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WO2013139375A1 - An apparatus for controlling the electric power transmission in an hvdc power transmission system - Google Patents

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

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Publication number
WO2013139375A1
WO2013139375A1 PCT/EP2012/054898 EP2012054898W WO2013139375A1 WO 2013139375 A1 WO2013139375 A1 WO 2013139375A1 EP 2012054898 W EP2012054898 W EP 2012054898W WO 2013139375 A1 WO2013139375 A1 WO 2013139375A1
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WO
WIPO (PCT)
Prior art keywords
converter
hvdc
direct current
distribution line
hvdc transmission
Prior art date
Legal status (The legal status 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 status listed.)
Ceased
Application number
PCT/EP2012/054898
Other languages
French (fr)
Inventor
Anirudh ACHARYA
Subhasish Mukherjee
Tomas Jonsson
Sasitharan Subramanian
Dimitris GIANNOCCARO
Soubhik AUDDY
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ABB Technology AG
Original Assignee
ABB Technology AG
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Publication date
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Priority to PCT/EP2012/054898 priority Critical patent/WO2013139375A1/en
Publication of WO2013139375A1 publication Critical patent/WO2013139375A1/en
Anticipated expiration legal-status Critical
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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
    • 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 or distribution lines for carrying direct current, DC. Further, the present invention relates to an HVDC power transmission system comprising a plurality of HVDC transmission or distribution lines for carrying direct current, DC, and a plurality of converter stations connected to the plurality of HVDC transmission or distribution lines, each of the converter stations being arranged to convert alternating current, AC, to direct current for input to the plurality of HVDC transmission or distribution lines, and/or direct current to alternating current, the system comprising at least one apparatus of the above- mentioned sort.
  • an HVDC power distribution network or an HVDC power transmission system uses direct current for the transmission of electrical power, in contrast to the more common AC systems. For long-distance transmission or distribution, HVDC systems may be less expensive and may suffer lower electrical losses.
  • an HVDC power transmission system comprises at least one long-distance HVDC link or cable for carrying direct current a long distance, e.g. under sea, and 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 an 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.
  • WO 2010/1 15452 discloses a meshed HVDC power transmission network comprising at least three HVDC converter stations interconnected in a first closed path by at least three transmission lines.
  • DE 1513827 discloses an apparatus for influencing the current distribution in an HVDC network.
  • EP 2 293 407 describes a direct current power transmission and distribu- tion system suitable for subsea electrical loads.
  • US-B2-7, 518,266 discloses a method and an apparatus for improving AC transmission system dispatchability, system stability, and power flow controllability using DC transmission systems.
  • EP 2 341 594 describes power collection and transmission systems com- prising DC-to-DC converters.
  • US-B2-7,633,770 discloses a converter topology for DC power transmission comprising DC-to-DC converters in order to increase system reliability.
  • WO 201 1/157300 and WO 201 1/160678 describe power electronic converters for HVDC transmission and reactive power compensation.
  • US-A1 -2006/0282239 discloses a method of setting-up steady state model of VSC-based multi-terminal HVDC transmission system which can be applied for analysis of power flow of large power system.
  • WO 2010/1 15452 describes power flow control in a meshed HVDC power transmission network
  • WO 2010/1 15453 discloses DC voltage compensation in a multi-terminal HVDC power transmission network.
  • each of the converter stations may be controlled, e.g. by controlling the DC voltage of each converter station.
  • the inventors of the present invention have found that the DC voltage control of the converter stations 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 an HVDC power transmission system. It is also an object of the present invention to provide an improved control of the electric power transmission in an 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.
  • the above-mentioned object of the present invention is 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 or distribution lines for carrying direct current, DC, wherein the apparatus comprises
  • each of the first and second DC-to-DC con- verters having a first DC side for output and/or input of direct current and a second DC side for output and/or input of direct current
  • a third converter for converting alternating current, AC, to direct current and/or direct current to alternating current
  • each of the third and fourth 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 first DC side of the first DC-to-DC converter is connectable in series with a first HVDC transmission or distribution line of the plurality of HVDC trans- mission or distribution lines
  • the first DC side of the second DC-to-DC converter is connectable in series with a second HVDC transmission or distribution line of the plurality of HVDC transmission or distribution lines
  • the third converter is connected via its DC side to the second DC side of the first DC-to-DC converter
  • the fourth converter is connected via its DC side to the second DC side of the second DC-to-DC converter
  • the third converter is connected via its AC side to the AC side of the fourth converter
  • the apparatus is arranged to control the direct current of the first HVDC transmission or distribution line by introducing a DC voltage in series with the first HVDC transmission or distribution line.
  • the apparatus of the present invention is especially advantageous and efficient for an HVDC power transmission system of the sort shown in Fig. 1 , which may be called a DC grid concept, where the system comprises a plurality of HVDC transmission or distribution lines for carrying direct current and a plurality of converter stations connected to the HVDC transmission or distribution lines.
  • the ap- paratus of the present invention is especially advantageous when the control of DC 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 or distribution line, to which the apparatus is connected can be increased or reduced in order to con- trol the power transmission.
  • the direct current control is attained by means of the apparatus' introduction, or injection, of a DC voltage in series with the first HVDC transmission or distribution line.
  • the injected DC voltage produces a fictive resistance, ARjnj.
  • the fictive resistance provides an active power extraction, or active power output, from the first HVDC transmission or distribution line when the fictive resistance corresponds to an increase in resistance, i.e. a positive AR in j, (since a resistance consumes power/energy), or an active power input to the first HVDC transmission or distribution line when the fictive resistance corresponds to a decrease in resistance, i.e. a negative ARinj.
  • a positive AR inJ is produced when the apparatus introduces a positive DC voltage in series with the first HVDC transmis- sion or distribution line
  • a negative ARinj is produced when the apparatus introduces a negative DC voltage in series with the first HVDC transmission or distribution line.
  • the apparatus of the present invention is arranged to regulate the voltage at its output to control the current flow in the first HVDC transmission or distribution line.
  • the apparatus according to the present invention is arranged to control the direct current of the first HVDC transmission or distribution line by introducing a fictive resistance in series with the first HVDC transmission or distribution line by introducing a DC voltage in series with the first HVDC transmission or distribution line.
  • the apparatus may be connectable to more than two HVDC transmission or distribution lines.
  • the apparatus is adapted to provide a transfer of power from the first HVDC transmission or distribution line to the second HVDC transmission or distribution line and to provide a transfer of power from the second HVDC transmission or distribution line to the first HVDC transmission or distribution line.
  • the direct current in a HVDC power transmission 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 detailed description of preferred embodiments.
  • the apparatus may also be arranged to control the direct current of the second HVDC transmission or distribution line by introducing a DC voltage in series with the second HVDC transmission or distribution line.
  • the control of the direct current of the second HVDC transmission or distribution line may be performed in a manner corresponding to the control of the direct current of the first HVDC transmission or distribution line as disclosed above.
  • 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.
  • the apparatus comprises control means for controlling the apparatus, wherein the control means are arranged to control the apparatus to introduce a positive DC voltage in series with the first HVDC transmission or distribution line for reducing the direct current of the first HVDC transmission or distribution line, and wherein the control means are arranged to control the apparatus to introduce a negative DC voltage in series with the first HVDC transmission or distribution line for increasing the direct current of the first HVDC transmission or distribution 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 or distribution line.
  • the control means may comprise a computer and/or a CPU.
  • the control means may be arranged to control the apparatus to introduce a positive fictive resistance in series with the first HVDC transmission or distribution line by introducing a positive DC voltage in series with the first HVDC transmission or dis- tribution line for reducing the direct current of the first HVDC transmission or distribution line
  • the control means may be arranged to control the apparatus to introduce a negative fictive resistance in series with the first HVDC transmission or distribution line by introducing a negative DC voltage in series with the first HVDC transmission or distribution line for increasing the direct current of the first HVDC transmission or distribution line.
  • the apparatus may comprise measuring means for measuring the DC load flow congestion of the HVDC power transmission system, and the measuring means may be are arranged to communicate with the control means.
  • the measuring means may be arranged to measure the direct current or direct voltage of the first or/and second HVDC transmission or distribution line/-s, and the measuring means per se may have a structure known to 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 is adapted for four quadrant operation.
  • 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 operations.
  • 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 or distribution lines for carrying direct current, DC, and a plurality of converter stations connected to the plurality of HVDC transmission or distribution lines, each of the converter stations being arranged to convert alternating current, AC, to direct current for input to the plurality of HVDC transmission or distribution lines, and/or direct current to alternating current
  • the system comprises at least one apparatus as claimed in any of the appended claims 1 -22 for controlling the electric power transmission in the system, and/or at least one apparatus according to any of the other disclosed embodiments of the apparatus.
  • the at least one HVDC transmission or distribution line may be one or a plurality of HVDC transmission or distribution lines
  • a plurality of HVDC transmission or distribution lines or converter stations may be two or more HVDC transmission or distribution 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 or distribution line, or to different HVDC transmission or distribution lines. For example, two apparatuses adapted for two quadrant operation may be connected to the same HVDC transmission or distribution line to attain four quadrant operation.
  • the at least one HVDC transmission or distribution line comprises at least one long-distance HVDC link or cable.
  • the HVDC transmission or distribution lines may comprise at least two long-distance HVDC links or cables.
  • High Voltage may be about 1 -1 .5 kV and above. However, for HVDC applications and systems, High Voltage may be about 320 kV and above, e.g. 500 kV, 800 kV or 1000 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 or distribution line voltage.
  • 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 definition of four quadrant operation of the apparatus of the present invention.
  • Fig. 6 is the schematic diagram of Fig. 4 but with reference currents illustrated;
  • Fig. 7 is a schematic diagram illustrating the voltage across the transformer of the apparatus of Fig. 4;
  • Fig. 8 is a schematic diagram illustrating the output voltage for a posi
  • Fig. 9 is a schematic diagram illustrating a second embodiment and further aspects of the apparatus according to the present invention
  • Fig. 10A is a schematic diagram illustrating a third embodiment and further aspects of the apparatus according to the present invention
  • Fig. 10B is a schematic diagram illustrating a fourth embodiment and fur- ther aspects of the apparatus according to the present invention.
  • Figs. 1 1 A and 1 1 B are schematic diagrams illustrating alternative electronic control devices.
  • Fig. 1 schematically shows aspects of the HVDC power transmission system and aspects of the apparatus 202; 402; 602 according to the present invention.
  • the HVDC power transmission system comprises a plurality of HVDC transmission or distribution lines 102, 104, 106, 108, 1 10, 1 12, 1 14 for carrying direct current, hereinafter called HVDC lines.
  • the HVDC lines may e.g. comprise HVDC cables, busbars, or other DC conductors.
  • the plurality of HVDC lines 102, 104, 106, 108, 1 10, 1 12, 1 14 may comprise at least one long-distance HVDC link.
  • a first and a second HVDC line 102, 108 of the plurality of HVDC lines 102, 104, 106, 108, 1 10, 1 12, 1 14 may be provided as long-distance HVDC links 102, 108.
  • HVDC 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 connected to the plurality of HVDC lines 102, 104, 106, 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.
  • the HVDC power transmission system may e.g.
  • Each of the converter stations 1 16, 1 18, 120, 122, 124 may be arranged to convert alternating current to direct current for input to the HVDC lines 102, 104, 106, 108, 1 10, 1 12, 1 14 and convert direct current to alternating current for input to neighbouring AC systems.
  • Each converter station 1 16, 1 18, 120, 122, 124 may be electrically connected to a conventional transformer 126, 128, 130, 132, 134, which may be an electric power transformer, in conventional ways known to the skilled person. Transformers and their function are well known to the person skilled in the art and therefore not discussed in more detail.
  • Each converter station 1 16, 1 18, 120, 122, 124 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 1 16, 1 18, 120, 122, 124 may be in the form of a symmetrical mo- nopolar converter, as illustrated in Fig. 2B.
  • the alternatives of Figs. 2A and 2B may also be combined in the same system.
  • the apparatus 202; 402; 602 is arranged 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 .
  • the apparatus 202, 402, 602 may e.g. be connected to any of the other HVDC lines.
  • Rn ne of the HVDC line 102 in Fig. 1 illustrates the resistance of the first HVDC line 102.
  • l D c in Fig. 1 is the direct current through the first HVDC line 102, i.e. the direct current carried by the first HVDC line 102.
  • I D C2 in Fig. 1 is the direct current through the second HVDC line 108, i.e. the direct current carried by the second HVDC 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 apparatus 202; 402; 602 may be configured accordingly in ways known to the skilled person.
  • the HVDC power transmission system comprises at least one embodiment of the apparatus 202; 402; 602 for controlling the electric power transmission in the system according to the present invention, and aspects of the apparatus 202; 402; 602 will hereinafter be disclosed.
  • the apparatus 202; 402; 602 may comprise a first bypass switch 136 con- nectable to the first HVDC line 102 and connected in parallel with a first DC-to-DC converter 204, 604 (see Figs. 3, 4, 9 and 10) of the apparatus 202; 402; 602.
  • the apparatus 202; 402; 602 may comprises a second bypass switch 137 (see Fig. 1 ) connectable to the second HVDC line 108 and connected in parallel with a second DC-to-DC converter 206, 606 (see Figs. 3, 4, 9 and 10) of the apparatus 202; 402; 602.
  • the second bypass switch 137 When the second bypass switch 137 is closed, it is arranged to conduct the direct current of the second HVDC line 108 to electrically bypass the second DC-to-DC converter 206, 606.
  • first and second bypass switches 136, 137 the first and the second DC-to-DC converter 204, 206; 604, 606, respectively, and the apparatus 202; 402; 602 may be bypassed during fault conditions, whereby the electric power transmission in an HVDC power transmission system and the control thereof are further improved.
  • Fig. 3 schematically shows a first embodiment of the apparatus 202 according to the present invention for controlling the electric power transmission in a HVDC power transmission system, e.g. as shown in Fig.
  • the apparatus 202 comprises a first DC-to-DC converter 204 and a second DC-to-DC converter 206.
  • Each of the first and second DC-to-DC converters 204, 206 has a first DC side 208, 212 for output and/or input of direct current and a second DC side 210, 214 for output and/or input of direct current.
  • the apparatus 202 comprises a third converter 216 for converting alternating current, AC, to direct current and/or direct current to alternating current, and a fourth converter 218 for converting direct current to alternating current and/or alternating current to direct current.
  • Each of the third and fourth converters 216, 218 has an AC side 220, 222 for output and/or input of alternating current and a DC side 224, 226 for output and/or input of direct current.
  • the first DC side 208 of the first DC-to-DC converter 204 is connectable in series with the first HVDC line 102 of the plurality of HVDC lines
  • the first DC side 212 of the second DC-to-DC converter 206 is connectable in series with the second HVDC line 108 of the plurality of HVDC lines.
  • the third converter 216 is connected via its DC side 224 to the second DC side 210 of the first DC-to-DC converter 204.
  • the fourth converter 218 is connected via its DC side 226 to the second DC side 214 of the second DC-to-DC converter 206.
  • the third converter 216 is connected via its AC side 220 to the AC side 222 of the fourth converter 218.
  • the apparatus 202 is arranged to control the direct current l D ci of the first HVDC line 102 by introducing a DC voltage V A B or V in ji in series with the first HVDC line 102.
  • the apparatus 202 may comprise control means 228, e.g. a computer or CPU, for controlling the apparatus 202 and its various components.
  • the control means 228 may be arranged to control the apparatus 202 to introduce a positive DC voltage, V inj i > 0, in series with the first HVDC line 102 for reducing the direct current, i.e.
  • the control means 228 may be arranged to control the apparatus 202 to introduce a negative DC voltage, V inj i ⁇ 0, in series with the first HVDC line 102 for increasing l D ci of the first HVDC line 102.
  • the apparatus 202 may be arranged to control the direct current l dc2 of the second HVDC line 108 by introducing a DC voltage V inj2 in series with the second HVDC line 108.
  • the apparatus 202 may be arranged to provide a transfer of power, more precisely active power, from the first HVDC line 102 to the second HVDC line 108 and to provide a transfer of power from the second HVDC line 108 to the first HVDC line 102.
  • the operation of the apparatus 202 may be schematically illustrated as follows: If it is assumed that voltage Vi (see Fig. 3) of the first HVDC line 102 is 320 kV, voltage V 2 (see Fig. 3) of the first HVDC line 102 is 300 kV, and the resistance R (see Fig.
  • a similar reasoning can be applied when the apparatus 202 introduces a negative DC voltage, V in ji, in series with the first HVDC line 102 to increase the direct current l dc i of the first HVDC line 102, whereby an active power input to the first HVDC line 102 is provided.
  • the apparatus 202 may comprise a transformer 230 connected between the third and fourth converters 216, 218, and each of the third and fourth converters may be connectable via its AC side 220, 222 to the transformer 230.
  • the transformer 230 may be an electric power transformer.
  • the transformer 230 may be a high or medium frequency transformer. The high or medium frequency of the transformer 230 will reduce the filter requirements for the reduction of voltage or current ripple.
  • Each of the third and fourth converters 216, 218 may be arranged to convert DC voltage to high or medium frequency AC voltage.
  • the transformer 230 may be arranged to isolate the first DC-to-DC converter 204 from the second DC- to-DC converter 206.
  • the transformer 230 may be arranged to isolate the first DC- to-DC converter 204 from the second HVDC line 108.
  • the transformer 230 may be arranged to isolate the second DC-to-DC converter 206 from the first HVDC line 102. By means of the transformer 230, the flexibility and efficiency of the electric power transmission in an HVDC power transmission system and the control thereof are further improved. The transformer 230 may also take part in fulfilling the voltage requirements of the apparatus 202.
  • the transformer 230 may comprise one or a plurality of transformers or transformer units.
  • the apparatus 202 of the present invention may be defined as floating between the first and the second HVDC line 102, 108, and the transformer 230 may therefore be free from DC insulation due to station voltage. Thus, the apparatus 202 and the HVDC power transmission system are less expensive to produce.
  • the apparatus 202 may comprise a first capacitor 232, to which the first DC-to-DC converter 204 and the third converter 216 may be connected.
  • the apparatus 202 may comprise a second capacitor 234, to which the second DC-to-DC converter 206 and the fourth converter 218 may be connected.
  • the first DC-to-DC converter 204 and the third converter 216 may share the first capacitor 232.
  • the second DC-to-DC converter 206 and the fourth converter 218 may share the sec- ond capacitor 234.
  • the first capacitor 232 may be arranged to act as a voltage source for the first DC-to-DC converter 204.
  • the second capacitor 234 may be arranged to act as a voltage source for the second DC-to-DC converter 206.
  • the third converter 216 may be arranged to control the charge balance of the first ca- pacitor 232.
  • the fourth converter 218 may be arranged to control the charge balance of the second capacitor 234.
  • Each of the first and second capacitors 232, 234 may comprise one or a plurality of capacitors, or capacitor components or units.
  • the apparatus 202 may be arranged to provide a transfer of power from the first HVDC line 102 to the second HVDC line 108 and to provide a transfer of power from the second HVDC line 108 to the first HVDC line 102.
  • the first DC-to-DC converter 204 may comprise four pairs 236, 238, 240, 242, also indicated as S ⁇ S 5 P, S ⁇ S 6 P, S 7 /S 7 P, SS SSP in Fig. 4 of electrically interconnected electronic switches 244, 246.
  • the second DC-to-DC converter 206 may comprise four pairs 248, 250, 252, 254, also indicated as Si3/Si3P : S SHP, Si5/Si5P : S Siep in Fig. 4 of electrically interconnected electronic switches 256, 258.
  • the third converter 216 may comprise four pairs 260, 262, 264, 266, also indicated as S S IP , S 2 /S 2 P, S/S 3 P, S/S P in Fig. 4 of electrically interconnected electronic switches 268, 270.
  • the fourth converter 218 may comprise four pairs 272, 274, 276, 278, also indicated as S S iP, S 2 /S 2 P, S3/S3P : S4/S4P in Fig. 4 of electrically interconnected electronic switches 280, 282.
  • Each of the first and second DC-to-DC converters 204, 206 may comprise filter means 284, 286 for smoothing out the voltage and current ripple caused by the switching of the electronic switches.
  • the filter means 284; 286 may be con- nected to the electronic switches 244, 246; 256, 258. 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, of each DC-to-DC converter 204, 206 may comprise a capacitor 288, 290, also indicated as C « and Cf 2 , respectively, in Fig. 4, and an inductor 292, 294, also indicated as L f1 and L f2 , respec- tively.
  • the capacitor 288; 290 may be connected in parallel with the electronic switches 244, 246; 256, 258.
  • the inductor 292; 294 may be connected in series with the electronic switches 244, 246; 256, 258.
  • the switching sequence and circuit condition are analyzed for first quadrant operation for the second HVDC line 108, i.e. current l dc2 of the second HVDC line 108 and voltage V inj2 are both positive (see Fig. 6). Then, power flows from the second HVDC line 108 to the first HVDC line 102 and the power flow should be supported by the converters 204, 206, 216, 218 of the apparatus 202.
  • This extracted power from the second HVDC line 108 will be added to the first HVDC line 102 and the current l dc1 of the first HVDC line 102 is taken as positive for the analysis which gives a requirement of negative V inj i (as positive l dc1 and negative V vinj i will make power negative, signifying power injection).
  • Some reference current di- rections are shown below in Fig. 6 and they will be used for the following circuit analysis. However, some other terminologies are that V c1 and V c2 denote voltage of the first capacitor 232, Ci, and the second capacitor 234, C 2 , respectively.
  • D1 , D2, D3 and D4 denote duty ratios of the first DC-to-DC converter 204, the third converter 216, the fourth converter 218 and the second DC-to-DC converter 206, respectively.
  • V inj2 will be positive and that will be obtained by switching the converter output between + V c2 and 0.
  • the path through which current flows is given below: The second DC-to-DC converter.
  • the fourth converter is the fourth converter
  • Vinji has to be negative, which is obtained by the following switching:
  • the third converter The third converter:
  • D3 is the duty ratio of each switch in the fourth converter 218 and hence the output voltage has effective duty of 2 ⁇ D3, also maximum value of D3 is 0.5 and maximum value of D4 and D1 can be 1 .
  • the power balance may be expressed by the following expression:
  • the analyses are done considering same polarity for V c1 and V c2 .
  • the analysis shows that the capacitor charge balance can be maintained by maintain- ing the power balance in the converters 204, 206, 216, 218, i.e. by correctly selecting the duty ratio for the converters 204, 206, 216, 218.
  • the first DC-to-DC converter is the first DC-to-DC converter
  • the third converter is the third converter
  • Si - Tx - S3P for zero voltage
  • S3 - Tx - S 4 - Ci for negative voltage
  • the fourth converter is the fourth converter
  • the second DC-to-DC converter is the second DC-to-DC converter
  • Switching operation for other converters can also be derived similarly.
  • the stage of energizing can be done from either of the apparatus sides.
  • separate power supply can be arranged as backup to energize the apparatus 202.
  • the first and second DC-to-DC converters 204, 206 of the second embodiment corre- spond to the first and second DC-to-DC converters 204, 206 of the first embodiment of Fig. 4, and are thus not further discussed in detail.
  • the structure of the third and fourth converters 416, 418 of the second embodiment is different from the third and fourth converters 216, 218 of Fig. 4.
  • the third converter 416 may comprise four pairs 460, 462, 464, 466, also indicated as S/D? , S2/D2, S3 D3, S4/D4 in Fig. 9 of electrically interconnected electronic control devices 468, 470.
  • the fourth converter 418 may comprise four pairs 472, 474, 476, 478, also indicated as SQ/D 9: S10/D10, Sii/Dii : S12/D12 in Fig. 9 of electrically interconnected electronic control devices 480, 482.
  • Each pair 460, 462, 464, 466, 472, 474, 476, 478 of electronic control devices 468, 470, 480, 482 may comprise an electronic switch 484 and a diode 486. Within each pair, the diode 486 may be anti-parallel to the electronic switch 484.
  • the apparatus 402 of Fig. 9 may comprise a transformer 430 connected between the third and fourth converters 416, 418.
  • the third and fourth converters 416, 418 of the third embodiment correspond to the third and fourth converters 416, 418 of the second embodiment of Fig. 9 and are thus not further discussed in detail.
  • the structure of the first and second DC-to-DC converters 604, 606 of the third embodiment is different from first and second DC-to-DC converters 204, 206 of the first and second embodiments of Figs. 4 and 9.
  • the first DC-to-DC converter 604 may comprise four pairs 660, 662, 664, 666, also indicated as S ⁇ D 5, S ⁇ D 6, S 7 /D 7, Se/D 8 in Fig.
  • the second DC-to-DC converter 606 may comprise four pairs 672, 674, 676, 678, also indicated as Si3/D 13i Si4/D 14i Si5/Di5 : Sie Di6 in Fig. 10A of electrically interconnected electronic control devices 680, 682.
  • Each pair 660, 662, 664, 666, 672, 674, 676, 678 of electronic control devices 668, 670, 680, 682 may comprise an electronic switch 684 and a diode 686. Within each pair, the diode 686 may be anti-parallel to the electronic switch 684.
  • the apparatus 602 of Fig. 10A may comprise a transformer 630 connected between the third and fourth converters 416, 418.
  • Each of the first and second DC-to-DC converters 604, 606 may comprise filter means 688, 690 corresponding to the filter means 284, 286 of the first and second DC-to-DC converters 204, 206 in Figs. 4.
  • the fourth embodiment of the apparatus 802 may essentially correspond to the apparatus of Fig. 10A and has a first and second DC-to-DC converters 604, 606 and a third and fourth converters 416, 418 which may correspond to the first and second DC-to-DC converters 604, 606 and the third and fourth converters 416, 418, respectively, of the apparatus 602 of Fig. 10A.
  • the apparatus 802 of Fig. 10B comprises a transformer 630 connected between the third and fourth converters 416, 418. Each of the third and fourth converters 416, 418 may be connectable via its AC side to the transformer 630.
  • the transformer 630 may be an electric power transformer.
  • the transformer 630 may be a high or medium frequency transformer.
  • the transformer 630 may comprise a first winding 804 connected to the third converter 416 and a second winding 806 connected to the fourth converter 418.
  • the apparatus 802 of Fig. 10B may comprise an auxiliary control device 808.
  • the transformer 630 may comprise a third winding 810 connected to the auxiliary control device 808.
  • the auxiliary control device 808 may be connectable to a load, or an electric power source 812.
  • the electric power source may comprise an AC source.
  • the auxiliary control device 808 may comprise at least one converter 814 for converting direct current to alternating current and/or alternating current to direct current, and the electric power source may comprise a DC source, e.g.
  • the auxiliary control device 808 may comprise a first converter 814 for converting direct current to alternating current and/or al- ternating current to direct current, and a DC-to-DC converter 816 having two DC sides for output and/or input of direct current.
  • the DC-to-DC converter 816 of the auxiliary control device 808 may be connectable to the electric power source 812, which e.g. comprises a DC source, e.g. an electric battery, or a third HVDC line.
  • the configuration of the DC-to-DC converter 816 of the auxiliary control device 808 may correspond to the configuration of the first DC-to-DC converter 204; 604 of any of the above-mentioned apparatuses.
  • the configuration of the first converter 814 of the auxiliary control device 808 may correspond to the configuration of the third converter 216; 416 of any of the above-mentioned apparatuses.
  • the auxiliary control device 808 is arranged to extract power, e.g. active power, from the first HVDC line 102 and/or the second HVDC line 108 and/or to input power, e.g.
  • the innovative introduction of a DC voltage in series with the first HVDC transmission or distribution line is further influenced.
  • the control of the electric power transmission in an HVDC power transmission system is further improved.
  • each electronic switch comprises a power semiconductor switch.
  • Each power semiconductor switch may comprise an Insulated Gate Bipolar Transistor, IGBT, or a Bi-Mode Insulated Gate Transistor, BiGT, or any other suitable power semiconductor switch.
  • each power semiconductor switch may comprise a thyristor, e.g. a gate turn-off thyristor, GTO, an Integrated Gate-Com- mutated Thyristor, IGCT, or a Forced Commutated Thyristor.
  • GTO gate turn-off thyristor
  • IGCT Integrated Gate-Com- mutated Thyristor
  • Forced Commutated Thyristor e.g. a Forced Commutated Thyristor.
  • other suit- able thyristors may also be used.
  • the inventors of the present invention have found that the structures of the converters as disclosed above further improve the flexibility and efficiency of the electric power transmission in an HVDC power transmission system and the control thereof.
  • a pair of anti-parallel power semiconductor switches e.g. IGBT
  • a pair of anti-series power semiconductor switches e.g. IGBT or BiGT, as shown in Figs. 1 1 A and 1 1 B may be used.
  • the advantage of the anti-series connection is that reverse blocking power semiconductor switches are not required.

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Description

AN APPARATUS FOR CONTROLLING THE ELECTRIC POWER TRANSMISSION IN AN 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 or distribution lines for carrying direct current, DC. Further, the present invention relates to an HVDC power transmission system comprising a plurality of HVDC transmission or distribution lines for carrying direct current, DC, and a plurality of converter stations connected to the plurality of HVDC transmission or distribution lines, each of the converter stations being arranged to convert alternating current, AC, to direct current for input to the plurality of HVDC transmission or distribution lines, and/or direct current to alternating current, the system comprising at least one apparatus of the above- mentioned sort.
Background of the Invention
An HVDC power distribution network or an HVDC power transmission system uses direct current for the transmission of electrical power, in contrast to the more common AC systems. For long-distance transmission or distribution, HVDC systems may be less expensive and may suffer lower electrical losses. In general, an HVDC power transmission system comprises at least one long-distance HVDC link or cable for carrying direct current a long distance, e.g. under sea, and 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 an 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.
WO 2010/1 15452 discloses a meshed HVDC power transmission network comprising at least three HVDC converter stations interconnected in a first closed path by at least three transmission lines.
DE 1513827 discloses an apparatus for influencing the current distribution in an HVDC network.
EP 2 293 407 describes a direct current power transmission and distribu- tion system suitable for subsea electrical loads.
US-B2-7, 518,266 discloses a method and an apparatus for improving AC transmission system dispatchability, system stability, and power flow controllability using DC transmission systems.
EP 2 341 594 describes power collection and transmission systems com- prising DC-to-DC converters.
US-B2-7,633,770 discloses a converter topology for DC power transmission comprising DC-to-DC converters in order to increase system reliability.
WO 201 1/157300 and WO 201 1/160678 describe power electronic converters for HVDC transmission and reactive power compensation.
US-A1 -2006/0282239 discloses a method of setting-up steady state model of VSC-based multi-terminal HVDC transmission system which can be applied for analysis of power flow of large power system.
WO 2010/1 15452 describes power flow control in a meshed HVDC power transmission network, and WO 2010/1 15453 discloses DC voltage compensation in a multi-terminal HVDC power transmission network.
The Object of the Invention
To control the electric power transmission in an 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 voltage of each converter station. However, the inventors of the present invention have found that the DC voltage control of the converter stations 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 an HVDC power transmission system. It is also an object of the present invention to provide an improved control of the electric power transmission in an 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 object of the present invention is 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 or distribution lines for carrying direct current, DC, wherein the apparatus comprises
a first DC-to-DC converter,
a second DC-to-DC converter, each of the first and second DC-to-DC con- verters having a first DC side for output and/or input of direct current and a second DC side for output and/or input of direct current,
a third converter for converting alternating current, AC, to direct current and/or direct current to alternating current, and
a fourth converter for converting direct current to alternating current and/or alternating current to direct current, each of the third and fourth 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 first DC side of the first DC-to-DC converter is connectable in series with a first HVDC transmission or distribution line of the plurality of HVDC trans- mission or distribution lines, the first DC side of the second DC-to-DC converter is connectable in series with a second HVDC transmission or distribution line of the plurality of HVDC transmission or distribution lines, the third converter is connected via its DC side to the second DC side of the first DC-to-DC converter, the fourth converter is connected via its DC side to the second DC side of the second DC-to-DC converter, and the third converter is connected via its AC side to the AC side of the fourth converter, and wherein
the apparatus is arranged to control the direct current of the first HVDC transmission or distribution line by introducing a DC voltage in series with the first HVDC transmission or distribution line. By means of the innovative apparatus of the present invention, the electric power transmission in an 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 apparatus of the present invention is especially advantageous and efficient for an HVDC power transmission system of the sort shown in Fig. 1 , which may be called a DC grid concept, where the system comprises a plurality of HVDC transmission or distribution lines for carrying direct current and a plurality of converter stations connected to the HVDC transmission or distribution lines. The ap- paratus of the present invention is especially advantageous when the control of DC voltage of the converter stations, or the control of shunt connected converter DC voltages of a DC grid, is not sufficient. By means of the apparatus of the present invention, the direct current of the HVDC transmission or distribution line, to which the apparatus is connected, can be increased or reduced in order to con- trol the power transmission. The direct current control is attained by means of the apparatus' introduction, or injection, of a DC voltage in series with the first HVDC transmission or distribution line. The injected DC voltage produces a fictive resistance, ARjnj. The fictive resistance provides an active power extraction, or active power output, from the first HVDC transmission or distribution line when the fictive resistance corresponds to an increase in resistance, i.e. a positive ARinj, (since a resistance consumes power/energy), or an active power input to the first HVDC transmission or distribution 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 transmis- sion or distribution line, and a negative ARinj is produced when the apparatus introduces a negative DC voltage in series with the first HVDC transmission or distribution line. Thus, by means of the apparatus of the present invention, the load of the first HVDC transmission or distribution line, to which the apparatus is connected, may be reduced or increased. The apparatus' active power extraction or output from the first HVDC transmission or distribution line results in a decrease in direct current of the line, and the apparatus' active power input to the first HVDC transmission or distribution line results in an increase in direct current of the line. By the increase and decrease in direct current of the first HVDC transmission or distribution line, the power transmission is controlled and load-flow congestion may be avoided, reduced or prevented. Thus, the apparatus of the present invention is arranged to regulate the voltage at its output to control the current flow in the first HVDC transmission or distribution line.
In alternative words, the apparatus according to the present invention is arranged to control the direct current of the first HVDC transmission or distribution line by introducing a fictive resistance in series with the first HVDC transmission or distribution line by introducing a DC voltage in series with the first HVDC transmission or distribution line. The apparatus may be connectable to more than two HVDC transmission or distribution lines.
To effect or introduce a positive fictive resistance, +ARinj, active power should be absorbed by the second HVDC transmission or distribution line, and to effect or introduce a negative fictive resistance, -ARinj, active power should be injected by and from the second HVDC transmission or distribution line. According to an advantageous embodiment of the apparatus according to the present inven- tion, the apparatus is adapted to provide a transfer of power from the first HVDC transmission or distribution line to the second HVDC transmission or distribution line and to provide a transfer of power from the second HVDC transmission or distribution line to the first HVDC transmission or distribution 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 detailed description of preferred embodiments.
The apparatus may also be arranged to control the direct current of the second HVDC transmission or distribution line by introducing a DC voltage in series with the second HVDC transmission or distribution line. The control of the direct current of the second HVDC transmission or distribution line may be performed in a manner corresponding to the control of the direct current of the first HVDC transmission or distribution line as disclosed above.
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.
According to an advantageous embodiment of the apparatus according to the present invention, the apparatus comprises control means for controlling the apparatus, wherein the control means are arranged to control the apparatus to introduce a positive DC voltage in series with the first HVDC transmission or distribution line for reducing the direct current of the first HVDC transmission or distribution line, and wherein the control means are arranged to control the apparatus to introduce a negative DC voltage in series with the first HVDC transmission or distribution line for increasing the direct current of the first HVDC transmission or distribution line. By the control means of this embodiment, the current flow in the first HVDC transmission or distribution 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 or distribution line. The control means may comprise a computer and/or a CPU. In alternative words, the control means may be arranged to control the apparatus to introduce a positive fictive resistance in series with the first HVDC transmission or distribution line by introducing a positive DC voltage in series with the first HVDC transmission or dis- tribution line for reducing the direct current of the first HVDC transmission or distribution line, and the control means may be arranged to control the apparatus to introduce a negative fictive resistance in series with the first HVDC transmission or distribution line by introducing a negative DC voltage in series with the first HVDC transmission or distribution line for increasing the direct current of the first HVDC transmission or distribution line.
The apparatus may comprise measuring means for measuring the DC load flow congestion of the HVDC power transmission system, and the measuring means may be are arranged to communicate with the control means. The measuring means may be arranged to measure the direct current or direct voltage of the first or/and second HVDC transmission or distribution line/-s, and the measuring means per se may have a structure known to 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 a further advantageous embodiment of the apparatus according to the present invention, the apparatus is adapted for four quadrant operation. 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 operations. 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 or distribution lines for carrying direct current, DC, and a plurality of converter stations connected to the plurality of HVDC transmission or distribution lines, each of the converter stations being arranged to convert alternating current, AC, to direct current for input to the plurality of HVDC transmission or distribution lines, and/or direct current to alternating current, wherein the system comprises at least one apparatus as claimed in any of the appended claims 1 -22 for controlling the electric power transmission in the system, and/or at least one apparatus according to any of the other disclosed embodiments of the apparatus. Positive technical effects of the HVDC power transmission system according to the present invention, and its embodiments, correspond to the technical effects mentioned in connection with the apparatus according to the present invention, and its embodiments. The at least one HVDC transmission or distribution line may be one or a plurality of HVDC transmission or distribution lines
A plurality of HVDC transmission or distribution lines or converter stations may be two or more HVDC transmission or distribution 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 or distribution line, or to different HVDC transmission or distribution lines. For example, two apparatuses adapted for two quadrant operation may be connected to the same HVDC transmission or distribution line to attain four quadrant operation.
According to a further advantageous embodiment of the HVDC power transmission system according to the present invention, the at least one HVDC transmission or distribution line comprises at least one long-distance HVDC link or cable. Advantageously, the HVDC transmission or distribution lines may comprise at least two long-distance HVDC links or cables.
In general, High Voltage may be about 1 -1 .5 kV and above. However, for HVDC applications and systems, High Voltage may be about 320 kV and above, e.g. 500 kV, 800 kV or 1000 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 or distribution line voltage.
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 appended claims and the detailed description of embodiments.
The embodiments and features of the apparatus and the HVDC power transmission system, respectively, according to the present invention may be com- bined in various possible ways providing further advantageous 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 definition of four quadrant operation of the apparatus of the present invention;
Fig. 6 is the schematic diagram of Fig. 4 but with reference currents illustrated; Fig. 7 is a schematic diagram illustrating the voltage across the transformer of the apparatus of Fig. 4;
Fig. 8 is a schematic diagram illustrating the output voltage for a posi
Figure imgf000011_0001
Fig. 9 is a schematic diagram illustrating a second embodiment and further aspects of the apparatus according to the present invention; Fig. 10A is a schematic diagram illustrating a third embodiment and further aspects of the apparatus according to the present invention; Fig. 10B is a schematic diagram illustrating a fourth embodiment and fur- ther aspects of the apparatus according to the present invention; and
Figs. 1 1 A and 1 1 B are schematic diagrams illustrating alternative electronic control devices.
Detailed Description of Preferred Embodiments
Abbreviations
Alternating Current AC
Bi-Mode Insulated Gate Transistor BiGT
Direct Current DC
Central Processing Unit CPU
Gate Turn-Off thyristor GTO
High Voltage Direct Current HVDC
Insulated Gate Bipolar Transistor IGBT
Integrated Gate-Commutated Thyristor IGCT
Fig. 1 schematically shows aspects of the HVDC power transmission system and aspects of the apparatus 202; 402; 602 according to the present invention. The HVDC power transmission system comprises a plurality of HVDC transmission or distribution lines 102, 104, 106, 108, 1 10, 1 12, 1 14 for carrying direct current, hereinafter called HVDC lines. The HVDC lines may e.g. comprise HVDC cables, busbars, or other DC conductors. The plurality of HVDC lines 102, 104, 106, 108, 1 10, 1 12, 1 14 may comprise at least one long-distance HVDC link. A first and a second HVDC line 102, 108 of the plurality of HVDC lines 102, 104, 106, 108, 1 10, 1 12, 1 14 may be provided as long-distance HVDC links 102, 108. HVDC 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 connected to the plurality of HVDC lines 102, 104, 106, 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. The HVDC power transmission system may e.g. comprise two, at least three, or at least four converter stations, or at least five converter stations. Each of the converter stations 1 16, 1 18, 120, 122, 124 may be arranged to convert alternating current to direct current for input to the HVDC lines 102, 104, 106, 108, 1 10, 1 12, 1 14 and convert direct current to alternating current for input to neighbouring AC systems. Each converter station 1 16, 1 18, 120, 122, 124 may be electrically connected to a conventional transformer 126, 128, 130, 132, 134, which may be an electric power transformer, in conventional ways known to the skilled person. Transformers and their function are well known to the person skilled in the art and therefore not discussed in more detail.
Each converter station 1 16, 1 18, 120, 122, 124, 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 1 16, 1 18, 120, 122, 124 may be in the form of a symmetrical mo- nopolar converter, as illustrated in Fig. 2B. The alternatives of Figs. 2A and 2B may also be combined in the same system.
The apparatus 202; 402; 602 according to the present invention is arranged 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 connections points are possible. The apparatus 202, 402, 602 may e.g. be connected to any of the other HVDC lines. Rnne of the HVDC line 102 in Fig. 1 illustrates the resistance of the first HVDC line 102. lDc in Fig. 1 is the direct current through the first HVDC line 102, i.e. the direct current carried by the first HVDC line 102. IDC2 in Fig. 1 is the direct current through the second HVDC line 108, i.e. the direct current carried by the second HVDC 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 apparatus 202; 402; 602 may be configured accordingly in ways known to the skilled person. The HVDC power transmission system comprises at least one embodiment of the apparatus 202; 402; 602 for controlling the electric power transmission in the system according to the present invention, and aspects of the apparatus 202; 402; 602 will hereinafter be disclosed.
The apparatus 202; 402; 602 may comprise a first bypass switch 136 con- nectable to the first HVDC line 102 and connected in parallel with a first DC-to-DC converter 204, 604 (see Figs. 3, 4, 9 and 10) of the apparatus 202; 402; 602.
When the first bypass switch 136 is closed, it is arranged to conduct the direct current of the first HVDC line 102 to electrically bypass the first DC-to-DC converter 204, 604. The apparatus 202; 402; 602 may comprises a second bypass switch 137 (see Fig. 1 ) connectable to the second HVDC line 108 and connected in parallel with a second DC-to-DC converter 206, 606 (see Figs. 3, 4, 9 and 10) of the apparatus 202; 402; 602. When the second bypass switch 137 is closed, it is arranged to conduct the direct current of the second HVDC line 108 to electrically bypass the second DC-to-DC converter 206, 606. By the first and second bypass switches 136, 137, the first and the second DC-to-DC converter 204, 206; 604, 606, respectively, and the apparatus 202; 402; 602 may be bypassed during fault conditions, whereby the electric power transmission in an HVDC power transmission system and the control thereof are further improved.
Fig. 3 schematically shows a first embodiment of the apparatus 202 according to the present invention for controlling the electric power transmission in a HVDC power transmission system, e.g. as shown in Fig. The apparatus 202 comprises a first DC-to-DC converter 204 and a second DC-to-DC converter 206. Each of the first and second DC-to-DC converters 204, 206 has a first DC side 208, 212 for output and/or input of direct current and a second DC side 210, 214 for output and/or input of direct current. The apparatus 202 comprises a third converter 216 for converting alternating current, AC, to direct current and/or direct current to alternating current, and a fourth converter 218 for converting direct current to alternating current and/or alternating current to direct current. Each of the third and fourth converters 216, 218 has an AC side 220, 222 for output and/or input of alternating current and a DC side 224, 226 for output and/or input of direct current.
The first DC side 208 of the first DC-to-DC converter 204 is connectable in series with the first HVDC line 102 of the plurality of HVDC lines, and the first DC side 212 of the second DC-to-DC converter 206 is connectable in series with the second HVDC line 108 of the plurality of HVDC lines. The third converter 216 is connected via its DC side 224 to the second DC side 210 of the first DC-to-DC converter 204. The fourth converter 218 is connected via its DC side 226 to the second DC side 214 of the second DC-to-DC converter 206. The third converter 216 is connected via its AC side 220 to the AC side 222 of the fourth converter 218.
The apparatus 202 is arranged to control the direct current lDci of the first HVDC line 102 by introducing a DC voltage VAB or Vinji in series with the first HVDC line 102. The apparatus 202 may comprise control means 228, e.g. a computer or CPU, for controlling the apparatus 202 and its various components. The control means 228 may be arranged to control the apparatus 202 to introduce a positive DC voltage, Vinji > 0, in series with the first HVDC line 102 for reducing the direct current, i.e. lDci, of the first HVDC line 102, and the control means 228 may be arranged to control the apparatus 202 to introduce a negative DC voltage, Vinji < 0, in series with the first HVDC line 102 for increasing lDci of the first HVDC line 102. The above-mentioned fictive resistance ARinj may be defined for the first HVDC line 102 by the following expression: ARinji = Vinji/Idci. In a corresponding way, the apparatus 202 may be arranged to control the direct current ldc2 of the second HVDC line 108 by introducing a DC voltage Vinj2 in series with the second HVDC line 108.
The apparatus 202 may be arranged to provide a transfer of power, more precisely active power, from the first HVDC line 102 to the second HVDC line 108 and to provide a transfer of power from the second HVDC line 108 to the first HVDC line 102. With reference to Fig. 3, the operation of the apparatus 202 may be schematically illustrated as follows: If it is assumed that voltage Vi (see Fig. 3) of the first HVDC line 102 is 320 kV, voltage V2 (see Fig. 3) of the first HVDC line 102 is 300 kV, and the resistance R (see Fig. 3) of the first HVDC line 102 is 5 Ω, then, without the presence of the apparatus 202, the current l'dci of the first HVDC line 102 could be expressed as l'dc1 = ( \A - V2)/R, which gives l'dc1 = (320-300)/5 kA = 4 kA, and the Power ΡΊ of the first HVDC line 102, when expressed as P? = Vi x Idd, would be ΡΊ = 320 χ 4 MW = 1280 MW through the line. When the apparatus 202 is connected to the first HVDC line 102, as shown in Fig. 3, and introduces a positive DC voltage, Vinji, in series with the first HVDC line 102, e.g. +10 kV, then ldci may be expressed as ldci = { Vi - V2 - Vinji)IR, which gives ldc1 = (320 - 300 - 10)/5 kA = 2 kA and Pi = 320 2 MW = 640 MW through the first HVDC line 102. Thus, by introducing a positive DC voltage, Vinji, in series with the first HVDC line 102, the apparatus 202 reduces the direct current ldci, and an active power extraction from the first HVDC line 102 is provided. A similar reasoning can be applied when the apparatus 202 introduces a negative DC voltage, Vinji, in series with the first HVDC line 102 to increase the direct current ldci of the first HVDC line 102, whereby an active power input to the first HVDC line 102 is provided.
The apparatus 202 may comprise a transformer 230 connected between the third and fourth converters 216, 218, and each of the third and fourth converters may be connectable via its AC side 220, 222 to the transformer 230. The transformer 230 may be an electric power transformer. The transformer 230 may be a high or medium frequency transformer. The high or medium frequency of the transformer 230 will reduce the filter requirements for the reduction of voltage or current ripple. Each of the third and fourth converters 216, 218 may be arranged to convert DC voltage to high or medium frequency AC voltage. The transformer 230 may be arranged to isolate the first DC-to-DC converter 204 from the second DC- to-DC converter 206. The transformer 230 may be arranged to isolate the first DC- to-DC converter 204 from the second HVDC line 108. The transformer 230 may be arranged to isolate the second DC-to-DC converter 206 from the first HVDC line 102. By means of the transformer 230, the flexibility and efficiency of the electric power transmission in an HVDC power transmission system and the control thereof are further improved. The transformer 230 may also take part in fulfilling the voltage requirements of the apparatus 202. The transformer 230 may comprise one or a plurality of transformers or transformer units.
The apparatus 202 of the present invention may be defined as floating between the first and the second HVDC line 102, 108, and the transformer 230 may therefore be free from DC insulation due to station voltage. Thus, the apparatus 202 and the HVDC power transmission system are less expensive to produce.
The apparatus 202 may comprise a first capacitor 232, to which the first DC-to-DC converter 204 and the third converter 216 may be connected. The apparatus 202 may comprise a second capacitor 234, to which the second DC-to-DC converter 206 and the fourth converter 218 may be connected. The first DC-to-DC converter 204 and the third converter 216 may share the first capacitor 232. The second DC-to-DC converter 206 and the fourth converter 218 may share the sec- ond capacitor 234. The first capacitor 232 may be arranged to act as a voltage source for the first DC-to-DC converter 204. The second capacitor 234 may be arranged to act as a voltage source for the second DC-to-DC converter 206. The third converter 216 may be arranged to control the charge balance of the first ca- pacitor 232. The fourth converter 218 may be arranged to control the charge balance of the second capacitor 234. Each of the first and second capacitors 232, 234 may comprise one or a plurality of capacitors, or capacitor components or units. The apparatus 202 may be arranged to provide a transfer of power from the first HVDC line 102 to the second HVDC line 108 and to provide a transfer of power from the second HVDC line 108 to the first HVDC line 102.
With reference to Fig. 4, aspects of the apparatus 202 of Fig. 3 are schematically illustrated in more detail. The first DC-to-DC converter 204 may comprise four pairs 236, 238, 240, 242, also indicated as S< S5P, S< S6P, S7/S7P, SS SSP in Fig. 4 of electrically interconnected electronic switches 244, 246. The second DC-to-DC converter 206 may comprise four pairs 248, 250, 252, 254, also indicated as Si3/Si3P: S SHP, Si5/Si5P: S Siep in Fig. 4 of electrically interconnected electronic switches 256, 258. The third converter 216 may comprise four pairs 260, 262, 264, 266, also indicated as S SIP, S2/S2P, S/S3P, S/S P in Fig. 4 of electrically interconnected electronic switches 268, 270. The fourth converter 218 may comprise four pairs 272, 274, 276, 278, also indicated as S SiP, S2/S2P, S3/S3P: S4/S4P in Fig. 4 of electrically interconnected electronic switches 280, 282.
Each of the first and second DC-to-DC converters 204, 206 may comprise filter means 284, 286 for smoothing out the voltage and current ripple caused by the switching of the electronic switches. The filter means 284; 286 may be con- nected to the electronic switches 244, 246; 256, 258. 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, of each DC-to-DC converter 204, 206 may comprise a capacitor 288, 290, also indicated as C« and Cf2, respectively, in Fig. 4, and an inductor 292, 294, also indicated as Lf1 and Lf2, respec- tively. The capacitor 288; 290 may be connected in parallel with the electronic switches 244, 246; 256, 258. The inductor 292; 294 may be connected in series with the electronic switches 244, 246; 256, 258. By the above-mentioned embodiments of the filter means, a further improved control of the power transmission is provided. In the following and with reference to Figs. 4-8, aspects of the operation of the apparatus 202 in all the four quadrants are discussed, where Tx refers to the transformer 230. Fig. 5 shows a schematic definition of four quadrant operation applicable to the apparatus 202 of the present invention. Fig. 6 is the schematic diagram of Fig. 4 but with reference currents illustrated. Fig. 7 is a schematic diagram illustrating the voltage across the transformer of the apparatus of Fig. 4, and Fig. 8 is a schematic diagram illustrating the output voltage for a positive Vinj2 in Fig. 4: First Quadrant Operation for the second HVDC line:
The switching sequence and circuit condition are analyzed for first quadrant operation for the second HVDC line 108, i.e. current ldc2 of the second HVDC line 108 and voltage Vinj2 are both positive (see Fig. 6). Then, power flows from the second HVDC line 108 to the first HVDC line 102 and the power flow should be supported by the converters 204, 206, 216, 218 of the apparatus 202. This extracted power from the second HVDC line 108 will be added to the first HVDC line 102 and the current ldc1 of the first HVDC line 102 is taken as positive for the analysis which gives a requirement of negative Vinji (as positive ldc1 and negative Vvinji will make power negative, signifying power injection). Some reference current di- rections are shown below in Fig. 6 and they will be used for the following circuit analysis. However, some other terminologies are that Vc1 and Vc2 denote voltage of the first capacitor 232, Ci, and the second capacitor 234, C2, respectively. D1 , D2, D3 and D4 denote duty ratios of the first DC-to-DC converter 204, the third converter 216, the fourth converter 218 and the second DC-to-DC converter 206, respectively.
During the first quadrant operation Vinj2 will be positive and that will be obtained by switching the converter output between + Vc2 and 0. For this, the path through which current flows is given below: The second DC-to-DC converter.
S-I5P - C2-S16P for +ve voltage and Si5P - S13 for zero voltage.
The fourth converter:
S11 - Tx - S12 - C-2 for +ve voltage, S-11 - Tx - Sgp for zero voltage, and
Sg - Tx - S-io - C2 for -ve voltage.
Vinji has to be negative, which is obtained by the following switching: The third converter:
SI P - Ci - S2P - Tx for +ve voltage,
SI P - S3 - Tx for 0 voltage,
S3P - Ci - S4p - Tx for -ve voltage. The first DC-to-DC converter:
Se - Ci - S7 for -ve voltage,
Ss - Sep for zero voltage.
The third and fourth converters 216, 218 should produce an AC voltage across the transformer 230 and for them D2 = D3, i.e. they should be operated at the same duty ratio (as the winding voltage across one transformer forward-biases switches in the other converter).
Figure imgf000018_0001
From power balance,
Vinj2 x ldc2= Vc2 X ld4= Vc2 X 2 D3 ld3 [1 ] D3 is the duty ratio of each switch in the fourth converter 218 and hence the output voltage has effective duty of 2 χ D3, also maximum value of D3 is 0.5 and maximum value of D4 and D1 can be 1 .
Thus, x D3) χ ldc2, and ld4=D4 χ idc2.
Again for the third converter 216 and the first DC-to-DC converter 204, the power balance may be expressed by the following expression:
D2 x Vc1 x {-id2) = Vc1 x {-ld4)=- Vinji x ldci [2] (-ve) sign for current is present due to current going out of dot in the transformer 230. From expressions [1 ] and [2], the following expressions are obtained: D4/(2 D3) ldc2 = D1 /(2 χ D2) χ ldc1, i.e.
D4 x /c/C2=D1 x ldd (because D2 = D3) [3]
From expression [3] it may be concluded that by controlling D1 and D4, currents in both the first and second HVDC lines 102, 108 can be fully controlled independently.
Now, if ldd is in reverse direction, i.e. from B to A (see Fig. 6), then for the same polarity of voltage for Vinj2 and Vinji polarity has to be reversed, which can be achieved by changing the switching of the first DC-to-DC converter 204. For the given switching sequence, the current path is given as follows:
S6 - C1 - S5 for -ve voltage and S6 - SSP for zero voltage.
The analyses are done considering same polarity for Vc1 and Vc2. The analysis shows that the capacitor charge balance can be maintained by maintain- ing the power balance in the converters 204, 206, 216, 218, i.e. by correctly selecting the duty ratio for the converters 204, 206, 216, 218.
Second Quadrant Operation for the second HVDC line when ldd is +ve, i.e. from A to B:
Power flow will be from the first HVDC line 102 to the second HVDC line 108 and Vinji will be positive.
The first DC-to-DC converter:
S5P - Ci - Sep for positive voltage,
S5P - S7 for zero voltage.
The third converter:
Si - Tx - S2 - Ci for positive voltage,
Si - Tx - S3P for zero voltage, S3 - Tx - S4 - Ci for negative voltage.
The fourth converter:
SU P - C-2 - Si2P - Tx for positive voltage,
SU P - S9 - Tx for zero voltage,
S10 - C2 - S11 - Tx for negative voltage.
The second DC-to-DC converter:
Si4 - C2 - Si3 for negative voltage,
S14 - S16P for zero voltage.
Switching operation for other converters can also be derived similarly. For the apparatus 202 of the present invention, the stage of energizing can be done from either of the apparatus sides. In case of both of the first and the second HVDC lines 102, 108 are de-energized, separate power supply can be arranged as backup to energize the apparatus 202.
With reference to Fig. 9, a second embodiment and further aspects of the apparatus 402 according to the present invention are schematically illustrated. The first and second DC-to-DC converters 204, 206 of the second embodiment corre- spond to the first and second DC-to-DC converters 204, 206 of the first embodiment of Fig. 4, and are thus not further discussed in detail. However, the structure of the third and fourth converters 416, 418 of the second embodiment is different from the third and fourth converters 216, 218 of Fig. 4. The third converter 416 may comprise four pairs 460, 462, 464, 466, also indicated as S/D?, S2/D2, S3 D3, S4/D4 in Fig. 9 of electrically interconnected electronic control devices 468, 470. The fourth converter 418 may comprise four pairs 472, 474, 476, 478, also indicated as SQ/D9: S10/D10, Sii/Dii: S12/D12 in Fig. 9 of electrically interconnected electronic control devices 480, 482. Each pair 460, 462, 464, 466, 472, 474, 476, 478 of electronic control devices 468, 470, 480, 482 may comprise an electronic switch 484 and a diode 486. Within each pair, the diode 486 may be anti-parallel to the electronic switch 484. The apparatus 402 of Fig. 9 may comprise a transformer 430 connected between the third and fourth converters 416, 418.
With reference to Fig. 10A, a third embodiment and further aspects of the apparatus 602 according to the present invention are schematically illustrated. The third and fourth converters 416, 418 of the third embodiment correspond to the third and fourth converters 416, 418 of the second embodiment of Fig. 9 and are thus not further discussed in detail. However, the structure of the first and second DC-to-DC converters 604, 606 of the third embodiment is different from first and second DC-to-DC converters 204, 206 of the first and second embodiments of Figs. 4 and 9. The first DC-to-DC converter 604 may comprise four pairs 660, 662, 664, 666, also indicated as S< D5, S< D6, S7/D7, Se/D8 in Fig. 10A of electrically interconnected electronic control devices 668, 670. The second DC-to-DC converter 606 may comprise four pairs 672, 674, 676, 678, also indicated as Si3/D13i Si4/D14i Si5/Di5: Sie Di6 in Fig. 10A of electrically interconnected electronic control devices 680, 682. Each pair 660, 662, 664, 666, 672, 674, 676, 678 of electronic control devices 668, 670, 680, 682 may comprise an electronic switch 684 and a diode 686. Within each pair, the diode 686 may be anti-parallel to the electronic switch 684. The topology of the apparatus 602 of Fig. 10A has the advantage of facili- tated implementation and control because of the absence of bi-directional switches. The apparatus 602 of Fig. 10A may comprise a transformer 630 connected between the third and fourth converters 416, 418. Each of the first and second DC-to-DC converters 604, 606 may comprise filter means 688, 690 corresponding to the filter means 284, 286 of the first and second DC-to-DC converters 204, 206 in Figs. 4.
With reference to Fig. 10B, a fourth embodiment and further aspects of the apparatus 802 according to the present invention are schematically illustrated. The fourth embodiment of the apparatus 802 may essentially correspond to the apparatus of Fig. 10A and has a first and second DC-to-DC converters 604, 606 and a third and fourth converters 416, 418 which may correspond to the first and second DC-to-DC converters 604, 606 and the third and fourth converters 416, 418, respectively, of the apparatus 602 of Fig. 10A. The apparatus 802 of Fig. 10B comprises a transformer 630 connected between the third and fourth converters 416, 418. Each of the third and fourth converters 416, 418 may be connectable via its AC side to the transformer 630. The transformer 630 may be an electric power transformer. The transformer 630 may be a high or medium frequency transformer. The transformer 630 may comprise a first winding 804 connected to the third converter 416 and a second winding 806 connected to the fourth converter 418. The apparatus 802 of Fig. 10B may comprise an auxiliary control device 808. The transformer 630 may comprise a third winding 810 connected to the auxiliary control device 808. The auxiliary control device 808 may be connectable to a load, or an electric power source 812. The electric power source may comprise an AC source. Alternatively, the auxiliary control device 808 may comprise at least one converter 814 for converting direct current to alternating current and/or alternating current to direct current, and the electric power source may comprise a DC source, e.g. an electric battery, or a third HVDC line, to which the at least one converter 814 is connectable. More precisely, the auxiliary control device 808 may comprise a first converter 814 for converting direct current to alternating current and/or al- ternating current to direct current, and a DC-to-DC converter 816 having two DC sides for output and/or input of direct current. The DC-to-DC converter 816 of the auxiliary control device 808 may be connectable to the electric power source 812, which e.g. comprises a DC source, e.g. an electric battery, or a third HVDC line. The configuration of the DC-to-DC converter 816 of the auxiliary control device 808 may correspond to the configuration of the first DC-to-DC converter 204; 604 of any of the above-mentioned apparatuses. The configuration of the first converter 814 of the auxiliary control device 808 may correspond to the configuration of the third converter 216; 416 of any of the above-mentioned apparatuses. The auxiliary control device 808 is arranged to extract power, e.g. active power, from the first HVDC line 102 and/or the second HVDC line 108 and/or to input power, e.g. active power, to the first HVDC line 102 and/or the second HVDC line 108 to influence the introduction of a DC voltage in series with the first HVDC line 102. By means of the auxiliary control device 808 of the apparatus of Fig. 10B, the innovative introduction of a DC voltage in series with the first HVDC transmission or distribution line is further influenced. By means of the apparatus of Fig. 10B, the control of the electric power transmission in an HVDC power transmission system is further improved.
According to advantageous aspects of the apparatus according to the present invention, each electronic switch comprises a power semiconductor switch. Each power semiconductor switch may comprise an Insulated Gate Bipolar Transistor, IGBT, or a Bi-Mode Insulated Gate Transistor, BiGT, or any other suitable power semiconductor switch. Alternatively, each power semiconductor switch may comprise a thyristor, e.g. a gate turn-off thyristor, GTO, an Integrated Gate-Com- mutated Thyristor, IGCT, or a Forced Commutated Thyristor. However, other suit- able thyristors may also be used. The inventors of the present invention have found that the structures of the converters as disclosed above further improve the flexibility and efficiency of the electric power transmission in an HVDC power transmission system and the control thereof.
Instead of a pair of anti-parallel power semiconductor switches, e.g. IGBT, used in the embodiments described above, a pair of anti-series power semiconductor switches, e.g. IGBT or BiGT, as shown in Figs. 1 1 A and 1 1 B may be used. The advantage of the anti-series connection is that reverse blocking power semiconductor switches are not required.
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 (202; 402; 602; 802) for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising a plurality of HVDC transmission or distribution lines (102, 104, 106, 108, 1 10, 1 12, 1 14) for carrying direct current, DC, wherein the apparatus comprises
a first DC-to-DC converter (204),
a second DC-to-DC converter (206), each of the first and second DC-to- DC converters having a first DC side (208, 212) for output and/or input of direct current and a second DC side (210, 214) for output and/or input of direct current, a third converter (216) for converting alternating current, AC, to direct current and/or direct current to alternating current, and
a fourth converter (218) for converting direct current to alternating current and/or alternating current to direct current, each of the third and fourth converters having an AC side (220, 222) for output and/or input of alternating current and a DC side (224, 226) for output and/or input of direct current, wherein
the first DC side of the first DC-to-DC converter is connectable in series with a first HVDC transmission or distribution line (102) of the plurality of HVDC transmission or distribution lines, the first DC side of the second DC-to-DC converter is connectable in series with a second HVDC transmission or distribution line (108) of the plurality of HVDC transmission or distribution lines, the third converter is connected via its DC side to the second DC side of the first DC-to-DC converter, the fourth converter is connected via its DC side to the second DC side of the second DC-to-DC converter, and the third converter is connected via its AC side to the AC side of the fourth converter, and wherein
the apparatus is arranged to control the direct current of the first HVDC transmission or distribution line by introducing a DC voltage in series with the first HVDC transmission or distribution line.
2. An apparatus according to claim 1 , ch aracterized in that the apparatus (202) comprises a transformer (230) connected between the third and fourth converters (216, 218), and in that each of the third and fourth converters is connectable via its AC side (220, 222) to the transformer.
3. An apparatus according to claim 2, characterized in that the transformer (230) is arranged to isolate the first DC-to-DC converter (204) from the second DC-to-DC converter (206).
4. An apparatus according to claim 2 or 3, characterized in that the transformer (230) is arranged to isolate the first DC-to-DC converter (204) from the second HVDC transmission or distribution line (108).
5. An apparatus according to any of the claims 2 to 4, characterized in that the transformer (230) is arranged to isolate the second DC-to-DC converter (206) from the first HVDC transmission or distribution line (102).
6. An apparatus according to any of the claims 2 to 5, characterized in that the transformer (630) comprises a first winding (804) connected to the third converter (416) and a second winding (806) connected to the fourth converter (418), in that the apparatus (802) comprise an auxiliary control device (808), in that the transformer comprises a third winding (810) connected to the auxiliary control device, in that the auxiliary control device is connectable to an electric power source (812), and in that auxiliary control device is arranged to extract power from the first HVDC transmission or distribution line (102) and/or the second HVDC transmission or distribution line (108) and/or to input power to the first HVDC transmission or distribution line and/or the second HVDC transmission or distribution line to influence the introduction of a DC voltage in series with the first HVDC transmission or distribution line.
7. An apparatus according to any of the claims 1 to 6, characterized in that the apparatus (202) comprises a first capacitor (232) to which the first DC-to- DC converter (204) and the third converter (216) are connected.
8. An apparatus according to claim 7, characterized in that the apparatus (202) comprises second capacitor (234) to which the second DC-to-DC converter (206) and the fourth converter (218) are connected.
9. An apparatus according to claim 8, characterized in that the first DC-to- DC converter (204) and the third converter (216) share the first capacitor (232), and in that the second DC-to-DC converter (206) and the fourth converter (218) share the second capacitor (234).
10. An apparatus according to claim 8 or 9, characterized in that the first capacitor (232) is arranged to act as a voltage source for the first DC-to-DC converter (204), and in that the second capacitor (234) is arranged to act as a voltage source for the second DC-to-DC converter (206).
1 1 . An apparatus according to any of the claims 8 to 10, characterized in that the third converter (216) is arranged to control the charge balance of the first capacitor (232), and in that the fourth converter (218) is arranged to control the charge balance of the second capacitor (234).
12. An apparatus according to any of the claims 1 to 1 1 , characterized in that the apparatus (202) is arranged to provide a transfer of power from the first HVDC transmission or distribution line (102) to the second HVDC transmission or distribution line (108) and arranged to provide a transfer of power from the second HVDC transmission or distribution line to the first HVDC transmission or distribution line.
13. An apparatus according to any of the claims 1 to 12, characterized in that each of the first and second DC-to-DC converters (204; 206) comprises four pairs (236, 238, 240, 242; 248, 250, 252, 254) of electronic switches (244, 246; 256, 258).
14. An apparatus (602) according to any of the claims 1 to 12, characterized in that each of the first and second DC-to-DC converters (604; 606) com- prises four pairs (660, 662, 664, 666; 672, 674, 676, 678) of electronic control devices (668, 670; 680, 682), each pair of electronic control devices comprising an electronic switch (684) and a diode (686).
15. An apparatus (202) according to any of the claims 1 to 14, characterized in that each of the third and fourth converters (216; 218) comprises four pairs (260, 262, 264, 266; 272, 274, 276, 278) of electronic switches (268, 270; 280, 282).
16. An apparatus (402; 602) according to any of the claims 1 to 14, characterized in that each of the third and fourth converters (416; 418) comprises four pairs (460, 462, 464, 466; 472, 474, 476, 478) of electronic control devices (468, 470; 480, 482), each pair of electronic control devices comprising an electronic switch (484) and a diode (486).
17. An apparatus according to any of the claims 13 to 16, characterized in that each of the first and second DC-to-DC converters (204; 206) comprises filter means (284; 286) for smoothing out the voltage and current ripple caused by the switching of the electronic switches (244, 246; 256, 258).
18. An apparatus according to any of the claims 13 to 17, characterized in that each electronic switch (244, 246; 256, 258; 268, 270; 280, 282) comprises a power semiconductor switch.
19. An apparatus according to any of the claims 1 to 18, characterized in that the apparatus comprises control means (228) for controlling the apparatus (202), in that the control means are arranged to control the apparatus to introduce a positive DC voltage in series with the first HVDC transmission or distribution line (102) for reducing the direct current of the first HVDC transmission or distribution line, and in that the control means are arranged to control the apparatus to introduce a negative DC voltage in series with the first HVDC transmission or distribution line for increasing the direct current of the first HVDC transmission or distribution line.
20. An apparatus according to any of the claims 1 to 19, characterized in that the apparatus (202) is arranged to control the direct current of the second HVDC transmission or distribution line (108) by introducing a DC voltage in series with the second HVDC transmission or distribution line (108).
21 . An apparatus according to any of the claims 1 to 20, characterized in that the apparatus (202; 402; 602; 802) comprises a first bypass switch (136) con- nectable to the first HVDC transmission or distribution line (102) and connected in parallel with a first DC-to-DC converter (204), and in that when closed, the first bypass switch is arranged to conduct the direct current of the first HVDC transmission or distribution line to electrically bypass the first DC-to-DC converter.
22. An apparatus according to any of the claims 1 to 21 , characterized in that the apparatus (202; 402; 602; 802) comprises a second bypass switch (137) connectable to the second HVDC transmission or distribution line (108) and connected in parallel with a second DC-to-DC converter (206), and in that when closed, the second bypass switch (137) is arranged to conduct the direct current of the second HVDC transmission or distribution line to electrically bypass the sec- ond DC-to-DC converter.
23. A high voltage direct current, HVDC, power transmission system comprising a plurality of HVDC transmission or distribution 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 plurality of HVDC transmission or distribution lines, each of the converter stations being arranged to convert alternating current, AC, to direct current for input to the plurality of HVDC transmission or distribution lines, and/or direct current to alternating current, wherein the system comprises at least one apparatus (202; 402; 602; 802) as claimed in any of the claims 1 -22 for controlling the electric power transmission in the system.
24. An HVDC power transmission system according to claim 23, characterized in that the system comprises at least three converter stations (1 16, 1 18, 120, 122, 124), or at least four converter stations (1 16, 1 18, 120, 122, 124).
25. An HVDC power transmission system according to claim 23 or 24,
characterized in that the plurality of HVDC transmission or distribution lines (102, 104, 106, 108, 1 10, 1 12, 1 14) comprises at least one long-distance HVDC link (102, 108).
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CN104283233A (en) * 2014-10-18 2015-01-14 国家电网公司 Flexible DC power transmission system
CN104377720A (en) * 2014-11-05 2015-02-25 无锡中汇汽车电子科技有限公司 Direct-current transmission current control method based on MMC converter station
EP2869419A1 (en) * 2013-10-29 2015-05-06 Alstom Technology Ltd Power transmission network
CN105281354A (en) * 2014-06-06 2016-01-27 南方电网科学研究院有限责任公司 A method for connecting the inverter side of DC power transmission to AC system

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