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WO2008036009A1 - Hvdc converter - Google Patents

Hvdc converter Download PDF

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Publication number
WO2008036009A1
WO2008036009A1 PCT/SE2006/050339 SE2006050339W WO2008036009A1 WO 2008036009 A1 WO2008036009 A1 WO 2008036009A1 SE 2006050339 W SE2006050339 W SE 2006050339W WO 2008036009 A1 WO2008036009 A1 WO 2008036009A1
Authority
WO
WIPO (PCT)
Prior art keywords
inlet
outlet
pulse
converter
transformers
Prior art date
Application number
PCT/SE2006/050339
Other languages
French (fr)
Inventor
Tommy Holmgren
Mats Hyttinen
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/SE2006/050339 priority Critical patent/WO2008036009A1/en
Publication of WO2008036009A1 publication Critical patent/WO2008036009A1/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
    • H02M5/00—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases
    • H02M5/40—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC
    • H02M5/42—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters
    • H02M5/44—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters using discharge tubes or semiconductor devices to convert the intermediate DC into AC
    • H02M5/443—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters using discharge tubes or semiconductor devices to convert the intermediate DC into AC using devices of a thyratron or thyristor type requiring extinguishing means
    • H02M5/45—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters using discharge tubes or semiconductor devices to convert the intermediate DC into AC using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
    • H02M5/451—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters using discharge tubes or semiconductor devices to convert the intermediate DC into AC using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only with automatic control of output voltage or frequency
    • 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
    • H02M5/00—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases
    • H02M5/40—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC
    • H02M5/42—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters
    • H02M5/44—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters using discharge tubes or semiconductor devices to convert the intermediate DC into AC
    • H02M5/443—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters using discharge tubes or semiconductor devices to convert the intermediate DC into AC using devices of a thyratron or thyristor type requiring extinguishing means
    • H02M5/45—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters using discharge tubes or semiconductor devices to convert the intermediate DC into AC using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
    • H02M5/452—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters using discharge tubes or semiconductor devices to convert the intermediate DC into AC using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only with automatic control of output waveform
    • 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
    • H02M7/02—Conversion of AC power input into DC power output without possibility of reversal
    • H02M7/04—Conversion of AC power input into DC power output without possibility of reversal by static converters
    • H02M7/12—Conversion of AC power input into DC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/145—Conversion of AC power input into DC power output without 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
    • H02M7/155—Conversion of AC power input into DC power output without 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
    • H02M7/162—Conversion of AC power input into DC power output without 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 in a bridge configuration
    • H02M7/1623—Conversion of AC power input into DC power output without 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 in a bridge configuration with control circuit
    • H02M7/1626—Conversion of AC power input into DC power output without 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 in a bridge configuration with control circuit with automatic control of the output voltage or current
    • 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
    • H02M7/4803—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 with means for reducing DC component from AC output voltage
    • 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
    • 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
    • H02M7/79—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 triode or transistor type requiring continuous application of a control signal
    • H02M7/81—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 triode or transistor type requiring continuous application of a control signal arranged for operation in parallel
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Definitions

  • the present invention relates to a High Voltage Direct Current (HVDC) converter, more in detail the invention relates to a back-to-back HVDC converter.
  • HVDC High Voltage Direct Current
  • HVDC components have been commercially exploited since 1954 when the first HVDC transmission was commissioned.
  • Mercury-arc valves were eventually replaced with high power thyristors and dc transmissions have reached several GW, over +/- 60OkV, and distances around 1000 kilometres.
  • 1997 a new breed of HVDC converter stations and HVDC transmissions were introduced.
  • HVDC converter bridges and lines or cables can be arranged into a number of configurations for effective utilization.
  • two HVDC converters are connected more or less directly to each other on the DC side, with the purpose of e.g. interconnecting two asynchronous AC power networks, or to regulate the flow of power in a AC power network.
  • Back-to-back DC. links are used in Japan for interconnections between power system networks of different frequencies (50 and 60 Hz).
  • the integral part of an HVDC power converter is the valve or valve arm. It may be non- controllable if constructed from one or more power diodes in series or controllable if constructed from one or more thyristors in series.
  • Fig. Ia schematically shows the electric circuit network for a conventional six-pulse converter unit 5.
  • the standard bridge or converter valve group 10 is defined as a double-way connection comprising six valves 20 or valve arms which are connected to a transformer 30 as illustrated in Fig. Ia.
  • Electric power flowing between the HVDC valve group and the AC system is three phase. When electric power flows into the DC valve group from the AC system then it is considered a rectifier. If power flows from the DC valve group into the AC system, it is an inverter.
  • Fig. Ia represents the electric circuit network depiction for the six pulse valve group configuration.
  • Fig. Ib is the graphical symbol of a 6 pulse converter unit. The six pulse valve group was usual when the valves were of mercury arc type.
  • Fig. 2a demonstrates a twelve pulse converter with two three phase converter transformers 31, 32 with one DC side winding as an ungrounded star connection 31 and the other a delta configuration 32. Consequently the AC voltages applied to each six pulse valve group 10 which make up the twelve pulse valve group 40 have a phase difference of 30 degrees which is utilized to cancel the AC side 5 th and 7 th harmonic currents and DC side 6 th harmonic voltage, thus resulting in a significant saving in harmonic filters.
  • Fig. 2 also shows the outline 50 around each of the three groups of four valves in a single vertical stack.
  • Fig. 2b is the graphical symbol of a 12 pulse converter unit.
  • Fig. 3 is a scheme over a conventional back-to-back 12 pulse AC- AC converter, comprising an AC inlet 50a, an AC outlet 50b, two 12-pulse ITVDC converter units 40 arranged in a back-to-back configuration.
  • the converter units are controlled by a control unit (not shown).
  • Each converter unit comprises two six-pulse valve groups in series according to fig. 2a.
  • the inlet valve groups being connected to the AC inlet via separate inlet transformers 30a, and the outlet valve groups being connected to the AC outlet via separate outlet transformers 30b.
  • the object of the invention is to provide a new back-to-back AC-AC converter which overcomes the drawbacks of the prior art. This is achieved by the AC-AC converter as defined in the independent claims.
  • One advantage with the AC- AC converter is that the transformers are not subjected to any DC voltage, which reduces the insulation required in the transformers, which in turn makes the transformers less expensive.
  • the transformers that can be used in the converter are of more standard type, whereby the transformer cost can be further reduced.
  • the DC voltage in the back-to-back connections will be half the voltage in a conventional 12- pulse converter.
  • the AC- AC converter can deliver half power in case of failure of one of the parallel converters.
  • Fig. Ia schematically shows the electric circuit network for a conventional six-pulse valve group converter unit.
  • Fig. Ib shows the graphical symbol of a 6-pulse converter unit according to fig. Ia.
  • Fig. 2a schematically shows the electric circuit network for a conventional 12-pulse valve group converter unit.
  • Fig. 2b shows the graphical symbol of a 12-pulse converter unit according to fig. 2a.
  • Fig. 3 is a scheme over a conventional back-to-back 12 pulse AC- AC converter.
  • Fig. 4 is a scheme over a back-to-back AC- AC converter according to the present invention.
  • Fig. 5 is a scheme over an alternative back-to-back AC- AC converter.
  • Fig. 6 is a scheme over an alternative back-to-back AC- AC converter.
  • Fig. 7 is a scheme over an alternative back-to-back AC- AC converter.
  • Fig. 4 shows an AC-AC converter 60 according to the present invention, comprising an AC inlet 50a, an AC outlet 50b, two six-pulse back-to-back ITVDC converter units 70 and a control unit 80. Due to the symmetric design of the AC- AC converter 60 its operation may be reversed, thereby switching sides of the AC-inlet and AC-outlet. As discussed above, the AC- inlet 50a and AC-outlet 50b are connected to two different AC networks, potentially of different voltage and/or frequency and controls the flow of power there between.
  • Each one of the converter units comprises an inlet and an outlet six-pulse valve group, 10a and 10b respectively, in back-to-back configuration.
  • the inlet valve group 10a is connected to the AC inlet 50a via an inlet transformer 30a
  • the outlet valve group is connected to the AC outlet 50b via an outlet transformer 30b.
  • the inlet and outlet transformers 30a and 30b are selected to provide transformation between the voltage in respective AC power network and the working DC-voltage of the valve groups.
  • the transformers may be of single phase, three phase, dual or more winding type with or without tap-changers. It is also possible to use an autotransformer as Y/Y transformer. According to one embodiment, the transformers are not provided with tap-changers, whereby all voltage regulation is performed by the valve groups. Such an arrangement may be economically feasible due to the fact that the cost reduction on the transformers is not fully met by the increased cost for the valves.
  • one transformer on each side is of star-star connection type (Y/Y), and the other of delta-star connection type (AfY), like in a conventional 12-pulse converter.
  • the ⁇ /Y provides a phase-shift of 30 degrees compared to the Y/Y transformer, which is used to cancel out unwanted harmonic currents in the AC-network.
  • transformers providing other phase-shifts such as an extended delta transformer, may be used to provide phase shifts of 30° between two transformers in a pair of six-pulse back-to-back ITVDC converter units 70, but phase-shifted with respect to the transformers in another pair of six-pulse back-to-back ITVDC converter units 70.
  • the transformers in the second AC-AC converter 60b are phase shifted by 15° with respect to the transformers in first AC- AC converter 60, whereby also harmonic currents of higher order (5 th and 7 th ) are cancelled out.
  • the extinction phase shift should be exactly 30°, however in reality the extinction phase shift may vary up to several degrees, due to non ideal components, defects, load situation etc. Therefore, according to the present invention, the term extinction phase shift, between two transformers in a pair of six-pulse back-to-back ITVDC converter units 70, shall be considered to embrace such situations.
  • the transformers on respective side of each AC-AC converter 60 should be of equal impedance in order to achieve complete cancellation of harmonic currents on that side. Moreover, in the embodiment shown in fig. 5 all transformers on the respective sides should be of equal impedance.
  • the control unit 80 may be an integrated part of the AC- AC converter 60, or alternatively it may e.g. be a part of a more general control system for the power network(s). In order to achieve cancellation of harmonic currents, the control unit 80 is arranged to perform coordinated control of the inlet six-pulse valve groups 10b in accordance with the phase shift(s) of the inlet transformer(s) 30a, and the outlet six-pulse valve groups 10b in accordance with the phase shift(s) of the outlet transformer(s) 30b. In the embodiment disclosed in fig. 4 the control unit 80 may follow the same control scheme as for a conventional 12-pulse converter, such as equidistant 12-pulse control.
  • the control scheme corresponds to a 24 pulse converter.
  • the control of two parallel AC- AC converters 60 that are not phase shifted with respect to each other do not need to be coordinated, even though it would be convenient to control them synchronously.
  • one inlet and one outlet transformer is of ⁇ /Y connection type, each with their ⁇ winding connected to the corresponding six-pulse valve group, and the other two transformers of Y/Y connection type.
  • the DC voltage in the back-to-back connection is reduced by a factor two compared to the conventional 12-pulse back-to-back converters. This greatly simplifies the design of the converter with respect to insulation properties etc.
  • the two parallel converter units 70 can be viewed as separate units, each of them can be run separately of the other one in case of failure of one converter unit 70, provided that a harmonics filter is arranged on the AC network side to compensate for the 5 th and 7 th harmonic currents produced by the sole 6 pulse converter.
  • each transformer in the present converter will be exposed to an equal amount of harmonic currents, but no DC voltage. Therefore the insulation requirements on the transformer are reduced and the transformer costs are reduced. Further, this makes it possible to select transformers from a wider range of standard transformers.
  • the DC voltage in each DC circuit of the present AC- AC converter will comprise a 6-pulse harmonic ripple component.
  • said ripple component is suppressed by a DC-reactor unit (not shown), such as an air coil or iron core coil.
  • the ripple components in the two converter units 70 of an AC-AC converter 60 are suppressed by a combined DC-reactor unit with two separate coil-windings about a common iron core. In said combined DC-reactor unit, the ripple components from the two DC-circuits will partly cancel out each other via the magnetic coupling through the iron core, and the reminder of the ripple component will be suppressed by the coils.
  • Fig. 6 shows an alternative embodiment of the present invention, wherein, the two converter units 70 of the AC- AC converter 60 share one conductor in the DC circuit. In this embodiment, it is sufficient with one reactor to suppress the ripple component in the combined DC-circuit.
  • Fig. 7 shows another embodiment, wherein several pairs of valve groups are connected in parallel to a common DC circuit.
  • the AC- AC converter illustrated in fig. 7 can be connected to one or more AC networks on each side, as illustrated in that the inlet side is connected to two separate AC networks and on the outlet side it is connected to one AC network.
  • This embodiment provides an AC- AC converter with a very high degree of flexibility as it can be used for interconnection and transmission of electric power between two or more separate AC-networks, with high redundancy.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Rectifiers (AREA)

Abstract

AC-AC converter (60) comprising an AC inlet (50a), an AC outlet (50b), one or more pairs of parallel six-pulse back-to-back HVDC converter units (70) and a control unit (80), each converter unit (70) comprising an inlet and an outlet six-pulse valve group (10a, 10b) in back- to-back configuration. The inlet valve group (10a) being connected to the AC inlet (50a) via an inlet transformer (30a), and the outlet valve group (10b) being connected to the AC outlet (50b) via an outlet transformer (30b). And wherein, for each pair of six-pulse back-to-back HVDC converter units (70), one inlet, and one outlet transformer provides an extinction phase shift with respect to the other transformer on respective side, the control unit (80) is arranged to perform coordinated control of the inlet six-pulse valve groups (10a) in accordance with the phase shift between the inlet transformers (30a), and coordinated control of the outlet six- pulse valve groups (10b) in accordance with the phase shift between the outlet transformers (30b).

Description

HVDC CONVERTER
The field of the invention
The present invention relates to a High Voltage Direct Current (HVDC) converter, more in detail the invention relates to a back-to-back HVDC converter.
Background of the Invention
The advantages of HVDC components have been commercially exploited since 1954 when the first HVDC transmission was commissioned. Mercury-arc valves were eventually replaced with high power thyristors and dc transmissions have reached several GW, over +/- 60OkV, and distances around 1000 kilometres. In 1997, a new breed of HVDC converter stations and HVDC transmissions were introduced.
HVDC converter bridges and lines or cables can be arranged into a number of configurations for effective utilization. In a Back-to-Back configuration two HVDC converters are connected more or less directly to each other on the DC side, with the purpose of e.g. interconnecting two asynchronous AC power networks, or to regulate the flow of power in a AC power network. Back-to-back DC. links are used in Japan for interconnections between power system networks of different frequencies (50 and 60 Hz).
The integral part of an HVDC power converter is the valve or valve arm. It may be non- controllable if constructed from one or more power diodes in series or controllable if constructed from one or more thyristors in series. Fig. Ia schematically shows the electric circuit network for a conventional six-pulse converter unit 5. The standard bridge or converter valve group 10 is defined as a double-way connection comprising six valves 20 or valve arms which are connected to a transformer 30 as illustrated in Fig. Ia. Electric power flowing between the HVDC valve group and the AC system is three phase. When electric power flows into the DC valve group from the AC system then it is considered a rectifier. If power flows from the DC valve group into the AC system, it is an inverter. Each valve consists of many series connected thyristors in thyristor modules. Fig. Ia represents the electric circuit network depiction for the six pulse valve group configuration. Fig. Ib is the graphical symbol of a 6 pulse converter unit. The six pulse valve group was usual when the valves were of mercury arc type.
Today nearly all ITVDC power converters with thyristor valves are assembled in a converter bridge of twelve pulse configuration. Fig. 2a demonstrates a twelve pulse converter with two three phase converter transformers 31, 32 with one DC side winding as an ungrounded star connection 31 and the other a delta configuration 32. Consequently the AC voltages applied to each six pulse valve group 10 which make up the twelve pulse valve group 40 have a phase difference of 30 degrees which is utilized to cancel the AC side 5th and 7th harmonic currents and DC side 6th harmonic voltage, thus resulting in a significant saving in harmonic filters. Fig. 2 also shows the outline 50 around each of the three groups of four valves in a single vertical stack. These are known as "quadrivalves" and are assembled as one valve structure by stacking four valves in series. Since the voltage rating of thyristors is several kV, a 500 kV quadrivalve may have hundreds of individual thyristors connected in series groups of valve or thyristor modules. Fig. 2b is the graphical symbol of a 12 pulse converter unit.
Fig. 3 is a scheme over a conventional back-to-back 12 pulse AC- AC converter, comprising an AC inlet 50a, an AC outlet 50b, two 12-pulse ITVDC converter units 40 arranged in a back-to-back configuration. The converter units are controlled by a control unit (not shown). Each converter unit comprises two six-pulse valve groups in series according to fig. 2a. The inlet valve groups being connected to the AC inlet via separate inlet transformers 30a, and the outlet valve groups being connected to the AC outlet via separate outlet transformers 30b.
Summary of the Invention
The object of the invention is to provide a new back-to-back AC-AC converter which overcomes the drawbacks of the prior art. This is achieved by the AC-AC converter as defined in the independent claims.
One advantage with the AC- AC converter is that the transformers are not subjected to any DC voltage, which reduces the insulation required in the transformers, which in turn makes the transformers less expensive. In addition, the transformers that can be used in the converter are of more standard type, whereby the transformer cost can be further reduced.
Earthing of the Y/Y-connected transformers on each side results in stable earth without circulation between the two converter sides.
The DC voltage in the back-to-back connections will be half the voltage in a conventional 12- pulse converter.
By the addition of 5th and 7th harmonic filters and dual wound transformers, the AC- AC converter can deliver half power in case of failure of one of the parallel converters.
Embodiments of the invention are defined in the dependent claims.
Brief Description of the Drawings
The invention will be described in detail below with reference to the drawings, in which:
Fig. Ia schematically shows the electric circuit network for a conventional six-pulse valve group converter unit.
Fig. Ib shows the graphical symbol of a 6-pulse converter unit according to fig. Ia.
Fig. 2a schematically shows the electric circuit network for a conventional 12-pulse valve group converter unit.
Fig. 2b shows the graphical symbol of a 12-pulse converter unit according to fig. 2a.
Fig. 3 is a scheme over a conventional back-to-back 12 pulse AC- AC converter.
Fig. 4 is a scheme over a back-to-back AC- AC converter according to the present invention.
Fig. 5 is a scheme over an alternative back-to-back AC- AC converter. Fig. 6 is a scheme over an alternative back-to-back AC- AC converter.
Fig. 7 is a scheme over an alternative back-to-back AC- AC converter.
Detailed Description of Preferred Embodiments
Fig. 4 shows an AC-AC converter 60 according to the present invention, comprising an AC inlet 50a, an AC outlet 50b, two six-pulse back-to-back ITVDC converter units 70 and a control unit 80. Due to the symmetric design of the AC- AC converter 60 its operation may be reversed, thereby switching sides of the AC-inlet and AC-outlet. As discussed above, the AC- inlet 50a and AC-outlet 50b are connected to two different AC networks, potentially of different voltage and/or frequency and controls the flow of power there between.
Each one of the converter units comprises an inlet and an outlet six-pulse valve group, 10a and 10b respectively, in back-to-back configuration. The inlet valve group 10a is connected to the AC inlet 50a via an inlet transformer 30a, and the outlet valve group is connected to the AC outlet 50b via an outlet transformer 30b.
The inlet and outlet transformers 30a and 30b are selected to provide transformation between the voltage in respective AC power network and the working DC-voltage of the valve groups. The transformers may be of single phase, three phase, dual or more winding type with or without tap-changers. It is also possible to use an autotransformer as Y/Y transformer. According to one embodiment, the transformers are not provided with tap-changers, whereby all voltage regulation is performed by the valve groups. Such an arrangement may be economically feasible due to the fact that the cost reduction on the transformers is not fully met by the increased cost for the valves.
In the disclosed embodiment, one transformer on each side is of star-star connection type (Y/Y), and the other of delta-star connection type (AfY), like in a conventional 12-pulse converter. The Δ/Y provides a phase-shift of 30 degrees compared to the Y/Y transformer, which is used to cancel out unwanted harmonic currents in the AC-network.In alternative embodiments, transformers providing other phase-shifts, such as an extended delta transformer, may be used to provide phase shifts of 30° between two transformers in a pair of six-pulse back-to-back ITVDC converter units 70, but phase-shifted with respect to the transformers in another pair of six-pulse back-to-back ITVDC converter units 70. Further, it is possible to connect several AC-AC converters 60 according to the present invention in parallel to increase the power handling capacity and to achieve better redundancy. According to one embodiment disclosed in fig. 5, with two AC- AC converters 60 connected in parallel, the transformers in the second AC-AC converter 60b are phase shifted by 15° with respect to the transformers in first AC- AC converter 60, whereby also harmonic currents of higher order (5th and 7th ) are cancelled out. Like in all three-phase systems, in order to achieve full extinction, the extinction phase shift should be exactly 30°, however in reality the extinction phase shift may vary up to several degrees, due to non ideal components, defects, load situation etc. Therefore, according to the present invention, the term extinction phase shift, between two transformers in a pair of six-pulse back-to-back ITVDC converter units 70, shall be considered to embrace such situations.
In all embodiments, the transformers on respective side of each AC-AC converter 60 should be of equal impedance in order to achieve complete cancellation of harmonic currents on that side. Moreover, in the embodiment shown in fig. 5 all transformers on the respective sides should be of equal impedance.
The control unit 80 may be an integrated part of the AC- AC converter 60, or alternatively it may e.g. be a part of a more general control system for the power network(s). In order to achieve cancellation of harmonic currents, the control unit 80 is arranged to perform coordinated control of the inlet six-pulse valve groups 10b in accordance with the phase shift(s) of the inlet transformer(s) 30a, and the outlet six-pulse valve groups 10b in accordance with the phase shift(s) of the outlet transformer(s) 30b. In the embodiment disclosed in fig. 4 the control unit 80 may follow the same control scheme as for a conventional 12-pulse converter, such as equidistant 12-pulse control. For the inlet and outlet sides respectively, this implies that the two valve groups 10a or 10b are phase shifted 30 degrees with respect to each other. Consequently, in the embodiment of fig. 5, the control scheme corresponds to a 24 pulse converter. However, the control of two parallel AC- AC converters 60 that are not phase shifted with respect to each other, do not need to be coordinated, even though it would be convenient to control them synchronously. In the embodiment of fig. 4 one inlet and one outlet transformer is of Δ/Y connection type, each with their Δ winding connected to the corresponding six-pulse valve group, and the other two transformers of Y/Y connection type. By Y-O earthing of the Y- winding of the Y/Y transformers that is connected to a six-pulse valve group a stable earthing is achieved without circulation from the inlet to the outlet side.
By the approach of parallel connection of two or more six-pulse back-to-back HVDC converter units 70, the DC voltage in the back-to-back connection is reduced by a factor two compared to the conventional 12-pulse back-to-back converters. This greatly simplifies the design of the converter with respect to insulation properties etc. Moreover, as the two parallel converter units 70 can be viewed as separate units, each of them can be run separately of the other one in case of failure of one converter unit 70, provided that a harmonics filter is arranged on the AC network side to compensate for the 5th and 7th harmonic currents produced by the sole 6 pulse converter.
Compared to a conventional 12-pulse converter, each transformer in the present converter will be exposed to an equal amount of harmonic currents, but no DC voltage. Therefore the insulation requirements on the transformer are reduced and the transformer costs are reduced. Further, this makes it possible to select transformers from a wider range of standard transformers.
The DC voltage in each DC circuit of the present AC- AC converter will comprise a 6-pulse harmonic ripple component. According to one embodiment, said ripple component is suppressed by a DC-reactor unit (not shown), such as an air coil or iron core coil. According to one embodiment, the ripple components in the two converter units 70 of an AC-AC converter 60 are suppressed by a combined DC-reactor unit with two separate coil-windings about a common iron core. In said combined DC-reactor unit, the ripple components from the two DC-circuits will partly cancel out each other via the magnetic coupling through the iron core, and the reminder of the ripple component will be suppressed by the coils.
Fig. 6 shows an alternative embodiment of the present invention, wherein, the two converter units 70 of the AC- AC converter 60 share one conductor in the DC circuit. In this embodiment, it is sufficient with one reactor to suppress the ripple component in the combined DC-circuit. Fig. 7 shows another embodiment, wherein several pairs of valve groups are connected in parallel to a common DC circuit. The AC- AC converter illustrated in fig. 7 can be connected to one or more AC networks on each side, as illustrated in that the inlet side is connected to two separate AC networks and on the outlet side it is connected to one AC network. This embodiment, provides an AC- AC converter with a very high degree of flexibility as it can be used for interconnection and transmission of electric power between two or more separate AC-networks, with high redundancy.

Claims

CLAIMS:
1. AC- AC converter (60) comprising an AC inlet (50a), an AC outlet (50b), one or more pairs of parallel six-pulse back-to-back ITVDC converter units (70) and a control unit
(80), each converter unit (70) comprising an inlet and an outlet six-pulse valve group (10a, 10b) in back-to-back configuration, the inlet valve group (10a) being connected to the AC inlet (50a) via an inlet transformer (30a), and the outlet valve group (10b) being connected to the AC outlet (50b) via an outlet transformer (30b),
wherein, for each pair of six-pulse back-to-back HVDC converter units (70), one inlet, and one outlet transformer provides an extinction phase shift with respect to the other transformer on respective side, the control unit (80) is arranged to perform coordinated control of the inlet six-pulse valve groups (10a) in accordance with the phase shift between the inlet transformers (30a), and coordinated control of the outlet six-pulse valve groups (10b) in accordance with the phase shift between the outlet transformers (30b).
2. AC- AC converter according to claim 1 wherein, for at least one pair of six-pulse back- to-back ITVDC converter units (70), one inlet transformer is of Δ/Y connection type, one outlet transformer is of Y/Δconnection type, each with their Δ winding connected to the six-pulse valve groups, and the other transformers of Y/Y connection type.
3. AC- AC converter according to claim 1 or 2 wherein the extinction phase shift is 30°.
4. AC- AC converter according to claim 1 wherein, for each pair of six-pulse back-to- back HVDC converter units (70), the coordinated control of the inlet and outlet six- pulse valve groups, each follow a twelve-pulse scheme.
5. AC- AC converter according to claim 4 comprising two pairs of parallel six-pulse back-to-back HVDC converter units (70), wherein the transformers in the second pair of HVDC converter units (70) are phase shifted by 15° with respect to the transformers in the first pair of HVDC converter units (70), and the coordinated control of the inlet and outlet six-pulse valve groups, each follow a 24-pulse scheme.
6. AC- AC converter according to claim 2 wherein the Y- winding of the Y/Y transformers that is connected to a six-pulse valve group is earthed.
PCT/SE2006/050339 2006-09-18 2006-09-18 Hvdc converter WO2008036009A1 (en)

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CN102157930A (en) * 2011-02-25 2011-08-17 浙江大学 Method for calculating DC side harmonic current of common-tower double-circuit DC transmission line
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US8994232B2 (en) 2009-07-17 2015-03-31 Siemens Aktiengesellschaft Star-point reactor
WO2011006796A3 (en) * 2009-07-17 2011-04-14 Siemens Aktiengesellschaft Device for inverting an electric parameter, having a star point reactor
RU2534027C2 (en) * 2009-07-17 2014-11-27 Сименс Акциенгезелльшафт Device for electric parameter conversion with zero-point reactor
DE102010034905A1 (en) * 2010-08-17 2012-02-23 Siemens Aktiengesellschaft High-voltage electric energy supply devices e.g. sea-side converter stations, for use in high-voltage direct current power transmission system, have two three-phase transformers connected in parallel with each other
CN102486482A (en) * 2010-12-01 2012-06-06 中国电力科学研究院 An RC Equivalent Load Device for Type Test of Multiple Valves of DC Converter Valve
CN102157930A (en) * 2011-02-25 2011-08-17 浙江大学 Method for calculating DC side harmonic current of common-tower double-circuit DC transmission line
US9948199B2 (en) 2012-02-24 2018-04-17 Abb Schweiz Ag HVDC converter system with transformer functions or arrangements integrated into a single transformer unit
US20140362619A1 (en) * 2012-02-24 2014-12-11 Mats Berglund HVDC Converter
CN103580053A (en) * 2012-07-23 2014-02-12 Abb技术有限公司 Series-connection multi-terminal direct current transmission system and operation method used during transmission capacity loss of system
WO2014044293A1 (en) * 2012-09-18 2014-03-27 Abb Technology Ltd Method of controlling a power transmission system and corresponding control system
CN107925358A (en) * 2015-08-12 2018-04-17 通用电气公司 Method and system for the current source high voltage direct current transmission system based on flue
CN106059396A (en) * 2016-07-04 2016-10-26 湖北工业大学 Soft starter of high-voltage motor
WO2019068311A1 (en) * 2017-10-03 2019-04-11 Abb Schweiz Ag Coordinating current stabilizing control with tap changer control
GB2581079A (en) * 2017-10-03 2020-08-05 Abb Power Grids Switzerland Ag Coordinating current stabilizing control with tap changer control
GB2581079B (en) * 2017-10-03 2022-05-04 Abb Power Grids Switzerland Ag Coordinating current stabilizing control with tap changer control
CN112202323A (en) * 2020-08-25 2021-01-08 中国南方电网有限责任公司超高压输电公司广州局 Redundancy improving method for flexible direct current valve control protection system
WO2023161897A1 (en) * 2022-02-27 2023-08-31 B. G. Negev Technologies And Applications Ltd., At Ben-Gurion University Resonant power conversion systems and methods

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