TECHNICAL FIELD

The present invention generally relates to the field of power transmission systems, e.g. High Voltage Direct Current (HVDC) power transmission systems. Specifically, the present invention relates to a converter arrangement configured to couple an alternating current (AC) power system with a direct current (DC) power system.

BACKGROUND

HVDC power transmission has become increasingly important due to increasing need for power supply or delivery and interconnected power transmission and distribution systems. HVDC power transmission systems are advantageous for long distance bulk power transmission. The power transferred may for example be between 1 GW and 10 GW, and power may be transmitted over distances of a few hundred kilometers or even several thousands of kilometers. HVDC technology may be classified as Current Source Converter (CSC) based HVDC or Voltage Source Converter (VSC) based HVDC. While CSC based HVDC converters employ thyristors as switches or switching elements (and/or other switches or switching elements that are not self-commutated), VSC based HVDC converters employ IGBTs as switches or switching elements (and/or other switches or switching elements that are self-commutated).

Interface arrangements or converter modules are known to be connected between an AC power system and a DC power system. Such an arrangement or module typically includes a converter, such as a voltage source converter or a current source converter, for conversion of AC power to DC power, or vice versa. The interface arrangement or converter module has a DC side for coupling to the DC power system and an AC side for coupling to the AC power system. The arrangement or module often includes a transformer having a primary side connected to the AC system and a secondary side for coupling to the converter.

For example in a HVDC power system, there is generally included an interface arrangement including or constituting an HVDC converter station, which is a type of station configured to convert high voltage DC to AC, or vice versa. An HVDC converter station may comprise a plurality of elements such as the converter itself (or a plurality of converters connected in series or in parallel), one or more transformers, capacitors, filters, and/or other auxiliary elements. Converters may comprise a plurality of solid-state based devices such as semiconductor devices and may as indicated in the foregoing be categorized as line- commutated converters or voltage source converters, e.g. depending on the type of switches (or switching devices) which are employed in the converter. A plurality of solid-state semiconductor devices such as IGBTs may be connected together, for instance in series, to form a building block, or cell, of an HVDC converter.

SUMMARY

Different HVDC converter topologies may be used, examples of which are parallel MMC and series MMC. In HVDC converters with parallel MMC topology, the converter phase legs are electrical connected in parallel between the DC poles (with reference to a bipole arrangement of the HVDC converter), and include cascaded converter cells, which for example may be half-bridge converter cells (two-level) or full-bridge converter cells (three-level). Each phase leg usually comprises two phase arms, which may be referred to as a positive converter arm and a negative converter arm, or an upper converter arm and a lower converter arm. Each converter arm may be constructed so as to be able to withstand DC pole to pole voltage. In HVDC converters with series MMC topology, the converter phase legs are electrically connected in series across the DC poles (again with reference to a bipole arrangement of the HVDC converter). As for parallel MMC topology converters, each phase leg usually comprises two phase arms, which may be referred to as a positive converter arm and a negative converter arm, or an upper converter arm and a negative converter arm.

The total number of converter cells which is required for series MMC topology converters may be half or about half of the total number of converter cells required for parallel MMC topology converters. Using series MMC topology converters may hence entail a lower cost compared to using parallel MMC topology converters, based both on a reduction of the total number of required components and on a smaller volume required for the converter station. Even though by using series MMC topology converters a significant reduction in the converter volume may be achieved compared to using parallel MMC topology converters, it would be desirable with converter stations which require even less space.

CSC based HVDC converters are widely used in HVDC applications. CSC based HVDC converters may be electrically connected in series. By electrically connecting converters in series, operational reliability and AC power system stability may be increased, and the need for maintenance may be reduced. Recently voltage source converters have attracted more attention for HVDC applications. In HVDC power transmission where the transferred power is relatively high, e.g., 3 GW or more, use of VSC based HVDC converters may be preferred over CSC based HVDC converters. An example of HVDC power transmission where the transferred power is relatively high is power transmission from relatively weak grids, or power transmission to isolated grids. In case the transferred power in HVDC power transmission is required or desired to be relatively high, for example about 3 GW or more, it may be advantageous - or even possibly required - to connect converters in series. However, in VSC based HVDC converters with parallel MMC topology, one or more of the converter arms (usually the uppermost converter arm) may need to be constructed so as to be able to withstand the full DC pole to pole voltage, and possibly so as to withstand high DC voltages that may possibly occur for example upon converter bus faults. Thereby, components of one or more of the converter arms may need to be overrated. For example, electrical energy storage elements such as cell capacitors in one or more converter arms may need to be overrated so as to have the capability to handle such high DC voltages that may possibly occur. However, such overrating may increase both the total volume of the HVDC converter station as well as its cost.

In view of the above, a concern of the present invention is to provide a converter arrangement configured to couple an alternating current (AC) power system with a direct current (DC) power system which may allow for facilitate a relatively high power transfer, such as 3 GW or even higher.

A further concern of the present invention is to provide a converter arrangement configured to couple an alternating current (AC) power system with a direct current (DC) power system which may allow for facilitate a reduction in converter volume.

A further concern of the present invention is to provide a converter arrangement configured to couple an AC power system with a DC power system which may reduce or even eliminate the need for overrating components of one or more of any converter arms.

To address at least one of these concerns and other concerns, a converter arrangement in accordance with the independent claim is provided. Preferred embodiments are defined by the dependent claims.

According to a first aspect, there is provided a converter arrangement which is configured to couple an AC power system with a DC power system. The converter arrangement comprises a plurality of converter modules electrically connected in series at a DC pole, for example between a first DC pole and a second DC pole, or between a DC pole and ground. Each of the converter modules comprises a plurality of phase modules for conversion of DC power to AC power, or vice versa. The phase modules may for example be electrically connected in series. Each phase module is configured to provide at least a portion of an AC waveform. Each phase module comprises at least one multi-level converter cell. Each multi-level converter cell is configured to provide a voltage contribution to the AC waveform based on voltage of the AC power system. Each phase module comprises at least one converter valve electrically connected to the at least one multi-level converter cell. At least one converter module is configured such that the at least one converter valve thereof is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state, so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the converter module.

By way of the phase modules in each converter module, each of which phase modules comprises at least one multi-level converter cell, and by way of the converter arrangement comprising a plurality of converter modules electrically connected in series, each of the converter modules may operate according to MMC principles, with several MMC- based converters connected in series. By way of the several MMC-based converters which are connected in series, the converter arrangement may be capable of transferring a relatively high power between the AC power system and the DC power system, for example about 3 GW or even more.

Each multi-level converter cell is configured to provide a voltage contribution to an AC voltage waveform based on, e.g., voltage of the AC power system and/or the DC power system. The multi-level converter cells of the respective converter modules can hence be used in order to synthesize a desired AC voltage waveform to satisfy the requirements of at least one of the AC power system and the DC power system. Each of the converter modules can hence be operated as a Voltage Source Converter, wherein DC side voltage establishes the AC side voltage.

At least one converter valve of at least one of the converter modules, which at least one converter valve is configured so that it is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the converter module, may be referred to as a director valve, or AC waveform shaper, since the at least one converter valve can selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell.

As described in the foregoing, in VSC based HVDC converters with series

MMC topology, one or more of the converter arms may need to be constructed so as to be able to withstand the full DC pole to pole voltage, and possibly so as to withstand high DC voltages that may possibly occur upon converter bus faults, whereby components of one or more of the converter arms - for example electrical energy storage elements such as cell capacitors in one or more converter arms - may need to be overrated. By means of at least one of the converter modules comprising a director valve, or AC waveform shaper, as mentioned in the foregoing, the need for such overrating may be reduced or even eliminated.

For example, during certain circumstances, generally during abnormal circumstances such as during a fault in the DC power system, the converter arrangement, and/or the AC power system, there may be relatively high currents, and hence voltages, which components of the converter arrangement may be subjected to. For example during a single phase converter bus to negative DC pole fault on the AC side or AC bus of the converter modules (e.g. in a current path between the converter modules and a transformer arranged between the converter modules and the AC power system), an upper, or positive, converter arm of (at least one of the phase modules of) a converter module (e.g. the one closest to a DC pole) may be directly exposed to a relatively high DC voltage due to the fault current, which DC voltage may charge an electrical energy storage element such as a capacitor in the converter arm. For example in a bipolar, multi-phase configuration, this may apply to the upper, or positive, converter arm of the converter module closest to the positive DC pole and also to the lower, negative converter arm of the converter module closest to the negative DC pole. One solution for handling such a situation is to overrate the electrical energy storage element so that is has a capability to handle such high DC voltages that may possibly occur. However, overrating of electrical energy storage elements such as cell capacitors in converter arms may increase both the total volume of the converter station as well as its cost.

As mentioned in the foregoing, at least one of the converter modules comprises a director valve, or AC waveform shaper. The director valve (or at least one converter valve of the converter module) may for example include at least one bidirectional switch, which for example may comprise at least two anti-parallel thyristors. Another or other types of bidirectional switches may however be used. Current can be selectively routed through the director valve for example so as to selectively bypass a converter cell or converter cell arm, as required or desired depending on the circumstances. This is in contrast to using converter valves including switches or switching devices such as an IGBT together with an anti-parallel diode, which may offer no or limited capability of selective routing of current so as to bypass another component. For example during a single phase converter bus to negative DC pole fault on the AC side or AC bus of the converter modules as mentioned in the foregoing, the at least one converter valve (or director valve) can be used to route a fault current through the at least one converter module so as to bypass for example an upper, or positive, converter arm of (at least one of the phase modules of) a converter module, thereby avoiding possibly overcharging of electrical energy storage element(s) such as a cell capacitor in the phase module, whereby the need for overrating such electrical energy storage element(s) in the phase module may be reduced or even avoided.

The plurality of converter modules may for example be electrically connected in series between a DC pole and ground, or between a first DC pole and a second DC pole.

As indicated in the foregoing, the at least one converter valve of the at least one converter module may for example include at least one bidirectional switch, which for example may comprise at least two anti-parallel thyristors.

The plurality of converter modules may be electrically connected in series between a first DC pole and a second DC pole. The at least one converter module may be the one or the ones of the plurality of converter modules that is or are electrically closest to the first DC pole or the second DC pole. The plurality of converter modules may be electrically connected in series between a DC pole and ground. The at least one converter module may be the converter module that is electrically closest to the DC pole.

In the context of the present application, by the one or the ones of the plurality of converter modules that is or are electrically closest to a DC pole it is meant the converter module(s) for which the reactance between the converter module(s) and the DC pole is the or one of the lowest as compared to the reactance between the other converter module(s) and the DC pole.

The converter arrangement may comprise a control unit. The control unit may for example be comprised in the at least one converter module. The control unit may be configured to control operation of one or more other components of the converter

arrangement. For example, the control unit may be configured to control operation of the at least one converter valve of the at least one converter module at least with respect to switching thereof.

In case of occurrence of a fault in the power system, such as for example a AC converter bus fault, the at least one converter valve of the at least one converter module may be used to control the fault current path within the converter module, e.g., so as to selectively route the fault current within the converter arrangement, by means of controlled switching of the at least one converter valve of the at least one converter module so as to selectively switch the at least one converter valve between conducting states with a selected current conduction direction and a non-conducting state.

For example, the control unit may be configured to, in response to receiving an indication indicating presence of a fault current in the converter arrangement, control switching of the at least one converter valve of the at least one converter module so as to route the fault current through the at least one converter valve of the at least one converter module and bypass at least a portion of the at least one multi-level converter cell of the at least one converter module. Preferably, any electrical energy storage element(s) such as a capacitor in the at least one multi-level converter cell of the at least one converter module can be bypassed in this way, so as to avoid overcharging of the electrical energy storage element(s) due to the fault current.

The indication indicating presence of a fault current in the converter arrangement may for example be transmitted to the control unit from some protection system or module for protecting, monitoring and controlling the operation and/or functionality of components included in the power system. The indication may be transmitted using wired and/or wireless communication means or techniques for example such as known in the art. The fault may for example be a fault at one of the first DC pole and the second DC pole, or a single phase converter bus to ground, converter bus to negative DC pole fault on the AC side or AC bus of the converter modules. The plurality of converter modules may be electrically connected in series between a first DC pole and a second DC pole. The control unit may be configured to, in response to receiving an indication indicating presence of a fault current in the converter arrangement caused by a fault at one of the first DC pole and the second DC pole, control switching of the at least one converter valve of the at least one converter module so as to route the fault current from the one of the first DC pole and the second DC pole at which there is a fault through the at least one converter valve of the at least one converter module to the other one of the first DC pole and the second DC pole, wherein the at least a portion of the at least one multi-level converter cell of the at least one converter module is bypassed.

For example during a single phase converter bus to negative DC pole fault on the AC side or AC bus of the converter modules, the at least one converter valve of the at least one converter module may be used to route a fault current through the at least one converter valve of the at least one converter module so as to bypass for example an upper, or positive, converter arm of the at least one converter module, thereby avoiding possibly overcharging of electrical energy storage element(s) such as a capacitor in the at least one converter module. The fault current can then be routed for example via a diode of the lower, or negative, converter arm of the at least one converter module. The diode surge current rating is preferably based on or defined by an estimated maximum fault current. The bypassing of the converter arm may be carried out until an AC circuit breaker arranged in a current path between the AC side, or AC bus, of the converter modules and the AC power system has been tripped, or opened.

As mentioned in the foregoing, the at least one converter valve of the at least one converter module is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state, so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the at least one converter module. Switching of the at least one converter valve of the at least one converter module may for example be based on a fundamental frequency of the AC power system. Switching of the at least one converter valve of the at least one converter module may for example be carried out by means of, or based on, generating control signals in a manner as such known in the art (e.g. by a control unit) and supplying the control signals to the (e.g., thyristors of the) at least one converter valve of the at least one converter module.

As indicated in the foregoing, the at least one converter valve of the at least one converter module may for example include at least one bidirectional switch, which for example may comprise at least two anti-parallel thyristors. In such a case, the at least one converter valve of the at least one converter module may for example be switched such that current is commutated from one of the thyristors in the pair of anti-parallel thyristors to the other thyristor, and the switching may preferably be controlled such that current commutates from one thyristor to the other seamlessly, and in principle under any load and power factor condition. The switching of the at least one converter valve of the at least one converter module such that current is commutated from one of the thyristors in the pair of anti-parallel thyristors to the other thyristor may be carried out at zero voltage or at a relatively low voltage, whereby switching losses can be kept relatively low. Thus, in view of the at least one converter valve comprising anti-parallel thyristors, switching of the (thyristors of the) at least one converter valve of the at least one converter module may be carried out at a relatively low frequency, current and/or voltage, which may be referred to as 'soft switching', and so there may be relatively small changes in voltage and/or current during a given period of time, whereby switching losses can be kept relatively low. Also, thyristors generally have low conduction losses as compared to for example IGBTs.

By means of the at least one converter valve of the at least one converter module including at least two anti-parallel thyristors, it may be facilitated to selectively route current through the at least one converter valve for example so as to selectively bypass a converter cell or converter cell arm, as required or desired depending on the circumstances. This is in contrast to using converter valves including switches or switching devices such as an IGBT together with an anti-parallel diode, which may offer no or limited capability of selective routing of current so as to bypass another component.

The converter arrangement may possibly comprise several control units.

Each of the converter modules may for example be included in or constitute a converter station, e.g. a HVDC converter station.

The DC power system may for example comprise at least one DC cable or a DC overhead line (OHL).

By way of the phase modules - each of which comprises at least one multilevel converter cell - being electrically connected in series, each of the converter modules employs a series MMC topology. Compared to employing parallel MMC topology, a reduction in the converter volume may be achieved. In turn, this may allow for a relatively low cost for the converter module, and further for a reduced cost of any installation in which the converter module is employed, e.g. as an HVDC converter in offshore applications.

Further, by way of the phase modules comprising multi-level converter cell, the converter module can operate according to MMC principles, whereby the total converter volume may be kept relatively low, since MMCs in general require no AC filters or DC passive filters.

The at least one converter valve of the at least one converter module may for example include at least two anti-parallel thyristors, and may according to one or more embodiments of the present invention only include anti-parallel thyristor pairs, or only include switching elements that are not self-commutated. Therefore, the at least one converter module may be configured such that the at least one converter valve of the at least one converter module is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state, so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the at least one converter module. To that end, the at least one converter module (possibly each phase module thereof) may for example comprise at least one commutation cell (which for example may comprise a full-bridge cell), which is electrically connected to the DC power system and electrically connected to the at least one converter valve of the at least one converter module. The commutation cell may be switchable so as to selectively cause the at least one converter valve of the at least one converter module, e.g. the at least one converter valve of a phase module in which the commutation cell is included, or with which the commutation cell is associated, to enter the non-conducting state. That is, the commutation cell may provide voltage for forced commutation of the (e.g., thyristors of the) at least one converter valve of the at least one converter module.

The AC power system may comprise a plurality of phases. For each of the converter arrangement's converter modules, each phase module of the converter module may correspond to one of the phases. The correspondence between the phase modules and the phases may be one-to-one, and so there may be a separate, or particular phase module corresponding to each phase. The converter arrangement may hence be a multi-phase arrangement.

The plurality of converter modules, which may be electrically connected to the DC power system, may for example be electrically connected in series between a first DC pole and a second DC pole, or between a DC pole and ground. The converter arrangement may for example be configured according to a monopole configuration, or a bipole configuration. The converter arrangement is however not limited thereto, but may for example in alternative be configured according to an asymmetrical monopole configuration.

In the context of the present application, by a multi-level converter cell it is meant a converter cell that is configured so as to be capable of providing a multiple of (two or more) voltage levels, which may be used in forming an AC voltage (waveform).

A multi-level converter cell may for example comprise a half-bridge, or two- level, cell or a full-bridge, or three-level, cell.

A multi-level converter cell may for example comprise at least one capacitor, and/or another type of electrical energy storage element, electrically connected, e.g. in parallel, with a series connection of switching elements, e.g. including Integrated Gate- Commutated Transistor (IGBT)-diode pairs, each IGBT-diode pair comprising one or more IGBTs and a diode arranged in anti-parallel fashion with respect to the IGBT(s).

In the context of the present application, by anti-parallel (or inverse-parallel) electrical devices such as thyristors, it is meant devices which are electrically connected in parallel but with their polarities reversed with respect to each other. Thus, in the context of the present application, by anti-parallel thyristors, it is meant thyristors which are arranged in anti-parallel fashion with respect to each other. The converter arrangement may comprise a DC side for coupling of the converter arrangement to the DC power system and an AC side for coupling of the converter arrangement to the AC power system. The AC side and/or the DC side may for example include at least one terminal.

In the context of the present application, by a non-conducting state of a converter valve it is meant a state where there is no or only very little conduction of current through the converter valve. Thus, the commutation cell may be switchable so as to

(substantially) stop the converter valve from conducting current.

As mentioned in the foregoing, the at least one converter valve of the at least one converter module may for example include at least two anti-parallel thyristors. The at least one commutation cell of the at least one converter module may for example comprise at least one electrical energy storage element, e.g. a capacitor, which can be selectively charged with DC power from the DC power system and selectively discharged. By switching of the commutation cell it may provide a selected voltage across at least one of the thyristors in the converter valve of the at least one converter module in order to switch the at least one thyristor into a non-conducting state. The other thyristor(s) in the converter valve of the at least one converter module may be in a conducting state. Thereby, the converter valve of the at least one converter module may be switched to a conducting state with a selected current conduction direction.

A multi-level converter cell may for example comprise at least one electrical energy storage element, e.g. a capacitor, which can be selectively charged with DC power from the DC power system and selectively discharged. For each of the plurality of converter modules, each multi-level converter cell of the converter module may comprise at least one electrical energy storage element which can be selectively charged with DC power from the DC power system and selectively discharged, and may be configured to provide a voltage contribution to the AC voltage waveform based on a voltage of the electrical energy storage element, e.g. the voltage across the electrical energy storage element.

For each of the plurality of converter modules, any one or each phase module may comprise a plurality of multi-level converter cells electrically connected in a multi-level converter cell arm, and/or a plurality of converter valves electrically connected in a converter valve arm. Each of the plurality of converter modules may comprise at least one transformer. For each of the plurality of converter modules, at least one transformer may be connected between the AC power system and a midpoint of the converter valve arm of the converter module and a midpoint of the multi-level converter cell arm of the converter module. The mult i- level converter cell arm and the converter valve arm of the converter module may for example be electrically connected in parallel.

According to one or more embodiments of the present invention, at least one of the multi-level converter cells in a multi-level converter cell arm may comprise a full-bridge cell. According to one example, the at least one commutation cell may comprise or be constituted by the at least one multi-level converter cell which comprises a full-bridge cell. According to another example, each of the plurality of multi-level converter cells in the multilevel converter cell arm may comprise a full-bridge cell, and the at least one commutation cell may comprise or be constituted by any one of the multi-level converter cells in the multi-level converter cell arm. According to the latter example, the commutation cell can hence comprise or be constituted by any one of the multi-level converter cells available in the multi-level converter cell arm.

For each or any one of the converter modules, at least one phase module of the converter module may comprise at least one surge protection device arranged so as to protect at least a portion of the at least one phase module, e.g. from any voltage transients which may occur. The at least one surge protection device may be arranged so as to protect at least a portion of the at least one phase module from any voltage transients which for example may occur on an electrical conductor electrically connected to the phase module. By way of the at least one surge protection device, components of the at least one phase module (such as, for example, one or more electrical energy storage elements such as capacitors) may be protected from relatively high transient currents, and hence voltages, which components of the converter module may be subjected to. As indicated in the foregoing, such relatively high transient currents, and hence voltages, may for example occur during a single phase converter bus to negative DC pole fault on the AC side or AC bus of the phase modules (e.g. in a current path between the phase modules and a transformer arranged between the phase modules and the AC power system).

The at least one surge protection device may for example comprise or be constituted by a surge arrester. The surge protection device or surge arrester may protect components of the at least one phase module from transients occurring on an electrical conductor electrically connected to the phase module. The surge protection device or surge arrester may also be connected to ground, or a ground point, and may, in case an over- voltage transient occurs, route or divert power from the over- voltage transient to ground. At nominal or 'normal' operating voltages of the converter module or phase module, the surge protection device or surge arrester may isolate the electrical conductor from the grounding point. This may for example be achieved or implemented by means of using a varistor, which may exhibit different resistances at different voltages. The surge arrester may for example comprise any type of surge arrester as known in the art. The surge arrester may for example comprise a surge arrester for high- voltage applications as made by the applicant.

In the context of the present application a surge protection device should be understood as substantially any device which is capable of protecting another electrical device from current or voltage spikes, e.g., relatively fast and short-duration electrical transients in voltage or current, or sustained overvoltage or overcurrent (e.g., overvoltage or overcurrent occurring over an extended period of time).

In the context of the present application, the term surge protection device encompasses devices or equipment the primary role of which may not be to protect another electrical device from current or voltage spikes or sustained overvoltage or overcurrent, but which may include such functionality or capability. The at least one surge protection device could for example comprise an uninterruptible power supply (UPS), which in addition to being capable of providing short-term power also may be capable of protecting another electrical device from current or voltage spikes or sustained overvoltage or overcurrent, as known in the art.

The at least one surge protection device may for example be comprised in - or at least electrically connected to - the at least one converter valve of the corresponding phase module (i.e. the phase module in which the at least one surge protection device is comprised). For example for the case where the at least one converter valve of the at least one converter module includes at least two anti-parallel thyristors, the at least one surge protection device may for example be electrically connected (e.g., in parallel) to the at least two anti-parallel thyristors of the at least one converter valve of the corresponding phase module.

According to another example, the at least one surge protection device may be electrically connected, for example in parallel, to the corresponding phase module (or converter arm).

Each or any one of the converter modules may comprise a plurality of surge protection devices. According to one or more embodiments of the present invention, each (or some) of the phase modules of the respective converter modules may for example comprise at least one surge protection device arranged so as to protect at least a portion of the respective phase module from any voltage transients which may occur, which voltage transients for example may occur on an electrical conductor electrically connected to the respective phase module.

According to a second aspect, there is provided a power system which includes an AC power system and a DC power system. The power system comprises a converter arrangement (or possibly several converter arrangements) according to the first aspect, configured to couple the AC power system with the DC power system. The power system may for example include an HVDC power system and/or a DC grid.

According to a third aspect, there is provided a HVDC converter station comprising at least one converter arrangement according to the first aspect.

According to a fourth aspect, there is provided a power transmission system comprising a DC power system that comprises two DC poles. The power transmission system may for example comprise an HVDC power transmission system. The power transmission system comprises two converter arrangements according to the first aspect, wherein the two converter arrangements are electrically interconnected by means of the DC power system. Each of the two converter arrangements is electrically connected to a respective one of two AC power systems for transmission of power between the two AC power systems. The plurality of converter modules of each of the two converter arrangements are electrically connected in series at a respective one of the two DC poles, for example between a respective DC pole and ground or between a respective DC pole and a neutral return line.

Further objects and advantages of the present invention are described in the following by means of exemplifying embodiments. It is noted that the present invention relates to all possible combinations of features recited in the claims. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the description herein. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the present invention will be described below with reference to the accompanying drawings.

Figures 1 to 3 are schematic circuit diagrams of power systems according to embodiments of the present invention.

Figure 4 is a schematic circuit diagram of a converter module according to an embodiment of the present invention.

Figure 5 is a schematic circuit diagram of a portion of a converter module in accordance with an embodiment of the present invention.

Figure 6 is a schematic circuit diagram of a mult i- level converter cell in accordance with an embodiment of the present invention.

Figure 7 is a schematic circuit diagram of a multi-level converter cell in accordance with an embodiment of the present invention.

Figure 8 is a schematic circuit diagram of a converter valve in accordance with an embodiment of the present invention.

Figure 9 is a schematic circuit diagram of a commutation cell in accordance with an embodiment of the present invention.

Figure 10 is a schematic circuit diagram of a converter valve in accordance with an embodiment of the present invention.

All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate embodiments of the present invention, wherein other parts may be omitted or merely suggested. DETAILED DESCRIPTION

The present invention will now be described hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments of the present invention set forth herein;

rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the present invention to those skilled in the art.

Figure 1 is a schematic circuit diagram of a power system 400 according to an embodiment of the present invention. In accordance with the embodiment of the present invention illustrated in Figure 1 the power system 400 comprises a HVDC power

transmission system. The HVDC power transmission system 400 comprises four converter arrangements 300, 301. In accordance with the embodiment of the present invention illustrated in Figure 1, each of the converter arrangements 300, 301 comprises a HVDC converter station. The HVDC power transmission system 400 comprises a DC power system, schematically indicated at 103, comprising two upper DC poles and two lower DC poles (not indicated by reference numerals in Figure 1; cf. Figures 2 and 3). The two upper converter arrangements 300, 301 and the two lower converter arrangements 300, 301, respectively, are electrically interconnected by means of the DC power system 103. Each of the converter arrangements 300, 301 is electrically connected to a respective one of four AC power systems, which are schematically indicated in Figure 1 at 102 and 302. In accordance with the embodiment of the present invention illustrated in Figure 1 - and as illustrated in Figure 1 - the AC power systems 102, 302 comprise three-phase AC power systems. Each of the converter arrangements 300, 301 is electrically connected to a respective one of the four AC power systems 102, 302 for transmission of power between the AC power systems 102 and the AC power systems 302. As illustrated in Figure 1, the HVDC power transmission system 400 may for example be configured in accordance with a bipole configuration with ground electrodes, or grounding points. As will be described further in the following with reference to Figures 2 and 3, any one or each of the converter arrangements 300, 301 may comprise a plurality of converter modules (not shown in Figure 1) which are electrically connected in series at a respective DC pole, for example between a respective DC pole and ground or between a respective DC pole and a neutral return line.

Figure 2 is a schematic circuit diagram of a power system 400 according to an embodiment of the present invention which comprises four converter arrangements 300, 301. The power system 400 illustrated in Figure 1 may be configured in accordance with the power system 400 illustrated in Figure 2. In particular, the converter arrangements 300, 301 illustrated in Figure 1 may be configured similarly or identically to the converter

arrangements 300, 301 illustrated in Figure 2 and as described in the following. With further reference to Figure 2, each of the converter arrangements 300 is configured to couple an AC power system, schematically indicated at 102, with a DC power system, schematically indicated at 103. Each of the converter arrangements 301 is configured to couple an AC power system, schematically indicated at 302, with the DC power system 103. Each of the converter arrangements 300 comprises a plurality of converter modules 100 which are electrically connected in series between a first DC pole or terminal Tl and ground, and between a second DC pole or terminal T2 and ground, respectively, as illustrated in Figure 2. Each of the converter arrangements 301 comprises a plurality of converter modules 100 which are electrically connected in series between a first DC pole or terminal T7 and ground, and between a second DC pole or terminal T8 and ground, respectively, as illustrated in Figure 2. The first and second DC poles Tl and T2 may or may not be comprised in the respective ones of the upper and lower converter arrangement 300. The first and second DC poles T7 and T8 may or may not be comprised in the respective ones of the upper and lower converter arrangement 301.

The DC poles Tl and T7 are electrically connected via a DC power transmission line 303, and the DC poles T2 and T8 are electrically connected via a DC power transmission line 304. According to the illustrated embodiment of the present invention, the converter arrangements 300, 301 are hence configured in accordance with a bipole configuration with ground electrodes, or grounding points.

Figure 2 illustrates a case where there are four converter modules 100 connected in series in each of the converter arrangements 300, 301. However, it is to be understood that this is according to a non-limiting example, and that each of the converter arrangements 300, 301 may comprise more than four converter modules connected in series or less than four converter modules connected in series.

Each of the converter modules 100 which are comprised in the respective ones of the converter arrangements 300, 301 comprises a plurality of phase modules (not shown in Figure 2) for conversion of DC power to AC power, or vice versa. The phase modules in the respective converter modules 100 may be electrically connected in series. Each phase module is configured to provide at least a portion of an AC waveform. Each phase module comprises at least one multi- level converter cell (not shown in Figure 2), wherein each multi- level converter cell is configured to provide a voltage contribution to the AC waveform based on voltage of the AC power system 103. Each phase module comprises at least one converter valve (not shown in Figure 2) electrically connected to the at least one multi-level converter cell in the phase module.

In one or both of the respective converter arrangements 300 and 301 at least one of the respective converter modules 100 is configured such that the at least one converter valve thereof is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the converter module 100. To that end, the at least one converter valve of the respective converter module 100 may for example comprise a bidirectional switch, such as, for example, at least two anti- parallel thyristors.

For example, in the converter arrangements 300, the uppermost and the lowermost of the converter modules 100 in the upper and lower converter arrangement 300, respectively, may be configured such that the at least one converter valve of the respective converter module 100 is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the converter module 100. For example during certain circumstances - generally during abnormal circumstances such as during a fault in the DC power system 103, the converter arrangement 300, and/or the AC power system 102 - there may be relatively high currents, and hence voltages, which components of the converter arrangement 300 or 301 may be subjected to. The fault could for example be a single phase converter bus to negative DC pole fault on the AC side or AC bus of the converter modules 100 of the converter arrangement 300, which for example could occur in a current path between the converter modules 100 and a transformer arranged between the converter modules 100 and the AC power system 102. In such a case, the uppermost converter module 100 of the upper converter arrangement 300, i.e. the converter module 100 closest to the DC pole Tl, may be directly exposed to a relatively high DC voltage. Fault current due to the high DC voltage could for example charge an electrical energy storage element such as a cell capacitor in a converter arm (or phase module) of the uppermost converter module 100. The same could also apply to the lowermost converter module 100 of the lower converter arrangement 300, i.e. the converter module 100 closest to the DC pole T2. By way of the uppermost and/or the lowermost converter modules 100 in the upper and lower converter arrangement 300, respectively, being configured such that the at least one converter valve of the respective converter module 100 is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the converter module 100, any fault current may be selectively routed through the at least one converter valve of the respective converter module 100 for example so as to selectively bypass a converter cell or converter cell arm, as required or desired depending on the circumstances. Thereby, overcharging of electrical energy storage element(s) such as a capacitor in the converter cell or converter cell arm may be reduced or even avoided, whereby the need for overrating such electrical energy storage element(s) in the converter cell or converter cell arm may be reduced or even avoided. This is in contrast to using converter valves including switches or switching devices such as an IGBT together with an anti-parallel diode, which may offer no or limited capability of selective routing of current so as to bypass another component. In accordance with the illustrated embodiment of the present invention, the converter modules 100 comprised in the converter arrangement 300 are electrically connected in series between a first DC pole Tl and a second DC pole T2. As indicated in the foregoing, the uppermost and/or the lowermost converter modules 100 in the upper and lower converter arrangement 300, respectively, may be configured such that the at least one converter valve of the respective converter module 100 is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the converter module 100. Thus, the converter modules 100 in the converter arrangements 300 which are configured such that the at least one converter valve of the respective converter module 100 is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the converter module 100 may for example be the converter modules 100 in the converter arrangements 300 which electrically closest to the first DC pole Tl and the second DC pole T2, respectively.

In alternative or in addition, and similarly to the converter arrangements 300, the uppermost and/or the lowermost of the converter modules 100 in the upper and lower converter arrangement 301, respectively, may be configured such that the at least one converter valve of the respective converter module 100 is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the converter module 100.

According to the illustrated embodiment of the present invention, each of the converter arrangements 300 and the converter arrangements 301 comprises a control unit 101. The control units 101 may be configured to control operation of one or more other

components of the respective converter arrangements 300 and 301.

With reference to the converter arrangements 300, the control unit 101 may for example be configured to control operation of the at least one converter valve of the at least one converter module 100 (i.e. the at least one converter module 100 which is configured such that the at least one converter valve of the converter module 100 is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the converter module 100) at least with respect to switching thereof. The control unit 101 may be configured to, in response to receiving an indication indicating presence of a fault current in the converter arrangement 300, control switching of the at least one converter valve of the at least one converter module 100 so as to route the fault current through the at least one converter valve of the at least one converter module 100 and bypass at least a portion of the at least one multi-level converter cell of the at least one converter module 100. According to the illustrated embodiment of the present invention, wherein the converter modules 100 of the converter arrangements 300 are electrically connected in series between a first DC pole Tl and ground, and between a second DC pole T2 and ground, respectively, the control unit 101 may be configured to, in response to receiving an indication indicating presence of a fault current in the converter arrangements 300 caused by a fault at one of the first DC pole Tl and the second DC pole T2, respectively, control switching of the at least one converter valve of the at least one converter module 100 of the respective converter arrangement 300 so as to route the fault current from the one of the first DC pole Tl and the second DC pole T2 at which there is a fault through the at least one converter valve of the at least one converter module 100 of the respective converter arrangement 300 to the other one of the first DC pole Tl and the second DC pole T2, wherein the at least a portion of the at least one multi-level converter cell of the at least one converter module 100 is bypassed. Possibly, the control unit 101 may for example be comprised in the at least one converter module 100 of the respective converter arrangement 300.

The same or similar considerations may apply to the control unit 101 comprised in the converter arrangements 301. Thus, with reference to the converter arrangements 301, the control unit 101 may for example be configured to control operation of the at least one converter valve of the at least one converter module 100 (i.e. the at least one converter module 100 which is configured such that the at least one converter valve of the converter module 100 is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the converter module 100) at least with respect to switching thereof. The control unit 101 may be configured to, in response to receiving an indication indicating presence of a fault current in the respective converter arrangement 301, control switching of the at least one converter valve of the at least one converter module 100 so as to route the fault current through the at least one converter valve of the at least one converter module 100 and bypass at least a portion of the at least one multi-level converter cell of the at least one converter module 100. According to the illustrated embodiment of the present invention, wherein the converter modules 100 of the converter arrangements 301 are electrically connected in series between a first DC pole T7 and ground, and between a second DC pole T8 and ground, respectively, the control unit 101 may be configured to, in response to receiving an indication indicating presence of a fault current in the converter arrangement 301 caused by a fault at one of the first DC pole T7 and the second DC pole T8, control switching of the at least one converter valve of the at least one converter module 100 of the respective converter arrangement 301 so as to route the fault current from the one of the first DC pole T7 and the second DC pole T8 at which there is a fault through the at least one converter valve of the at least one converter module 100 of the respective converter arrangement 301 to the other one of the first DC pole T7 and the second DC pole T8, wherein the at least a portion of the at least one multi-level converter cell of the at least one converter module 100 is bypassed. Possibly, the control unit 101 may for example be comprised in the at least one converter module 100 of the respective converter arrangement 301.

Figure 3 is a schematic circuit diagram of a power system 400 according to another embodiment of the present invention. The power system 400 illustrated in Figure 3 is similar to the power system 400 illustrated in Figure 2, and identical reference numerals in Figures 2 and 3 denote the same or similar components, having the same or similar function. The power system 400 illustrated in Figure 3 differs from the power system 400 illustrated in Figure 2 in that the converter arrangements 300, 301 in the power system 400 illustrated in Figure 3 are configured in accordance with a bipole configuration having a metallic neutral return - schematically indicated at 305 - whereas the converter arrangements 300, 301 in the power system 400 illustrated in Figure 2 are configured in accordance with a bipole configuration with ground electrodes, or grounding points.

Each of the converter modules 100 of the respective ones of the converter arrangements 300 and 301 in the power systems 400 illustrated in Figures 2 and 3 may be configured similarly or the same, or substantially similar or the same. An example

configuration of the converter modules 100 in accordance with one or more embodiments of the present invention will be described in the following with reference to Figures 4 to 10.

Figure 4 is a schematic circuit diagram of a converter module 100 according to an embodiment of the present invention. The converter module 100 is configured to couple an AC power system 102 with a DC power system 103, or vice versa. The converter module 100 comprises three phase modules 104, 105, 106 for conversion of AC power to DC power, or vice versa.

The phase modules 104, 105, 106 are electrically connected in series. For example, in accordance with the embodiment of the present invention illustrated in Figure 4, the phase modules 104, 105, 106 may be electrically connected in series between a first DC pole or terminal Tl and a second DC pole or terminal T2. Although Figure 4 illustrates only one converter module 100 between the first DC pole or terminal Tl and the second DC pole or terminal T2, it is to be understood that that there may be several converter modules, for example being electrically connected in series between the first DC pole Tl and the second DC pole T2 (or between a DC pole and ground) such as illustrated in Figure 2. Since Figure 4 is intended to illustrate the configuration of the converter modules 100, any other converter modules are omitted in Figure 4.

Each of the phase modules 104, 105, 106 may be configured to provide at least a portion of an AC waveform, e.g. an AC voltage waveform. To that end, each of the phase modules 104, 105, 106 may comprise at least one multi-level converter cell (not shown in Figure 4), wherein each multi-level converter cell is configured to provide a voltage contribution to the AC voltage waveform, e.g. based on (at least) voltage of the AC power system.

The AC power system 102 may comprise a plurality of phases. According to an example, the AC power system 102 may be a three-phase power system. In accordance with the embodiment of the present invention illustrated in Figure 4, the AC power system 102 is a three-phase power system, comprising three conductors or phases for coupling the AC power system 102 with the DC power system 103, or vice versa, and each of the phase modules 104, 105, 106 corresponds to one phase, such that there is a one-to-one correspondence between the phase modules 104, 105, 106 and the three phases. However, it is to be understood that the number of phases as well as the number of phase modules as illustrated in Figure 4 (and also in Figure 5 described in the following) are according to examples, and that in principle any number of phases and any number of phase modules are possible, e.g. one or two phases, and/or two or four phase modules.

As illustrated in Figure 4, the (three) phases, and the (three) phase modules

104, 105, 106 may be electrically connected in series on the DC side so as to share the DC link voltage.

The converter module 100 may comprise a transformer, which may comprise a primary side for coupling of the transformer to the AC power system 102 and a secondary side for coupling of the transformer to the phase modules 104, 105, 106. In accordance with the embodiment of the present invention illustrated in Figure 4, the transformer is a three- phase transformer, which can be considered as comprising three (separate) 'phase

transformers' 107, 108, 109, one for each phase. Each of the phase transformers 107, 108, 109 may comprise a primary side for coupling of phase the transformer 107, 108, 109 to the AC power system 102 and a secondary side for coupling of the phase transformer 107, 108, 109 to the respective phase modules 104, 105, 106. The primary side of the respective phase transformers 107, 108, 109 may comprise a set of primary windings, arranged to be coupled to the AC power system 102. The secondary side of the respective phase transformers 107, 108, 109 may comprise a set of secondary windings, arranged to be coupled to the respective phase modules 104, 105, 106. Each of the phase transformers 107, 108, 109 may be controlled and/or operated independently of the others.

The converter module 100 may comprise a circuit breaker arranged in a current path between the AC side, or AC bus, of the phase modules 104, 105, 106 and the AC power system 102. Hence, the circuit breaker may be an AC circuit breaker. In accordance with the embodiment of the present invention illustrated in Figure 4, the circuit breaker can be considered as comprising three (separate) 'phase circuit breakers' 110, 111, 112, one for each phase. Further in accordance with the embodiment of the present invention illustrated in Figure 4, the phase circuit breakers 110, 111, 1 12 may be arranged in a current path between the respective phase transformers 107, 108, 109 and the AC power system 102. Each of the phase circuit breakers 110, 111, 112 is configured to controllably effect discontinuation of flow of current in the current path upon opening of contacts (not shown in Figure 4) of the phase circuit breaker 110, 11 1, 112. The phase circuit breakers 110, 111, 112 may for example be arranged in a current path between the set of primary windings of the respective phase transformers 107, 108, 109 and the AC power system 102. Each of the phase circuit breakers 110, 111, 112 may possibly be controlled and/or operated independently of the others.

As illustrated in Figure 4, each of the phases may include a reactor or inductor 113, 114, 115 arranged in a current path between the respective phase transformers 107, 108, 109 and the AC power system 102. For example, according to Figure 4, the reactors or inductors 113, 114, 115 may be arranged in a current path between the respective phase circuit breakers 110, 111, 112 and the AC power system 102.

Further as illustrated in Figure 4, each of the phases may be coupled to the AC power system 102 by way of terminals T3, T4 and T5, respectively.

It is to be understood that various components which are not illustrated in Figure 4 may be included in the converter module 100. Such components, which thus are not shown in Figure 4, may for example include resistors, capacitors, filters, additional transformers and/or other auxiliary elements.

Figure 5 is a schematic circuit diagram of a portion of the converter module

100 illustrated in Figure 4, illustrating an exemplary configuration of the phase modules 104, 105, 106. As mentioned in the foregoing with respect to Figure 4, each of the phase modules 104, 105, 106 may be configured to provide at least a portion of an AC waveform, e.g. an AC voltage waveform. To that end, each of the phase modules 104, 105, 106 may comprise a plurality of multi-level converter cells, each multi-level converter cell being configured to provide a voltage contribution to the AC voltage waveform, e.g. based on (at least) voltage of the DC power system 103.

The phase module 104 may include a plurality of multi-level converter cells 141-1, 141-N and 142-1, 142-N, electrically connected, e.g. in series as illustrated in Figure 5, and arranged in a mult i- level converter cell arm 161. The mult i- level converter cells

141- 1, 141-N of the multi-level converter cell arm 161 constitute an upper mult i- level converter cell arm of the phase module 104, and the multi-level converter cells 142-1, ...,

142- N of the multi-level converter cell arm 161 constitute a lower multi-level converter cell arm of the phase module 104.

Similarly, the phase module 105 may include a plurality of multi-level converter cells 143-1, 143-N and 144-1, 144-N, electrically connected, e.g. in series as illustrated in Figure 5, and arranged in a mult i- level converter cell arm 162. The mult i- level converter cells 143-1, 143-N of the multi-level converter cell arm 162 constitute an upper multi-level converter cell arm of the phase module 105, and the multi-level converter cells 144- 1 , ... , 144-N of the multi-level converter cell arm 162 constitute a lower multi- level converter cell arm of the phase module 105.

Similarly, the phase module 106 may include a plurality of multi-level converter cells 145-1, ..., 145-N and 146-1, ..., 146-N, electrically connected, e.g. in series as illustrated in Figure 5, and arranged in a multi-level converter cell arm 163. The multi-level converter cells 145-1, ..., 145-N of the multi-level converter cell arm 163 constitute an upper multi-level converter cell arm of the phase module 106, and the multi-level converter cells 146-1, ..., 146-N of the multi-level converter cell arm 163 constitute a lower multi- level converter cell arm of the phase module 106.

In the embodiment of the present invention illustrated in Figure 5, the upper multi-level converter cell arm and the lower multi-level converter cell arm of the phase modules 104, 105, 106 each includes N multi-level converter cells, where N is an integer, such as ten, fifteen, or twenty. However, it is to be understood that each of the phase modules 104, 105, 106 may include in principle any number of multi-level converter cells. According to an example, each of the phase modules 104, 105, 106 may include a single multi-level converter cell.

Referring now to Figure 6, there is shown an example configuration of the multi-level converter cell 141-1 shown in Figure 5. It is to be understood that any one of the other multi-level converter cells 141-N, 142-1, 142-N, 143-1, 143-N, 144-1, 144- N, 145-1, ..., 145-N, 146-1, ..., 146-N shown in Figure 5 may be configured in the same manner or in a similar manner as the multi-level converter cell 141-1 illustrated in Figure 6. According to the example illustrated in Figure 6, the multi-level converter cell 141-1 comprises two switches or switching elements 191, 192 and a capacitor 193. According to the example illustrated in Figure 6, each of the switches or switching elements 191, 192 comprises a transistor together with a diode. The transistors may for example comprise insulated gate bipolar transistors (IGBTs). It is to be understood that the switching elements 191, 192 shown in Figure 6 are according to an example, and that other types of switching elements can be used. Also, the multi-level converter cell 141-1 is not limited to using a capacitor 193 as electrical energy storage element, but other types of electrical energy storage elements may be employed. With reference to Figure 5, the capacitor 193 can be selectively charged with DC power from the DC power system 103 and selectively discharged. The multi-level converter cell 141-1 can thereby be controlled so as to provide a voltage contribution to the AC voltage waveform based on a voltage of the capacitor 193 (or another electrical energy storage element).

Figure 6 illustrates the multi-level converter cell 141-1 configured as a half- bridge circuit, wherein the two switches or switching elements 191, 192 are connected in series across the electrical energy storage element 193, with a midpoint connection between the switches or switching elements 191, 192 and one of the electrical energy storage element 193 terminals as external connections. However, it is to be understood that this configuration is according to a non-limiting example and that variations are possible. For example, the multi-level converter cell 141-1 could be configured as a full-bridge circuit. Configuring the multi-level converter cell 141-1 as a full-bridge circuit may allow for or facilitate insertion of the electrical energy storage element 193 into the circuit in either polarity. Figure 7 illustrates the multi-level converter cell 141-1 configured as a full-bridge circuit. The multi-level converter cell 141-1 illustrated in Figure 7 comprises four switches or switching elements 191, 192, 196, 197, each comprising a transistor (e.g., an IGBT) together with a diode. The multi-level converter cell 141-1 illustrated in Figure 7 further comprises an electrical energy storage element in the form of a capacitor 193.

With further reference to Figure 5, any one of the mult i- level converter cells 141-1, 141-N, 142-1, 142-N, 143-1, 143-N, 144-1, 144-N, 145-1, 145-N and 146-1, ..., 146-N may for example comprise a half-bridge, or two-level, cell or a full- bridge, or three-level, cell.

With further reference to Figure 5, each of the phase modules 104, 105, 106 may comprise a plurality of converter valves. The plurality of converter valves are electrically connected to the multi-level converter cells and are controllably switchable between conducting states with a selected current conduction direction and a non-conducting state, so as to selectively control polarity of any voltage contribution provided by the respective multilevel converter cells.

The phase module 104 may include a plurality of converter valves 151-1, ... , 151-N and 152-1, 152-N electrically connected, e.g. in series as illustrated in Figure 5, and arranged in a converter valve arm 171. The converter valves 151-1, 151-N of the converter valve arm 171 constitute an upper converter valve arm of the phase module 104, and the converter valves 152-1, ..., 152-N of the converter valve arm constitute a lower converter valve arm of the phase module 104.

Similarly, the phase module 105 may include a plurality of converter valves 153-1, 153-N and 154-1, 154-N electrically connected, e.g. in series as illustrated in Figure 5, and arranged in a converter valve arm 172. The converter valves 153-1, 153-N of the converter valve arm 172 constitute an upper converter valve arm of the phase module 105, and the converter valves 154-1, ..., 154-N of the converter valve arm constitute a lower converter valve arm of the phase module 105.

Similarly, the phase module 106 may include a plurality of converter valves 155-1, ..., 155-N and 156-1, ..., 156-N electrically connected, e.g. in series as illustrated in Figure 5, and arranged in a converter valve arm 173. The converter valves 155-1, 155-N of the converter valve arm 173 constitute an upper converter valve arm of the phase module 106, and the converter valves 156-1, ..., 156-N of the converter valve arm constitute a lower converter valve arm of the phase module 106.

In the embodiment of the present invention illustrated in Figure 5, the upper converter valve arm and the lower converter valve arm of the phase modules 104, 105, 106 each includes N converter valves, where N is an integer, such as ten, fifteen, or twenty.

However, it is to be understood that each of the phase modules 104, 105, 106 may include in principle any number of converter valves. According to an example, each of the phase modules 104, 105, 106 may include a single converter valve.

Referring now to Figure 8, there is shown an example configuration of the converter valve 151-1 shown in Figure 5. It is to be understood that any one of the other converter valves 151-N, 152-1, 152-N, 153-1, 153-N, 154-1, 154-N, 155-1, 155-N, 156-1, ..., 156-N shown in Figure 5 may be configured in the same manner or in a similar manner as the converter valve 151-1 illustrated in Figure 8. According to the example illustrated in Figure 8, the converter valve 151-1 includes at least two anti-parallel thyristors 194, 195. As illustrated in Figure 8, the thyristors 194, 195 are electrically connected in parallel and have their polarities reversed with respect to each other. Thereby, the converter valve 151-1 may exhibit a capability or capacity of a controllable, bidirectional switch.

With further reference to Figures 4 and 5, the phase transformers 107, 108, 109 may be connected between the AC power system 102 and a midpoint of the corresponding converter valve arm 171, 172, 173 and a midpoint of the corresponding multi-level converter cell arm 161, 162, 163.

The midpoint of the multi-level converter cell arms 161, 162, 163 may be defined as being a point connecting the upper multi-level converter cell arm of the respective multi-level converter cell arm 161, 162, 163 on one side, and the lower multi-level converter cell arm of the respective multi-level converter cell arm 161, 162, 163 on the other side. For example when the multi-level converter cells in the multi-level converter cell arm 161, 162, 163 are electrically connected in series, the midpoint may be defined as a point where half or approximately half of the multi-level converter cells are provided on one side of the midpoint and the remaining ones of the multi-level converter cells are provided on the other side of the midpoint.

Similarly, the midpoint of the converter valve arms 171, 172, 173 may be defined as being a point connecting the upper converter valve arm of the respective converter valve arm 171, 172, 173 on one side, and the lower converter valve arm of the respective converter valve arm 171, 172, 173 on the other side. For example when the converter valves in the converter valve arm 171, 172, 173 are electrically connected in series, the midpoint may be defined as a point where half or approximately half of the converter valves are provided on one side of the midpoint and the remaining ones of the converter valves are provided on the other side of the midpoint. As illustrated in Figure 5, for any one of the phase modules 104, 105, 106, the multi-level converter cell arm 171, 172, 173 and the converter valve arm 161, 162, 163 in the respective phase module 104, 105, 106 may for example be electrically connected in parallel.

With further reference to Figure 5, any one of the phase modules 104, 105, 106 may comprise a commutation cell 181, 182, 183 electrically connected to the respective converter valves 151-1, 151-N, 152-1, 152-N, 153-1, 153-N, 154-1, 154-N, 155-1, ..., 155-N, 156-1, ..., 156-N, and switchable so as to cause the respective converter valves 151-1, 151-N, 152-1, 152-N, 153-1, 153-N, 154-1, 154-N, 155-1, 155-N, 156-1, 156-N to enter the non-conducting state.

Referring now to Figure 9, there is shown an example configuration of the commutation cell 181 shown in Figure 5. It is to be understood that any one of the other commutation cells 182, 183 shown in Figure 5 may be configured in the same manner or in a similar manner as the commutation cell 181 illustrated in Figure 9. In general, each of the commutation cells 181, 182, 183 may include at least one electrical energy storage element, such as a capacitor, which can be selectively charged with DC power from the DC power system 102 and selectively discharged, wherein by switching of the commutation cell 181, 182, 183, it may provide a selected voltage across at least one of the thyristors 194, 195 in a converter valve 151-1, 151-N, 152-1, 152-N, 153-1, 153-N, 154-1, 154-N, 155-1, ..., 155-N, 156-1, ..., 156-N, and switchable so as to cause the respective converter valves 151-1, 151-N, 152-1, 152-N, 153-1, 153-N, 154-1, 154-N, 155-1, 155-N, 156-1, ..., 156-N in order to switch the at least one thyristor 194, 195 into a nonconducting state. According to the example illustrated in Figure 9, the commutation cell 181 comprises two electrical energy storage elements 186-1, 186-2 in the form of capacitors. Each of the capacitors 186-1, 186-2 is arranged in a corresponding full-bridge cell 184, 185 having four switching elements 187-1 to 187-4 and 187-5 to 187-8, respectively. According to the example illustrated in Figure 9, the switching elements 187-1 to 187-8 comprise a transistor together with a diode. The transistors may for example comprise IGBTs. It is to be understood that the switching elements 187-1 to 187-8 shown in Figure 9 are according to an example, and that other types of switching elements can be used. Thus, any one of the commutation cells 181, 182, 183 may preferably comprise a full-bridge cell, which is in accordance with the embodiment of the present invention illustrated in Figure 9. However, this is not necessary. Any one of the commutation cells 181, 182, 183 may for example comprise a half- bridge cell. The commutation cells 181, 182, 183 are not limited to using a capacitors 186-1, 186-2 as electrical energy storage elements, but other types of electrical energy storage elements may be employed. As illustrated in Figure 5, the commutation cells 181, 182, 183 may for example be arranged at the midpoint of the multi-level converter cell arms 161, 162, 163, respectively. As indicated in the foregoing, the multi-level converter cells 141-1, ..., 141-N, 142-1, 142-N, 143-1, 143-N, 144-1, 144-N, 145-1, 145-N and 146-1, 146- N of the phase modules 104, 105, 106 are configured to provide a voltage contribution to the AC voltage waveform, e.g. based on (at least) voltage of the DC power system 103. The multi-level converter cells 141-1, 141-N, 142-1, 142-N, 143-1, 143-N, 144-1, 144-N, 145-1, ..., 145-N and 146-1, ..., 146-N can hence be used in order to synthesize a desired AC voltage waveform in order to satisfy the requirements of either the AC power system 102 or the DC power system 103. The converter module 100 can hence be operated as a Voltage Source Converter, wherein DC side voltage establishes the AC side voltage. By way of the converter cells 141-1, 141-N, 142-1, 142-N, 143-1, 143-N, 144-1,

144- N, 145-1, 145-N and 146-1, 146-N being multi-level converter cells, each multilevel converter cell 141-1, 141-N, 142-1, 142-N, 143-1, 143-N, 144-1, 144-N,

145- 1, 145-N and 146-1, 146-N is configured so as to be capable ofproviding a multiple of voltage levels, such as two or more voltage levels, which may be used in forming the AC voltage waveform.

The forming of the AC voltage waveform by operation and control of the (components of the) phase modules 104, 105, 106 may be carried out using general principles which as such are known in the art. For each phase, depending on which of the switches or switching elements 191, 192 in each multi-level converter cell is switched on (i.e. is in a conducting state), the electrical energy storage element 193 can either be bypassed or connected into the circuit. Each multi-level converter cell can thereby act as a possibly independent, separate, controllable voltage source. In accordance with the embodiment of the present invention, the multi-level converter cell 141-1 is a two-level converter, which can generate either zero voltage or the voltage across the electrical energy storage element (e.g., a capacitor) 193. With a number of multi-level converter cells electrically connected, e.g. in series such as illustrated in Figure 5, with the series-connected multi-level converter cells forming multi-level converter cell arms 161, 162, 163, a number of voltage levels can be provided which can be used to synthesize a stepped voltage waveform. Another way to describe this is that the voltage at the AC output of each phase may be controllably switched between a number of discrete voltage levels which are based on or correspond to the electrical potentials at the first DC pole or terminal Tl and at the second DC pole or terminal T2. The polarity of voltage contributions provided by the multi-level converter cells can be controlled by the converter valves (so as to produce positive or negative voltage contributions), thereby allowing for a stepped voltage waveform to be synthesized which for example can

approximate a sine wave or sinusoid.

Referring now to Figure 10, there is shown another example configuration of the converter valve 151-1 shown in Figure 5. It is to be understood that any one, or each, of the other converter valves 151-N, 152-1, .... 152-N, 153-1, 153-N, 154-1, 154-N, 155- 1, ..., 155-N, 156-1, ..., 156-N shown in Figure 5 may be configured in the same manner or in a similar manner as the converter valve 151-1 illustrated in Figure 10. The converter valve 151-1 illustrated in Figure 10 is similar to the converter valve 151-1 illustrated in Figure 8. According to the example illustrated in Figure 10, the converter valve 151-1 includes at least two anti-parallel thyristors 194, 195, which are electrically connected in parallel and have their polarities reversed with respect to each other. Thereby, the converter valve 151-1 may exhibit a capability or capacity of a controllable, bidirectional switch. In addition, the converter valve 151-1 illustrated in Figure 10 comprises a surge protection device constituted by a surge arrester 198. According to the example illustrated in Figure 10, the surge arrester 198 is electrically connected to the anti-parallel thyristors 194, 195. Hence, the surge arrester 198 and the anti-parallel thyristors 194 and 195 are mutually electrically connected in parallel with respect to each other. It is however to be understood that the particular electrical coupling of the surge arrester 198 with the anti-parallel thyristors 194 and 195 illustrated in Figure 10 is according to an example and that the surge arrester 198 could be electrically connected to the anti-parallel thyristors 194 and 195 in some other way than illustrated in Figure 10. It is further to be understood that even though Figure 10 depicts one surge protection device 198 electrically connected to the anti-parallel thyristors 194, 195, there may possibly be one or more additional surge protection devices comprised in - or at least electrically connected to - the converter valve 151-1.

In conclusion a converter arrangement is disclosed, which is configured to couple an AC power system with a DC power system. The converter arrangement comprises a plurality of converter modules electrically connected in series at a DC pole, for example between a first DC pole and a second DC pole, or between a DC pole and ground. At least one converter module is configured such that the at least one converter valve thereof is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell of the converter module.

While the present invention has been illustrated in the appended drawings and the foregoing description, such illustration is to be considered illustrative or exemplifying and not restrictive; the present invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the appended claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.