WO2013182231A1 - A method and an arrangement for limiting the current in an electrical power transmission system - Google Patents

A method and an arrangement for limiting the current in an electrical power transmission system Download PDF

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Publication number
WO2013182231A1
WO2013182231A1 PCT/EP2012/060620 EP2012060620W WO2013182231A1 WO 2013182231 A1 WO2013182231 A1 WO 2013182231A1 EP 2012060620 W EP2012060620 W EP 2012060620W WO 2013182231 A1 WO2013182231 A1 WO 2013182231A1
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WO
WIPO (PCT)
Prior art keywords
current
direct current
distribution line
cell
partially based
Prior art date
Application number
PCT/EP2012/060620
Other languages
French (fr)
Inventor
Jyoti Sastry
Frans Dijkhuizen
Original Assignee
Abb Technology Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Abb Technology Ltd filed Critical Abb Technology Ltd
Priority to PCT/EP2012/060620 priority Critical patent/WO2013182231A1/en
Publication of WO2013182231A1 publication Critical patent/WO2013182231A1/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01C—RESISTORS
    • H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
    • H01C7/10—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material voltage responsive, i.e. varistors
    • H01C7/12—Overvoltage protection resistors
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
    • H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
    • H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/79—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/797—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Definitions

  • the present invention relates to a method for limiting the current in an electrical power transmission system, the electrical power transmission system comprising a direct current power transmission system comprising at least one direct current transmission or distribution line for carrying direct current, DC, and at least one converter connected to the at least one DC transmission or distribution line and connectable to an alternating current power system, the converter com- prising at least one cascaded cell and being arranged to convert alternating current to direct current for input to the at least one DC transmission or distribution line and/or direct current to alternating current for input to the alternating current power system, where each cascaded cell comprises at least one cell capacitor.
  • the present invention relates to an arrangement for limiting the current in an electrical power transmission system of the above-mentioned sort.
  • the present invention also relates to an electrical power transmission system comprising at least one arrangement of the above-mentioned sort.
  • An HVDC (high voltage direct current) power distribution network or an HVDC power transmission system uses direct current for the transmission of electrical power, in contrast to the more common AC system. For long-distance distribution, HVDC systems may be less expensive and may suffer lower electrical losses.
  • an HVDC power transmission system comprises at least one long-distance HVDC link or cable for carrying direct current a long distance, e.g. under sea, and converters, also called converter stations, for converting alternating current to direct current for input to the HVDC power transmission system and converters or converter stations for converting direct current back to alternating current.
  • US 7,408,755 B1 discloses a circuit for inrush/transient current limitation and overload/short circuit protection for a DC voltage power supply.
  • US 7,974,059 B2 describes a power supply apparatus comprising a DC power source that outputs DC voltage, a power supply circuit to which the DC voltage output from the DC power source is applied, and a current limiting unit connected in series between the DC power source and the power supply circuit.
  • US 6,624,993 B1 discloses a fault current limiting system for DC and pulsed circuit systems.
  • EP 1 526 624 A2 describes an assembly which rapidly identifies a short circuit in an alternating current or direct current power supply system and shuts down the system at once.
  • the short circuit current flows through a high temperature super-conductor current limiter (HTS) in series with a switchgear station.
  • HTS high temperature super-conductor current limiter
  • US 5,596,472 discloses an apparatus for controlling an over-current tripping device of a high-speed DC circuit breaker, at least partially based on an out- put signal generated by a current transformer.
  • JP 54137646 describes a protective circuit for DC current transformer.
  • WO 201 1/057675 A1 discloses a breaking device for breaking and limiting a direct current flowing through an HVDC power transmission or distribution line.
  • the breaking device comprises a parallel connection of a main breaker and a non- linear resistor, the main breaker comprising at least one power semiconductor switch of a first current direction, and a series connection of a high speed switch and an auxiliary breaker, the high speed switch comprising at least one mechanical switch, and the auxiliary breaker has a smaller on-resistance than the main breaker and comprises at least one power semiconductor switch of the first current direction, where the series connection is connected in parallel to the parallel connection.
  • US 201 1/0235375 describes an apparatus having a converter connected to a DC voltage circuit via a short-circuit protection device arranged to limit the current in the DC voltage circuit.
  • US 5,999,388 discloses a method and apparatus for limiting current in a DC network of a power transmission system, where at least one parallel connection of at least one semiconductor element of turn-off type and a surge diverter is provided in the DC network.
  • US 2009/0161277 discloses a method and device for preventing damage to a semiconductor switch circuit during a failure.
  • WO 201 1/060812 describes a DC voltage source converter for HVDC power transmission, where a voltage is provided to oppose the flow of current created by a fault in a DC network.
  • a converter for converting alternating current to direct current for input to a direct current power transmission system and/or direct current to alternating cur- rent for input to an alternating current power system may comprise one or a plurality of cascaded cells.
  • Each cascaded cell includes a cell capacitor and may in general include power semiconductor switches and diodes, as disclosed in more detail in the detailed description of embodiments.
  • the inventors of the present invention have found that faults in the DC power system result in problems with un- controlled charging of the cell capacitor of the cascaded cell so that the cell capacitor may be overloaded and the converter needs to be shut down.
  • the converter may operate in a so called blocked mode for a finite period of time.
  • the converter may essentially be a diode bridge, thereby allowing the AC side, i.e. the AC power system or network, to feed the DC fault.
  • the collapse in the voltage on the DC side may result in uncontrolled charging of the cell capacitor of each cascaded cell.
  • the over-voltage across the cascaded cell or cells result in tripping of the converter, i.e. the converter is shut down, because each cascaded cell exceeds a safe oper- ating voltage threshold.
  • the above-mentioned object of the present invention is attained by providing a method for limiting the current in an electrical power transmission system, the electrical power transmission system comprising a direct current power trans- mission system comprising at least one direct current transmission or distribution line for carrying direct current, DC, and at least one converter connected to the at least one DC transmission or distribution line and connectable to an alternating current power system, the converter comprising at least one cascaded cell and being arranged to convert alternating current to direct current for input to the at least one DC transmission or distribution line and/or direct current to alternating current for input to the alternating current power system, where each cascaded cell comprises at least one cell capacitor, wherein at least one current limiting apparatus is connected to the at least one direct current transmission or distribution line for limiting the current of the at least one direct current transmission or distri- bution line, the current limiting apparatus comprising electrical energy dissipating means, and wherein the method comprises the step of limiting the current of the at least one direct current transmission or distribution line by means of the current limiting apparatus and at
  • the current is limited such that the voltage of each cascaded cell is prevented from exceeding a safe operating voltage, whereby a shut-down, or tripping, of the converter is avoided.
  • interruption of power transmission is prevented.
  • an improved control of the electric power transmission in a DC power transmission system is provided, and the electric power transmission in a DC power transmission system, especially in an HVDC power transmission system, is improved.
  • an improved electrical power transmission system including a DC power transmission system is provided.
  • the step of limiting the current of the at least one direct current transmission or distribution line is performed after a fault in the DC power transmission system has been cleared.
  • the method is adapted to limit the current in an HVDC power transmission system comprising at least one HVDC transmission or distribution line, or limit the current in an electrical power transmission system comprising the HVDC power transmission system.
  • an arrangement for limiting the current in an electrical power transmis- sion system comprising a direct current power transmission system comprising at least one direct current transmission or distribution line for carrying direct current, DC, and at least one converter connected to the at least one DC transmission or distribution line and connectable to an alternating current power system, the converter comprising at least one cas- caded cell and being arranged to convert alternating current to direct current for input to the at least one DC transmission or distribution line, and/or direct current to alternating current for input to the alternating current power system, where each cascaded cell comprises at least one cell capacitor, wherein the arrangement comprises at least one current limiting apparatus for limiting the current of the at least one direct current transmission or distribution line, the current limiting apparatus comprising electrical energy dissipating means, and wherein the arrangement comprises a control unit arranged to control the at least one current limiting apparatus to limit the current of the at least one direct current transmission or distribution line at least partially based on a
  • the arrangement is adapted to limit the current in an HVDC power transmission system comprising at least one HVDC transmission or distribution line, or limit the current in an electrical power transmission system comprising an HVDC power transmission system.
  • the arrangement may be connectable to the at least one cascaded cell.
  • the arrangement may be connectable to the converter.
  • the current limiting apparatus may be connectable to the at least one DC transmission or distribution line.
  • the control unit may be connected to the at least one current limiting apparatus.
  • the control unit may be connectable to the at least one cascaded cell.
  • the arrangement may comprise the at least one cascaded cell.
  • an electrical power transmission system comprising a direct current power transmission system, the direct current power transmission system comprising at least one direct current transmission or distribution line for carrying direct current, DC, and at least one converter connected to the at least one DC transmission or distribution line and connectable to an alternating current power system, the converter comprising at least one cascaded cell and being arranged to convert alternating current to direct current for input to the at least one DC transmission or distribution line, and/or direct current to alternating current for input to the alternating current power system, each cascaded cell comprises at least one cell capacitor, wherein the electrical power transmission system comprises at least one arrangement as claimed in any of the claims 13-21 for limiting the current in the electrical power transmission system, and/or at least one arrangement according to any of the em- bodiments of the arrangement disclosed below.
  • the at least one DC transmission or distribution line may be one or a plurality of DC transmission or distribution lines.
  • the at least one converter which may also be called a converter station, may be one or a plurality of converters.
  • the direct current power transmission system may be an HVDC power transmission system, and the at least one direct current transmission or distribution line may be an HVDC transmission or distribution line.
  • High Voltage may be about 1 -1 .5 kV and above.
  • High Voltage may be about 1 -1 .5 kV and above.
  • High Voltage may be about 150 kV and above, e.g. 320 kV, 640 kV, 800 kV or 1000 kV, and above.
  • the method, arrangement and/or the system according to the present invention are advantageously adapted for the above-mentioned HVDC voltage levels and above.
  • the features and embodiments of the method, arrangement and electrical power transmission system, respectively, may be combined in various possible ways providing further advantageous embodiments.
  • Fig. 1 is a schematic diagram illustrating aspects of the arrangement according to the present invention, where the arrangement is connected to a an electrical power transmission system;
  • Fig. 2 is a schematic representation of a positive arm of the converter of electrical power transmission system shown in Fig. 1 during normal operation;
  • Fig. 3 is a schematic representation of the positive arm of Fig. 2 in
  • Fig. 4 is a schematic graph illustrating the voltage of the cascaded cell when a DC fault occurs and after the DC fault has been cleared, when the present invention is not applied;
  • Fig. 5 is a schematic graph illustrating the voltage of the cascaded cell when a DC fault occurs and after the DC fault has been cleared, when the present invention is applied;
  • Fig. 6 is a flow chart illustrating aspects of the method according to the present invention.
  • Fig. 7-9 are schematic diagrams illustrating embodiments of the current limiting apparatus, each of which may be included in the ar- rangement according to the present invention or to each of which the method according to the present invention may be applied.
  • Detailed Description of Preferred Embodiments are schematic diagrams illustrating embodiments of the current limiting apparatus, each of which may be included in the ar- rangement according to the present invention or to each of which the method according to the present invention may be applied.
  • the electrical power transmission system 204 comprises a direct current power transmission system 206, e.g. a DC network, comprising at least one direct current transmission or distribution line 208, hereinafter called DC line 208, for carrying direct current, DC.
  • the direct current power transmission system 206 may be an HVDC power transmission system.
  • the DC line 208 may be an HVDC line.
  • the DC line 208 may e.g. comprise a DC cable, busbar, or any other electrical conductor or electrically conductive element or means for carrying direct current.
  • the electrical power transmission system 204 comprises at least one converter 210, which also may be called converter station, connected to the DC line 208.
  • the converter 210 is connectable to an alternating current, AC, power system 212, e.g. an AC network.
  • the converter 210 is ar- ranged to convert alternating current to direct current for input to the DC line 208 and/or direct current to alternating current for input to the AC power system 212.
  • the converter 210 comprises at least one cascaded cell 214, 216.
  • the converter 210 may comprise a plurality of cascaded cells 214, 216. Each cascaded cell 214, 216 comprises at least one cell capacitor 218, 220.
  • Each cascaded cell 214, 216 may be a cascaded half-bridge cell (also called Cascaded Two-Level, CTL, cell). Each cascaded cell 214, 216 may be a cascaded full-bridge cell.
  • a cascaded half-bridge cell and a cascaded full-bridge cell per se and their structure are well known to the person skilled in the art and therefore not disclosed or discussed in more detail.
  • the converter 210 may comprise a plurality of cascaded cells, which may be half-bridge cells, or cascaded full-bridge cells, or a mixture thereof.
  • the converter 210 may thus comprise a mixture of cascaded half-bridge cells and cascaded full-bridge cells.
  • the plurality of cascaded cells may be interconnected to one another.
  • the electrical power transmission system 204 may be adapted for single phase power or multi-phase power, e.g. three-phase power, and the components of the system and the arrangement 202 may be configured accordingly in ways known to the skilled person.
  • the arrangement 202 comprises at least one current limiting apparatus 234 for limiting the current of the at least one DC line 208.
  • the current limiting apparatus 234 comprises electrical energy dissipating means 236 for dissipating electrical energy when current is applied to it.
  • the electrical energy dissipating means 236 may be in the form of an electrical energy dissipating element.
  • the electrical energy dissipating means 236, or element may comprise at least one electrical device 238 of a group of electrical devices comprising a nonlinear resistor, an inductor and a linear resistor.
  • the electrical energy dissipating means 236 may comprise nonlinear resistor, an inductor and/or a linear resistor.
  • the nonlinear resistor may comprise a voltage-dependent nonlinear resistor, for example a surge arrester.
  • the electrical energy dissipating means 236 may comprise a plurality of the above-mentioned electrical devices 238.
  • the electrical energy dissipating means 236 or element comprises a voltage-dependent nonlin- ear resistor, e.g. surge arrester.
  • the arrangement 202 comprises a control unit 240 arranged to control the at least one current limiting apparatus 234 to limit the current of the at least one DC line 208.
  • the control unit 240 may be in the form of control equipment.
  • the control unit may comprise a CPU or a computer.
  • the control unit 240 may be con- nected to the current limiting apparatus 234 and may be connectable or connected to each cascaded cell 214.
  • the control unit 240 may be connectable or connected to the converter 210.
  • the arrangement 202 may comprise measuring equipment 242 for determining the voltage of each cascaded cell 214.
  • the control unit 240 may be connected to the measuring equipment 242.
  • the measuring equipment 242 may be conventional equipment known to the person skilled in the art and may comprise conventional sensors or detectors.
  • the measuring equipment 242 may determine the voltage of each cascaded cell 214, e.g. the voltage of each cell capacitor 218. It is to be understood that the control unit 240 and the measuring equipment 242 may be connected to the nega- tive arm 224 of the converter 210 and the cascaded cells 216 of the negative arm 224.
  • the control unit 240 is arranged to control the at least one current limiting apparatus 234 to limit the current of the at least one DC line 208 at least partially based on a value of a parameter corresponding to or related to the voltage capability of the cascaded cell 214 in order to prevent the voltage of the cascaded cell 214 from exceeding a safe operating voltage.
  • the control unit 240 may be arranged to control the current limiting apparatus 234 to limit the current of the DC line 208 after a fault in the direct current power transmission system 206 has been cleared, e.g. cleared by means of the current limiting apparatus 234, or any other current limiting and/or breaking apparatus.
  • the current limiting apparatus 234 may clear the fault by breaking and/or limiting the current carried by the DC line 208.
  • the control unit 240 may be arranged to control the current limiting apparatus 234 to limit the current of the DC line 208 at least partially based on a value of the parameter that is at least partially based on a predetermined protection voltage level V max of the cascaded cell 214, whereby an efficient limitation of the current is provided such that the voltage of each cascaded cell is prevented from exceeding a safe operating voltage.
  • the predetermined protection voltage level V max may be a maximum cell voltage level which may be a design characteristic of the cascaded cell 214 based on the configuration of each cascaded cell 214.
  • the predetermined protection voltage level V max may be the maximum voltage that the cascade cell can manage.
  • the control unit 240 may be arranged to control the current limiting apparatus 234 to limit the current of the DC line 208 at least partially based on a value of the parameter that is at least partially based on a safe operating voltage threshold V safe of each cascaded cell 214, whereby an efficient limitation of the current is provided such that the voltage of each cascaded cell is prevented from exceeding a safe operating voltage.
  • the safe operating voltage threshold V safe may be a threshold above which the converter 210 is tripped, i.e. shut down, to protect the converter 210 and its components and to avoid that the cascaded cell 214 reaches the predetermined protection voltage level V max .
  • the measuring equipment 242 may be arranged to determine or measure the voltage of each cascaded cell 214 for determining the safe operating voltage threshold Vsafe-
  • the control unit 240 may be arranged to control the current limiting apparatus 234 to limit the current of the DC line 208 at least partially based on a value of the parameter that is at least partially based on the difference between the safe operating voltage threshold V safe and the predetermined protection voltage level Vmax, whereby an efficient limitation of the current is provided such that the voltage of each cascaded cell is prevented from exceeding a safe operating voltage.
  • the control unit 240 may be arranged to control the current limiting apparatus 234 to limit the current of the DC line 208 at least partially based on the capacitance C of the cell capacitor 218 of the cascaded cell 214, whereby an efficient limitation of the current is provided such that the voltage of each cascaded cell is prevented from exceeding a safe operating voltage.
  • the control unit 240 may be arranged to control the current limiting apparatus 234 to limit the current of the DC line 208 so that the current of an arm, e.g. the positive arm 222, of the converter is given by the equation:
  • equation [1] ⁇ arm safe 1 ⁇ 4nax
  • Fig. 2 illustrates normal operation without any fault and the current i arm is carried through the positive arm 222 and through the three cascaded cells 214.
  • a fault e.g. a line fault, such as a short circuit
  • the converter 210, and also the positive arm 222 may operate in a so called blocked mode for a finite period of time.
  • Fig. 3 illustrates blocked mode of the positive arm 222 of the converter 210.
  • the converter 210 may essentially be a diode bridge, thereby allowing the AC side, i.e. the AC power system 212, to feed the DC fault.
  • This provides protection of the power semiconductor switches 226, 228 of the cascaded cell 214 during the fault transient and results in the AC side 212 feeding the DC fault through the diodes 230, 232 of each cascaded cell 214.
  • the state of each diode 230, 232 may be determined by the DC voltage, the AC voltage and the polarity of the AC current. In most cases the diodes 230, 323 are biased such that the AC side 212 feeds the DC side 206.
  • the inventors of the present invention have found that in some cases, the instantaneous voltages result in a condition where the path of current is through the cell capacitor 218 and a diode 232 of each cascaded cell 214. This results in uncontrolled charging of the cell capacitor 218.
  • a typical DC line fault e.g. a short circuit
  • a fault current also called surge current or over-current.
  • the fault current is illustrated by arrow A.
  • the current limiting apparatus 234, or any other current limiting and/or breaking apparatus may be arranged to limit and/or break the current carried by the DC line 208.
  • the inventors of the present invention have found that an uncontrolled opposite charging current is arises, i.e. opposite to the fault current A.
  • the opposite charging current is illustrated by arrow B.
  • the opposite charging current B charges the cell capacitor 218 of each cascaded cell 214 and may overcharge or overload the cell capacitor 218, resulting in tripping, i.e. shut down, of the entire converter 210.
  • the electrical energy dissipating means 236 dissipates some energy and thus limits the uncontrolled opposite charging current B.
  • the amount of energy to be dissipated in order to limit the opposite charging current B, after the fault has been cleared, is small in relation to amount of energy involved during the fault condition, i.e. when fault current A is to be limited.
  • Fig. 4 is a schematic graph illustrating the voltage of the cascaded cell when a DC fault occurs and after the DC fault has been cleared, when the present invention is not applied.
  • the voltage of the cascaded cell is illustrated by graph 301 and the arm current i arm of an arm 222 of converter 210 is illustrated by graph 302.
  • a DC fault e.g. a line fault, results in a fault current and in a transient current increase indicated by A.
  • the fault current A is taken care of and the fault is cleared by means of a current limiting and/or breaking apparatus, and the voltage of the cascaded cell is kept below a safe operating voltage.
  • Fig. 5 is a schematic graph illustrating the voltage of the cascaded cell when a DC fault occurs and after the DC fault has been cleared, when the present invention is applied.
  • the safe operating voltage threshold is indicated by 401 .
  • the voltage of the cascaded cell 214 when applying the present invention is illustrated by graph 403.
  • the voltage of the cascaded cell when the present invention is not applied is illus- trated by graph 404.
  • a DC fault results in a fault current and in a transient current increase indicated by A.
  • the fault current A is taken care of and the fault is cleared by means of the current limiting apparatus 234, or any other current limiting and/or breaking apparatus, and the voltage of the cascaded cell is kept at a level below the safe operating voltage threshold 401 .
  • the above-mentioned opposite charging current B arises, and for the case without the present invention, i.e. graph 404, the opposite charging current B overcharges the cell capacitor of each cascaded cell and the voltage of the cascaded cell increases greatly and exceeds the safe operating voltage threshold 401 , which results in tripping of the converter.
  • the present invention applied i.e.
  • the electrical energy dissipating means 236 of the current limiting apparatus 234 dissipates some energy and thus limits the uncontrolled opposite charging current B, whereby the voltage of each cascaded cell is prevented from exceeding the safe operating voltage threshold 401 .
  • the opposite charging current B does not deviate a lot from the cur- rent during normal operation and is much smaller than the fault current A.
  • the current limiting apparatus 234 is used to limit the current while the current is close to the current during normal operation.
  • Fig. 6 schematically illustrates aspects of method for limiting the current in an electrical power transmission system 204, e.g. as disclosed above in connec- tion with Fig. 1 , according to the present invention.
  • the at least one current limiting apparatus 234, which comprises the electrical energy dissipating means 236, is connected to the DC line 208, and the method comprises the step of limiting the current of the DC line 208 by means of the current limiting apparatus 234 and at least partially based on the above-mentioned value of a parameter corresponding to or related to the voltage capability of the cascaded cell 214 in order to prevent the voltage of the cascaded cell 214 from exceeding a safe operating voltage.
  • the step of limiting the current of the at least one DC line 208 may be performed after a fault in the direct current power transmission system 206 has been cleared.
  • the current of the at least one DC line 208 may be limited at least partially based on a value of the parameter that is at least partially based on the above-mentioned predetermined protection voltage level V max of each cascaded cell 214.
  • the current of the at least one DC line 208 may be limited at least partially based on a value of the parameter that is at least partially based on the above-mentioned safe operat- ing voltage threshold V sa fe Of each cascaded cell 214.
  • the current of the at least one DC line 208 may be limited at least partially based on a value of the parameter that is at least partially based on the difference between the safe operating voltage threshold V safe and the predetermined protection voltage level V max .
  • the current of the at least one DC line 208 may be limited at least partially based on the capacitance C of each cell capacitor 218 of the cascaded cell 214.
  • the method may comprise the step of determining setting of the at least one current limiting apparatus 234 at least partially based on said value of the parameter corresponding to or related to the voltage capability of the cascaded cell 234, and the step of setting the current limiting apparatus 234 to limit the current according to the determined setting.
  • the setting of the current limiting apparatus 234 may be determined at least partially based on a value of the parameter that is at least partially based on the above-mentioned predetermined protection voltage level V max of the cascaded cell 214.
  • the setting of the current limiting apparatus 234 may be determined at least partially based on a value of the parameter that is at least partially based on the above-mentioned safe operating voltage threshold V sa fe of the cascaded cell 214.
  • the setting of the current limiting apparatus 234 may be determined at least partially based on a value of the parameter that is at least partially based on the difference between the safe operating voltage threshold V sa fe and the predetermined protection voltage level V max .
  • the setting of the current limiting apparatus may be determined at least partially based on the capacitance C of the cell capacitor of the cascaded cell.
  • the stage of determining setting of the at least one current limiting apparatus 234 may comprise the following steps:
  • the voltage of each cascaded cell 214 is measured and/or determined, at step 601 .
  • the safe operating voltage threshold V safe of each cascaded cell 214 may be determined, at step 602.
  • the predetermined protection voltage level V max of each cascaded cell 214 is obtained at step 603.
  • the difference between the safe operating voltage threshold V safe and the predetermined protection voltage level V max is determined at step 604.
  • the capacitance C of the cell capacitor of each cascaded cell 214 is obtained at step 605.
  • a current reference i arm is calculated based on the following equation:
  • the setting of the current limiting apparatus 234 is determined so that the current i arm of an arm 222 of the converter 210 is given by the equation [1 ].
  • the current limiting apparatus 234 is set as mentioned above, and the current of the DC line 208 is limited by means of the current limiting apparatus 234 according to the determined setting, i.e. so that the current i arm will be carried by the arm 222. It is to be understood that some of the above-mentioned steps may be performed in a different order.
  • each current limiting apparatus 702 connected in series with the DC line 208, may comprise a parallel connection of a main breaker 706 and of electrical energy dissipating means 736, the main breaker 706 comprising at least one power semiconductor switch 708, which may be of a first current direction 710.
  • the electrical energy dissipating means 736 may be any of the electrical devices 238 mentioned above, e.g. non-linear resistor, such as an arrester.
  • Each current limiting apparatus 702 may comprise at least one mechanical switch 712 connected in parallel with the parallel connection.
  • Each current limiting apparatus 702 may comprise a series connection of the at least one mechanical switch 712 and of an auxiliary breaker 714.
  • the auxiliary breaker 714 comprises at least one power semiconductor switch 708, which may be of the first current direction 710.
  • the series connection is connected in parallel with the parallel connection.
  • the main breaker 706 may comprise a plurality of power semiconductor switches 708, which may be of the first current direction 710. In Fig. 7, each power semiconductor switch 708 is included in a base element 716.
  • the at least one mechanical switch 712 may be a high speed switch.
  • a high speed mechanical switch may be a mechanical switch that is arranged to switch to its conducting mode within 5 ms, or even within 4 ms.
  • the arrangement 202 may also comprise a reactor 718, e.g. a coil, connected in series with current limiting apparatus 702. However, the reactor 718 may be excluded from the arrangement.
  • the series connection of the at least one mechanical switch 712 and of the auxiliary breaker 714 may be excluded from the embodiment shown in Fig. 7.
  • the arrangement 202 may comprise a plurality of current limiting apparatuses 702 connected in series with one another.
  • the arrangement 801 may comprise a second series connection of at least one mechanical switch 712 and of an auxiliary breaker 714, the auxiliary breaker 714 of the second series connection comprising at least one power semiconductor switch 708.
  • the second series connection may be connected in parallel with a third series connection comprising a plurality of current limiting apparatuses 802, each apparatus 802 comprising a parallel connection of a main breaker 706 and of electrical energy dissipating means 736 as dis- closed in connection with Fig. 7.
  • the current of the at least one DC line 208 may be limited by means of one or a plurality of the current limiting apparatuses disclosed above.
  • Each current limiting apparatus 702, 802 of Figs. 7-9 may correspond to any of the embodiments of the device or the current limiting arrangement disclosed in WO-A1 -201 1/057675 which is hereby incorporated by reference.
  • each power semiconductor switch may comprise an Insulated Gate Bipolar Transistor, IGBT, or a Bi-Mode Insulated Gate Transistor, BiGT, or any other suitable power semiconductor switch.
  • each power semiconductor switch may comprise a thyristor, e.g. a gate turn-off thyristor, GTO, an Integrated Gate-Commutated Thyristor, IGCT, or a Forced Commutated Thyristor.
  • a thyristor e.g. a gate turn-off thyristor, GTO, an Integrated Gate-Commutated Thyristor, IGCT, or a Forced Commutated Thyristor.
  • GTO gate turn-off thyristor
  • IGCT Integrated Gate-Commutated Thyristor
  • Forced Commutated Thyristor e.g. a Forced Commutated Thyristor
  • the various components of the apparatus and the system of the present invention which are connected or connectable to one another or to other units, may be electrically connected, or connectable, to one another or to other units, e.g. via electrical conductors, e.g. busbars or DC lines, and/or may be indirectly connected, or connectable, e.g. electrically, to one another or to other units, e.g. via additional intermediate electric equipment or units located and con- nected/connectable between the components.

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Abstract

An arrangement (202; 801) and a method for limiting the current in an electrical power transmission system, the electrical power transmission system (204) comprising a direct current power transmission system (206) comprising at least one direct current transmission or distribution line (208) for carrying direct current, DC, and at least one converter (210) connected to the at least one DC transmission or distribution line and connectable to an alternating current power system (212), the converter comprising at least one cascaded cell (214, 216) and being arranged to convert alternating current to direct current for input to the at least one DC transmission or distribution line and/or direct current to alternating current for input to the alternating current power system, where each cascaded cell comprises at least one cell capacitor (218, 220), wherein at least one current limiting apparatus (234) is connected to the at least one direct current transmission or distribution line for limiting the current of the at least one direct current transmission or distribution line, the current limiting apparatus comprising electrical energy dissipating means (236), and wherein the method comprises the step of limiting (608) the current of the at least one direct current transmission or distribution line by means of the current limiting apparatus and at least partially based on a value of a parameter corresponding to or related to the voltage capability of the cascaded cell in order to prevent the voltage of the cascaded cell from exceeding a safe operating voltage. An electrical power transmission system (204) comprising the arrangement (202; 801).

Description

A METHOD AND AN ARRANGEMENT FOR LIMITING THE CURRENT IN AN ELECTRICAL POWER TRANSMISSION SYSTEM
Technical Field
The present invention relates to a method for limiting the current in an electrical power transmission system, the electrical power transmission system comprising a direct current power transmission system comprising at least one direct current transmission or distribution line for carrying direct current, DC, and at least one converter connected to the at least one DC transmission or distribution line and connectable to an alternating current power system, the converter com- prising at least one cascaded cell and being arranged to convert alternating current to direct current for input to the at least one DC transmission or distribution line and/or direct current to alternating current for input to the alternating current power system, where each cascaded cell comprises at least one cell capacitor. Further, the present invention relates to an arrangement for limiting the current in an electrical power transmission system of the above-mentioned sort. The present invention also relates to an electrical power transmission system comprising at least one arrangement of the above-mentioned sort.
Background of the Invention
An HVDC (high voltage direct current) power distribution network or an HVDC power transmission system uses direct current for the transmission of electrical power, in contrast to the more common AC system. For long-distance distribution, HVDC systems may be less expensive and may suffer lower electrical losses. In general, an HVDC power transmission system comprises at least one long-distance HVDC link or cable for carrying direct current a long distance, e.g. under sea, and converters, also called converter stations, for converting alternating current to direct current for input to the HVDC power transmission system and converters or converter stations for converting direct current back to alternating current.
However, in HVDC power transmission systems, it may be necessary to rapidly limit the current in the DC power transmission system and also quickly interrupt the current, since the alternating current power system is through a converter directly connected to the DC power transmission system, which may mean suddenly very high currents, transient currents, directly into the DC power trans- mission system, when for example a ground fault occurs there. Therefore, DC breakers have been provided in the DC power transmission system. Mechanical breakers in the DC power transmission system may often not be enough, since the time available for breaking the current may be too short for such a mechanical breaker. Thus, high speed DC breakers or current limiters have been developed.
US 7,408,755 B1 discloses a circuit for inrush/transient current limitation and overload/short circuit protection for a DC voltage power supply.
US 7,974,059 B2 describes a power supply apparatus comprising a DC power source that outputs DC voltage, a power supply circuit to which the DC voltage output from the DC power source is applied, and a current limiting unit connected in series between the DC power source and the power supply circuit.
US 6,624,993 B1 discloses a fault current limiting system for DC and pulsed circuit systems.
EP 1 526 624 A2 describes an assembly which rapidly identifies a short circuit in an alternating current or direct current power supply system and shuts down the system at once. The short circuit current flows through a high temperature super-conductor current limiter (HTS) in series with a switchgear station.
US 5,596,472 discloses an apparatus for controlling an over-current tripping device of a high-speed DC circuit breaker, at least partially based on an out- put signal generated by a current transformer.
JP 54137646 describes a protective circuit for DC current transformer.
WO 201 1/057675 A1 discloses a breaking device for breaking and limiting a direct current flowing through an HVDC power transmission or distribution line. The breaking device comprises a parallel connection of a main breaker and a non- linear resistor, the main breaker comprising at least one power semiconductor switch of a first current direction, and a series connection of a high speed switch and an auxiliary breaker, the high speed switch comprising at least one mechanical switch, and the auxiliary breaker has a smaller on-resistance than the main breaker and comprises at least one power semiconductor switch of the first current direction, where the series connection is connected in parallel to the parallel connection.
US 201 1/0235375 describes an apparatus having a converter connected to a DC voltage circuit via a short-circuit protection device arranged to limit the current in the DC voltage circuit. US 5,999,388 discloses a method and apparatus for limiting current in a DC network of a power transmission system, where at least one parallel connection of at least one semiconductor element of turn-off type and a surge diverter is provided in the DC network.
WO 201 1/012174 describes a converter with active fault current limitation for HVDC power transmission.
US 2009/0161277 discloses a method and device for preventing damage to a semiconductor switch circuit during a failure.
WO 201 1/060812 describes a DC voltage source converter for HVDC power transmission, where a voltage is provided to oppose the flow of current created by a fault in a DC network.
The Object of the Invention
A converter for converting alternating current to direct current for input to a direct current power transmission system and/or direct current to alternating cur- rent for input to an alternating current power system may comprise one or a plurality of cascaded cells. Each cascaded cell includes a cell capacitor and may in general include power semiconductor switches and diodes, as disclosed in more detail in the detailed description of embodiments. The inventors of the present invention have found that faults in the DC power system result in problems with un- controlled charging of the cell capacitor of the cascaded cell so that the cell capacitor may be overloaded and the converter needs to be shut down. When faults occur on the DC side, i.e. in the DC power system or network, the converter may operate in a so called blocked mode for a finite period of time. During the blocked mode the converter may essentially be a diode bridge, thereby allowing the AC side, i.e. the AC power system or network, to feed the DC fault. The collapse in the voltage on the DC side may result in uncontrolled charging of the cell capacitor of each cascaded cell. For longer duration DC faults, i.e. faults on the DC side, the over-voltage across the cascaded cell or cells result in tripping of the converter, i.e. the converter is shut down, because each cascaded cell exceeds a safe oper- ating voltage threshold.
The above-mentioned problems identified by the inventors of the present invention impair the electric power transmission in a DC power transmission system. Thus, the object of the present invention is to improve the electric power transmission in a DC power transmission system, especially in an HVDC power transmission system. It is also an object of the present invention to provide an improved control of the electric power transmission in a DC power transmission system. Another object of the present invention is to provide an improved electrical power transmission system including a DC power transmission system, especially an HVDC power transmission system.
Summary of the Invention
The above-mentioned object of the present invention is attained by providing a method for limiting the current in an electrical power transmission system, the electrical power transmission system comprising a direct current power trans- mission system comprising at least one direct current transmission or distribution line for carrying direct current, DC, and at least one converter connected to the at least one DC transmission or distribution line and connectable to an alternating current power system, the converter comprising at least one cascaded cell and being arranged to convert alternating current to direct current for input to the at least one DC transmission or distribution line and/or direct current to alternating current for input to the alternating current power system, where each cascaded cell comprises at least one cell capacitor, wherein at least one current limiting apparatus is connected to the at least one direct current transmission or distribution line for limiting the current of the at least one direct current transmission or distri- bution line, the current limiting apparatus comprising electrical energy dissipating means, and wherein the method comprises the step of limiting the current of the at least one direct current transmission or distribution line by means of the current limiting apparatus and at least partially based on a value of a parameter corresponding to or related to the voltage capability of the cascaded cell in order to pre- vent the voltage of the cascaded cell from exceeding a safe operating voltage.
By means of the method according to the present invention, the current is limited such that the voltage of each cascaded cell is prevented from exceeding a safe operating voltage, whereby a shut-down, or tripping, of the converter is avoided. Thus, interruption of power transmission is prevented. By means of the method according to the present invention, an improved control of the electric power transmission in a DC power transmission system is provided, and the electric power transmission in a DC power transmission system, especially in an HVDC power transmission system, is improved. By means of the apparatus according to the present invention, an improved electrical power transmission system including a DC power transmission system is provided. Advantageously, the step of limiting the current of the at least one direct current transmission or distribution line is performed after a fault in the DC power transmission system has been cleared. Advantageously, the method is adapted to limit the current in an HVDC power transmission system comprising at least one HVDC transmission or distribution line, or limit the current in an electrical power transmission system comprising the HVDC power transmission system.
Further, the above-mentioned objects of the present invention are also attained by an arrangement for limiting the current in an electrical power transmis- sion system, the electrical power transmission system comprising a direct current power transmission system comprising at least one direct current transmission or distribution line for carrying direct current, DC, and at least one converter connected to the at least one DC transmission or distribution line and connectable to an alternating current power system, the converter comprising at least one cas- caded cell and being arranged to convert alternating current to direct current for input to the at least one DC transmission or distribution line, and/or direct current to alternating current for input to the alternating current power system, where each cascaded cell comprises at least one cell capacitor, wherein the arrangement comprises at least one current limiting apparatus for limiting the current of the at least one direct current transmission or distribution line, the current limiting apparatus comprising electrical energy dissipating means, and wherein the arrangement comprises a control unit arranged to control the at least one current limiting apparatus to limit the current of the at least one direct current transmission or distribution line at least partially based on a value of a parameter corresponding to or related to the voltage capability of the cascaded cell in order to prevent the voltage of the cascaded cell from exceeding a safe operating voltage.
Advantageously, the arrangement is adapted to limit the current in an HVDC power transmission system comprising at least one HVDC transmission or distribution line, or limit the current in an electrical power transmission system comprising an HVDC power transmission system.
Positive technical effects of the arrangement according to the present invention, and its embodiments, correspond to the technical effects mentioned in connection with the method according to the present invention, and its embodiments, and vice versa. The arrangement may be connectable to the at least one cascaded cell. The arrangement may be connectable to the converter. The current limiting apparatus may be connectable to the at least one DC transmission or distribution line. The control unit may be connected to the at least one current limiting apparatus. The control unit may be connectable to the at least one cascaded cell. The arrangement may comprise the at least one cascaded cell.
The above-mentioned objects of the present invention are also attained by an electrical power transmission system comprising a direct current power transmission system, the direct current power transmission system comprising at least one direct current transmission or distribution line for carrying direct current, DC, and at least one converter connected to the at least one DC transmission or distribution line and connectable to an alternating current power system, the converter comprising at least one cascaded cell and being arranged to convert alternating current to direct current for input to the at least one DC transmission or distribution line, and/or direct current to alternating current for input to the alternating current power system, each cascaded cell comprises at least one cell capacitor, wherein the electrical power transmission system comprises at least one arrangement as claimed in any of the claims 13-21 for limiting the current in the electrical power transmission system, and/or at least one arrangement according to any of the em- bodiments of the arrangement disclosed below. Positive technical effects of the electrical power transmission system according to the present invention, and its embodiments, correspond to the technical effects mentioned in connection with the arrangement and the method according to the present invention, and their embodiments. The at least one DC transmission or distribution line may be one or a plurality of DC transmission or distribution lines. The at least one converter, which may also be called a converter station, may be one or a plurality of converters. The direct current power transmission system may be an HVDC power transmission system, and the at least one direct current transmission or distribution line may be an HVDC transmission or distribution line.
In general, High Voltage may be about 1 -1 .5 kV and above. However, for
HVDC applications and systems, High Voltage may be about 150 kV and above, e.g. 320 kV, 640 kV, 800 kV or 1000 kV, and above. The method, arrangement and/or the system according to the present invention are advantageously adapted for the above-mentioned HVDC voltage levels and above. The features and embodiments of the method, arrangement and electrical power transmission system, respectively, may be combined in various possible ways providing further advantageous embodiments.
Further advantageous embodiments of the method, apparatus and the electrical power transmission system according to the present invention and further advantages with the present invention emerge from the appended claims and the detailed description of embodiments.
Brief Description of the Drawings
The present invention will now be described, for exemplary purposes, in more detail by way of embodiments and with reference to the enclosed drawings, in which:
Fig. 1 is a schematic diagram illustrating aspects of the arrangement according to the present invention, where the arrangement is connected to a an electrical power transmission system;
Fig. 2 is a schematic representation of a positive arm of the converter of electrical power transmission system shown in Fig. 1 during normal operation;
Fig. 3 is a schematic representation of the positive arm of Fig. 2 in
blocked mode;
Fig. 4 is a schematic graph illustrating the voltage of the cascaded cell when a DC fault occurs and after the DC fault has been cleared, when the present invention is not applied;
Fig. 5 is a schematic graph illustrating the voltage of the cascaded cell when a DC fault occurs and after the DC fault has been cleared, when the present invention is applied;
Fig. 6 is a flow chart illustrating aspects of the method according to the present invention; and
Fig. 7-9 are schematic diagrams illustrating embodiments of the current limiting apparatus, each of which may be included in the ar- rangement according to the present invention or to each of which the method according to the present invention may be applied. Detailed Description of Preferred Embodiments
With reference to Fig. 1 , aspects of the arrangement 202 for limiting the current in an electrical power transmission system 204 according to the present invention are schematically illustrated. The electrical power transmission system 204 comprises a direct current power transmission system 206, e.g. a DC network, comprising at least one direct current transmission or distribution line 208, hereinafter called DC line 208, for carrying direct current, DC. The direct current power transmission system 206 may be an HVDC power transmission system. The DC line 208 may be an HVDC line. The DC line 208 may e.g. comprise a DC cable, busbar, or any other electrical conductor or electrically conductive element or means for carrying direct current. The electrical power transmission system 204 comprises at least one converter 210, which also may be called converter station, connected to the DC line 208. The converter 210 is connectable to an alternating current, AC, power system 212, e.g. an AC network. The converter 210 is ar- ranged to convert alternating current to direct current for input to the DC line 208 and/or direct current to alternating current for input to the AC power system 212. The converter 210 comprises at least one cascaded cell 214, 216. The converter 210 may comprise a plurality of cascaded cells 214, 216. Each cascaded cell 214, 216 comprises at least one cell capacitor 218, 220.
Each cascaded cell 214, 216 may be a cascaded half-bridge cell (also called Cascaded Two-Level, CTL, cell). Each cascaded cell 214, 216 may be a cascaded full-bridge cell. A cascaded half-bridge cell and a cascaded full-bridge cell per se and their structure are well known to the person skilled in the art and therefore not disclosed or discussed in more detail. As stated above, the converter 210 may comprise a plurality of cascaded cells, which may be half-bridge cells, or cascaded full-bridge cells, or a mixture thereof. The converter 210 may thus comprise a mixture of cascaded half-bridge cells and cascaded full-bridge cells. The plurality of cascaded cells may be interconnected to one another.
The converter 210 may comprise a plurality of arms 222, 224, e.g. one positive arm 222 and one negative arm 224. Each of the arms 222, 224 may comprise one or a plurality of cascaded cells 214, 216. In Fig. 1 , each arm 222, 224 comprises three interconnected cascaded cells 214, 216. However, each arm may comprise more or fewer cascaded cells. Each cascaded cell 214, 216 may com- prise one or a plurality of power semiconductor switches 226, 228. Each cascaded cell 214, 216 may comprise one or a plurality of diodes 230, 232.
The electrical power transmission system 204 may be adapted for single phase power or multi-phase power, e.g. three-phase power, and the components of the system and the arrangement 202 may be configured accordingly in ways known to the skilled person.
The arrangement 202 comprises at least one current limiting apparatus 234 for limiting the current of the at least one DC line 208. The current limiting apparatus 234 comprises electrical energy dissipating means 236 for dissipating electrical energy when current is applied to it. The electrical energy dissipating means 236 may be in the form of an electrical energy dissipating element. The electrical energy dissipating means 236, or element, may comprise at least one electrical device 238 of a group of electrical devices comprising a nonlinear resistor, an inductor and a linear resistor. Thus, the electrical energy dissipating means 236 may comprise nonlinear resistor, an inductor and/or a linear resistor. The nonlinear resistor may comprise a voltage-dependent nonlinear resistor, for example a surge arrester. The electrical energy dissipating means 236 may comprise a plurality of the above-mentioned electrical devices 238. Advantageously, the electrical energy dissipating means 236 or element comprises a voltage-dependent nonlin- ear resistor, e.g. surge arrester.
The arrangement 202 comprises a control unit 240 arranged to control the at least one current limiting apparatus 234 to limit the current of the at least one DC line 208. The control unit 240 may be in the form of control equipment. The control unit may comprise a CPU or a computer. The control unit 240 may be con- nected to the current limiting apparatus 234 and may be connectable or connected to each cascaded cell 214. The control unit 240 may be connectable or connected to the converter 210. The arrangement 202 may comprise measuring equipment 242 for determining the voltage of each cascaded cell 214. The control unit 240 may be connected to the measuring equipment 242. The measuring equipment 242 may be conventional equipment known to the person skilled in the art and may comprise conventional sensors or detectors. Based on signals from the sensors the measuring equipment 242 may determine the voltage of each cascaded cell 214, e.g. the voltage of each cell capacitor 218. It is to be understood that the control unit 240 and the measuring equipment 242 may be connected to the nega- tive arm 224 of the converter 210 and the cascaded cells 216 of the negative arm 224.
The control unit 240 is arranged to control the at least one current limiting apparatus 234 to limit the current of the at least one DC line 208 at least partially based on a value of a parameter corresponding to or related to the voltage capability of the cascaded cell 214 in order to prevent the voltage of the cascaded cell 214 from exceeding a safe operating voltage. The control unit 240 may be arranged to control the current limiting apparatus 234 to limit the current of the DC line 208 after a fault in the direct current power transmission system 206 has been cleared, e.g. cleared by means of the current limiting apparatus 234, or any other current limiting and/or breaking apparatus. The current limiting apparatus 234 may clear the fault by breaking and/or limiting the current carried by the DC line 208.
The control unit 240 may be arranged to control the current limiting apparatus 234 to limit the current of the DC line 208 at least partially based on a value of the parameter that is at least partially based on a predetermined protection voltage level Vmax of the cascaded cell 214, whereby an efficient limitation of the current is provided such that the voltage of each cascaded cell is prevented from exceeding a safe operating voltage. The predetermined protection voltage level Vmax may be a maximum cell voltage level which may be a design characteristic of the cascaded cell 214 based on the configuration of each cascaded cell 214. The predetermined protection voltage level Vmax may be the maximum voltage that the cascade cell can manage. The control unit 240 may be arranged to control the current limiting apparatus 234 to limit the current of the DC line 208 at least partially based on a value of the parameter that is at least partially based on a safe operating voltage threshold Vsafe of each cascaded cell 214, whereby an efficient limitation of the current is provided such that the voltage of each cascaded cell is prevented from exceeding a safe operating voltage. The safe operating voltage threshold Vsafe may be a threshold above which the converter 210 is tripped, i.e. shut down, to protect the converter 210 and its components and to avoid that the cascaded cell 214 reaches the predetermined protection voltage level Vmax. The measuring equipment 242 may be arranged to determine or measure the voltage of each cascaded cell 214 for determining the safe operating voltage threshold Vsafe- The control unit 240 may be arranged to control the current limiting apparatus 234 to limit the current of the DC line 208 at least partially based on a value of the parameter that is at least partially based on the difference between the safe operating voltage threshold Vsafe and the predetermined protection voltage level Vmax, whereby an efficient limitation of the current is provided such that the voltage of each cascaded cell is prevented from exceeding a safe operating voltage. The control unit 240 may be arranged to control the current limiting apparatus 234 to limit the current of the DC line 208 at least partially based on the capacitance C of the cell capacitor 218 of the cascaded cell 214, whereby an efficient limitation of the current is provided such that the voltage of each cascaded cell is prevented from exceeding a safe operating voltage. The control unit 240 may be arranged to control the current limiting apparatus 234 to limit the current of the DC line 208 so that the current of an arm, e.g. the positive arm 222, of the converter is given by the equation:
^arm safe ¼nax) Π]> where iarm is the current of an arm 222 of the converter 210. By applying the above-mentioned equation [1 ], an efficient limitation of the current is provided such that the voltage of each cascaded cell is prevented from exceeding a safe operating voltage. Further, equation [1] provides a dynamic variation of the setting of the current limiting apparatus 234.
With reference to Figs. 2-3, the operation of the positive arm 222 of the converter 210 of Fig. 1 is illustrated in more detailed. Fig. 2 illustrates normal operation without any fault and the current iarm is carried through the positive arm 222 and through the three cascaded cells 214. When a fault, e.g. a line fault, such as a short circuit, occurs on the DC side, i.e. in the DC power transmission system 206, the converter 210, and also the positive arm 222, may operate in a so called blocked mode for a finite period of time. Fig. 3 illustrates blocked mode of the positive arm 222 of the converter 210. During blocked mode the converter 210 may essentially be a diode bridge, thereby allowing the AC side, i.e. the AC power system 212, to feed the DC fault. This provides protection of the power semiconductor switches 226, 228 of the cascaded cell 214 during the fault transient and results in the AC side 212 feeding the DC fault through the diodes 230, 232 of each cascaded cell 214.The state of each diode 230, 232 may be determined by the DC voltage, the AC voltage and the polarity of the AC current. In most cases the diodes 230, 323 are biased such that the AC side 212 feeds the DC side 206. However, the inventors of the present invention have found that in some cases, the instantaneous voltages result in a condition where the path of current is through the cell capacitor 218 and a diode 232 of each cascaded cell 214. This results in uncontrolled charging of the cell capacitor 218.
A typical DC line fault, e.g. a short circuit, implies that a voltage decrease occurs and an increase in current occurs and produces a fault current, also called surge current or over-current. In Fig. 3, the fault current is illustrated by arrow A. When blocked mode of the converter 210 occurs, the current limiting apparatus 234, or any other current limiting and/or breaking apparatus, may be arranged to limit and/or break the current carried by the DC line 208. After the fault has been cleared, the inventors of the present invention have found that an uncontrolled opposite charging current is arises, i.e. opposite to the fault current A. In Fig. 3, the opposite charging current is illustrated by arrow B. The opposite charging current B charges the cell capacitor 218 of each cascaded cell 214 and may overcharge or overload the cell capacitor 218, resulting in tripping, i.e. shut down, of the entire converter 210. By means of the arrangement 202 of the present invention, the electrical energy dissipating means 236 dissipates some energy and thus limits the uncontrolled opposite charging current B. The amount of energy to be dissipated in order to limit the opposite charging current B, after the fault has been cleared, is small in relation to amount of energy involved during the fault condition, i.e. when fault current A is to be limited.
Fig. 4 is a schematic graph illustrating the voltage of the cascaded cell when a DC fault occurs and after the DC fault has been cleared, when the present invention is not applied. The voltage of the cascaded cell is illustrated by graph 301 and the arm current iarm of an arm 222 of converter 210 is illustrated by graph 302. A DC fault, e.g. a line fault, results in a fault current and in a transient current increase indicated by A. The fault current A is taken care of and the fault is cleared by means of a current limiting and/or breaking apparatus, and the voltage of the cascaded cell is kept below a safe operating voltage. After the DC fault has been cleared, the opposite charging current, indicated by B, arises and overcharges the cell capacitor of each cascaded cell, and the voltage of the cascaded cell in- creases greatly and exceeds the safe operating voltage threshold, resulting in tripping of the converter.
Fig. 5 is a schematic graph illustrating the voltage of the cascaded cell when a DC fault occurs and after the DC fault has been cleared, when the present invention is applied. The safe operating voltage threshold is indicated by 401 . The current carried by the DC line 208 and the current limiting apparatus 234, respectively, is illustrated by graph 402. The voltage of the cascaded cell 214 when applying the present invention is illustrated by graph 403. For illustrative purposes, the voltage of the cascaded cell when the present invention is not applied is illus- trated by graph 404. A DC fault results in a fault current and in a transient current increase indicated by A. The fault current A is taken care of and the fault is cleared by means of the current limiting apparatus 234, or any other current limiting and/or breaking apparatus, and the voltage of the cascaded cell is kept at a level below the safe operating voltage threshold 401 . However, after the DC fault has been cleared, the above-mentioned opposite charging current B arises, and for the case without the present invention, i.e. graph 404, the opposite charging current B overcharges the cell capacitor of each cascaded cell and the voltage of the cascaded cell increases greatly and exceeds the safe operating voltage threshold 401 , which results in tripping of the converter. For the case with the present invention applied, i.e. graph 403, the electrical energy dissipating means 236 of the current limiting apparatus 234 dissipates some energy and thus limits the uncontrolled opposite charging current B, whereby the voltage of each cascaded cell is prevented from exceeding the safe operating voltage threshold 401 . With reference to Fig. 4, it can be seen that the opposite charging current B does not deviate a lot from the cur- rent during normal operation and is much smaller than the fault current A. Thus, the current limiting apparatus 234 is used to limit the current while the current is close to the current during normal operation.
Fig. 6 schematically illustrates aspects of method for limiting the current in an electrical power transmission system 204, e.g. as disclosed above in connec- tion with Fig. 1 , according to the present invention. The at least one current limiting apparatus 234, which comprises the electrical energy dissipating means 236, is connected to the DC line 208, and the method comprises the step of limiting the current of the DC line 208 by means of the current limiting apparatus 234 and at least partially based on the above-mentioned value of a parameter corresponding to or related to the voltage capability of the cascaded cell 214 in order to prevent the voltage of the cascaded cell 214 from exceeding a safe operating voltage. The step of limiting the current of the at least one DC line 208 may be performed after a fault in the direct current power transmission system 206 has been cleared. The current of the at least one DC line 208 may be limited at least partially based on a value of the parameter that is at least partially based on the above-mentioned predetermined protection voltage level Vmax of each cascaded cell 214. The current of the at least one DC line 208 may be limited at least partially based on a value of the parameter that is at least partially based on the above-mentioned safe operat- ing voltage threshold Vsafe Of each cascaded cell 214. The current of the at least one DC line 208 may be limited at least partially based on a value of the parameter that is at least partially based on the difference between the safe operating voltage threshold Vsafe and the predetermined protection voltage level Vmax. The current of the at least one DC line 208 may be limited at least partially based on the capacitance C of each cell capacitor 218 of the cascaded cell 214.
The method may comprise the step of determining setting of the at least one current limiting apparatus 234 at least partially based on said value of the parameter corresponding to or related to the voltage capability of the cascaded cell 234, and the step of setting the current limiting apparatus 234 to limit the current according to the determined setting. The setting of the current limiting apparatus 234 may be determined at least partially based on a value of the parameter that is at least partially based on the above-mentioned predetermined protection voltage level Vmax of the cascaded cell 214. The setting of the current limiting apparatus 234 may be determined at least partially based on a value of the parameter that is at least partially based on the above-mentioned safe operating voltage threshold Vsafe of the cascaded cell 214. The setting of the current limiting apparatus 234 may be determined at least partially based on a value of the parameter that is at least partially based on the difference between the safe operating voltage threshold Vsafe and the predetermined protection voltage level Vmax. The setting of the current limiting apparatus may be determined at least partially based on the capacitance C of the cell capacitor of the cascaded cell.
With reference to Fig. 6, the stage of determining setting of the at least one current limiting apparatus 234 may comprise the following steps: The voltage of each cascaded cell 214 is measured and/or determined, at step 601 . Based on the voltage of each cascaded cell 214, the safe operating voltage threshold Vsafe of each cascaded cell 214 may be determined, at step 602. The predetermined protection voltage level Vmax of each cascaded cell 214 is obtained at step 603. The difference between the safe operating voltage threshold Vsafe and the predetermined protection voltage level Vmax is determined at step 604. The capacitance C of the cell capacitor of each cascaded cell 214 is obtained at step 605. At step 606, a current reference iarm is calculated based on the following equation:
Figure imgf000016_0001
At step 607, the setting of the current limiting apparatus 234 is determined so that the current iarm of an arm 222 of the converter 210 is given by the equation [1 ]. At step 608, the current limiting apparatus 234 is set as mentioned above, and the current of the DC line 208 is limited by means of the current limiting apparatus 234 according to the determined setting, i.e. so that the current iarm will be carried by the arm 222. It is to be understood that some of the above-mentioned steps may be performed in a different order.
Figs. 7 to 9, schematically illustrate various embodiments of the current limiting apparatus, each of which may be included in the arrangement according to the present invention or to each of which the method according to the present invention may be applied. With reference to Fig. 7, each current limiting apparatus 702, connected in series with the DC line 208, may comprise a parallel connection of a main breaker 706 and of electrical energy dissipating means 736, the main breaker 706 comprising at least one power semiconductor switch 708, which may be of a first current direction 710. The electrical energy dissipating means 736 may be any of the electrical devices 238 mentioned above, e.g. non-linear resistor, such as an arrester. Each current limiting apparatus 702 may comprise at least one mechanical switch 712 connected in parallel with the parallel connection. Each current limiting apparatus 702 may comprise a series connection of the at least one mechanical switch 712 and of an auxiliary breaker 714. The auxiliary breaker 714 comprises at least one power semiconductor switch 708, which may be of the first current direction 710. The series connection is connected in parallel with the parallel connection. The main breaker 706 may comprise a plurality of power semiconductor switches 708, which may be of the first current direction 710. In Fig. 7, each power semiconductor switch 708 is included in a base element 716. The at least one mechanical switch 712 may be a high speed switch. A high speed mechanical switch may be a mechanical switch that is arranged to switch to its conducting mode within 5 ms, or even within 4 ms. Further, the arrangement 202 may also comprise a reactor 718, e.g. a coil, connected in series with current limiting apparatus 702. However, the reactor 718 may be excluded from the arrangement. The series connection of the at least one mechanical switch 712 and of the auxiliary breaker 714 may be excluded from the embodiment shown in Fig. 7.
With reference to Fig. 8, the arrangement 202 may comprise a plurality of current limiting apparatuses 702 connected in series with one another.
With reference to Fig. 9, the arrangement 801 may comprise a second series connection of at least one mechanical switch 712 and of an auxiliary breaker 714, the auxiliary breaker 714 of the second series connection comprising at least one power semiconductor switch 708. The second series connection may be connected in parallel with a third series connection comprising a plurality of current limiting apparatuses 802, each apparatus 802 comprising a parallel connection of a main breaker 706 and of electrical energy dissipating means 736 as dis- closed in connection with Fig. 7.
According to embodiments of the method according the present invention, the current of the at least one DC line 208 may be limited by means of one or a plurality of the current limiting apparatuses disclosed above.
Each current limiting apparatus 702, 802 of Figs. 7-9 may correspond to any of the embodiments of the device or the current limiting arrangement disclosed in WO-A1 -201 1/057675 which is hereby incorporated by reference.
The inventors of the present invention have found that the above-mentioned embodiments of the current limiting apparatus and the arrangement, respectively, illustrated in Figs. 7-9 are advantageous for performing the method ac- cording to the present invention and for limiting the current in an electrical power transmission system in an efficient way. By means of the embodiments of Figs. 7- 9, and also the embodiment of Fig. 1 , the current may be efficiently limited such that the voltage of each cascaded cell is prevented from exceeding a safe operating voltage, whereby a shut-down, or tripping, of the converter is avoided. Each power semiconductor switch may comprise an Insulated Gate Bipolar Transistor, IGBT, or a Bi-Mode Insulated Gate Transistor, BiGT, or any other suitable power semiconductor switch. Alternatively, each power semiconductor switch may comprise a thyristor, e.g. a gate turn-off thyristor, GTO, an Integrated Gate-Commutated Thyristor, IGCT, or a Forced Commutated Thyristor. However, other suitable thyristors may also be used.
The various components of the apparatus and the system of the present invention, which are connected or connectable to one another or to other units, may be electrically connected, or connectable, to one another or to other units, e.g. via electrical conductors, e.g. busbars or DC lines, and/or may be indirectly connected, or connectable, e.g. electrically, to one another or to other units, e.g. via additional intermediate electric equipment or units located and con- nected/connectable between the components.
The features of the different embodiments of arrangement, the electrical power transmission system and the method disclosed above may be combined in various possible ways providing further advantageous embodiments.
The invention shall not be considered limited to the embodiments illustrated, but can be modified and altered in many ways by one skilled in the art, without departing from the scope of the appended claims.

Claims

1 . A method for limiting the current in an electrical power transmission system (204), the electrical power transmission system comprising a direct current power transmission system (206) comprising at least one direct current transmission or distribution line (208) for carrying direct current, DC, and at least one converter (210) connected to the at least one DC transmission or distribution line and connectable to an alternating current power system (212), the converter comprising at least one cascaded cell (214, 216) and being arranged to convert alternating current to direct current for input to the at least one DC transmission or distribution line and/or direct current to alternating current for input to the alternating current power system, where each cascaded cell comprises at least one cell capacitor (218, 220), wherein at least one current limiting apparatus (234) is connected to the at least one direct current transmission or distribution line for limiting the cur- rent of the at least one direct current transmission or distribution line, the current limiting apparatus comprising electrical energy dissipating means (236), and wherein the method comprises the step of limiting (608) the current of the at least one direct current transmission or distribution line by means of the current limiting apparatus and at least partially based on a value of a parameter corresponding to or related to the voltage capability of the cascaded cell in order to prevent the voltage of the cascaded cell from exceeding a safe operating voltage.
2. A method according to claim 1 , ch aracterized in that the step of limiting the current (608) of the at least one direct current transmission or distribution line (208) is performed after a fault in the direct current power transmission system (206) has been cleared.
3. A method according to claim 1 or 2, characterized in that the current of the at least one direct current transmission or distribution line (208) is limited at least partially based on a value of the parameter that is at least partially based on a predetermined protection voltage level (Vmax) of the cascaded cell (214).
4. A method according to claim 3, ch aracterized in that the current of the at least one direct current transmission or distribution line (208) is limited at least partially based on a value of the parameter that is at least partially based on a safe operating voltage threshold Vsafe of the cascaded cell (214).
5. A method according to claim 4, characterized in that the the current of the at least one direct current transmission or distribution line (208) is limited at least partially based on a value of the parameter that is at least partially based on the difference between the safe operating voltage threshold {Vsafe) and the predetermined protection voltage level (Vmax).
6. A method according to claim 4 or 5, characterized in that the current of the at least one direct current transmission or distribution line (208) is limited at least partially based on the capacitance (C) of the cell capacitor (218) of the cascaded cell (214).
7. A method according to any of the claims 1 or 6, characterized in that the step of limiting (608) the current of the at least one direct current transmission or distribution line (208) comprises
the step of determining setting (607) of the at least one current limiting apparatus (234) at least partially based on a value of the parameter corresponding to or related to the voltage capability of the cascaded cell (214), and
the step of setting (608) the current limiting apparatus (234) to limit the current according to the determined setting.
8. A method according to claim 7, characterized in that the setting of the current limiting apparatus (234) is determined at least partially based on a value of the parameter that is at least partially based on a predetermined protection voltage level (Vmax) of the cascaded cell (214).
9. A method according to claim 8, characterized in that the setting of the current limiting apparatus (234) is determined at least partially based on a value of the parameter that is at least partially based on a safe operating voltage threshold {Vsafe) of the cascaded cell (214).
10. A method according to claim 9, ch aracterized in that the setting of the current limiting apparatus (234) is determined at least partially based on a value of the parameter that is at least partially based on the difference between the safe operating voltage threshold {Vsafe) and the predetermined protection voltage level
1 1 . A method according to claim 9 or 10, characterized in that the setting of the current limiting apparatus (234) is determined at least partially based on the capacitance (C) of the cell capacitor (218) of the cascaded cell (214).
12. A method according to claim 6 or 1 1 , characterized in that the current of the at least one direct current transmission or distribution line (208) is limited or the setting of the current limiting apparatus is determined so that the current of an arm of the converter is given by the equation: i-arm ~ ^ (Ysafe K ax) >
where iarm is the current of an arm of the converter, C is the capacitance of the cell capacitor, Vsafe is the safe operating voltage threshold of the cascaded cell, and Vmax is the predetermined protection voltage level of the cascaded cell.
13. An arrangement (202) for limiting the current in an electrical power transmission system (204), the electrical power transmission system comprising a direct current power transmission system (206) comprising at least one direct current transmission or distribution line (208) for carrying direct current, DC, and at least one converter (210) connected to the at least one DC transmission or distri- bution line and connectable to an alternating current power system (212), the converter comprising at least one cascaded cell (214, 216) and being arranged to convert alternating current to direct current for input to the at least one DC transmission or distribution line and/or direct current to alternating current for input to the alternating current power system, where each cascaded cell comprises at least one cell capacitor (218, 220), wherein the arrangement comprises at least one current limiting apparatus (234) for limiting the current of the at least one direct current transmission or distribution line, the current limiting apparatus comprising electrical energy dissipating means (236), and wherein the arrangement com- prises a control unit (240) arranged to control the at least one current limiting apparatus to limit the current of the at least one direct current transmission or distribution line at least partially based on a value of a parameter corresponding to or related to the voltage capability of the cascaded cell in order to prevent the voltage of the cascaded cell from exceeding a safe operating voltage.
14. An arrangement according to claim 13, characterized in that the control unit (240) is arranged to control the current limiting apparatus (234) to limit the current of the at least one direct current transmission or distribution line (208) after a fault in the direct current power transmission system (206) has been cleared.
15. An arrangement according to claim 13 or 14, characterized in that the control unit (240) is arranged to control the current limiting apparatus (234) to limit the current of the at least one direct current transmission or distribution line (208) at least partially based on a value of the parameter that is at least partially based on a predetermined protection voltage level (Vmax) of the cascaded cell (214).
16. An arrangement according to claim 15, characterized in that the control unit (240) is arranged to control the current limiting apparatus (234) to limit the cur- rent of the at least one direct current transmission or distribution line (208) at least partially based on a value of the parameter that is at least partially based on a safe operating voltage threshold (Vsafe) of the cascaded cell (214).
17. An arrangement according to claim 16, characterized in that the control unit (240) is arranged to control the current limiting apparatus (234) to limit the current of the at least one direct current transmission or distribution line (208) at least partially based on a value of the parameter that is at least partially based on the difference between the safe operating voltage threshold {Vsafe) and the predetermined protection voltage level (Vmax).
18. An arrangement according to claim 16 or 17, characterized in that the control unit (249) is arranged to control the current limiting apparatus (234) to limit the current of the at least one direct current transmission or distribution line (208) at least partially based on the capacitance (C) of the cell capacitor (218) of the cascaded cell (214).
19. An arrangement according to claim 18, characterized in that the control unit (240) is arranged to control the current limiting apparatus (234) to limit the current of the at least one direct current transmission or distribution line (208) so that the current of an arm of the converter is given by the equation: i-arm ~ ^ (Ysafe K ax) >
where iarm is the current of an arm of the converter, C is the capacitance of the cell capacitor, Vsafe is the safe operating voltage threshold of the cascaded cell, and Vmax is the predetermined protection voltage level of the cascaded cell.
20. An arrangement according to any of the claims 13 to 19, characterized in that the electrical energy dissipating means (236) comprise an electrical device (238) of a group of electrical devices comprising a nonlinear resistor, an inductor and a linear resistor.
21 . An arrangement according to claim 20, characterized in that the nonlinear resistor comprises a voltage-dependent nonlinear resistor, for example a surge arrester.
22. A method according to any of the claims 1 to 12, characterized in that the current of the at least one direct current transmission or distribution line is limited by means of the current limiting apparatus included in the arrangement as claimed in any of the claims 13 to 21 .
23. An electrical power transmission system (204) comprising a direct current power transmission system (206), the direct current power transmission system comprising at least one direct current transmission or distribution line (208) for car- rying direct current, DC, and at least one converter (210) connected to the at least one DC transmission or distribution line and connectable to an alternating current power system (212), the converter comprising at least one cascaded cell (214, 216) and being arranged to convert alternating current to direct current for input to the at least one DC transmission or distribution line and/or direct current to alternating current for input to the alternating current power system, each cascaded cell comprises at least one cell capacitor (218, 220), wherein the electrical power transmission system comprises at least one arrangement (202; 801 ) as claimed in any of the claims 13 to 21 for limiting the current in the electrical power transmission system.
PCT/EP2012/060620 2012-06-05 2012-06-05 A method and an arrangement for limiting the current in an electrical power transmission system WO2013182231A1 (en)

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