KR101292991B1 - Power converter with multi-level voltage output and harmonics compensator - Google Patents

Abstract

A multilevel voltage converter which converts a DC voltage to an AC voltage and an AC voltage to a DC voltage has at least one phase leg between the first control unit 30 and the first DC terminal 4 and the second DC terminal 5. (1), wherein the at least one phase leg (1) comprises a first voltage source Uvp1 and the first AC terminal 6 between the first DC terminal 4 and the first AC terminal 6; And a second voltage source Uvn1 between the second DC terminal 5, wherein the first control unit 30 controls the first and second voltage sources Uvp1, Uvn1. The transducer comprises at least one coupling inductor 18 coupled in series with the at least one phase leg 1, an active controlled harmonic compensator 21 connected to the at least one coupling inductor 18, and the And further comprising a second control unit 23 adapted to control the output of the harmonic compensator 21 to reduce the harmonics in the circulating current i _c flowing through the at least one phase leg 1. .

Description

Power converter with multilevel voltage output and harmonic compensator {POWER CONVERTER WITH MULTI-LEVEL VOLTAGE OUTPUT AND HARMONICS COMPENSATOR}

The present invention relates to a power converter having a multilevel voltage output, called a multi-level converter, and is applied to converting a DC voltage into an AC voltage and an AC voltage into a DC voltage. The multilevel converter comprises a first control unit and at least one phase leg between the first DC terminal and the second DC terminal, the phase leg being the first between the first DC terminal and the first AC terminal. A voltage source, and a second voltage source between the first AC terminal and the second DC terminal, wherein the first control unit controls the first and second voltage sources.

In the art, multilevel converters are known to be used to reduce harmonic distortion at the output of voltage source converters. Multilevel converters each phase in which the output voltage (or output voltages in the case of a multi-phase converter) is switched in such a way that it can take several individual levels, as can be seen, for example, in DE10103031. It is a converter with power semiconductor switches in the leg. In the multilevel converter described in DE10103031, each of the first and second voltage sources comprises at least first and second submodules in series, each submodule being connected in parallel with a capacitor in the form of a half-bridge. Power electronic switches.

In WO 2008/067785 A1 a multilevel converter according to DE10103031 is disclosed which additionally comprises at least one inductor in each phase leg. In general, converter adjustment means for controlling a multilevel converter via power electronic switches further adjusts the circulating current flowing through the phase leg. The circulating current is a current which is closed between the phase legs but does not enter the AC grid through the AC terminal.

If the circulating current is controlled via ordinary converter adjusting means as described in WO 2008/067785 A1, the voltage rating of the power electronic switches of the converter must take into account the extra voltage required to adjust the circulating currents in the desired manner.

It is therefore an object of the present invention to propose a power converter which takes into account the adjustment of the circulating currents in a preferred manner in which the required voltage rating of the power semiconductor switches is affected as little as possible.

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

As mentioned above, a device for converting a DC voltage to an AC voltage and an AC voltage to a DC voltage comprises, according to the invention, at least one coupling inductor coupled in series with at least one phase leg, at least An active controlled harmonic compensator coupled to one coupling inductor and a second control unit adapted to control the output of the harmonic compensator to reduce harmonics in the circulating current flowing through the at least one phase leg.

The present invention is based on the recognition that the preferred way in which the circulating currents are best adjusted is to reduce the harmonics occurring at certain frequencies in the circulating current rather than to generally reduce the circulating currents. The inventors have realized that at each switching event in the power electronic switches of the converter, harmonics appear in the circulating current resulting in increased losses. In the worst case, some of the significantly higher magnitude harmonics in the circulating currents could even lead to system instability. As described in WO 2008/067785 A1, the introduction of additional phase inductors helps to obtain a general current limit in the converter circuit, but does not help to avoid such pronounced harmonics.

By introducing an active controlled harmonic compensator that reduces the harmonics with the highest magnitude or, in the best case, completely shuts off, the first control unit that controls the power semiconductor switches of the multilevel converter knows or considers the highest disturbing components of the circulating currents. Is avoided, and as a result the requirements on the voltage rating of the power semiconductor switches can be reduced thereby.

A closer look at the harmonics in the circulating currents of a multilevel converter according to DE10103031 reveals the following: The sum of the voltage ripples for the submodules of both voltage sources in one phase leg is the AC voltage in its frequency spectrum. It represents the main component that is twice the fundamental frequency of. This major frequency component also produces parasitic harmonic components at circulating currents that are twice the fundamental frequency. If this component is not limited in any way, the losses will result in an increase, perhaps even a loss of system stability.

Thus, according to a preferred embodiment of the present invention, the control of the harmonic compensator is processed such that 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 in conjunction with the accompanying drawings in which:
1 illustrates a multilevel converter topology as known from the art,
FIG. 2 shows the setup of voltage sources in the phase legs of the converter of FIG. 1 as known in the art;
3 shows two different embodiments of submodules in the converter of FIGS. 1 and 2;
4 shows one phase leg of a multilevel converter having a harmonic compensator and a second control unit according to an embodiment of the invention,
5, 6 and 7 each show a three phase multilevel converter according to another embodiment of the invention with phase inductors in series with each phase leg,
8A and 8B show one embodiment of a harmonic compensator having a current-stiff power converter,
9 shows a three phase multilevel converter according to another embodiment of the present invention having phase inductors connected to AC terminals,
10A and 10B show one embodiment of a harmonic compensator having a voltage-stiff power converter,
11 shows a three phase multilevel converter according to another embodiment of the present invention with phase inductors connected to AC terminals.

A multilevel power converter for converting a DC voltage to an AC voltage and an AC voltage to a DC voltage according to the present invention may contain one phase leg or multiple phase legs depending on how many phases the AC voltage has. 1 shows a three phase transducer known in the art. Each of the three phase legs 1, 2 and 3 of the device of FIG. 1 comprises two so-called arms of series connection: a first DC of positive voltage level Positive upper arm and second DC terminal 5 of zero or negative voltage level and three AC terminals respectively connected between terminal 4 and one of three AC terminals 6, 7 or 8 Negative lower arms connected between one of the fields 6, 7, or 8, respectively. Each positive arm comprises a first voltage source Uvpi and a first phase inductor 9 of a series connection, each negative arm comprising a second phase inductor 10 and a second voltage source Uvni, where i is Means the number of corresponding phase legs, for example i = 1, 2 or 3. An intermediate point or connection point between the first and second phase inductors of each phase leg is connected to one of the AC terminals 6, 7 or 8, respectively. All phase legs are connected to each other and in parallel with the two DC terminals 4 and 5. By appropriately controlling the voltage sources of the phase legs in time via the first control unit 30 an AC to DC conversion is achieved.

As shown in Fig. 2, each voltage source consists of submodules 15 in one string state connected in series, where at least two submodules 15 are included in one such column.

Two other embodiments 15a and 15b of the submodules 15, known in the art from FIG. 3, can be seen. The submodules are in the form of rectifying cells, each cell containing two valves and a large capacity DC capacitor that accepts a DC voltage. The valves are equipped with a power electronic switch 16 with turn-off capability and a free-wheeling diode anti-parallel connected to the switch. Depending on which of the two power electronic switches 16 goes through, the corresponding submodule may take one of two switching states, with a zero voltage in state 1 or a capacitor voltage in state 2 Is applied to. Combinations of any of these or improved submodules are possible in each voltage source 15. In the application of the present invention, essentially each of the submodules can generate a number of individual levels of the output voltage of the converter.

According to one embodiment of the invention, the converter of FIGS. 1 to 3 is additionally provided with an active control harmonic compensator 21 in each phase leg as depicted in FIG. 4 with respect to the phase leg 1. 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 coupling inductor 18. In the following, the term " connected " clearly means " galvanically coupled ". The coupling inductor 18 is coupled to the first inductor 9 via coupling means 19 and to the second inductor 10 via coupling means 20. The AC terminal 6 is symmetrically connected to the coupling inductor 18 such that portions of the same size of the coupling inductor 18 are part of the positive phase arm and the negative phase arm, respectively. The coupling means 19 and 20 can be galvanic and / or magnetic, and the magnetic coupling can be combined with the coupling inductor and the respective inductor through air or a magnetic material such as iron. It can be realized between one or second phase inductors.

The symbols in FIG. 4 have the following meanings:

u _{vp / n} The voltage of the voltage source at each of the positive or negative arms;

i _{vp / n} current in positive / negative phase arms;

i _v Output current at the AC terminal;

u _f the voltage at the AC terminal (AC voltage);

current through i _m coupling inductor;

L _h Inductance of the first and second inductors;

Inductance of the L _m coupling inductor.

In the following, the analysis performed by the inventors on the action of the circulating current in the depicted phase leg 1 is briefly described.

By applying a controlled electric current cycle for equation _c = i (i _{vp vn} + i) / 2 is Kirchhoff (Kirchhoff) voltage law to the straight path from u to u _{vp vn} can be obtained, where

 Is given.

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

을 따르도록 할 수 있지만, 추가적으로 전압 u _vc 는 또한 다음과 같이 기생 항 Δu _vc 를 포함할 수 있다. It is shown that u _vc is the driving voltage for the circulating current. As is known from the art, this voltage is controllable by appropriately controlling the switching of the submodules 15. Therefore the voltage u _vc is the reference

In addition, the voltage u _vc may also include the parasitic term Δ u _vc as follows.

If the model of the system analyzes the real frequency dependent behavior of the system described by equations (2) and (3), then the parasitic term Δ u _vc consists of the following three harmonic components: appear:

1) a first component whose magnitude is twice the fundamental frequency of the AC voltage, which is generally a few percent of the rated AC voltage;

2) a second component whose magnitude is four times the fundamental frequency, fraction of 1);

3) a third component comprising switching harmonics.

Since the first component has the highest magnitude of the three components, this component will result in harmonics of circulating current with the highest peaks. Thus, to reduce the losses resulting from these peaks, it is desirable to reduce the first component.

With the harmonic compensator 21 according to FIG. 4, the compensation of component 1) is possible. Preferably, the coupling desired current flow through the inductor (18) (i _m), the control unit 23 so that the generation and controls the power electronic converter 22. In order to produce the desired compensation current in the phase leg, here in particular in the first and second phase inductors, such that harmonics in the circulating current i _{c which} is twice the fundamental frequency of the AC voltage U _f are reduced. current flow (i _m) by taking into account the desired type of coupling between the 18 and the phase leg are determined by the control unit 23.

The harmonic compensator 21 is a circulating current i _c By processing the harmonics in, the voltage rating of the power semiconductor switches 16 can thereby be reduced, resulting in a reduction in the overall costs for the power converter shown in FIG. 1. This would of course be an offset for the additional costs induced by the introduction of harmonic compensators. To keep these additional costs below the cost savings mentioned above, the first and second phase inductors are in such a way that the voltage across the coupling inductor 18 is significantly lower than the voltage across the first and second inductors. It is proposed in a particular embodiment of the invention to select the inductance L _m of the coupling inductor 18 with respect to the inductance L _h of the poles 9 and 10. If all inductors are coils, the number of turns of the coil of the coupling inductor 18 will be chosen to be appropriately small compared to the number of turns of the first and second phase inductors 9 and 10.

 5, 6, 7, 9 and 11, embodiments of a three phase multilevel converter are shown, in which each coupling inductors 18 are galvanically galvanized to their respective phase legs. Are combined. However, not only the manner in which the harmonic compensators 21 are connected to the corresponding coupling inductors 18, but also the arrangement of the phase inductors 9, 10, 32 also changes, respectively.

The transducer shown in FIG. 5 is made of three phase legs according to FIG. 4.

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 intermediate points of the delta connected first and second phase inductors 9 and 10 are connected to the corresponding first or second DC terminals 4 or 5, respectively. Each phase inductor 9 or 10 may include only one inductor element or two or more inductor elements in series connection.

In Fig. 7, the first and second phase inductors are replaced with one AC phase inductor 32 for each phase. The AC phase inductor 32 is moved from the phase legs to three corresponding AC phases, which are connected in series to each of the AC terminals 6, 7 and 8.

8A and 8B show one embodiment of the harmonic compensators 21 of FIGS. 5, 6 and 7. The harmonic compensator 21 shown in FIG. 8A, connected in parallel with the corresponding coupling inductor 18, includes an AC / DC power electronic converter 22 controlled by the second control unit 23 as described above. . The second control unit 23 is integrated in the harmonic compensator 21. Alternatively, the second control unit can be installed separately. The power electronic converter 22 is arranged in the form of a current-stiff power converter, and the DC current is a DC current source formed by the DC voltage source 36 and the auxiliary inductor 34 connected in series with the DC voltage source 36. Through the converter. An example of how the AC / DC power electronic converter 22 is arranged is shown in principle and schematically in FIG. 8B, where the AC / DC power electronic converter 22 has 3 with ideal power electronic switching switches 38. -Level converter, the switching of the changeover switches 38 being controlled by the second control unit 23.

9 shows one embodiment of a three phase multilevel converter having the same arrangement of phase inductors 32 as in FIG. However, each phase leg comprises only one coupling inductor 18 and one corresponding harmonic compensator 21, both arranged in series with each other and in series with a second voltage source Uvni at the corresponding negative phase arm. Are arranged. Alternatively, coupling inductors 18 and harmonic compensators 21 may also be arranged in series with the corresponding positive phase arm.

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

One embodiment of the harmonic compensators 21 of FIGS. 9 and 11 is shown in FIGS. 10A and 10B. The harmonic compensator 21 connected in series with the corresponding coupling inductor 18 comprises an AC / DC power electronic converter 22 and a second control unit 23 for controlling the power electronic converter 22 in the manner described above. Include again. In this embodiment, the power electronic converter 22 is arranged as a voltage-stiff converter supplied by the DC voltage source 36. An example of how the AC / DC power electronic converter 22 is arranged is shown in principle and outline in FIG. 10B, where the AC / DC power electronic converter 22 has three ideal power electronic switching switches 38. -Level converter, the switching of the changeover switches 38 being controlled by the second control unit 23. As an alternative, the harmonic compensators 21 of FIGS. 9 and 11 may also be arranged as current-stiff converters according to FIGS. 8A and 8B.

In FIGS. 9 and 11, the three harmonic compensators 21 of the positive and / or negative phase arms are all connected by wire connection to the same potential, respectively. This makes it possible in an alternative embodiment to combine the power electronics converters of the three harmonic compensators 21 with a three phase power electronics converter controlled by one second control unit.

Claims (13)

A DC voltage to an AC voltage and an AC voltage to a DC voltage, comprising a first control unit 30 and at least one phase leg 1 between the first DC terminal 4 and the second DC terminal 5. A multilevel voltage converter to convert,
The at least one phase leg 1 comprises a first voltage source Uvp1 between the first DC terminal 4 and the first AC terminal 6 and the first AC terminal 6 and the second DC terminal ( 5) a second voltage source Uvn1 between,
The first control unit 30 controls the first and second voltage sources Uvp1, Uvn1,
The converter is connected to at least one coupling inductor 18 galvanically coupled in series with the at least one phase leg 1, the output side of which is connected in parallel with the at least one coupling inductor 18. An active control harmonic compensator 21 comprising a power electronic converter 22 directly connected to the coupling inductor 18, and a second control unit 23 adapted to control the output of the harmonic compensator 21. Further comprising, reducing harmonics in a circulating current (i _c ) flowing through the at least one phase leg (1). The method of claim 1,
A multilevel voltage, in which the second control unit 23 controls the output of the harmonic compensator 21 so that harmonics in the circulating current i _{c which} is twice the fundamental frequency of the AC voltage U _f are reduced. converter. 3. The method according to claim 1 or 2,
The harmonic compensator (21) by the second control unit 23, to generate a flow of current (i _m), the desired through the coupled inductor (18) which controls the output of the harmonic compensator (21), a multi- Level voltage converter. 3. The method according to claim 1 or 2,
The second control unit (23) controls the output of the harmonic compensator (21) via closed loop control. The method of claim 1,
The power electronic converter (22) is a current-stiff converter. The method according to any one of claims 1, 2 or 5,
At least one phase inductor (9) is connected in series with said phase leg (1). The method according to any one of claims 1, 2 or 5,
At least one phase inductor (9) is connected in series with said first AC terminal (6).

delete delete delete delete delete delete

Images