WO2012037965A1 - An apparatus for controlling the electric power transmission in a hvdc power transmission system - Google Patents

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

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Publication number
WO2012037965A1
WO2012037965A1 PCT/EP2010/063879 EP2010063879W WO2012037965A1 WO 2012037965 A1 WO2012037965 A1 WO 2012037965A1 EP 2010063879 W EP2010063879 W EP 2010063879W WO 2012037965 A1 WO2012037965 A1 WO 2012037965A1
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WO
WIPO (PCT)
Prior art keywords
converter
hvdc
direct current
transmission line
voltage
Prior art date
Application number
PCT/EP2010/063879
Other languages
French (fr)
Inventor
Subhasish Mukherjee
Tomas U Jonsson
Sasitharan Subramanian
Original Assignee
Abb Technology Ag
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Filing date
Publication date
Application filed by Abb Technology Ag filed Critical Abb Technology Ag
Priority to PCT/EP2010/063879 priority Critical patent/WO2012037965A1/en
Publication of WO2012037965A1 publication Critical patent/WO2012037965A1/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J1/00—Circuit arrangements for dc mains or dc distribution networks
    • H02J1/08—Three-wire systems; Systems having more than three wires
    • 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 at least one HVDC transmission line for carrying direct current, DC. Further, the present invention relates to a HVDC power transmission system comprising at least one HVDC transmission line for carrying direct current, and a plurality of converter stations connected to the at least one HVDC transmission line, each of the converter stations being adapted to convert alternating current, AC, to direct current for input to the at least one HVDC transmission line, and/or direct current to alternating current, the system comprising an apparatus for controlling the electric power transmission in the system.
  • a HVDC power distribution network or a HVDC power transmission system uses direct current for the transmission of electrical power, in contrast to the more common AC systems. For long-distance distribution, HVDC systems may be less expensive and may suffer lower electrical losses.
  • a HVDC power transmission system comprises at least one long-distance HVDC link or cable for carrying direct current a long distance, e.g. under sea, and converter stations for converting alternating current to direct current for input to the HVDC power transmission system and converter stations for converting direct current back to alternating current.
  • US-B2-6,788,033 and US-A-5, 734,258 disclose DC to DC conversion and relate to stationary or portable systems powered by a DC battery, and to electric vehicles.
  • US-B2-6, 914,420 describes a power converter for converting power between a first and a second voltage, and relates to electric vehicles.
  • US-B2-7, 518,266 discloses an AC power transmission system, where a DC transmission ring is used, utilizing controllable AC-DC converters in a multi-in- feed/out-feed arrangement.
  • each of the converter stations may be controlled, e.g. by controlling the DC node voltage of each converter station.
  • the inventors of the present invention have found that the DC node voltage control of the converter stations, or the control of shunt connected converter DC voltages of a DC grid, may not be sufficient in order to avoid or reduce load- flow congestion of the system.
  • the object of the present invention is to improve the electric power transmission in a HVDC power transmission system. It is also an object of the present invention to provide an improved control of the electric power transmission in a HVDC power transmission system. A further object of the present invention is to avoid, reduce or prevent load-flow congestion in the system. Another object of the present invention is to provide an improved HVDC power transmission system.
  • an apparatus for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising at least one HVDC transmission line for carrying direct current, DC
  • the apparatus comprises a first converter for converting alternating current, AC, to direct current and/or direct current to alternating current, a second converter for converting direct current to alternating current and/or alternating current to direct current, each of the first and second converters having an AC side for output and/or input of alternating current and a DC side for output and/or input of direct current, and an electric power transformer connected between the first and second converters, wherein the first converter is connectable via its DC side to the HVDC transmission line, wherein each of the first and second converters is connected via its AC side to the electric power transformer, wherein the second converter is connectable via its DC side to a DC source, and wherein the apparatus is adapted to control the direct current of the HVDC transmission line by introducing a DC voltage in series with the HV
  • the electric power transmission in a HVDC power transmission system and the control thereof are efficiently improved, and load-flow congestion in the system may be avoided, reduced or prevented.
  • the AC side of the second converter may be adapted to pro- vide, directly or indirectly, alternating current to the AC side of the first converter, and/or vice versa.
  • the apparatus of the present invention is especially advantageous and efficient for a HVDC power transmission system of the sort shown in Fig. 1 , which may be called a DC grid concept, where the system comprises several HVDC transmission lines for carrying direct current and several converter stations connected to the HVDC transmission lines.
  • the apparatus of the present invention is especially advantageous when the control of DC node voltage of the converter stations, or the control of shunt connected converter DC voltages of a DC grid, is not sufficient.
  • the direct current of the HVDC transmission line, to which the apparatus is connected can be increased or reduced in order to control the power transmission.
  • the direct current control is attained by the apparatus' introduction, or injection, of a DC voltage in series with the HVDC transmission line.
  • the injected DC voltage produces a fictive resistance, ARinj.
  • the fictive resistance provides an active power extraction or output from the HVDC transmission line when the fictive resistance corresponds to an increase in resistance, i.e. a positive AR lnj , (since a resistance consumes power/energy), or an active power input to the HVDC transmission line when the fictive resistance corresponds to a decrease in resistance, i.e. a negative ARinj.
  • a positive ARinj is produced when the apparatus introduces a positive DC voltage in series with the HVDC transmission line
  • a negative AR lnj is produced when the apparatus introduces a negative DC voltage in series with the HVDC transmission line.
  • the load of the HVDC transmission line, to which the apparatus is connected may be reduced or increased.
  • the apparatus' active power extraction or output from the HVDC transmission line results in a decrease in direct current of the line
  • the apparatus' active power input to the HVDC transmission line results in an increase in direct current of the line.
  • the apparatus of the present invention is adapted to re- gulate the voltage at its output to control the current flow in the HVDC transmission line.
  • the apparatus according to the present invention is adapted to control the direct current of the HVDC transmission line by introducing a fictive resistance in series with the HVDC transmission line by introducing a DC voltage in series with the HVDC transmission 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 electric power transformer By the electric power transformer, the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof are improved.
  • the electric power transformer may also take part in fulfilling the voltage requirements of the apparatus.
  • the electric power transformer may be in the form of a high frequency electric power transformer.
  • the 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 connectable, e.g. electrically or inductively, via additional intermediate electric equipment or units located and connected/connectable between the components, e.g. a transformer, another converter etc.
  • High Voltage may be about 1 -1 .5 kV and above.
  • High Voltage may be about 1 -1 .5 kV and above.
  • High Voltage may be about 500 kV and above, e.g. 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 line voltage.
  • the apparatus comprises control means for controlling the apparatus, wherein the control means are adapted to control the apparatus to introduce a positive DC voltage in series with the HVDC transmission line for re- ducing the direct current of the HVDC transmission line, and wherein the control means are adapted to control the apparatus to introduce a negative DC voltage in series with the HVDC transmission line for increasing the direct current of the HVDC transmission line.
  • the control means may be in form of a control unit and may be connectable to the HVDC power transmission system, e.g. to the HVDC transmission line.
  • the control means may comprise a computer and/or a CPU.
  • control means may be adapted to control the apparatus to introduce a positive fictive resistance in series with the HVDC transmission line by introducing a positive DC voltage in series with the HVDC transmission line for reducing the direct current of the HVDC transmission line
  • control means may be adapted to control the apparatus to introduce a negative fictive resistance in series with the HVDC transmission line by introducing a negative DC voltage in series with the HVDC transmission line for increasing the direct current of the HVDC transmission line.
  • the apparatus comprises measuring means for measuring the DC load flow congestion of the HVDC power transmission system, and the measuring means are adapted to communicate with the control means.
  • the measuring means may be adapted to measure the direct current or direct voltage of the HVDC line, and the measuring means per se may have a structure known the person skilled in the art.
  • the measuring means, or measuring equipment may comprise conventional sensors, e.g. sensors for measuring direct current or direct voltage.
  • the apparatus comprises a bypass switch connectable to the HVDC transmission line and connected in parallel with the first converter, and when closed the bypass switch is adapted to conduct the direct current of the HVDC transmission line to electrically bypass the first converter.
  • the bypass switch By the bypass switch, the first converter, and the apparatus, may be bypassed during fault conditions, whereby the electric power transmission in a HVDC power transmission system and the control thereof are further improved.
  • the apparatus comprises the DC source to which the second converter is connected via its DC side.
  • active power should be absorbed by the DC source, and to effect or introduce a negative fictive resistance, -AR in j, active power should be injected by and from the DC source.
  • the apparatus is adapted to be connected to a DC source comprising an electric battery, to which the second converter is connectable via its DC side.
  • the DC source may comprise a capacitor.
  • the inventors of the present invention have found that the use of an electric battery for the DC source efficiently improves the electric power transmission in a HVDC power transmission system and the control thereof.
  • other suitable DC sources may be used.
  • the DC source may for example comprise photovoltaic cells and/or flywheels etc.
  • the apparatus is adapted to be connected to a DC source being part of a HVDC grid, to which the second converter is connectable via its DC side.
  • a DC source being part of a HVDC grid
  • the second converter is connectable via its DC side.
  • the apparatus is adapted to be connected to a DC source comprising a first cascaded half-bridge cell, to which the second converter is connectable via its DC side.
  • the cascaded half-bridge cell also called Cascaded Two-Level, CTL, cell
  • CTL cascaded Two-Level
  • a cascaded half-bridge cell and a cascaded full-bridge cell per se and their structure are well known to the person skilled in the art and therefore not disclosed or discussed in more detail.
  • the inventors of the present invention have found that the use of a cascaded half-bridge cell for the DC source efficiently improves the electric power transmission in a HVDC power transmission system and the control thereof.
  • the first cascaded half-bridge cell may be adapted to be part of a con- verter station included in the HVDC power transmission system, the converter station being adapted to convert alternating current to direct current, for input to the HVDC transmission line, and/or direct current to alternating current.
  • the apparatus may be adapted to be connected to a DC source comprising a plurality of cascaded half-bridge cells to which the first cascaded half-bridge cell is connectable.
  • the second converter comprises a Voltage
  • VSC Source Converter
  • the second converter comprises four pairs of electronic control switches.
  • the electronic control switches may be connected to one another.
  • the inventors of the present invention have found that this structure of the second converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
  • the second converter comprises four pairs of electronic control devices, each pair of electronic control devices comprising an electronic control switch and a diode.
  • the electronic control devices may be connected to one another.
  • the inventors of the present invention have found that this structure of the second converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
  • the first converter comprises a full-bridge converter.
  • the inventors of the present invention have found that this structure of the first converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
  • the first converter may comprise a full-bridge converter with a bypass switch.
  • the second converter is adapted to convert DC voltage to high frequency AC voltage.
  • the electric power transformer may be a high frequency transformer.
  • the first converter comprises four pairs of electronic control switches.
  • the electronic control switches may be connected to one another.
  • the inventors of the present invention have found that this structure of the first converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
  • the first converter may also comprise a fifth pair of electronic control switches.
  • Each electronic control switch of the fifth pair may comprise a transistor.
  • the fifth pair of electronic control switches may be connected in parallel with the four pairs of electronic control switches.
  • the first converter comprises filter means for smoothing out the voltage and current ripple caused by the switching of the electronic control switches.
  • the filter means may be connected to the electronic control switches.
  • the filter means, or filter components may comprise a capacitor and an inductor.
  • the capacitor may be connected in parallel with the electronic control switches.
  • the inductor may be connected in series with the electronic control switches.
  • each electronic control switch comprises a transistor, e.g. an Insulated Gate Bipolar Transistor, IGBT, or a Bi-Mode Insulated Gate Transistor, BIGT, or any other suitable transistor.
  • each electronic control switch may also comprise a thyristor, e.g. a gate turn-off thyristor, GTO, an Integrated Gate-Commutated Thyristor, IGCT, or a Forced Commutated Thyristor.
  • GTO gate turn-off thyristor
  • IGCT Integrated Gate-Commutated Thyristor
  • Forced Commutated Thyristor e.g. a Forced Commutated Thyristor
  • other suitable thyristors may also be used. The inventors of the present invention have found that these structures of first and/or second converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
  • the electric power transformer is adapted to isolate the first converter from the DC source.
  • the HVDC transmission line, to which the apparatus is connected is also efficiently isolated from the DC source.
  • the first converter is connectable in series with the HVDC transmission line.
  • the apparatus is adapted for four quadrant operation. Aspects of the four quadrant operation are disclosed in the detailed description of preferred embodiments.
  • the apparatus may be adapted for one quadrant operation, two quadrant operation or three quadrant operation, where the quadrant operation/-s may be any of the first to fourth quadrant opera- tions e.g. as disclosed in the detailed description of preferred embodiments.
  • the one, two or three quadrant operation may be attained by replacing suitable
  • IGBT/IGBTs with diode/diodes of a four quadrant converter.
  • the above-mentioned objects of the present invention are also attained by providing a high voltage direct current, HVDC, power transmission system com- prising at least one HVDC transmission line for carrying direct current, DC, and a plurality of converter stations connected to the at least one HVDC transmission line, each of the converter stations being adapted to convert alternating current, AC, to direct current for input to the at least one HVDC transmission line, and/or direct current to alternating current, wherein the system comprises at least one apparatus as claimed in any of the claims 1 -17 for controlling the electric power transmission in the system, and/or at least one apparatus according to any of the above-mentioned embodiments of the apparatus.
  • the at least one HVDC transmission line may be one or a plurality of HVDC transmission lines
  • the system comprises a plurality of HVDC transmission lines.
  • a plurality of HVDC transmission lines or converter stations may be two or more HVDC transmission lines or converter stations, respectively.
  • the at least one apparatus may be one or a plurality of apparatuses, e.g. two or more apparatuses.
  • a plurality of apparatuses may be connected to the same HVDC transmission line, or to different HVDC transmission lines. For example, two apparatuses adapted for two quadrant operation may be connected to the same HVDC transmission line to attain four quadrant operation.
  • the system comprises at least three converter stations.
  • the system comprises at least four converter stations, or at least five converter stations.
  • the at least one HVDC transmission line comprises at least one long-distance HVDC link or cable.
  • the HVDC transmission lines may comprise at least two long-distance HVDC links or cables.
  • Fig. 1 is a schematic block diagram illustrating aspects of the HVDC power transmission system and aspects of the apparatus according to the present invention
  • Fig. 2A is schematic block diagram illustrating a first embodiment of a converter station shown in Fig. 1 ;
  • Fig. 2B is schematic block diagram illustrating a second embodiment of a converter station shown in Fig. 1 ;
  • Fig. 3 is a schematic block diagram illustrating a first embodiment of the apparatus according to the present invention
  • Fig 4 is a schematic diagram illustrating aspects of the apparatus of Fig. 3 in more detail
  • Fig. 5 is a schematic diagram illustrating the four quadrant operation of the apparatus of Fig. 4;
  • Fig. 6 is a schematic diagram illustrating an equivalent circuit for first quadrant operation of the apparatus of Fig. 4;
  • Fig. 7 is a schematic graph illustrating the first quadrant operation of the apparatus of Fig. 4;
  • Fig. 8 is a schematic graph illustrating the second quadrant operation of the apparatus of Fig. 4;
  • Fig. 9 is a schematic diagram illustrating a second embodiment and further aspects of the apparatus according to the present invention.
  • Fig. 10 is a schematic diagram illustrating a third embodiment and further 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 302, 602, 902 according to the present inven- tion.
  • the HVDC power transmission system comprising a plurality of HVDC transmission lines 102, 104, 106, 108, 1 10, 1 12, 1 14 for carrying direct current.
  • the HVDC transmission lines may e.g. comprise HVDC cables, busbars, or other DC conductors.
  • the HVDC transmission lines may comprise at least one long-dis- tance HVDC link.
  • a first and second long-distance HVDC link 102, 108 are provided.
  • HVDC transmission lines and links are well known to the skilled person and thus not discussed in further detail.
  • the HVDC power transmission system comprises a plurality of converter stations 1 16, 1 18, 120, 122, 124 electrically connected to the HVDC transmission lines 102, 104, 106, 108, 1 10, 1 12, 1 14.
  • five converter stations 1 16, 1 18, 120, 122, 124 are provided, but there may be more or fewer converter stations.
  • Each of the converter stations may be adapted to convert alternating current to direct current for input to the transmission lines and convert direct current to alternating current for input to neighbouring AC systems.
  • Each converter station 1 16, 1 18, 120, 122, 124 may be electrically con- nected to a conventional electric power transformer 126, 128, 130, 132, 134 in conventional ways known to the skilled person.
  • Each converter station which may be called a DC Grid converter station, may have asymmetrical monopoles with separate converters for positive and negative polarity, as illustrated in Fig. 2A.
  • each converter station may be in the form of a balanced bipolar converter, as illustrated in Fig. 2B.
  • the alternatives of Figs. 2A and 2B may also be combined in the same system.
  • the apparatus 302, 602, 902 according to the present invention is adapted to be electrically connected to the HVDC system, e.g. by being connected between positions A and B as illustrated in Fig. 1 .
  • the apparatus may e.g.
  • 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 may be configured accordingly in ways known to the skilled person.
  • the HVDC power transmission system comprises an embodiment of the apparatus 302, 602, 902 for controlling the electric power transmission in the system ac- cording to the present invention, and aspects of the apparatus 302, 602, 902 will hereinafter be disclosed.
  • the apparatus 302, 602, 902 may comprise a bypass switch 1 36 (see Fig. 1 ) electrically connectable to the HVDC transmission line 102 to which the appa- ratus 302, 602, 902 is connected and electrically connected in parallel with a first converter 304, 604, 904 (see Figs. 3, 4, 9 and 10) of the apparatus 302, 602, 902.
  • a bypass switch 136 When the bypass switch 136 is closed, it is adapted to conduct the direct current of HVDC transmission line to electrically bypass the first converter 304, 604, 904.
  • the bypass switch 136 By the bypass switch 136, the first converter 304, 604, 904 and the apparatus 302, 602, 902 may be bypassed during fault conditions.
  • Fig. 3 schematically shows a first embodiment of the apparatus according to the present invention for controlling the electric power transmission in a HVDC power transmission system, e.g. as shown in Fig. 1
  • the apparatus comprises a first converter 304 for converting alternating current to direct current and/or direct current to alternating current, and a second converter 306 for converting direct current to alternating current and/or alternating current to direct current.
  • Each of the first and second converters 304, 306 has an AC side 308, 310 for output and/or input of alternating current and a DC side 312, 314 for output and/or input of direct current.
  • the first converter 304 is electrically connectable via its DC side 312 to the HVDC transmission line 102, and the first converter 304 may be electrically connectable in series with the HVDC transmission line 102.
  • the AC side 308 of the first converter 304 is adapted to provide alternating current to the AC side 310 of the second converter 306, and vice versa.
  • the second converter 306 is connectable via its DC side 314 to a DC source 316, e.g. an electric battery or a HVDC grid. An electric battery and a HVDC grid are well known to the skilled person and therefore not discussed in more detail.
  • the apparatus 302 may comprise the DC source 316.
  • the apparatus 302 comprises an electric power transformer 318 connected between the first and second converters 304, 306, each of the first and second converters 304, 306 being electrically connectable, or connected, via its AC side 308, 310 to the electric power transformer 318.
  • the electric power transformer 318 may be a high frequency transformer, and the second converter 306 may be adapted to convert DC voltage to high frequency AC voltage.
  • the electric power transformer 318 may be adapted to isolate the first converter 304 from the DC source 316, and may thus also be adapted to isolate the HVDC line 102 from the DC source 316.
  • the DC source 316 may comprise an apparatus capacitor 320 (see Fig. 4) to which the second converter 306 is electrically connect- able via its DC side 314.
  • the apparatus 302 is adapted to control the direct current of the HVDC transmission line 102 by introducing a DC voltage V A B in series with the HVDC transmission line 102.
  • the apparatus 302 may comprise control means 324, e.g. a computer or CPU, for controlling the apparatus and its various components.
  • the control means 324 are adapted to control the apparatus 302 to introduce a positive DC voltage, V A B > 0, in series with the HVDC transmission line 102 for reducing the direct current, i.e. be , of the HVDC transmission line 102, and the control means 324 are adapted to control the apparatus 302 to introduce a negative DC voltage, V AB ⁇ 0, in series with the HVDC transmission line 102 for increasing be of the HVDC transmission line 102.
  • the second converter 306 may comprise a VSC and may comprise four pairs 402, 404, 406, 408, also indicated as Ss/S's , Se/S' 6, S 7 /S' 7, Ss/S's in Fig. 4, of electrically interconnected electronic control switches 410, 412.
  • the first converter 304 may comprise a full-bridge converter.
  • the first converter 304 may comprise four pairs 414, 416, 418, 420, also indicated as
  • the first converter 304 may also comprise a fifth pair 430 of electronic control switches 431 , 433, also indicated as SAB/S'AB ' M Fig. 4.
  • the fifth pair 430 of electronic control switches may be electrically connected in parallel with the four pairs 414, 416, 418, 420 of electronic control switches.
  • the fifth pair 430 of electronic control switches may be used to give a path to the direct current when the first converter 304 is bypassed to give zero voltage.
  • the first converter 304 may comprise filter means 426, 428, connected to the electronic control switches 422, 424, for smoothing out the voltage and current ripple caused by the switching of the electronic control switches 422, 424.
  • the filter means may comprise a capacitor 426, also indicated as d in Fig. 4, and an inductor 428, also indicated as L f ,.
  • the capacitor 426 may be connected in parallel with the electronic control switches 422, 424.
  • the inductor 428 may be electrically connected in se- ries with the electronic control switches 422, 424.
  • the capacitor 426 may be connected in parallel with the fifth pair 430 of electronic control switches.
  • the filter inductor 428 may be connected in series with the first converter DC terminal with a first end connected to the common point of 414, 41 8 and 430, and with a second end connected to the filter capacitor 426.
  • the other end of the filter capacitor 426 may be connected to the common point of 420, 41 6 and 430.
  • This connection may also be reversed, i.e. the first end of the filter inductor 428 may be connected to the common point of 420, 41 6 and 430, and the second end of the filter inductor 428 may be connected to the filter capacitor 426.
  • the other end of the filter capacitor 426 may be connected to the common point of 414, 41 8 and 430.
  • the four quadrant operation of the apparatus may be supported by bidirectional valves.
  • PWM switching By introducing PWM switching, the injected voltage V A B may be regulated to a desired value or level in an efficient way. PWM switching per se is well known to the skilled person and is thus not discussed in further detail.
  • the power requirement of first converter 304 is supplied from the second converter 306 via the electric power transformer 31 8.
  • the VSC of the second converter 306 may comprise at least two legs which convert direct current to alternating current and/or vice-versa. To effect or introduce a positive fictive resistance, +AR m j, active power should be absorbed by the DC source, and to effect or introduce a negative fictive resistance, -AR in j, active power should be injected by and from the DC source.
  • the apparatus 302 can operate in all the four quadrants as shown in Fig. 5, the voltage and current polarity being as shown in Fig. 1 , 3 or 4.
  • the HVDC line current is flowing from position A to position V (see Fig. 1 ). Since the voltage/potential in position A is greater than in position B, the switches S l , S 2 , S 3 , S 4 are forward-biased.
  • the equivalent circuit for the first quadrant operation is illustrated in Fig. 6, where 1 ⁇ 2 is the voltage in position A, V 2 is the voltage at station 2, which correspond to a converter station on the right side in Fig.
  • V dcs is the transformer secondary voltage.
  • the transistor e.g. IGBT
  • the voltage across the transistor switches should be positive. Kirchoff s voltage law for the first quadrant operation is given as
  • V 1 -V 2 -V fa 0
  • V t -V 2 > V dcs V dcs
  • V IOBT > 0 the transistor switches are forward-biased and the transistor switches may be turned ON.
  • V IOBT ⁇ 0 the transistor switches are reverse- biased and the transistor switches may not be turned ON.
  • the output vol- tage across the positions A-B will be the DC voltage V DCS .
  • zero voltage is inserted by bypassing the DC source at the first converter 304 as illustrated in Fig. 7. In the first converter 304, bypassing can be effected by turning O/ the transistor SAB- In the second converter 306, the DC capacitor C dc ⁇ s bypassed through S 5 , S' 7 or S 6 , S' 8 . With appropriate duty ratio the voltage across the positions A-B may be controlled to give desired positive resistance.
  • transistors S 3 , S 4 are turned ON for positive half cycle and transistors S t , S 2 are turned ON during negative half cycle.
  • the bypass path S A B in the first converter 304 is used to achieve zero voltage across the positions A-B.
  • the switches S' 5 - S' 6 or S' 7 - S' 8 are forward-biased in the second converter 306 since there is a voltage difference between the positions A-B.
  • the voltage across the positions A-B ( V A B) may be regulated by PWM operation as shown in Fig. 8.
  • the PWM voltage may be averaged by the filter means and injected in series with the HVDC line 102.
  • V V I -V V 2 > ⁇ V V dcs
  • Fig. 5 and the current direction is from position Vto position A.
  • the positive voltage VAB ' ⁇ S obtained between positions A-B by turning CW the S S 2 , S 3 , S 4 bridge.
  • the AC square voltage is converted to DC voltage by this bridge.
  • the duty ratio turning ON transistor S'AB
  • the voltage across positions A-B, V A B may be regulated to a desired value.
  • the average voltage between positions A-B may reduced to a desired value which gives a "positive" resistance.
  • the current direction is from position Vto position A, and injected voltage is negative.
  • S'i-S' 2 is O/Vduring posi- tive half cycle and S' 3 -S' 4 is O/Vduring negative half cycle
  • the voltage across positions A-B ( V AB ) is equal to positive DC ( V DCS ) voltage.
  • the DC capacitor 320 may be bypassed using the electronic control switch S'AB-
  • the average voltage across positions A-B may be regulated to desired negative value which gives a "negative" resistance.
  • Fig. 9 schematically shows a second embodiment and further aspects of the apparatus 602 according to the present invention for controlling the electric power transmission in a HVDC power transmission system, e.g. as shown in Fig. 1 .
  • the first converter 604 of this embodiment corresponds to the first converter 304 of the first embodiment of Figs. 3 and 4, and is thus not further discussed.
  • the second converter 606 of the second embodiment is different from the second converter of Fig. 4 and may comprise four pairs 702, 704, 706, 708, also indicated as Ss D ⁇ Se/D ⁇ S 7 /D 7, Ss/D 8 in Fig. 9 , of electrically interconnected electronic control devices 710, 712.
  • Each pair of electronic control devices 710, 712 may comprise an electronic control switch 710 and a diode 712.
  • Fig. 10 schematically shows a third embodiment and further aspects of the apparatus 902 according to the present invention for controlling the electric power transmission in a HVDC power transmission system, e.g. as shown in Fig. 1 .
  • the second converter 906 of the third embodiment may comprise four pairs 802, 804, 806, 808, also indicated as Ss/S's, S ⁇ /S' 6 , S/S , Ss/S's in Fig. 10, of electrically interconnected electronic control switches 810, 812.
  • the first converter 904 may comprise four pairs 814, 816, 818, 820, also indicated as S S' f , S ⁇ S' 2, Ss/S's , S/S 4 in Fig. 10, of electrically interconnected electronic control switches 822, 824.
  • the first converter 904 may also comprise a fifth pair 830 of electronic control switches 831 , 833, also indicated as SAB S'AB-
  • the first converter 904 of the third embodiment may comprise filter means 826, 828, connected to the electronic control switches 822, 824, for smoothing out the voltage and current ripple caused by the switching of the electronic control switches 822, 824.
  • the filter means may comprise a capacitor 826, also indicated as C f in Fig. 10, and an inductor 828, also indicated as L F .
  • the components of the third embodiment are differently interconnected in relation to the first embodiment of Fig. 4, as illustrated in Fig. 10.
  • Figs. 9 and 10 may comprise any of the features mentioned in connection with Fig. 3.
  • a pair of anti-parallel transistors e.g. IGBT
  • a pair of anti-series transistors 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 transistors are not required.
  • Each of the above-mentioned electronic control switches may comprises a transistor, e.g. an IGBT, a BIGT or any other suitable transistor.
  • each of the above-mentioned electronic control switches may comprise a thyristor, e.g. a GTO, an IGCT, or a Forced Commutated Thyristor.

Abstract

An apparatus (302; 602; 902) for controlling the electric power transmission in a high voltage direct current, power transmission system comprising at least one HVDC transmission line (102, 104, 106, 108, 110, 112, 114) for carrying direct current. The apparatus (302) comprises a first converter (304) a second converter (306) each of the first and second converters having an AC side (308, 310) for output and/or input of alternating current and a DC side (312, 314) for output and/or input of direct current, and an electric power transformer (318) connected between the first and second converters. The first converter is connectable via its DC side to the HVDC transmission line (102), each of the first and second converters is connected via its AC side to the electric power transformer, and the second converter is connectable via its DC side to a DC source (316). The apparatus is adapted to control the direct current of the HVDC transmission line by introducing a DC voltage in series with the HVDC transmission line (102).

Description

AN APPARATUS FOR CONTROLLING THE ELECTRIC POWER
TRANSMISSION IN A HVDC POWER TRANSMISSION SYSTEM
Technical Field
The present invention relates to an apparatus for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising at least one HVDC transmission line for carrying direct current, DC. Further, the present invention relates to a HVDC power transmission system comprising at least one HVDC transmission line for carrying direct current, and a plurality of converter stations connected to the at least one HVDC transmission line, each of the converter stations being adapted to convert alternating current, AC, to direct current for input to the at least one HVDC transmission line, and/or direct current to alternating current, the system comprising an apparatus for controlling the electric power transmission in the system.
Background of the Invention
A HVDC power distribution network or a HVDC power transmission system uses direct current for the transmission of electrical power, in contrast to the more common AC systems. For long-distance distribution, HVDC systems may be less expensive and may suffer lower electrical losses. In general, a HVDC power transmission system comprises at least one long-distance HVDC link or cable for carrying direct current a long distance, e.g. under sea, and converter stations for converting alternating current to direct current for input to the HVDC power transmission system and converter stations for converting direct current back to alternating current.
US-B2-6,788,033 and US-A-5, 734,258 disclose DC to DC conversion and relate to stationary or portable systems powered by a DC battery, and to electric vehicles. US-B2-6, 914,420 describes a power converter for converting power between a first and a second voltage, and relates to electric vehicles.
US-B2-7, 518,266 discloses an AC power transmission system, where a DC transmission ring is used, utilizing controllable AC-DC converters in a multi-in- feed/out-feed arrangement.
US 3,694,728 describes a HVDC mesh-operated network comprising several interconnected stations for effecting an exchange of power by means of converters located at the stations and which are connected to AC networks. The Object of the Invention
To control the electric power transmission in a HVDC power transmission system comprising at least one HVDC line and a plurality of converter stations for converting between alternating current and direct current in order to avoid or re- duce DC load-flow congestion in the system, each of the converter stations may be controlled, e.g. by controlling the DC node voltage of each converter station. However, the inventors of the present invention have found that the DC node voltage control of the converter stations, or the control of shunt connected converter DC voltages of a DC grid, may not be sufficient in order to avoid or reduce load- flow congestion of the system.
The object of the present invention is to improve the electric power transmission in a HVDC power transmission system. It is also an object of the present invention to provide an improved control of the electric power transmission in a HVDC power transmission system. A further object of the present invention is to avoid, reduce or prevent load-flow congestion in the system. Another object of the present invention is to provide an improved HVDC power transmission system.
Summary of the Invention
The above-mentioned objects of the present invention are attained by providing an apparatus for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising at least one HVDC transmission line for carrying direct current, DC, wherein the apparatus comprises a first converter for converting alternating current, AC, to direct current and/or direct current to alternating current, a second converter for converting direct current to alternating current and/or alternating current to direct current, each of the first and second converters having an AC side for output and/or input of alternating current and a DC side for output and/or input of direct current, and an electric power transformer connected between the first and second converters, wherein the first converter is connectable via its DC side to the HVDC transmission line, wherein each of the first and second converters is connected via its AC side to the electric power transformer, wherein the second converter is connectable via its DC side to a DC source, and wherein the apparatus is adapted to control the direct current of the HVDC transmission line by introducing a DC voltage in series with the HVDC transmission line. By the innovative apparatus of the present invention, the electric power transmission in a HVDC power transmission system and the control thereof are efficiently improved, and load-flow congestion in the system may be avoided, reduced or prevented. The AC side of the second converter may be adapted to pro- vide, directly or indirectly, alternating current to the AC side of the first converter, and/or vice versa.
The apparatus of the present invention is especially advantageous and efficient for a HVDC power transmission system of the sort shown in Fig. 1 , which may be called a DC grid concept, where the system comprises several HVDC transmission lines for carrying direct current and several converter stations connected to the HVDC transmission lines. The apparatus of the present invention is especially advantageous when the control of DC node voltage of the converter stations, or the control of shunt connected converter DC voltages of a DC grid, is not sufficient. By the apparatus of the present invention, the direct current of the HVDC transmission line, to which the apparatus is connected, can be increased or reduced in order to control the power transmission. The direct current control is attained by the apparatus' introduction, or injection, of a DC voltage in series with the HVDC transmission line. The injected DC voltage produces a fictive resistance, ARinj. The fictive resistance provides an active power extraction or output from the HVDC transmission line when the fictive resistance corresponds to an increase in resistance, i.e. a positive ARlnj, (since a resistance consumes power/energy), or an active power input to the HVDC transmission line when the fictive resistance corresponds to a decrease in resistance, i.e. a negative ARinj. A positive ARinj is produced when the apparatus introduces a positive DC voltage in series with the HVDC transmission line, and a negative ARlnj is produced when the apparatus introduces a negative DC voltage in series with the HVDC transmission line. Thus, by the apparatus of the present invention, the load of the HVDC transmission line, to which the apparatus is connected, may be reduced or increased. The apparatus' active power extraction or output from the HVDC transmission line results in a decrease in direct current of the line, and the apparatus' active power input to the HVDC transmission line results in an increase in direct current of the line. By the increase and decrease in direct current of HVDC transmission line, the power transmission is controlled and load-flow congestion may be avoided, reduced or prevented. Thus, the apparatus of the present invention is adapted to re- gulate the voltage at its output to control the current flow in the HVDC transmission line.
In alternative words, the apparatus according to the present invention is adapted to control the direct current of the HVDC transmission line by introducing a fictive resistance in series with the HVDC transmission line by introducing a DC voltage in series with the HVDC transmission line.
Further, the direct current in a HVDC power transmission system, e.g. a DC grid system, may reverse, and therefore, voltage polarity reversal for maintained fictive resistance is required, which is also attained by the apparatus of the present invention. Further, the apparatus of the present invention has the capability to operate in all the four quadrants, which is discussed in more detail in the detailed description of preferred embodiments.
By the electric power transformer, the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof are improved. The electric power transformer may also take part in fulfilling the voltage requirements of the apparatus. The electric power transformer may be in the form of a high frequency electric power transformer.
The 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 connectable, e.g. electrically or inductively, via additional intermediate electric equipment or units located and connected/connectable between the components, e.g. a transformer, another converter etc.
In general, High Voltage may be about 1 -1 .5 kV and above. However, for
HVDC applications and systems, High Voltage may be about 500 kV and above, e.g. 800 kV or 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 line voltage.
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 adapted to control the apparatus to introduce a positive DC voltage in series with the HVDC transmission line for re- ducing the direct current of the HVDC transmission line, and wherein the control means are adapted to control the apparatus to introduce a negative DC voltage in series with the HVDC transmission line for increasing the direct current of the HVDC transmission line. By the control means of this embodiment, the current flow in the HVDC transmission line is efficiently controlled. The control means may be in form of a control unit and may be connectable to the HVDC power transmission system, e.g. to the HVDC transmission line. The control means may comprise a computer and/or a CPU. In alternative words, the control means may be adapted to control the apparatus to introduce a positive fictive resistance in series with the HVDC transmission line by introducing a positive DC voltage in series with the HVDC transmission line for reducing the direct current of the HVDC transmission line, and the control means may be adapted to control the apparatus to introduce a negative fictive resistance in series with the HVDC transmission line by introducing a negative DC voltage in series with the HVDC transmission line for increasing the direct current of the HVDC transmission line.
According to a further advantageous embodiment of the apparatus according to the present invention, the apparatus comprises measuring means for measuring the DC load flow congestion of the HVDC power transmission system, and the measuring means are adapted to communicate with the control means. The measuring means may be adapted to measure the direct current or direct voltage of the HVDC line, and the measuring means per se may have a structure known the person skilled in the art. The measuring means, or measuring equipment, may comprise conventional sensors, e.g. sensors for measuring direct current or direct voltage.
According to another advantageous embodiment of the apparatus according to the present invention, the apparatus comprises a bypass switch connectable to the HVDC transmission line and connected in parallel with the first converter, and when closed the bypass switch is adapted to conduct the direct current of the HVDC transmission line to electrically bypass the first converter. By the bypass switch, the first converter, and the apparatus, may be bypassed during fault conditions, whereby the electric power transmission in a HVDC power transmission system and the control thereof are further improved.
According to yet another advantageous embodiment of the apparatus according to the present invention, the apparatus comprises the DC source to which the second converter is connected via its DC side. To effect or introduce a positive fictive resistance, +AF?/ny, active power should be absorbed by the DC source, and to effect or introduce a negative fictive resistance, -ARinj, active power should be injected by and from the DC source.
According to an advantageous embodiment of the apparatus according to the present invention, the apparatus is adapted to be connected to a DC source comprising an electric battery, to which the second converter is connectable via its DC side. The DC source may comprise a capacitor. The inventors of the present invention have found that the use of an electric battery for the DC source efficiently improves the electric power transmission in a HVDC power transmission system and the control thereof. However, other suitable DC sources may be used. The DC source may for example comprise photovoltaic cells and/or flywheels etc.
According to a further advantageous embodiment of the apparatus according to the present invention, the apparatus is adapted to be connected to a DC source being part of a HVDC grid, to which the second converter is connectable via its DC side. The inventors of the present invention have found that the use of a DC source being part of a HVDC grid provides efficient electric power transmission in a HVDC power transmission system and an efficient control thereof.
According to another advantageous embodiment of the apparatus accord- ing to the present invention, the apparatus is adapted to be connected to a DC source comprising a first cascaded half-bridge cell, to which the second converter is connectable via its DC side. The cascaded half-bridge cell (also called Cascaded Two-Level, CTL, cell) may be provided by a cascaded full-bridge cell. A cascaded half-bridge cell and a cascaded full-bridge cell per se and their structure are well known to the person skilled in the art and therefore not disclosed or discussed in more detail. The inventors of the present invention have found that the use of a cascaded half-bridge cell for the DC source efficiently improves the electric power transmission in a HVDC power transmission system and the control thereof. The first cascaded half-bridge cell may be adapted to be part of a con- verter station included in the HVDC power transmission system, the converter station being adapted to convert alternating current to direct current, for input to the HVDC transmission line, and/or direct current to alternating current. The apparatus may be adapted to be connected to a DC source comprising a plurality of cascaded half-bridge cells to which the first cascaded half-bridge cell is connectable. According to still another advantageous embodiment of the apparatus according to the present invention, the second converter comprises a Voltage
Source Converter, VSC. By this embodiment the electric power transmission in a HVDC power transmission system and the control thereof are further improved. However, other suitable converters sorts may be used.
According to yet another advantageous embodiment of the apparatus according to the present invention, the second converter comprises four pairs of electronic control switches. The electronic control switches may be connected to one another. The inventors of the present invention have found that this structure of the second converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
According to still another advantageous embodiment of the apparatus according to the present invention, the second converter comprises four pairs of electronic control devices, each pair of electronic control devices comprising an electronic control switch and a diode. The electronic control devices may be connected to one another. The inventors of the present invention have found that this structure of the second converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
According to an advantageous embodiment of the apparatus according to the present invention, the first converter comprises a full-bridge converter. The inventors of the present invention have found that this structure of the first converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof. The first converter may comprise a full-bridge converter with a bypass switch.
According to another advantageous embodiment of the apparatus according to the present invention, the second converter is adapted to convert DC voltage to high frequency AC voltage. Advantageously, the electric power transformer may be a high frequency transformer. By this embodiment, the electric power transmission in a HVDC power transmission system and the control thereof are further improved.
According to yet another advantageous embodiment of the apparatus according to the present invention, the first converter comprises four pairs of electronic control switches. The electronic control switches may be connected to one another. The inventors of the present invention have found that this structure of the first converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof. Advantageously, the first converter may also comprise a fifth pair of electronic control switches. Each electronic control switch of the fifth pair may comprise a transistor. The fifth pair of electronic control switches may be connected in parallel with the four pairs of electronic control switches.
According to still another advantageous embodiment of the apparatus according to the present invention, the first converter comprises filter means for smoothing out the voltage and current ripple caused by the switching of the electronic control switches. The filter means may be connected to the electronic control switches. By smoothing out the voltage and current ripple, a further improved control of the electric power transmission is attained. The filter means, or filter components, may comprise a capacitor and an inductor. The capacitor may be connected in parallel with the electronic control switches. The inductor may be connected in series with the electronic control switches. By the above-mentioned embodiments of the filter means, a further improved control of the power transmission is provided.
According to an advantageous embodiment of the apparatus according to the present invention, each electronic control switch comprises a transistor, e.g. an Insulated Gate Bipolar Transistor, IGBT, or a Bi-Mode Insulated Gate Transistor, BIGT, or any other suitable transistor. Alternatively, each electronic control switch may also comprise a thyristor, e.g. a gate turn-off thyristor, GTO, an Integrated Gate-Commutated Thyristor, IGCT, or a Forced Commutated Thyristor. However, other suitable thyristors may also be used. The inventors of the present invention have found that these structures of first and/or second converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
According to a further advantageous embodiment of the apparatus accord- ing to the present invention, the electric power transformer is adapted to isolate the first converter from the DC source. By this embodiment, the HVDC transmission line, to which the apparatus is connected, is also efficiently isolated from the DC source. According to another advantageous embodiment of the apparatus according to the present invention, the first converter is connectable in series with the HVDC transmission line.
According to yet another advantageous embodiment of the apparatus ac- cording to the present invention, the apparatus is adapted for four quadrant operation. Aspects of the four quadrant operation are disclosed in the detailed description of preferred embodiments. Advantageously, the apparatus may be adapted for one quadrant operation, two quadrant operation or three quadrant operation, where the quadrant operation/-s may be any of the first to fourth quadrant opera- tions e.g. as disclosed in the detailed description of preferred embodiments. The one, two or three quadrant operation may be attained by replacing suitable
IGBT/IGBTs with diode/diodes of a four quadrant converter.
The above-mentioned objects of the present invention are also attained by providing a high voltage direct current, HVDC, power transmission system com- prising at least one HVDC transmission line for carrying direct current, DC, and a plurality of converter stations connected to the at least one HVDC transmission line, each of the converter stations being adapted to convert alternating current, AC, to direct current for input to the at least one HVDC transmission line, and/or direct current to alternating current, wherein the system comprises at least one apparatus as claimed in any of the claims 1 -17 for controlling the electric power transmission in the system, and/or at least one apparatus according to any of the above-mentioned embodiments of the apparatus. Positive technical effects of the HVDC power transmission system according to the present invention, and its embodiments, correspond to the above-mentioned technical effects mentioned in connection with the apparatus according to the present invention, and its embodiments. The at least one HVDC transmission line may be one or a plurality of HVDC transmission lines
According to an advantageous embodiment of the HVDC power transmission system according to the present invention, the system comprises a plurality of HVDC transmission lines.
A plurality of HVDC transmission lines or converter stations may be two or more HVDC transmission lines or converter stations, respectively. The at least one apparatus may be one or a plurality of apparatuses, e.g. two or more apparatuses. A plurality of apparatuses may be connected to the same HVDC transmission line, or to different HVDC transmission lines. For example, two apparatuses adapted for two quadrant operation may be connected to the same HVDC transmission line to attain four quadrant operation.
According to an advantageous embodiment of the HVDC power transmis- sion system according to the present invention, the system comprises at least three converter stations. Advantageously, the system comprises at least four converter stations, or at least five converter stations.
According to a further advantageous embodiment of the HVDC power transmission system according to the present invention, the at least one HVDC transmission line comprises at least one long-distance HVDC link or cable. Advantageously, the HVDC transmission lines may comprise at least two long-distance HVDC links or cables.
The above-mentioned embodiments and features of the apparatus and the HVDC power transmission system, respectively, according to the present invention may be combined in various possible ways providing further advantageous embodiments.
Further advantageous embodiments of the apparatus and the HVDC power transmission system, respectively, according to the present invention and further advantages with the present invention emerge from the detailed description of embodiments.
Brief Description of the Drawings
The present invention will now be described, for exemplary purposes, in more detail by way of embodiments and with reference to the enclosed drawings, in which:
Fig. 1 is a schematic block diagram illustrating aspects of the HVDC power transmission system and aspects of the apparatus according to the present invention;
Fig. 2A is schematic block diagram illustrating a first embodiment of a converter station shown in Fig. 1 ;
Fig. 2B is schematic block diagram illustrating a second embodiment of a converter station shown in Fig. 1 ;
Fig. 3 is a schematic block diagram illustrating a first embodiment of the apparatus according to the present invention; Fig 4 is a schematic diagram illustrating aspects of the apparatus of Fig. 3 in more detail;
Fig. 5 is a schematic diagram illustrating the four quadrant operation of the apparatus of Fig. 4;
Fig. 6 is a schematic diagram illustrating an equivalent circuit for first quadrant operation of the apparatus of Fig. 4;
Fig. 7 is a schematic graph illustrating the first quadrant operation of the apparatus of Fig. 4;
Fig. 8 is a schematic graph illustrating the second quadrant operation of the apparatus of Fig. 4;
Fig. 9 is a schematic diagram illustrating a second embodiment and further aspects of the apparatus according to the present invention;
Fig. 10 is a schematic diagram illustrating a third embodiment and further 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
Pulse Width Modulation PWM
Voltage Source Converter VSC
Fig.1 schematically shows aspects of the HVDC power transmission system and aspects of the apparatus 302, 602, 902 according to the present inven- tion. The HVDC power transmission system comprising a plurality of HVDC transmission lines 102, 104, 106, 108, 1 10, 1 12, 1 14 for carrying direct current. The HVDC transmission lines may e.g. comprise HVDC cables, busbars, or other DC conductors. The HVDC transmission lines may comprise at least one long-dis- tance HVDC link. In Fig. 1 , a first and second long-distance HVDC link 102, 108 are provided. HVDC transmission lines and links are well known to the skilled person and thus not discussed in further detail. The HVDC power transmission system comprises a plurality of converter stations 1 16, 1 18, 120, 122, 124 electrically connected to the HVDC transmission 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. Each of the converter stations may be adapted to convert alternating current to direct current for input to the transmission lines and convert direct current to alternating current for input to neighbouring AC systems. Each converter station 1 16, 1 18, 120, 122, 124 may be electrically con- nected to a conventional electric power transformer 126, 128, 130, 132, 134 in conventional ways known to the skilled person. Electric power transformers and their function are well known to the person skilled in the art and therefore not discussed in more detail. Each converter station, which may be called a DC Grid converter station, may have asymmetrical monopoles with separate converters for positive and negative polarity, as illustrated in Fig. 2A. Alternatively, each converter station may be in the form of a balanced bipolar converter, as illustrated in Fig. 2B. The alternatives of Figs. 2A and 2B may also be combined in the same system. The apparatus 302, 602, 902 according to the present invention is adapted to be electrically connected to the HVDC system, e.g. by being connected between positions A and B as illustrated in Fig. 1 . However, other locations and connections points are possible, and the apparatus may e.g. be connected to any of the other HVDC transmission lines. Rnne of the HVDC transmission line 102 in Fig. 1 illustrates the resistance of the line 102, and lDC in Fig. 1 is the direct current through the line 102, i.e. the direct current carried by the line 102. 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 may be configured accordingly in ways known to the skilled person. The HVDC power transmission system comprises an embodiment of the apparatus 302, 602, 902 for controlling the electric power transmission in the system ac- cording to the present invention, and aspects of the apparatus 302, 602, 902 will hereinafter be disclosed.
The apparatus 302, 602, 902 may comprise a bypass switch 1 36 (see Fig. 1 ) electrically connectable to the HVDC transmission line 102 to which the appa- ratus 302, 602, 902 is connected and electrically connected in parallel with a first converter 304, 604, 904 (see Figs. 3, 4, 9 and 10) of the apparatus 302, 602, 902. When the bypass switch 136 is closed, it is adapted to conduct the direct current of HVDC transmission line to electrically bypass the first converter 304, 604, 904. By the bypass switch 136, the first converter 304, 604, 904 and the apparatus 302, 602, 902 may be bypassed during fault conditions.
Fig. 3 schematically shows a first embodiment of the apparatus according to the present invention for controlling the electric power transmission in a HVDC power transmission system, e.g. as shown in Fig. 1 The apparatus comprises a first converter 304 for converting alternating current to direct current and/or direct current to alternating current, and a second converter 306 for converting direct current to alternating current and/or alternating current to direct current. Each of the first and second converters 304, 306 has an AC side 308, 310 for output and/or input of alternating current and a DC side 312, 314 for output and/or input of direct current. The first converter 304 is electrically connectable via its DC side 312 to the HVDC transmission line 102, and the first converter 304 may be electrically connectable in series with the HVDC transmission line 102. The AC side 308 of the first converter 304 is adapted to provide alternating current to the AC side 310 of the second converter 306, and vice versa. The second converter 306 is connectable via its DC side 314 to a DC source 316, e.g. an electric battery or a HVDC grid. An electric battery and a HVDC grid are well known to the skilled person and therefore not discussed in more detail. The apparatus 302 may comprise the DC source 316. The apparatus 302 comprises an electric power transformer 318 connected between the first and second converters 304, 306, each of the first and second converters 304, 306 being electrically connectable, or connected, via its AC side 308, 310 to the electric power transformer 318. The electric power transformer 318 may be a high frequency transformer, and the second converter 306 may be adapted to convert DC voltage to high frequency AC voltage. The electric power transformer 318 may be adapted to isolate the first converter 304 from the DC source 316, and may thus also be adapted to isolate the HVDC line 102 from the DC source 316. The DC source 316 may comprise an apparatus capacitor 320 (see Fig. 4) to which the second converter 306 is electrically connect- able via its DC side 314.
The apparatus 302 is adapted to control the direct current of the HVDC transmission line 102 by introducing a DC voltage VAB in series with the HVDC transmission line 102. The apparatus 302 may comprise control means 324, e.g. a computer or CPU, for controlling the apparatus and its various components. The control means 324 are adapted to control the apparatus 302 to introduce a positive DC voltage, VAB > 0, in series with the HVDC transmission line 102 for reducing the direct current, i.e. be, of the HVDC transmission line 102, and the control means 324 are adapted to control the apparatus 302 to introduce a negative DC voltage, VAB < 0, in series with the HVDC transmission line 102 for increasing be of the HVDC transmission line 102. The above-mentioned fictive resistance ARinj may be defined by the following expression: ARinj = VABI be- With reference to Fig. 4, aspects of the apparatus of Fig. 3 are schematically illustrated in more detail. The second converter 306 may comprise a VSC and may comprise four pairs 402, 404, 406, 408, also indicated as Ss/S's, Se/S'6, S7/S'7, Ss/S's in Fig. 4, of electrically interconnected electronic control switches 410, 412. The first converter 304 may comprise a full-bridge converter. The first converter 304 may comprise four pairs 414, 416, 418, 420, also indicated as
Sj/S'i, S^S'2, S3/S'3, S4 S in Fig. 4, of electrically interconnected electronic control switches 422, 424. The first converter 304 may also comprise a fifth pair 430 of electronic control switches 431 , 433, also indicated as SAB/S'AB 'M Fig. 4. The fifth pair 430 of electronic control switches may be electrically connected in parallel with the four pairs 414, 416, 418, 420 of electronic control switches. The fifth pair 430 of electronic control switches may be used to give a path to the direct current when the first converter 304 is bypassed to give zero voltage. The first converter 304 may comprise filter means 426, 428, connected to the electronic control switches 422, 424, for smoothing out the voltage and current ripple caused by the switching of the electronic control switches 422, 424. The filter means may comprise a capacitor 426, also indicated as d in Fig. 4, and an inductor 428, also indicated as Lf,. The capacitor 426 may be connected in parallel with the electronic control switches 422, 424. The inductor 428 may be electrically connected in se- ries with the electronic control switches 422, 424. The capacitor 426 may be connected in parallel with the fifth pair 430 of electronic control switches.
The filter inductor 428 may be connected in series with the first converter DC terminal with a first end connected to the common point of 414, 41 8 and 430, and with a second end connected to the filter capacitor 426. The other end of the filter capacitor 426 may be connected to the common point of 420, 41 6 and 430. This connection may also be reversed, i.e. the first end of the filter inductor 428 may be connected to the common point of 420, 41 6 and 430, and the second end of the filter inductor 428 may be connected to the filter capacitor 426. The other end of the filter capacitor 426 may be connected to the common point of 414, 41 8 and 430.
The four quadrant operation of the apparatus may be supported by bidirectional valves. By introducing PWM switching, the injected voltage VAB may be regulated to a desired value or level in an efficient way. PWM switching per se is well known to the skilled person and is thus not discussed in further detail. The power requirement of first converter 304 is supplied from the second converter 306 via the electric power transformer 31 8. The VSC of the second converter 306 may comprise at least two legs which convert direct current to alternating current and/or vice-versa. To effect or introduce a positive fictive resistance, +ARmj, active power should be absorbed by the DC source, and to effect or introduce a negative fictive resistance, -ARinj, active power should be injected by and from the DC source.
With reference to Figs. 5-8, aspects of the four quadrant operation of the apparatus of Fig. 4 will now be illustrated. As mentioned above, the apparatus 302 can operate in all the four quadrants as shown in Fig. 5, the voltage and current polarity being as shown in Fig. 1 , 3 or 4. In the first quadrant operation, the HVDC line current is flowing from position A to position V (see Fig. 1 ). Since the voltage/potential in position A is greater than in position B, the switches Sl , S2 , S3 , S4 are forward-biased. The equivalent circuit for the first quadrant operation is illustrated in Fig. 6, where ½ is the voltage in position A, V2 is the voltage at station 2, which correspond to a converter station on the right side in Fig. 3 {station 1 in Fig. 6 corresponds to a converter station on the left side in Fig. 3), and Vdcs is the transformer secondary voltage. To assure forward-biasing for the transistor (e.g. IGBT) switches, the voltage across the transistor switches should be positive. Kirchoff s voltage law for the first quadrant operation is given as
V1 -V2 -Vfa > 0
This implies that Vt -V2 > Vdcs , and the voltage across the transistor is positive. When VIOBT > 0 , the transistor switches are forward-biased and the transistor switches may be turned ON. When VIOBT < 0 , the transistor switches are reverse- biased and the transistor switches may not be turned ON.
If transistor switches St , S2 are turning ON during positive half cycle and transistor switches S3 , S4 are turning ON during negative half cycle, the output vol- tage across the positions A-B will be the DC voltage VDCS. To regulate the injected voltage to the HVDC line 102, zero voltage is inserted by bypassing the DC source at the first converter 304 as illustrated in Fig. 7. In the first converter 304, bypassing can be effected by turning O/ the transistor SAB- In the second converter 306, the DC capacitor Cdc\s bypassed through S5, S'7 or S6, S'8. With appropriate duty ratio the voltage across the positions A-B may be controlled to give desired positive resistance.
During the second quadrant operation, to get negative voltage across the positions A-B, transistors S3 , S4 are turned ON for positive half cycle and transistors St , S2 are turned ON during negative half cycle. The bypass path SAB in the first converter 304 is used to achieve zero voltage across the positions A-B. The switches S'5- S'6 or S'7- S'8 are forward-biased in the second converter 306 since there is a voltage difference between the positions A-B. The voltage across the positions A-B ( VAB) may be regulated by PWM operation as shown in Fig. 8. The PWM voltage may be averaged by the filter means and injected in series with the HVDC line 102. If the duty ratio is increased, the voltage VAB will be more negative and a "negative" resistance, i.e. a decrease in resistance, is introduced in the HVDC line 102. Since -V2 > -Vdcs is always true, the forward-biased condition for the first converter 304 is fulfilled. However, it may not true for the second converter 306 since the DC polarity is reversed with respect to the first converter 304. Thus, the condition for forward-biasing may be given as
nV1 -nV2 >Vdc where n is the transformation ratio of the transformer 318. The above condition may be rewritten on the secondary side as
V VI -V V2 > ^ V Vdcs
Thus, the same condition is implied on the second quadrant operation.
During the third quadrant operation, the switching is effected as shown in
Fig. 5 and the current direction is from position Vto position A. The positive voltage VAB '\S obtained between positions A-B by turning CW the S S2, S3, S4 bridge.
The AC square voltage is converted to DC voltage by this bridge. By inserting zero voltage by varying the duty ratio (turning ON transistor S'AB), the voltage across positions A-B, VAB, may be regulated to a desired value. Thus, the average voltage between positions A-B may reduced to a desired value which gives a "positive" resistance.
During the fourth quadrant operation, the current direction is from position Vto position A, and injected voltage is negative. When S'i-S'2 is O/Vduring posi- tive half cycle and S'3-S'4 is O/Vduring negative half cycle, the voltage across positions A-B ( VAB) is equal to positive DC ( VDCS) voltage. To regulate VAB to a desired value, the DC capacitor 320 may be bypassed using the electronic control switch S'AB- Thus, the average voltage across positions A-B may be regulated to desired negative value which gives a "negative" resistance.
Fig. 9 schematically shows a second embodiment and further aspects of the apparatus 602 according to the present invention for controlling the electric power transmission in a HVDC power transmission system, e.g. as shown in Fig. 1 . The first converter 604 of this embodiment corresponds to the first converter 304 of the first embodiment of Figs. 3 and 4, and is thus not further discussed. However, the second converter 606 of the second embodiment is different from the second converter of Fig. 4 and may comprise four pairs 702, 704, 706, 708, also indicated as Ss D^ Se/D^ S7/D7, Ss/D8 in Fig. 9, of electrically interconnected electronic control devices 710, 712. Each pair of electronic control devices 710, 712 may comprise an electronic control switch 710 and a diode 712.
Fig. 10 schematically shows a third embodiment and further aspects of the apparatus 902 according to the present invention for controlling the electric power transmission in a HVDC power transmission system, e.g. as shown in Fig. 1 . The second converter 906 of the third embodiment may comprise four pairs 802, 804, 806, 808, also indicated as Ss/S's, S</S'6, S/S , Ss/S's in Fig. 10, of electrically interconnected electronic control switches 810, 812. The first converter 904 may comprise four pairs 814, 816, 818, 820, also indicated as S S'f, S^S'2, Ss/S's, S/S 4 in Fig. 10, of electrically interconnected electronic control switches 822, 824. The first converter 904 may also comprise a fifth pair 830 of electronic control switches 831 , 833, also indicated as SAB S'AB- The first converter 904 of the third embodiment may comprise filter means 826, 828, connected to the electronic control switches 822, 824, for smoothing out the voltage and current ripple caused by the switching of the electronic control switches 822, 824. The filter means may comprise a capacitor 826, also indicated as Cf in Fig. 10, and an inductor 828, also indicated as LF. The components of the third embodiment are differently interconnected in relation to the first embodiment of Fig. 4, as illustrated in Fig. 10.
Otherwise, the second and third embodiments of Figs. 9 and 10 may comprise any of the features mentioned in connection with Fig. 3.
Instead of a pair of anti-parallel transistors, e.g. IGBT, used in the embodiments described above, a pair of anti-series transistors, 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 transistors are not required.
Each of the above-mentioned electronic control switches may comprises a transistor, e.g. an IGBT, a BIGT or any other suitable transistor. Alternatively, each of the above-mentioned electronic control switches may comprise a thyristor, e.g. a GTO, an IGCT, or a Forced Commutated Thyristor.
The invention shall not be considered limited to the embodiments illustrated, but can be modified and altered in many ways by one skilled in the art, without departing from the scope the appended claims. For example, the disclosed embodiments may be combined in various possible ways, and additional electric equipment, devices or units may be connected to and between the components of the embodiments.

Claims

1 . An apparatus (302; 602; 902) for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system compris- ing at least one HVDC transmission line (102, 104, 106, 108, 1 10, 1 12, 1 14) for carrying direct current, DC, characterized in that the apparatus (302) comprises a first converter (304) for converting alternating current, AC, to direct current and/or direct current to alternating current, a second converter (306) for converting direct current to alternating current and/or alternating current to direct current, each of the first and second converters having an AC side (308, 310) for output and/or input of alternating current and a DC side (312, 314) for output and/or input of direct current, and an electric power transformer (318) connected between the first and second converters, in that the first converter is connectable via its DC side to the HVDC transmission line (102), in that each of the first and second con- verters is connected via its AC side to the electric power transformer, in that the second converter is connectable via its DC side to a DC source (316), and in that the apparatus is adapted to control the direct current of the HVDC transmission line by introducing a DC voltage in series with the HVDC transmission line (102).
2. An apparatus according to claim 1 , characterized in that the apparatus (302) comprises control means (324) for controlling the apparatus, in that the control means are adapted to control the apparatus to introduce a positive DC voltage in series with the HVDC transmission line (102) for reducing the direct current of the HVDC transmission line, and in that the control means are adapted to control the apparatus to introduce a negative DC voltage in series with the HVDC transmission line for increasing the direct current of the HVDC transmission line.
3. An apparatus according to claim 1 or 2, characterized in that the apparatus (302; 602; 902) comprises a bypass switch (136) connectable to the HVDC transmission line (102) and connected in parallel with the first converter (304), and in that when closed the bypass switch is adapted to conduct the direct current of the HVDC transmission line to electrically bypass the first converter.
4. An apparatus according to any of the claims 1 to 3, characterized in that the apparatus (302; 602; 902) comprises the DC source (316) to which the second converter (306) is connected via its DC side (314).
5. An apparatus according to any of the claims 1 to 4, characterized in that the apparatus is adapted to be connected to a DC source (316) comprising an electric battery, to which the second converter (306) is connectable via its DC side (314).
6. An apparatus according to any of the claims 1 to 5, characterized in that the apparatus is adapted to be connected to a DC source (316) being part of a HVDC grid, to which the second converter (306) is connectable via its DC side (314).
7. An apparatus according to any of the claims 1 to 6, characterized in that the second converter (306) comprises a Voltage Source Converter, VSC.
8. An apparatus according to any of the claims 1 to 7, characterized in that the second converter (306) comprises four pairs (402, 404, 406, 408) of elec- tronic control switches (410, 412).
9. An apparatus according to any of the claims 1 to 7, characterized in that the second converter (606) comprises four pairs (702, 704, 706, 708) of electronic control devices (710, 712), each pair of electronic control devices compris- ing an electronic control switch (710) and a diode (712).
10. An apparatus according to any of the claims 1 to 9, characterized in that the first converter (304) comprises a full-bridge converter.
1 1 . An apparatus according to any of the claims 1 to 10, characterized in that the second converter (306) is adapted to convert DC voltage to high frequency AC voltage.
12. An apparatus according to any of the claims 1 to 1 1 , characterized in that the first converter (304) comprises four pairs (414, 416, 418, 420) of electronic control switches (422, 424).
13. An apparatus according to claim 12, characterized in that the first converter (304) comprises a fifth pair (430) of electronic control switches (431 , 433).
14. An apparatus according to claim 12 or 13, characterized in that the first converter (304) comprises filter means (426, 428) for smoothing out the voltage and current ripple caused by the switching of the electronic control switches (422, 424).
15. An apparatus according to claim 8, 9, 12, 13 or 14, characterized in that each electronic control switch (410, 422, 424; 710; 810, 812) comprises a transistor.
16. An apparatus according to any of the claims 1 to 15, characterized in that the electric power transformer (318) is adapted to isolate the first converter (304) from the DC source (316).
17. An apparatus according to any of the claims 1 to 16, characterized in that the first converter (304) is connectable in series with the HVDC transmission line (102).
18. A high voltage direct current, HVDC, power transmission system comprising at least one HVDC transmission line (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 at least one HVDC transmission line, each of the converter stations being adapted to convert alternating current, AC, to direct current for input to the at least one HVDC transmission line, and/or direct current to alternating current, wherein the system comprises at least one apparatus (302; 602; 902) as claimed in any of the claims 1 -17 for controlling the electric power transmission in the system.
19. A HVDC power transmission system according to claim 18, characterized in that the system comprises a plurality of HVDC transmission lines (102, 104, 106, 108, 1 10, 1 12, 1 14).
20. A HVDC power transmission system according to claim 18 or 19, characterized in that the system comprises at least three converter stations (1 16, 1 18, 120, 122, 124).
21 . A HVDC power transmission system according to any of the claims 18 to 20, characterized in that the at least one HVDC transmission line (102, 104, 106, 108, 1 10, 1 12, 1 14) comprises at least one long-distance HVDC link (102, 108).
PCT/EP2010/063879 2010-09-21 2010-09-21 An apparatus for controlling the electric power transmission in a hvdc power transmission system WO2012037965A1 (en)

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CN102801178A (en) * 2012-07-20 2012-11-28 南方电网科学研究院有限责任公司 Direct-current transmission additional control method in frequency domain analysis
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CN104332984A (en) * 2014-10-28 2015-02-04 国网宁夏电力公司电力科学研究院 Online identification method for node voltage interaction factors based on noise-like
CN104362661A (en) * 2014-11-10 2015-02-18 国家电网公司 Flexible DC transmission bipolar main junction device
CN107065570A (en) * 2017-06-05 2017-08-18 国家电网公司 A kind of MMC HVDC system control parameters hierarchy optimization method
CN107154630A (en) * 2017-07-05 2017-09-12 国家电网公司 A kind of hybrid power electrical substation
WO2018059664A1 (en) * 2016-09-27 2018-04-05 Siemens Aktiengesellschaft Arrangement and method for transmitting electrical power

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Cited By (8)

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Publication number Priority date Publication date Assignee Title
CN102801178A (en) * 2012-07-20 2012-11-28 南方电网科学研究院有限责任公司 Direct-current transmission additional control method in frequency domain analysis
CN103151792A (en) * 2013-03-15 2013-06-12 西安杰能电力科技股份有限公司 Harmonic current suppression device for ultrahigh-voltage direct current power transmission system
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CN104332984B (en) * 2014-10-28 2016-08-17 国网宁夏电力公司电力科学研究院 A kind of node voltage based on noise like influences each other the on-line identification method of the factor
CN104362661A (en) * 2014-11-10 2015-02-18 国家电网公司 Flexible DC transmission bipolar main junction device
WO2018059664A1 (en) * 2016-09-27 2018-04-05 Siemens Aktiengesellschaft Arrangement and method for transmitting electrical power
CN107065570A (en) * 2017-06-05 2017-08-18 国家电网公司 A kind of MMC HVDC system control parameters hierarchy optimization method
CN107154630A (en) * 2017-07-05 2017-09-12 国家电网公司 A kind of hybrid power electrical substation

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