JP5511947B2 - Power converter with multi-level voltage output and harmonic compensator - Google Patents

Description

  The present invention relates to a power converter (referred to as a multi-level converter) having a multi-level voltage output adapted to convert a DC voltage to an AC voltage and an AC voltage to a DC voltage. The multi-level voltage converter comprises a first control unit and at least one phase leg between a first DC terminal and a second DC terminal, the phase leg being connected to the first DC terminal and the first DC terminal. A first voltage source between the first AC terminal and a second voltage source between the first AC terminal and the second DC terminal, wherein the first control unit includes the first and second voltages. Control the source.

  It is known in the art to use multi-level converters to reduce harmonic distortion in the output of voltage source converters. For example, as can be seen in German Patent No. 10103031, the multilevel converter is a converter with a power semiconductor switch in each phase leg, and these switches are connected to the output voltage (multiphase conversion). Are switched so that multiple discrete levels can be taken. In the multilevel converter described in German Patent No. 10103031, the first and second voltage sources each include at least a first submodule and a second submodule in series connection, Each sub-module comprises two power electronic switches connected in parallel to a capacitor in the form of a half bridge.

  WO 2008 / 067785A1 (Patent Document 2) discloses a multilevel converter according to German Patent No. 10103031 (Patent Document 1), further comprising at least one inductor in each phase leg. Has been. In addition, converter adjustment means, usually controlling the multi-level converter via a power electronic switch, adjust the circulating current flowing through the phase leg. The circulating current is the current that is closed between the phase legs but does not enter the AC grid through the AC terminal.

  When the circulating current is controlled by normal converter adjusting means as described in WO 2008 / 067785A1, the rated voltage of the converter's power electronic switch is as desired. The additional voltage required to adjust the voltage must be taken into account.

German Patent No. 10103031 International Publication No. 2008 / 067785A1

  Accordingly, an object of the present invention is to propose a power converter that makes it possible to adjust the circulating current as desired and that has as little influence on the required rated voltage of the power semiconductor switch as possible. .

  This object is achieved by a device according to claim 1.

  According to the present invention, a device as described above for converting a DC voltage to an AC voltage and an AC voltage to a DC voltage comprises at least one coupled inductor coupled in series with at least one phase leg and at least one coupled A circulating current flowing through the at least one phase leg, comprising an actively controlled harmonic compensator connected to the inductor and a second control unit adapted to control the output of the harmonic compensator Reduce the harmonics in it.

  The present invention is based on the recognition of the fact that the preferred way in which the circulating current can best be adjusted is not to reduce the circulating current in general, but to reduce the harmonics occurring at a particular frequency of the circulating current. . The inventor has noticed that at each switching event in the power electronic switch of the converter, harmonics appear in the circulating current and increase the loss. In the worst case, some of the harmonics with significantly higher amplitude in the circulating current can even destabilize the system. The introduction of the additional phase inductor described in WO 2008 / 067785A1 helps to achieve general current limiting in the converter circuit, but the significant harmonics themselves It does not prevent.

  First control unit for controlling a power semiconductor switch of a multi-level converter by introducing an actively controlled harmonic compensator that reduces or at best cuts off the harmonics having the maximum amplitude Prevents the most disturbing circulating current component from being sensed and taken into account, thereby reducing the requirements on the rated voltage of the power semiconductor switch.

  From a more detailed observation of harmonics in the circulating current of the multilevel converter according to DE 10103031, the sum of the voltage ripples in the sub-modules of both voltage sources in one phase leg is It was found that the frequency spectrum shows a main component that is twice the fundamental frequency of the AC voltage. This main frequency component produces a parasitic harmonic component of the circulating current that is also twice the fundamental frequency. Unless this component is limited in any way, an increase in loss occurs. In some cases, system stability may even be lost.

  Thus, according to a preferred embodiment of the present invention, the harmonic compensator control is configured such that the harmonics in the circulating current, which is twice the fundamental frequency of the AC voltage, are reduced.

  Other features and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description, taken in conjunction with the accompanying drawings.

FIG. 3 shows a multilevel converter topology known in the art. FIG. 2 shows the voltage source mechanism at the phase leg of the converter of FIG. 1 as known in the art. FIG. 3 shows two different embodiments of sub-modules in the converter of FIGS. 1 and 2. FIG. 4 shows one phase leg of a multi-level converter comprising a harmonic compensator and a second control unit according to an embodiment of the invention. FIG. 3 shows a three-phase multilevel converter according to a different embodiment of the invention comprising a phase inductor in series with each phase leg. FIG. 3 shows a three-phase multilevel converter according to a different embodiment of the invention comprising a phase inductor in series with each phase leg. FIG. 3 shows a three-phase multilevel converter according to a different embodiment of the invention comprising a phase inductor in series with each phase leg. It is a figure which shows one Embodiment of a harmonic compensator provided with a current type power converter. It is a figure which shows one Embodiment of a harmonic compensator provided with a current type power converter. FIG. 6 shows a three-phase multilevel converter according to a further embodiment of the invention comprising a phase inductor connected to an AC terminal. It is a figure which shows one Embodiment of a harmonic compensator provided with a voltage type power converter. It is a figure which shows one Embodiment of a harmonic compensator provided with a voltage type power converter. FIG. 6 shows a three-phase multilevel converter according to a further embodiment of the invention comprising a phase inductor connected to an AC terminal.

  A multi-level power converter according to the invention for converting a DC voltage into an AC voltage and an AC voltage into a DC voltage comprises only one phase leg or a plurality of phase legs depending on the number of phases that the AC voltage has. be able to. FIG. 1 shows a three-phase converter known in the art. The three phase legs 1, 2, and 3 of the device of FIG. 1 each comprise two so-called arms in series connection. These two arms have a positive upper arm connected between the first DC terminal 4 at the positive voltage level and one of the three AC terminals 6, 7, or 8, respectively, and a zero or negative voltage level. A negative lower arm connected respectively between the second DC terminal 5 and one of the three AC terminals 6, 7 or 8. Each positive arm comprises a series connection of a first voltage source Uvpi and a first phase inductor 9, and each negative arm comprises a second phase inductor 10 and a second voltage source Uvni, where i Represents the number of corresponding phase legs, i.e., 1, 2, or 3. Each midpoint or connection point between the first phase inductor and the second phase inductor of each phase leg is connected to one of the AC terminals 6, 7, or 8, respectively. All phase legs are connected in parallel to each other and connected to the two DC terminals 4 and 5. The first control unit 30 performs the AC to DC conversion by appropriately controlling the voltage source of the phase leg over time.

  As shown in FIG. 2, each voltage source is formed from a series connected string of sub-modules 15, where one such string includes at least two sub-modules 15.

  In FIG. 3, two different embodiments 15a and 15b of a submodule 15 known in the art can be seen. The submodule has the form of a rectifying cell, each cell comprising two valves and a large DC capacitor holding a direct voltage. The valve comprises a power electronic switch 16 with a turn-off function and a freewheeling diode connected in reverse parallel to the switch. Depending on which of the two power electronic switches 16 is conducting, the corresponding sub-module can take one of two switching states: in state 1 a zero voltage is applied to the output, in state 2 the capacitor voltage is Applied to output. Any combination of these sub-modules or sophisticated sub-modules is possible at each voltage source 15. For the purposes of the present invention, it is only important that each sub-module is capable of generating a plurality of discrete levels of one step in the output voltage of the converter.

  According to one embodiment of the present invention, as shown in FIG. 4 for phase leg 1, the converter of FIGS. 1-3 is further provided with an active control harmonic compensator 21 in each phase leg. The harmonic compensator 21 includes a power electronic converter 22 and a second control unit 23 for controlling the power electronic converter 22. The power electronic converter 22 is connected to the coupled inductor 18. In the following, the term “connected” means in particular “electrochemically coupled”. The coupled inductor 18 is coupled to the first inductor 9 via the coupling means 19 and is coupled to the second inductor 10 via the coupling means 20. AC terminal 6 is symmetrically connected to coupled inductor 18 such that equal sized portions of coupled inductor 18 are part of a positive phase arm and a negative phase arm, respectively. The coupling means 19 and 20 may be electrochemical and / or magnetic means, the magnetic coupling being realized by air or by a magnetic material, for example iron, between the coupling inductor and the first or second phase inductor, respectively. be able to.

The symbols in FIG. 4 represent the following.
u _{vp / n} voltage source voltage at positive or negative arm respectively i _{vp / n} current at positive / negative phase arm i _v output current at AC terminal u _f voltage at AC terminal (AC voltage)
i Current through _m coupled inductor L _h Inductance of first and second inductor L Inductance of _{m m} coupled inductor

  An analysis of the behavior of the circulating current in the illustrated phase 1 performed by the inventor will be briefly described below.

The governing equation for the circulating current i _c = (i _vp + i _vn ) / 2 can be obtained by applying Kirchhoff's voltage law in the DC path from u _vp to u _vn , giving:

By introducing the differential voltage u _vc = (u _vp −u _vn ) / 2, equation (1) can be simplified as follows.

に従うように発生させることができるが、さらに寄生項Δｕ_ｖｃも含む。 It can be seen that u _vc is the drive voltage for the circulating current. As is known from the art, this voltage can be controlled by appropriately controlling the switching of the submodule 15. Therefore, the voltage u _vc is the reference voltage

In addition to the parasitic term Δu _vc .

Analysis of the actual frequency dependent behavior of the system whose model is described by equations (2) and (3) shows that the parasitic term Δu _vc consists of the following three harmonic components:
1) A first component that is twice the fundamental frequency of the AC voltage. The amplitude of this component is usually a few percent of the rated AC voltage.
2) A second component that is four times the fundamental frequency. The amplitude of this component is a fraction of 1).
3) A third component including switching harmonics.

  Since the first component has the largest amplitude of the three components, this component provides the harmonic of the circulating current with the largest peak.

  Therefore, it is desirable to reduce the first component to reduce the losses caused by these peaks.

Using the harmonic compensator 21 according to FIG. 4, it is possible to compensate for component 1). Preferably, the control unit 23 controls the power electronic converter 22 to the desired current i _m through the coupling inductor 18 is generated. Desired current i _m is determined by the control unit 23 by considering the coupling type between the coupled inductor 18 and the phase leg, phase leg, the desired compensation, especially in the first and second phase inductor here the current generated, whereby harmonics in a circulating current i _c is twice the fundamental frequency of the AC voltage U _f decreases.

Since the harmonic compensator 21 processes the harmonics in the circulating current _ic , it can reduce the rated voltage of the power semiconductor switch 16, thus reducing the overall cost for the power converter shown in FIG. To do. Of course, this must offset the additional costs incurred by the introduction of harmonic compensators. In order to keep these additional costs below the cost savings described above, in a particular embodiment of the present invention, the voltage at the coupled inductor 18 is much lower than the voltage at the first and second inductors. It has been proposed to select the inductance L _m of the coupled inductor 18 relative to the inductance L _{h of} the first and second phase inductors 9 and 10. If all the inductors are coils, the number of turns of the coupled inductor 18 is selected to be appropriately less than the number of turns of the first and second phase inductors 9 and 10.

  5, 6, 7, 9, and 11 illustrate three-phase multi-level converter embodiments, where in each embodiment, a coupled inductor 18 is electrochemically coupled to a respective phase leg. However, the arrangement of the phase inductors 9, 10, and 32 and the manner in which the harmonic compensator 21 is connected to the corresponding coupled inductor 18 are different.

  The converter shown in FIG. 5 consists of three phase legs according to FIG.

  The converter of FIG. 6 shows a delta connection of three first phase inductors 9 of three positive phase arms and a similar delta connection of three second phase inductors 10 of three negative phase arms. . The midpoints of the first phase inductor 9 or the second phase inductor 10 that are delta-connected are connected to the corresponding first DC terminal 4 or second DC terminal 5, respectively. Each phase inductor 9 or 10 may comprise only one inductor element or a series connection of two or more inductor elements.

  In FIG. 7, the first and second phase inductors are replaced by one AC phase inductor 32 for each phase. The AC phase inductor 32 is repositioned from the phase leg to three corresponding AC phase lines, where it is connected in series with AC terminals 6, 7, and 8, respectively.

  FIGS. 8a and 8b show one embodiment of the harmonic compensator 21 of FIGS. The harmonic compensator 21 shown in FIG. 8a connected in parallel to the corresponding coupled inductor 18 comprises an AC / DC power electronic converter 22, which is the second as described above. It is controlled by the control unit 23. The second control unit 23 is incorporated in the harmonic compensator 21. Alternatively, the second control unit can be installed separately. The power electronic converter 22 is configured in the form of a current type power converter, and the DC current is converted through a DC current source formed by a DC voltage source 36 and an auxiliary inductor 34 connected in series to the DC voltage source 36. Injected into the vessel. An example of how the AC / DC power electronic converter 22 can be constructed is shown schematically in FIG. 8b in principle, where the AC / DC power electronic converter 22 is an ideal power source. A three-level converter including an electronic changeover switch 38, and the switching of the changeover switch 38 is controlled by the second control unit 23.

  FIG. 9 shows one embodiment of a three-phase multilevel converter with the same phase inductor 32 configuration as FIG. However, each phase leg comprises only one coupling inductor 18 and a corresponding harmonic compensator 21, which are arranged in series with each other and with a corresponding negative compensator 21. The second voltage source Uvni in the phase arm is also arranged in series. In the alternative, the coupled inductor 18 and the harmonic compensator 21 are also arranged in series with the corresponding positive phase arm.

  The embodiment in FIG. 11 differs from FIG. 9 in that it shows a symmetrical configuration of the coupled inductor 18 and the corresponding series-connected harmonic compensator 21 in each phase arm of the three phase legs.

  One embodiment of the harmonic compensator 21 of FIGS. 9 and 11 is shown in FIGS. 10a and 10b. The harmonic compensator 21 connected in series to the corresponding coupled inductor 18 is again the AC / DC power electronic converter 22 and the second control for controlling the power electronic converter 22 as described above. A unit 23. In this embodiment, the power electronic converter 22 is configured as a voltage type converter powered by a DC voltage source 36. An example of how the AC / DC power electronic converter 22 can be constructed is shown schematically in FIG. 10b in principle, where the AC / DC power electronic converter 22 is an ideal power. A three-level converter including an electronic changeover switch 38, and the switching of the changeover switch 38 is controlled by the second control unit 23. In an alternative solution, the harmonic compensator 21 of FIGS. 9 and 11 can also be configured as a current source converter according to FIGS. 8a and 8b.

  In FIG. 9 and FIG. 11, the three harmonic compensators 21 of the positive and / or negative phase arms are all connected to the same potential by Y connection. Thereby, in an alternative embodiment, the power electronic converters of the three harmonic compensators 21 can be combined into a three-phase power electronic converter controlled by only one second control unit. .

Claims (7)

A multi-level voltage converter for converting a DC voltage into an AC voltage and an AC voltage into a DC voltage, the first control unit (30), a first DC terminal (4) and a second DC At least one phase leg (1) between the terminals (5), said at least one phase leg (1) between the first DC terminal (4) and the first AC terminal (6) A first voltage source (Uvp1); a second voltage source (Uvn1) between the first AC terminal (6) and the second DC terminal (5); and the first control unit (30 In the multi-level voltage converter for controlling the first and second voltage sources (Uvp1, Uvn1),
The converter is further connected in parallel to at least one coupled inductor (18) electrochemically coupled in series to the at least one phase leg (1) and to the at least one coupled inductor (18). Thus, an active control type harmonic compensator (21) including a power electronic converter (22) directly connected to the coupled inductor (18) and an output of the harmonic compensator (21) are controlled. And a second control unit (23) adapted to reduce the harmonics in the circulating current (i _c ) flowing through the at least one phase leg (1). Level voltage converter. The harmonic compensator (21) so that the second control unit (23) reduces harmonics in the circulating current (i _c ) that is twice the fundamental frequency of the AC voltage (U _f ). The multi-level voltage converter according to claim 1, wherein the multi-level voltage converter is controlled. The output of the harmonic compensator (21) so that the second control unit (23) causes the harmonic compensator (21) to generate a desired current (i _m ) through the coupled inductor (18). The multi-level voltage converter according to claim 1 or 2, wherein the multi-level voltage converter is controlled.   The multilevel voltage converter according to any one of claims 1 to 3, wherein the second control unit (23) controls the output of the harmonic compensator (21) by closed-loop control.   The multi-level voltage converter according to claim 1, wherein the power electronic converter (22) is a current type converter.   The multi-level voltage converter according to any one of the preceding claims, wherein at least one phase inductor (9) is connected in series with the phase leg (1).   Multi-level voltage converter according to any one of the preceding claims, wherein at least one phase inductor (9) is connected in series with the first AC terminal (6).

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