WO2013135300A1

WO2013135300A1 - Arrangement for conversion between ac and dc

Info

Publication number: WO2013135300A1
Application number: PCT/EP2012/054625
Authority: WO
Inventors: Staffan Norrga; Alireza NAMI; Liwei Wang
Original assignee: Abb Research Ltd
Priority date: 2012-03-16
Filing date: 2012-03-16
Publication date: 2013-09-19

Abstract

The invention concerns an arrangement (10) for conversion between AC and DC, which arrangement comprises a primary converter module with a first two-5 level voltage source converter (12) and a second two- level voltage source converter (14), where the two- level voltage source converters are connected in series between a first and a second DC pole, a first and a second transformer (T1, T2), each having a primary side 10 connected to an AC power line (18A, 18B, 18C) and a secondary side connected to a corresponding two-level voltage source converter, and a secondary converter module with a first group (16) of voltage source converter cells (C1A, C1B, C1C, C1D) and a second group 15 (17) of voltage source converter cells (C2A, C2B, C2C, C2D), where the cells of the first and second groups are connected in cascade between the first and the second DC pole.

Description

ARRANGEMENT FOR CONVERSION BETWEEN AC AND DC

FIELD OF INVENTION The present invention generally relates to voltage source converters. More particularly the present invention relates to an arrangement for conversion between AC and DC. BACKGROUND

Arrangements for conversion between AC and DC are needed in various situations, for instance when

interfacing a Direct Current (DC) power transmission system with an Alternating Current (AC) transmission system, for instance a three-phase system. Other areas are in Static VAR compensators

One type of arrangement that has found interest

recently in high power application is the voltage reinjection converter arrangement.

One type of voltage reinjection converter arrangement is described in WO 2010/0888968. This document

describes a number of H-bridge converters, one for each AC phase, connected in series between two DC poles. In this arrangement there is furthermore a series of voltage source converter cells connected in parallel with the H bridge converters.

Another known voltage reinjection converter arrangement has been proposed by J. Arrillaga, Y. H. Liu, N. R. Watson in "Flexible Power Transmission The HVDC Options", John Wiley and Sons, 2007. The arrangement is made up of a primary converter module made up of two two-level voltage source converters in series between two DC poles and a secondary converter module made up of diode clamped multiple level converters in parallel with the two-level converters. Each two-level converter is connected to a corresponding transformer, which in turn is connected to an AC line.

In this type of converter, the two-level converters are soft switching converters, i.e. switching at zero voltage and fundamental frequency, while the diode clamped converters are provided for providing

triangular wave variations on the voltage obtained at each two-level converter. The waves from the two two- level converters are then combined after transformation for obtaining an AC voltage. In this type of

arrangement most of the current will flow through the two-level converters. Because of the soft switching of these converters, the losses of the converter

arrangement are low.

However, the secondary module is unnecessarily complex and may furthermore cause ripple on the DC voltage. It does furthermore not functioning that well at high voltages .

Arrillaga describes variations of the above mentioned arrangement for operating at high voltages. Such variations involve the use of additional transformers between the primary and secondary switching modules, which if anything provided an even more complex

structure . There is therefore a need for improvement on the converter arrangement proposed by Arrillaga. The present invention has the object of improving on the converter arrangement presented by Arrillaga et al .

SUMMARY OF THE INVENTION The present invention is directed towards providing an improved arrangement for conversion between AC and DC.

This object is according to the present invention achieved through an arrangement for conversion between AC and DC, said arrangement comprising:

a primary converter module comprising a first two-level voltage source converter and a second two-level voltage source converter, the two-level voltage source

converters being connected in series between a first and a second DC pole,

a first and a second transformer, each having a primary side connected to an AC power line and a secondary side connected to a corresponding two-level voltage source converter, and

a secondary converter module comprising a first group of voltage source converter cells and a second group of voltage source converter cells, where the cells of the first and second groups are connected in cascade between the first and the second DC pole.

The present invention has a number of advantages. Power can then be transferred in either direction. The losses in the operation of the arrangement are low. Another advantage of the invention is that it is possible to obtain a high resolution of a re-injected voltage with a high number of cells and switch in a very low number of pulses with a minimum loss. This in turn allows the high resolution of the output voltage to be obtained. The use of cells has the further advantage and that is that ripple occurring on the DC side has a high

frequency and a low amplitude. The use of cells also provides excellent scalability and modularity.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will in the following be

described with reference being made to the accompanying drawings, where fig. 1 schematically shows a converter arrangement for converting between AC and DC according to a first embodiment of the invention,

fig. 2 schematically shows the structure of a first type of voltage source converter cell,

fig. 3 schematically shows the structure of a second type of voltage source converter cell,

fig. 4 schematically shows the structure of a third type of voltage source converter cell,

fig. 5 schematically shows a number of voltages in the converter arrangement of the first embodiment,

fig. 6 schematically shows a converter arrangement according to a second embodiment of the present

invention, and

fig. 7 schematically shows a converter arrangement according to a third embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of preferred embodiments of the invention will be given.

Fig. 1 schematically shows an arrangement 10 for conversion between AC and DC. For this reason the converter arrangement 10 may be connected between a DC power line and an AC power line. The DC power line may here comprise a first and a second pole PI and P2, while the AC power line 18 may be a three-phase power line comprising phase conductors 18A, 18B and 18C. The DC power line may furthermore be a part of a DC system such as a High Voltage Direct Current (HVDC) system, while the AC power line may be a part of an AC system such as a Flexible Alternating Current Transmission System (FACTS) .

The conversion may thus be a conversion between high DC voltages, such as above 800 kV and high AC voltages such as 400 kV. The conversion arrangement may because of this be provided in a power transmission system, which system may be an AC power transmission system or a DC power transmission system.

A DC power line may be a power line covering a long distance for transferring power. One exemplifying distance is the distance of 500 km. It should here also be realized that a DC power system may include several power lines, converters and converter arrangements. More particularly, the converter arrangement 10

comprises a primary converter module comprising a first and a second two-level voltage source converter 12 and 14 connected in series between the two poles PI and P2. In this first embodiment the AC power line is a three- phase power line and therefore the first two-level voltage source converter comprises three parallel branches each with a pair of switches S1A, SIB, SIC, SID, S1E and S1F, where the midpoint between the two switches of a branch is connected to a secondary side winding of a first transformer Tl, where the

corresponding primary side winding is connected to a phase conductor 18A, 18b and 18C of the AC power line. The midpoint of the switches S1A and SIB of a first branch is connected to a first secondary winding of the first transformer Tl, with the corresponding first primary winding being connected to a first phase conductor 18A of the AC power line. The midpoint of the switches SIC and SID of a second branch is connected to a second secondary winding of the first transformer Tl, with the corresponding second primary winding being connected to a second phase conductor 18B of the AC power line. The midpoint of the switches S1E and S1F of a third branch is connected to a third secondary winding of the first transformer Tl, with the

corresponding third primary winding being connected to a third phase conductor 18C of the AC power line. The primary windings may more particularly be connected in series with the phase conductors 18A, 18B, 18C.

In a similar manner the second two-level converter 14 also comprises three parallel branches each with a pair of switches S2A, S2B, S2C, S2D, S2E and S2F, where the midpoint between the two switches of a branch is connected to a secondary side winding of a second transformer T2, where the corresponding primary side winding is connected to a phase conductor 18A, 18B and 18C of the AC power line. The midpoint of the switches S2A and S2B of a first branch is connected to a first secondary winding of the second transformer T2, with the corresponding first primary winding being connected to the first phase conductor 18A of the AC power line. The midpoint of the switches S2C and S2D of a second branch is connected to a second secondary winding of the second transformer T2, with the corresponding second primary winding being connected to the second phase conductor 18B of the AC power line. The midpoint of the switches S2E and S2F of a third branch is connected to a third secondary winding of the second transformer T2, with the corresponding third primary winding being connected to the third phase conductor 18C of the AC power line. The primary windings may more particularly be connected in series with the phase conductors 18A, 18B, 18C.

A branch in the first converter can also be seen as being connected in series with a branch of the second converter between the first and the second pole PI and P2. Each such two-level converter branch is thus provided for one corresponding AC phase.

The primary windings of the first transformer Tl are in this embodiment connected in series with the primary windings of the second transformer T2, while the secondary windings of the first transformer Tl are delta-connected. The primary windings of the second transformer T2 are wye-connected, as are also the secondary windings of the second transformer T2.

There is also a secondary converter module connected in parallel with the primary converter module. The

secondary converter module is in this embodiment made up of a first group 16 of voltage source converter cells CIA, C1B, C1C, C1D and a second group 17 of voltage source converter cells C2A, C2B, C2C and C2D. The cells of these two groups are thus connected in series or cascade between the first and the second poles PI and P2. In this way there may be provided a chain link of cells. Furthermore in parallel with the first and second group of cells 16 and 17 there is also an inductor L and a DC capacitor C_Dc-

The midpoint or junction between the first and the second group of cells 16 and 17 is here connected to the junction between the first and the second two-level converter 12 and 14 via a first interconnection branch 20. The first interconnection 20 branch is thus

provided between the point where the first group 16 of cells is connected to the second group 17 of cells and the point where the first two-level converter 12 is connected to the second two-level converter 14.

The first converter 12 is connected to the first pole using a second interconnecting branch 22 in series with said inductor L, while the second converter 14 is connected to the second pole via a third

interconnecting branch 24. The inductor L, which is optional, may be a smoothing inductor. Finally there is a control unit 25, which provides control signals for controlling the two-level

converters and the groups of cells. The control is indicated by arrows leading from the control unit 25 to the two-level converters 12 and 14 and the first and second groups 16 and 17.

The switching elements of the two-level converters are all provided in the form of a transistor with anti- parallel diode, where the transistor may be Insulated Gate Bipolar Transistors (IGBT) .

The cells may be half-bridge or full bridge cells with a cell capacitor and can be switched to provide at least one voltage contribution or zero voltage, where the available voltage contributions from a cell may be a positive, a negative or both a positive and negative contribution . Fig. 2 shows a first type of half-bridge cell

CCA that may be used in the arrangement. The cell CCA is also denoted a two-level converter cell, and

includes an energy storage element, here in the form of a capacitor C, which is connected in parallel with a first group of switches. The switches in the first group are connected in series with each other. The first group here includes a first switch SW1 and a second switch SW2 (shown as dashed boxes), where each switch SW1, SW2 may be realized in the form of a transistor, such as an IGBT (Insulated Gate Bipolar

Transistor) together with an anti-parallel diode. The first switch SW1 thus includes a first transistor Tl together with anti-parallel first diode Dl . The second switch SW2 includes a second transistor T2 with an anti-parallel second diode D2. In fig. 2 the second diode D2 of the second switch SW2 is oriented upwards in the figure, which is towards the capacitor C, and connected in parallel between emitter and collector of the second transistor T2. The second switch SW2 is connected in series with and followed by the first switch SW1 that has the first diode Dl with the same orientation as the second diode D2 and connected in parallel with the first transistor Tl.

The cell has a first connection terminal TE1A and a second connection terminal TE2A, each providing a connection for the cell in cascade with other cells of the secondary converter module. In this first type of cell the first connection terminal TE1A more

particularly provides a connection to the junction between the first and the second switch SW1 and SW2, while the second connection terminal TE2A provides a connection to the junction between the first switch SW1 and the capacitor C. These connection terminals TE1A and TE2A thus provide points where the cell can be connected between the first and second pole. The secondary converter module thus includes a suitable number of such cells in cascade with each other between the two poles. In the described example the first and second groups thus provide a cascaded two-level (CTL) secondary converter module

A secondary converter module employing cells of the first type may for instance be obtained through

connecting a first connection terminal of a first cell CIA in the first group 16 to the first pole, connecting a first connection terminal of a second cell C1B of the first group 16 to the second connection terminal of the first cell CIA of the first group 16, connecting a first connection terminal of a third cell C1C to the second connection terminal of the second cell C1B, connecting a first connection terminal of a fourth cell C1D to the second connection terminal of the third cell C1C, connecting a first connection terminal of a first cell C2A of the second group 17 to the second

connection terminal of the fourth cell C1D of the first group 16, connecting a first connection terminal of a second cell C2B of the second group 17 to the second connection terminal of the first cell C2A of the second group 17, connecting a first connection terminal of a third cell C2C to the second connection terminal of the second cell C2B, connecting a first connection terminal of a fourth cell C2D to the second connection terminal of the third cell C2C and finally connecting a second terminal of the fourth cell C2D of the second group 17 to the second pole P2.

The first type of cell provides a voltage contribution being either zero or a voltage with one type of polarity, which in this case is a positive polarity.

Fig. 3 schematically shows a second type of half-bridge converter cell CCB having the same type of components as the first type and being interconnected in the same way. However, here the first switch SW1 is followed by the second switch SW2. There is also in this second type of cell CCB a connection terminal, a second connection terminal TE2B, which provides a connection to the connection point between the first and the second switches SW1 and SW2 as well as a connection terminal, a first connection terminal TE1B, which provides a connection to the junction between the first switch SW1 and the capacitor C. This type of cell provides a voltage contribution being either zero or a voltage having the opposite polarity to the polarity of the voltage contribution provided by the first type of cell .

Fig. 4 schematically shows a third type of converter cell CCC including the same type of components having the same orientation in the same way as in the first type of cell, i.e. first and a second switches SW1 and SW2 each including a first and a second transistor Tl and T2 with anti-parallel first and second diodes Dl and D2 in a first group of switches provided in

parallel with an energy storage element, also here realized as a capacitor C. However here there is also a second group of switches connected in series with each other. This second group is here connected in parallel with the first group as well as with the energy storage element. The second group here includes a third and a fourth switch SW3 and SW4, provided through a third transistor T3 with anti-parallel third diode D3 and through a fourth transistor T4 with anti-parallel fourth diode D4 having the same orientation as the first and second diodes. This second group is thus provided in a further branch in parallel with the capacitor C. The third and fourth switches SW3 and SW4 are here provided in the same way as the first and second switches of the second type of cell. As in the first type of cell a first connection terminal TE1C is here provided to the junction between the first and the second switches SW1 and SW2. The second connection terminal TE2C is here provided in the same way as the second connection terminal in the second type of cell. In this third type of cell the second connection terminal TE2C thus provides a connection to the

junction between the third and fourth switches SW3 and SW4. As opposed to the first and second types of cells, this cell CCC is a full-bridge cell providing a voltage contribution being zero, a positive voltage

contribution or a negative voltage contribution, i.e. being zero, a positive polarity or a negative polarity.

The cells in the arrangement in fig. 1 may here be either cells of the first type, cells of the second type or cells of the third type. It is also possible with combinations of any of the three types of cells.

Fig. 5 schematically shows the voltages provided by the arrangement according to the first embodiment, where the voltages shown comprise the first pole voltage V_Pi, the voltage V1 applied by the first group of cells 16 to the first two-level converter 12, the voltage V12 applied by the second group of cells 17 to the second two-level converter 14 and finally the resulting AC voltage V_AC on the AC power line 18. The shown voltage levels are here normalized.

These voltages are obtained through the control unit 25 applying control signals to the gates of the transistors in the switches of the two-level converters and cells. Pulse width modulation (PWM) control may be employed. The control may furthermore be implemented using space vector modulation.

If the desired AC voltage has a period T and thus a fundamental frequency of f = 1/T then the second group of cells 17 provide a triangular wave V1 that is repeated every 60 degrees of the AC voltage of the AC line 18, i.e. having a period that is T/6. Also the first group of cells 16 provide a wave V12 having the same repetition rate or period. However, this wave V12 is displaced thirty degrees from the wave of the second group of cells 17.

The operation can also be described in the following way. For a given phase, such as a first phase provided by a first branch with switches S1A and SIB in the first two-level converter 12, the control unit 25 controls the first switch S1A in the upper part of this branch to be switched on while another of the switches SID and/or SIF in the lower part of the other branches is turned on, while controlling the cells CIA, C1B, C1C and C1D of the first group 16 to form a voltage varying in time, where the variation is from zero to a peak value and then back to zero. In the first group some cells may be bypassed (through providing a zero voltage contribution) and others are controlled to provide a voltage in order to provide this variation. They are thus controlled to provide thirty degrees of a

resulting AC phase. If then the second switch SIB in the lower part of the first branch is turned on while the first switch S1A is turned off at the same time as another of the switches SIC and/or SIE in the upper part of the other branches is turned on, which is done as the voltage across the first transformer Tl is zero and only once in the period T, and thereafter the voltage provided by the first group 16 of cells is varied in the same way as described above, the first transformer experiences a voltage varying from zero to the peak value, however now with a negative polarity, and then back to zero.

If the second two-level converter and second group of cells operate in the same way but phase shifted thirty degrees, then the AC voltage V_AC is obtained.

With this type wave forming it is then possible to switch a two-level converter at zero voltage and fundamental frequency i.e. at the frequency f = 1/T. Through the phase shift of the triangular waves in relation to each other it is furthermore guaranteed that the DC voltage at the DC pole is kept even. The basic switching is thus performed by the two level converters, each switching once in the period T, i.e. at the fundamental frequency. The additional switching needed to obtain an output voltage is obtained through switching of the cells for adding or subtracting a cell voltage contribution that assists in the forming of the triangular wave.

Power can then be transferred in either direction between the AC line 18 and the DC poles PI and P2. The majority of the current will in this type of operation then be running through the two-level converters and, because these are switched at zero voltages, the switching losses will be low. The switching of the cells will not have as low losses. However, the current running through the secondary converter module is much lower than the current running through the primary converter module. Therefore these losses will be insignificant and consequently the total losses will be low. Another advantage with the cells is that they have low on-state losses. The use of cells also provides excellent scalability and modularity, where the size of standard modules and mechanical layout can be kept small . The use of wye-connected transformers has the advantage of enabling the removal of triple harmonics. The phase shift of thirty degrees also provides better overall ac waveform. Another advantage of the use of cells is that it is possible to obtain a high resolution of the re- injected voltage with a high number of cells and switch in a very low number of pulses with a minimum loss. This in turn allows the high resolution of the output voltage to be obtained. The use of cells has the further advantage and that is that the ripple that occurs on the DC side has a higher frequency and lower amplitude than when diode-clamped multilevel converters are used.

As may be observed in fig. 5, the control of the cells in the first embodiment has to be made for

simultaneously ensuring a stable DC voltage and an AC voltage that has a desirable shape. An arrangement according to a second embodiment, but without control unit is schematically shown in fig. 6. This second embodiment differs from the first

embodiment in that the first interconnecting branch 20 comprises a third group 26 of cells C3A C3B and C3C.

An arrangement according to a third embodiment of the invention that is shown in fig. 6 comprises a fourth group 28 of cells C4A, C4B and C4C in the second interconnecting branch 22 as well as a fifth group 30 of cells C5A, C5B and C5C in the third interconnecting branch 24. If the sum of voltage contributions of these as seen between the two poles cancel out then they can also be used for controlling the shape and content of the AC voltage without influencing the DC voltage.

The second and third embodiments both have the

advantage of allowing the AC voltage output to be controlled independently of the DC voltage. This means that the AC voltage can be controlled decoupled from the DC voltage. This AC control could as an alternative be provided using the modulation performed via the control of the first and second group of cells. This type of modulation may for instance be used in a variation of the arrangement according to the first embodiment .

The switches that are provided in the two-level

converters as well as in the cells may be realized in the form of IGBT (Insulated Gate Bipolar Transistor) transistors together with anti-parallel diodes. It should however be realized that other types of switches may be used, such as Integrated Gate-Commutated Thyristors (IGCT), Bi-Mode Insulated Gate Transistors (BIGT) , gate turn-off thyristors (GTO) and Forced

Commutated Thyristors. Also other suitable thyristors may be used.

The control unit may be realized in the form of

discrete components. However, it may also be

implemented in the form of a processor with

accompanying program memory comprising computer program code that performs the desired control functionality when being run on the processor. A computer program product carrying this code can be provided as a data carrier such as one or more CD ROM discs or one or more memory sticks carrying the computer program code, which performs the above-described control functionality when being loaded into a control unit of a voltage source converter .

It should also be realized that the number of cells described as being provided in a group is only

exemplifying and that each group may include more than the three or four being shown for instance 8, 10 or 12.

It should also be realized that the DC capacitor may be omitted and that the way in which the transformers are connected may also be varied.

From the foregoing discussion it is evident that the present invention can be varied in a multitude of ways. It shall consequently be realized that the present invention is only to be limited by the following claims .

Claims

1. An arrangement (10) for conversion between AC and DC, said arrangement comprising:

a primary converter module comprising a first two-level voltage source converter (12) and a second two-level voltage source converter (14), said two-level voltage source converters being connected in series between a first and a second DC pole (PI, P2),

a first and a second transformer (Tl, T2), each having a primary side connected to an AC power line (18A, 18B, 18C) and a secondary side connected to a corresponding two-level voltage source converter, and

a secondary converter module comprising a first group (16) of voltage source converter cells (CIA, C1B, C1C, C1D) and a second group (17) of voltage source

converter cells (C2A, C2B, C2C, C2D) , where the cells of the first and second groups are connected in cascade between the first and the second DC pole.

2. Arrangement according to claim 1, further comprising a first interconnection branch (20) between the point where the first group of cells is connected to the second group of cells and the point where the first two-level converter is connected to the second two-level converter.

3. Arrangement according to claim 2, wherein the first interconnection branch comprises a third group (26) of cells (C3A, C3B, C3C) .

4. Arrangement according to any previous claim, further comprising a second interconnecting branch (22) between the first two-level converter (12) and the first DC pole (PI) and a third interconnecting branch (24) between the second two-level converter (14) and the second DC pole (P2) .

5. Arrangement according to claim 4, wherein the second interconnection branch comprises a fourth group (28) of cells (C4A, C4B, C4C) and the third

interconnecting branch comprises a fifth group (30) of cells (C5A, C5B, C5C) .

6. Arrangement according to claim 4 or 5, wherein the first interconnecting branch is connected to the first DC pole via an inductor (L) .

7. Arrangement according to any previous claim, wherein the two-level converters are three-phase converters, the transformers are three-phase

transformers and the AC line is a three-phase AC line.

8. Arrangement according to any previous claim, further comprising a control unit (25) configured to control the two-level voltage source converters the primary converter module to switch at zero voltage and fundamental frequency.

9. Arrangement according to claim 8, wherein the control unit, when performing said control is

configured to control the first and second two level converter to switch with a separation of thirty

degrees .

10. Arrangement according to claim 8 or 9, wherein the control unit in the control of the cells in the first and the second groups is configured to control the cells of a group to periodically vary from zero to a peak voltage and back within sixty degrees.

11. Arrangement according to claim 10, wherein the control unit in the control of the cells in the first and the second groups is configured to control the cells of a group to perform the same type of variation with a phase shift of thirty degrees.

12. Arrangement according to any previous claim, wherein the primary and secondary windings of the second transformer are wye connected, while the primary windings of the first transformer are connected in series with the primary windings of the second

transformer and the secondary windings of the first transformer are delta connected.

13. Arrangement according to any previous claim, wherein the cells comprise half-bridge converter cells.

14. Arrangement according to any previous claim, wherein the cells comprise full-bride converter cells.