TECHNICAL FIELD

The invention relates to a power converter for converting power to or from a high voltage AC (Alternating Current) connection, a first high voltage DC (Direct Current) connection, and a second high voltage DC connection.

BACKGROUND

High voltage power conversion between is known in the art for a variety of different applications. One such application is for links related to HVDC (high voltage DC). However, there is a problem with connecting various grids of various structures. Most converters that are known are concerned with power conversion between power networks of the same type, or at most, conversion between DC and AC.

SUMMARY

It is an object to provide a power converter for converting power between a voltage stiff high voltage DC connection, a current stiff high voltage DC connection and a high voltage AC connection.

According to a first aspect, it is presented a power converter for transferring power between a first high voltage DC connection being a voltage stiff DC connection, a second high voltage DC connection being a current stiff DC connection and a high voltage AC connection. The power converter comprises a power converter assembly comprising: a first voltage source converter, a first current source converter, and a second voltage source converter, connected serially in the mentioned order between a positive terminal and a negative terminal of the first high voltage DC connection; and a plurality of transformers connected on one side to the high voltage AC connection and on a second side to respective AC sides of all but one of the first voltage source converter, the first current source converter and the second voltage source converter. The positive terminal of the second high voltage DC connection is connected on to a point between the first current source converter and the first voltage source converter, and the negative terminal of the second high voltage DC connection is connected to a point between the first current source converter and the second voltage source converter. Using this arrangement, it is possible to transfer power in arbitrary direction between a current stiff DC network, a voltage stiff DC network and an AC grid. This is particularly useful as a connector between traditional HVDC, which is typically current stiff, and later generation HVDC, which is typically voltage stiff. On top of that, the AC connection allows for additional flexibility.

The high voltage AC connection may be arranged to be used for active power transfer. In this way, the AC connection can be used either as a power source or a load.

Each one of the voltage source converters may comprise a plurality of converter cells and each one of the converter cells may comprise at least one switching element and an energy storage element.

The power converter may further comprise a second current source converter connected betw ^r een the first current source converter and the second voltage source converter and the negative terminal of the second high voltage DC connection is connected to a point between the second current source converter and the second voltage source converter. This provides a

symmetrical to symmetrical DC/DC conversion.

A point between the first current source converter and the second current source converter may be grounded. In this way, the second high voltage DC connection can be a bipole connection.

Any current source converters may comprise at least one controllable switch.

Each controllable switch of a current source converter may be a thyristor. This structure provides low losses and relatively high ratings. Each controllable switch of a current source converter may be a transistor. This structure provides excellent ability to control the current source converter, even when biased.

The plurality of transformers may comprise transformers connected on one side to the high voltage AC connection (AC) and on a second side to respective AC sides of all voltage source converters (3a-b), and current source converters. In other words, transformers are provided connected to each converter, providing a uniform structure. Moreover, this decouples any DC bias from the converters from the AC connection, eliminating the need for further DC blocking devices.

The power converter may further comprise at least one filter arranged to reduce any alternating current through the negative terminal and the positive terminal of the second high voltage DC connection. This reduces any AC component (ripple) on the second high voltage DC connection. Each one of the at least one filter may comprise a first inductor and a second inductor.

The power converter may comprise a plurality of the power converter assemblies. For instance, the power converter may comprise three power converter assemblies. Each power converter assembly may provide its own high voltage AC connection, such that each power converter assembly corresponds to one phase of a combined multiphase AC connection. This allows for an efficient structure to support multiphase (e.g. three phase) AC connection.

A capacitor may be connected between the terminals of the first high voltage DC connection. This reduces any AC component (ripple) on the first high voltage DC connection.

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 DRA WINGS

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

Fig l is a schematic diagram of a power converter for converting between DC, DC and AC; Fig 2 is a schematic diagram of a three phase power converter for converting between DC, DC and AC;

Figs 3A-B are schematic diagrams of embodiments of a power converter assembly of Figs 1-2;

Fig 4 is a schematic diagram of an embodiment of a power converter assembly of Figs 1-2;

Figs 5A-B are schematic graphs illustrating the power flow in the power converter through a phase shift device of Figs 3A-B and 4;

Fig 6 is a schematic diagram illustrating possible converter cell arrangements of voltage source converters of Figs 3A-B and 4; Figs 7A-C are schematic diagrams illustrating embodiments of converter cells of the voltage source converters of Fig 6; and

Figs 8A-B are schematic diagrams illustrating embodiments of the current source converters of Figs 3A-B and 4.

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 l is a schematic diagram of a power converter l for converting between DC, DC and AC. The power converter l converts power in either direction between the first high voltage DC connection DCi and the second high voltage DC connection, DC ₂ . The first high voltage DC connection DG comprises a positive terminal DG+ and a negative terminal DG-. Analogously, the second high voltage connection comprises a positive terminal DC ₂ + a negative terminal DC ₂ -. Furthermore, there is a high voltage AC connection AC which can either consumes or provides (active) power. Hence, each one of the connections DG, DC ₂ , and AC can either consume or provide power, as long as there is at least one of these that consumes power and at least one of these that provides power. The power converter 1 comprises a power converter assembly 9 which performs the actual power conversion.

Fig 2 is a schematic diagram of a three phase power converter 1 for converting between DC, DC and AC. The three phase power converter 1 here comprises three power converter assemblies 9a-c. In this way, the AC connection here comprises three connectors AG, AC ₂ and AC ₃ to be able to provide a three phase connection, e.g. to an AC grid, an AC power source or an AC power load. It is to be noted that a multiphase power converter can be provided with fewer or more phases than what is shown in Fig 2, arranged in an analogous way that is shown in Fig 2.

Figs 3A-B are schematic diagrams of embodiments of a power converter assembly of Figs 1-2. Firstly, the structure of Fig 3A will be described.

The power converter assembly 9 comprises a first voltage source converter 3a, a first current source converter 4a, and a second voltage source converter 3b connected serially in the mentioned order between the positive terminal DCi ⁺ and the negative terminal DC of the first high voltage DC connection DCi. Hence, the first voltage source converter 3a is connected between the positive terminal DCi ⁺ and one DC side of the first current source converter 4a, and second voltage source converter 3b is connected between the negative terminal DC and the other DC side of the first current source converter 4a.

Each one of these two voltage source converters 3a-b have two DC sides, situated on the upper and lower parts in Fig. 3A, and one AC side, situated on the left side in Fig. 3A. The respective AC sides 5a-b of the first voltage source converter 3a and the second voltage source converter 3b are connected to a common AC bus 7.

A first transformer 20a is provided between the AC side of the first voltage source converter 3a and the AC bus 7. Similarly, a second transformer 20b is provided between the AC side of the first current source converter 4a and the AC bus.

The positive terminal DC ₂ ⁺ of the second high voltage DC connection DC ₂ is connected on one side of the second voltage source converter 3b, between the first voltage source converters 3a and the first current source converter 4a. The negative terminal DC ₂ ^~ of the second high voltage DC connection DC ₂ is connected to the other side of the first current source converter 4a, at a point between the first current source converter 4a and the second voltage source converter 3b. The AC connection (AC) is connected to the AC bus 7 for active power transfer with an AC grid.

Each one of the transformers 2oa-b will result in a phase shift, whereby there is a transfer of active AC power through the transformers 20a-b. Moreover, since the transformers 20a-b also block DC, the transformers 20a-b allows for different DC biases on the AC sides of the two voltage source converters

3a-b.

The DC bias on the bus 7 is thus the DC bias of the AC output of the second voltage source converter 3b. Optionally, a DC blocking device such as a capacitor can be provided serially with the AC connection in order to eliminate the DC bias of the second voltage source converter 3b on the AC connection.

Using the structure of the power converter assembly 9 of Fig 3A, a

DC/DC/AC power conversion is provided to connect DC grids of different types. The voltage source converters 3a-b and the current source converter 4a are controlled in concert to maintain an essentially constant voltage on the first high voltage DC voltage connection DCi and an essentially constant current on the second high voltage DC connection DC ₂ . With this control strategy and since there are voltage source converters 3a-b connected to the first high voltage DC connection DG, the first high voltage DC connection DCi is a voltage stiff DC connection. Moreover, with the control strategy and since the second high voltage DC connection DC ₂ is connected on either side of the first current source converter 4a, the second high voltage connection DC ₂ is a current stiff DC connection. Combined with the AC connection, the power converter assembly 9 provides an efficient power transfer device between voltage stiff DC, current stiff DC and AC. Additionally, through appropriate control of the power converter assembly 9, a desired power factor can be provided on the AC connection. In Fig 3B, the power converter assembly 9 is equivalent to the power converter assembly of Fig 3A. One difference is that there is a third

transformer 20c is connected between the AC side of the second voltage source converter 3b and the DC bus 7. Using this structure, the three transformers 20a-c block DC from the converters 3a-b, 4a, whereby the converters 3a-b, 4a do not contribute to any DC bias on the DC bus.

Fig 4 is a schematic diagram of an embodiment of a power converter assembly of Figs 1-2. The power converter 9 is similar to the power converter assembly 9 of Fig 3B. Here however, a second current source converter 4b is provided between the first current source converter 4a and the second voltage source converter 3a. Hence, the negative terminal DC ₂ ^" of the second high voltage DC connection is connected to a point between the second current source converter 4b and the second voltage source converter 3b. By connecting a point between the first and second current source converters 4a- b to ground, a bipole DC connection can be provided on the second high voltage DC connection DC ₂ . Optionally, a capacitor 2 is provided between the two terminals DCi+, D&- of the first high voltage DC connection DCi. The capacitors facilitate circulation of any AC current without affecting the high voltage DC connection DCi and reduces AC ripple on the high voltage DC connections. If there are several power converter assemblies provided in a power converter, e.g. as shown in Fig 2, the need for the capacitor 2 is reduced, as the AC current can circulate between the power converter assemblies.

Optionally, a filter I2a-b is provided to reduce any alternating current through the terminals DC ₂ ^" , DC ₂ ⁺ of the second high voltage DC connection DC ₂ . Here, the filter is depicted in two parts I2a-b, respectively comprising an inductor I3a-b, provided serially to the terminals DC ₂ ^" , DC ₂ ⁺ of the second high voltage DC connection DC ₂ . Alternatively or additionally, a capacitor (not shown) can be provided between the terminals DC ₂ ^" , DC ₂ ⁺ of the second high voltage DC connection DC ₂ , analogously to what is shown for the first high voltage DC connection. As for the first high voltage DC connection DCi, if there are several power converter assemblies provided in a power converter, e.g. as shown in Fig 2, the need for the filter is reduced, as the AC current can circulate between the power converter assemblies.

Figs 5A-B are schematic graphs illustrating the power flow through any of the transformers 20a-c of Figs 3A-B and 4. The same principle applies for any equivalent phase shift device. Fig 5A is shows the voltage v _r at one side of the transformer and the voltage v ₂ at the other side of the transformer over time.

Fig 5B shows the current ii ₂ from one side of the transformer to the other side of the transformer over time. The time scales of Figs 5A and 5B are the same. It can thus be seen at time ti, i is equal to v ₂ , whereby the derivative of current ii ₂ equal zero and going negative corresponding to the current being at its maximum point. At a time t2, there is a maximum difference between the voltages Vi and v ₂ , whereby the derivative of current ii ₂ is maximum which corresponds to zero crossing of the current. At a time t3, v ₂ is again equal to Vi, whereby the derivative of current i ₁₂ is zero and going positive

corresponding to the current being at a minimum (negative) point. The phase difference between Vx and v ₂ is thus handled by the transformer

Fig 6 is a schematic diagram illustrating possible converter cell arrangements of voltage source converters of Figs 3A-B and 4. Fig 6 illustrates the structure of any one of the voltage source converters 3a-b, here represented by a single voltage source converter 3. The voltage source converter 3 comprises two converter arms 33, each comprising a plurality of converter cells 32a-b, wherein each converter cell 32a-b is controlled by the controller 50. The converter cells 32a-b can be connected in series to increase voltage rating and/or in parallel (not shown) to increase current rating. The serially connected converter cells 32a-b can optionally be individually controlled by the controller 50 to achieve a finer granularity in the

conversion, e.g. to achieve a more sinusoidal (or square, saw tooth shaped, etc.) power conversion. While the voltage source converter 3 is here illustrated to have two converter cells 32a-b, any number of converter cells is possible, including one, three or more. In one embodiment, the number of converter cells in each voltage source converter 3 is in the range from 30 to 1000 converter cells.

One or more smoothing inductors 31 are provided in the voltage source converter on to provide a smoother current on the AC connection AC.

Additionally, the voltage drop across the inductor(s) is helpful in providing the desired number of levels in the AC voltage. Optionally, the two inductors 31 can be combined in one inductor (not shown) with a centre tap for the AC connection.

Figs 7A-C are schematic diagrams illustrating embodiments of converter cells 32a-b of the voltage source converters of Fig 6. Any one of the converter cells 32a-b is here represented as a single converter cell 32. A converter cell 32 is a combination of semiconductor switches, such as transistors, and energy storing elements, such as capacitors, supercapacitors, inductors, batteries, flywheels etc. Optionally, a converter cell can be a multilevel converter structure such as a flying capacitor or MPC (Multi-Point-Clamped) or ANPC (Active Neutral-Point-Clamped) multilevel structure.

Fig 7A illustrates a converter cell comprising an active component in the form of a switch 40 and an energy storage component 41 in the form of a capacitor. The switch 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 32 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 32 implementing a half bridge structure. The converter cell 32 here comprises a leg of two serially connected active components in the form of switches 4oa-b, e.g. IGBTs, IGCTs, GTOs, etc. A leg of two serially connected diodes 42a-b is connected with the leg of serially connected switches 40a-b as shown in the figure. An energy storage component 41 is also provided in parallel with the leg of transistors 4oa-b and with the leg of diodes 32a-b. The output voltage synthesized by the converter cell can thus either be zero or the voltage of the energy storage component 41.

Fig 7C illustrates a converter cell 32 implementing a full bridge structure. The converter cell 32 here comprises four switches 40a-d, e.g. IGBTs, IGCTs,

GTOs, etc. An energy storage component 41 is also provided in parallel across a first leg of two transistors 40a-b and a second leg of two transistors 40c-d. Compared to the half bridge of Fig 7B, the full bridge structure allows the synthesis of an output voltage capable of assuming both signs, whereby the voltage of the converter cell can either be zero, the voltage of the energy storage component 41, or a reversed voltage of the energy storage component

41.

Figs 8A-B are schematic diagrams illustrating embodiments of the current source converters 4a-b of Figs 3A-B and 4. Either one of the current source converters 4a-b is here represented as a single current source converter 4. In Fig 8A, the current source converter 4 comprises two serially connected transistors 2oa-b. The transistors 20a-b are of a type which is any suitable high power transistor, e.g. an IGBT (Insulated Gate Bipolar Transistor) or power FET (Field Effect Transistor). The operation of the transistors 20a-b is controlled by a controller 50. From a midpoint between the transistors 2oa-b, an AC port AC is provided, via an inductor 24. Optionally, a pole reverser 23 is provided. This is particularly useful when bidirectional power transfer is desired.

In Fig 8B, the current converter 4 is instead implemented using two thyristors 2ia-b, which is also controlled by the controller 50. Otherwise the structure is similar to the current converter of Fig 8A.

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.