WO2014071597A1 - A step-down dc autotransformer for hvdc and a system thereof - Google Patents

A step-down dc autotransformer for hvdc and a system thereof Download PDF

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Publication number
WO2014071597A1
WO2014071597A1 PCT/CN2012/084348 CN2012084348W WO2014071597A1 WO 2014071597 A1 WO2014071597 A1 WO 2014071597A1 CN 2012084348 W CN2012084348 W CN 2012084348W WO 2014071597 A1 WO2014071597 A1 WO 2014071597A1
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WO
WIPO (PCT)
Prior art keywords
converter
transformer
phase
autotransformer
converters
Prior art date
Application number
PCT/CN2012/084348
Other languages
French (fr)
Inventor
Dawei YAO
Xiaobo Yang
Chunming YUAN
Original Assignee
Abb Technology Ltd.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Abb Technology Ltd. filed Critical Abb Technology Ltd.
Priority to PCT/CN2012/084348 priority Critical patent/WO2014071597A1/en
Publication of WO2014071597A1 publication Critical patent/WO2014071597A1/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/49—Combination of the output voltage waveforms of a plurality of 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
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
    • H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
    • H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/75—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means
    • H02M7/757—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
    • H02M7/7575—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only for high voltage direct transmission 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
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0083—Converters characterised by their input or output configuration
    • H02M1/0093—Converters characterised by their input or output configuration wherein the output is created by adding a regulated voltage to or subtracting it from an unregulated input
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0095—Hybrid converter topologies, e.g. NPC mixed with flying capacitor, thyristor converter mixed with MMC or charge pump mixed with buck
    • 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 invention relates to the technical field of HVDC applications as a step-down DC autotransformer for HVDC and a system thereof.
  • DC/DC transformer would become a fundamental component for HVDC applications in the future. Actually, there is no essentially practical DC/DC transformer solution for HVDC system.
  • DC/DC transformer There is potential requirement for DC/DC transformer at present.
  • One application is to step down DC voltage of one HVDC line from a higher voltage level to a lower voltage level (say from 800kV to 400kV) for power supply of load centers or interconnection of HVDC lines with difference voltage.
  • Another application is to tapping power from HVDC line to different DC voltage levels.
  • DC-DC converter provided with an AC link (US4462070, Katsuji Eida) is designed as alternative solution of low voltage chopper converter, but with only 1200V input level and the topology comprises single phase inverters and cannot be used for high voltage high power HVDC transmission system. Further, US4462070 needs controlled rectifiers which have forced turn off capacity (GTO or forced turn off thyristor) for the current inverter, additional smoothing reactor and capacitors to realize the control function.
  • GTO forced turn off capacity
  • the invention proposes a step-down DC autotransformer for HVDC and a system thereof, where the power capacity of each converter and transformer is only part of the transmitted power in case of low cost and high performance.
  • the monopolar topology of the DC autotransformer comprises: a first AC/DC converter operated as an inverter, a second AC/DC converter and AC transformer with at least two windings; in which the first AC/DC converter can be a converter with single-phase, three-phase or multi-phase; the second AC/DC converter can be a converter with single-phase, three-phase or multi-phase; the AC transformer can be composed of a AC transformer or a number of AC transformers with three-phase or multi-phase; the DC side of the first AC/DC converter is connected between a high DC voltage line and a low DC voltage line; the DC side of the second AC/DC converter is connected between the low DC voltage line and a neutral bus; and the AC sides of the first AC/DC converter and second AC/DC converter are connected via the AC transformer.
  • the an AC transformer is composed of a number of AC transformers with single-phase, three-phase or multi-phase; each the AC side of the first and second AC/DC converter is connected to one side of each the a number of AC transformers; and the other side of each the a number of AC transformers is connected together with a AC bus.
  • an additional winding is installed in the AC transformer which can be an AC transformer with single-phase, three-phase or multi-phase.
  • the other side of a number of AC transformers or the additional winding of the AC transformer is connected to AC system; and the AC system can be local AC network, energy storage system, StatCom or condenser.
  • the first AC/DC converter is a voltage source converter operated as an inverter; and the second AC/DC converter is a voltage source converter, a line commutated converter or a diode rectifier.
  • the first AC/DC converter can be a voltage source converter or a line commutated converter operated as an inverter; and the second AC/DC converter can be a voltage source converter, a line commutated converter or a diode rectifier.
  • the first AC/DC converter is line commutated converter; and the second AC/DC converter is voltage source converter.
  • both the first and second AC/DC converters are voltage source converters with single-phase, three-phase or multi-phase.
  • both the first and second AC/DC converters are line commutated converters with single-phase, three-phase or multi-phase.
  • the AC bus can exchange the power bi-directionally between the first and second AC/DC converter by controlling the rectified power or inverted power of the first or second AC/DC converter.
  • the bipolar topology of the DC autotransformer comprises: a first pole comprising a first AC/DC converter operated as an inverter, a second AC/DC converter and a first AC transformer with at least two windings; a second pole comprising a third AC/DC converter operated as an inverter, a fourth AC/DC converter and second AC transformer with at least two windings; and a neutral bus; in which each the first, second, third and fourth AC/DC converters can be converter with single-phase, three-phase or multi-phase; each the first and second AC transformers can be composed of an AC transformer or a number of AC transformers with single-phase, three-phase or multi-phase; the DC side of the first AC/DC converter is connected between a first high DC voltage line and a first low DC voltage line; the DC side of the second AC/DC converter is connected between the first low DC voltage line and the neutral bus; and the AC sides of the first
  • each of the first AC transformer and the second AC transformer is composed of a number of AC transformers; in which each the AC side of the first, second, third and fourth AC/DC converters is connected to one side of the a number of AC transformers; and the other side of each the a number of AC transformers is connected together with an AC bus.
  • an additional winding is installed in the first AC transformer; and besides the windings connected to the AC sides of the third and fourth AC/DC converters, an additional winding is installed in the second AC transformer.
  • the other side of the a number of AC transformers or the additional winding of the AC transformers is connected to AC system; and the AC system can be local AC network, energy storage system, StatCom or condenser.
  • the first and third AC/DC converters are voltage source converters; and each the second and fourth AC/DC converters is operated as rectifier, which can be voltage source converter, line commutated converter or diode rectifier.
  • the first and third AC/DC converters are voltage source converters or line commutated converters; and each the second and fourth AC/DC converters can be voltage source converter, line commutated converter or diode rectifier.
  • each the first and third AC/DC converter is line commutated converter; and each the second and fourth AC/DC converters is voltage source converter.
  • all the first, second, third and fourth AC/DC converters are voltage source converters with single-phase, three-phase or multi-phase.
  • all the first, second, third and fourth AC/DC converters are line commutated converters with single-phase, three-phase or multi-phase.
  • the AC bus can exchange the power bi-directionally between the first pole and second pole by controlling rectified power or inverted power of the first, second, third and/or fourth AC/DC converter.
  • the currents of the second AC/DC converter and the fourth AC/DC converter are equal to avoid unbalanced current through earth via the neutral bus.
  • the step-down DC autotransformer further comprises: an energizing component, which is connected between the low DC voltage line and the neutral bus.
  • the step-down DC autotransformer further comprises: a first energizing component and a second energizing component; the first energizing component is connected between the first low DC voltage line and the neutral bus; and the second energizing component is connected between the second low DC voltage line and the neutral bus.
  • the line commutated converter can be replaced by a capacitor commutated converter.
  • the high DC voltage line and low DC voltage line are connected to different DC systems with different voltage levels.
  • it provides a step-down DC transformer for HVDC.
  • the DC transformer comprises: a first AC/DC converter used as an inverter, a second AC/DC converter used as a rectifier and an AC transformer; in which the first AC/DC converter is voltage source converter; the second AC/DC converter is line commutated converter; the DC side of the first AC/DC converter is connected between a high DC voltage line and a neutral bus; the DC side of the second AC/DC converter is connected between a low DC voltage line and a neutral bus; and the AC sides of the first AC/DC converter and second AC/DC converter are connected via the AC transformer.
  • an additional winding is installed in the AC transformer which can be an AC transformer with single-phase, three-phase or multi-phase; the additional winding of the AC transformer is connected to AC system; and the AC system can be local AC network, energy storage system, StatCom or condenser.
  • it provides a system comprising at least one of the step-down DC autotransformer or transformer above mentioned.
  • Embodiments of the present invention provide a step-down DC autotransformer for HVDC and a system thereof, which facilitates the development of HVDC applications at high altitude area where the lower DC voltage level is desired to decrease insulation cost.
  • the lower DC voltage will be stepped up at low altitude area to realize high power and long distance transmission.
  • Figs.1a-1 h individually illustrate a monopolar topology of a step-down DC autotransformer for HVDC according to an embodiment of the present invention
  • Fig.2 illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention
  • Figs.3a-3j individually illustrate a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention
  • Figs.4a-4e individually illustrate a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention
  • Fig.5 illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention
  • Figs.6a-6g individually illustrate a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • Fig.7 illustrates a monopolar topology of a full-power step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the nominal power flow direction of the step-down DC autotransformer is from high DC voltage side to low DC voltage side in terms of absolute voltage value. Therefore, the direction of reversed power flow is from low DC voltage side to high DC voltage side.
  • VSC voltage source converter
  • LCC line commutated converter
  • AC transformer technology will be employed in this invention.
  • the topology of VSC can be two-level converter, multi-level converter (MLC), MMC (modular multi-level converter), CTL (cascaded two-level converter) or AMMC (alternating arm multi-level converter) and so forth. All VSC topologies employ fully controllable semiconductor devices, such as IGBT, IGCT, IEGT, GTO, MOSFET and so forth.
  • the LCC topology can be a single phase converter, a 6 pulse converter, a 12 pulse converter and so forth.
  • VSC or LCC can be realized by other various topologies.
  • the said converter, either AC/DC VSC or AC/DC LCC can be operated at rectifier mode (power flows from AC side to DC side) or inverter mode (power flows from DC side to AC side), according to function requirement.
  • the frequency of the AC transformer in the invention may be not limited to be 50 or 60Hz.
  • the frequency can be optimally designed based on the comprehensive consideration on the cost, efficiency, weight and footprint etc.
  • the size of reactive power compensation equipment for LCC is decreased significantly, as the reactive power can be provided by VSC.
  • the AC transformer in the invention is also not limited to be a three-phase transformer, the descriptions of winding of AC transformer below can be single phase winding, three phase winding or any other winding types to form a multiphase AC transformer.
  • Fig. 1a illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to an embodiment of the present invention.
  • the step-down DC autotransformer for HVDC is a monopolar topology which comprises a VSC 1 used as an inverter, a LCC 2 used as a rectifier, and a three-winding AC transformer 3.
  • the DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line;
  • the DC side of the LCC 2 is connected between the low DC voltage line and a neutral bus;
  • the AC sides of the VSC 1 and LCC 2 are connected via the AC transformer 3, in which one winding is connected to the AC side of the VSC 1 , and the other two windings are connected to the AC side of the LCC 2.
  • the DC autotransformer is a cascaded type, which consists of a VSC, a LCC and an AC transformer which frequency can be designed optimally.
  • the power capacity of each converter or transformer is only part of the transmitted power, which depends on the DC/DC step-down ratio. Therefore the cost and footprint are low.
  • Fig.1 b illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the step-down DC autotransformer for HVDC comprises a VSC 1 configured as an inverter, a LCC 2 used as a rectifier and a two-winding AC transformer 3.
  • the DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line;
  • the DC side of the LCC 2 is connected between the low DC voltage line and a neutral bus;
  • the AC sides of the VSC 1 and LCC 2 are connected via the AC transformer 3, in which one winding is connected to the AC side of the VSC 1 , and the other winding is connected to the AC side of the LCC 2.
  • Fig. 1c illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to an embodiment of the present invention.
  • the step-sown DC autotransformer for HVDC is configured to realize bi-directional power flow, which comprises two VSC converters VSC 1 and VSC 2, and a two-winding AC transformer 3, in which DC side of VSC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of VSC 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the VSC 1 and VSC 2 are connected via the AC transformer 3, in which one winding is connected to the AC side of the VSC 1 , and the other winding is connected to the AC side of the VSC 2.
  • VSC 1 is used as an inverter
  • VSC 2 is used as a rectifier in the step-down DC autotransformer for HVDC.
  • DC current direction of VSC 1 and VSC 2 are changed: conversion mode of VSC 1 is changed to rectifier and conversion mode of VSC 2 is changed to inverter.
  • Fig.ld illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the step-down DC autotransformer for HVDC comprises a VSC 1 configured as an inverter, a diode rectifier 2, and a two-winding AC transformer 3.
  • the DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the diode rectifier 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the VSC 1 and diode rectifier 2 are connected via the AC transformer 3, in which one winding is connected to the AC side of the VSC 1 , and the other winding is connected to the AC side of the diode rectifier 2.
  • Fig.le illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the step-down DC autotransformer for HVDC comprises a LCC 1 configured as an inverter, a VSC 2 used as a rectifier and a two-winding AC transformer 3.
  • the DC side of the LCC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the VSC 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the VSC 1 and LCC 2 are connected via the AC transformer 3, in which one winding is connected to the AC side of the LCC 1 , and the other winding is connected to the AC side of the VSC 2.
  • Fig. 1f illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to an embodiment of the present invention.
  • the step-down DC autotransformer for HVDC is a monopolar topology which comprises a LCC 1 used as an inverter, a VSC 2 used as a rectifier, and a three-winding AC transformer 3.
  • the DC side of the LCC 1 is connected between a high DC voltage line and a low DC voltage line;
  • the DC side of the VSC 2 is connected between the low DC voltage line and a neutral bus;
  • the AC sides of the LCC 1 and VSC 2 are connected via the AC transformer 3, in which two windings are connected to the AC side of the LCC 1 , and the other winding is connected to the AC side of the VSC 2.
  • Fig.lg illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the step-down DC autotransformer for HVDC comprises a VSC 1 configured as an inverter, a LCC 2 used as a rectifier and three single phase AC transformers 3.
  • the DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the LCC 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the VSC 1 and LCC 2 are connected via the AC transformer 3, in which one side of those AC transformers is connected to VSC 1 and the other side of those AC transformers is connected to LCC 2.
  • Fig.lh illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the step-down DC autotransformer for HVDC comprises a VSC 1 configured as an inverter, a LCC 2 used as a rectifier, a two-winding AC transformer 3 and an energizing component 5.
  • the DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line;
  • the DC side of the LCC 2 is connected between the low DC voltage line and a neutral bus;
  • the AC sides of the VSC 1 and LCC 2 are connected via the AC transformer 3, in which one winding is connected to the AC side of the VSC 1 , and the other winding is connected to the AC side of the LCC 2.
  • the DC autotransformer further comprises an energizing component, which is used for start-up process.
  • the energizing component can be connected in parallel with LCC 2 between the low DC voltage line and the neutral bus.
  • Fig.2 illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the step-down DC autotransformer for HVDC is a monopolar topology which comprises a VSC 1 used as an inverter and a LCC 2 used as a rectifier, three AC transformers 3 and an AC bus 4.
  • the DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line;
  • the DC side of the LCC 2 is connected between the low DC voltage line and a neutral bus;
  • one side of the all AC transformers are connected to the AC sides of the VSC 1 and LCC 2, and the other side of all AC transformers 3 are connected to the AC bus 4.
  • Fig.3a, Fig.3b, Fig.3c, Fig.3d, Fig.3e and Fig.3f individually illustrates a topology of step sown DC autotransformer for HVDC according to an embodiment of the present invention
  • Fig.3e and Fig.3f is similar to respective topology shown in Fig.1a, Fig.1 b, Fig.1c, Fig.ld, Fig.le and Fig.lf except that an additional winding is added into the AC transformer 3.
  • the additional winding of AC transformer 3 can be connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser.
  • Fig. 3g illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the step-down DC autotransformer for HVDC is a monopolar topology which comprises a VSC 1 used as an inverter and a LCC 2 used as a rectifier, three AC transformers 3 and an AC bus 4.
  • the DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the LCC 2 is connected between the low DC voltage line and a neutral bus; one side of the all three AC transformers is connected to the AC sides of VSC 1 and LCC 2, and the other side of AC transformers is connected to the AC bus 4.
  • the AC bus 4 can be standalone AC bus or connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser.
  • Fig.3h illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • another step-down DC autotransformer for HVDC is configured to realize bi-directional power flow, which comprises two LCC converters LCC 1 and LCC 2, and a three-winding AC transformer 3, in which DC side of LCC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of LCC 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the LCC 1 and LCC 2 are connected via the AC transformer 3, in which the first winding is connected to the AC side of the LCC 1 , the second winding is connected to the AC side of the LCC 2, and the third winding is connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser.
  • an AC system for example a local AC network, an energy storage system, a StatCom or a condenser.
  • the LCC 1 is used as an inverter, and LCC 2 is used as a rectifier in the step-down DC autotransformer.
  • DC voltage polarities of both LCC 1 and LCC 2 are changed: operation mode of LCC 1 is changed to rectifier and operation mode of LCC 2 is changed to inverter.
  • Fig.3i illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • FIG.3i another step-down DC autotransformer for HVDC is configured to realize bi-directional power flow.
  • the topology shown in Fig. 3i is similar to that shown in Fig. 3g except that the connect direction of LCC 2 is reversed.
  • both LCC 1 and LCC 2 are operated as inverters in the step-down DC autotransformer.
  • DC voltage polarity of both LCC 1 and LCC 2 are changed; operation modes of LCC 1 and LCC 2 are changed to rectifiers.
  • Fig.3 illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • FIG.3j another step-down DC autotransformer for HVDC is configured to realize bi-directional power flow.
  • the topology shown in Fig. 3j is similar to that shown in Fig. 3c except that the VSC 2 is replaced by LCC 2.
  • Fig.4a illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the bipolar topology of a step-down DC autotransformer for HVDC comprises: a first pole comprising a VSC 1 operated as an inverter, a LCC 2 operated as a rectifier and a first AC transformer 3 and a second pole comprising a VSC V operated as an inverter, a LCC 2' operated as a rectifier and a second AC transformer 3'; in which the DC side of VSC 1 is connected between a first high DC voltage line (High DC voltage Pole 1 , Udc1+) and a first low DC voltage line (Low DC voltage Pole 1 , Udc2+); the DC side of LCC 2 is connected between the first low DC voltage line (Udc2+) and a neutral bus; and the AC sides of VSC 1 and LCC 2 are connected via the first AC transformer 3; and the DC side of VSC V is connected between a second high DC voltage line (High DC voltage Pole 2, Udc1-) and a second low DC voltage line (Low DC
  • both the first and second AC transformers (3, 3') are three-winding transformers, in which one winding of the first AC transformer 3 is connected to the AC side of VSC 1 , the other two windings are connected to the AC side of LCC 2; and one winding of the second AC transformer 3' is connected to the AC side of VSC 1 ', the other two windings are connected to the AC side of LCC 2'.
  • the AC transformers (3, 3') shown in Fig.4a can also be placed by a number of transformers.
  • Fig.4b illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • Fig.4b is similar to Fig.4a except that the first and second AC transformers (3, 3') are two-winding transformers.
  • One winding of the first AC transformer 3 is connected to the AC side of the first VSC 1 , the other winding is connected to the AC side of the first LCC 2; and one winding of the second AC transformer 3' is connected to the AC side of the second VSC 1', the other winding is connected to the AC side of the second LCC 2 ⁇
  • Fig.4c illustrates a bipolar topology of a step-down DC autotransformer for
  • HVDC according to another embodiment of the present invention.
  • the step-down DC autotransformer for HVDC is configured to realize bi-directional power flow; in detail, all the first, second third and fourth converters are VSCs (1 , V, 2, 2').
  • the DC side of VSC 1 is connected between a first high DC voltage line (High DC voltage Pole 1 , Udc1 +) and a first low DC voltage line (Low DC voltage Pole 1 , Udc2+);
  • the DC side of VSC 2 is connected between the first low DC voltage line (Udc2+) and a neutral bus; and the AC sides of VSC 1 and VSC 2 are connected via the first AC transformer 3; and the DC side of VSC V is connected between a second high DC voltage line (High DC voltage Pole 2, Udc1-) and a second low DC voltage line (Low DC voltage Pole 2, Udc2-);
  • the DC side of VSC 2' is connected between the second low DC voltage line (Low DC voltage Pole 2, Udc2-) and the neutral bus; and the AC sides of VSC 1'
  • Fig.4d illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the bipolar topology of a step-down DC autotransformer for HVDC comprises: a first pole comprising LCC 1 operated as an inverter, VSC 2 operated as a rectifier and provide the commutation voltage for LCC 1 , and a first AC transformer 3 and a second pole comprising LCC 1 ' operated as an inverter, VSC 2' operated as a rectifier and provide the commutation voltage for LCC 1 ⁇ and a second AC transformer 3'; in which the DC side of the first LCC 1 is connected between a first high DC voltage line (High DC voltage Pole 1 , Udc1 +) and a first low DC voltage line (Low DC voltage Pole 1 , Udc2+); the DC side of the first VSC 2 is connected between the first low DC voltage line (Udc2+) and a neutral bus; and the AC sides of the first LCC 1 and first VSC 2 are connected via the first AC transformer 3; and the DC side of the second LCC 1' is connected between
  • both the first and second AC transformers (3, 3') are three-winding transformers, in which one winding of the first AC transformer 3 is connected to the AC side of the first LCC 1 , the other two windings are connected to the AC side of the first VSC 2; and one winding of the second AC transformer 3' is connected to the AC side of the second LCC 1 ', the other two windings are connected to the AC side of the second VSC 2'.
  • the bipolar topology of a step-down DC autotransformer for HVDC comprises: a first pole comprising a VSC 1 operated as an inverter, a LCC 2 operated as a rectifier and a first AC transformer 3 and a second pole comprising a VSC V operated as an inverter, a LCC 2' operated as a rectifier and a second AC transformer 3', and a common AC bus 4; in which the DC side of VSC 1 is connected between a first high DC voltage line (High DC voltage Pole 1 , Udc1 +) and a first low DC voltage line (Low DC voltage Pole 1 , Udc2+); the DC side of LCC 2 is connected between the first low DC voltage line (Udc2+) and a neutral bus; one side of the all three AC transformers 3 are connected to the AC sides of the VSC 1 and LCC 2, and the other side of AC transformers 3 are connected to the common AC bus 4.
  • a first pole comprising a VSC 1 operated as an inverter,
  • the DC side of VSC 1 ' is connected between a second high DC voltage line (High DC voltage Pole 2, Udc1 -) and a second low DC voltage line (Low DC voltage Pole 2, Udc2-); the DC side of LCC 2' is connected between the second low DC voltage line (Low DC voltage Pole 2, Udc2-) and the neutral bus; one side of the all three AC transformers 3' is connected to the AC sides of the VSC ' and LCC 2', and the other side of AC transformers 3' is connected to the common AC bus 4.
  • the AC bus 4 can be either a standalone bus, or connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser.
  • Fig.5 illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the step-down DC autotransformer for HVDC comprises two energizing components besides the LCCs (2, 2'), VSCs (1 , V), AC transformers (3, 3') and similar connection relationship is shown in Fig.4a.
  • the DC autotransformer further comprises a first energizing component, which is configured to be used for start-up process and connected in parallel with the first LCC 2 between the first low DC voltage line and the neutral bus; and a second energizing component, which is configured to be used for start-up process and connected in parallel with the second LCC 2' between the second low DC voltage line and the neutral bus.
  • Fig.6a, Fig.6b, Fig.6c and Fig.6d individually illustrates a topology of step down DC autotransformer for HVDC according to an embodiment of the present invention
  • Each DC autotransformer topology shown in Fig.6a, Fig.6b, Fig.6c and Fig.6d is similar to respective topology shown in Fig.4a, Fig.4b, Fig.4c and Fig.4d except that an additional winding is added into the AC transformer 3.
  • the additional winding of AC transformer 3 can be connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser.
  • Fig.6e illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the step-down DC autotransformer for HVDC comprises: a first pole comprising a LCC 1 operated as an inverter, a LCC 2 operated as a rectifier and a first AC transformer 3 and a second pole comprising a LCC V operated as an inverter, a LCC 2' operated as a rectifier and a second AC transformer 3'; in which the DC side of LCC 1 is connected between a first high DC voltage line (High DC voltage Pole 1 , Udc1+) and a first low DC voltage line (Low DC voltage Pole 1 , Udc2+); the DC side of LCC 2 is connected between the first low DC voltage line (Udc2+) and a neutral bus; and the AC sides of LCC 1 and LCC 2 are connected via the first AC transformer 3, in which the first winding is connected to the AC side of the LCC 1 , the second winding is connected to the AC side of the LCC 2, and the third winding is connected to an AC system, for example a
  • the DC side of LCC 1' is connected between a second high DC voltage line (High DC voltage Pole 2, Udc1-) and a second low DC voltage line (Low DC voltage Pole 2, Udc2-); the DC side of LCC 2' is connected between the second low DC voltage line (Low DC voltage Pole 2, Udc2-) and the neutral bus; and the AC sides of LCC 1 ' and LCC 2' are connected via the second AC transformer 3', in which the first winding is connected to the AC side of the LCC 1', the second winding is connected to the AC side of the LCC 2', and the third winding is connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser.
  • an AC system for example a local AC network, an energy storage system, a StatCom or a condenser.
  • LCC 1 and LCC 1' are used as inverters, and LCC 2 and LCC 2' are used as rectifiers in the step-down DC autotransformer.
  • DC voltage polarity of both LCC 1 , LCC 1 ", LCC 2 and LCC 2' are changed: operation mode of LCC 1 and LCC V is changed to rectifier and operation mode of LCC 2 and LCC 2' is changed to inverter.
  • Fig.6f illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • FIG.6f another step-down DC autotransformer for HVDC is configured to realize bi-directional power flow.
  • the topology shown in Fig. 6f is similar to that shown in Fig. 6e except that the connection direction of LCC 2 and LCC 2' is reversed.
  • LCC 1 , LCC1 ', LCC 2 and LCC 2 are operated as inverters in the step-down DC autotransformer.
  • DC voltage polarities of LCC 1 , LCC 1', LCC 2 and LCC 2' are changed; operation modes of LCC 1 , LCC V, LCC2 and LCC 2 are changed to rectifier.
  • Fig.6g illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • FIG.6g another step-down DC autotransformer for HVDC is configured to realize bi-directional power flow.
  • the topology shown in Fig. 6g is similar to that shown in Fig. 6c except that VSC 2 and VSC 2' are replaced by LCC 2 and LCC 2'.
  • Fig.7 illustrates a monopolar topology of a full-power step-down DC autotransformer for HVDC according to another embodiment of the present invention.
  • the DC autotransformer comprises: a voltage source converter 1 used as an inverter, a line commutated converter 2 used as a rectifier and an AC transformer 3; in which the DC side of VSC 1 is connected between a high DC voltage line and a neutral bus; the DC side of LCC 2 is connected between a low DC voltage line and the neutral bus; and the AC sides of the VSC 1 and LCC 2 are connected via the AC transformer 3.
  • the AC transformer 3 is a two-winding transformer, in which one winding is connected to the AC side of the VSC 1 , and the other winding is connected to the AC side of the LCC 2.
  • any converter is a LCC and operates as rectifier, it can be replaced by a diode rectifier.

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Abstract

A step-down DC autotransformer for HVDC and a system thereof are disclosed. The monopolar topology of the DC autotransformer comprises a first AC/DC converter, a second AC/DC converter, an AC transformer and an energizing component; in which the DC side of the first AC/DC converter is connected between a high DC voltage line and a low DC voltage line; the DC side of the second AC/DC converter is connected between the low DC voltage line and a neutral bus; the AC sides of the first AC/DC converter and second AC/DC converter are connected via the AC transformer; and the energizing component is connected between the low DC voltage line and the neutral bus. The step-down DC autotransformer also can be a bipolar topology. For the proposed solutions, the power capacity of each converter or AC transformer is only part of the transmitted power, which depends on the DC/DC step-down ratio. Therefore the step-down DC autotransformer has low cost, small footprint and high performance.

Description

A Step-down DC Autotransformer for HVDC and a System Thereof
FIELD OF THE INVENTION
The invention relates to the technical field of HVDC applications as a step-down DC autotransformer for HVDC and a system thereof.
BACKGROUND OF THE INVENTION
DC/DC transformer would become a fundamental component for HVDC applications in the future. Actually, there is no essentially practical DC/DC transformer solution for HVDC system.
There is potential requirement for DC/DC transformer at present. One application is to step down DC voltage of one HVDC line from a higher voltage level to a lower voltage level (say from 800kV to 400kV) for power supply of load centers or interconnection of HVDC lines with difference voltage. Another application is to tapping power from HVDC line to different DC voltage levels.
Existing solutions for DC/DC transformer, for example prior arts: "HVDC DC to DC converter with commutating transformer" (US5311418A, Magnus Lalander) needs local AC grid for active and reactive power exchange; or "Step-up DC-DC converter for megawatt size applications" (D. Jovcic, IET Power Electronics, Vol. 2, issue 6, 2009: pp 675-685) is not verified for GW level application. Besides, both the prior arts use the full power converter, that's to say, each converter has the same capacity as the transmission capacity. Hence such DC/DC converter is of high cost. Another example: "DC-DC converter provided with an AC link" (US4462070, Katsuji Eida) is designed as alternative solution of low voltage chopper converter, but with only 1200V input level and the topology comprises single phase inverters and cannot be used for high voltage high power HVDC transmission system. Further, US4462070 needs controlled rectifiers which have forced turn off capacity (GTO or forced turn off thyristor) for the current inverter, additional smoothing reactor and capacitors to realize the control function.
Due to problems mentioned above, a simple and low cost solution for DC voltage conversion is proposed in the present invention and mainly used for step-down DC voltage application in a HVDC system.
SUMMARY OF THE INVENTION
The invention proposes a step-down DC autotransformer for HVDC and a system thereof, where the power capacity of each converter and transformer is only part of the transmitted power in case of low cost and high performance.
According to an aspect of the present invention, it provides a step-down DC autotransformer for HVDC system. The monopolar topology of the DC autotransformer comprises: a first AC/DC converter operated as an inverter, a second AC/DC converter and AC transformer with at least two windings; in which the first AC/DC converter can be a converter with single-phase, three-phase or multi-phase; the second AC/DC converter can be a converter with single-phase, three-phase or multi-phase; the AC transformer can be composed of a AC transformer or a number of AC transformers with three-phase or multi-phase; the DC side of the first AC/DC converter is connected between a high DC voltage line and a low DC voltage line; the DC side of the second AC/DC converter is connected between the low DC voltage line and a neutral bus; and the AC sides of the first AC/DC converter and second AC/DC converter are connected via the AC transformer.
According to a preferred embodiment of the present invention, the an AC transformer is composed of a number of AC transformers with single-phase, three-phase or multi-phase; each the AC side of the first and second AC/DC converter is connected to one side of each the a number of AC transformers; and the other side of each the a number of AC transformers is connected together with a AC bus.
According to a preferred embodiment of the present invention, besides the windings connected to the AC sides of the first AC/DC converter and the second AC/DC converter, an additional winding is installed in the AC transformer which can be an AC transformer with single-phase, three-phase or multi-phase.
According to a preferred embodiment of the present invention, the other side of a number of AC transformers or the additional winding of the AC transformer is connected to AC system; and the AC system can be local AC network, energy storage system, StatCom or condenser.
According to a preferred embodiment of the present invention, the first AC/DC converter is a voltage source converter operated as an inverter; and the second AC/DC converter is a voltage source converter, a line commutated converter or a diode rectifier.
According to a preferred embodiment of the present invention, the first AC/DC converter can be a voltage source converter or a line commutated converter operated as an inverter; and the second AC/DC converter can be a voltage source converter, a line commutated converter or a diode rectifier.
According to a preferred embodiment of the present invention, the first AC/DC converter is line commutated converter; and the second AC/DC converter is voltage source converter.
According to a preferred embodiment of the present invention, both the first and second AC/DC converters are voltage source converters with single-phase, three-phase or multi-phase.
According to a preferred embodiment of the present invention, both the first and second AC/DC converters are line commutated converters with single-phase, three-phase or multi-phase.
According to a preferred embodiment of the present invention, the AC bus can exchange the power bi-directionally between the first and second AC/DC converter by controlling the rectified power or inverted power of the first or second AC/DC converter.
According to the other aspect of the present invention, it provides a step-down DC autotransformer for HVDC. The bipolar topology of the DC autotransformer comprises: a first pole comprising a first AC/DC converter operated as an inverter, a second AC/DC converter and a first AC transformer with at least two windings; a second pole comprising a third AC/DC converter operated as an inverter, a fourth AC/DC converter and second AC transformer with at least two windings; and a neutral bus; in which each the first, second, third and fourth AC/DC converters can be converter with single-phase, three-phase or multi-phase; each the first and second AC transformers can be composed of an AC transformer or a number of AC transformers with single-phase, three-phase or multi-phase; the DC side of the first AC/DC converter is connected between a first high DC voltage line and a first low DC voltage line; the DC side of the second AC/DC converter is connected between the first low DC voltage line and the neutral bus; and the AC sides of the first AC/DC converter and second AC/DC converter are connected via the first AC transformer; and the DC side of the third AC/DC converter is connected between a second low DC voltage line and a second high DC voltage line; the DC side of the fourth AC/DC converter is connected between the neutral bus and the second low DC voltage line; and the AC sides of the third AC/DC converter and the fourth AC/DC converter are connected via the second AC transformer.
According to a preferred embodiment of the present invention, each of the first AC transformer and the second AC transformer is composed of a number of AC transformers; in which each the AC side of the first, second, third and fourth AC/DC converters is connected to one side of the a number of AC transformers; and the other side of each the a number of AC transformers is connected together with an AC bus.
According to a preferred embodiment of the present invention, besides the windings connected to the AC sides of the first and second AC/DC converters, an additional winding is installed in the first AC transformer; and besides the windings connected to the AC sides of the third and fourth AC/DC converters, an additional winding is installed in the second AC transformer.
According to a preferred embodiment of the present invention, the other side of the a number of AC transformers or the additional winding of the AC transformers is connected to AC system; and the AC system can be local AC network, energy storage system, StatCom or condenser.
According to a preferred embodiment of the present invention, the first and third AC/DC converters are voltage source converters; and each the second and fourth AC/DC converters is operated as rectifier, which can be voltage source converter, line commutated converter or diode rectifier. According to a preferred embodiment of the present invention, the first and third AC/DC converters are voltage source converters or line commutated converters; and each the second and fourth AC/DC converters can be voltage source converter, line commutated converter or diode rectifier.
According to a preferred embodiment of the present invention, each the first and third AC/DC converter is line commutated converter; and each the second and fourth AC/DC converters is voltage source converter.
According to a preferred embodiment of the present invention, all the first, second, third and fourth AC/DC converters are voltage source converters with single-phase, three-phase or multi-phase.
According to a preferred embodiment of the present invention, all the first, second, third and fourth AC/DC converters are line commutated converters with single-phase, three-phase or multi-phase.
According to a preferred embodiment of the present invention, the AC bus can exchange the power bi-directionally between the first pole and second pole by controlling rectified power or inverted power of the first, second, third and/or fourth AC/DC converter.
According to a preferred embodiment of the present invention, the currents of the second AC/DC converter and the fourth AC/DC converter are equal to avoid unbalanced current through earth via the neutral bus.
According to a preferred embodiment of the present invention, the step-down DC autotransformer further comprises: an energizing component, which is connected between the low DC voltage line and the neutral bus.
According to a preferred embodiment of the present invention, the step-down DC autotransformer further comprises: a first energizing component and a second energizing component; the first energizing component is connected between the first low DC voltage line and the neutral bus; and the second energizing component is connected between the second low DC voltage line and the neutral bus.
According to a preferred embodiment of the present invention, the line commutated converter can be replaced by a capacitor commutated converter.
According to a preferred embodiment of the present invention, the high DC voltage line and low DC voltage line are connected to different DC systems with different voltage levels. According to another aspect of the present invention, it provides a step-down DC transformer for HVDC. The DC transformer comprises: a first AC/DC converter used as an inverter, a second AC/DC converter used as a rectifier and an AC transformer; in which the first AC/DC converter is voltage source converter; the second AC/DC converter is line commutated converter; the DC side of the first AC/DC converter is connected between a high DC voltage line and a neutral bus; the DC side of the second AC/DC converter is connected between a low DC voltage line and a neutral bus; and the AC sides of the first AC/DC converter and second AC/DC converter are connected via the AC transformer.
According to a preferred embodiment of the present invention, besides the windings connected to the AC sides of the first AC/DC converter and the second AC/DC converter, an additional winding is installed in the AC transformer which can be an AC transformer with single-phase, three-phase or multi-phase; the additional winding of the AC transformer is connected to AC system; and the AC system can be local AC network, energy storage system, StatCom or condenser.
According to another aspect of the present invention, it provides a system comprising at least one of the step-down DC autotransformer or transformer above mentioned.
Embodiments of the present invention provide a step-down DC autotransformer for HVDC and a system thereof, which facilitates the development of HVDC applications at high altitude area where the lower DC voltage level is desired to decrease insulation cost. With the proposed invention, the lower DC voltage will be stepped up at low altitude area to realize high power and long distance transmission.
BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter of the invention will be explained in more details in the following description with reference to preferred exemplary embodiments which are illustrated in the drawings, in which:
Figs.1a-1 h individually illustrate a monopolar topology of a step-down DC autotransformer for HVDC according to an embodiment of the present invention;
Fig.2 illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention;
Figs.3a-3j individually illustrate a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention;
Figs.4a-4e individually illustrate a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention;
Fig.5 illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention;
Figs.6a-6g individually illustrate a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention; and
Fig.7 illustrates a monopolar topology of a full-power step-down DC autotransformer for HVDC according to another embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Exemplary embodiments of the present invention are described in conjunction with the accompanying drawings hereinafter. For the sake of clarity and conciseness, not all the features of actual implementations are described in the specification.
It is defined that the nominal power flow direction of the step-down DC autotransformer is from high DC voltage side to low DC voltage side in terms of absolute voltage value. Therefore, the direction of reversed power flow is from low DC voltage side to high DC voltage side.
Fully controlled semiconductor based voltage source converter (VSC) technology or thyristor bridge based line commutated converter (LCC) technology and AC transformer technology will be employed in this invention. The topology of VSC can be two-level converter, multi-level converter (MLC), MMC (modular multi-level converter), CTL (cascaded two-level converter) or AMMC (alternating arm multi-level converter) and so forth. All VSC topologies employ fully controllable semiconductor devices, such as IGBT, IGCT, IEGT, GTO, MOSFET and so forth. The LCC topology can be a single phase converter, a 6 pulse converter, a 12 pulse converter and so forth. It's obvious to the person skilled in art that VSC or LCC can be realized by other various topologies. The said converter, either AC/DC VSC or AC/DC LCC can be operated at rectifier mode (power flows from AC side to DC side) or inverter mode (power flows from DC side to AC side), according to function requirement.
It shall be noted that the frequency of the AC transformer in the invention may be not limited to be 50 or 60Hz. For some embodiments, the frequency can be optimally designed based on the comprehensive consideration on the cost, efficiency, weight and footprint etc. The size of reactive power compensation equipment for LCC is decreased significantly, as the reactive power can be provided by VSC. Besides, the AC transformer in the invention is also not limited to be a three-phase transformer, the descriptions of winding of AC transformer below can be single phase winding, three phase winding or any other winding types to form a multiphase AC transformer.
Fig. 1a illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to an embodiment of the present invention.
As shown in Fig.1a, the step-down DC autotransformer for HVDC is a monopolar topology which comprises a VSC 1 used as an inverter, a LCC 2 used as a rectifier, and a three-winding AC transformer 3. The DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the LCC 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the VSC 1 and LCC 2 are connected via the AC transformer 3, in which one winding is connected to the AC side of the VSC 1 , and the other two windings are connected to the AC side of the LCC 2.
Such a simple and low cost solution of the DC autotransformer shown in Fig.1a can be used for HVDC voltage step-down in the case of unidirectional power flow.
It's obvious to the person skilled in art that the DC autotransformer is a cascaded type, which consists of a VSC, a LCC and an AC transformer which frequency can be designed optimally. Especially, the power capacity of each converter or transformer is only part of the transmitted power, which depends on the DC/DC step-down ratio. Therefore the cost and footprint are low.
Fig.1 b illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.1b, the step-down DC autotransformer for HVDC comprises a VSC 1 configured as an inverter, a LCC 2 used as a rectifier and a two-winding AC transformer 3. The DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the LCC 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the VSC 1 and LCC 2 are connected via the AC transformer 3, in which one winding is connected to the AC side of the VSC 1 , and the other winding is connected to the AC side of the LCC 2.
Fig. 1c illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to an embodiment of the present invention.
As shown in Fig.1c, the step-sown DC autotransformer for HVDC is configured to realize bi-directional power flow, which comprises two VSC converters VSC 1 and VSC 2, and a two-winding AC transformer 3, in which DC side of VSC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of VSC 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the VSC 1 and VSC 2 are connected via the AC transformer 3, in which one winding is connected to the AC side of the VSC 1 , and the other winding is connected to the AC side of the VSC 2. For nominal power flow operation, the VSC 1 is used as an inverter, and VSC 2 is used as a rectifier in the step-down DC autotransformer for HVDC. For power flow reverse operation, DC current direction of VSC 1 and VSC 2 are changed: conversion mode of VSC 1 is changed to rectifier and conversion mode of VSC 2 is changed to inverter.
Fig.ld illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.ld, the step-down DC autotransformer for HVDC comprises a VSC 1 configured as an inverter, a diode rectifier 2, and a two-winding AC transformer 3. The DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the diode rectifier 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the VSC 1 and diode rectifier 2 are connected via the AC transformer 3, in which one winding is connected to the AC side of the VSC 1 , and the other winding is connected to the AC side of the diode rectifier 2.
Fig.le illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.le, the step-down DC autotransformer for HVDC comprises a LCC 1 configured as an inverter, a VSC 2 used as a rectifier and a two-winding AC transformer 3. The DC side of the LCC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the VSC 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the VSC 1 and LCC 2 are connected via the AC transformer 3, in which one winding is connected to the AC side of the LCC 1 , and the other winding is connected to the AC side of the VSC 2.
Fig. 1f illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to an embodiment of the present invention.
As shown in Fig.lf, the step-down DC autotransformer for HVDC is a monopolar topology which comprises a LCC 1 used as an inverter, a VSC 2 used as a rectifier, and a three-winding AC transformer 3. The DC side of the LCC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the VSC 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the LCC 1 and VSC 2 are connected via the AC transformer 3, in which two windings are connected to the AC side of the LCC 1 , and the other winding is connected to the AC side of the VSC 2.
Fig.lg illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.lg, the step-down DC autotransformer for HVDC comprises a VSC 1 configured as an inverter, a LCC 2 used as a rectifier and three single phase AC transformers 3. The DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the LCC 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the VSC 1 and LCC 2 are connected via the AC transformer 3, in which one side of those AC transformers is connected to VSC 1 and the other side of those AC transformers is connected to LCC 2.
Fig.lh illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.l h, the step-down DC autotransformer for HVDC comprises a VSC 1 configured as an inverter, a LCC 2 used as a rectifier, a two-winding AC transformer 3 and an energizing component 5. The DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the LCC 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the VSC 1 and LCC 2 are connected via the AC transformer 3, in which one winding is connected to the AC side of the VSC 1 , and the other winding is connected to the AC side of the LCC 2.
The DC autotransformer further comprises an energizing component, which is used for start-up process. In detail, the energizing component can be connected in parallel with LCC 2 between the low DC voltage line and the neutral bus.
Fig.2 illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig. 2, the step-down DC autotransformer for HVDC is a monopolar topology which comprises a VSC 1 used as an inverter and a LCC 2 used as a rectifier, three AC transformers 3 and an AC bus 4. The DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the LCC 2 is connected between the low DC voltage line and a neutral bus; one side of the all AC transformers are connected to the AC sides of the VSC 1 and LCC 2, and the other side of all AC transformers 3 are connected to the AC bus 4.
Fig.3a, Fig.3b, Fig.3c, Fig.3d, Fig.3e and Fig.3f individually illustrates a topology of step sown DC autotransformer for HVDC according to an embodiment of the present invention;
Each DC autotransformer topology shown in Fig.3a, Fig.3b, Fig.3c, Fig.3d,
Fig.3e and Fig.3f is similar to respective topology shown in Fig.1a, Fig.1 b, Fig.1c, Fig.ld, Fig.le and Fig.lf except that an additional winding is added into the AC transformer 3. The additional winding of AC transformer 3 can be connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser.
As shown in Fig.3a, Fig.3b, Fig.3c, Fig.3d, Fig.3e and Fig.3f, various power flow direction can be realized with these topologies.
Fig. 3g illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig. 3g, the step-down DC autotransformer for HVDC is a monopolar topology which comprises a VSC 1 used as an inverter and a LCC 2 used as a rectifier, three AC transformers 3 and an AC bus 4. The DC side of the VSC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of the LCC 2 is connected between the low DC voltage line and a neutral bus; one side of the all three AC transformers is connected to the AC sides of VSC 1 and LCC 2, and the other side of AC transformers is connected to the AC bus 4. The AC bus 4 can be standalone AC bus or connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser.
Fig.3h illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.3h, another step-down DC autotransformer for HVDC is configured to realize bi-directional power flow, which comprises two LCC converters LCC 1 and LCC 2, and a three-winding AC transformer 3, in which DC side of LCC 1 is connected between a high DC voltage line and a low DC voltage line; the DC side of LCC 2 is connected between the low DC voltage line and a neutral bus; and the AC sides of the LCC 1 and LCC 2 are connected via the AC transformer 3, in which the first winding is connected to the AC side of the LCC 1 , the second winding is connected to the AC side of the LCC 2, and the third winding is connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser.
For nominal power flow operation, the LCC 1 is used as an inverter, and LCC 2 is used as a rectifier in the step-down DC autotransformer. For power flow reverse operation, DC voltage polarities of both LCC 1 and LCC 2 are changed: operation mode of LCC 1 is changed to rectifier and operation mode of LCC 2 is changed to inverter.
Fig.3i illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.3i, another step-down DC autotransformer for HVDC is configured to realize bi-directional power flow. The topology shown in Fig. 3i is similar to that shown in Fig. 3g except that the connect direction of LCC 2 is reversed.
For nominal power flow operation, both LCC 1 and LCC 2 are operated as inverters in the step-down DC autotransformer. For power flow reverse operation, DC voltage polarity of both LCC 1 and LCC 2 are changed; operation modes of LCC 1 and LCC 2 are changed to rectifiers.
Fig.3] illustrates a monopolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.3j, another step-down DC autotransformer for HVDC is configured to realize bi-directional power flow. The topology shown in Fig. 3j is similar to that shown in Fig. 3c except that the VSC 2 is replaced by LCC 2.
Fig.4a illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.4a, the bipolar topology of a step-down DC autotransformer for HVDC comprises: a first pole comprising a VSC 1 operated as an inverter, a LCC 2 operated as a rectifier and a first AC transformer 3 and a second pole comprising a VSC V operated as an inverter, a LCC 2' operated as a rectifier and a second AC transformer 3'; in which the DC side of VSC 1 is connected between a first high DC voltage line (High DC voltage Pole 1 , Udc1+) and a first low DC voltage line (Low DC voltage Pole 1 , Udc2+); the DC side of LCC 2 is connected between the first low DC voltage line (Udc2+) and a neutral bus; and the AC sides of VSC 1 and LCC 2 are connected via the first AC transformer 3; and the DC side of VSC V is connected between a second high DC voltage line (High DC voltage Pole 2, Udc1-) and a second low DC voltage line (Low DC voltage Pole 2, Udc2-); the DC side of LCC 2' is connected between the second low DC voltage line (Low DC voltage Pole 2, Udc2-) and the neutral bus; and the AC sides of VSC 1 ' and LCC 2' are connected via the second AC transformer 3'.
According to a preferred embodiment of the present invention, both the first and second AC transformers (3, 3') are three-winding transformers, in which one winding of the first AC transformer 3 is connected to the AC side of VSC 1 , the other two windings are connected to the AC side of LCC 2; and one winding of the second AC transformer 3' is connected to the AC side of VSC 1 ', the other two windings are connected to the AC side of LCC 2'.
The AC transformers (3, 3') shown in Fig.4a can also be placed by a number of transformers.
Fig.4b illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
Fig.4b is similar to Fig.4a except that the first and second AC transformers (3, 3') are two-winding transformers. One winding of the first AC transformer 3 is connected to the AC side of the first VSC 1 , the other winding is connected to the AC side of the first LCC 2; and one winding of the second AC transformer 3' is connected to the AC side of the second VSC 1', the other winding is connected to the AC side of the second LCC 2\
Fig.4c illustrates a bipolar topology of a step-down DC autotransformer for
HVDC according to another embodiment of the present invention.
As shown in Fig.4c, the step-down DC autotransformer for HVDC is configured to realize bi-directional power flow; in detail, all the first, second third and fourth converters are VSCs (1 , V, 2, 2'). The DC side of VSC 1 is connected between a first high DC voltage line (High DC voltage Pole 1 , Udc1 +) and a first low DC voltage line (Low DC voltage Pole 1 , Udc2+); the DC side of VSC 2 is connected between the first low DC voltage line (Udc2+) and a neutral bus; and the AC sides of VSC 1 and VSC 2 are connected via the first AC transformer 3; and the DC side of VSC V is connected between a second high DC voltage line (High DC voltage Pole 2, Udc1-) and a second low DC voltage line (Low DC voltage Pole 2, Udc2-); the DC side of VSC 2' is connected between the second low DC voltage line (Low DC voltage Pole 2, Udc2-) and the neutral bus; and the AC sides of VSC 1' and VSC 2' are connected via the second AC transformer 3'.
For nominal power flow operation, conversion mode VSC 1 and VSC 1' are inverters and conversion mode of VSC 2 and VSC 2' are rectifiers. For power flow reverse operation, DC current direction of all converters (1 , 1\ 2, 2') is changed, conversion mode of VSC 1 and VSC V is changed to rectifier and conversion mode of VSC 2 and VSC 2' is changed to inverter. Fig.4d illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.4d, the bipolar topology of a step-down DC autotransformer for HVDC comprises: a first pole comprising LCC 1 operated as an inverter, VSC 2 operated as a rectifier and provide the commutation voltage for LCC 1 , and a first AC transformer 3 and a second pole comprising LCC 1 ' operated as an inverter, VSC 2' operated as a rectifier and provide the commutation voltage for LCC 1\ and a second AC transformer 3'; in which the DC side of the first LCC 1 is connected between a first high DC voltage line (High DC voltage Pole 1 , Udc1 +) and a first low DC voltage line (Low DC voltage Pole 1 , Udc2+); the DC side of the first VSC 2 is connected between the first low DC voltage line (Udc2+) and a neutral bus; and the AC sides of the first LCC 1 and first VSC 2 are connected via the first AC transformer 3; and the DC side of the second LCC 1' is connected between a second high DC voltage line (High DC voltage Pole 2, Udd -) and a second low DC voltage line (Low DC voltage Po!e2, Udc2-); the DC side of the second VSC 2' is connected between the second low DC voltage line (Low DC voltage Pole 2, Udc2-) and the neutral bus; and the AC sides of the second LCC 1 ' and the second VSC 2' are connected via the second converter transformer 3'.
According to a preferred embodiment of the present invention, both the first and second AC transformers (3, 3') are three-winding transformers, in which one winding of the first AC transformer 3 is connected to the AC side of the first LCC 1 , the other two windings are connected to the AC side of the first VSC 2; and one winding of the second AC transformer 3' is connected to the AC side of the second LCC 1 ', the other two windings are connected to the AC side of the second VSC 2'.
As shown in Fig.4e, the bipolar topology of a step-down DC autotransformer for HVDC comprises: a first pole comprising a VSC 1 operated as an inverter, a LCC 2 operated as a rectifier and a first AC transformer 3 and a second pole comprising a VSC V operated as an inverter, a LCC 2' operated as a rectifier and a second AC transformer 3', and a common AC bus 4; in which the DC side of VSC 1 is connected between a first high DC voltage line (High DC voltage Pole 1 , Udc1 +) and a first low DC voltage line (Low DC voltage Pole 1 , Udc2+); the DC side of LCC 2 is connected between the first low DC voltage line (Udc2+) and a neutral bus; one side of the all three AC transformers 3 are connected to the AC sides of the VSC 1 and LCC 2, and the other side of AC transformers 3 are connected to the common AC bus 4. The DC side of VSC 1 ' is connected between a second high DC voltage line (High DC voltage Pole 2, Udc1 -) and a second low DC voltage line (Low DC voltage Pole 2, Udc2-); the DC side of LCC 2' is connected between the second low DC voltage line (Low DC voltage Pole 2, Udc2-) and the neutral bus; one side of the all three AC transformers 3' is connected to the AC sides of the VSC ' and LCC 2', and the other side of AC transformers 3' is connected to the common AC bus 4. The AC bus 4 can be either a standalone bus, or connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser.
Fig.5 illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention. As shown in Fig.5, the step-down DC autotransformer for HVDC comprises two energizing components besides the LCCs (2, 2'), VSCs (1 , V), AC transformers (3, 3') and similar connection relationship is shown in Fig.4a. In detail, the DC autotransformer further comprises a first energizing component, which is configured to be used for start-up process and connected in parallel with the first LCC 2 between the first low DC voltage line and the neutral bus; and a second energizing component, which is configured to be used for start-up process and connected in parallel with the second LCC 2' between the second low DC voltage line and the neutral bus. Fig.6a, Fig.6b, Fig.6c and Fig.6d individually illustrates a topology of step down DC autotransformer for HVDC according to an embodiment of the present invention;
Each DC autotransformer topology shown in Fig.6a, Fig.6b, Fig.6c and Fig.6d is similar to respective topology shown in Fig.4a, Fig.4b, Fig.4c and Fig.4d except that an additional winding is added into the AC transformer 3. The additional winding of AC transformer 3 can be connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser.
As shown in Fig.6a, Fig.6b, Fig.6c and Fig.6d, various power flow direction can be realized with these topologies.
Fig.6e illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.6e, the step-down DC autotransformer for HVDC comprises: a first pole comprising a LCC 1 operated as an inverter, a LCC 2 operated as a rectifier and a first AC transformer 3 and a second pole comprising a LCC V operated as an inverter, a LCC 2' operated as a rectifier and a second AC transformer 3'; in which the DC side of LCC 1 is connected between a first high DC voltage line (High DC voltage Pole 1 , Udc1+) and a first low DC voltage line (Low DC voltage Pole 1 , Udc2+); the DC side of LCC 2 is connected between the first low DC voltage line (Udc2+) and a neutral bus; and the AC sides of LCC 1 and LCC 2 are connected via the first AC transformer 3, in which the first winding is connected to the AC side of the LCC 1 , the second winding is connected to the AC side of the LCC 2, and the third winding is connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser. The DC side of LCC 1' is connected between a second high DC voltage line (High DC voltage Pole 2, Udc1-) and a second low DC voltage line (Low DC voltage Pole 2, Udc2-); the DC side of LCC 2' is connected between the second low DC voltage line (Low DC voltage Pole 2, Udc2-) and the neutral bus; and the AC sides of LCC 1 ' and LCC 2' are connected via the second AC transformer 3', in which the first winding is connected to the AC side of the LCC 1', the second winding is connected to the AC side of the LCC 2', and the third winding is connected to an AC system, for example a local AC network, an energy storage system, a StatCom or a condenser.
For nominal power flow operation, the LCC 1 and LCC 1' are used as inverters, and LCC 2 and LCC 2' are used as rectifiers in the step-down DC autotransformer. For power flow reverse operation, DC voltage polarity of both LCC 1 , LCC 1 ", LCC 2 and LCC 2' are changed: operation mode of LCC 1 and LCC V is changed to rectifier and operation mode of LCC 2 and LCC 2' is changed to inverter.
As shown in Fig.6e, various power flow direction can be realized with the topology.
Fig.6f illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.6f, another step-down DC autotransformer for HVDC is configured to realize bi-directional power flow. The topology shown in Fig. 6f is similar to that shown in Fig. 6e except that the connection direction of LCC 2 and LCC 2' is reversed.
For nominal power flow operation, LCC 1 , LCC1 ', LCC 2 and LCC 2 are operated as inverters in the step-down DC autotransformer. For power flow reverse operation, DC voltage polarities of LCC 1 , LCC 1', LCC 2 and LCC 2' are changed; operation modes of LCC 1 , LCC V, LCC2 and LCC 2 are changed to rectifier.
As shown in Fig.6f, various power flow direction can be realized with the topology.
Fig.6g illustrates a bipolar topology of a step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.6g, another step-down DC autotransformer for HVDC is configured to realize bi-directional power flow. The topology shown in Fig. 6g is similar to that shown in Fig. 6c except that VSC 2 and VSC 2' are replaced by LCC 2 and LCC 2'.
Fig.7 illustrates a monopolar topology of a full-power step-down DC autotransformer for HVDC according to another embodiment of the present invention.
As shown in Fig.7, a full-power step-down DC autotransformer for HVDC is provided. The DC autotransformer comprises: a voltage source converter 1 used as an inverter, a line commutated converter 2 used as a rectifier and an AC transformer 3; in which the DC side of VSC 1 is connected between a high DC voltage line and a neutral bus; the DC side of LCC 2 is connected between a low DC voltage line and the neutral bus; and the AC sides of the VSC 1 and LCC 2 are connected via the AC transformer 3. The AC transformer 3 is a two-winding transformer, in which one winding is connected to the AC side of the VSC 1 , and the other winding is connected to the AC side of the LCC 2.
It shall be noted that the proposed solution in Fig.7 employs full power converters, which means in spite of the DC/DC ratio, the capacity of each converter as well as converter transformer is always the same as power to be transmitted. In all the solutions proposed above, if any converter is a LCC, it can be replaced by CCC or a fully controlled semiconductor based current source converter (CSC).
In all the solutions proposed above, if any converter is a LCC and operates as rectifier, it can be replaced by a diode rectifier.
According to another aspect of the present invention, it provides a system comprising at least one of the DC autotransformer above embodiments mentioned.
Though the present invention has been described on the basis of some preferred embodiments, those skilled in the art should appreciate that those embodiments should by no means limit the scope of the present invention. Without departing from the spirit and concept of the present invention, any variations and modifications to the embodiments should be within the apprehension of those with ordinary knowledge and skills in the art, and therefore fall in the scope of the present invention which is defined by the accompanied claims.

Claims

1. A step-down DC autotransformer for HVDC, wherein the monopolar topology of said DC autotransformer comprises:
a first AC/DC converter operated as an inverter, a second AC/DC converter and AC transformer with at least two windings;
in which
said first AC/DC converter can be a converter with single-phase, three-phase or multi-phase;
said second AC/DC converter can be a converter with single-phase, three-phase or multi-phase;
said AC transformer can be composed of a AC transformer or a number of AC transformers with three-phase or multi-phase;
the DC side of said first AC/DC converter is connected between a high DC voltage line and a low DC voltage line;
the DC side of said second AC/DC converter is connected between said low DC voltage line and a neutral bus; and
the AC sides of said first AC/DC converter and second AC/DC converter are connected via said AC transformer.
2. The step-down DC autotransformer according to Claim 1 , wherein
said an AC transformer is composed of a number of AC transformers with single-phase, three-phase or multi-phase;
each said AC side of said first and second AC/DC converter is connected to one side of each said a number of AC transformers; and
the other side of each said a number of AC transformers is connected together with a AC bus.
3. The step-down DC autotransformer according to Claim 1 , wherein besides the windings connected to said AC sides of said first AC/DC converter and said second AC/DC converter, an additional winding is installed in said AC transformer which can be an AC transformer with single-phase, three-phase or multi-phase.
4. The step-down DC autotransformer according to Claim 2 or 3, wherein said other side of said a number of AC transformers or said additional winding of said AC transformer is connected to AC system; and said AC system can be local AC network, energy storage system, StatCom or condenser.
5. The step-down DC autotransformer according to Claim 1 or 2, wherein said first AC/DC converter is a voltage source converter operated as an inverter; and
said second AC/DC converter is a voltage source converter, a line commutated converter or a diode rectifier.
6. The step-down DC autotransformer according to Claim 4, wherein
said first AC/DC converter can be a voltage source converter or a line commutated converter operated as an inverter; and
said second AC/DC converter can be a voltage source converter, a line commutated converter or a diode rectifier.
7. The step-down DC autotransformer according to Claim 1 or 2, wherein said first AC/DC converter is line commutated converter; and
said second AC/DC converter is voltage source converter.
8. The step-down DC autotransformer according to any one of Claims 1-4, wherein both said first and second AC/DC converters are voltage source converters with single-phase, three-phase or multi-phase.
9. The step-down DC autotransformer according to Claim 4, wherein both said first and second AC/DC converters are line commutated converters with single-phase, three-phase or multi-phase.
10. The step-down DC autotransformer according to any one of Claims 4-9, wherein said AC bus can exchange the power bi-directionally between said first and second AC/DC converter by controlling the rectified power or inverted power of said first or second AC/DC converter.
11. A step-down DC autotransformer for HVDC, wherein the bipolar topology of said DC autotransformer comprises:
a first pole comprising a first AC/DC converter operated as an inverter, a second AC/DC converter and a first AC transformer with at least two windings;
a second pole comprising a third AC/DC converter operated as an inverter, a fourth AC/DC converter and second AC transformer with at least two windings; and a neutral bus;
in which
each said first, second, third and fourth AC/DC converters can be converter with single-phase, three-phase or multi-phase;
each said first and second AC transformers can be composed of an AC transformer or a number of AC transformers with single-phase, three-phase or multi-phase;
the DC side of said first AC/DC converter is connected between a first high DC voltage line and a first low DC voltage line; the DC side of said second AC/DC converter is connected between said first low DC voltage line and said neutral bus; and the AC sides of said first AC/DC converter and second AC/DC converter are connected via said first AC transformer; and the DC side of said third AC/DC converter is connected between a second low DC voltage line and a second high DC voltage line; the DC side of said fourth AC/DC converter is connected between said neutral bus and said second low DC voltage line; and the AC sides of said third AC/DC converter and said fourth AC/DC converter are connected via said second AC transformer.
12. The step-down DC autotransformer according to Claim 11 , wherein each of said first AC transformer and said second AC transformer is composed of a number of AC transformers; in which
each said AC side of said first, second, third and fourth AC/DC converters is connected to one side of said a number of AC transformers; and the other side of each said a number of AC transformers is connected together with an AC bus.
13. The step-down DC autotransformer according to Claim 11 , wherein besides the windings connected to said AC sides of said first and second AC/DC converters, an additional winding is installed in said first AC transformer; and
besides the windings connected to said AC sides of said third and fourth AC/DC converters, an additional winding is installed in said second AC transformer.
14. The step-down DC autotransformer according to Claim 12 or 13, wherein said other side of said a number of AC transformers or said additional winding of said AC transformers is connected to AC system; and
said AC system can be local AC network, energy storage system, StatCom or condenser.
15. The step-down DC autotransformer according to Claim 11 or 12, wherein said first and third AC/DC converters are voltage source converters; and each said second and fourth AC/DC converters is operated as rectifier, which can be voltage source converter, line commutated converter or diode rectifier.
16. The step-down DC autotransformer according to Claim 14, wherein
said first and third AC/DC converters are voltage source converters or line commutated converters; and
each said second and fourth AC/DC converters can be voltage source converter, line commutated converter or diode rectifier.
17. The step-down DC autotransformer according to claim 11 or 12, wherein each of said first and third AC/DC converters is line commutated converter; and
each of said second and fourth AC/DC converters is voltage source converter.
18. The step-down DC autotransformer according to any one of Claims 11-14, wherein all said first, second, third and fourth AC/DC converters are voltage source converters with single-phase, three-phase or multi-phase.
19. The step-down DC autotransformer according to Claim 14, wherein all said first, second, third and fourth AC/DC converters are line commutated converters with single-phase, three-phase or multi-phase.
20. The step-down DC autotransformer according to any one of Claims 14-19, wherein said AC bus can exchange the power bi-directionally between said first pole and second pole by controlling rectified power or inverted power of said first, second, third and/or fourth AC/DC converter.
21. The step-down DC autotransformer according to any one of Claims 11-20, wherein the currents of said second AC/DC converter and said fourth AC/DC converter are equal to avoid unbalanced current through earth via said neutral bus.
22. The step-down DC autotransformer according to Claim 1 , wherein said step-down DC autotransformer further comprises: an energizing component, which is connected between said low DC voltage line and said neutral bus.
23. The step-down DC autotransformer according to claim 11 , wherein said step-down DC autotransformer further comprises:
a first energizing component and a second energizing component;
said first energizing component is connected between said first low DC voltage line and said neutral bus; and
said second energizing component is connected between said second low DC voltage line and said neutral bus.
24. The step-down DC autotransformer according to any one of Claims 5, 6, 15 and 16, wherein said line commutated converter can be replaced by a capacitor commutated converter.
25. The step-down DC autotransformer according to any one of Claims 1 , 11 and 23, wherein the high DC voltage line and low DC voltage line are connected to different DC systems with different voltage levels.
26. A step-down DC transformer for HVDC, wherein said DC transformer comprises:
a first AC/DC converter used as an inverter, a second AC/DC converter used as a rectifier and an AC transformer; in which
said first AC/DC converter is voltage source converter;
said second AC/DC converter is line commutated converter;
the DC side of said first AC/DC converter is connected between a high DC voltage line and a neutral bus;
the DC side of said second AC/DC converter is connected between a low DC voltage line and a neutral bus; and
the AC sides of said first AC/DC converter and second AC/DC converter are connected via said AC transformer.
27. The step-down DC transformer according to Claim 26, wherein
besides the windings connected to said AC sides of said first AC/DC converter and said second AC/DC converter, an additional winding is installed in said AC transformer which can be an AC transformer with single-phase, three-phase or multi-phase;
said additional winding of said AC transformer is connected to AC system; and
said AC system can be local AC network, energy storage system, StatCom or condenser.
28. A system, wherein said system comprises at least one of said step-down DC autotransformer or transformer according to any one of above claims.
PCT/CN2012/084348 2012-11-09 2012-11-09 A step-down dc autotransformer for hvdc and a system thereof WO2014071597A1 (en)

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US3942089A (en) * 1973-10-10 1976-03-02 Brown, Boveri & Cie., Ag D.C. voltage transformation in high tension systems
US4462070A (en) * 1982-01-29 1984-07-24 Toyo Denki Seizo Kabushiki Kaisha DC-DC Converter provided with an AC link

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US3942089A (en) * 1973-10-10 1976-03-02 Brown, Boveri & Cie., Ag D.C. voltage transformation in high tension systems
US4462070A (en) * 1982-01-29 1984-07-24 Toyo Denki Seizo Kabushiki Kaisha DC-DC Converter provided with an AC link

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SE543496C2 (en) * 2018-12-21 2021-03-09 Abb Power Grids Switzerland Ag Power supporting arrangement for an ac network

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