EP2449668A1

EP2449668A1 - Power converter with multi-level voltage output and harmonics compensator

Info

Publication number: EP2449668A1
Application number: EP09780106A
Authority: EP
Inventors: Lars-Erik Juhlin; Hans Peter Nee; Lennart Harnefors; Björn Jacobsson
Original assignee: ABB Technology AG
Current assignee: ABB Technology AG
Priority date: 2009-07-02
Filing date: 2009-07-02
Publication date: 2012-05-09
Also published as: KR20120016669A; KR101292991B1; CN102474201A; JP2012531878A; JP5511947B2; WO2011000428A1

Abstract

A multi-level voltage converter for converting a DC voltage into an AC voltage and vice versa comprisesa first control unit (30) and at least one phase leg (1) between a first DC terminal (4) and a second DC terminal (5), where the at least one phase leg (1) comprises a first voltage source (Uvp1) between the first DC terminal (4) and a first AC terminal (6) and a second voltage source (Uvn1) between the first AC terminal (6) and the second DC terminal (5) and where the first control unit (30) controls the first and second voltage sources (Uvp1, Uvn1). The converter comprises further at least one coupling inductor (18) being coupled in series with the at least one phase leg (1), an actively controlled harmonics compensator (21) being connected to the at least one coupling inductor (18), and a second control unit (23) being adapted to control the output of the harmonics compensator (21) so as to reduce harmonics in a circulating current (i_c ) flowing through the at least one phase leg (1).

Description

Power converter with multi-level voltage output and harmonics compensator

The invention is related to a power converter with multi-level voltage output, called a multi-level converter, which is adapted to convert a DC voltage into an AC voltage and vice versa. The multi-level converter comprises a first control unit and at least one phase leg between a first DC terminal and a second DC terminal, where the phase leg comprises a first voltage source between the first DC terminal and a first AC terminal and a second voltage source between the first AC terminal and the second DC terminal and where the first control unit controls the first and second voltage sources.

In the art, multi-level converters are known to be used in order to reduce harmonic distortion in the output of voltage source converters. A multi-level converter is a converter with power semiconductor switches in each phase leg which are switched in such a way that the output voltage - or, in case of a multi-phase converter, the output voltages - can assume several discrete levels, as can be seen for example in

DE10103031. In the multi-level converter described in DE10103031 , each of the first and second voltage sources comprises at least a first and a second submodule in series-connection, where each submodule comprises two power electronic switches connected in parallel with a capacitor in form of a half-bridge.

In WO 2008/067785 A1 , a multi-level converter according to DE10103031 is disclosed which in addition comprises at least one inductor in each phase leg. Converter regulating means which normally control the multi-level converter via the power electronic switches regulate in addition a circulating current flowing through the phase legs. The circulating current is the current which closes between the phase legs but does not enter the AC grid through the AC terminal.

If the circulating-current is controlled via the usual converter regulating means as described in WO 2008/067785 A1 , the voltage rating of the power electronic switches of the converter must allow for the extra voltage needed to regulate the circulating currents in the desired manner. It is therefore an object of the invention to propose a power converter which allows for regulation of the circulating currents in a desired manner, where the required voltage rating of the power semiconductor switches is affected as little as possible. This object is achieved by a device according to claim 1.

The device for converting a DC voltage into an AC voltage and vice versa as described above, comprises according to the invention at least one coupling inductor, which is coupled in series with the at least one phase leg, an actively controlled harmonics compensator, which is connected to the at least one coupling inductor, and a second control unit, which is adapted to control the output of the harmonics compensator so as to reduce harmonics in a circulating current flowing through the at least one phase leg The invention is based on the recognition of the fact that the desired manner in which the circulating currents should best be regulated is to reduce the harmonics which occur at specific frequencies in the circulating current, rather than to reduce the circulating currents in general. What the inventors realized is that at each switching event in the power electronic switches of the converter, harmonics appear in the circulating current causing increased losses. As a worst case, some of the harmonics with distinctively high amplitude in the circulating currents could even lead to system instability. The introduction of additional phase inductors, as described in WO

2008/067785 A1 , helps to obtain a general current limitation in the converter circuit but does nothing to avoid the distinctive harmonics as such.

By introducing an actively controlled harmonics compensator that reduces or in the best case completely blocks the harmonics with the highest amplitude, it is avoided that the first control unit which controls the power semiconductor switches of the multilevel converter sees and takes into account the most disturbing components of the circulating currents, so that the requirements on the voltage rating of the power semiconductor switches can thereby be reduced. A closer look at the harmonics in the circulating currents of a multi-level converter according to DE10103031 revealed the following: The sum of the voltage ripple over the submodules of both voltage sources in one phase leg shows in its frequency spectrum a main component at twice the fundamental frequency of the AC voltage. This main frequency component creates a parasitic harmonic component in the circulating current that is also of twice the fundamental frequency. Unless this component is somehow limited, increased losses will result; possibly even loss of system stability. Therefore, according to a preferred embodiment of the invention, the control of the harmonics compensator is arranged so that harmonics in the circulating current at twice the fundamental frequency of the AC voltage are reduced.

Other features and advantages of the present invention will become more apparent to a person skilled in the art from the following detailed description in conjunction with the appended drawings in which:

Fig. 1 shows a multi-level converter topology as is known from the art,

Fig. 2 shows the setup of the voltage sources in the phase legs of the converter of Fig. 1 as known from the art,

Fig. 3 shows two different embodiments of the submodules in the converter of

Figs. 1 and 2,

Fig. 4 shows one phase leg of a multi-level converter with a harmonics

compensator and a second control unit according to one embodiment of the invention,

Fig. 5, 6, 7 each show a three-phase multi-level converter according to different embodiments of the invention with phase inductors in series with each phase leg,

Figs. 8a, b shows an embodiment of the harmonics compensator with a current-stiff power converter,

Fig. 9 shows a three-phase multi-level converter according to a further

embodiment of the invention with phase inductors connected to the AC terminals, Figs. 10a,b shows an embodiment of the harmonics compensator with a voltage-stiff power converter,

Fig. 11 shows a three-phase multi-level converter according to a further

embodiment of the invention with phase inductors connected to the AC terminals.

The multi-level power converter for converting a DC voltage into an AC voltage and vice versa according to the invention can contain either a single phase leg or multiple phase legs, depending on how many phases the AC voltage has. 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: a positive, upper arm which is connected between a first DC terminal 4 at a positive voltage level and one of three AC terminal 6, 7 or 8, respectively, and a negative, lower arm, which is connected between a second DC terminal 5 at zero or a negative voltage level and one of the three AC terminals 6, 7 or 8, respectively. 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 stands for the number of the corresponding phase leg, i.e. i=1 , 2 or 3. The midpoint or connection point between the first and second phase inductors of each phase leg is each connected to one of the AC terminals 6, 7 or 8, respectively. All the phase legs are connected in parallel to each other and to the two DC terminals 4 and 5. By appropriately controlling the voltage sources of the phase legs over time, via a first control unit 30, the AC to DC conversion is made. As is shown in Fig. 2, each voltage source is made up of a series connected string of submodules 15, where at least two submodules 15 are comprised in one such string.

From Fig. 3, two different embodiments 15a and 15b of the submodules 15, which are known in the art, can be seen. The submodules have the form of commutation cells, each cell comprising two valves and a large DC capacitor holding a direct voltage. The valves are equipped with a power electronic switch 16 with turn-off capability and a free-wheeling diode in anti-parallel connection to the switch. Depending on which of the two power electronic switches 16 is conducting, the corresponding submodule can assume one of two switching states, where in state one zero voltage or in state two the capacitor voltage is applied to the output. Any combination of these or refined submodules is possible within each voltage source 15, For the application of the invention it is only essential that the submodules each can generate a step of the multiple discrete levels the output voltage of the converter.

According to an embodiment of the invention, the converter of Figs. 1 to 3 is additionally equipped with an actively controlled harmonics compensator 21 in each phase leg, as is depicted in Fig. 4 for phase leg 1. The harmonics compensator 21 comprises a power electronic converter 22 and a second control unit 23 to control the power electronic converter 22. The power electronic converter 22 is connected to a coupling inductor 18. In the following, the term "connected" stands expressly for "galvanically coupled". The coupling inductor 18 is coupled via coupling means 19 to the first inductor 9 and via coupling means 20 to the second inductor 10. The AC terminal 6 is symmetrically connected to the coupling inductor 18 so that equally big parts of the coupling inductor 18 become part of the positive and negative phase arms, respectively. The coupling means 19 and 20 can be galvanic and/or magnetic, where a magnetic coupling could be realized via air or via a magnetic material between the coupling inductor and the respective first or second phase inductors, such as iron.

The symbols in Fig. 4 have the following meaning:

u_vp/n voltage of the voltage source in positive or negative arm, respectively; i_{vpl n} current in positive/negative phase arm;

i_v output current at AC terminal;

u_f voltage at AC terminal (AC voltage);

i_m current through coupling inductor;

L_h inductance of first and second inductor;

L_n inductance of coupling inductor. In the following an analysis performed by the inventors of the behaviour of the circulating current in the depicted phase 1 is described briefly.

The governing equation for the circulating current i_c = (i_vp + i_vn)l2 can be obtained by applying Kirchhoff's voltage law to the direct path from u_vp to u_vn , giving u "_wvp -L^ -ri -L_h ^-ri_vn = u_vn . (1 )

dt dt

Introducing the differential voltage u_vc = (u_vp -u_vn)l2 allows equation (1 ) to be simplified to It is seen that u_vc is the driving voltage for the circulating current. This voltage is controllable, as is known from the art, by appropriately controlling the switching of the submodules 15. Accordingly, the voltage u_vc can be made to follow a reference u"^f , but in addition the voltage u_vc also contains a parasitic term Δu_vc as Analysis of the real frequency dependent behaviour of the system, a model of which 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 at twice the fundamental frequency of the AC voltage whose amplitude is normally a few percents of the rated AC voltage;

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

3) a third component comprising switching harmonics.

Since the first component has the biggest amplitude of the three components, this component would result in harmonics of the circulating current with the highest peaks. Accordingly, in order to reduce the losses resulting from these peaks, it is desirable to reduce the first component.

With the harmonics compensator 21 according to Fig. 4, compensation of the component 1 ) becomes possible. Preferrably, the control unit 23 controls the power electronic converter 22 so that a desired current flow i_m through the coupling inductor

18 is generated. The desired current flow i_m is determined by the control unit 23 by taking into account the kind of coupling between the coupling inductor 18 and the phase leg in order to generate a desired compensating current in the phase leg, and here in particular in the first and second phase inductors so that the harmonics in the circulating current i_c at twice the fundamental frequency of the AC voltage U_f are reduced.

Since the harmonics compensator 21 takes care of the harmonics in the circulating current i_c, the voltage rating of the power semiconductor switches 16 can thereby be reduced, which results in a decrease of the overall costs for the power converter shown in Fig. 1. This, of course, has to be offset against the additional costs induced by the introduction of the harmonics compensator. In order to keep these additional costs below the above named cost savings, it is suggested in a special embodiment of the invention to choose the inductance L_m of the coupling inductor 18 in relation to the inductances L_h of the first and second phase inductors, 9 and 10, in such a way, that the voltage across the coupling inductor 18 is considerably lower than the voltage across the first and second inductors. I n case that all the inductors are coils, the number of turns of the coil of the coupling inductor 18 would be chosen to be appropriately small compared to the number of turns of the first and second phase inductors, 9 and 10.

In Figs. 5, 6, 7, 9 and 1 1 , embodiments of a three-phase multi-level converter are shown, where in each embodiment the coupling inductors 18 are galvanically coupled to their respective phase leg. The placement of the phase inductors 9, 10, 32, respectively, as well as the way the harmonics compensator 21 is connected to the corresponding coupling inductor 18 vary, however. The converter shown in Fig. 5 is built of three phase legs according to Fig. 4.

The converter of Fig. 6 shows a delta-connection of the three first phase inductors 9 of the three positive phase arms as well as a similar delta-connection of the three second phase inductors 10 of the three negative phase arms. The midpoint of the delta-connected first and second phase inductors 9 and 10, respectively, is connected to the corresponding first or second DC terminal 4 or 5. Each phase inductor 9 or 10 may comprise just one inductor element or a series connection of two or more inductor elements.

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

Figs. 8a and 8b show an embodiment of the harmonics compensators 21 of Figs. 5, 6 and 7. The harmonics compensator 21 , shown in Fig. 8a, which is connected in parallel with the corresponding coupling inductor 18, comprises an AC/DC power electronic converter 22 which is controlled by a second control unit 23 as described above. The second control unit 23 is integrated in the harmonics compensator 21.

Alternatively, a second control unit could be installed separately. The power electronic converter 22 is arranged in form of a current-stiff power converter, where a DC current is injected into the converter via a DC current source formed by a DC voltage source 36 and an auxiliary inductor 34 connected in series with the DC voltage source 36. An example for how the AC/DC power electronic converter 22 may be arranged is shown in principle and schematically in Fig. 8b, where the AC/DC power electronic converter 22 is a 3-level converter with ideal power electronic change-over switches 38, where the switching of the change-over switches 38 is controlled by the second control unit 23.

Fig. 9 shows an embodiment of the three-phase multi-level converter with the same arrangement of the phase inductors 32 as in Fig. 7. However, each phase leg comprises only one coupling inductor 18 and one corresponding harmonics compensator 21 , and the two are arranged in series with each other and in series with the second voltage source Uvni in the corresponding negative phase arm. In the alternative, the coupling inductors 18 and the harmonics compensators 21 could as well be placed in series with the corresponding positive phase arm.

The embodiment in Fig. 11 differs from Fig. 9 in that it depicts a symmetrical arrangement of a coupling inductor 18 and a corresponding, series-connected harmonics compensator 21 in each phase arm of the three phase legs. An embodiment of the harmonics compensators 21 of Figs. 9 and 11 is shown in Figs. 10a and 10b. The harmonics compensator 21 which is connected in series with the corresponding coupling inductor 18 comprises again an AC/DC power electronic converter 22 and a second control unit 23 to control the power electronic converter 22 in a way described above. In this embodiment, the power electronic converter 22 is arranged as a voltage-stiff converter being supplied by a DC voltage source 36. An example for how the AC/DC power electronic converter 22 may be arranged is shown in principle and schematically in Fig. 10b, where the AC/DC power electronic converter 22 is a 3-level converter with ideal power electronic change-over switches 38, where the switching of the change-over switches 38 is controlled by the second control unit 23. In an alternative solution, the harmonics compensators 21 of Fig. 9 and 11 could also be arranged as current-stiff converters according to Figs. 8a and 8b.

In Figs. 9 and 11 , the three harmonics compensators 21 of the positive and/or negative phase arms, respectively, are all connected in wye-connection to the same potential. This makes it possible in an alternative embodiment to combine the power electronic converters of the three harmonics compensators 21 to a three-phase power electronics converter which is controlled by a single second control unit.

Claims

1. Multi-level voltage converter for converting a DC voltage into an AC voltage and vice versa comprising a first control unit (30) and at least one phase leg (1 ) between a first DC terminal (4) and a second DC terminal (5), where the at least one phase leg (1 ) comprises a first voltage source (Uvp1 ) between the first DC terminal (4) and a first AC terminal (6) and a second voltage source (Uvn1 ) between the first AC terminal (6) and the second DC terminal (5) and where the first control unit (30) controls the first and second voltage sources (Uvp1 , Uvn1 ), characterized in that

the converter further comprises at least one coupling inductor (18) being coupled in series with the at least one phase leg (1 ), an actively controlled harmonics

compensator (21 ) being connected to the at least one coupling inductor (18), and a second control unit (23) being adapted to control the output of the harmonics compensator (21 ) so as to reduce harmonics in a circulating current (i_c) flowing through the at least one phase leg (1 ).

2. Multi-level voltage converter according to claim 1 , where the second control unit (23) controls the output of the harmonics compensator (21 ) so that harmonics in the circulating current (i_c) at twice the fundamental frequency of the AC voltage (U_f) are reduced.

3. Multi-level voltage converter according to claim 1 or 2, where the second control unit (23) controls the output of the harmonics compensator (21 ) so that the harmonics compensator (21 ) generates a desired current flow (i_m) through the coupling inductor (18).

4. Multi-level voltage converter according to any of the previous claims, where the

second control unit (23) controls the output of the harmonics compensator (21 ) via closed-loop control.

5. Multi-level voltage converter according to any of the previous claims, where the

coupling inductor (18) is coupled galvanically to the at least one phase leg (1 ).

6. Multi-level voltage converter according to any of the claims 1 to 4, where the

coupling inductor (18) is coupled magnetically via air or via a magnetic material to the at least one phase leg (1 ).

7. Multi-level voltage converter according to any of the previous claims, where the harmonics compensator (21 ) comprises a power electronic converter (22) the output side of which is directly connected to the coupling inductor (18).

8. Multi-level voltage converter according to claim 7, where the power electronic

converter (22) is connected in parallel with the coupling inductor (18).

9. Multi-level voltage converter according to claim 8, where the power electronic

converter (22) is a current-stiff converter.

10. Multi-level voltage converter according to claim 7, where the power electronic

converter (22) is connected in series with the coupling inductor (18)..

1 1. Multi-level voltage converter according to claim 10, where the power electronic

converter (22) is a voltage-stiff converter.

12. Multi-level voltage converter according to any of the previous claims, where at least one phase inductor (9) is connected in series with the phase leg (1 ).

13. Multi-level voltage converter according to any of the previous claims 1 to 11 , where at least one phase inductor (9) is connected in series with the first AC terminal (6).