WO2012037967A1 - An apparatus for controlling the electric power transmission in a hvdc power transmission system - Google Patents

An apparatus for controlling the electric power transmission in a hvdc power transmission system Download PDF

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Publication number
WO2012037967A1
WO2012037967A1 PCT/EP2010/063882 EP2010063882W WO2012037967A1 WO 2012037967 A1 WO2012037967 A1 WO 2012037967A1 EP 2010063882 W EP2010063882 W EP 2010063882W WO 2012037967 A1 WO2012037967 A1 WO 2012037967A1
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WO
WIPO (PCT)
Prior art keywords
converter
hvdc
transmission line
hvdc transmission
direct current
Prior art date
Application number
PCT/EP2010/063882
Other languages
French (fr)
Inventor
Kalpesh Hirjibhai Bhalodi
Subhasish Mukherjee
Tomas U Jonsson
Sasitharan Subramanian
Original Assignee
Abb Technology Ag
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 Ag filed Critical Abb Technology Ag
Priority to PCT/EP2010/063882 priority Critical patent/WO2012037967A1/en
Publication of WO2012037967A1 publication Critical patent/WO2012037967A1/en

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Classifications

    • 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
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J1/00—Circuit arrangements for dc mains or dc distribution networks
    • H02J1/08—Three-wire systems; Systems having more than three wires
    • 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
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33569—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
    • H02M3/33576—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer
    • H02M3/33584—Bidirectional converters
    • 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 an apparatus for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising a plurality of HVDC transmission lines for carrying direct current, DC. Further, the present invention relates to a HVDC power transmission system comprising a plurality of HVDC transmission lines for carrying direct current, and a plurality of converter stations connected to the HVDC transmission lines, each of the converter stations being adapted to convert alternating current, AC, to direct current for input to the HVDC transmission lines, and/or direct current to alternating current, the system comprising an apparatus for controlling the electric power transmission in the system.
  • a HVDC power distribution network or a HVDC power transmission system uses direct current for the transmission of electrical power, in contrast to the more common AC systems. For long-distance distribution, HVDC systems may be less expensive and may suffer lower electrical losses.
  • a 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 converter stations for converting alternating current to direct current for input to the HVDC power transmission system and converter stations for converting direct current back to alternating current.
  • US-B2-6,788,033 and US-A-5, 734,258 disclose DC to DC conversion and relate to stationary or portable systems powered by a DC battery, and to electric vehicles.
  • US-B2-6, 914,420 describes a power converter for converting power between a first and a second voltage, and relates to electric vehicles.
  • US-B2-7, 518,266 discloses an AC power transmission system, where a DC transmission ring is used, utilizing controllable AC-DC converters in a multi-in- feed/out-feed arrangement.
  • each of the converter stations may be controlled, e.g. by controlling the DC node voltage of each converter station.
  • the inventors of the present invention have found that the DC node voltage control of the converter stations, or the control of shunt connected converter DC voltages of a DC grid, may not be sufficient in order to avoid or reduce load- flow congestion of the system.
  • the object of the present invention is to improve the electric power transmission in a 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 HVDC power transmission system. A further object of the present invention is to avoid, reduce or prevent load-flow congestion in the system. Another object of the present invention is to provide an improved HVDC power transmission system.
  • an apparatus for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising a plurality of HVDC transmission lines for carrying direct current, DC
  • the apparatus comprises a first converter for converting alternating current, AC, to direct current and/or direct current to alternating current, and a second converter for converting direct current to alternating current and/or alternating current to direct current, each of the first and second converters having an AC side for output and/or input of alternating current and a DC side for output and/or input of direct current, wherein the AC side of the first converter is connected to the AC side of the second converter, the first converter is connectable via its DC side to a first HVDC transmission line of said plurality of HVDC transmission lines, the second con- verter is connectable via its DC side to a second HVDC transmission line of said plurality of HVDC transmission lines, and wherein the apparatus is adapted to control the direct current of the first HVDC transmission line
  • the electric power transmission in a HVDC power transmission system and the control thereof are efficiently improved, and load-flow congestion in the system may be avoided, reduced or prevented.
  • the AC side of the second converter may be adapted to pro- vide, directly or indirectly, alternating current to the AC side of the first converter, and/or vice versa.
  • the apparatus of the present invention is especially advantageous and efficient for a HVDC power transmission system of the sort shown in Fig. 1 , which may be called a DC grid concept, where the system comprises several HVDC transmission lines for carrying direct current and several converter stations connected to the HVDC transmission lines.
  • the apparatus of the present invention is especially advantageous when the control of DC node voltage of the converter stations, or the control of shunt connected converter DC voltages of a DC grid, is not sufficient.
  • the direct current of the HVDC transmission line, to which the apparatus is connected can be increased or reduced in order to control the power transmission.
  • the direct current control is attained by the apparatus' introduction, or injection, of a DC voltage in series with the first HVDC transmission line.
  • the injected DC voltage produces a fictive resistance, ARinj.
  • the fictive resistance provides an active power extraction or output from the first HVDC transmission line when the fictive resistance corresponds to an increase in resistance, i.e. a positive AR mj , (since a resistance consumes power/energy), or an active power input to the first HVDC transmission line when the fictive resistance corresponds to a decrease in resistance, i.e. a negative AR in j.
  • a positive AR inj is produced when the apparatus introduces a positive DC voltage in series with the first HVDC transmission line
  • a negative AR mj is produced when the apparatus introduces a negative DC voltage in series with the first HVDC transmission line.
  • the load of the first HVDC transmission line, to which the apparatus is connected may be reduced or increased.
  • the apparatus' active power extraction or output from the first HVDC transmission line results in a decrease in direct current of the line
  • the apparatus' active power input to the first HVDC transmission line results in an increase in direct current of the line.
  • the apparatus of the present invention is adapted to regulate the voltage at its output to control the current flow in the first HVDC transmission line.
  • the apparatus according to the present invention is adapted to control the direct current of the first HVDC transmission line by intro- ducing a fictive resistance in series with the first HVDC transmission line by introducing a DC voltage in series with the first HVDC transmission line.
  • the apparatus is adapted to transfer power from the first HVDC transmission line to the second HVDC transmission line and to transfer power from the second HVDC transmission line to the first HVDC transmission line.
  • a HVDC power transmission system e.g. a
  • DC grid system may reverse, and therefore, voltage polarity reversal for maintained fictive resistance is required, which is also attained by the apparatus of the present invention.
  • the apparatus of the present invention has the capability to operate in all the four quadrants, which is discussed in more detail in the de- tailed description of preferred embodiments.
  • the various components of the apparatus of the present invention 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 connect- able, e.g. electrically or inductively, via additional intermediate electric equipment or units located and connected/connectable between the components, e.g. a transformer, another converter etc.
  • High Voltage may be about 1 -1 .5 kV and above. However, for HVDC applications and systems, High Voltage may be about 500 kV and above, e.g. 800 kV or 1 000 kV, and above.
  • the apparatus and/or the system according to the present invention are advantageously adapted for the above-mentioned HVDC voltage levels and above.
  • the voltage rating of the apparatus may be 1 -5 % of the HVDC transmission line voltage.
  • the apparatus comprises control means for controlling the apparatus, wherein the control means are adapted to control the apparatus to introduce a positive DC voltage in series with the first HVDC transmission line for reducing the direct current of the first HVDC transmission line, and wherein the control means are adapted to control the apparatus to introduce a negative DC voltage in series with the first HVDC transmission line for increasing the direct current of the first HVDC transmission line.
  • the control means may be in form of a control unit and may be connectable to the HVDC power transmission system, e.g. to the first HVDC transmission line.
  • the control means may comprise a computer and/or a CPU.
  • control means may be adapted to control the apparatus to introduce a positive fictive resistance in series with the first HVDC transmission line by introducing a positive DC voltage in series with first the HVDC transmission line for reducing the direct current of the first HVDC transmission line
  • control means may be adapted to control the apparatus to introduce a negative fictive resistance in series with the first HVDC transmission line by introducing a negative DC voltage in series with the first HVDC transmission line for increasing the direct current of the first HVDC transmission line.
  • the apparatus comprises measuring means for measuring the DC load flow congestion of the HVDC power transmission system, and the measuring means are adapted to communicate with the control means.
  • the measuring means may be adapted to measure the direct current or direct voltage of the first HVDC transmission line, and the measuring means per se may have a structure known the person skilled in the art.
  • the measuring means, or measuring equipment may comprise conventional sensors, e.g. sensors for measuring direct current or direct voltage.
  • the apparatus comprises a first bypass switch connectable to the first HVDC transmission line and connected in parallel with the first converter, and when closed the first bypass switch is adapted to conduct the direct current of the first HVDC transmission line to electrically bypass the first converter.
  • the first bypass switch, the first converter, and the apparatus may be bypassed during fault conditions, whereby the electric power transmission in a HVDC power transmission system and the control thereof are further improved.
  • the apparatus comprises a second bypass switch connectable to the second HVDC transmission line and connected in parallel with the second converter, and when closed the second bypass switch is adapted to conduct the direct current of the second HVDC transmission line to electrically bypass the second converter.
  • the second bypass switch, the second converter, and the apparatus may be bypassed during fault conditions, whereby the electric power transmission in a HVDC power transmission system and the control thereof are further improved.
  • the apparatus comprises an electric power trans- former connected between the first and the second converter, and each of the first and second converters is connected via its AC side to the electric power transformer.
  • the electric power transformer By the electric power transformer, the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof are improved.
  • the electric power transformer may also take part in fulfilling the voltage requirements of the apparatus.
  • the electric power transformer may be in the form of a high frequency electric power transformer.
  • the electric power transformer is adapted to isolate the first converter from the second converter.
  • the first HVDC trans- mission line, to which the apparatus is connected is also efficiently isolated from the second HVDC transmission line, and the second HVDC transmission line is also efficiently isolated from the first HVDC transmission line.
  • the electric power transformer is adapted to isolate the first converter from the second HVDC transmission line.
  • the electric power transformer is adapted to isolate the second converter from the first HVDC transmission line.
  • the first converter comprises a full-bridge converter.
  • the inventors of the present invention have found that this structure of the first converter further improves the flexibility and efficiency of the electric power trans- mission in a HVDC power transmission system and the control thereof.
  • the first converter may comprise a full-bridge converter with a bypass switch.
  • the second converter comprises a full-bridge con- verter.
  • the inventors of the present invention have found that this structure of the second converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
  • the second converter may comprise a full-bridge converter with a bypass switch.
  • the first converter comprises four pairs of electronic control switches.
  • the electronic control switches may be connected to one another.
  • the inventors of the present invention have found that this structure of the first converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
  • the first converter may also comprise a fifth pair of electronic control switches.
  • Each electronic control switch of the fifth pair may comprise a transistor.
  • the fifth pair of electronic control switches may be connected in parallel with the four pairs of electronic control switches.
  • the first converter comprises filter means for smoothing out the voltage and current ripple caused by the switching of the electronic control switches.
  • the filter means may be connected to the electronic control switches.
  • the filter means, or filter components may comprise a capacitor and an inductor.
  • the capacitor may be connected in parallel with the electronic control switches.
  • the inductor may be connected in series with the electronic control switches.
  • the second converter comprises four pairs of electronic control switches.
  • the electronic control switches of the second converter may be connected to one another.
  • the inventors of the present invention have found that this structure of the second converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
  • the second converter may also comprise a fifth pair of electronic control switches.
  • Each electronic control switch of the second converter's fifth pair may comprise a transistor.
  • the second converter's fifth pair of electronic control switches may be connected in parallel with the second converter's four pairs of electronic control switches.
  • the second converter comprises filter means for smoothing out the voltage and current ripple caused by the switching of the elec- tronic control switches.
  • the second converter's filter means may be connected to the electronic control switches of the second converter.
  • the second converter's filter means, or filter components may comprise a capacitor and an inductor.
  • the second converter's capacitor may be connected in parallel with the second converter's electronic control switches.
  • the second converter's inductor may be connected in series with the second converter's electronic control switches.
  • each electronic control switch comprises a transistor, e.g. an Insulated Gate Bipolar Transistor, IGBT, or a Bi-Mode Insulated Gate Transistor, BIGT, or any other suitable transistor.
  • each electronic control switch may also comprise 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.
  • other suitable thyristors may also be used. The inventors of the present invention have found that these structures of first and/or second converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
  • the second converter is adapted to convert DC voltage to high frequency AC voltage.
  • the electric power transformer may be a high frequency transformer.
  • the first converter is adapted to convert DC volt- age to high frequency AC voltage.
  • the electric power transformer may be a high frequency transformer.
  • the first converter is connectable in series with the first HVDC transmission line.
  • the second converter is connectable in series with the second HVDC transmission line.
  • the apparatus is adapted to control the direct current of the second HVDC transmission line by introducing a DC voltage in series with the second HVDC transmission line.
  • the apparatus may be adapted to control the direct current of the second HVDC transmission in corresponding ways as dis- closed above for the control of the direct current of the first HVDC transmission.
  • the apparatus is adapted for four quadrant operation. Aspects of the four quadrant operation are disclosed in the detailed description of preferred embodiments.
  • the apparatus may be adapted for one quadrant operation, two quadrant operation or three quadrant operation, where the quadrant operation/-s may be any of the first to fourth quadrant opera- tions e.g. as disclosed in the detailed description of preferred embodiments.
  • the one, two or three quadrant operation may be attained by replacing suitable
  • IGBT/IGBTs with diode/diodes of a four quadrant converter.
  • a high voltage direct current, HVDC, power transmission system com- prising a plurality of HVDC transmission lines for carrying direct current, DC, and a plurality of converter stations connected to the HVDC transmission lines, each of the converter stations being adapted to convert alternating current, AC, to direct current for input to the HVDC transmission lines, and/or direct current to alternat- ing current
  • the system comprises at least one apparatus as claimed in any of the claims 1 -23 for controlling the electric power transmission in the system, and/or at least one apparatus according to any of the above-mentioned embodiments of the apparatus.
  • a plurality of HVDC transmission lines or converter stations may be two or more HVDC transmission lines or converter stations, respectively.
  • the at least one apparatus may be one or a plurality of apparatuses, e.g. two or more apparatuses.
  • a plurality of apparatuses may be connected to the same HVDC transmission line, or to different HVDC transmission lines. For example, two apparatuses adapted for two quadrant operation may be connected to the same HVDC transmission line to attain four quadrant operation.
  • the system comprises at least three converter stations.
  • the system comprises at least four converter stations, or at least five converter stations.
  • the HVDC transmission lines comprise at least one long-distance HVDC link or cable.
  • the HVDC transmission lines may comprise at least two long-distance HVDC links or cables.
  • Fig. 1 is a schematic block diagram illustrating aspects of the HVDC power transmission system and aspects of the apparatus according to the present invention
  • Fig. 2A is schematic block diagram illustrating a first embodiment of a converter station shown in Fig. 1 ;
  • Fig. 2B is schematic block diagram illustrating a second embodiment of a converter station shown in Fig. 1 ;
  • Fig. 3 is a schematic block diagram illustrating a first embodiment of the apparatus according to the present invention.
  • Fig 4 is a schematic diagram illustrating aspects of the apparatus of
  • Fig. 5 is a schematic diagram illustrating the four quadrant operation of the apparatus of Fig. 4;
  • Fig. 6 is a schematic diagram illustrating an equivalent circuit for the first quadrant operation of the first converter of Fig. 4;
  • Fig. 7 is a schematic diagram illustrating an equivalent circuit for the first quadrant operation of the first converter and for the second quadrant operation of the second converter of Fig. 4;
  • Fig. 8 is a schematic graph illustrating the first quadrant operation of the first converter and the second quadrant operation of the second converter of Fig. 4, where the direction of the direct current of the second HVDC transmission line is from position C to D;
  • Fig. 9 is a schematic graph illustrating the first quadrant operation of the first converter and the fourth quadrant operation of the second converter of Fig. 4, where the direction of the direct cur- rent of the second HVDC transmission line is from position D to C;
  • Fig. 10 is a schematic graph illustrating the second quadrant operation of the first converter and the first quadrant operation of the second converter of Fig. 4, where the direction of the direct current of the second HVDC transmission line is from position C to D;
  • Fig. 1 1 is a schematic diagram illustrating a second embodiment and further aspects of the apparatus according to the present invention.
  • Figs. 12A and 12B are schematic diagrams illustrating alternative electronic control switches.
  • Fig.1 schematically shows aspects of the HVDC power transmission system and aspects of the apparatus 302, 602 according to the present invention.
  • the HVDC power transmission system comprising a plurality of HVDC transmission lines 102, 104, 106, 108, 1 10, 1 12, 1 14 for carrying direct current.
  • the HVDC transmission lines may e.g. comprise HVDC cables, busbars, or other DC conductors.
  • the HVDC transmission lines may comprise at least one long-distance HVDC link.
  • a first and a second HVDC transmission line 102, 108 of said plurality of HVDC transmission lines are in the form of long-distance HVDC links 102, 108.
  • the HVDC power transmission system comprises a plurality of converter stations 1 16, 1 18, 120, 122, 124 electrically con- nected to the HVDC transmission lines 102, 104, 1 06, 108, 1 10, 1 12, 1 14.
  • five converter stations 1 16, 1 18, 120, 122, 124 are provided, but there may be more or fewer converter stations.
  • Each of the converter stations may be adapted to convert alternating current to direct current for input to the transmission lines and convert direct current to alternating current for input to neighbouring AC sys- terns.
  • Each converter station 1 16, 1 18, 1 20, 122, 124 may be electrically connected to a conventional electric power transformer 126, 128, 130, 132, 134 in conventional ways known to the skilled person. Electric power transformers and their function are well known to the person skilled in the art and therefore not discussed in more detail.
  • Each converter station which may be called a DC Grid converter station, may have asymmetrical monopoles with separate converters for positive and negative polarity, as illustrated in Fig. 2A.
  • each converter station may be in the form of a balanced bipolar converter, as illustrated in Fig. 2B.
  • the alternatives of Figs. 2A and 2B may also be combined in the same system.
  • the apparatus 302, 602 is adapted to be electrically connected to the HVDC system, e.g. by being connected between positions A and B and between positions C and D as illustrated in Fig. 1 .
  • Rn ne i of the first HVDC transmission line 102 in Fig. 1 illustrates the resistance of the first line 102.
  • l DC i in Fig. 1 is the direct current through the first line 102, i.e. the direct current carried by the first line 102
  • bc2 in Fig. 1 is the direct current through the second line 108, i.e.
  • the HVDC power transmission system may be adapted for single phase power or multi-phase power, e.g. three-phase power, and the components of the system and the appa- ratus may be configured accordingly in ways known to the skilled person.
  • the HVDC power transmission system comprises an embodiment of the apparatus 302, 602 for controlling the electric power transmission in the system according to the present invention, and aspects of the apparatus 302, 602 will hereinafter be disclosed.
  • the apparatus 302, 602 may comprise a first bypass switch 136 (see Fig. 1 ) electrically connectable to the first HVDC transmission line 102, to which the apparatus 302, 602 is connected, and electrically connected in parallel with a first converter 304, 604 (see Figs.
  • the apparatus 302, 602 may comprise a second bypass switch 137 (see Fig. 1 ) electrically connectable to the second HVDC transmission line 108, to which the apparatus 302, 602 is connected, and electrically connected in parallel with a sec- ond converter 306, 606 (see Figs. 3, 4 and 1 1 ) of the apparatus 302, 602.
  • the second bypass switch 137 When the second bypass switch 137 is closed, it is adapted to conduct the direct current of the second HVDC transmission line 108 to electrically bypass the second converter 306, 606.
  • the first and the second bypass switch 136, 137 the first and the second converter 304, 306, 604, 606, respectively, and the apparatus 302, 602 may be bypassed during fault conditions.
  • Fig. 3 schematically shows a first embodiment of the apparatus according to the present invention for controlling the electric power transmission in a HVDC power transmission system, e.g. as shown in Fig. 1
  • the apparatus comprises a first converter 304 for converting alternating current to direct current and/or direct current to alternating current, and a second converter 306 for converting direct current to alternating current and/or alternating current to direct current.
  • Each of the first and second converters 304, 306 has an AC side 308, 310 for output and/or input of alternating current and a DC side 312, 314 for output and/or input of direct current.
  • the first converter 304 is electrically connectable via its DC side 312 to the first HVDC transmission line 102, and the first converter 304 may be electrically connectable in series with the first HVDC transmission line 102.
  • the AC side 308 of the first converter 304 is adapted to provide alternating current to the AC side 310 of the second converter 306, and vice versa.
  • the second converter 306 is connectable via its DC side 314 to the second HVDC transmission line 108, and the second converter 306 may be electrically connectable in series with the second HVDC transmission line 108.
  • the apparatus 302 may comprise an electric power transformer 318, also indicated as T x in Fig. 4, connected between the first and second converters 304, 306, each of the first and second converters 304, 306 being electrically connect- able, or connected, via its AC side 308, 310 to the electric power transformer 318.
  • the electric power transformer 318 may be a high frequency transformer.
  • the second converter 306 may be adapted to convert DC voltage to high frequency AC voltage
  • the first converter 304 may be adapted to convert DC voltage to high frequency AC voltage.
  • the electric power transformer 318 may be adapted to isolate the first converter 304 from the second converter 306, and may thus be adapted to isolate the first converter 304 from the second HVDC transmission line 108 and to isolate the second converter 306 from the first HVDC transmission line 102. Consequently, the electric power transformer 318 may be adapted to isolate the first HVDC transmission line 102 from the second HVDC transmission line 108.
  • the apparatus 302 is adapted to control the direct current foci of the first HVDC transmission line 102 by introducing a DC voltage V / ⁇ or V inj1 in series with the first HVDC transmission line 102.
  • the apparatus 302 may comprise control means 324, e.g. a computer or CPU, for controlling the apparatus and its various components.
  • the control means 324 are adapted to control the apparatus 302 to introduce a positive DC voltage, V lnj1 > 0, in series with the first HVDC transmission line 102 for reducing the direct current, i.e.
  • the apparatus 302 may be adapted to control the direct current of the second HVDC transmission line 108 by introducing a DC voltage V ⁇ in series with the second HVDC transmission line 108.
  • the apparatus 302 may be adapted to transfer power, more precisely active power, from the first HVDC transmission line 102 to the second HVDC transmission line 108 and to transfer power from the second HVDC transmission line 108 to the first HVDC transmission line 102.
  • the first converter 304 may comprise a full-bridge converter.
  • the first converter 304 may comprise four pairs 402, 404, 406, 408, also indicated as S S'r, S ⁇ S' 2, S ⁇ S e , S S'4 in Fig. 4, of electrically interconnected electronic control switches 410, 412.
  • the first converter 304 may also comprise a fifth pair 434 of electronic control switches 431 , 433, also indicated as SAB/S AB ' ⁇ Fig. 4.
  • the fifth pair 434 of electronic control switches 431 , 433 may be electrically connected in parallel with the four pairs 402, 404, 406, 408 of electronic control switches.
  • the fifth pair 434 of electronic control switches may be used to give a path to the direct current when the first converter 304 is bypassed to give zero voltage.
  • the first converter 304 may comprise filter means 426, 428, connected to the electronic control switches 41 0, 41 2, for smoothing out the voltage and current ripple caused by the switching of the electronic control switches 41 0, 41 2.
  • the filter means may comprise a capacitor 426, also indicated as C ⁇ in Fig. 4, and an inductor 428, also indicated as L f .
  • the capacitor 426 may be connected in parallel with the electronic control switches 41 0, 41 2.
  • the inductor 428 may be electrically connected in series with the electronic control switches 41 0, 412.
  • the capacitor 426 may be connected in parallel with the fifth pair 434 of electronic control switches.
  • the second converter 306 may comprise a full-bridge converter.
  • the sec- ond converter 306 may comprise four pairs 414, 41 6, 418, 420, also indicated as S5/S ' 5, Se/S'e , S S , Ss/S's in Fig. 4, of electrically interconnected electronic control switches 422, 424.
  • the second converter 306 may also comprise a fifth pair 436 of electronic control switches 438, 440, also indicated as SCD S'CD ' W Fig. 4.
  • the second converter's 306 fifth pair 436 of electronic control switches 438, 440 may be electrically connected in parallel with the second converter's 306 four pairs 414, 416, 418, 420 of electronic control switches.
  • the second converter's 306 fifth pair 436 of electronic control switches may be used to give a path to the direct current when the second converter 306 is bypassed to give zero voltage.
  • the second converter 306 may comprise filter means 430, 432, connected to the second con- verter's 306 electronic control switches 422, 424, for smoothing out the voltage and current ripple caused by the switching of the second converter's 306 electronic control switches 422, 424.
  • the second converter's 306 filter means may comprise a capacitor 430, also indicated as C ⁇ in Fig. 4, and an inductor 432, also indicated as L f ⁇ n Fig. 4.
  • the second converter's 306 capacitor 430 may be con- nected in parallel with the second converter's 306 electronic control switches 422, 424.
  • the second converter's 306 inductor 432 may be electrically connected in series with the second converter's 306 electronic control switches 422, 424.
  • the second converter's 306 capacitor 430 may be connected in parallel with the second converter's 306 fifth pair 436 of electronic control switches 438, 440.
  • the four quadrant operation of the apparatus may be supported by bidirectional valves. By introducing PWM switching, the injected voltage V in ji may be regulated to a desired value or level in an efficient way. PWM switching per se is well known to the skilled person and is thus not discussed in further detail.
  • the power requirement of first converter 304 is supplied from the second converter 306 via the electric power transformer 318, and vice versa.
  • active power should be absorbed by the first converter 304 and fed to the second HVDC transmission line 108 by the second converter 306.
  • active power should be injected in series with the first HVDC transmission line 102, the active power for injection being obtained from the second HVDC transmission line 108.
  • the apparatus 302 can operate in all the four quadrants as shown in Fig. 5, the voltage and current polarity being as shown in Fig. 1 , 3 or 4.
  • the first converter 304 corresponds to "Converter 1”
  • the second converter 306 corresponds to "Converter 2”.
  • the equivalent circuit for the first quadrant operation of the first converter 304 is illustrated in Fig. 6, where Rn ne2 in Fig. 6 illustrates the resistance of the second HVDC transmission line 108 of Figs. 3 and 4.
  • the direct current I dd of the first HVDC transmission line 102 is flowing from position A to position Y (see Fig. 1 ). Since the voltage/potential in position A is greater than in position Y, the switches
  • the second converter 306 may be possible only if
  • the second converter 306 has to be operated according to the second or the fourth quadrant depending on the current direction, i.e. if the first converter 304 "acts" as "positive” resistance, the second converter 306 will be at "negative” resistance.
  • the first converter 304 may always have an impact on the power flow of the second HVDC transmission line 108.
  • This topology is mostly suitable for the re- duction of power flow of one HVDC transmission line and the increase of power flow of another HVDC transmission line.
  • both of the HVDC line currents l dc i and l dc2 should be equal and the injected voltages are equal with reverse polarity, as illustrated by the following expression:
  • the same voltage with reverse polarity is injected in the second converter 306, Vjnji.
  • the power flow of the first HVDC transmission line 102 is controlled by the apparatus 302 with a change in power flow in the second HVDC transmission line 108:
  • the second quadrant operation of the first converter 304 may be possible only if
  • S 3 , S 4 , S 5 , S 6 are turned ON during positive half cycle and S l , S 2 , S 7 , S 8 are turned ON during negative half cycle.
  • the bypass paths S ⁇ and S C D may be used to achieve zero voltage across the positions A-B.
  • the voltage across positions A-B ( V AB ) may be regulated by PWM operation as shown in Fig. 1 0.
  • the PWM voltage may be averaged by the filter means and injected in series with the first HVDC line 102.
  • the voltage 1 ⁇ 2 3 ⁇ 4s will be more negative and a "negative" re- sistance, i.e. a decrease in resistance, is introduced into the first HVDC line 102.
  • Voltage control at the second converter 306 may not possible since it may fully depend on the operation of the first converter 304.
  • the second converter injected voltage V inj2 may be equal and of reverse polarity in relation to the first converter injected voltage V inj1 . If the first converter 304 is in the second quadrant, the second converter 306 should be operated according to the first or the third quadrant to satisfy the power balance equation.
  • the direct current of the second HVDC transmission line 108 is in the direction from C to D (/dc2>0), it will be the first quadrant, and if the direct current of the second HVDC transmission line 108 is in the direction from D to C (/dc2 ⁇ 0), it will be the third quadrant.
  • the first and second converters 604, 606 may be symmetrical on both sides.
  • the transformer 618 may be connected on the outer side of the converters 604, 606, and the HVDC transmission line 102, 108 may be connected between two leg midpoints as shown in Fig. 1 1 , which illustrates a second embodiment and further aspects of the apparatus 602 according to the present invention.
  • the components T X , L F , d, SAB, S'AB, SCD and S'CD of the apparatuses 602 may correspond to the corresponding components T x , L f , d, SAB, S'AB, SCD and S'CD of Fig.
  • the components Si-S 8 and S'rS'e of the apparatus 602 may correspond to the components S S s and S'rS'e of Fig. 4 as disclosed above.
  • the interconnection of the components T x , L f , Cf, Cdc, SAB, S'AB, SCD,S'CD SI-S 8 and S'i-S' 8 of the apparatus 602 is schematically illustrated in Fig. 1 1 .
  • the apparatus 602 of Fig 1 1 may be adapted to be connected to the HVDC transmission lines as disclosed for the embodiment shown in Figs. 3 and 4.
  • a pair of anti-parallel transistors e.g. IGBT
  • a pair of anti-series transistors e.g. IGBT or BIGT, as shown in Figs. 12A and 12B may be used.
  • the advantage of the anti-series connection is that reverse blocking transistors are not required.
  • Each of the above-mentioned electronic control switches may comprises a transistor, e.g. an IGBT, a BIGT or any other suitable transistor.
  • each of the above-mentioned electronic control switches may comprise a thyristor, e.g. a GTO, an IGCT, or a Forced Commutated Thyristor.

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Abstract

An apparatus (302; 602) for controlling the electric power transmission in a high voltage direct current, power transmission system comprising a plurality of HVDC transmission lines (102, 104, 106, 108, 110, 112, 114) for carrying direct current, wherein the apparatus (302) comprises a first converter (304) and a second converter (306). Each of the first and second converters having an AC side (308, 310) for output and/or input of alternating current and a DC side (312, 314) for output and/or input of direct current. The AC side of the first converter is connected to the AC side of the second converter, the first converter is connectable via its DC side to a first HVDC transmission line (102) and the second converter is connectable via its DC side to a second HVDC transmission line (108). The apparatus is adapted to control the direct current of the first HVDC transmission line (102) by introducing a DC voltage in series with the first HVDC transmission line (102).

Description

AN APPARATUS FOR CONTROLLING THE ELECTRIC POWER
TRANSMISSION IN A HVDC POWER TRANSMISSION SYSTEM
Technical Field
The present invention relates to an apparatus for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising a plurality of HVDC transmission lines for carrying direct current, DC. Further, the present invention relates to a HVDC power transmission system comprising a plurality of HVDC transmission lines for carrying direct current, and a plurality of converter stations connected to the HVDC transmission lines, each of the converter stations being adapted to convert alternating current, AC, to direct current for input to the HVDC transmission lines, and/or direct current to alternating current, the system comprising an apparatus for controlling the electric power transmission in the system.
Background of the Invention
A HVDC power distribution network or a HVDC power transmission system uses direct current for the transmission of electrical power, in contrast to the more common AC systems. For long-distance distribution, HVDC systems may be less expensive and may suffer lower electrical losses. In general, a 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 converter stations for converting alternating current to direct current for input to the HVDC power transmission system and converter stations for converting direct current back to alternating current.
US-B2-6,788,033 and US-A-5, 734,258 disclose DC to DC conversion and relate to stationary or portable systems powered by a DC battery, and to electric vehicles. US-B2-6, 914,420 describes a power converter for converting power between a first and a second voltage, and relates to electric vehicles.
US-B2-7, 518,266 discloses an AC power transmission system, where a DC transmission ring is used, utilizing controllable AC-DC converters in a multi-in- feed/out-feed arrangement.
US 3,694,728 describes a HVDC mesh-operated network comprising several interconnected stations for effecting an exchange of power by means of converters located at the stations and which are connected to AC networks. The Object of the Invention
To control the electric power transmission in a HVDC power transmission system comprising at least one HVDC line and a plurality of converter stations for converting between alternating current and direct current in order to avoid or re- duce DC load-flow congestion in the system, each of the converter stations may be controlled, e.g. by controlling the DC node voltage of each converter station. However, the inventors of the present invention have found that the DC node voltage control of the converter stations, or the control of shunt connected converter DC voltages of a DC grid, may not be sufficient in order to avoid or reduce load- flow congestion of the system.
The object of the present invention is to improve the electric power transmission in a 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 HVDC power transmission system. A further object of the present invention is to avoid, reduce or prevent load-flow congestion in the system. Another object of the present invention is to provide an improved HVDC power transmission system.
Summary of the Invention
The above-mentioned objects of the present invention are attained by providing an apparatus for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising a plurality of HVDC transmission lines for carrying direct current, DC, wherein the apparatus comprises a first converter for converting alternating current, AC, to direct current and/or direct current to alternating current, and a second converter for converting direct current to alternating current and/or alternating current to direct current, each of the first and second converters having an AC side for output and/or input of alternating current and a DC side for output and/or input of direct current, wherein the AC side of the first converter is connected to the AC side of the second converter, the first converter is connectable via its DC side to a first HVDC transmission line of said plurality of HVDC transmission lines, the second con- verter is connectable via its DC side to a second HVDC transmission line of said plurality of HVDC transmission lines, and wherein the apparatus is adapted to control the direct current of the first HVDC transmission line by introducing a DC voltage in series with the first HVDC transmission line. By the innovative apparatus of the present invention, the electric power transmission in a HVDC power transmission system and the control thereof are efficiently improved, and load-flow congestion in the system may be avoided, reduced or prevented. The AC side of the second converter may be adapted to pro- vide, directly or indirectly, alternating current to the AC side of the first converter, and/or vice versa.
The apparatus of the present invention is especially advantageous and efficient for a HVDC power transmission system of the sort shown in Fig. 1 , which may be called a DC grid concept, where the system comprises several HVDC transmission lines for carrying direct current and several converter stations connected to the HVDC transmission lines. The apparatus of the present invention is especially advantageous when the control of DC node voltage of the converter stations, or the control of shunt connected converter DC voltages of a DC grid, is not sufficient. By the apparatus of the present invention, the direct current of the HVDC transmission line, to which the apparatus is connected, can be increased or reduced in order to control the power transmission. The direct current control is attained by the apparatus' introduction, or injection, of a DC voltage in series with the first HVDC transmission line. The injected DC voltage produces a fictive resistance, ARinj. The fictive resistance provides an active power extraction or output from the first HVDC transmission line when the fictive resistance corresponds to an increase in resistance, i.e. a positive ARmj, (since a resistance consumes power/energy), or an active power input to the first HVDC transmission line when the fictive resistance corresponds to a decrease in resistance, i.e. a negative ARinj. A positive ARinj is produced when the apparatus introduces a positive DC voltage in series with the first HVDC transmission line, and a negative ARmj is produced when the apparatus introduces a negative DC voltage in series with the first HVDC transmission line. Thus, by the apparatus of the present invention, the load of the first HVDC transmission line, to which the apparatus is connected, may be reduced or increased. The apparatus' active power extraction or output from the first HVDC transmission line results in a decrease in direct current of the line, and the apparatus' active power input to the first HVDC transmission line results in an increase in direct current of the line. By the increase and decrease in direct current of the first HVDC transmission line, the power transmission is controlled and load- flow congestion may be avoided, reduced or prevented. Thus, the apparatus of the present invention is adapted to regulate the voltage at its output to control the current flow in the first HVDC transmission line.
In alternative words, the apparatus according to the present invention is adapted to control the direct current of the first HVDC transmission line by intro- ducing a fictive resistance in series with the first HVDC transmission line by introducing a DC voltage in series with the first HVDC transmission line.
To effect or introduce a positive fictive resistance, +ARinj, active power should be absorbed by the second HVDC transmission line, and to effect or introduce a negative fictive resistance, -ARmj, active power should be injected by and from the second HVDC transmission line. According to an advantageous embodiment of the apparatus according to the present invention, the apparatus is adapted to transfer power from the first HVDC transmission line to the second HVDC transmission line and to transfer power from the second HVDC transmission line to the first HVDC transmission line.
Further, the direct current in a HVDC power transmission system, e.g. a
DC grid system, may reverse, and therefore, voltage polarity reversal for maintained fictive resistance is required, which is also attained by the apparatus of the present invention. Further, the apparatus of the present invention has the capability to operate in all the four quadrants, which is discussed in more detail in the de- tailed description of preferred embodiments.
The various components of the apparatus 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 connect- able, e.g. electrically or inductively, via additional intermediate electric equipment or units located and connected/connectable between the components, e.g. a transformer, another converter etc.
In general, High Voltage may be about 1 -1 .5 kV and above. However, for HVDC applications and systems, High Voltage may be about 500 kV and above, e.g. 800 kV or 1 000 kV, and above. The apparatus and/or the system according to the present invention are advantageously adapted for the above-mentioned HVDC voltage levels and above. The voltage rating of the apparatus may be 1 -5 % of the HVDC transmission line voltage. According to another advantageous embodiment of the apparatus according to the present invention, the apparatus comprises control means for controlling the apparatus, wherein the control means are adapted to control the apparatus to introduce a positive DC voltage in series with the first HVDC transmission line for reducing the direct current of the first HVDC transmission line, and wherein the control means are adapted to control the apparatus to introduce a negative DC voltage in series with the first HVDC transmission line for increasing the direct current of the first HVDC transmission line. By the control means of this embodiment, the current flow in the first HVDC transmission line is efficiently controlled. The control means may be in form of a control unit and may be connectable to the HVDC power transmission system, e.g. to the first HVDC transmission line. The control means may comprise a computer and/or a CPU. In alternative words, the control means may be adapted to control the apparatus to introduce a positive fictive resistance in series with the first HVDC transmission line by introducing a positive DC voltage in series with first the HVDC transmission line for reducing the direct current of the first HVDC transmission line, and the control means may be adapted to control the apparatus to introduce a negative fictive resistance in series with the first HVDC transmission line by introducing a negative DC voltage in series with the first HVDC transmission line for increasing the direct current of the first HVDC transmission line.
According to a further advantageous embodiment of the apparatus according to the present invention, the apparatus comprises measuring means for measuring the DC load flow congestion of the HVDC power transmission system, and the measuring means are adapted to communicate with the control means. The measuring means may be adapted to measure the direct current or direct voltage of the first HVDC transmission line, and the measuring means per se may have a structure known the person skilled in the art. The measuring means, or measuring equipment, may comprise conventional sensors, e.g. sensors for measuring direct current or direct voltage.
According to another advantageous embodiment of the apparatus according to the present invention, the apparatus comprises a first bypass switch connectable to the first HVDC transmission line and connected in parallel with the first converter, and when closed the first bypass switch is adapted to conduct the direct current of the first HVDC transmission line to electrically bypass the first converter. By the first bypass switch, the first converter, and the apparatus, may be bypassed during fault conditions, whereby the electric power transmission in a HVDC power transmission system and the control thereof are further improved.
According to yet another advantageous embodiment of the apparatus ac- cording to the present invention, the apparatus comprises a second bypass switch connectable to the second HVDC transmission line and connected in parallel with the second converter, and when closed the second bypass switch is adapted to conduct the direct current of the second HVDC transmission line to electrically bypass the second converter. By the second bypass switch, the second converter, and the apparatus, may be bypassed during fault conditions, whereby the electric power transmission in a HVDC power transmission system and the control thereof are further improved.
According to still another advantageous embodiment of the apparatus according to the present invention, the apparatus comprises an electric power trans- former connected between the first and the second converter, and each of the first and second converters is connected via its AC side to the electric power transformer. By the electric power transformer, the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof are improved. The electric power transformer may also take part in fulfilling the voltage requirements of the apparatus. The electric power transformer may be in the form of a high frequency electric power transformer.
According to an advantageous embodiment of the apparatus according to the present invention, the electric power transformer is adapted to isolate the first converter from the second converter. By this embodiment, the first HVDC trans- mission line, to which the apparatus is connected, is also efficiently isolated from the second HVDC transmission line, and the second HVDC transmission line is also efficiently isolated from the first HVDC transmission line. Advantageously, the electric power transformer is adapted to isolate the first converter from the second HVDC transmission line. Advantageously, the electric power transformer is adapted to isolate the second converter from the first HVDC transmission line.
According to a further advantageous embodiment of the apparatus according to the present invention, the first converter comprises a full-bridge converter. The inventors of the present invention have found that this structure of the first converter further improves the flexibility and efficiency of the electric power trans- mission in a HVDC power transmission system and the control thereof. The first converter may comprise a full-bridge converter with a bypass switch.
According to another advantageous embodiment of the apparatus according to the present invention, the second converter comprises a full-bridge con- verter. The inventors of the present invention have found that this structure of the second converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof. The second converter may comprise a full-bridge converter with a bypass switch.
According to yet another advantageous embodiment of the apparatus ac- cording to the present invention, the first converter comprises four pairs of electronic control switches. The electronic control switches may be connected to one another. The inventors of the present invention have found that this structure of the first converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof. Ad- vantageously, the first converter may also comprise a fifth pair of electronic control switches. Each electronic control switch of the fifth pair may comprise a transistor. The fifth pair of electronic control switches may be connected in parallel with the four pairs of electronic control switches.
According to still another advantageous embodiment of the apparatus ac- cording to the present invention, the first converter comprises filter means for smoothing out the voltage and current ripple caused by the switching of the electronic control switches. The filter means may be connected to the electronic control switches. By smoothing out the voltage and current ripple, a further improved control of the electric power transmission is attained. The filter means, or filter components, may comprise a capacitor and an inductor. The capacitor may be connected in parallel with the electronic control switches. The inductor may be connected in series with the electronic control switches. By the above-mentioned embodiments of the filter means, a further improved control of the power transmission is provided.
According to an advantageous embodiment of the apparatus according to the present invention, the second converter comprises four pairs of electronic control switches. The electronic control switches of the second converter may be connected to one another. The inventors of the present invention have found that this structure of the second converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof. Advantageously, the second converter may also comprise a fifth pair of electronic control switches. Each electronic control switch of the second converter's fifth pair may comprise a transistor. The second converter's fifth pair of electronic control switches may be connected in parallel with the second converter's four pairs of electronic control switches.
According to a further advantageous embodiment of the apparatus according to the present invention, the second converter comprises filter means for smoothing out the voltage and current ripple caused by the switching of the elec- tronic control switches. The second converter's filter means may be connected to the electronic control switches of the second converter. By smoothing out the voltage and current ripple, a further improved control of the electric power transmission is attained. The second converter's filter means, or filter components, may comprise a capacitor and an inductor. The second converter's capacitor may be connected in parallel with the second converter's electronic control switches. The second converter's inductor may be connected in series with the second converter's electronic control switches. By the above-mentioned embodiments of the second converter's filter means, a further improved control of the power transmission is provided.
According to another advantageous embodiment of the apparatus according to the present invention, each electronic control switch comprises a transistor, e.g. an Insulated Gate Bipolar Transistor, IGBT, or a Bi-Mode Insulated Gate Transistor, BIGT, or any other suitable transistor. Alternatively, each electronic control switch may also 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 inventors of the present invention have found that these structures of first and/or second converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
According to yet another advantageous embodiment of the apparatus according to the present invention, the second converter is adapted to convert DC voltage to high frequency AC voltage. Advantageously, the electric power transformer may be a high frequency transformer. By this embodiment, the electric power transmission in a HVDC power transmission system and the control thereof are further improved.
According to still another advantageous embodiment of the apparatus according to the present invention, the first converter is adapted to convert DC volt- age to high frequency AC voltage. Advantageously, the electric power transformer may be a high frequency transformer. By this embodiment, the electric power transmission in a HVDC power transmission system and the control thereof are further improved.
According to an advantageous embodiment of the apparatus according to the present invention, the first converter is connectable in series with the first HVDC transmission line.
According to a further advantageous embodiment of the apparatus according to the present invention, the second converter is connectable in series with the second HVDC transmission line.
According to another advantageous embodiment of the apparatus according to the present invention, the apparatus is adapted to control the direct current of the second HVDC transmission line by introducing a DC voltage in series with the second HVDC transmission line. The apparatus may be adapted to control the direct current of the second HVDC transmission in corresponding ways as dis- closed above for the control of the direct current of the first HVDC transmission. By this embodiment, the electric power transmission in a HVDC power transmission system and the control thereof are further improved, and load-flow congestion in the system may be avoided, reduced or prevented.
According to yet another advantageous embodiment of the apparatus ac- cording to the present invention, the apparatus is adapted for four quadrant operation. Aspects of the four quadrant operation are disclosed in the detailed description of preferred embodiments. Advantageously, the apparatus may be adapted for one quadrant operation, two quadrant operation or three quadrant operation, where the quadrant operation/-s may be any of the first to fourth quadrant opera- tions e.g. as disclosed in the detailed description of preferred embodiments. The one, two or three quadrant operation may be attained by replacing suitable
IGBT/IGBTs with diode/diodes of a four quadrant converter.
The above-mentioned objects of the present invention are also attained by providing a high voltage direct current, HVDC, power transmission system com- prising a plurality of HVDC transmission lines for carrying direct current, DC, and a plurality of converter stations connected to the HVDC transmission lines, each of the converter stations being adapted to convert alternating current, AC, to direct current for input to the HVDC transmission lines, and/or direct current to alternat- ing current, wherein the system comprises at least one apparatus as claimed in any of the claims 1 -23 for controlling the electric power transmission in the system, and/or at least one apparatus according to any of the above-mentioned embodiments of the apparatus. Positive technical effects of the HVDC power transmission system according to the present invention, and its embodiments, correspond to the above-mentioned technical effects mentioned in connection with the apparatus according to the present invention, and its embodiments. A plurality of HVDC transmission lines or converter stations may be two or more HVDC transmission lines or converter stations, respectively. The at least one apparatus may be one or a plurality of apparatuses, e.g. two or more apparatuses. A plurality of apparatuses may be connected to the same HVDC transmission line, or to different HVDC transmission lines. For example, two apparatuses adapted for two quadrant operation may be connected to the same HVDC transmission line to attain four quadrant operation.
According to an advantageous embodiment of the HVDC power transmis- sion system according to the present invention, the system comprises at least three converter stations. Advantageously, the system comprises at least four converter stations, or at least five converter stations.
According to a further advantageous embodiment of the HVDC power transmission system according to the present invention, the HVDC transmission lines comprise at least one long-distance HVDC link or cable. Advantageously, the HVDC transmission lines may comprise at least two long-distance HVDC links or cables.
The above-mentioned embodiments and features of the apparatus and the HVDC power transmission system, respectively, according to the present invention may be combined in various possible ways providing further advantageous embodiments.
Further advantageous embodiments of the apparatus and the HVDC power transmission system, respectively, according to the present invention and further advantages with the present invention emerge from 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 block diagram illustrating aspects of the HVDC power transmission system and aspects of the apparatus according to the present invention;
Fig. 2A is schematic block diagram illustrating a first embodiment of a converter station shown in Fig. 1 ;
Fig. 2B is schematic block diagram illustrating a second embodiment of a converter station shown in Fig. 1 ;
Fig. 3 is a schematic block diagram illustrating a first embodiment of the apparatus according to the present invention;
Fig 4 is a schematic diagram illustrating aspects of the apparatus of
Fig. 3 in more detail;
Fig. 5 is a schematic diagram illustrating the four quadrant operation of the apparatus of Fig. 4;
Fig. 6 is a schematic diagram illustrating an equivalent circuit for the first quadrant operation of the first converter of Fig. 4;
Fig. 7 is a schematic diagram illustrating an equivalent circuit for the first quadrant operation of the first converter and for the second quadrant operation of the second converter of Fig. 4;
Fig. 8 is a schematic graph illustrating the first quadrant operation of the first converter and the second quadrant operation of the second converter of Fig. 4, where the direction of the direct current of the second HVDC transmission line is from position C to D;
Fig. 9 is a schematic graph illustrating the first quadrant operation of the first converter and the fourth quadrant operation of the second converter of Fig. 4, where the direction of the direct cur- rent of the second HVDC transmission line is from position D to C;
Fig. 10 is a schematic graph illustrating the second quadrant operation of the first converter and the first quadrant operation of the second converter of Fig. 4, where the direction of the direct current of the second HVDC transmission line is from position C to D;
Fig. 1 1 is a schematic diagram illustrating a second embodiment and further aspects of the apparatus according to the present invention; and
Figs. 12A and 12B are schematic diagrams illustrating alternative electronic control switches.
Detailed Description of Preferred Embodiments
Abbreviations
Alternating Current AC
Bi-Mode Insulated Gate Transistor BIGT
Direct Current DC
Cascaded Two- Level cell CTL cell
Central Processing Unit CPU
Gate Turn-Off thyristor GTO
High Voltage Direct Current HVDC
Insulated Gate Bipolar Transistor IGBT
Integrated Gate-Commutated Thyristor IGCT
Pulse Width Modulation PWM
Voltage Source Converter VSC
Fig.1 schematically shows aspects of the HVDC power transmission system and aspects of the apparatus 302, 602 according to the present invention. The HVDC power transmission system comprising a plurality of HVDC transmission lines 102, 104, 106, 108, 1 10, 1 12, 1 14 for carrying direct current. The HVDC transmission lines may e.g. comprise HVDC cables, busbars, or other DC conductors. The HVDC transmission lines may comprise at least one long-distance HVDC link. In Fig. 1 , a first and a second HVDC transmission line 102, 108 of said plurality of HVDC transmission lines are in the form of long-distance HVDC links 102, 108. HVDC transmission lines and links are well known to the skilled person and thus not discussed in further detail. The HVDC power transmission system comprises a plurality of converter stations 1 16, 1 18, 120, 122, 124 electrically con- nected to the HVDC transmission lines 102, 104, 1 06, 108, 1 10, 1 12, 1 14. In Fig. 1 , five converter stations 1 16, 1 18, 120, 122, 124 are provided, but there may be more or fewer converter stations. Each of the converter stations may be adapted to convert alternating current to direct current for input to the transmission lines and convert direct current to alternating current for input to neighbouring AC sys- terns. Each converter station 1 16, 1 18, 1 20, 122, 124 may be electrically connected to a conventional electric power transformer 126, 128, 130, 132, 134 in conventional ways known to the skilled person. Electric power transformers and their function are well known to the person skilled in the art and therefore not discussed in more detail. Each converter station, which may be called a DC Grid converter station, may have asymmetrical monopoles with separate converters for positive and negative polarity, as illustrated in Fig. 2A. Alternatively, each converter station may be in the form of a balanced bipolar converter, as illustrated in Fig. 2B. The alternatives of Figs. 2A and 2B may also be combined in the same system. The apparatus 302, 602 according to the present invention is adapted to be electrically connected to the HVDC system, e.g. by being connected between positions A and B and between positions C and D as illustrated in Fig. 1 . However, other locations and connection points are possible, and the apparatus may e.g. be connected to any of the other HVDC transmission lines. Rnnei of the first HVDC transmission line 102 in Fig. 1 illustrates the resistance of the first line 102. lDCi in Fig. 1 is the direct current through the first line 102, i.e. the direct current carried by the first line 102, and bc2 in Fig. 1 is the direct current through the second line 108, i.e. the direct current carried by the second line 108. The HVDC power transmission system may be adapted for single phase power or multi-phase power, e.g. three-phase power, and the components of the system and the appa- ratus may be configured accordingly in ways known to the skilled person. The HVDC power transmission system comprises an embodiment of the apparatus 302, 602 for controlling the electric power transmission in the system according to the present invention, and aspects of the apparatus 302, 602 will hereinafter be disclosed. The apparatus 302, 602 may comprise a first bypass switch 136 (see Fig. 1 ) electrically connectable to the first HVDC transmission line 102, to which the apparatus 302, 602 is connected, and electrically connected in parallel with a first converter 304, 604 (see Figs. 3, 4 and 1 1 ) of the apparatus 302, 602. When the first bypass switch 136 is closed, it is adapted to conduct the direct current of the first HVDC transmission line 102 to electrically bypass the first converter 304, 604. The apparatus 302, 602 may comprise a second bypass switch 137 (see Fig. 1 ) electrically connectable to the second HVDC transmission line 108, to which the apparatus 302, 602 is connected, and electrically connected in parallel with a sec- ond converter 306, 606 (see Figs. 3, 4 and 1 1 ) of the apparatus 302, 602. When the second bypass switch 137 is closed, it is adapted to conduct the direct current of the second HVDC transmission line 108 to electrically bypass the second converter 306, 606. By the first and the second bypass switch 136, 137, the first and the second converter 304, 306, 604, 606, respectively, and the apparatus 302, 602 may be bypassed during fault conditions.
Fig. 3 schematically shows a first embodiment of the apparatus according to the present invention for controlling the electric power transmission in a HVDC power transmission system, e.g. as shown in Fig. 1 The apparatus comprises a first converter 304 for converting alternating current to direct current and/or direct current to alternating current, and a second converter 306 for converting direct current to alternating current and/or alternating current to direct current. Each of the first and second converters 304, 306 has an AC side 308, 310 for output and/or input of alternating current and a DC side 312, 314 for output and/or input of direct current. The first converter 304 is electrically connectable via its DC side 312 to the first HVDC transmission line 102, and the first converter 304 may be electrically connectable in series with the first HVDC transmission line 102. The AC side 308 of the first converter 304 is adapted to provide alternating current to the AC side 310 of the second converter 306, and vice versa. The second converter 306 is connectable via its DC side 314 to the second HVDC transmission line 108, and the second converter 306 may be electrically connectable in series with the second HVDC transmission line 108.
The apparatus 302 may comprise an electric power transformer 318, also indicated as Txin Fig. 4, connected between the first and second converters 304, 306, each of the first and second converters 304, 306 being electrically connect- able, or connected, via its AC side 308, 310 to the electric power transformer 318. The electric power transformer 318 may be a high frequency transformer. The second converter 306 may be adapted to convert DC voltage to high frequency AC voltage, and the first converter 304 may be adapted to convert DC voltage to high frequency AC voltage. The electric power transformer 318 may be adapted to isolate the first converter 304 from the second converter 306, and may thus be adapted to isolate the first converter 304 from the second HVDC transmission line 108 and to isolate the second converter 306 from the first HVDC transmission line 102. Consequently, the electric power transformer 318 may be adapted to isolate the first HVDC transmission line 102 from the second HVDC transmission line 108.
The apparatus 302 is adapted to control the direct current foci of the first HVDC transmission line 102 by introducing a DC voltage V/^ or Vinj1 in series with the first HVDC transmission line 102. The apparatus 302 may comprise control means 324, e.g. a computer or CPU, for controlling the apparatus and its various components. The control means 324 are adapted to control the apparatus 302 to introduce a positive DC voltage, Vlnj1 > 0, in series with the first HVDC transmission line 102 for reducing the direct current, i.e. foci, of the first HVDC transmission line 102, and the control means 324 are adapted to control the apparatus 302 to introduce a negative DC voltage, Vinji < 0, in series with the first HVDC transmis- sion line 102 for increasing foci of the first HVDC transmission line 102. The above-mentioned fictive resistance ARinj may be defined for the first HVDC transmission line 102 by the following expression: ARinji = Vinji/ldci- In a corresponding way, the apparatus 302 may be adapted to control the direct current of the second HVDC transmission line 108 by introducing a DC voltage V^ in series with the second HVDC transmission line 108. The apparatus 302 may be adapted to transfer power, more precisely active power, from the first HVDC transmission line 102 to the second HVDC transmission line 108 and to transfer power from the second HVDC transmission line 108 to the first HVDC transmission line 102.
With reference to Fig. 4, aspects of the apparatus of Fig. 3 are schemati- cally illustrated in more detail. The first converter 304 may comprise a full-bridge converter. The first converter 304 may comprise four pairs 402, 404, 406, 408, also indicated as S S'r, S^S'2, S^S e, S S'4 in Fig. 4, of electrically interconnected electronic control switches 410, 412. The first converter 304 may also comprise a fifth pair 434 of electronic control switches 431 , 433, also indicated as SAB/S AB '^ Fig. 4. The fifth pair 434 of electronic control switches 431 , 433 may be electrically connected in parallel with the four pairs 402, 404, 406, 408 of electronic control switches. The fifth pair 434 of electronic control switches may be used to give a path to the direct current when the first converter 304 is bypassed to give zero voltage. The first converter 304 may comprise filter means 426, 428, connected to the electronic control switches 41 0, 41 2, for smoothing out the voltage and current ripple caused by the switching of the electronic control switches 41 0, 41 2. The filter means may comprise a capacitor 426, also indicated as C^ in Fig. 4, and an inductor 428, also indicated as Lf. The capacitor 426 may be connected in parallel with the electronic control switches 41 0, 41 2. The inductor 428 may be electrically connected in series with the electronic control switches 41 0, 412. The capacitor 426 may be connected in parallel with the fifth pair 434 of electronic control switches.
The second converter 306 may comprise a full-bridge converter. The sec- ond converter 306 may comprise four pairs 414, 41 6, 418, 420, also indicated as S5/S '5, Se/S'e, S S , Ss/S's in Fig. 4, of electrically interconnected electronic control switches 422, 424. The second converter 306 may also comprise a fifth pair 436 of electronic control switches 438, 440, also indicated as SCD S'CD 'W Fig. 4. The second converter's 306 fifth pair 436 of electronic control switches 438, 440 may be electrically connected in parallel with the second converter's 306 four pairs 414, 416, 418, 420 of electronic control switches. The second converter's 306 fifth pair 436 of electronic control switches may be used to give a path to the direct current when the second converter 306 is bypassed to give zero voltage. The second converter 306 may comprise filter means 430, 432, connected to the second con- verter's 306 electronic control switches 422, 424, for smoothing out the voltage and current ripple caused by the switching of the second converter's 306 electronic control switches 422, 424. The second converter's 306 filter means may comprise a capacitor 430, also indicated as C^ in Fig. 4, and an inductor 432, also indicated as Lf \n Fig. 4. The second converter's 306 capacitor 430 may be con- nected in parallel with the second converter's 306 electronic control switches 422, 424. The second converter's 306 inductor 432 may be electrically connected in series with the second converter's 306 electronic control switches 422, 424. The second converter's 306 capacitor 430 may be connected in parallel with the second converter's 306 fifth pair 436 of electronic control switches 438, 440. The four quadrant operation of the apparatus may be supported by bidirectional valves. By introducing PWM switching, the injected voltage Vinji may be regulated to a desired value or level in an efficient way. PWM switching per se is well known to the skilled person and is thus not discussed in further detail. The power requirement of first converter 304 is supplied from the second converter 306 via the electric power transformer 318, and vice versa. To effect or introduce a positive fictive resistance, +ARinji, in the first HVDC transmission line 102, active power should be absorbed by the first converter 304 and fed to the second HVDC transmission line 108 by the second converter 306. To effect or introduce a nega- tive fictive resistance, -ARmji, in the first HVDC transmission line 102, active power should be injected in series with the first HVDC transmission line 102, the active power for injection being obtained from the second HVDC transmission line 108.
With reference to Figs. 5-10, aspects of the four quadrant operation of the apparatus of Fig. 4 will now be illustrated. As mentioned above, the apparatus 302 can operate in all the four quadrants as shown in Fig. 5, the voltage and current polarity being as shown in Fig. 1 , 3 or 4. In Fig. 5, the first converter 304 corresponds to "Converter 1 " and the second converter 306 corresponds to "Converter 2".
The equivalent circuit for the first quadrant operation of the first converter 304 is illustrated in Fig. 6, where Rnne2 in Fig. 6 illustrates the resistance of the second HVDC transmission line 108 of Figs. 3 and 4.
In the first quadrant operation, the direct current Idd of the first HVDC transmission line 102 is flowing from position A to position Y (see Fig. 1 ). Since the voltage/potential in position A is greater than in position Y, the switches
St , S2 , S3 , S4 are forward-biased. The first quadrant operation of the first converter
304 may be possible only if | /dc | > \ldc2\. Once the first converter 304 is in the first quadrant operation, the second converter 306 has to be operated according to the second or the fourth quadrant depending on the current direction, i.e. if the first converter 304 "acts" as "positive" resistance, the second converter 306 will be at "negative" resistance. This implies that power of the first HVDC transmission line 102 should be injected to the second HVDC transmission line 108 and vice versa. Thus, the first converter 304 may always have an impact on the power flow of the second HVDC transmission line 108. This topology is mostly suitable for the re- duction of power flow of one HVDC transmission line and the increase of power flow of another HVDC transmission line.
The equivalent circuit for the first quadrant operation of the first converter 304 and the second quadrant operation of the second converter 306 are illustrated in Fig. 7.
When the first and second converters 304, 306 are connected to the electric power transformer 318, both of the HVDC line currents ldci and ldc2 should be equal and the injected voltages are equal with reverse polarity, as illustrated by the following expression:
Y -v, -y.
When both of the first and second converters 304, 306 are bypassed, the direct current depends on individual line parameters, as
V -V
and L v3 v4
linel R line2
From the equations above the injected voltage at the first converter 304 for < cycle of position D may be expressed as
Vinjl=
Figure imgf000019_0001
The same voltage with reverse polarity is injected in the second converter 306, Vjnji. Thus the power flow of the first HVDC transmission line 102 is controlled by the apparatus 302 with a change in power flow in the second HVDC transmission line 108:
(X 1
-V2) - D (^(Χ-ν,κν,
Rlinel + ¾ ine2
I del
1
(V3 -V4) + D (^(Χ-ν,κν,
Rlinel + ¾ ine2
Ldc2
All of the above-mentioned expressions/equations represent steady state conditions. The same condition is applied if the current direction is from position/point D to position C, and the second converter 306 will be at the fourth quadrant operation as shown in Fig 9. To keep the transformer flux linkage within certain limits, both of the side currents are routed at high frequency using switches Sl , S2 , S5 , S6 turning ON for an half cycle and switches S3 , S4 , S7 , S8 turning ON for the next half cycle. To regulate the injection voltage, zero voltage is inserted by bypassing the input at the first converter 304 and the second converter 306 by turning ON the switches and SCD- With appropriate duty ratio, the voltage across positions A-
B is regulated to give a desired "positive" resistance.
The second quadrant operation of the first converter 304 may be possible only if | /dc | < \ldc2\. To attain negative voltage across positions A-B, S3 , S4 , S5 , S6 are turned ON during positive half cycle and Sl , S2 , S7 , S8 are turned ON during negative half cycle. The bypass paths S^and SCD may be used to achieve zero voltage across the positions A-B. The voltage across positions A-B ( VAB) may be regulated by PWM operation as shown in Fig. 1 0. The PWM voltage may be averaged by the filter means and injected in series with the first HVDC line 102. If the duty ratio is increased, the voltage ½¾s will be more negative and a "negative" re- sistance, i.e. a decrease in resistance, is introduced into the first HVDC line 102. Voltage control at the second converter 306 may not possible since it may fully depend on the operation of the first converter 304. The second converter injected voltage Vinj2 may be equal and of reverse polarity in relation to the first converter injected voltage Vinj1. If the first converter 304 is in the second quadrant, the second converter 306 should be operated according to the first or the third quadrant to satisfy the power balance equation. If the direct current of the second HVDC transmission line 108 is in the direction from C to D (/dc2>0), it will be the first quadrant, and if the direct current of the second HVDC transmission line 108 is in the direction from D to C (/dc2<0), it will be the third quadrant.
The same operating principle should be applied for the other two quadrants.
When using bi-directional IGBTs, the first and second converters 604, 606 may be symmetrical on both sides. Thus, the transformer 618 may be connected on the outer side of the converters 604, 606, and the HVDC transmission line 102, 108 may be connected between two leg midpoints as shown in Fig. 1 1 , which illustrates a second embodiment and further aspects of the apparatus 602 according to the present invention. The components TX, LF, d, SAB, S'AB, SCD and S'CD of the apparatuses 602 may correspond to the corresponding components Tx, Lf, d, SAB, S'AB, SCD and S'CD of Fig. 4 as disclosed above, and the components Si-S8 and S'rS'e of the apparatus 602 may correspond to the components S Ss and S'rS'e of Fig. 4 as disclosed above. The interconnection of the components Tx, Lf, Cf, Cdc, SAB, S'AB, SCD,S'CD SI-S8 and S'i-S'8 of the apparatus 602 is schematically illustrated in Fig. 1 1 . The apparatus 602 of Fig 1 1 may be adapted to be connected to the HVDC transmission lines as disclosed for the embodiment shown in Figs. 3 and 4.
Instead of a pair of anti-parallel transistors, e.g. IGBT, used in the embodi- ments described above, a pair of anti-series transistors, e.g. IGBT or BIGT, as shown in Figs. 12A and 12B may be used. The advantage of the anti-series connection is that reverse blocking transistors are not required.
Each of the above-mentioned electronic control switches may comprises a transistor, e.g. an IGBT, a BIGT or any other suitable transistor. Alternatively, each of the above-mentioned electronic control switches may comprise a thyristor, e.g. a GTO, an IGCT, or a Forced Commutated Thyristor.
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 the appended claims. For example, the disclosed embodiments may be combined in various possible ways, and additional electric equipment, devices or units may be connected to and between the components of the embodiments.

Claims

1 . An apparatus (302; 602) for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising a plu- rality of HVDC transmission lines (102, 1 04, 106, 108, 1 10, 1 12, 1 14) for carrying direct current, DC, characterized in that the apparatus (302) comprises a first converter (304) for converting alternating current, AC, to direct current and/or direct current to alternating current, and a second converter (306) for converting direct current to alternating current and/or alternating current to direct current, each of the first and second converters having an AC side (308, 310) for output and/or input of alternating current and a DC side (312, 314) for output and/or input of direct current, in that the AC side of the first converter is connected to the AC side of the second converter, in that the first converter is connectable via its DC side to a first HVDC transmission line (102) of said plurality of HVDC transmission lines, in that the second converter is connectable via its DC side to a second HVDC transmission line (108) of said plurality of HVDC transmission lines, and in that the apparatus is adapted to control the direct current of the first HVDC transmission line (102) by introducing a DC voltage in series with the first HVDC transmission line (102).
2. An apparatus according to claim 1 , characterized in that the apparatus (302) comprises control means (324) for controlling the apparatus, in that the control means are adapted to control the apparatus to introduce a positive DC voltage in series with the first HVDC transmission line (102) for reducing the direct current of the first HVDC transmission line, and in that the control means are adapted to control the apparatus to introduce a negative DC voltage in series with the first HVDC transmission line for increasing the direct current of the first HVDC transmission line.
3. An apparatus according to claim 1 or 2, characterized in that the apparatus (302; 602) comprises a first bypass switch (136) connectable to the first HVDC transmission line (102) and connected in parallel with the first converter (304), and in that when closed the first bypass switch is adapted to conduct the di- rect current of the first HVDC transmission line to electrically bypass the first converter.
4. An apparatus according to any of the claims 1 to 3, characterized in that the apparatus (302; 602) comprises a second bypass switch (137) connect- able to the second HVDC transmission line (108) and connected in parallel with the second converter (306), and in that when closed the second bypass switch is adapted to conduct the direct current of the second HVDC transmission line to electrically bypass the second converter.
5. An apparatus according to any of the claims 1 to 4, characterized in that the apparatus (302; 602) comprises an electric power transformer (318) connected between the first and the second converter (304, 306), and in that each of the first and second converters is connected via its AC side (308, 310) to the elec- trie power transformer.
6. An apparatus according to claim 5, characterized in that the electric power transformer (318) is adapted to isolate the first converter (304) from the second converter (306).
7. An apparatus according to claim 5 or 6, characterized in that the electric power transformer (318) is adapted to isolate the first converter (304) from the second HVDC transmission line (108).
8. An apparatus according to any of the claims 5 to 7, characterized in that the electric power transformer (318) is adapted to isolate the second converter (306) from the first HVDC transmission line (102).
9. An apparatus according to any of the claims 1 to 8, characterized in that the apparatus (302; 602) is adapted to transfer power from the first HVDC transmission line (102) to the second HVDC transmission line (108) and to transfer power from the second HVDC transmission line (108) to the first HVDC transmission line (102).
10. An apparatus according to any of the claims 1 to 9, characterized in that the first converter (304) comprises a full-bridge converter.
1 1 . An apparatus according to any of the claims 1 to 10, characterized in that the second converter (306) comprises a full-bridge converter.
12. An apparatus according to any of the claims 1 to 1 1 , characterized in that the first converter (304) comprises four pairs (402, 404, 406, 408) of electronic control switches (410, 412).
13. An apparatus according to claim 12, characterized in that the first converter (304) comprises a fifth pair (434) of electronic control switches (431 ,433).
14. An apparatus according to claim 1 2 or 13, characterized in that the first converter (304) comprises filter means (426, 428) for smoothing out the voltage and current ripple caused by the switching of the electronic control switches (410, 412).
15. An apparatus according to any of the claims 1 to 1 4, characterized in that the second converter (306) comprises four pairs (414, 416, 418, 420) of electronic control switches (422, 424).
16. An apparatus according to claim 15, characterized in that the second converter (306) comprises a fifth pair (436) of electronic control switches (438, 440).
17. An apparatus according to claim 1 5 or 16, characterized in that the second converter (306) comprises filter means (430, 432) for smoothing out the voltage and current ripple caused by the switching of the electronic control switches (422, 424).
18. An apparatus according to any of the claims 12 to 17, characterized in that each electronic control switch (410, 412, 422, 424, 431 ,433, 438, 440) comprises a transistor.
19. An apparatus according to any of the claims 1 to 18, characterized in that the second converter (306) is adapted to convert DC voltage to high frequency AC voltage.
20. An apparatus according to any of the claims 1 to 19, characterized in that the first converter (304) is adapted to convert DC voltage to high frequency AC voltage.
21 . An apparatus according to any of the claims 1 to 20, characterized in that the first converter (304) is connectable in series with the first HVDC transmission line (102).
22. An apparatus according to any of the claims 1 to 21 , characterized in that the second converter (306) is connectable in series with the second HVDC transmission line (108).
23. An apparatus according to any of the claims 1 to 22, characterized in that the apparatus is adapted to control the direct current of the second HVDC transmission line (108) by introducing a DC voltage in series with the second HVDC transmission line (108).
24. A high voltage direct current, HVDC, power transmission system comprising a plurality of HVDC transmission lines (102, 104, 106, 108, 1 10, 1 12, 1 14) for carrying direct current, DC, and a plurality of converter stations (1 16, 1 18, 120, 122, 124) connected to the HVDC transmission lines, each of the converter stations being adapted to convert alternating current, AC, to direct current for input to the HVDC transmission lines, and/or direct current to alternating current, wherein the system comprises at least one apparatus (302; 602) as claimed in any of the claims 1 -23 for controlling the electric power transmission in the system.
25. A HVDC power transmission system according to claim 24, characterized in that the system comprises at least three converter stations (1 16, 1 18, 120, 122, 124).
26. A HVDC power transmission system according to claim 24 or 25, characterized in that the HVDC transmission lines (102, 104, 106, 108, 1 10, 1 12, 1 14) comprise at least one long-distance HVDC link (102, 108).
PCT/EP2010/063882 2010-09-21 2010-09-21 An apparatus for controlling the electric power transmission in a hvdc power transmission system WO2012037967A1 (en)

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Publication number Priority date Publication date Assignee Title
WO2013139375A1 (en) * 2012-03-20 2013-09-26 Abb Technology Ltd An apparatus for controlling the electric power transmission in an hvdc power transmission system
EP2670013A1 (en) * 2012-06-01 2013-12-04 Alstom Technology Ltd Current flow controller
WO2013178807A1 (en) * 2012-06-01 2013-12-05 Alstom Technology Ltd Current flow controller
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