High-voltage dc-insulated electric power plants TECHNICAL FIELD 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 con- version 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. Specific to the electric power systems which are used in connection with he present invention is that 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.

As far as the dc transmission line is concerned, it 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.

BACKGROUND ART, THE PROBLEMS The electric power plants which are described under the technical field, that is, electric power plants which are included in a dc series link and where the electric power plants comprise a driven ac machine and at least one ac-dc converter as well as associated plant components, will, from the point of view of insulation engineering, be stressed by -an ac voltage, generated in the driven ac machine, at low-voltage or medium-voltage level, and -a high-voltage dc potential relative to ground because of the connection of the ac machine and the ac-dc converter to the electric power system.

Documentation for solving the problems of a high dc potential stress on series-connected dc electric power plants is very limited, probably because it has not been possible to demonstrate that such electric power systems/plants are, or have been, in operation.

From a purely technical point of view, US 4057736, "Electrical power generation and distribution system", provides a solution. First, a summary of the embodiments described in the US document will be given. Because the embodiments described entail problems from the point of view of systems engineering, which have not been considered in the US document, a relatively detailed description of these problems, related to the embodiments described, will then be given below.

From US 4057736 it is clear that"... a plurality of rela- tively low voltage generating stations are connected in series to cumulatively produce the high voltage needed for long distance transmission line delivery. Power-generating devices of successive stations are supported on insulative structures of progressively greater height... The genera-

ting devices may take various forms including, for example ... 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". Otherwise, it is clear from the text that the high-voltage"long distance transmission line"may be a single conductor and that ground is used as return conduc- tor. Thus, primarily according to Figures 1 and 16 of the US document, there is formed a series connection of the generating stations with intermediate increasingly highly dc-stressed transmission lines, the long highly dc-stressed transmission line, the series-connected consumers with intermediate dc-ac machines, which are subjected to increasingly lower dc stress relative to ground and the ground return conductor. Both one end of the series- connected generating stations and one end of the series- connected consumers are thus directly connected to ground.

To overcome the problems concerning dc insulation enginee- ring, according to the US document, machines and converters are supported by some form of insulators dimensioned for increasingly higher dc potential stress ("insulative struc- tures or progressively greater height") and the shafts of both generators and consuming motors have an insulating intermediate link towards ground. It is further clear from Figures 9 and 15 of the US document that the dc system may also be designed with two single conductors and so-called centre grounding, whereby the earth crust is used as return conductor only if the current of the two single conductors differs during operation or during a fault. Figure 9 also shows that more than two single conductors may be used, for example in connection with internal parallel connection of generating stations.

As indicated above, however, problems of a not insigni- ficant nature are associated with the embodiments described in the above-mentioned US patent document. Some of these

problems will be described in the following lines. With the embodiments described, it will be difficult to -maintain the transmission of the accumulated power of the electric power plants in case of a ground fault in the dc loop and/or an insulation fault inside the series- connected electric power plants -limit fault currents to values which are harmless for the electric power system/plants in case of a ground fault in the dc loop and/or an insulation fault inside the series- connected electric power plants, -achieve a simple and reliable bypass of a power plant in the event of a fault or during service, -limit the electric fields around electrical and/or passive plant components, such as machines, converters, etc., to harmless values, -utilize established members for computerized control, protection, communication, etc.

In addition, it can be determined that the US document does not comprise any control strategy, neither during normal operation, nor under various fault conditions.

The consequences of the above-mentioned problem areas will be briefly described.

It is very important to be able to maintain the generating transmission power of the electric power plants also in the event of a fault on the electric power plants and/or on the transmission lines included therein. If a ground or insula- tion fault occurs, no embodiments or measures are described according to the US document for maintaining the fault currents occurring at harmless values. Bypassing of a faulty"generating station"is described only when the 'station"comprises four generating parts and then in an

exceedingly complicated way (according to Figure 13) with the aid of logic circuits and electromagnetic switching devices. Also if one"station", for example driven by a renewable energy source, cannot deliver the generated power to the dc loop, then, within a few seconds, the rotating parts of the turbine and the ac machine included will accelerate to such speed of rotation that their mechanical strength limits will be exceeded, resulting in severe mechanical and electrical damage. Nor are any measures described for preventing this. An electric power system with mixed series-and parallel-connected stations also implies that it may be difficult to determine where a fault has occurred.

With regard to the distribution of the electric fields around all electric and/or passive plant components which are located at potential, that is, machines, converters, etc., these are determined not only by their constructive design. To a great extent, the field distribution depends on how grounded objects, temporarily or continuously, in the form of mounting or the like, approach the plant com- ponents mentioned, or are in the vicinity of the plant components. This means that the distribution and concen- tration of the electric field may be essentially changed when unintentionally approaching grounded objected, for example in connection with accidents, service, rebuilding, and so on. From this follows that considerable demands must be placed on the surrounding device for protection against hazardous contact. In addition, field concentrations may lead to considerable problems with partial discharges.

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 appli- cation describes an embodiment with an ac-dc converter for feeding dc power to an ac power network without describing

the system grounding. This application as well as SE 9904753-2 (PCT/SEOO/02617),"Use of HVDC-insulated conduc- tor in magnetic flux carriers", describes an embodiment relating to the ac machines which overcomes the high dc potential which occurs in these systems. These two docu- ments show that cables with extruded insulation with a capacity to withstand a high dc potential have been pro- duced recently, described, inter alia, in"Extruded DC power cables and accessories for use in HVDC transmission system", published in ICC Fall Meeting 1999, pp. 1-7, written by P. Carstensen, K. Johannesson and A. Gustafsson.

By using such cables and conductors, respectively, with an inner and an outer semiconductor layer, possibly with so- called MIND cables, that is, a cable with a paper insula- tion impregnated by viscous oil, as conductors in the windings of the magnetic circuits of electric machines, the high dc potential stress of the machines in question may be overcome. The use of such a cable in the winding of the machines, however, implies that classical mechanical engi- neering, insulation engineering, design engineering and manufacturing engineering cannot be used directly. However, such an extruded cable for alternating current has already been used, as described, for example, in WO 97/45919, "Rotating electric machine with magnetic circuit for high voltage and method for manufacturing the same".

The ac-dc converters which are used in connection with the technique which will be used in the above-mentioned SE 99044740-9, that is, with ac machines with windings of cables with extruded insulation with a capacity to with- stand a high dc potential, must also be insulated taking into consideration the high dc potential to which they will be subjected. 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.

A similar problem with a high dc potential appears in the so-called HVDC transmissions. Here, a series connection is

made of a number of converter modules which are supplied with ac voltage from so-called converter transformers.

These are transformers which are different from conven- tional transformers by being oil-filled and provided with special bushings in order for the windings to withstand the high dc potential to which they are subjected from the point of view of insulation. These problems are described in WO 97/45907,"Rotating electrical machine plants".

SUMMARY OF THE INVENTION According to the description of the technical field, 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. In such an electric power system, as previously pointed out, the electric power plants may, from the point of view of insulation engineering, become located at a high dc poten- tial 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.

As indicated above, an electric power plant according to the invention comprises an ac machine and at least one ac- dc converter. When several ac-dc converters are to be used, these are preferably connected in parallel. To be able to describe the invention, a more detailed descrip- tion of the ac machine and the converter, and of their mutual connection, will first be made. 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.

To be able to achieve the object of the invention in other respects, the following applies: -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 concen- trically 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 differen- ces within the converter are thus kept down at levels which are determined only by the ac voltage of the rota- ting 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 corre- sponding 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 com- prise 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.

In a preferred embodiment of the invention, 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.

An embodiment of electric power plants/systems in accor- dance with the described invention implies that -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 ac and dc insulations are thus separated from each other.

By using a technique with a capacity to withstand a high dc potential as well as high-ohmic potential connections in electrically passive plant components according to the invention, considerable advantages may be obtained both in rotating electric machines and in converters which are subjected to a high dc potential relative to ground. With regard to -the ac machines, 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 manufac- tured using conventional and well-known insulation systems according to the state of the art; -the ac-dc converters, power conductors, connections, inductors, filters, control circuits, computers, communi- cation members, etc., according to existing and conven- tional technique may be utilized directly inside the inner shell without expensive reconstructions to adapt to the high dc potential.

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 con- verter, resulting in the physical size, weight, volume, cost, etc., of the machine and the converter, are deter- mined by the ac criteria of the relevant well-known rated voltage.

Another advantage of embodiments according to the inven- tion is that functional disorders in the electric power system, caused by flashover, will also be minimized with embodiments according to the invention.

An additional and important advantage of embodiments according to the invention is that the electric fields around electrically active and/or passive plant compo- nents, that is, machines, converters, etc., may be deter- mined 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, as opposed to the background art described above, 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 semi- conductors such as power diodes, which block because of inverse voltage during normal operation of the electric power system.

Another and exceedingly important advantage of the embo- diments according to the invention is that, in addition to the use of a conventional grounding system, a system, defined here as an"equipotential connection system", is obtained and that these systems together permit a consi- derable number of improvements relative to only so-called normal plant grounding. A more detailed description of the combination of the grounding and equipotential connection systems, as they may be applied to embodiments according to the invention, and the advantages permitted by the combination, will be given under the description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a preferred embodiment of an electric power plant according to the invention.

Figure 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 trans- mission 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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 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. As mentioned above, 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.

Since 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 14, and the 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 19. The task of the control unit 17 is to control both the converter 2 and the rotor winding of the ac machine. In the embodiment shown, the electric power plant is intended to be controlled via a wireless communi- cation 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 insu- lating 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. As will have been clear from the above, the cable which forms the transmission lines is provided with inner and outer semiconductive layers (not shown). As mentioned, 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.

With regard to the shape of 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.

From the point of view of electrical conductivity, there are a plurality of materials and combinations of materials, respectively, that may be used. In addition to a purely electrically conductive, mechanically strong material in the form of sheet and the like, conductive plastic mate- rials, conductive metallic nettings applied to plastic materials, etc., may be used. An inner and outer shell included in the same pair of shells may very well be shaped from different electrically conductive materials.

As regards the mechanical supporting capacity, the thick- ness 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.

Figure 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. Although the ac voltage which is to be transmitted between the different pairs of shells, as will have been clear from the descrip- tion, 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 same is true of the corresponding high-voltage insulating three- phase bushings 25a and 25b. Connection of the semicon- ductive layers is carried out in the same way as described for the corresponding layers in Figure 1. To potentially fix the dc outputs of the converter to the inner shell, in this embodiment, both the outputs must be connected to the inner shell via high-ohmic resistors 16a and 16b, respec- tively. 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.

In alternative embodiments of the invention, the shells may be toroidal. For such embodiments, the shells are prefera- bly formed so as to conform closely to the outer surroun- ding surface of the stator of the ac machine. In an embo- diment corresponding to that of Figure 1, that is, when the ac machine and the ac-dc converter are within the same pair of shells, 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. In its simplest toroidal embodiment, only 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. One advantage of toroidal shells is that the insulating shaft part 11 may be replaced by insulating spokes 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.

As indicated above, 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. To be able to demonstrate more of the advantages when electric power plants according to the invention form an electric power system, the "grounding possibilities"which are available will be described, based on Figure 3.

Figure 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. This implies that, in each electric power plant, a resistive fault-current-limiting, normally currentless, equipotential connection is formed between the neutral point of the windings of the ac machines and the reference plane, that is, the inner shell. As previously mentioned, the casing and body of both the ac machine and the ac-dc converter are low-ohmically connected to the inner shell.

The advantages offered by these"grounding systems"are, among other things, the following: -dangerous fault currents to a ground plane or a reference plane are eliminated; -unintentional voltage towards a ground plane or a reference plane is eliminated; -persons and property are protected; -electric power plants and systems are protected against any overvoltages/transients; -electric power plants and systems are protected against discharges of static electricity; -electric power plants and systems are protected against lightning strokes.

Together, these"grounding systems"ensure conversion from alternative energy to electrical energy and also ensure the transmission of the energy. They also facilitate the possi- bilities of monitoring, measurement, fault disconnection, and signal transmission.