WO2003069757A1 - Equipment and method for exchanging power, in shunt connection, with an electric power network, and use of such equipment - Google Patents
Equipment and method for exchanging power, in shunt connection, with an electric power network, and use of such equipment Download PDFInfo
- Publication number
- WO2003069757A1 WO2003069757A1 PCT/SE2003/000015 SE0300015W WO03069757A1 WO 2003069757 A1 WO2003069757 A1 WO 2003069757A1 SE 0300015 W SE0300015 W SE 0300015W WO 03069757 A1 WO03069757 A1 WO 03069757A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- voltage
- power network
- converter
- power
- phase position
- Prior art date
Links
Classifications
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
- H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
- H02J3/1835—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control
- H02J3/1842—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters
-
- 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
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/20—Active power filtering [APF]
Definitions
- the present invention relates to equipment for exchanging power, in shunt connection, with an electric power network, a method for this purpose, and use of such equipment for this purpose.
- the equipment comprises a reactive impedance element and a voltage source converter for mutual connection in series.
- capacitors may be respectively connected to and disconnected from the power network by means of electric switching devices, for example thyristor switches, whereby the reactive power supplied to the power network may be controlled in steps (Thyristor Switched Capacitor - TSC) .
- Electric switching devices for example thyristor switches
- the reactive power supplied to the power network may be controlled in steps
- Thyristor Switched Capacitor - TSC Fixed capacitors also often occur in combination with TCRs and combinations of TSCs and TCRs, which makes possible a continuous control of the reactive power exchange with the power network.
- Capacitors connected in shunt connection are used primarily in industrial networks to compensate for the reactive power consumption in, for example, large asynchronous motors.
- Other typical applications, where often a combination of fixed capacitors and TCRs is advantageous, is in connection with loads with a greatly varying reactive power consumption, such as, for example, in arc furnaces. In certain cases, it may be suitable to connect the compensation equipment to the industrial network via a transformer.
- VSCs Voltage Source Converters
- 1GBT series-connected transistors
- PWM pulse width modulation
- Converters of this kind thus constitute a device that enables a rapid control of a generated ac voltage, both with respect to amplitude and phase position.
- the voltage generated by the converter is brought to comprise a component of the fundamental frequency, in the following referred to as the fundamental voltage, but in addition thereto, because of the pulse-width modulation, also components of other frequencies.
- phase inductors which are normally dimensioned such that, at the rated current of the converter, they take up about 10-30% of the network's nominal voltage of fundamental frequency.
- the converter is brought to generate a voltage, the fundamental component of which, both with regard to frequency and phase position, essentially coincides with the voltage of the network (to cover active losses in converters and phase inductors, the phase position must deviate somewhat from the phase position for the vol- tage of the network; this is disregarded in this reasoning on principles) , and by varying the amplitude of the generated voltage, the converter may be brought to consume reactive power, if its voltage has a lower amplitude than that of the network, and to generate reactive power, respective- ly, if its voltage has a higher amplitude than that of the network. Since, in industrial networks, the task is normally to generate reactive power, the voltage source converter is normally supplemented by a capacitor, which may possibly be connectible in steps.
- the converter must be dimensioned for a voltage equal to the nominal voltage of the network plus a control range for generation of reactive power.
- Inductors coupled in shunt are primarily used in high- voltage transmission networks with overhead lines but also in transmission networks with cables, in the latter case also at lower voltages .
- these objects are achieved by dimensioning the voltage source converter for a control range that limits the amplitude of the fundamental voltage to a value that is lower than the nominal voltage of the power network and comprises generation of a reactive component of the fundamental voltage with a phase position that either coincides with the phase position for the voltage of the power network or that deviates by 180° electrically from the phase position for the voltage of the power network.
- control range of the converter comprises, in addition thereto, generation of an active component of the fundamental voltage with a phase position that deviates from the phase position for the voltage of the power network by +90° electrically or by -90° electrically and with an amplitude that brings about an exchange of active power with the power network.
- the equipment comprises a control system for controlling, in dependence on electric variables sensed in the power network, the fundamental voltage generated by the converter with regard to amplitude and phase position within the control range, whereby the control system comprises a signal-processing member with a phase-advancing characteristic in a frequency interval surrounding the frequency 8.8 Hz and means for forming a reference value for the current of the converter in dependence on an output signal from said signal- processing member.
- Figure 1 shows a piece of equipment with a voltage source converter and an inductive impedance element for compensation of reactive power according to the prior art
- Figure 2 shows an embodiment of the invention with a vol- tage source converter and a capacitive impedance element
- Figure 3 shows an embodiment of the invention with a voltage source converter and an inductive impedance element
- Figure 4 shows a relationship between reactive power consumption and voltage generated by the converter in an embodiment of the invention according to Figure 3,
- Figure 5 shows a further embodiment of the invention with a voltage source converter and an inductive impedance element
- Figure 6 shows in a vector diagram a relationship between the voltage of the power network and voltage generated by the converter in a further development of the invention
- FIG 7A shows in more detail an embodiment of the invention according to Figure 3 .
- Figure 7B shows details of an embodiment of a control system for an embodiment of the invention according to Figure 7A.
- FIG. 1 shows the main circuits in three-phase equipment for compensation of reactive power carried out according to the prior art.
- the equipment comprises a capacitor bank C and a voltage source converter VSC , connected to a power network N with a voltage U and with a fundamental frequency ( / ) .
- the nominal voltage of the power network is referred to in the following as Un.
- the power network N may be an industrial network with a voltage 36 kV.
- the capacitor bank is shown in this context simplified as a capacitor C connected to the power network via a filter inductor Lf .
- the capacitor C is usually distributed among a plurality of capacitors, each one being connected to the power network via a respective filter inductor for tuning and filtering of selected harmonics in the power network.
- the converter is connected to the power network via phase inductors Lph. Through the capacitor C, a current IC ' flows, and through the converter a current IS' flows. The voltage on the phase terminal of the converter is designated US' .
- the equipment is to be able to generate a reactive power that is continuously controllable in the range of 0.5-1.0 per unit.
- a preferred dimensioning of the components included is then for the capacitor C' to generate a reactive power of 0.75 per unit at a voltage of the power network of 1.0 per unit. Consequently, the reactive power flow from the converter VSC and the phase inductor Lph shall then be continuously controllable within the interval +/- 0.25 per unit, which is equivalent to these components, at a voltage of the power network equal to 1.0 per unit, being traversed by a current IS' equal to +/- 0.25 per unit.
- phase inductor Lph is dimensioned such that, at the current 0.25 per unit, the voltage drop across this amounts to 0.20 per unit .
- the converter when the converter is to generate a reactive power of 0.25 per unit, its voltage US' must be equal to the line voltage plus the voltage drop across the phase inductor, that is, equal to 1.20 per unit.
- the voltage generated by the converter has the same phase position as the line voltage U.
- a voltage source converter VSC is now connected to the power network N via a capacitor C and a filter inductor Lf .
- a current IC flows, and the voltage of the phase terminal of the conver- ter is designated US.
- the voltage drop of fundamental frequency across the filter inductor Lf is omitted.
- the capacitor C is dimensioned preferably such that, at the current IC equal to 1.0 per unit, it generates a reactive power of 1.33 per unit.
- the impedance of the capacitor expressed in per unit is thus 1.33 and at the current IC equal to 1.0 per unit, the voltage UC across the capacitor is thus equal to 1.33 per unit.
- the current IC will thus be equal to 1.0 per unit, and by controlling the fundamental voltage of the converter to +0.33 per unit, the current IC becomes equal to 0.5 per unit.
- the converter is thus dimensioned for a control range in voltage amplitude of +/- 0.33 per unit .
- the fundamental voltage US generated by the converter has a phase position that either coincides with the phase position of the voltage U of the power network or that deviates by 180° electrically from the phase position for the voltage of the power network (in this reasoning on principles, the fact that the phase position must deviate somewhat from the above-mentioned phase positions to cover active losses in the equipment is disregarded) .
- the capacitor C is dimensioned for a voltage corresponding to the line voltage plus the voltage that the converter generates with an opposite phase position in relation to the line voltage.
- Figure 3 shows an embodiment of the invention for consumption of reactive power.
- the capacitor bank C and the filter inductor Lf are replaced by an inductor LC, dimensioned for a voltage corresponding to the line voltage plus the voltage that the converter generates with an opposite phase position in relation to the line voltage.
- a current IL flows.
- the mode of operation of this embodiment of the invention is completely analogous to what is described with reference to Figure 2.
- Figure 4 shows reactive power consumption and on the vertical axis the magnitude of the fundamental voltage US generated by the converter.
- the thick line on the horizontal axis illustrates how the reactive power consumption varies between a minimum value Qmin and a maximum value Qmax in dependence on the amplitude and phase position of the voltage generated by the converter.
- the power network is in the form of a high- voltage transmission line, typically of a voltage level in the range of 132 - 500 kV, it is desirable to achieve a continuous and rapid control of a shunt-connected inductor.
- This may be advantageously achieved by utilizing a voltage source converter in a manner similar to that described above with reference to Figures 2 and 3, whereby, in this embodiment of the invention, a transformer is connected between the inductor and the converter in the manner illustrated in Figure 5.
- the equipment described in Figure 5 shows, in addition thereto, a transformer T, connected between the inductor LC and the converter VSC.
- the transformer may preferably be designed with a transformation ratio 132/20 to 36 kV.
- the converter must then be connected via a transformer with the transformation ratio 500/20 to 36 kV.
- Such a transformer is more expensive than a transformer dimensioned for, for example, a primary voltage of 132 kV. With equipment and a method according to the invention, a significant saving in transformer cost is thus obtained.
- vol- tage generated by the converter be limited within a control range that does not bring about a voltage overload of the reactive impedance element.
- control range of the converter with respect to amplitude is to be limited to 0.33 per unit but with a phase position of the generated fundamental voltage that comprises a phase position that deviates by 180 2 electrically from the phase position for the voltage of the power network.
- a phase position that deviates by 180 s from the phase position for the voltage of the power network and an amplitude larger than 0.33 per unit the voltage across the capacitor C would otherwise exceed 1.33 per unit.
- the corresponding reasoning applies also to the embodiments of the invention described with reference to Figures 3 and 5.
- the converter must also generate a certain active power to cover resistive losses in the equipment. This is achieved in a manner known per se by varying the phase position of the generated voltage relative to the line voltage to phase positions that differ from 0° electrically and 180° electrically. The energy associated therewith is covered by discharge of the dc voltage capacitor, which is therefore arranged with a voltage control for maintaining its dc voltage constant.
- the voltage generated by the converter will therefore comprise a component with a phase position coinciding with the phase position of the power network or differing therefrom by 180° electrically, and a component with a phase position differing from the phase position for the voltage of the power network by 90° electrically.
- control range for the converter shall thus comprise generation of a voltage that has a component with a phase position differing by 180° electrically from the phase position for the voltage of the power network.
- the fundamental voltage generated by the converter is controlled, in a manner known per se, also to phase positions that lie in either of the intervals 0°-180° electrically and 180°-360° electrically relative to the phase position of the line voltage, and then with an amplitude and a phase angle that, in addition to covering losses in the equipment, permit an exchange of active power with the power network .
- FIG. 6 A vector representation of the voltage ratio in this further development of the invention is illustrated in Figure 6 (for equipment according to Figure 5 with the voltage US generated by the converter transformed to the high-voltage side of the transformer) .
- the intersection point between the vertical and horizontal axes constitutes the origin of coordina- tes for a vector representation of the line voltage U and the horizontal axis represents the phase position of the line voltage.
- the line voltage and the voltage generated by the converter are shown in the figure by a vector U and a vector US, respectively.
- the voltage US has the phase posi- tion ⁇ relative to the line voltage.
- the voltage UL across the inductor LC ( Figures 4 and 5) is represented in the figure by the vectorial difference between vector U and vector US .
- the circle in the figure indicates the control range A within which the voltage generated by the converter may be controlled with respect to amplitude and phase position.
- the amplitude of the reactive component of the current IL ( Figures 4 and 5) , and hence the magnitude of the reactive power that is exchanged with the power network, is dependent on the reactive component USr of the fundamental voltage US along the horizontal axis.
- the active power exchange with the power network is dependent on the active component USa of the voltage US along the vertical axis .
- both reactive and active power may thus be exchanged with the network.
- the extent of the exchange of active power with the power network is determined by the energy storage capacity in the dc circuit of the converter.
- This dc circuit generally comprises a capacitor CD ( Figures 1-3 and 5) J and based on a given energy that is to be exchanged with the power network, this capacitor may be dimensioned so that the energy exchange may take place with a substantially retained voltage. Briefly, however, the voltage across the capacitor may be allowed to vary typically in an interval of from 0.7 to 1.25 per unit. With regard to stored energy, the capacitor CD may be typically dimensioned such that, at nominal active current, it is discharged from nominal voltage to zero voltage in 5 to 20 ms .
- So-called flicker consists of voltage variations in the power network within a frequency range that may be observed by and is disturbing to the human eye in case of electric lighting supplied by the ac network.
- the dc voltage capacitor is arranged with a voltage control for maintaining its dc voltage constant, and for achieving the above-mentioned transient ex- change of active power with the power network for reduction of flicker, it must be ensured that the voltage control of the dc voltage capacitor does not essentially counteract the interventions from the control system for reduction of flicker.
- a specified and standardized disturbance curve weighed in dependence on the frequency, exhibits a maximum at a frequency of about 8.8 Hz, and a system for reducing flicker should therefore advantageously be active in an interval around this frequency.
- FIG. 7A shows in more detail a piece of equipment of the kind described with reference to Figure 3.
- a circuit breaker CBl and a surge arrester ZD connected in parallel therewith are connected between ground and the connection point between the inductor LC and the converter VSC.
- the inductor LC is connected to the power network via a circuit breaker CB2, whereupon the circuit breaker CBl is opened.
- the dc voltage capacitor CD is then not charged to substantially nominal voltage, for example by means of an external voltage source, the capaci- tor will be charged via the diodes which, in a known manner, are arranged in an antiparallel connection with the controllable semiconductor elements of the converter.
- the voltage US will initially increase due to the current IL through the inductor being rectified via the mentioned diodes .
- the surge arrester ZD limits the voltage US to permissible values until the circuit breaker CBl, via protective equipment (not shown) , is closed.
- a surge arrester may be connected across the dc voltage capacitor CD (not shown in the figure) to limit the voltage thereof.
- a further converter (not shown in the figure) may be connected to the capacitor to transmit therefrom active power to an electric power network.
- the current IL is measured by means of a current-measuring device Mil.
- a superordinate control member 71 forms, in some manner known per se, in dependence on electric variables such as current and voltage measured in the power network and on reference values not shown, a reference value ILR for the current IL. In Figure 7A, this is exemplified such that a value of the voltage U, sensed by means of a voltage- measuring device MU, is applied to the control member 71.
- the reference value ILR for the current IL and the sensed value of the current thereof are supplied to a difference- forming member 72, and the difference is supplied to current control system 73 designed in some manner known per se.
- the current control system forms, in a manner also known per se, reference values USR for the three phase voltages of the converter.
- the reference values USR are supplied to a modulating unit 74, which, in accordance with a chosen pattern for pulse-width modulation, forms firing pulses Fp to the controllable semiconductor elements of the converter.
- FIG. 7B parts of a control system, known per se, for reducing flicker are illustrated.
- the voltage U and the current I are sensed at a point in the power network N by means of the voltage-measuring device MU and a current- measuring device MI2.
- the sensed values of current and voltage are supplied to a calculating member 75 which, in some manner known per se, forms values p(t) of active power flow and q(t) of reactive power flow at the measuring point.
- a signal-processing member 76 is provided with a transfer function that renders the control system active in the frequency interval that is of interest for reducing flicker.
- the amplitude amplification for the member 76 as a function of the frequency is indicated in the block 76 in the figure.
- the signal-processing member 76 is supplied with the value p(t) and forwards to the superordinate member 71 essentially those components of the active power flow that lie within the mentioned frequency interval.
- the superordinate control member 71 is also supplied with the value q(t) and forms from the values supplied thereto, in some manner known per se, a reference value ILR for the current I .
- the reference value thus formed will thus comprise one active and one reactive component of the current IL and will result in both an active and a reactive component of the fundamental voltage .
- this transformer may be designed for a primary voltage that is considerably lower than the nominal voltage of the power network.
- control may be utilized for reducing overvoltages caused, for example, by switching operations in the power network, damping of power oscillations in the transmission line, and for voltage control in case of a varying transmission of power in the transmission line.
- control range of the converter also comprises generation of an active component (USa) of the fundamental voltage with a phase position that devia'tes from the phase position for the voltage of the power network by +90 s electrically or -90 a electrically and with an amplitude that brings about an exchange of active power with the power network
- the equipment may be utilized for damping flicker, by being provided with a superordinate control member in a manner known per se, by a transient exchange of active power with the power network.
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Control Of Electrical Variables (AREA)
- Supply And Distribution Of Alternating Current (AREA)
Abstract
Description
Claims
Priority Applications (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EP03701188A EP1464103A1 (en) | 2002-01-09 | 2003-01-07 | Equipment and method for exchanging power, in shunt connection, with an electric power network, and use of such equipment |
US10/501,076 US7173349B2 (en) | 2002-01-09 | 2003-01-07 | Equipment and method for exchanging power, in shunt connection, with an electric power network, and use of such equipment |
AU2003202179A AU2003202179A1 (en) | 2002-01-09 | 2003-01-07 | Equipment and method for exchanging power, in shunt connection, with an electric power network, and use of such equipment |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
SE0200050A SE519413C2 (en) | 2002-01-09 | 2002-01-09 | Equipment and method for exchanging power with an electric power grid in a shunt coupling and using such equipment |
SE0200050-3 | 2002-01-09 |
Publications (1)
Publication Number | Publication Date |
---|---|
WO2003069757A1 true WO2003069757A1 (en) | 2003-08-21 |
Family
ID=20286618
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/SE2003/000015 WO2003069757A1 (en) | 2002-01-09 | 2003-01-07 | Equipment and method for exchanging power, in shunt connection, with an electric power network, and use of such equipment |
Country Status (6)
Country | Link |
---|---|
US (1) | US7173349B2 (en) |
EP (1) | EP1464103A1 (en) |
CN (1) | CN100370672C (en) |
AU (1) | AU2003202179A1 (en) |
SE (1) | SE519413C2 (en) |
WO (1) | WO2003069757A1 (en) |
Families Citing this family (8)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US7253403B2 (en) * | 2004-04-21 | 2007-08-07 | Indiana University Research And Technology Corporation | Control system for a power supply |
US8674666B2 (en) * | 2005-12-30 | 2014-03-18 | Abb Technology Ltd. | Device for balancing a transmission network |
JP2009177914A (en) * | 2008-01-23 | 2009-08-06 | Mitsubishi Electric Corp | Flicker improvement effect evaluation system |
AU2009344065A1 (en) * | 2009-04-09 | 2011-10-20 | Abb Technology Ag | An arrangement for exchanging power |
EP2526605A2 (en) * | 2010-01-22 | 2012-11-28 | ABB Inc. | Method and apparatus for improving power generation in a thermal power plant |
DK2607692T3 (en) * | 2011-12-22 | 2015-05-04 | Siemens Ag | Method for determining a voltage limiting range |
WO2014147294A1 (en) * | 2013-03-19 | 2014-09-25 | Merus Power Dynamics Oy | Method and apparatus for compensating non-active currents in electrical power networks |
FR3044486B1 (en) * | 2015-12-01 | 2019-01-25 | Schneider Electric Industries Sas | ACTIVE FILTRATION SYSTEM |
Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO1997049157A1 (en) | 1996-06-17 | 1997-12-24 | Asea Brown Boveri Ab | Method and device for compensation of reactive power |
DE19738125A1 (en) | 1997-09-01 | 1999-03-04 | Siemens Ag | Method and device for compensating for mains voltage distortions occurring in a network |
Family Cites Families (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
CN2302498Y (en) | 1997-06-12 | 1998-12-30 | 周维保 | Multi-function measuring instrument |
SE517777C2 (en) * | 2000-11-30 | 2002-07-16 | Abb Ab | Device and method of voltage control in an electrical transmission network |
US6411067B1 (en) * | 2001-02-20 | 2002-06-25 | Abb Ab | Voltage source converters operating either as back-to-back stations or as parallel static var compensators |
-
2002
- 2002-01-09 SE SE0200050A patent/SE519413C2/en unknown
-
2003
- 2003-01-07 US US10/501,076 patent/US7173349B2/en not_active Expired - Fee Related
- 2003-01-07 EP EP03701188A patent/EP1464103A1/en not_active Withdrawn
- 2003-01-07 AU AU2003202179A patent/AU2003202179A1/en not_active Abandoned
- 2003-01-07 WO PCT/SE2003/000015 patent/WO2003069757A1/en not_active Application Discontinuation
- 2003-01-07 CN CNB038056208A patent/CN100370672C/en not_active Expired - Fee Related
Patent Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO1997049157A1 (en) | 1996-06-17 | 1997-12-24 | Asea Brown Boveri Ab | Method and device for compensation of reactive power |
DE19738125A1 (en) | 1997-09-01 | 1999-03-04 | Siemens Ag | Method and device for compensating for mains voltage distortions occurring in a network |
Non-Patent Citations (3)
Title |
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ASHARI M. ET AL.: "Mitigation of line and neutral current harmonics in three-phase distribution systems", PUB-CONFERENCE RECORD OF THE 2000 IEEE INDUSTRY..., vol. 5, 8 October 2000 (2000-10-08) - 12 October 2000 (2000-10-12), pages 3166 - 3171, XP001042715 * |
BIJLENGA BO ET AL.: "SVC light-a powerful tool for power quality improvement", ABB REVIEW, June 1998 (1998-06-01), pages 21 - 30, XP002962589 * |
CHENG PO-TAI ET AL.: "Operations of the dominant harmonic active filter (DHAF) under realistic utility conditions", PUB-CONFERENCE RECORD OF THE 2000 IEEE INDUSTRY..., vol. 4, 8 October 2000 (2000-10-08) - 12 October 2000 (2000-10-12), pages 2135 - 2142, XP001042613 * |
Also Published As
Publication number | Publication date |
---|---|
SE0200050D0 (en) | 2002-01-09 |
CN1639941A (en) | 2005-07-13 |
CN100370672C (en) | 2008-02-20 |
EP1464103A1 (en) | 2004-10-06 |
SE0200050L (en) | 2003-02-25 |
AU2003202179A1 (en) | 2003-09-04 |
SE519413C2 (en) | 2003-02-25 |
US7173349B2 (en) | 2007-02-06 |
US20050116691A1 (en) | 2005-06-02 |
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