WO2015124209A1

WO2015124209A1 - Ac/dc converter with serially connected phase converters

Info

Publication number: WO2015124209A1
Application number: PCT/EP2014/053471
Authority: WO
Inventors: Alireza NAMI; Frans Dijkhuizen
Original assignee: Abb Technology Ltd
Priority date: 2014-02-21
Filing date: 2014-02-21
Publication date: 2015-08-27

Abstract

It is presented an AC/DC converter (l) for converting power between a DC connection and an AC connection. The AC/DC converter (l) comprises: a plurality of phase converters (3, 3a-c), each comprising two DC connections (9a-b) and two AC connections (6a-b). The plurality of phase converters (3, 3a-c) are serially connected between the positive DC terminal (20) and the negative DC terminal (21). Each phase converter (3, 3a-c) comprises a first path (7) and a second path (8) between the DC connections (9a-b), the first path (7) comprising a first arm (5a) and a second arm (5b), the second path (8) comprising a third arm (5c) and a fourth arm (sd). A first AC connection (6a) is provided between the first arm (5a) and the second arm (5b) and a second AC connection (6b) is provided between the third arm (5c) and the fourth arm (sd); and each converter cell comprises a switching element and an energy storage element.

Description

AC/DC CONVERTER WITH SERIALLY CONNECTED PHASE

CONVERTERS

TECHNICAL FIELD

The invention relates to an AC/DC converter with serially connected phase converters for converting power between a DC (Direct Current) connection and an AC (Alternating Current) connection.

BACKGROUND

AC (Alternating Current) grids have been used for a long time for power transmission. In more recent years, there is an increase in interest for DC (Direct Current) links, and in particular HVDC (High Voltage Direct Current) for power transmission. In some cases there are several interconnected HVDC links, creating a DC grid. AC/DC converters are used to convert electrical power e.g. from the AC grid to the DC link/DC grid or vice versa.

US 2013/0070495 discloses an AC/DC converter comprising at least two phase legs connected in series between first and second DC connection terminals of the AC/DC converter, wherein each phase leg comprises: an AC connection having first and second terminals arranged to connect the phase leg to a phase of an AC system; a phase branch comprising at least one converter cell and having first and second branch end terminals; and a capacitor. The capacitor is connected the between the first branch end terminal and the first AC connection terminal, so that the capacitor forms a DC blocking capacitor.

However, such DC blocking capacitor is a large and expensive component, typically in millifarad size for high voltage applications. SUMMARY

It is an object to provide an AC/DC converter allowing the omission of AC side capacitors.

According to a first aspect, it is presented an AC/DC converter for converting power between a DC connection comprising a positive DC terminal and a negative DC terminal, and an AC connection comprising a plurality of AC phase connections. The AC/DC converter comprises: a plurality of phase converters, each comprising two DC connections and two AC connections, the plurality of phase converters being serially connected via their respective DC connections between the positive DC terminal and the negative DC terminal; wherein each phase converter comprises a first path and a second path connected in parallel between the DC connections, the first path comprising a first arm comprising a plurality of converter cells and a second arm

comprising a plurality of converter cells, serially connected between the DC connections, the second path comprising a third arm and a fourth arm serially connected between the DC connections, whereby a first AC

connection of each phase converter is provided between the first arm and the second arm and a second AC connection of each phase converter is provided between the third arm and the fourth arm. By having, in each phase converter, the first AC connection from the middle point between the first and second arm, and the second AC connection from the middle point between the third and fourth arm, the converter cells can be controlled such that there is no DC bias between the AC connections. In this way, AC side capacitors are not needed, which is a great improvement since the AC side capacitors are large and costly. Moreover, using this structure, the voltage rating for each phase converter is reduced to one third, compared to if each phase converter were to be connected between the DC poles. Also, this structure of the AC/DC converter results in a mechanically more simple layout, reducing complexity and size when installed due to the omission of large and bulky AC side DC blocking capacitors, thereby increasing the flexibility of the mechanical layout design. Additionally, the series connection of the phase converters does not have a circulating AC current between phases, which is the case when parallel connection of phase converters is employed. This reduces control needs and ratings on inductors in the phase converters.

Each converter cell may comprise the energy storage element provided in parallel to a serial connection of at least two switching elements. The third arm may comprise a plurality of converter cells and the fourth arm may comprise a plurality of converter cells. This embodiment provides great control of the conversion.

The third arm may consist of a plurality of serially connected switches and the fourth arm may consist of a plurality of serially connected switches. This provides a more simple structure, albeit with a cost of less control compared to using converter cells in the third arm and the fourth arm.

The third arm may consist of a capacitor device and the fourth arm may consist of a capacitor device. This provides an even more simple structure, albeit with a cost of less control compared to using converter cells in the third arm and the fourth arm.

Each capacitor device may comprise one or more capacitors connected serially and/or in parallel. In other words, each capacitor device can be made up of any suitable number of capacitors. For each phase converter, the two AC connections may be configured to be connected to either side of a respective transformer winding of a multi-phase transformer. This allows zero phase components to be cancelled.

Each phase converter may further comprise a third path provided in parallel with the first path and the second path, the third path comprising a fifth arm comprising a plurality of converter cells and a sixth arm comprising a plurality of converter cells, serially connected between the DC connections, such that a third AC connection is provided between the fifth arm and the sixth arm. In this way, the three AC connections are configured to be connected to one side of a three phase transformer. Also this embodiment allows zero phase components to be cancelled.

At least a third of the converter cells may be four quadrant cells, such as full bridge converter cells, and the remaining of the converter cells may be two quadrant converter cells, such as half bridge converter cells and/or three quadrant cells. In one embodiment, at least a third but less than half of the converter cells are four quadrant cells and the remaining converter cells are two quadrant converter cells and/or three quadrant cells. In one

embodiment, a third of the converter cells are four quadrant cells and two thirds of the converter cells are two quadrant converter cells and/or three quadrant cells.

The AC/DC converter may comprise at least one converter cell being a four quadrant cell at least one converter cell being a two quadrant converter cell.

The four quadrant converter cells may be configured to be controlled to provide a constant voltage on the AC connections when the voltage on the DC connection varies.

If one phase converter fails into a shortcut mode on the DC side, the other two phase converters can compensate for the failure, using the four quadrant converter cells in the other two phase converters which can reverse the voltage. Note however, that it is sufficient that one third of the converter cells are four quadrant converter cells to compensate for the a third of the DC voltage which is lost with the failed phase converters. This is a great cost saving compared to the scenario of having all converter cells being four quadrant converter cells. Moreover, with the fault blocking capability provided by the reversible voltage of the four quadrant converter cells when appropriately controlled, a shortcut between the DC terminals can be absorbed by the phase converters while still being connected to the AC connection. In this way, the need for a DC breaker is reduced or even eliminated.

Each phase converter may further comprise a capacitor connected between its DC connections. The capacitor can be quite small and is used to absorb transients of the phase converters.

The AC/DC converter may comprise three phase converters. According to a second aspect, it is presented a converter device comprising the AC/DC converter according to the first aspect and a multiphase transformer connected to the AC/DC converter.

Primary windings of the multiphase transformer may be connected to the AC/DC converter, in which case there are two secondary windings for each primary winding.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig 1 is a schematic diagram illustrating an embodiment of an AC/DC converter with a connected multiphase transformer;

Fig 2 is a schematic diagram illustrating an embodiment of the phase converters of Fig 1 comprising two paths with converter cells;

Fig 3 is a schematic diagram illustrating an embodiment of the phase converters of Fig 1 comprising one path with converter cells and one path with switches;

Fig 4 is a schematic diagram illustrating an embodiment of the phase converters of Fig 1 comprising one path with converter cells and one path with capacitors; Fig 5 is a schematic diagram illustrating an embodiment of the phase converters of Fig l comprising three paths with converter cells, connected to a three-phase transformer;

Fig 6 is a schematic diagram illustrating the AC/DC converter of Fig l with a connected multiphase transformer according to one embodiment with multiple secondary windings; and

Figs 7A-C are schematic diagrams illustrating embodiments of converter cells of the phase converters of Figs 2-5.

DETAILED DESCRIPTION

The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

Fig 1 is a schematic diagram illustrating an embodiment of an AC/DC converter 1 with a connected multiphase transformer. The AC/DC converter 1 is used for converting power between a DC connection comprising a positive DC terminal 20 and a negative DC terminal 21, and an AC connection comprising a plurality of AC phase connections 25a-c. Both the DC

connection and the AC connection are high voltage connections. The voltage difference between the DC terminals is here denoted va. The AC/DC converter 1 comprises a first phase converter 3a, a second phase converter 3b and a third phase converter 3c. Each one of the phase converters 3a-c comprises two DC connections 9a-b and two AC connections 6a-b. The phase converters 3a-c are serially connected via their respective DC connections 9a-b between the positive DC terminal 20 and the negative DC terminal 21. While Fig 1 shows three phase converters 3a-c, the AC/DC converter can be provided with any suitable number of serially connected phase converters as long as there are at least two phase converters.

It is to be noted that the terms positive and negative are to be interpreted as mutually relative terms and not absolute. In other words, the negative DC terminal 20 can have a positive voltage as long as it is less than the positive DC terminal 21.

For each phase converter 3a-c, the two AC connections 6a-b are connected to either side of a respective primary winding 3ia-c of a multi -phase

transformer 30. The multiphase transformer also comprises a corresponding number of secondary windings 32a-c. In this way, AC terminals 42a-c are provided e.g. for connection to an AC grid. It is to be noted that with the structure provided here, the AC/DC converter 1 can be connected to the transformer windings without any intermediate filters.

The AC/DC converter 1 can be configured for unidirectional or bidirectional power transfer between the AC side and the DC side.

The three phase converters 3a-c share the DC side pole to pole voltage.

Hence, using this structure, the voltage rating for each phase converter is reduced to one third, compared to if each phase converter were to be connected between the DC poles. This would reduce the required silicon area to a third if the current through each converter cell would be the same.

However, by placing the converter cells in series, the current increases, which increases the required silicon area. These two effects essentially take each other out, whereby the total silicon area of the semiconductors of a similar size compared to phase converters connected in parallel between the DC terminals. Moreover, since the voltage across each phase converter is smaller when connected in parallel and the number of converter cells of a phase converter depends on the DC voltage rating, fewer number of converter cells are required by connecting the phase converters serially, resulting in a comparatively compact physical design. Moreover, this structure of the AC/DC converter results in a mechanically more simple layout, reducing complexity and size when installed due to the omission of large and bulky AC side DC blocking capacitors.

Also, the series connection of the phase converters does not have a circulating AC current between phases which is the case when parallel connection of phase converters is employed. This reduces control needs and ratings on inductors in the phase converters.

A controller 15 is connected to each one of the phase converters 3a-c to control the converter cells of the phase converters 3a-c and any other switches that may be present, such as in the embodiment of the phase converter shown in Fig 3.

It is to be noted that while the AC/DC converter 1 is here exemplified with three phase converters (and therefore three AC phases), the AC/DC converter 1 can be provided with any number of phase converters (and therefore AC phases) as long as there are at least two phase converters.

Fig 2 is a schematic diagram illustrating an embodiment of the phase converters of Fig 1 comprising two paths with converter cells. Each one of the phase converters 3a-c of Fig 1 may be embodied by a phase converter 3 as illustrated in Fig 2. The phase converter 3 comprises a first path 7 and a second path 8. The two paths 7, 8 are connected in parallel between the DC connections 9a-b. The first path 7 comprises a first arm 5a and second arm 5b, each comprising a plurality of converter cells 2. The first arm 5a and the second arm are serially connected between the DC connections 9a-b. Each converter cell 2 comprises a switching element and an energy storage element (see e.g. Figs 7A-C and its corresponding description below). Moreover, each converter cell is controllable to either be in a short circuit state or a contribution state. In the short circuit state, the converter cell is essentially bypassed whereas in the contribution state, the converter cell provides a voltage contribution.

Depending on the converter cell type, the voltage contribution can be positive or negative. By suitable control from the controller (15 of Fig 1), the converter cells 2 can be controlled to provide a suitable wave form on the AC

connection 6a or 6b, e.g. sinusoidal. With many converter cell

The second path 8 comprises a third arm 5c and a fourth arm 5d which are also serially connected between the DC connections 9a-b. In this

embodiment, the third arm 5c and the fourth arm 5d each comprises a plurality of converter cells 2.

A first AC connection 6a is provided between the first arm 5a and the second arm 5b and a second AC connection 6b is provided between the third arm 5c and the fourth arm 5d. Inductors 4 are provided on either or both sides of the AC connections 6a-b.

The converter cells can be any type of suitable converter cell. For example, the converter cells can be two quadrant converter cells such as half bridge cells, four quadrant converter cells such as full bridge cells or a combination of both (see below for details on the cell types). The quadrants relate to direction of current and polarity of voltage. For instance a four quadrant converter cell is reversible in current direction and voltage polarity. However, a two quadrant converter cell is only reversible in one of current direction and voltage polarity. In the applications shown herein, the two quadrant converter cell is a converter cell which is reversible in current direction but not voltage polarity.

The first path 7 (and in this embodiment also the second path 8) consisting of the first arm 4a and the second arm 4b can be seen as a modular multilevel converter (MMC). By employing the MMC, the switching frequency of each converter cell can be kept low, while the accuracy of the AC side waveform is kept of a high quality.

The first arm 5a and the second arm 5b are controlled to operate with a 180 degree phase shift relative to each other. The third arm is controlled to operate with a 180 degree phase shift relative to the first arm. Moreover, the third arm 5c and the fourth arm 5d are controlled to operate with a 180 degree phase shift relative to each other. In this way, an AC voltage is provided on the AC connections 6a-b, relative to each other.

The control can be expressed analytically using the following. The voltage across the first arm is expressed as v_Ui, the voltage across the second arm is expressed as vh, the voltage across the third arm is expressed as v_u2 and the voltage across the fourth arm is expressed as vi₂. With the voltage across the DC connectors of the whole AC/DC converter being expressed as va, and the number of phases in this example being three, the voltages across the arms can be expressed as follows, where each phase converter holds vd/3: Vui= Vd/6 - (vd/6)*m*sin (cot) (1)

Vh= Vd/6 + (vd/6)*m*sin (cot) (2)

Vu2= Vd/6 + (vd/6)*m*sin (cot) (3) vi2= Vd/6 - (vd/6)*m*sin (cot) (4) where m is a modulation factor and ω is the angular velocity of the AC connection. The voltage across the first path Vi, i.e. the first and second arm is then given by:

Vi = Vui + Vh = Vd/6 - (vd/6)*m*sin (cot) + Vd/6 + (vd/6)*m*sin (cot)

Analogously, the voltage across the first path v₂, i.e. the third and fourth arm is given by: v₂ = Vu2 + V12 = Vd/6 + (vd/6)*m*sin (cot) + Vd/6 - (vd/6)*m*sin (cot)

In other words, the voltage across the first path and second path are controlled to be equal and with (essentially) no AC component. The voltage v_s across the two AC connections 6a, 6b is for instance given by: Vs = Vh - vi2 = Vd/6 + (vd/6)*m*sin (cot) - (vd/6 - (vd/6)*m*sin (cot)) = (vd/6)*m*sin (cot) + (vd/6)*m*sin (cot)

= (vd/3)*m*sin (cot) (γ)

It is thus shown how the DC component is removed for the AC connection, thereby removing the need for any capacitor or other component on the AC side of the phase converter for DC isolation. This remains true for any embodiment of the arms (i.e. also for the embodiment shown in Figs 3, 4 and 5), as long as the DC level of the two AC connections 6a-b are kept at about the same. Moreover, there is no circulating current in the phase converter with the control shown above in normal operation. However, in some cases there may be transients that need to be taken care of. For this purpose, a small capacitor 14 is optionally provided between the DC connections 9a-b. Since the transients are fast in nature, the small capacitor 14 only needs to be in the order of microfarads for DC and AC voltages of hundreds of kilovolts, which does not add any significant cost or size to the phase converter 3. In this way, the small capacitor 14 handles any transients that may occur between the DC connections 9a-b.

Since the MMC structure is used, with appropriate control, the transformer windings will be exposed to waveforms of a good quality, e.g. sinusoidal. Moreover, the balancing of the energy storage elements is done on the converter cell level with a simple modulation strategy and low frequency switching elements. This eliminates the need to have any AC side DC blocking capacitor between the AC/DC converter 1 and the transformer 30 as is needed in the prior art. Removing the capacitor is a significant advantage for high voltage applications where the size and cost of capacitors is substantial.

Significantly, there is no circulating current between arms as it is cancelled out when the AC connections 6a-b are connected to the transformer winding. This reduces rating requirements on any inductor 4 in the arms. Moreover, control complexity is reduced due to the absence of the circulating current. Looking to the control of the first path now, there can be a DC bias applied to the whole phase converter 3 (depending on where in the series connection of Fig 2 that the phase converter is provided). However, the same DC bias is applied to both the first path 7 and the second path 8 since these are connected in parallel to the DC connections 9a-b. Since the first AC

connection 6a and the second AC connection 6b are provided in the middle of the first and second paths 7-8, the DC voltage difference between the first AC connection 6a and the second AC connection 6b can be controlled to be essentially zero. Hence, there is no need to provide any AC side DC blocking capacitor between the phase converter and any transformer winding provided between the AC connections 6a-b. Even if one or more of the converter cells 2 fail and fall into a short circuit mode, the converter cells can be controlled such that there is no significant DC difference between the AC connections 6a-b. Lowering impedance requirements on inductors has a great effect on complexity and cost when applied at high voltages, such as in an HVDC environment where the size and cost of inductors have great impact.

Fig 3 is a schematic diagram illustrating an embodiment of the phase converters of Fig 1 comprising one path with converter cells and one path with switches. Here, in the second path 8, the only elements in the third arm 5c and the fourth arm 5d (i.e. apart from conductors) are a plurality of serially connected switches 12, where each switch is optionally provided with an anti-parallel diode. Each switch 12 can e.g. be implemented using an insulated gate bipolar transistor (IGBT), Integrated Gate-Commutated Thyristor (IGCT), a Gate Turn-Off thyristor (GTO), BIGT (Bi-Mode Insulated Gate Transistor), RC-IGBT (Reverse Conduction IGBT) or any other suitable high power semiconductor component. This embodiment requires fewer components than the embodiment shown in Fig 2, which reduces cost and complexity.

Fig 4 is a schematic diagram illustrating an embodiment of the phase converters of Fig 1 comprising one path with converter cells and one path with capacitors. Here, in the second path 8, the only elements in the third arm 5c and the fourth arm 5d (i.e. apart from conductors) are capacitor devices 13. Each capacitor device 13 provides a capacitance using one or more capacitors connected serially and/or in parallel. This embodiment reduces the number of components needed, but the capacitors need to support the full current across the phase converter 3, which may result in capacitors of a large size. Still, the middle point of the first path, being the first AC

connection 6a can be controlled such that the DC component of the first AC connection is the same as the DC component of the second AC connection 6b. Hence, there is no need for any AC side DC blocking capacitors between the AC connections 6a-b and any connected transformer winding.

Fig 5 is a schematic diagram illustrating an embodiment of the phase converters of Fig 1 comprising three paths with converter cells, connected to a three-phase transformer. This embodiment can be seen as an extension of the embodiment of Fig 2. Here, the phase converter 3 further comprises a third path 17 provided in parallel with the first path 7 and the second path 8. The third path 17 comprises a fifth arm 5e and a sixth arm 5f, each comprising a plurality of converter cells 2. The fifth arm 5e and the sixth arm 5f are serially connected between the DC connections 9a-b. A third AC connection 6c is thus provided between the fifth arm 5e and the sixth arm 5f. In this way, the three AC connections 6a-c can be connected to a primary side of a three phase transformer 32a-c. The three AC connections 6a-c can e.g. be connected to three respective points of a three phase transformer 30' in a delta

configuration. A wye configuration is equally possible. On a secondary side of the transformer 30', the configuration of its windings 45a-c can be the same or it can differ. For instance, the windings of the secondary side can be in a wye configuration, but could equally well be in a delta configuration, depending on installation requirements.

The phase shift between the three AC connections 6a-c can e.g. be 120 degrees relative to each other to provide a three phase AC connection to the three phase transformer 30'. When multiple phase converters are employed serially as shown in Fig 1, the respective windings of the secondary side of the phase converters can be connected serially, such that the AC components of all three phase converters are accumulated. It is to be noted that the primary and secondary side of the transformer is only used herein to distinguish between the two sides of the transformer and are mutually exchangeable.

By introducing the three-phase transformer 30', zero sequence components can be blocked as long as the windings on the converter side are connected in delta.

Fig 6 is a schematic diagram illustrating the AC/DC converter 1 of Fig 1 with a connected multiphase transformer 30 according to one embodiment with multiple secondary windings. Here, the multiphase transformer 30 comprises two secondary windings 32a, 32a' for the first primary winding 31a.

Analogously, there are two secondary windings 32b, 32b' for the second primary winding 31b and two secondary windings 32c, 32c' for the third primary winding 31c. With the arrangement shown, third harmonics are reduced or even eliminated. Using this embodiment, zero sequence current can be cancelled, which can increase the converter power by 15%. Figs 7A-C are schematic diagrams illustrating embodiments of converter cells of the phase converters of Figs 2-5. It is to be noted that the embodiments of Figs 7A-C are only examples and converter cells of any suitable structure forming part of a multilevel bridge configuration could be used. In one embodiment, each converter cell comprises an energy storage element provided in parallel to a serial connection of at least two switching elements.

A converter cell 2 is a combination of one or more semiconductor switching elements, such as transistors or thyristors, and one or more energy storing elements 41, such as capacitors, supercapacitors, inductors, batteries, etc. Optionally, a converter cell 2 can be a multilevel converter structure in itself, such as a flying capacitor or MPC (Multi-Point-Clamped) or ANPC (Active - Neutral-Point-Clamped) multilevel structure.

Fig 7A illustrates a converter cell 2 comprising a switching element 40 and an energy storage element 41 in the form of a capacitor. The switching element 40 can for example be implemented using an insulated gate bipolar transistor (IGBT), Integrated Gate-Commutated Thyristor (IGCT), a Gate Turn-Off thyristor (GTO), or any other suitable high power semiconductor component. In fact, the converter cell 2 of Fig 7A can be considered to be to be a more general representation of the converter cell shown in Fig 7B, which will be described here next.

Fig 7B illustrates a converter cell 2 being a two quadrant converter cell in the form of a half bridge structure. The converter cell 2 here comprises a leg of two serially connected switching elements 40a-b, e.g. in the form of IGBTs, IGCTs, GTOs, etc. Optionally, there is an antiparallel diode connected across each switching element 40a-b (not shown). An energy storage element 41 is also provided in parallel with the leg of switching elements 4oa-b. The voltage synthesised by the converter cell 2 can thus either be zero or the voltage of the energy storage element 41.

Fig 7C illustrates a converter cell 2 being a four quadrant converter cell in the form of a full bridge structure. The converter cell 2 here comprises four switching elements 40a-d, e.g. IGBTs, IGCTs, GTOs, etc. Optionally, there is an antiparallel diode connected across each switching element 4oa-d (not shown). An energy storage element 41 is also provided in parallel across a first leg of two switching elements 4oa-b and a second leg of two switching elements 40c-d. Compared to the half bridge of Fig 7B, the full bridge structure allows the synthesis of a voltage capable of assuming both signs, whereby the voltage of the converter cell can either be zero, the voltage of the energy storage element 41, or a reversed voltage of the energy storage element 41. A three quadrant converter cell can be achieved by replacing one of the switching elements 4oa-d of the full bridge converter cell with a diode.

Fig 8 shows the AC/DC converter of Fig 1 where DC voltages are shown.

Across the first phase converter 3a there is a voltage vda, across the second phase converter there is a voltage vab, and across the third phase converter, there is a voltage vac Across the DC connection DC, there is a voltage va.

Looking now to a first fault condition, if one phase converter fails into a shortcut 75 on the DC side, the other two phase converters can compensate for the failure if there is an over dimensioning of the number of converter cells of the other two phase converters of at least 25 per cent. Looking now to a second fault condition where there is a short circuit between the DC poles 20, 21, with the fault blocking capability provided by the reversible voltage of the four quadrant converter cells when appropriately controlled, the DC short circuit can be absorbed by the phase converters while still being connected to the AC connection with full normal voltage. This can work as long as at least a third of the converter cells are four quadrant converter cells. In this way, the need for a DC breaker is reduced or even eliminated, since no DC current flows during the short circuit. This isolates the fault on the DC side such that the fault is not propagated to the AC side. This allows the converter to operate in what is called a STATCOM operation with maximum AC voltage and support the reactive power to the AC grid during the temporary fault at the DC side.

When four-quadrant converter cells are employed, these can be controlled to provide a constant voltage on the AC connections when the voltage on the DC connection varies. For instance, the AC and DC side voltages can be decoupled in a boosting operation. This means that the converter cells are controlled to provide a higher voltage on the AC connections when required, e.g. due to the voltage on the DC connection dropping or due to a higher voltage required on the AC side. The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims

Claims

l8 CLAIMS

1. An AC/DC converter (1) for converting power between a DC connection comprising a positive DC terminal (20) and a negative DC terminal (21), and an AC connection comprising a plurality of AC phase connections (25a-c), the AC/DC converter (1) comprising:

a plurality of phase converters (3, 3a-c), each comprising two DC connections (9a-b) and two AC connections (6a-b), the plurality of phase converters (3, 3a-c) being serially connected via their respective DC connections (9a-b) between the positive DC terminal (20) and the negative DC terminal (21);

wherein each phase converter (3, 3a-c) comprises a first path (7) and a second path (8) connected in parallel between the DC connections (9a-b), the first path (7) comprising a first arm (5a) comprising a plurality of converter cells (2) and a second arm (5b) comprising a plurality of converter cells (2), serially connected between the DC connections (9a-b), the second path (8) comprising a third arm (5c) and a fourth arm (sd) serially connected between the DC connections (9a-b), whereby a first AC connection (6a) of each phase converter (3, 3a-c) is provided between the first arm (5a) and the second arm (5b) and a second AC connection (6b) of each phase converter (3, 3a-c) is provided between the third arm (5c) and the fourth arm (sd); and

each converter cell comprises a switching element and an energy storage element.

2. The AC/DC converter (1) according to claim 1, wherein each converter cell comprises the energy storage element provided in parallel to a serial connection of at least two switching elements.

3. The AC/DC converter (1) according to claim 1 or 2, wherein the third arm (5c) comprises a plurality of converter cells (2) and the fourth arm (sd) comprises a plurality of converter cells (2).

4. The AC/DC converter (1) according to claim 1 or 2, wherein the third arm (5c) consists of a plurality of serially connected switches (12) and the fourth arm (sd) consists of a plurality of serially connected switches (12).

5. The AC/DC converter (1) according to claim 1 or 2, wherein the third arm (5c) consists of a capacitor (13) device and the fourth arm (sd) consists of a capacitor device (13).

6. The AC/DC converter (1) according to claim 5, wherein each capacitor device comprises one or more capacitors connected serially and/or in parallel.

7. The AC/DC converter (1) according to any one of the preceding claims, wherein, for each phase converter (3, 3a-c), the two AC connections (6a-b) are configured to be connected to either side of a respective transformer winding (3ia-c) of a multi-phase transformer (30).

8. The AC/DC converter (1) according to claim 3, wherein each phase converter further comprises a third path (17) provided in parallel with the first path (7) and the second path (8), the third path (17) comprising a fifth arm (se) comprising a plurality of converter cells (2) and a sixth arm (51) comprising a plurality of converter cells (2), serially connected between the DC connections (9a-b), such that a third AC connection (6c) is provided between the fifth arm (se) and the sixth arm (51), whereby the three AC connections (6a-c) are configured to be connected to one side of a three phase transformer (32a-c).

9. The AC/DC converter (1) according to any one of the preceding claims, wherein at least a third of the converter cells are four quadrant converter cells and the remaining of the converter cells are two quadrant converter cells.

10. The AC/DC converter (1) according to claim 9, comprising at least one converter cell being a four quadrant cell at least one converter cell being a two quadrant converter cell.

11. The AC/DC converter (l) according to claim 9 or 10, wherein the four quadrant converter cells are configured to be controlled to provide a constant voltage on the AC connections when the voltage on the DC connection varies.

12. The AC/DC converter (1) according to any one of the preceding claims, wherein each phase converter further comprises a capacitor (14) connected between its DC connections (9a-b).

13. The AC/DC converter (1) according to any one of the preceding claims, comprising three phase converters (3, 3a-c).

14. A converter device (50) comprising the AC/DC converter (1) according to any one of the preceding claims and a multiphase transformer (30) connected to the AC/DC converter (1).

15. The converter device (50) according to claim 14, wherein primary windings (3ia-c) of the multiphase transformer (30) are connected to the AC/DC converter (1), and wherein there are two secondary windings (32a- 32a', 32b-32b', 32C-32C') for each primary winding.