a direct voltage sink receiving the direct voltage DCG, can be connected to the DC connection 2. An alternating voltage ACG can be applied to the AC connection 3 during operation of the power converter 1. An alternating voltage source providing the alternating voltage ACG, or an alternating voltage sink receiving the alternating voltage ACG, can be connected to the AC connection 3. The alternating voltage ACG can be single-phase or multi-phase, for example three-phase as in the exemplary embodiment shown in Fig. 1.Therefore, at least two connection contacts (not shown) are provided on the AC connection 3.

[0025] On the AC side, upstream of the AC terminal 3, a separating unit 4, such as a separating relay, can also be provided in the power converter 1 to electrically separate the AC terminal 3 of the power converter 1, or an AC voltage source connected to it during operation, from the other parts of the power converter 1. Such a design is shown in Fig. 2.

[0026] Likewise, further circuit components can be provided in the converter 1 on the AC side, preferably upstream of an isolating unit 4, such as a known electrical output filter, which generally consists of a circuit of capacitors and chokes. However, since such circuit components are irrelevant to the invention and are also well known, they are not shown in the figures.

[0027] A DC / AC main converter 5 is provided on the AC side of the power converter 1. The DC / AC main converter 5 serves to transfer energy to the power converter 1 and, during operation of the power converter 1, converts a DC voltage into an AC voltage, which is applied to an AC converter terminal 6 of the DC / AC main converter 5, or vice versa in the case of a bidirectional first DC / AC main converter 5 (in the opposite case, the DC / AC main converter 5 operates as an AC / DC converter). The AC converter connection 6 of the DC / AC main converter 5 is connected to the AC connection 3 of the power converter 1, if necessary via a separation unit 4 and / or further circuit components of the power converter 1. The AC energy transmission side of the power converter 1 is thus formed by the AC part of the DC / AC main converter 5 and the AC connection 3, and if necessary the separation unit 4 and / or further circuit components between the separation unit 4 and the DC / AC main converter 5.

[0028] The DC converter terminal 7 of the DC / AC main converter 5 is connected to a DC intermediate circuit 8. The DC intermediate circuit 8 consists of at least two intermediate circuit capacitors CZK, which preferably have the same capacitance values. The at least two intermediate circuit capacitors CZK are connected in series. During operation of the power converter 1, an intermediate circuit voltage UZK is applied to the DC intermediate circuit 8. Due to the serial connection of the intermediate circuit capacitors CZK, this voltage is divided into capacitor voltages Uci, Uc2, each applied to an intermediate circuit capacitor CZK. The DC / AC main converter 5 thus receives the intermediate circuit voltage UZK as a DC voltage for conversion into the AC voltage at the AC converter terminal 6.

[0029] In one possible embodiment of the power converter 1, as in the embodiment according to Fig. 1, during operation of the power converter 1 the intermediate circuit voltage UZK corresponds to the direct voltage DCG applied to the DC connection 2, in this case a direct voltage. In another possible embodiment of the power converter 1, as shown in Fig. 2, at least one at least unidirectional DC / DC main converter 9 is arranged between the DC connection 2 and the direct voltage intermediate circuit 8, which, during operation of the power converter 1, serves to transfer energy to the power converter 1 and converts a direct voltage DCG applied to the DC connection 2, in this case a direct voltage, into the intermediate circuit voltage UZK, or vice versa in the case of a bidirectional converter. For example, in a PV power converter, several PV strings can be provided, each of which is applied to a DC / DC main converter 9.Or several batteries could be provided, each connected to a main DC / DC converter 9.

[0030] The DC energy transmission side of the power converter 1 is thus formed by the DC intermediate circuit 8 and the DC part of the DC / AC main converter 5, or by the DC / DC main converter 9, the DC intermediate circuit 8 and the DC part of the DC / AC main converter 5.

[0031] During operation, the energy transfer of the power converter 1 thus takes place from the DC connection 2 via the DC energy transfer side and the AC energy transfer side to the AC connection 3, or vice versa in the case of a bidirectional converter. For the operation of the power converter 1, a control unit 10 is provided which controls the individual parts of the power converter 1 via control lines S1, S3, in particular the DC / AC main converter 5 via a control line S1 and optionally also the at least one DC / DC main converter 9 via a control line S3. If present, the control unit 10 can also control the isolation unit 4 via a control line S2. The type of control signals sent via the control lines S1, S2, S3 is irrelevant for the invention. During operation, the control unit 10 controls, in the DC / AC main converter 5, and possibly also in the DC / DC main converter 9, in a known manner, in particular switching elements, such as semiconductor switches, such as transistors.

[0032] The control unit 10 is preferably implemented as a microprocessor-based component or as an integrated circuit, such as an application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA), which requires an electrical power supply for operation.

[0033] For operation, the power converter 1 therefore requires its own power supply 11, which in particular provides the electrical energy for operating the control unit 10, and possibly also for other basic supply components or peripherals. The power supply 11 of the power converter 1 comprises an AC / DC supply converter 12, whose AC converter connection 13 is connected to the AC connection 3 of the power converter 1. This ensures, even in the case of a separation unit 4, that a voltage supply via the AC voltage source or AC voltage sink connected to the power converter 1 is possible even when the separation unit 4 is open. The DC converter connection 14 of the AC / DC supply converter 12 is connected to a DC internal supply bus 15 of the power converter 1.The internal power supply 11 of the power converter 1 also includes a DC / DC supply converter 16, whose first DC converter terminal 17 is connected to the internal DC supply bus 15 and whose second DC converter terminal 18 is connected to the DC intermediate circuit 8, so that during operation of the power converter 1 in this embodiment according to Fig. 1, the intermediate circuit voltage UZK is applied to the second DC converter terminal 18 of the DC / DC supply converter 16. The control unit 10 has a supply terminal 19 connected to the internal DC supply bus 15. However, a separate power supply unit could also be provided for the electrical power supply of the control unit 10. In this case, the supply terminal 19 would be connected to the power supply unit or switchably connected to the power supply unit or the internal DC supply bus 15.

[0034] This allows the DC internal power supply bus 15 to be supplied with a DC supply voltage Uv from the AC / DC supply converter 12 via the AC side of the power converter 1, as well as via the DC / DC supply converter 16 and the DC intermediate circuit 8. The power supply to the control unit 10 can thus be provided during operation of the power converter 1 from both the AC side and the DC side of the power converter 1. The AC / DC supply converter 12 and / or the DC / DC supply converter 16 can also be controlled by the control unit 10 via a control line S4, S5. However, it can also be provided that the AC / DC supply converter 12 is always active, thus eliminating the need for a control line S5 (as in Fig. 2).

[0035] During operation of the power converter 1, the DC supply voltage Uv is provided, for example, via the AC / DC supply converter 12 and the AC connection 3 of the power converter 1. If necessary, the DC internal supply bus 15 can be additionally or alternatively supplied with the DC supply voltage Uv via the DC / DC supply converter 16. However, according to the invention, the DC / DC supply converter 16 has another function in addition to the optional voltage supply of the DC internal supply bus 15, as explained below.

[0036] A DC / DC charging converter 20 is also provided in the power converter 1. A first DC converter terminal 22 of the DC / DC charging converter 20 is connected to the DC internal supply bus 15, so that the DC supply voltage Uv is applied to this first DC converter terminal 22 during operation. At the second DC converter terminal 21 of the DC / DC charging converter 20, a plurality of DC charging voltages ULI, UL2 are provided at DC charging terminals 21a, 21b of the DC converter terminal 21 of the DC / DC charging converter 20, depending on the number of intermediate circuit capacitors CZK. In the embodiment according to Fig. 1, two DC charging voltages ULI, UL2 are thus provided. The DC / DC charging converter 20 thus converts the DC supply voltage Uv of the DC self-supply bus 15 into the plurality of DC charging voltages ULI, UL2.The DC / DC charging converter 20 thus has a DC converter connection 22 with a plurality of DC charging connections 21a, 21b, wherein each of the DC charging connections 21a, 21b is connected to one of the intermediate circuit capacitors CZK. During operation of the power converter 1, the DC / DC charging converter 20 can generate a DC charging voltage U1, UL2 at each of the DC charging connections 21a, 21b, at least as needed, which is applied to the respectively connected intermediate circuit capacitor CZK.

[0037] Although it should of course be noted that a low DC voltage, for example in the range of 10-50V, preferably 20V or 24V, is applied to the DC internal supply bus 15 as the DC supply voltage Uv, while the intermediate circuit voltage UZK is generally in the range of a few hundred volts to a thousand volts. The DC internal supply bus 15 is not used, in particular, for the energy transmission of the power converter 1. This circuit with AC / DC supply converter 12, DC / DC supply converter 16, and DC / DC charging converter 20 enables the internal energy supply of the power converter 1 (as already explained above), but also the balancing of the DC intermediate circuit 8 and also the precharging of the DC intermediate circuit 8 from the AC side in a simple and energy-efficient manner, as explained below.

[0038] Precharging the DC link 8 from the AC side of the power converter 1 is necessary, for example, when no DC voltage DCG is available at the DC terminal 2 of the power converter 1 (for example, at night in the case of a PV system at the DC terminal 2) and the DC link 8 therefore cannot be supplied via the DC terminal 2 or the DC / DC main converter 9. In this case, the AC / DC supply converter 12 supplies the DC internal supply bus 15 via the AC terminal 3, to which an AC voltage ACG is present, for example from an AC grid. To precharge the DC intermediate circuit 8, the DC supply voltage Uv of the DC internal supply bus 15 is converted by the DC / DC charging converter 20 into the DC charging voltages ULI, UL2, which are applied to the intermediate circuit capacitors CZK, whereby the intermediate circuit capacitors CZK are charged to the DC charging voltages ULI, UL2.During the pre-charging of the DC intermediate circuit 8 via the DC / DC charging converter 20, the DC / DC supply converter 16, which is also connected to the DC intermediate circuit 8 and to the DC internal supply bus 15, is switched off in order to prevent the DC / DC supply converter 16 from simultaneously drawing energy from the DC intermediate circuit 8 and discharging the DC intermediate circuit 8 again, which would prevent pre-charging.

[0039] The pre-charging function of the power converter 1 enables another operating mode of the power converter 1 in certain configurations. For example, if a PV system is connected to the DC connection 2, the PV system cannot provide any DC voltage DCG at night. This means that the DC link 8 is discharged. However, the power converter 1 is also connected to an AC grid via the AC connection 3 at night. The DC link 8 can be pre-charged via the AC / DC supply converter 12 and the DC / DC charging converter 20, and the power converter 1 can thus essentially be put into operation. When the power converter 1 is in operation, for example, reactive power could be generated with the power converter 1 and fed into the AC grid. The ability of a power converter 1 for PV applications to feed reactive power is a requirement of grid operators in many countries.The energy for generating the reactive power could be taken from the AC grid by the DC / AC main converter 5, for example as active power, and loaded via the DC / AC main converter 5 into the DC intermediate circuit 8, specifically into the intermediate circuit capacitors CZK.

[0040] With the power converter 1 according to the invention, reactive power could be fed into an AC network via the AC / DC supply converter 12, the DC internal supply bus 15 and the DC / DC charging converter 20 even when no DC voltage is present at the DC connection 2.

[0041] If an asymmetry of the capacitor voltages Uci, Uc2 occurs at the intermediate circuit capacitors CZK, for example due to component values ​​of the intermediate circuit capacitors CZK not being exactly the same, such an asymmetry can be compensated according to the invention as follows.

[0042] If one of the intermediate circuit capacitors CZK is TOO low (for example, one capacitor voltage Uci, Uc2 at nominal voltage and another below it), for example due to different capacitance values ​​of the intermediate circuit capacitors CZK, SO, the DC charging voltage ULI, UL2 provided by the DC / DC charging converter 20 ensures that the undercharged intermediate circuit capacitor CZK is recharged with a capacitor voltage Uci, Uc2 that is lower than the associated DC charging voltage ULI, UL2. The electrical output power provided by the DC / DC charging converter 20 thus flows into the DC intermediate circuit 8 and recharges the undercharged intermediate circuit capacitor CZK. At the same time, however, the DC intermediate circuit 8 is also discharged via the DC / DC supply converter 16, with all intermediate circuit capacitors CZK being discharged together because its second DC converter connection 18 is connected to the DC intermediate circuit 8.The DC / DC supply converter 16 is thus active simultaneously with the DC / DC charging converter 20. The discharged electrical energy is not lost, but is made available to the DC / DC charging converter 20 for recharging via the DC internal supply bus 15. The electrical energy is thus circulated. Accordingly, it can also be the case that energy that was taken from an intermediate circuit capacitor CZK is fed back into it. This (ideally) achieves that both intermediate circuit capacitors CZK are charged to the nominal voltage (e.g., 500V). In other words, this recharging and simultaneous discharging can also be referred to as balancing the intermediate circuit capacitors CZK.

[0043] If only the intermediate circuit capacitor CZK were recharged, the undercharged intermediate circuit capacitor CZK would be charged to the same level as the other intermediate circuit capacitor CZK, which, however, may be above the nominal voltage and is therefore undesirable. If only a discharge were to occur, both intermediate circuit capacitors CZK would be reduced by the same level, and the asymmetry would remain. However, by simultaneously recharging and discharging, the capacitor voltage Uci, Uc2 can be balanced.

[0044] During operation of the power converter 1, the control unit 10 can coordinate the recharging and discharging, i.e., balancing. Measured values ​​of the capacitor voltages Uci, Uc2, which the DC / AC main converter 5 can provide, can serve as the basis for this. The DC / DC supply converter 16 and the DC / DC charging converter 20 can be controlled by the control unit 10 via the control lines S4, S6 to the DC / DC charging converter 20 and the DC / DC supply converter 16.

[0045] Another example of balancing occurs when the capacitor voltages Uci and Uc2 are different, but both are lower than the nominal voltage, for example, if the voltages are distributed differently due to component tolerances. In this case, balancing is also achieved as described above via the DC / DC supply converter 16, the DC internal power supply bus 15, and the DC / DC charging converter 20. The DC / DC supply converter 16 is connected on the input side via the DC converter connection 18 to the DC intermediate circuit 8, i.e., to the intermediate circuit voltage UZK. The DC / DC supply converter 16, which feeds the DC internal supply bus 15, draws electrical power from the DC link 8, whereby the DC / DC supply converter 16 discharges the intermediate circuit capacitors CZK and lowers the capacitor voltages Uci, Uc2 on all intermediate circuit capacitors CZK.Since this extracted electrical power flows into the internal power supply of the power converter 1, this is very energy-efficient. If one of the capacitor voltages Uci, Uc2 drops below the assigned DC charging voltage ULI, UL2 of the DC / DC charging converter 20, the DC / DC charging converter 20 ensures balancing again by recharging as described above. During balancing, the electrical energy of the overcharged intermediate circuit capacitor CZK is thus circulated via the DC / DC supply converter 16, the internal DC power supply bus 15, and the DC / DC charging converter 20 and supplied to the now undercharged intermediate circuit capacitor CZK.

[0046] Since the DC / DC power converter 16 is also responsible for the internal power supply, it is generally designed with a higher power rating than the charging converter 20, i.e., with a higher rated output power. For example, the DC / DC charging converter 20 could be designed with a power rating of 20W, and the DC / DC power converter 16 with a power rating of 50W.

[0047] It is obvious that the voltage levels of the various converters, especially the DC / DC supply converter 16 and the DC / DC charging converter 20, must be coordinated with each other and with the rated intermediate circuit voltage UZK and the rated capacitor voltages Uci, Uc2, respectively. The rated voltages are the voltages that should be achieved during proper operation of the power converter 1. Such voltage-based dimensioning of the converters can easily be performed by a specialist.

[0048] It is sensible, for example, if the achievable DC charging voltages ULI, UL2 of the DC / DC charging converter 20 each correspond at least to the nominal capacitor voltages Uci, Uc2 desired at the intermediate circuit capacitors CZK, whereby the sum of the DC charging voltages ULI, UL2 advantageously corresponds to the nominal intermediate circuit voltage UZK.

[0049] To ensure symmetrization in a simple manner, it could be provided that the output voltage of the DC / DC supply converter 16, which is fed into the DC internal supply bus 15 as the DC supply voltage Uv, is greater than the output voltage of the AC / DC supply converter 12, which is also fed into the DC internal supply bus 15. For example, the DC / DC supply converter 16 could provide an output voltage of 24V and the AC / DC supply converter 12 could provide an output voltage of 20V. Thus, the DC internal supply bus 15 is primarily supplied by the DC / DC supply converter 16 during operation of the power converter 1, and the symmetrization described above is ensured. Of course, it could also be provided that the control unit 10 controls the DC / DC supply converter 16 and the AC / DC supply converter 12 in order to enable symmetrization.

[0050] A voltage overload (more than the rated voltage of a DC link capacitor CZK) of each individual DC link capacitor CZK is generally preventable. This can also be considered an asymmetry in the case of differently charged DC link capacitors CZK and can be prevented by balancing via the DC / DC supply converter 16, the DC internal power supply bus 15, and the DC / DC charging converter 20. This is because the DC / DC supply converter 16 lowers the capacitor voltage Uci, Uc2 at each DC link capacitor CZK and uses it via the DC internal power supply bus 15 and the DC / DC charging converter 20 to compensate for the asymmetry. It can also be provided that the capacitor voltages Uci, Uc2 are detected by the control unit 10.If at least one of the intermediate circuit capacitors CZK reaches its specified nominal capacitor voltage, it can be provided to reduce the electrical output power of the DC / DC charging converter 20, for example by the control unit 10, so that a voltage overload of the intermediate circuit capacitors CZK by the DC / DC charging converter 20 is avoided.

[0051] To prevent a voltage overload of the intermediate circuit capacitor CZK at symmetrical capacitor voltages Uci, Uc2, it can also be provided that the capacitor voltages Uci, Uc2 are detected by the control unit 10. If at least one of the intermediate circuit capacitors CZK reaches its nominal capacitor voltage, it can be provided to reduce the electrical output power of the DC / DC charging converter 20, for example, by the control unit 10, so that a voltage overload of the intermediate circuit capacitors CZK is avoided.

[0052] Even during pre-charging of the DC voltage intermediate circuit 8, an asymmetry of the capacitor voltages Uci, Uc2 can occur, as described above. This means that even during pre-charging via the DC / DC charging converter 20, balancing can be achieved via the DC / DC supply converter 16, the DC internal power supply bus 15, and the DC / DC charging converter 20, as described above. This means that the DC / DC supply converter 16 can also be activated during pre-charging for balancing purposes. This operating case therefore represents balancing as described above. During pre-charging, the power consumed by the DC / DC supply converter 16 can also be limited, for example by the control unit 10, in order not to hinder pre-charging but still to reduce the capacitor voltages Uci, Uc2 if necessary.

[0053] Likewise, during precharging, the capacitor voltages Uci, Uc2 can be detected by the control unit 10. If at least one of the intermediate circuit capacitors CZK reaches its nominal voltage, the electrical output power of the DC / DC charging converter 20 can be reduced, for example, by the control unit 10, so that a voltage overload of the intermediate circuit capacitors CZK is avoided. This can occur with or without switching on the DC / DC supply converter 16.

[0054] During pre-charging, for example, in preparation for reactive power feed-in, the AC / DC supply converter 12 is active because it feeds the DC self-supply bus 15. All of this can be controlled via the control unit 10, for example, via the corresponding control lines.

[0055] Fig. 2 shows a power converter 1 according to the invention in operation with a DC voltage source 30, in this case a PV system, which is connected to the DC connection 2 and provides the DC voltage DCG. In the embodiment of the power converter 1 according to Fig. 2, a DC / DC main converter 9 is also provided, as already described above. An AC voltage sink 31, in this case a three-phase AC grid, is connected to the AC connection 3. The power converter 1 enables an energy flow from the DC voltage source 30 to the AC voltage sink 31, for example, a feed-in operation of the PV system into the AC grid. However, it is self-evident that the DC voltage source 30 could also be another voltage source, such as a battery, a DC grid, etc. In this case, the energy flow could also be reversed and electrical energy could also be transferred from the AC side to the DC side of the power converter 1.For example, the power converter 1 could be designed to charge a battery connected to the DC terminal 2 from an AC grid connected to the AC terminal 3 or from a wind turbine. With such a configuration, bidirectional operation is also conceivable, with energy flowing from the battery or a DC grid into an AC grid, or vice versa, from the AC grid into a battery or a DC grid. A combination of unidirectional and bidirectional operation is also possible, since multiple DC / DC main converters 9 are possible.

[0056] In the power converters 1 described with reference to Figs. 1 and 2, the DC / DC supply converter 16 is connected to the intermediate circuit voltage UZK of the DC voltage intermediate circuit 8 via the DC converter connection 18.

[0057] However, it would also be conceivable in principle for the DC converter terminal 18 of the DC / DC supply converter 16 to be connected to one of the capacitor voltages Uci, Uc2 or to several capacitor voltages Uci, Uc2. In this case, the DC / DC supply converter 16 also has at least one center terminal, depending on the number of connected capacitor voltages Uci, Uc2. It can therefore be operated in such a way that a more heavily charged intermediate circuit capacitor CZK is discharged. Accordingly, energy taken from one intermediate circuit capacitor CZK can essentially be fed directly to another intermediate circuit capacitor CZK. Thus, the DC / DC supply converter 16 involved in balancing the intermediate circuit capacitors CZK accelerates the balancing duration.

[0058] For the invention, it is irrelevant how the individual converters of the power converter 1 are designed. In principle, all possible and known circuit topologies for the DC / AC main converter 5, the DC / DC main converter(s) 9 (if present), the AC / DC supply converter 12, the DC / DC supply converter 16, and the DC / DC charging converter 20 are conceivable and applicable.

[0059] Fig. 3 shows a particularly advantageous embodiment of a DC / DC charging converter 20. In this embodiment, the DC / DC charging converter 20 comprises a transformer 35 with a primary winding 36 on a transformer core 39, which is connected to the DC internal supply bus 15 via a clock switching element 37. The clock switching element 37 can be controlled, for example, via a control line S6 by the control unit 10 in order to clock the supply voltage Uv of the DC internal supply bus 15 at the primary winding 36 at a predetermined clock frequency. On the secondary side of the transformer 35, a plurality of secondary windings 38a, 38b corresponding to the number of intermediate circuit capacitors CZK are provided on the same transformer core 39; in the exemplary embodiment in Fig. 3, two secondary windings 38a, 38b are provided.The turns ratio between the primary winding 36 and the respective secondary winding 38a, 38b is preferably selected such that the DC charging voltage ULI, UL2 induced at a secondary winding 38a, 38b during operation essentially corresponds (except for a forward voltage of the diode D1, D2) to the capacitor voltage Uci, Uc2 desired at the respectively assigned intermediate circuit capacitor CZK. A diode D1, D2 is connected in series with each secondary winding 38a, 38b in such a way that the diode D1, D2 conducts when the capacitor voltage Uci, Uc2 is lower than the respective DC charging voltages ULI, UL2. A diode D1, D2 blocks when a capacitor voltage Uci, Uc2 is equal to or greater than the respective DC charging voltages ULI, UL2 and conducts when the capacitor voltage Uci, Uc2 is smaller than the respective DC charging voltages ULI, UL2 by the breakdown voltage of the diode D1, D2.This circuit of the DC / DC charging converter 20 thus ensures automatic recharging until the capacitor voltages Uci, Uc2 of the intermediate circuit capacitors CZK are balanced if the intermediate circuit capacitors CZK are undercharged, as described above. The energy transmitted by the DC / DC charging converter 20 can also be controlled via the timing of the clock switching element 37 or a suitable operational management strategy (e.g., a set peak current) in the control unit 10, for example, to influence how quickly an asymmetry at the intermediate circuit capacitors CZK is compensated.

[0060] In the illustrated embodiments, a control unit 10 is shown in each case. However, it is understood that a distributed control system with a plurality of individual control units 10 can also be provided in the power converter 1. In a distributed control system, at least one control unit 10, preferably all control units 10, can be electrically supplied via the internal power supply 11 as described above.

[0061] A significant advantage of the invention is that the AC / DC supply converter 12, the DC / DC supply converter 16, and the DC / DC charging converter 20 all need to be unidirectional, meaning they only need to allow electrical energy to flow in one direction. For the AC / DC supply converter 12 and the DC / DC supply converter 16, the energy flows into the DC internal power supply bus 15, and for the DC / DC charging converter 20, the energy flows out of the DC internal power supply bus 15 (and into the DC voltage intermediate circuit 8). This enables the use of simply constructed converters. Last but not least, the invention also enables simple and robust control, since the AC / DC supply converter 12, the DC / DC supply converter 16 and the DC / DC charging converter 20 can each be operated in a simple voltage-controlled manner, wherein only a specific output voltage of the respective converter has to be set.This advantageously distributes the tasks to be implemented among appropriately dimensioned and controllable converters. This means that the AC / DC supply converter 12 and the DC / DC supply converter 16 can be dimensioned according to their supply task, while the DC / DC charging converter 20 can be dimensioned smaller, since it only has to compensate for the capacitance differences of the intermediate circuit capacitors CZK.

Claims

Patent claims 1. Power converter with a DC connection (2), to which an electrical direct voltage (DCG) can be applied during operation, and an AC connection (3), to which an electrical alternating voltage (ACG) can be applied during operation, wherein a DC / AC main converter (5) with a DC converter connection (7) and an AC converter connection (6) is provided in the power converter (1), and the AC converter connection (6) is connected to the AC connection (3) of the power converter (1), and the DC converter connection (7) is connected to a DC intermediate circuit (8), to which an intermediate circuit voltage (UZK) is applied during operation of the power converter (1), wherein the DC intermediate circuit (8) comprises at least two series-connected intermediate circuit capacitors (CZK), and a capacitor voltage (Uci, Ucz) is applied to each of the intermediate circuit capacitors (CZK) during operation of the power converter (1), wherein the DC connection (2) of the converter (1) is connected to the DC link (8),wherein an AC / DC supply converter (12) is provided in the power converter (1), which is connected to an AC converter terminal (13) of the AC / DC supply converter (12) with the AC terminal (3) of the power converter (1) and is connected to a DC internal supply bus (15) of the power converter (1) with a DC / DC supply converter terminal (14), characterized in that a DC / DC supply converter (16) is provided in the power converter (1), wherein a first DC converter terminal (17) of the DC / DC supply converter (16) is connected to the DC internal supply bus (15) and a second DC converter terminal (18) of the DC / DC supply converter (16) is connected to the DC intermediate circuit (8) and, during operation of the power converter (1), is connected to the second DC converter connection (18) of the DC / DC supply converter (16) the intermediate circuit voltage (UZK) or at least one capacitor voltage (Uci, Ucz) is applied, and that a DC / DC charging converter (20) is provided in the power converter (1),wherein a first DC converter terminal (22) of the DC / DC charging converter (20) is connected to the DC self-supply bus (15) and a second DC converter terminal (21) of the DC / DC charging converter (20) is designed with a number of DC charging terminals (21a, 21b) corresponding to the number of intermediate circuit capacitors (CZK), wherein each DC charging terminal (21a, 21b) is connected to a respective intermediate circuit capacitor (CZK).

2. Power converter according to claim 1, characterized in that between the DC connection (2) and the DC voltage intermediate circuit (8) at least one DC / DC main converter (9) with a first DC converter connection (25) of the DC / DC main converter (9) and a second DC converter connection (26) of the DC / DC main converter (9) is provided, wherein the DC connection (2) of the power converter (1) via the at least one DC / DC main converter (9) is connected to the DC voltage intermediate circuit (8) by connecting the first DC converter terminal (25) of the DC / DC main converter (9) to the DC terminal (2) of the power converter (1) and connecting the second DC converter terminal (26) of the DC / DC main converter (9) to the DC voltage intermediate circuit (8).

3. Power converter according to claim 1 or 2, characterized in that the DC / DC charging converter (20) comprises a transformer (35) with a primary winding (36) on a transformer core (39), wherein the primary winding (36) is connected to the DC internal supply bus (15) via at least one clock switching element (37), that on the secondary side of the transformer (35) on the transformer core (39) a number of secondary windings (38a, 38b) corresponding to the number of intermediate circuit capacitors (CZK) are provided, wherein a turns ratio between the primary winding (36) and the respective secondary winding (38a, 38b) is selected such that during operation of the power converter (1) at a secondary winding (38a, 38b) a DC charging voltage (ULI , U1.2) is induced, which substantially corresponds to the voltage desired at the respectively assigned intermediate circuit capacitor (CZK). capacitor voltage (Uci, Ucz) and each DC charging voltage (ULI , U1.2) is applied to an intermediate circuit capacitor (CZK) in each case, and that a diode (D1, D2) is connected in series with each secondary winding (38a, 38b), so that during operation of the power converter (1) the diode (D1, D2) conducts when the capacitor voltage (Uci, Ucz) is lower than the respective DC charging voltage (ULI, UL2).

4. Power converter according to one of claims 1 to 3, characterized in that at least one control unit (10) with a supply input (19) is provided in the power converter (1), wherein the supply input (19) is connected to the DC internal supply bus (15).

5. Method for operating a power converter (1) with a DC connection (2) to which an electrical direct voltage (DCG) is applied, and with an AC connection (3) to which an electrical alternating voltage (ACG) is applied, wherein in the power converter (1) an intermediate circuit voltage (UZK) of a direct voltage intermediate circuit (8) with at least two series-connected intermediate circuit capacitors (CZK), to each of which a capacitor voltage (Uci, Ucz) is applied, is converted into the alternating voltage (ACG) by a DC / AC main converter (5), or vice versa, wherein the intermediate circuit voltage (UZK) of the direct voltage intermediate circuit (8) is provided via the DC connection (2) during normal operation, or vice versa, wherein a DC internal supply bus (15) of the power converter (1) is supplied with a DC supply voltage via an AC / DC supply converter (12) connected to the AC connection (3). (Uv), characterized in thatthat a DC / DC charging converter (20) converts the DC supply voltage (Uv) of the DC self-supply bus (15) into one of the number of intermediate circuit capacitors (CZK), corresponding number of DC charging voltages (ULI , U1.2) and each DC charging voltage (ULI , U1.2) is applied to an intermediate circuit capacitor (CZK) and that a DC / DC supply converter (16) connected to the DC voltage intermediate circuit (8) is switched off in order to charge the intermediate circuit capacitors (CZK) of the DC voltage intermediate circuit (8) via the AC / DC supply converter (12), the DC internal supply bus (15) and to pre-charge the DC / DC charging converter (20) in the absence of a DC voltage (DCG) at the DC connection (2) or the DC / DC supply converter (16) is switched on, so that the intermediate circuit voltage (UZK) or at least one capacitor voltage (Uci, Uc2) of an intermediate circuit capacitor (CZK) of the DC voltage intermediate circuit (8) is converted into the DC supply voltage (Uv) via the DC / DC supply converter (16) and is fed into the DC internal supply bus (15) in order to charge the intermediate circuit capacitors (CZK) of the DC voltage intermediate circuit (8) via the DC / DC supply converter (16), the DC self-supply bus (15) and the DC / DC charging converter (20) when a DC voltage (DCG) is applied to the DC connection (2).

6. Method according to claim 5, characterized in that during precharging and / or during balancing the capacitor voltages (Uci, Ucz) are detected and an output power of the DC / DC charging converter (20) is reduced if one of the capacitor voltages (Uci, Ucz) becomes greater than a predetermined nominal capacitor voltage.

7. Method according to claim 5 or 6, characterized in that the DC voltage quantity (DCG) at the DC connection (2) is converted into the intermediate circuit voltage (UZK) of the DC voltage intermediate circuit (8), or vice versa, by a DC / DC main converter (9) of the power converter (1).

8. Method according to one of claims 5 to 7, characterized in that a control unit (10) of the power converter (1) is supplied with the DC supply voltage (Uv) via the DC self-supply bus (15).

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