WO2012037964A1 - Series - connected dc / dc converter for controlling the power flow in a hvdc power transmission system - Google Patents

Series - connected dc / dc converter for controlling the power flow in a hvdc power transmission system Download PDF

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Publication number
WO2012037964A1
WO2012037964A1 PCT/EP2010/063878 EP2010063878W WO2012037964A1 WO 2012037964 A1 WO2012037964 A1 WO 2012037964A1 EP 2010063878 W EP2010063878 W EP 2010063878W WO 2012037964 A1 WO2012037964 A1 WO 2012037964A1
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WO
WIPO (PCT)
Prior art keywords
converter
hvdc
direct current
transmission line
voltage
Prior art date
Application number
PCT/EP2010/063878
Other languages
French (fr)
Inventor
Praveen Kumar Barupati
Subhasish Mukherjee
Tomas Jonsson
Sasitharan Subramanian
Original Assignee
Abb Technology Ag
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Abb Technology Ag filed Critical Abb Technology Ag
Priority to PCT/EP2010/063878 priority Critical patent/WO2012037964A1/en
Publication of WO2012037964A1 publication Critical patent/WO2012037964A1/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
    • H02J1/10—Parallel operation of dc sources
    • H02J1/102—Parallel operation of dc sources being switching converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
    • H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
    • H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
    • H02M3/158—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators including plural semiconductor devices as final control devices for a single load
    • 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 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, AC, 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.
  • 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 DC-to-DC converter and a second converter for converting alternating current, AC, to direct current and/or direct current to alternating current
  • the DC-to-DC converter having two DC sides for output and/or input of direct current
  • the second converter having an AC side for output and/or input of alternating current and a DC side for output and/or input of direct current
  • the DC-to-DC converter is connectable to the HVDC transmission line
  • the second converter is connected via its DC side to the DC-to-DC converter
  • the second converter is connectable via its AC side to an AC source
  • 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.
  • the electric power transmission in a HVDC power transmission system and the control thereof are ef- ficiently improved, and load-flow congestion in the system may be avoided, reduced or prevented.
  • the DC side of the second converter may be adapted to provide, directly or indirectly, direct current to the DC-to-DC converter, and/or vice versa.
  • the DC-to-DC converter may be adapted to convert direct current from a first voltage level to a second voltage level.
  • the DC-to-DC converter may be adapted to regulate its output voltage.
  • 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 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 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 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 reducing 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 transmis- sion 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 voltage.
  • the apparatus comprises a bypass switch connectable to the HVDC transmission line and connected in parallel with the DC-to-DC converter, and when closed the bypass switch is adapted to conduct the direct current of the HVDC transmission line to electrically bypass the DC-to-DC converter.
  • the bypass switch By the bypass switch, the DC-to-DC 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 AC source.
  • active power should be absorbed by the AC source, and to effect or introduce a negative fictive resistance, -ARinj, active power should be injected by and from the AC source.
  • the apparatus is adapted to be connected to an AC source comprising an AC grid.
  • an AC grid for the AC source efficiently improves the electric power transmission in a HVDC power transmission system and the control thereof.
  • other suitable AC sources are possible.
  • the second converter comprises a Voltage Source Converter, VSC.
  • VSC Voltage Source Converter
  • 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 second converter comprises six pairs of electronic control devices, each pair of electronic control devices comprising an elec- tronic control switch and a diode.
  • the inventors of the present invention have found that also 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 DC-to-DC converter comprises a full-bridge converter.
  • the inventors of the present invention have found that this structure of the DC-to- DC converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
  • the DC-to-DC 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 DC-to-DC converter further improves the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof.
  • the DC-to-DC converter comprises a capacitor.
  • the second converter is adapted to control the voltage of the capacitor.
  • the DC-to-DC 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 con- trol switches.
  • the filter means, or filter components may comprise a filter capacitor and an inductor.
  • the capacitor may be connected in parallel with the electronic control switches.
  • the inductor may be connected in series with the electronic control switches.
  • the apparatus comprises an electric power transformer connected to the AC side of the second converter, and the electric power transformer is connectable to the AC source.
  • 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 is adapted to isolate the DC-to-DC converter from the AC source.
  • the HVDC transmission line, to which the apparatus is connected is also efficiently isolated from the DC source.
  • the DC-to-DC 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.
  • a high voltage direct current, 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 to direct current for input to the at least one HVDC transmission line, and/or direct current to alternating current
  • 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-men- tioned embodiments of the apparatus.
  • the at least one HVDC transmission line may be one or 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.
  • Fig. 1 is a schematic block diagram illustrating aspects of the
  • 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 aspects 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
  • Figs. 5A-5B are schematic diagrams illustrating the four quadrant opera ⁇ tion of the apparatus of Fig. 4;
  • Fig. 6 is a schematic graph illustrating the first quadrant operation of the apparatus of Fig. 4;
  • Fig. 7 is a schematic diagram illustrating an equivalent circuit for first quadrant operation of the apparatus of Fig. 4.
  • Fig. 8 is a schematic graph illustrating the second quadrant opera ⁇ tion of the apparatus of Fig. 4.
  • 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 con- verter 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 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 apparatus 302 may comprise a bypass switch 136 (see Fig. 1 ) electrically connectable to the HVDC transmission line 102 to which the apparatus 302 is connected and electrically connected in parallel with a DC-to-DC converter 304 (see Figs. 3 and 4) of the apparatus 302.
  • 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 DC-to-DC converter 304.
  • the bypass switch 136 By the bypass switch 136, the DC-to-DC converter 304 and the apparatus 302 may be bypassed during fault conditions.
  • Fig. 3 schematically shows aspects of the apparatus 302 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 302 comprises a DC- to-DC converter 304 having two DC sides for output and/or input of direct current and may be adapted to convert direct current from a first voltage level to a second voltage level.
  • the DC-to-DC converter is electrically connectable to the HVDC transmission line 102, and the DC-to-DC converter 304 may be electrically con- nectable in series with the HVDC transmission line 102.
  • the DC-to-DC converter 304 may be adapted to regulate its output voltage.
  • the apparatus 302 comprises a second converter 306 for converting direct current to alternating current and/or alternating current to direct current.
  • the second converter 306 has an AC side 308 for output and/or input of alternating current and a DC side 310 for output and/or input of direct current.
  • the second converter 306 is connected via its DC side 310 to the DC-to-DC converter 304.
  • the DC side 310 of the second converter 306 is adapted to provide direct current to the DC-to-DC converter 304, and/or vice versa.
  • the second converter 306 is connectable via its AC side 308 to an AC source 314.
  • the apparatus 302 may comprise an electric power transformer 312 electrically connected to the AC side 308 of the second converter 306.
  • the electric power transformer 3 2 may be a high frequency transformer.
  • the electric power transformer is electrically connectable to the AC source 314, e.g. an AC grid.
  • An AC grid is well known to the skilled person and therefore not discussed in more detail.
  • the apparatus 302 may comprise the AC source 314.
  • the apparatus 302 is adapted to control the direct current of the HVDC transmission line 102 by introducing a DC voltage in series with the HVDC transmission line 102.
  • the electric power transformer 312 may be adapted to isolate the DC-to-DC converter 304 from the AC source 314, and may thus also be adapted to isolate the HVDC line 102 from the AC source 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 316, e.g. a computer or CPU, for controlling the apparatus and its various components.
  • the control means 316 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. he, of the HVDC transmission line 102, and the control means 316 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 l DC of the HVDC transmission line 102.
  • the second converter 306 may comprise a VSC and may comprise six pairs 402, 404, 406, 408, 410, 412 of electrically intercon- nected electronic control devices 414, 416. Each pair of electronic control devices 414, 416 may comprise an electronic control switch 414 and a diode 416.
  • the DC- to-DC converter 304 may comprise a full-bridge converter.
  • the DC-to-DC converter 304 may comprise four pairs 418, 420, 422, 424, also indicated as
  • Each pair of electronic control devices 426, 428 of the DC-to-DC converter 304 may comprise an electronic control switch 426 and a diode 428.
  • the DC-to-DC converter 304 may comprise filter means 430, 432 connected to the electronic control switches 426, for smoothing out the voltage and current ripple caused by the switching of the electronic control switches 426.
  • the filter means may comprise a filter capacitor 430, also indicated as C ⁇ in Fig. 4, and an inductor 432, also indicated as L f .
  • the filter capacitor 430 may be connected in parallel with the electronic control switches 426, and/or connected in parallel with the four pairs 418, 420, 422, 424 of electronic control devices of the DC-to-DC converter 304.
  • the inductor 432 may be electrically connected in series with the electronic control switches 426, and/or connected in parallel with the four pairs 418, 420, 422, 424 of electronic control devices of the DC-to-DC converter 304.
  • the filter inductor 432 may be connected by connecting one end to the midpoint of a first leg (e.g. common point of 418 and 424) and by connecting the other end to one end of the filter capacitor 430, where the other end of the filter capacitor 430 may be connected between the midpoint of a second leg (e.g.
  • the DC-to-DC converter 304 may also comprise a DC capacitor 434 electrically connected to the electronic control devices of the DC-to-DC converter 304.
  • the second converter 306 may be adapted to control the voltage V dc of the DC capacitor 434.
  • Each of the above-mentioned electronic control switches may comprise 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.
  • the four quadrant operation of the apparatus may be supported by bidirectional valves.
  • 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 DC-to-DC converter 304 is supplied from the second con- verter 306 which in turn is supplied from the AC source 314 via the transformer 312.
  • the VSC of the second converter 306 may comprise at least two legs which convert direct current to alternating current and/or vice-versa.
  • active power should be absorbed by the AC source 314, and to effect or introduce a negative fictive resistance, -ARjnj, ac- tive power should be injected by and from the AC source 314.
  • the apparatus 302 can operate in all the four quadrants as shown in Figs. 5A and 5B, the voltage and current polarity being as shown in Fig. 1 , 3 or 4.
  • the first quadrant operation current is flowing from position A to position Y. Since the voltage/potential in position A is greater than in position Y, in order to operate the first quadrant the diodes D D 2 should be forward-biased. This will result in the voltage V dc across positions A-B.
  • zero voltage is inserted by bypassing the DC capacitor 434.
  • the HVDC line current is flowing from position A to position Y. Since voltage in position A is greater than in position B, the diodes D and D 2 should be forward-biased.
  • the equivalent circuit for the first qua- drant operation is given in Fig. 7, where Vi is the voltage in position A, V 2 is the voltage in position Y, and V dc is the DC capacitor voltage V dc .
  • Station 1 in Fig. 7 may correspond to a converter station on the left side in Fig. 3, and station 2 may correspond to a converter station on the right side in Fig. 3.
  • the voltage over positions A-B should be a positive voltage. Kirchoff's voltage law for the first quadrant operation for forward-biasing may be given as
  • the DC capacitor voltage V dc may be maintained at a desired level by the second converter 306.
  • the second converter 306 may act as an inverter when the DC-to-DC converter 304 is working in the first or third quadrant, i.e. the power absorbed by the DC-to-DC converter 304 should be taken out from the DC capacitor 434.
  • the DC capacitor 434 should be replenished by the second converter 306 and will thus act as a rectifier.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Inverter Devices (AREA)

Abstract

An apparatus (302) for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising at least one HVDC transmission line (102, 104, 106, 108, 110, 112, 114) for carrying direct current, wherein the apparatus (302) comprises a DC-to-DC converter (304) and a second converter (306) for converting alternating current to direct current and/or direct current to alternating current, the DC-to-DC converter having two DC sides for output and/or input of direct current, the second converter having an AC side (308) for output and/or input of alternating current and a DC side (310) for output and/or input of direct current. The DC-to-DC converter is connectable to the HVDC transmission line (102), and the second converter is connected via its DC side to the DC-to-DC converter. The second converter is connectable via its AC side to an AC source (314), and 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. A HVDC power transmission system comprising the above-mentioned apparatus (302).

Description

SERIES - CONNECTED DC/DC CONVERTER FOR CONTROLLING THE
POWER FLOW 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 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, AC, 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 DC-to-DC converter and a second converter for converting alternating current, AC, to direct current and/or direct current to alternating current, the DC-to-DC converter having two DC sides for output and/or input of direct current, the second converter 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 DC-to-DC converter is connectable to the HVDC transmission line, the second converter is connected via its DC side to the DC-to-DC converter, and the second converter is connectable via its AC side to an AC 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 ef- ficiently improved, and load-flow congestion in the system may be avoided, reduced or prevented. The DC side of the second converter may be adapted to provide, directly or indirectly, direct current to the DC-to-DC converter, and/or vice versa. The DC-to-DC converter may be adapted to convert direct current from a first voltage level to a second voltage level. The DC-to-DC converter may be adapted to regulate its output voltage.
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, ARjnj. 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 ARinj, (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 Rjnj. A positive ARjnj is produced when the apparatus introduces a positive DC voltage in series with the HVDC transmission line, and a negative ARinj 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.
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.
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 reducing 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 transmis- sion 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 voltage.
According to another advantageous embodiment of the apparatus accord- ing to the present invention, the apparatus comprises a bypass switch connectable to the HVDC transmission line and connected in parallel with the DC-to-DC converter, and when closed the bypass switch is adapted to conduct the direct current of the HVDC transmission line to electrically bypass the DC-to-DC converter. By the bypass switch, the DC-to-DC 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 AC source. To effect or introduce a positive fictive resistance, +ARinj, active power should be absorbed by the AC source, and to effect or introduce a negative fictive resistance, -ARinj, active power should be injected by and from the AC source.
According to an advantageous embodiment of the apparatus according to the present invention, the apparatus is adapted to be connected to an AC source comprising an AC grid. The inventors of the present invention have found that the use of an AC grid for the AC source efficiently improves the electric power transmission in a HVDC power transmission system and the control thereof. However, other suitable AC sources are possible.
According to another advantageous embodiment of the apparatus accord- ing 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 still another advantageous embodiment of the apparatus ac- cording 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 yet another advantageous embodiment of the apparatus according to the present invention, the second converter comprises six pairs of electronic control devices, each pair of electronic control devices comprising an elec- tronic control switch and a diode. The inventors of the present invention have found that also 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 DC-to-DC converter comprises a full-bridge converter. The inventors of the present invention have found that this structure of the DC-to- DC 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 ac- cording to the present invention, the DC-to-DC 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 DC-to-DC 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 DC-to-DC converter comprises a capacitor. By this em- bodiment, the inventors of the present invention have found that the flexibility and efficiency of the electric power transmission in a HVDC power transmission system and the control thereof are further improved.
According to another advantageous embodiment of the apparatus according to the present invention, the second converter is adapted to control the voltage of the capacitor.
According to a further advantageous embodiment of the apparatus according to the present invention, the DC-to-DC 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 con- trol 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 filter capacitor and an inductor. The capacitor may be connected in parallel with the electronic control switches. The inductor may be connected in series with the electronic control switches. By the above-mentioned embodiments of the filter means, a further improved control of the power transmission is provided.
According to an advantageous embodiment of the apparatus according to the present invention, the apparatus comprises an electric power transformer connected to the AC side of the second converter, and the electric power transformer is connectable to the AC source. 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.
According to a further advantageous embodiment of the apparatus accord- ing to the present invention, the electric power transformer is adapted to isolate the DC-to-DC converter from the AC 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 DC-to-DC 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 above-mentioned objects of the present invention are also attained by providing a high voltage direct current, 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 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-men- tioned 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 transmission 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 aspects 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;
Figs. 5A-5B are schematic diagrams illustrating the four quadrant opera¬ tion of the apparatus of Fig. 4;
Fig. 6 is a schematic graph illustrating the first quadrant operation of the apparatus of Fig. 4;
Fig. 7 is a schematic diagram illustrating an equivalent circuit for first quadrant operation of the apparatus of Fig. 4; and
Fig. 8 is a schematic graph illustrating the second quadrant opera¬ tion of the apparatus of Fig. 4.
Detailed Description of Preferred Embodiments
Abbreviations
Alternating Current AC
Bi-Mode Insulated Gate Transistor BIGT
Direct Current DC
Cascaded Two- Level cell CTL cell
Central Processing Unit CPU
Gate Turn-Off thyristor GTO
High Voltage Direct Current HVDC
Insulated Gate Bipolar Transistor IGBT
Integrated Gate-Commutated Thyristor IGCT
Pulse Width Modulation PWM
Voltage Source Converter VSC
Fig.1 schematically shows aspects of the HVDC power transmission system and aspects of the apparatus 302 according to the present invention. The HVDC power transmission system comprising a plurality of HVDC transmission lines 02, 04, 06, 08, 0, 2, 4 for carrying direct current. The HVDC transmission lines may e.g. comprise HVDC cables, busbars, or other DC con¬ ductors. The HVDC transmission lines may comprise at least one long-distance HVDC link. In Fig. 1 , a first and second long-distance HVDC link 102, 108 are pro¬ vided. 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, 1 20, 122, 124 may be electrically connected to a conventional electric power transformer 126, 128, 130, 132, 134 in conventional ways known to the skilled person. Electric power transformers and their function are well known to the person skilled in the art and therefore not discussed in more detail. Each converter station, which may be called a DC Grid converter station, may have asymmetrical monopoles with separate converters for positive and negative polarity, as illustrated in Fig. 2A. Alternatively, each con- verter 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 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. Rrme of the HVDC transmission line 02 in Fig. 1 illustrates the resistance of the line 102, and be 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 for controlling the electric power transmission in the system according to the present invention, and aspects of the apparatus 302 will hereinafter be disclosed.
The apparatus 302 may comprise a bypass switch 136 (see Fig. 1 ) electrically connectable to the HVDC transmission line 102 to which the apparatus 302 is connected and electrically connected in parallel with a DC-to-DC converter 304 (see Figs. 3 and 4) of the apparatus 302. When the bypass switch 136 is closed, it is adapted to conduct the direct current of HVDC transmission line to electrically bypass the DC-to-DC converter 304. By the bypass switch 136, the DC-to-DC converter 304 and the apparatus 302 may be bypassed during fault conditions.
Fig. 3 schematically shows aspects of the apparatus 302 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 302 comprises a DC- to-DC converter 304 having two DC sides for output and/or input of direct current and may be adapted to convert direct current from a first voltage level to a second voltage level. The DC-to-DC converter is electrically connectable to the HVDC transmission line 102, and the DC-to-DC converter 304 may be electrically con- nectable in series with the HVDC transmission line 102. The DC-to-DC converter 304 may be adapted to regulate its output voltage. The apparatus 302 comprises a second converter 306 for converting direct current to alternating current and/or alternating current to direct current. The second converter 306 has an AC side 308 for output and/or input of alternating current and a DC side 310 for output and/or input of direct current. The second converter 306 is connected via its DC side 310 to the DC-to-DC converter 304. The DC side 310 of the second converter 306 is adapted to provide direct current to the DC-to-DC converter 304, and/or vice versa. The second converter 306 is connectable via its AC side 308 to an AC source 314. The apparatus 302 may comprise an electric power transformer 312 electrically connected to the AC side 308 of the second converter 306. The electric power transformer 3 2 may be a high frequency transformer. The electric power transformer is electrically connectable to the AC source 314, e.g. an AC grid. An AC grid is well known to the skilled person and therefore not discussed in more detail. The apparatus 302 may comprise the AC source 314. The apparatus 302 is adapted to control the direct current of the HVDC transmission line 102 by introducing a DC voltage in series with the HVDC transmission line 102. The electric power transformer 312 may be adapted to isolate the DC-to-DC converter 304 from the AC source 314, and may thus also be adapted to isolate the HVDC line 102 from the AC source 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 316, e.g. a computer or CPU, for controlling the apparatus and its various components. The control means 316 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. he, of the HVDC transmission line 102, and the control means 316 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 lDC 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 six pairs 402, 404, 406, 408, 410, 412 of electrically intercon- nected electronic control devices 414, 416. Each pair of electronic control devices 414, 416 may comprise an electronic control switch 414 and a diode 416. The DC- to-DC converter 304 may comprise a full-bridge converter. The DC-to-DC converter 304 may comprise four pairs 418, 420, 422, 424, also indicated as
Figure imgf000014_0001
D2/S2, D3/S3, S4/D4 in Fig. 4, of electrically interconnected electronic control devices 426, 428. Each pair of electronic control devices 426, 428 of the DC-to-DC converter 304 may comprise an electronic control switch 426 and a diode 428. The DC-to-DC converter 304 may comprise filter means 430, 432 connected to the electronic control switches 426, for smoothing out the voltage and current ripple caused by the switching of the electronic control switches 426. The filter means may comprise a filter capacitor 430, also indicated as C^ in Fig. 4, and an inductor 432, also indicated as Lf. The filter capacitor 430 may be connected in parallel with the electronic control switches 426, and/or connected in parallel with the four pairs 418, 420, 422, 424 of electronic control devices of the DC-to-DC converter 304. The inductor 432 may be electrically connected in series with the electronic control switches 426, and/or connected in parallel with the four pairs 418, 420, 422, 424 of electronic control devices of the DC-to-DC converter 304. The filter inductor 432 may be connected by connecting one end to the midpoint of a first leg (e.g. common point of 418 and 424) and by connecting the other end to one end of the filter capacitor 430, where the other end of the filter capacitor 430 may be connected between the midpoint of a second leg (e.g. common point of 420 and 422). The DC-to-DC converter 304 may also comprise a DC capacitor 434 electrically connected to the electronic control devices of the DC-to-DC converter 304. The second converter 306 may be adapted to control the voltage Vdcof the DC capacitor 434. Each of the above-mentioned electronic control switches may comprise 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 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 DC-to-DC converter 304 is supplied from the second con- verter 306 which in turn is supplied from the AC source 314 via the transformer 312. 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, +ARinj, active power should be absorbed by the AC source 314, and to effect or introduce a negative fictive resistance, -ARjnj, ac- tive power should be injected by and from the AC source 314.
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 Figs. 5A and 5B, the voltage and current polarity being as shown in Fig. 1 , 3 or 4. In the first quadrant operation, current is flowing from position A to position Y. Since the voltage/potential in position A is greater than in position Y, in order to operate the first quadrant the diodes D D2 should be forward-biased. This will result in the voltage Vdc across positions A-B. To regulate the injected voltage, zero voltage is inserted by bypassing the DC capacitor 434. This can be done by turning on transistor S3 or S4 and direct current flows through D1-S3 or S4-D2. With appropriate duty ratio for transistor S3 or S4, the voltage across positions A-B is regulated to give desired "positive" resistance.
In the first quadrant operation, the HVDC line current is flowing from position A to position Y. Since voltage in position A is greater than in position B, the diodes D and D2 should be forward-biased. The equivalent circuit for the first qua- drant operation is given in Fig. 7, where Vi is the voltage in position A, V2 is the voltage in position Y, and Vdc is the DC capacitor voltage Vdc. Station 1 in Fig. 7 may correspond to a converter station on the left side in Fig. 3, and station 2 may correspond to a converter station on the right side in Fig. 3. To assure forward-biasing for the diodes, the voltage over positions A-B should be a positive voltage. Kirchoff's voltage law for the first quadrant operation for forward-biasing may be given as
V, - V2 -Vdc > 0
This implies that -V2 > Vdc , and the voltage across the diode is positive. For the second and the fourth quadrants, this condition is always satisfied since the DC capacitor 434 is reversely connected, i.e. for example for the second quadrant,
V vl -V v2 > ^ -V vdc■
During the second quadrant operation, transistors S3-S4 are turned on to attain a negative voltage across positions A-B. The bypass path S3-D or S4-D2 is used to achieve zero voltage across positions A-B. The voltage across positions A-B ( VAB) may be regulated by PWM operation as shown in Fig. 8. The PWM voltage may be averaged by filter means and injected in series with the HVDC line 102. During this mode, when there is very low duty PWM for S3-S4 (about 0%), VAB is almost zero. When the duty ratio is increased, ½¾s will be negative and a "nega- tive" resistance is introduced in to the line 102.
During the third quadrant operation, the switching conditions are as a shown in Fig, 5B, and current is flowing from position Vto position A. Even though all the switches are OFF, the diodes D3-D4 may be forward-biased since there is a voltage difference between positions A-B. By inserting zero voltage by varying the duty ratio of S1-D3 or S2-D4, the voltage VAB across positions A-B may be regulated to a desired value. Thus, the average voltage between positions A-B may be reduced to a desired value which gives "positive" resistance.
During the fourth quadrant operation, the current is flowing from position Y to position A. When transistors Si-S2 are ON, the voltage VAB across positions A-B is equal to the DC capacitor voltage Vdc- To regulate VAB to a desired value, the DC capacitor 434 may be bypassed using switches S-i-D3 or S2-D4. Thus, the average voltage across positions A-B may be regulated to a desired negative value which gives "negative" resistance.
The DC capacitor voltage Vdc may be maintained at a desired level by the second converter 306. The second converter 306 may act as an inverter when the DC-to-DC converter 304 is working in the first or third quadrant, i.e. the power absorbed by the DC-to-DC converter 304 should be taken out from the DC capacitor 434. In a corresponding same way, when the DC-to-DC converter 304 is working in the second or fourth quadrant, the DC capacitor 434 should be replenished by the second converter 306 and will thus act as a rectifier.
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) for controlling the electric power transmission in 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, characterized in that the apparatus (302) comprises a DC-to-DC converter (304) and a second converter (306) for converting alternating current, AC, to direct current and/or direct current to alternating current, the DC-to-DC converter having two DC sides for output and/or input of direct current, the second converter having an AC side (308) for output and/or input of alternating current and a DC side (310) for output and/or input of direct current, in that the DC-to-DC converter is connectable to the HVDC transmission line (102), in that the second converter is connected via its DC side to the DC-to-DC converter, in that the second converter is connectable via its AC side to an AC source (314), 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.
2. An apparatus according to claim 1 , characterized in that the apparatus comprises control means (316) for controlling the apparatus (302), in that the con- trol means are adapted to control the apparatus to introduce a positive DC voltage in series with the HVDC transmission line ( 02) 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) comprises a bypass switch (136) connectable to the HVDC transmission line and connected in parallel with the DC-to-DC converter (304), and in that when closed the bypass switch is adapted to conduct the direct current of HVDC transmission line to electrically bypass the DC-to-DC converter.
4. An apparatus according to any of the claims 1 to 3, characterized in that the apparatus (302) comprises the AC source (314).
5. An apparatus according to any of the claims 1 to 4, characterized in that the apparatus is adapted to be connected to an AC source (314) comprising an AC grid. 6. An apparatus according to any of the claims 1 to 5, characterized in that the second converter (306) comprises a Voltage Source Converter, VSC.
7. An apparatus according to any of the claims 1 to 6, characterized in that the second converter (306) comprises four pairs (402, 404, 406, 408, 410, 412) of electronic control devices (414, 416), each pair of electronic control devices comprising an electronic control switch (414) and a diode (416).
8. An apparatus according to claim 7, characterized in that the second converter comprises six pairs (402, 404, 406, 408, 410, 412) of electronic control devices (414, 416), each pair of electronic control devices comprising an electronic control switch (414) and a diode (416).
9. An apparatus according to any of the claims 1 to 8, characterized in that the DC-to-DC converter (304) comprises a full-bridge converter. 0. An apparatus according to any of the claims 1 to 9, characterized in that the DC-to-DC converter (304) comprises four pairs (418, 420, 422, 424) of electronic control devices (426, 428), each pair of electronic control devices comprising an electronic control switch (426) and a diode (428).
1 1 . An apparatus according to claim 7, 8 or 10, characterized in that each electronic control switch (424, 426) comprises a transistor.
12. An apparatus according to claim 10 or 1 1 , characterized in that the DC- to-DC converter (304) comprises filter means (430, 432) for smoothing out the voltage and current ripple caused by the switching of the electronic control switches (424).
13. An apparatus according to any of the claims 1 to 12, characterized in that the DC-to-DC converter (304) comprises a capacitor (434).
14. An apparatus according to claim 13, characterized in that the second converter (306) is adapted to control the voltage of the capacitor (434).
15. An apparatus according to any of the claims 1 to 14, characterized in that the apparatus (302) comprises an electric power transformer (312) connected to the AC side of the second converter, and in that the electric power transformer is connectable to the AC source (314).
16. An apparatus according to claim 15, characterized in that the electric power transformer (312) is adapted to isolate the DC-to-DC converter (304) from the AC source (314).
17. An apparatus according to any of the claims 1 to 16, characterized in that the DC-to-DC 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 ( 02, 104, 06, 08, 0, 2, 4) for carrying direct current 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 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) 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/EP2010/063878 2010-09-21 2010-09-21 Series - connected dc / dc converter for controlling the power flow in a hvdc power transmission system WO2012037964A1 (en)

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WO2013091700A1 (en) * 2011-12-21 2013-06-27 Abb Technology Ltd An arrangement for controlling the electric power transmission in a hvdc power transmission system
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