WO2015090365A1

WO2015090365A1 - Integrated series converter and circuit breaker in a power system

Info

Publication number: WO2015090365A1
Application number: PCT/EP2013/076893
Authority: WO
Inventors: Mats Hyttinen; Reza DERAKHSHANFAR
Original assignee: Abb Technology Ltd
Priority date: 2013-12-17
Filing date: 2013-12-17
Publication date: 2015-06-25
Also published as: WO2015090365A9

Abstract

Embodiments of the present invention disclose an apparatus (100) adapted to be connected to a current path in at least one transmission line (101) which interconnects at least two power systems. The apparatus (100) is adapted to control electric power trans- mission in the at least one transmission line (101) between the power systems and controllably effect discontinuation of flow of electrical current in the at least one transmission line (101). The apparatus (100) comprises electrical converter circuitry (104, 120) adapted to controllably introduce current and/or voltage into the current path so as to adjust flow of electrical current in the at least one transmission line(101). The apparatus (100) comprises circuit breaking circuitry (106, 107, 108, 109, 120) adapted to interrupt electrical current in the current path.

Description

INTEGRATED SERIES CONVERTER AND CIRCUIT BREAKER IN A POWER

SYSTEM

TECHNICAL FIELD

The present invention generally relates to power systems such as electrical power distribution or transmission systems, e.g. High Voltage Direct Current (HVDC) power transmission systems. Specifically, the present invention relates to an apparatus for use in a power system, which apparatus integrates series converter and circuit breaker functionalities and/or capabilities in one device. The apparatus may have both capability as a load flow controller or series converter with respect to power transmission between at least two interconnected DC power systems and circuit breaking capability so as to for example allow to isolate the at least two interconnected DC power systems from each other.

BACKGROUND

Power systems such as electrical power distribution or transmission systems are used to supply, transmit and use electric power. High Voltage Direct Current (HVDC) power transmission is becoming increasingly important due to increasing need for power supply or delivery and interconnected power transmission and distribution systems.

Power systems such as electrical power distribution or transmission systems generally include a protection system for protecting, monitoring and controlling the operation and/or functionality of other components included in the power system. Such protection systems may for example be able to detect short circuits, overcurrents and overvoltages in power lines, transformers and/or other parts or components of the power system. The protection systems can include protection equipment such as circuit breakers for isolating any possible faults for example occurring in power transmission and distribution lines by opening or tripping the circuit breakers. After the fault has been cleared, e.g. by performing repairs and/or maintenance on the component in which the fault has been detected, the power flow can be restored by closing the circuit breakers. In alternative or in addition, the protection systems can be arranged to, upon detection of a fault in a particular route for power flow, isolate the route in which the fault has been detected and select an alternative route for the power flow. Operation of the circuit breakers may be responsive to detection of a fault condition or fault current. Upon detection of a fault condition or fault current, a mechanism may operate the circuit breaker so as to interrupt the current flowing there through. Once a fault has been detected, contacts within the circuit breaker may separate in order to interrupt the current there through. Spring arrangements, pneumatic arrangements or some other means utilizing mechanically stored energy may be employed to separate the contacts. Mechanical current interrupters and/or solid-state interrupters based on semiconductor devices may for example be employed in circuit breakers. Once the fault condition has been mitigated or eliminated the contacts can be closed so as to resume flow of current through the circuit breaker.

An HVDC grid or a DC grid may comprise multiple alternating current (AC)/DC converter terminals interconnected by transmission lines, e.g., underground cables and/or overhead lines. Within the grid, a terminal may be connected to multiple terminals resulting in different types of topologies. Hence, within the grid there may be several interconnected DC power systems. DC circuit breakers can be used for isolating faulty

components, such as transmission lines, in HVDC and DC grids.

SUMMARY

It may be difficult or even impossible to electrically connect two direct current (DC) power systems, e.g. High Voltage Direct Current (HVDC) power transmission systems, to each other if the DC power systems have not been specifically designed or constructed so as to facilitate or enable electrically connecting them together. As an example scenario, there may be provided two different, separate DC power systems, e.g. HVDC power transmission systems, for which DC power systems it may be desired or even required to electrically connect them to each other, e.g. so as to form a part or portion of a DC or HVDC grid.

However, interconnecting the two DC power systems may be difficult or even impossible, for example in case the DC voltages of the respective DC power systems are not the same. A relatively small voltage difference may be needed between the two DC power systems, or between terminals of the respective DC power systems, in order to create a desired or required power flow between the two DC power systems on their DC sides. In case the DC power systems have not been specifically designed or constructed so as to facilitate or enable electrically connecting them together, the DC power systems will probably require different DC voltages in the connection point between the DC power systems. In case of a fault or failure occurring in one or both of the DC power systems, it may be desired or even required to relatively quickly isolate or disconnect the DC power systems from each other, as one DC system may not be designed or constructed to withstand stresses, e.g. current and/voltage stresses, from the other DC power system, or it may have a limited capacity for withstanding such stresses. In view of the above, in order to facilitate or enable an interconnection between the two DC power systems, both a functionality of a load flow controller or converter with respect to power transmission between the DC power systems and a functionality of a circuit breaker with respect to interruption of power flow between the DC power systems may be desired or even required. The same or similar considerations may apply for a scenario where it is desired or even required to electrically interconnect more than two different, separate DC power systems.

Furthermore, there is an increasing need for more compact constructions for use in power systems, which may allow for decreasing the overall size of various power system components, e.g., electrical substations such as switchyards, HVDC terminals, etc.

In view of the above, a concern of the present invention is to achieve a relatively high efficiency in power transmission between power systems, e.g. DC power systems, and/or to simplify power transmission between such power systems.

A further concern of the present invention is to simplify and/or facilitate, or even enable, interconnection between two or more power systems, e.g. DC power systems.

A further concern of the present invention is to decrease the overall size of power system components.

A further concern of the present invention is to achieve a relatively high efficiency in power transmission between power systems, e.g. DC power systems, and/or to simplify power transmission between such power systems, while at the same time achieving a relatively small overall size of power system components.

A further concern of the present invention is to facilitate, or even enable, interconnection between two or more power systems, e.g. DC power systems, while at the same time achieving a relatively small overall size of power system components.

A further concern of the present invention is to achieve a relatively high efficiency in power transmission between power systems, simplify power transmission between such power systems, and/or facilitate, or even enable, interconnection between two or more power systems, while at the same time allowing for achieving a relatively low cost of the power system.

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

According to a first aspect, there is provided an apparatus adapted to be connected to a current path in at least one transmission line arranged to carry an electrical current. The at least one transmission line interconnects at least two DC power systems. The apparatus is adapted to control electric power transmission in the at least one transmission line between the at least two DC power systems and controllably effect discontinuation of flow of electrical current in the at least one transmission line. The apparatus comprises electrical converter circuitry adapted to controllably introduce current and/or voltage into the current path so as to adjust flow of electrical current in the at least one transmission line between the at least two DC power systems. The apparatus comprises circuit breaking circuitry adapted to interrupt electrical current in the current path. The electrical converter circuitry and the circuit breaking circuitry may be arranged such that the apparatus can be selectively switched between at least two modes of operation. The at least two modes of operation may include a first mode in which current and/or voltage is controllably introduced into the current path so as to adjust flow of electrical current in the at least one transmission line between the at least two DC power systems, and a second mode in which electrical current in the current path is interrupted.

According to one example, the electrical converter circuitry and the circuit breaking circuitry may be Overlapping' with respect to component(s), unit(s), element(s), etc., included in the respective ones of the electrical converter circuitry and the circuit breaking circuitry, in the sense that at least one component in the apparatus at least in part enables or facilitates, or at least contributes to, achieving both electrical series converter functionality or capability and circuit breaking functionality or capability, in a single device. Hence, by means of an apparatus according to the first aspect, both electrical series converter functionality or capability and circuit breaking functionality or capability can be 'integrated' in one apparatus. Thereby, a relatively small and/or compact arrangement can be achieved for providing both electrical series converter functionality or capability and circuit breaking functionality or capability in a power system. For example, the size of an apparatus according to the first aspect may be smaller as compared to the total size of separate devices which provide electrical series converter functionality or capability and circuit breaking functionality or capability, respectively, in the power system.

In view of the above, by means of an apparatus according to the first aspect, it may be facilitated or even enabled to electrically connect two DC power systems, e.g. two HVDC power transmission systems, to each other even if the DC power systems have not been specifically designed or constructed so as to facilitate or enable electrically connecting them together. This can be achieved by the apparatus according to the first aspect providing both electrical series converter functionality or capability and circuit breaking functionality or capability.

Since according to the first aspect both electrical series converter functionality or capability and circuit breaking functionality or capability may be achieved by means of a single device, or at least by means of a smaller number of components as compared to utilizing separate devices which provide electrical series converter functionality or capability and circuit breaking functionality or capability, an apparatus according to the first aspect may allow for using common resources, e.g. control, monitoring and/or auxiliary systems, housing and/or infrastructure, for both functionalities. In other words, by means of an apparatus according to the first aspect, any further resources in a power system, such as buildings and/or equipment, which may be required or desired to supplement electrical series converter functionality or capability and circuit breaking functionality or capability may be relatively few. In turn, this may allow for a relatively inexpensive power system as compared to a power system using separate devices which provide electrical series converter functionality or capability and circuit breaking functionality or capability, in which any further resources such as mentioned above may require separate control and auxiliary systems, housing and/or infrastructure, etc., for the two functionalities.

The electrical converter circuitry can for example be used for load flow control, series tapping and/or DC voltage regulation. DC voltage may alternatively may be referred to as direct voltage.

The apparatus may for example be connected in series to the current path in the at least one transmission line and/or to the at least one transmission line.

The electrical converter circuitry and the circuit breaking circuitry may share at least one common component, or the electrical converter circuitry and the circuit breaking circuitry may be integrally arranged via at least one common component.

The at least one common component may for example include a common circuitry which is adapted to be connected to the current path.

The common circuitry may for example comprise load flow controller or converter circuitry, and/or at least one power semiconductor switching element. The load flow controller circuitry, or load flow converter circuitry, may be adapted to selectively and controllably block current therethrough.

The common circuitry may be adapted to, when the apparatus is operating in the first mode, controllably introduce current and/or voltage into the current path, and, when the apparatus is operating in the second mode, interrupt electrical current in the current path.

According to one example, the common circuitry may be adapted to, when the apparatus is operating in the first mode, operate as a multiphase converter, wherein the phases, or conductors (of the multiphase converter), may be connected in series to the at least one transmission line.

The circuit breaking circuitry may include a first circuit breaking unit and a second circuit breaking unit connected in parallel to the first circuit breaking unit. The first circuit breaking unit may comprise at least one power semiconductor switching element. The second circuit breaking unit may comprise a series connection of a disconnector and load flow controller or converter circuitry. The second circuit breaking unit may have a smaller on- resistance than the first circuit breaking unit. The load flow controller or converter circuitry may be adapted to selectively and controllably block current therethrough. The common circuitry may comprise the load flow controller or converter circuitry. The disconnector may for example comprise a mechanical switch or a series connection of several mechanical switches and/or another type of disconnector.

In the context of the present application, by the second circuit breaking unit having a smaller on-resistance than the first circuit breaking unit it is meant that the second circuit breaker unit has a smaller resistance for an electrical current flowing therethrough when it is turned on, i.e. is in a conducting state, than the first circuit breaker unit. Hence, the second circuit breaker unit may have a lower conduction voltage drop compared to the first circuit breaker unit.

According to another example, the common circuitry may be adapted to, when the apparatus is operating in the first mode, operate as a multiphase converter, wherein the phases or conductors are connected in parallel to the at least one transmission line.

The common circuitry may according to an example comprise a plurality of interconnected power semiconductor switching elements. At least some of the plurality of interconnected power semiconductor switching elements may be controllable with respect to switching operation. The plurality of interconnected power semiconductor switching elements may be arranged such that, when the apparatus is operating in the second mode, the phases or conductors are connected in series to the at least one transmission line. Thus, during 'normal' operation, i.e. when e.g. a fault condition or fault condition, for example occurring in the transmission line, has not been detected, the apparatus may operate as a load flow controller or converter with respect to power transmission between the at least two DC power systems by means of a multiphase, e.g. three-phase, parallel connection to the at least one transmission line. Then, according to one example, when a fault current or fault condition occurs, for example in the at least one transmission line, one or several of the plurality of interconnected power semiconductor switching elements may be switched such that the phases or conductors become connected in series to the at least one transmission line.

At least one of the power semiconductor switching element or elements may be controllable with respect to switching operation. At least one of the power semiconductor switching element or elements may comprise at least one power semiconductor switch, which may be controllable with respect to switching operation.

For example, at least one of the power semiconductor switching element or elements may comprise a power semiconductor switch and a diode connected in anti-parallel with the power semiconductor switch. According to another example, in alternative or in addition at least one of the power semiconductor switching elements may comprise a power semiconductor switch of a first current direction and a power semiconductor switch of a second current direction, which is opposite to the first current direction.

The power semiconductor switch may for example be based on, or be selected from, an insulated-gate bipolar transistor (IGBT), a metal-oxide semiconductor field effect transistor (MOSFET), an integrated gate-commutated thyristor (IGCT) and/or a gate turn-off thyristor (GCT). Switches of these types belong to the group of power semiconductor switches which are controllable with respect to switching operation, or have selective turn-on and turn-off capability. Hence, the at least one of the power semiconductor switching element or elements which is controllable with respect to switching operation may for example comprise at least one power semiconductor switch which is IGBT-, MOSFET-, IGCT- and/or GCT-based.

In alternative or in addition, at least one of the power semiconductor switching element or elements, or power semiconductor switch or switches, may include or be constituted by a thyristor or the like.

According to a second aspect, there is provided a power system which comprises at least two DC power systems interconnected by means of at least one

transmission line arranged to carry an electrical current. The power system according to the second aspect comprises an apparatus according to the first aspect. The apparatus is connected to a current path in the at least one transmission line for controlling electric power

transmission in the at least one transmission line between the at least two DC power systems and controllably effecting discontinuation of flow of electrical current in the at least one transmission line.

The power system may comprise auxiliary equipment for operation of the electrical converter circuitry and the circuit breaking circuitry. The auxiliary equipment may be common to the electrical converter circuitry and the circuit breaking circuitry. Hence, both of the electrical converter circuitry and the circuit breaking circuitry may utilize the same auxiliary equipment, which may be facilitated or enabled by the power system comprising an apparatus according to the first aspect, which provide both electrical series converter functionality or capability and circuit breaking functionality or capability. The auxiliary equipment may for example include equipment for controlling and/or monitoring operation of the electrical converter circuitry and the circuit breaking circuitry. In alternative or in addition, the auxiliary equipment may include for example housing and/or infrastructure for both the electrical converter circuitry and the circuit breaking circuitry.

The power system may for example comprise an HVDC power transmission system and/or a DC power grid.

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 following description. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the invention will be described below with reference to the accompanying drawings. Fig. 1 is a schematic block diagram of a power system according to an embodiment of the present invention.

Figs. 2-6 are schematic block diagrams of apparatuses according to embodiments of the present invention.

In the accompanying drawings, the same reference numerals denote the or similar elements throughout the views.

DETAILED DESCRIPTION

The present invention will now be described more fully 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 set forth herein; rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the invention to those skilled in the art. Furthermore, like numbers refer to the same or similar elements or components throughout.

Referring now to Fig. 1, there is shown a schematic block diagram of a power system 200 according to an embodiment of the present invention. The power system 200 includes two direct current (DC) power systems 102, 103 interconnected by means of a transmission line 101 arranged to carry an electrical current. The power system 200 includes an apparatus 100 according to an embodiment of the present invention, e.g. such as described in the following with reference to any one of Figs. 2-6. The apparatus 100 is connected to a current path in the transmission line 101 for controlling electric power transmission in the transmission line 101 between the DC power systems 102, 103 and controllably effecting dis- continuation of flow of electrical current in the transmission line 101. It is to be noted that the Fig. 1 is schematic. For example, even though Fig. 1 indicates that the distance between the apparatus 100 and the DC power system 102 is smaller than and the distance between the apparatus 100 and the DC power system 103, this is not necessary. The distance between the apparatus 100 and the DC power system 102 may be larger than the distance between the apparatus 100 and the DC power system 103, or the distances may be the same or

substantially the same.

It is to be understood that the two DC power systems 102, 103, the transmission line 101 and the apparatus 100 may constitute only a part or portion of the power system 200. Hence, it is to be understood that power system 200 may include further components other than the two DC power systems 102, 103, the transmission line 101 and the apparatus 100 depicted in Fig. 1. Any other such component which may be included in the power system 200 are not shown in Fig. 1. The power system 200 may for example include or be constituted by a power transmission system such as an High Voltage Direct Current (HVDC) power transmission system, or an HVDC grid or a DC grid. The transmission line 101 may for example be a power transmission line such as a DC cable, an overhead line (OHL), or a combination of DC cable and OHL.

Referring now to Fig. 2, there is shown a schematic block diagram of an apparatus 100 according to an embodiment of the present invention. The apparatus 100 comprises electrical converter circuitry adapted to controUably introduce current and/or voltage into the current path, so as to adjust flow of electrical current in the transmission line 101 between the DC power systems 102, 103. The apparatus 100 comprises circuit breaking circuitry adapted to interrupt electrical current in the current path. According to the embodiment depicted in Fig. 2, the electrical converter circuitry and the circuit breaking circuitry are arranged such that the apparatus 100 can be selectively switched between at least two modes of operation. The two modes of operation include a first mode, in which current and/or voltage is controUably introduced into the current path so as to adjust flow of electrical current in the transmission line 101 between the DC power systems 102, 103, and a second mode, in which electrical current in the current path is interrupted.

According to the embodiment depicted in Fig. 2, the electrical converter circuitry includes or is constituted by a transformer 104, or converter transformer, which may be connected to an appropriate power source (not shown in Fig. 2), and circuitry 105, which will be further described in the following. Such a power source may be included in the apparatus 100. According to the embodiment depicted in Fig. 2, the circuit breaking circuitry is realized or implemented also by means of the circuitry 105. In accordance with the embodiment depicted in Fig. 2, the electrical converter circuitry and the circuit breaking circuitry may hence share at least one common component, which according to the example depicted in Fig. 2 is constituted at least in part by a common circuitry 105.

As will be further described in the following e.g. with reference to Figs. 5 and

6, the common circuitry 105 may comprise at least one power semiconductor switching element which is adapted to be connected to the current path. The common circuitry 105, or the at least one power semiconductor switching element, may be adapted to, when the apparatus 100 is operating in the first mode, controUably introduce current and/or voltage into the current path in the transmission line 101, and, when the apparatus 100 is operating in the second mode, interrupt electrical current in the current path in the transmission line 101.

It is to be noted that although inputs and outputs from the transformer 104 are indicated in Fig. 2 as including or being constituted by several conductors, or phases, this is according to an example and does not limit the present invention. Inputs and outputs from the transformer 104 may according to other examples be single-phase, or have more phases or less phases than the three-phase arrangement illustrated in Fig. 2, as appropriate.

Referring now to Fig. 3, there is shown a schematic view of an apparatus 100 according to another embodiment of the present invention. The apparatus 100 comprises electrical converter circuitry adapted to controllably introduce current and/or voltage into the current path, so as to adjust flow of electrical current in the transmission line 101. The apparatus 100 comprises circuit breaking circuitry adapted to interrupt electrical current in the current path.

According to the embodiment depicted in Fig. 3, the electrical converter circuitry includes a transformer 104, which may be connected to an appropriate power source (not shown in Fig. 3), and load flow controller circuitry 120, comprising e.g. at least one power semiconductor switching element The load flow controller circuitry 120, or circuitry 120, is adapted to selectively and controllably block current therethrough. The circuitry 120 is connected to the current path. The power source may be included in the apparatus 100.

According to the embodiment depicted in Fig. 3, the circuit breaking circuitry includes a first circuit breaking unit 106 and a second circuit breaking unit 107 connected in parallel to the first circuit breaking unit 106. The first circuit breaking unit 106 comprises four power semiconductor switching elements 108 connected in series. However, according to other embodiments of the present invention the first circuit breaking unit 106 may comprise fewer than or more than four power semiconductor switching elements 108 connected in series. The second circuit breaking unit 107 comprises a series connection of a disconnector 109 and the circuitry 120. The disconnector 109 may for example include at least one mechanical switch, a series connection of several mechanical switches, and/or another type of appropriate disconnector. The first circuit breaking unit 106 may be arranged such that the second circuit breaking unit 107 has a smaller on-resistance than the first circuit breaking unit 106.

As illustrated in Fig. 3, each of the power semiconductor switching elements 108 may have a non- linear resistor 111 connected in parallel thereto.

Further, as also illustrated in Fig. 3, the circuitry 120 may have a non- linear resistor 112 connected in parallel thereto. As indicated in Fig. 3, the non-linear resistor 112 may be included in the second circuit breaking unit 107.

For example for current rate limitation purposes, as illustrated in Fig. 3, a reactor 113, e.g. an inductor, may be arranged in series with the first circuit breaking unit 106. The reactor 113 may be included in the apparatus 100 or it may be connected e.g. in series to the apparatus 100.

The first circuit breaking unit 106 may be referred to as a main circuit breaker unit and the second circuit breaking unit 107 may be referred to as an auxiliary circuit breaker unit.

The circuitry 120, which is connected to the current path, is adapted to, when the apparatus 100 is operating in the first mode, operate as a multiphase converter. The phases may be connected in series to the transmission line 101. As further described in the following, when the apparatus 100 is operating in the second mode, the circuitry 120 may enable or facilitate interruption of electrical current in the current path and thus may be adapted to interrupt electrical current in the current path. Thus, circuitry 120 is a common component or common circuitry which is shared by the electrical converter circuitry and the circuit breaking circuitry of the apparatus 100, which electrical converter circuitry and circuit breaking circuitry are arranged such that the apparatus 100 can be selectively switched between at least two modes of operation, including a first mode in which current and/or voltage is controllably introduced into the current path so as to adjust flow of electrical current in the transmission line 101, and a second mode in which electrical current in the current path is interrupted.

It is to be noted that although inputs and outputs from the transformer 104 are indicated in Fig. 3 as including or being constituted by several conductors, or phases, this is according to an example and does not limit the present invention. Inputs and outputs from the transformer 104 may for example have more phases than the three-phase arrangement illustrated in Fig. 3, as appropriate.

An exemplifying, non- limiting way of operating the apparatus 100 in the second mode is now described. According to the example, the auxiliary circuit breaker unit or second circuit breaking unit 107 has a smaller on-resistance than the main circuit breaker unit or first circuit breaking unit 106, i.e. the auxiliary circuit breaker unit has a smaller resistance for an electrical current flowing therethrough when it is in a conducting state, than the main circuit breaker unit. Hence, the auxiliary circuit breaker unit has according to the example a lower conduction voltage drop than the main circuit breaker unit.

Further according to this example, during normal operation, i.e. when no fault current or fault condition, for example occurring in the transmission line 101, has been detected, electrical current passes through the auxiliary circuit breaker unit or second circuit breaking unit 107, since during normal operation the disconnector 109 is turned on and the circuitry 120 does not block current therethrough, and hence both are in a conducting state.

On detection of a fault current or fault condition, an opening signal can be generated for the auxiliary circuit breaker unit or second circuit breaking unit 107 and/or for the main circuit breaker unit or first circuit breaking unit 106. A opening signal for the auxiliary circuit breaker unit may be generated and transmitted to the auxiliary circuit breaker unit prior to generation of an opening signal for the main circuit breaker unit and transmission the opening signal to the main circuit breaker unit. Such generation and transmission of opening signals may be achieved e.g. by means of some protection system for protecting, monitoring and controlling the operation and/or functionality of other components included in the power system. Such a protection system may for example in case of detection of a fault current or fault condition in a wired and/or wireless manner transmit the opening signals to the auxiliary circuit breaker unit and the main circuit breaker unit. Responsive to an opening signal being generated for the auxiliary circuit breaker unit or second circuit breaking unit 107, the disconnector 109is turned off so as to be in a non-conductive state or substantially non-conductive state, and/or the circuitry 120 is controlled so as to block current therethrough.

Usually, when a fault current or fault condition has been detected, first of all the circuitry 120 is controlled so as to block current therethrough. The main circuit breaker unit is closed, i.e. its power semiconductor switches 108 are turned on so as to be in a conducting state, at least some time before the circuitry 120 is controlled so as to block current therethrough. When the circuitry 120 is controlled so as to block current therethrough, electrical current is commutated to the current to the main circuit breaker unit or first circuit breaking unit 106. Sometime later the disconnector 109 may then be opened, and then the main circuit breaker unit is opened, i.e. all or substantially all of its power semiconductor switches 108 are turned off so as to be in a non-conductive state or substantially non- conductive state. The electrical current may then commutate over to the non-linear resistors 111, whereby the current level can be reduced and the voltage can be limited.

Referring now to Fig. 4, there is shown a schematic view of an apparatus 100 according to another embodiment of the present invention. The apparatus 100 depicted in Fig. 4 is similar to the apparatus 100 depicted in Fig. 3. The difference between the apparatus 100 depicted in Fig. 4 and the apparatus 100 depicted in Fig. 3 is that according to the apparatus 100 depicted in Fig. 4, a non- linear resistor 121 is connected in parallel to the series connection of power semiconductor switching elements 108 in the first circuit breaker unit 106, instead of the arrangement depicted in Fig. 3 where each of the power semiconductor switching elements 108 has a non- linear resistor 111 connected in parallel thereto.

Referring now to Fig. 5, there is shown a schematic view of an apparatus 100 according to another embodiment of the present invention. The apparatus 100 comprises electrical converter circuitry adapted to controllably introduce current and/or voltage into the current path, so as to adjust flow of electrical current in the transmission line 101. The apparatus 100 comprises circuit breaking circuitry adapted to interrupt electrical current in the current path. According to the embodiment depicted in Fig. 5, the electrical converter circuitry includes a transformer 104, which may be connected to an appropriate power source (not shown in Fig. 5). The power source may be included in the apparatus 100. Further according to the embodiment depicted in Fig. 5, the electrical converter circuitry includes circuitry 110 comprising a plurality of power semiconductor switching elements 117 connected in series which are connected to the current path. At least some of power semiconductor switching elements 117 may be controllable with respect to switching operation. As illustrated in Fig. 5, each of the power semiconductor switching elements 117 may have a non- linear resistor 118 connected in parallel thereto.

As illustrated in Fig. 5, the apparatus 100 may comprise a plurality of electrical storage elements, e.g. in the form of capacitors 1 14, which are connected in series, which series connection of capacitors 114 may be connected in parallel to the series connection of power semiconductor switching elements 117.

The circuitry 110, which according to the embodiment depicted in Fig. 5 comprises a series connection of power semiconductor switching elements 117 and is connected to the current path, is adapted to, when the apparatus 100 is operating in the first mode, operate as a multiphase converter, with the phases connected in series to the transmission line 101. As further described in the following, when the apparatus 100 is operating in the second mode, the circuitry 110 may enable or facilitate interruption of electrical current in the current path, and may hence be adapted to interrupt electrical current in the current path. Thus, circuitry 110 is a common component or common circuitry which is shared by the electrical converter circuitry and the circuit breaking circuitry of the apparatus 100, which electrical converter circuitry and circuit breaking circuitry are arranged such that the apparatus 100 can be selectively switched between at least two modes of operation, including a first mode, in which current and/or voltage is controllably introduced into the current path, so as to adjust flow of electrical current in the transmission line 101, and a second mode, in which electrical current in the current path is interrupted.

It is to be noted that although inputs and outputs from the transformer 104 are indicated in Fig. 5 as including or being constituted by several conductors, or phases, this is according to an example and does not limit the present invention. Inputs and outputs from the transformer 104 may for example have more phases than the three-phase arrangement illustrated in Fig. 5, as appropriate.

During 'normal' operation, e.g. when the apparatus 100 is operating in the first mode, when no fault condition or fault condition, which for example may occur in the transmission line 101, has been detected, the apparatus 100 may operate as a load flow controller or converter with respect to power transmission between at least two interconnected DC power systems (cf. Fig. 1), by means of a multiphase, e.g. three-phase, connection in series to the transmission line 101. Then, according to one example, when a fault current or fault condition occurs, for example in the transmission line 101, some or all of the power semiconductor switching elements 117 may be turned off, i.e. brought into a non-conducting or substantially non-conducting state. The electrical current may then commutate over to the respective ones of the non- linear resistors 118, whereby the current level can be reduced and the voltage can be limited. Both of these functionalities or capabilities, i.e. electrical series converter functionality or capability and circuit breaking functionality or capability, can be achieved at least in part by means of common circuitry 110 comprising at least some of the power semiconductor switching elements 117. Hence, at least some of the power

semiconductor switching elements 117 can be considered as a common component or common circuitry 110 which is shared by the electrical converter circuitry and the circuit breaking circuitry of the apparatus 100, which electrical converter circuitry and circuit breaking circuitry are arranged such that the apparatus 100 can be selectively switched between at least two modes of operation, including a first mode in which current and/or voltage is controllably introduced into the current path so as to adjust flow of electrical current in the transmission line 101, and a second mode in which electrical current in the current path is interrupted.

Referring now to Fig. 6, there is shown a schematic view of an apparatus 100 according to another embodiment of the present invention. The apparatus 100 comprises electrical converter circuitry adapted to controllably introduce current and/or voltage into the current path, so as to adjust flow of electrical current in the transmission line 101. The apparatus 100 comprises circuit breaking circuitry adapted to interrupt electrical current in the current path. According to the embodiment depicted in Fig. 6, the electrical converter circuitry includes a transformer 104, which may be connected to an appropriate power source (not shown in Fig. 6). The power source may be included in the apparatus 100.

Further according to the embodiment depicted in Fig. 6, the electrical converter circuitry includes circuitry 110 comprising a plurality of interconnected power semiconductor switching elements 115, 116, which for example be arranged such as illustrated in Fig. 6, which are connected to the current path. At least some of power semiconductor switching elements 115, 116 may be controllable with respect to switching operation. As illustrated in Fig. 6, at least some of the power semiconductor switching elements 115, 116 may be arranged such that they, when the apparatus 100 is operating in the first mode, operate as a multiphase converter, wherein the phases are connected in parallel to the transmission line 101. As illustrated in Fig. 6, this can for example be achieved by the power semiconductor switching elements 116 being turned on, i.e. being in a conducting state. It is to be noted that although inputs and outputs from the transformer 104 are indicated in Fig. 6 as including or being constituted by several conductors, or phases, this is according to an example and does not limit the present invention. Inputs and outputs from the transformer 104 may for example have more phases than the three-phase arrangement illustrated in Fig. 6, as appropriate.

One or both of the power semiconductor switching elements 116 may in alternative or in addition include a disconnector or a series connection of several

disconnectors, each of which is capable of relatively fast operation. The disconnector may for example comprise a mechanical switch.

The power semiconductor switching elements 115, 116 may be arranged such that, when the apparatus 100 is operating in the second mode, the phases or conductors are connected in series to the transmission line 101. As illustrated in Fig. 6, this can be achieved by the power semiconductor switching elements 116 being turned off, i.e. being in a nonconducting or substantially non-conducting state. Responsive to a fault condition or fault condition having been detected, the power semiconductor switching elements 115 may then all be turned off, i.e. brought into a non-conducting or substantially non-conducting state. A fault condition or fault condition may for example occur in the transmission line 101, and may for example be detected by some protection system for protecting, monitoring and controlling the operation and/or functionality of other components included in the power system, such as mentioned in the foregoing.

Thus, during 'normal' operation, e.g. when the apparatus 100 is operating in the first mode, when no fault condition or fault condition has been detected the apparatus 100 may operate as a load flow controller or converter with respect to power transmission between at least two interconnected DC power systems (cf. Fig. 1) by means of a multiphase, e.g. three-phase, parallel connection to the transmission line 101. Then, according to one example, when a fault current or fault condition occurs, one or several of the plurality of interconnected power semiconductor switching elements 115, 116 may be switched such that the phases or conductors become connected in series to the transmission line 101. Both of these

functionalities or capabilities, i.e. electrical series converter functionality or capability and circuit breaking functionality or capability, can be achieved at least in part by means of common circuitry 110 comprising at least some of the power semiconductor switching elements 115, 116. Hence, at least some of the power semiconductor switching elements 115, 116 can be considered as a common component or common circuitry 110 which is shared by the electrical converter circuitry and the circuit breaking circuitry of the apparatus 100, which electrical converter circuitry and circuit breaking circuitry are arranged such that the apparatus 100 can be selectively switched between at least two modes of operation, including a first mode in which current and/or voltage is controllably introduced into the current path so as to adjust flow of electrical current in the transmission line 101, and a second mode in which electrical current in the current path is interrupted.

With reference to any one of the embodiments described herein, at least one of any power semiconductor switching elements may comprise at least one power semiconductor switch and may for example be based on, or be selected from, an IGBT, a MOSFET, an IGCT and/or a GCT. Switches of these types belong to the group of power semiconductor switches which are controllable with respect to switching operation, or have selective turn-on and turn- off capability.

According to the embodiments of the present invention which have been described in the foregoing with reference to Figs. 2-6, the electrical converter circuitry includes or is constituted by a transformer 104, or converter transformer. However, it is to be noted that a transformer, or converter transformer, is not necessary for embodiments of the present invention, and may be omitted or replaced with other component(s), while still achieving among other things an electrical series converter functionality or capability such as described in the foregoing.

Example configurations of the electrical converter circuitry have been described herein, e.g. in the foregoing with reference to embodiments of the present invention depicted in Figs. 2-6. However, it is to be understood that various other configurations or arrangements of the electrical converter circuitry are possible and are encompassed by embodiments of the present invention. For example, configurations or arrangements of the electrical converter circuitry may be based on a line-commutated and/or voltage-source converter configuration or arrangement, including features of, e.g., a two-level converter, a three-level converter, a modular multi-level converter (MMC), a half-bridge MMC, a full- bridge MMC, a cascaded two-level (CST) converter, a half-bridge CST converter, a full- bridge CST converter, a DC-DC converter, a thyristor bridge, or any combination thereof.

In conclusion, embodiments of the present invention disclose an apparatus adapted to be connected to a current path in at least one transmission line which interconnects at least two power systems. The apparatus is adapted to control electric power transmission in the at least one transmission line between the power systems and controllably effect discontinuation of flow of electrical current in the at least one transmission line. The apparatus comprises electrical converter circuitry adapted to controllably introduce current and/or voltage into the current path so as to adjust flow of electrical current in the at least one transmission line. The apparatus comprises circuit breaking circuitry adapted to interrupt electrical current in the current path.

While the present invention has been illustrated and described in detail in the appended drawings and the foregoing description, such illustration and description are 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. 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.

Claims

CLAIMS:

1. An apparatus (100) adapted to be connected to a current path in at least one transmission line (101) arranged to carry an electrical current and which interconnects at least two DC power systems (102, 103), the apparatus being adapted to control electric power transmission in the at least one transmission line between the at least two DC power systems and controllably effect discontinuation of flow of electrical current in the at least one transmission line, the apparatus comprising:

electrical converter circuitry (104, 105, 110) adapted to controllably introduce current and/or voltage into the current path so as to adjust flow of electrical current in the at least one transmission line between the at least two DC power systems; and

circuit breaking circuitry (105; 106, 107, 108, 110; 117; 115, 116) adapted to interrupt electrical current in the current path;

wherein the electrical converter circuitry and the circuit breaking circuitry are arranged such that the apparatus can be selectively switched between at least two modes of operation including a first mode in which current and/or voltage is controllably introduced into the current path so as to adjust flow of electrical current in the at least one transmission line between the at least two DC power systems, and a second mode in which electrical current in the current path is interrupted.

2. An apparatus according to claim 1, wherein the electrical converter circuitry and the circuit breaking circuitry share at least one common component.

3. An apparatus according to claim 1 or 2, wherein the at least one common component includes a common circuitry (105; 110) which is adapted to be connected to the current path and further is adapted to, when the apparatus is operating in the first mode, controllably introduce current and/or voltage into the current path, and, when the apparatus is operating in the second mode, interrupt electrical current in the current path.

4. An apparatus according to claim 3, wherein the common circuitry is adapted to, when the apparatus is operating in the first mode, operate as a multiphase converter, wherein the phases are connected in series to the at least one transmission line.

5. An apparatus according to claim 3 or 4, wherein the circuit breaking circuitry includes a first circuit breaking unit (106) and a second circuit breaking unit (107) connected in parallel to the first circuit breaking unit, wherein the first circuit breaking unit comprises at least one power semiconductor switching element (108) and the second circuit breaking unit comprises a series connection of a disconnector (109) and load flow controller circuitry (120), wherein the second circuit breaking unit has a smaller on-resistance than the first circuit breaking unit.

6. An apparatus according to any one of claims 3-6, wherein the load flow controller circuitry is adapted to selectively and controllably block current therethrough, and wherein the common circuitry comprises the load flow controller circuitry.

7. An apparatus according to claim 3 or 4, wherein the common circuitry comprises at least one power semiconductor switching element (117).

8. An apparatus according to claim 3, 4 or 7, wherein the common circuitry comprises load flow controller circuitry (120) adapted to selectively and controllably block current therethrough.

9. An apparatus according to claim 3, wherein the common circuitry is adapted to, when the apparatus is operating in the first mode, operate as a multiphase converter, wherein the phases are connected in parallel to the at least one transmission line.

10. An apparatus according to claim 9, wherein the common circuitry comprises a plurality of interconnected power semiconductor switching elements (115, 116) arranged such that, when the apparatus is operating in the second mode, the phases are connected in series to the at least one transmission line.

11. An apparatus according to any one of claims 5-7 or 10, wherein at least one of the power semiconductor switching element or elements is controllable with respect to switching operation.

12. An apparatus according to any one of claim 11, wherein the at least one of the power semiconductor switching element or elements which is controllable with respect to switching operation comprises at least one power semiconductor switch which is insulated- gate bipolar transistor-, metal-oxide semiconductor field effect transistor-, integrated gate- commutated thyristor-, and/or gate turn-off thyristor-based.

13. A power system (200) comprising :

at least two DC power systems (102. 103) interconnected by means of at least one transmission line (101) arranged to carry an electrical current; and

an apparatus (100) according to any one of claims 1-12 connected to a current path in the at least one transmission line for controlling electric power transmission in the at least one transmission line between the at least two DC power systems and controllably effecting discontinuation of flow of electrical current in the at least one transmission line.

14. A power system according to claim 13, further comprising auxiliary equipment for operation of the electrical converter circuitry and the circuit breaking circuitry, wherein the auxiliary equipment is common to the electrical converter circuitry and the circuit breaking circuitry.

15. A power system according to claim 13 or 14, wherein the power system comprises a High Voltage Direct Current power transmission system.

16. A power system according to any one of claims 13-15, wherein the power system comprises a DC power grid.