Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
  • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
  • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Definitions

• the present invention relates to an electric power system comprising at least two electric power plants with a dc output, a dc transmission line and means for dc-ac conversion of the dc power, transmitted by means of the dc transmission line, to a distribution or transmission net- work.
• the electric power plants include a driven ac machine and at least one ac-dc converter.
• a series link is formed, consisting of a series connection of the dc outputs of the ac-dc converters of the electric power plants via intermediate transmission lines, the dc transmission line and the means for dc-ac conversion. This means that the electric power plants, from the point of view of insulation engineering, may be located at a high dc potential relative to ground.
• the dc transmission line can preferably be designed using HVDC single-conductor cables or, in a known manner, comprise a single-conductor HVDC cable/overhead line with ground as return conductor.
• Typical technical fields where series-connected dc electric power plants may be subjected to a high dc potential are electric power systems/installations where the electric power plants are driven by renewable energy sources, for example wind power, solar cells, etc.
• the present invention relates to embodiments of electric power plants included in the above-described electric power system, which permit them to operate at a high dc potential relative to ground.
• DC generators ... or AC generators connected to power converters (fig. 4) driven through insulative drive shafts
• Power may be converted to smaller voltages at the distribution region by coupling a plurality of electrical motors in series, each being supported on insulative structure".
• the high-voltage "long distance transmission line” may be a single conductor and that ground is used as return conduc- tor.
• the US document does not comprise any control strategy, neither during normal operation, nor under various fault conditions.
• SE 9904740-9 (PCT/SE00/02616 ) , "Electric power system based on renewable energy sources", describes a series-connected dc electric power system which relates to the same techni- cal field as the present invention, that is, an electric power system in which the included ac machines and ac-dc converters are subjected to a high dc potential.
• the application describes an embodiment with an ac-dc converter for feeding dc power to an ac power network without describing the system grounding.
• SE 9904740-9 PCT/SE00/02616
• the problems which have been described in connection with the description of US 4057736 will thus be equally apparent also as far as the ac-dc converters in an embodiment according to SE 99044740-9 are concerned.
• the present invention relates to an electric power system which comprises at least two electric power plants with a dc output, each one including an ac machine and at least one ac-dc converter, a dc transmission line and means for dc-ac conversion, and a series connection is formed of the dc outputs of the ac-dc converters of the electric power plants via intermediate transmission lines, the dc trans- mission line and the means for the dc-ac conversion.
• the electric power plants may, from the point of view of insulation engineering, become located at a high dc potential relative to ground.
• the object of the invention is to provide the ac machines of the electric power plants, having magnetic flux carriers and current-carrying ac windings, and the associated ac-dc converters with control electronics, with such a high dc insulation level that the risk of partial discharges (PD) , or flashover between the parts included and between the parts included and ground, becomes minimal .
• PD partial discharges
• an electric power plant comprises an ac machine and at least one ac- dc converter.
• ac-dc converters When several ac-dc converters are to be used, these are preferably connected in parallel.
• the ac machine comprises a driven shaft and is provided with magnetic flux carriers with a winding with an available neutral point and with an ac output which is connected to an ac input of at least one ac-dc converter with dc outputs.
• the ac machine including the magnetic flux carriers with the ac-carrying windings, are arranged within a surrounding "inner" electric field-controlling shell which is connected at high-voltage dc potential relative to ground, for example with a high-ohmic potential connection to the neutral point of the ac machine and an "outer" electric field-controlling shell, substantially concentrically arranged in relation to the inner shell, with a preferably low-ohmic connection to ground.
• the casing and the body of the ac machine are low-ohmically connected to the inner shell. The potential differences within the rotating electric ac machine are thus kept down at a level which is determined only by the ac voltage, generated in the rotating electric machine, at low-voltage or medium- voltage level;
• the ac-dc converter including power semiconductors with voltage- and current-carrying parts, such as connections, inductors, etc., as well as signal and control circuits, are arranged within a surrounding "inner" electric field- controlling shell which is connected at a high-voltage dc potential relative to the ground plane, for example with a high-ohmic potential connection to the outputs of the ac- dc converter, or instead only a high-ohmic potential connection from the neutral point of the ac machine, and an "outer" electric field-controlling shell, substantially concentrically arranged in relation to the inner shell, with a preferably low-ohmic connection to ground.
• the casing and the body of the ac-dc converter are connected low-ohmically to the inner shell.
• the potential differences within the converter are thus kept down at levels which are determined only by the ac voltage of the rotating electric machine, connected to the ac inputs of the converter.
• the potential differences within the converter will hence be of the same order of magnitude as the rated voltage of the ac machine ,-
• the shells shall have an electrically conducting function.
• a more detailed description of the shells will be given in the description of the embodiments ,-
• the insulation between the inner and outer shells shall be dimensioned for the nominal voltage of the dc-ac con- verters towards the ac power network, that is, the corresponding nominal operating voltage between the terminals of the dc-ac converter, provided that the outer casings are connected low-ohmically to the ground plane;
• the surrounding field-controlling shells shall be provided with high-voltage insulating bushings for the transmission lines between the electric power plants;
• the transmission lines between the electric power plants and the transmission line between the electric power plant, which is located nearest the return conductor, and the return conductor shall preferably be in the form of the above-mentioned extruded dc power cable, that is, a cable with a capacity to withstand a high dc potential;
• the mechanical shaft of the driven ac machine shall comprise an electrically insulating part towards the shaft of a drive device to be able to galvanically separate the shaft of the ac machine from ground potential.
• both the ac machine and the ac-dc converter are placed within one and the same pair of surrounding inner and outer field-con- trolling shells, whereby only the neutral point of the ac machine is high-ohmically connected, with regard to the potential, to the inner shell.
• Another embodiment will also be described under the description of embodiments.
• the potential difference inside the electric power plant is of the same order of magnitude as the rated voltage of the ac machine
• the ac component in the electric fields is of the same order of magnitude as the rated voltage of the ac machine and is maintained internally in the electric power plant and its innermost surrounding high-voltage dc-insulated equipotential surfaces, that is, the inner shell;
• the background art described above shows how to proceed and provides suggestions for overcoming these problems with, for example, extensive "supported (on) insulative structures of progressively greater height" or with special dc-extruded insulated conductors with an inner and an outer semiconductive screen in the windings of the magnetic flux carriers .
• the great advantage of using a technique according to the invention in relation to that described in SE 9904740-9 and in SE 9904753-2 is that the slot insulation of the ac machine no longer needs to be carried out for full dc voltage but may be designed, from the insulation stress point of view, for the relevant rated voltage of the ac machine. This implies that the ac machines may be manufactured using conventional and well-known insulation systems according to the state of the art;
• One advantage of using a technique according to the invention is thus that the criteria for dimensioning and insulation of the rotating ac machine and the ac-dc converter, resulting in the physical size, weight, volume, cost, etc., of the machine and the converter, are determined by the ac criteria of the relevant well-known rated voltage .
• an additional and important advantage of embodiments according to the invention is that the electric fields around electrically active and/or passive plant components, that is, machines, converters, etc., may be determined at the design stage.
• the electric fields as far as the dc part is concerned are controlled, respectively, by equipotential surfaces in the shells and by equipotential surfaces in cables and cable accessories such as jointing devices and bushings.
• the devices for protection against hazardous contact are thus integrated into the shells and the cables with accessories .
• Still another advantage of the embodiments according to the invention is that it is simple to arrange bypass of a faulty electric power plant. This can be made by means of power electronics and, for example, only based on power semiconductors such as power diodes, which block because of inverse voltage during normal operation of the electric power system.
• Figure 1 shows a preferred embodiment of an electric power plant according to the invention.
• FIG. 2 shows an alternative embodiment of an electric power plant according to the invention.
• Figure 3 shows how a plurality of electric power plants according to the invention, together with a dc transmission line and means for dc-ac conversion of the dc power transmitted to a distribution or transmission network by means of the transmission line, can form a dc electric power system.
• FIG. 1 A preferred embodiment of an electric power plant according to the invention, and showing the principle of such a plant, is shown in Figure 1.
• the preferred embodiment comprises an ac machine 1 and an ac-dc converter 2 within one and the same pair of shells, that is, within an inner shell 3 and an outer shell 4, arranged substantially concentrically thereto.
• the shells are fixed relative to each other by means of spacing insulators 5.
• the space 6 between the casings is filled with a gaseous and/or solid insulation.
• the outer shell is low-ohmically connected to the ground plane 7.
• the outer shell of the electric power plant is here shown placed on a bracket 8, for example belonging to a wind power plant.
• the ac machine is fixed to the inner shell 3 by means of fixing devices 9.
• the shaft 10 of the ac machine shall be galvanically separated from ground potential, the shaft consists of an electrically insulating part 11, preferably located in the space between the two shells.
• the driven part 12 of the shell is journalled in a support 13.
• the rotor of the ac machine is here shown provided with an excitation winding
• stator of the ac machine is provided with a Y- connected three-phase winding 15, the neutral point of which is high-ohmically potential-connected to the inner shell by means of a resistance 16.
• the converter 2 with the associated control unit 17 and protection unit 18 is also fixed to the inner shell 3 by means of fixing devices 1 .
• the task of the control unit 17 is to control both the converter 2 and the rotor winding of the ac machine.
• the electric power plant is intended to be controlled via a wireless communication link 20.
• the three-phase voltage of the ac machine which voltage is generated in the stator winding, is connected to the ac input of the converter.
• the two-pole dc output of the converter is led, via the high-voltage insulating bushings 21 and 22 arranged through the shells, out into the other electric power plants of the dc loop via HVDC transmission lines 23 and 24.
• the cable which forms the transmission lines is provided with inner and outer semiconductive layers (not shown) .
• the inner layer is in direct contact with the electric conductor.
• the outer layer shall have a galvanic connection to the outer shell and hence to the ground plane.
• the solid insulation of the cable, within the bushings 21 and 22 and further in towards the dc outputs of the ac-dc converter, is preferably given a conically tapering shape.
• the outer shell is suitably provided with a lightning conductor 7a which is connected together with the connection of the outer shell to the ground plane .
• the shells As will have been clear, it is important that they should be electrically conductive. Another, and mechanical, requirement is that they shall have a certain mechanical supporting capacity, especially as regards those parts of the shells which are to support the ac machine and the ac-dc converter with associated control units.
• the thickness of the shells may be dimensioned depending on the specific load in question. Those parts of the shells which support the ac machine and the ac-dc converter should therefore suitably have a material thickness which is larger than the other parts of the shell.
• the mechanical supporting capacity of the shells thus also places corresponding demands on the shape of the spacing elements which, in addition to permitting a sufficient insulation distance, also must be able to take up various mechanical loads .
• FIG 2 shows an alternative embodiment in which the ac machine 1 and the converter 2 are placed in separate pairs of shells 3a, 4a and 3b, 4b, respectively.
• the ac voltage which is to be transmitted between the different pairs of shells is considerably lower than the high dc potential to which the electric power plant will be exposed, this ac transmission, in the form of a transmission cable 23a, must be HVDC-insulated for a maximum dc voltage.
• the corresponding high-voltage insulating three- phase bushings 25a and 25b connection of the semiconductive layers is carried out in the same way as described for the corresponding layers in Figure 1.
• both the outputs must be connected to the inner shell via high-ohmic resistors 16a and 16b, respectively.
• the design of the two pairs of shells as far as bearing capacity, intermediate insulation 6, fixing devices 9, 19 and other plant components included, such as spacing insulators 5, are concerned, is identical with that shown in Figure 1.
• the shells may be toroidal.
• the shells are prefera- bly formed so as to conform closely to the outer surrounding surface of the stator of the ac machine.
• the converter may, especially if it is designed as a diode rectifier, very well be arranged more or less integrated with the stator of the ac machine.
• the ac machine is surrounded by a pair of toroidal shells, that is, corre- sponding to the shells 3a and 4a according to Figure 2.
• toroidal shells are toroidal shells which thus may be dimensioned to take up respective proportions of the driving torque on the shaft 12. This may have great signi- ficance, especially in the case of low-speed embodiments, where, for obvious reasons, the shaft torque may be high.
• embodiments of the electric power plants according to the invention permit special and new "grounding systems" when they together form a dc-series- connected electric power system.
• grounding possibilities which are available will be described, based on Figure 3.
• FIG 3 shows an electric power system comprising a number, here shown as two, of electric power plants 26 and 27, shown in a somewhat simplified form of the electric power plants according to Figure 1 and Figure 2, an HVDC transmission line 28, a dc-ac converter 29 for conversion of the transmitted dc power to a Y/D-connected transformer 30 for feeding a distribution or transmission network 31. Since the dc-ac converter will be subjected to the same high dc potential as the rest of the electric power plants, also this converter will have to be placed within an inner shell 32 and an outer shell 33 arranged practically concentrically with the inner shell.
• the electric power system is preferably resistance- grounded, that is, it has a connection over a resistive fault current-limiting member to the ground plane at one point. According to Figure 3, this takes place with a fault current-limiting resistor 34 connected to the neutral point of the transformer winding.
• the neutral point also has a high-ohmic connection 35 to the inner shell 32.
• the outer shells of all the electric power plants are direct-grounded via the connections 36 and 37.
• the corresponding direct grounding of the outer shells 33 of the ac-dc converter takes place via the connection 38.
• the neutral points of the windings 39 and 40 of the ac machine are connected, via high-ohmic connections 16c and 16d, to their own inner shells 3c and 3d, respectively, each of which forms its own reference plane.
• grounding systems ensure conversion from alternative energy to electrical energy and also ensure the transmission of the energy. They also facilitate the possibilities of monitoring, measurement, fault disconnection, and signal transmission.

Landscapes

• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Rectifiers (AREA)
• Control Of Eletrric Generators (AREA)
• Inverter Devices (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=20278824

Family Applications (1)

Country Status (4)

Families Citing this family (4)

Family Cites Families (4)

• 2000
  • 2000-03-16 SE SE0000865A patent/SE515953C2/sv not_active IP Right Cessation
• 2001
  • 2001-02-09 EP EP01904737A patent/EP1282930A1/en not_active Withdrawn
  • 2001-02-09 AU AU2001232562A patent/AU2001232562A1/en not_active Abandoned
  • 2001-02-09 WO PCT/SE2001/000259 patent/WO2001069758A1/en not_active Application Discontinuation

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events