WO1999031520A1

WO1999031520A1 - A plant and a method in connection therewith

Info

Publication number: WO1999031520A1
Application number: PCT/SE1998/002150
Authority: WO
Inventors: Mats Leijon; Maria Lundmark
Original assignee: Abb Ab
Priority date: 1997-11-27
Filing date: 1998-11-27
Publication date: 1999-06-24
Also published as: SE9704391L; AU1515699A; JP2002508520A; SE512698C2; DE19882835T1; SE9704391D0

Abstract

An electric plant comprises an electric device connected to an electric distribution or transmission network, said electric device having a magnetic circuit and at least one winding, and a measuring apparatus (10) adapted to supervise the electric device. The measuring apparatus comprises at least one capacitive sensor (11) including an inner electrode (12) and a screen electrode (14), which screens the inner electrode from disturbing electric fields. The sensor is adapted to provide voltage measurement by sensing, by means of the inner electrode (12), such a part of an electric field which penetrates into the inner electrode. The sensor (11) is directed towards an electric line connecting the electric device with the distribution or transmission network so as to sense, at insulation distance, the voltage, including overtones and transients, of this electric line via the electric field surrounding the line. Furthermore, the invention comprises a method for supervising the electric device by means of the measuring apparatus.

Description

A plant and a method in connection therewith

TECHNICAL FIELD

This invention is related to an electric plant comprising an elect- ric device connected to an electric distribution or transmission network, said electric device having a magnetic circuit and at least one winding, and a measuring apparatus adapted to supervise the electric device. In addition, the invention is related to a method for supervising the electric device.

The electric device may be used in any electrotechnical connections. The power range may be from VA up to the 1000-MVA range. The present invention is primarily intended to be used for medium and high voltage. According to IEC standard, medium voltage is 1 -72,5 kV whereas high voltage is above 72,5 kV. Thus, the transmission, subtransmission and distribution levels are included.

A difficult problem with respect to quality of electricity is the oc- currence of undesired variations. Quality of voltage concerns how sinusoidal and symmetrical the voltage is. One of the big problems concerns the occurrence of harmonics. Harmonics in an electric system are voltages or currents which in time describe a sine wave having a frequency which is a multiple (2, 3, 4, 5,...) of the system frequency (in Sweden 50 Hz). The normally occurring harmonics in electric systems are the odd harmonics third, fifth, seventh and so on.

The third older harmonics, the third order voltage harmonics, causes normally the most severe problem. Harmonics in a power network may cause a number of problems for the equipment connected to the power network and to the objects which themselves cause the harmonics. These problems are transmission losses, resonance and overheating.

It is known to try to reduce harmonics problems by providing filters (active or passive) at harmonics generating devices.

The voltage measuring apparatuses which are currently avai- lable are often costly and complicated devices which must be brought into contact with the voltage carrying device. Examples on such solutions are voltage transformers. These solutions entail large investments and therefore imply a great departure from the low investment which is desirable. A further problem with prior voltage measuring apparatuses is that they are often restricted to measurement at system frequency. Thus, these known apparatuses are not adapted to detect harmonics voltages on the electric power network. The harmonics' problem is often directly associated with how the connected devices are grounded.

According to a first aspect of the invention a rotating electric machine is contemplated. Such electric machines comprise synchronous machines which are mainly used as generators for connection to distribution and transmission networks, commonly referred to below as power networks. The synchronous machines are also used as motors and for phase compensation and voltage control, in that case as mechanically idling machines. The technical field also comprises double-fed machines, asyn- chronous converter cascades, external pole machines, synchronous flux machines and asynchronous machines.

According to another aspect of the invention, said electric device is formed by a power transformer or reactor. For all trans- mission and distribution of electric energy, transformers are used and their task is to allow exchange of electric energy bet- ween two or more electric systems and for this, electromagnetic induction is utilized in a well-known manner. The transformers primarily intended with the present invention belong to the so- called power transformers with a rated power of from a few hundred kVA up to more than 1000 MVA with a rated voltage of from 3-4 kV and up to very high transmission voltages, 400 kV to 800 kV or higher.

Although the following description of the prior art with respect to the second aspect mainly refers to power transformers, the present invention is also applicable to reactors, which, as is well- known, may be designed as single-phase and three-phase reactors. As regards insulation and cooling there are, in principle, the same embodiments as for transformers. Thus, air-insulated and oil-insulated, self-cooled, pressure-oil cooled, etc., reactors are available. Although reactors have one winding (per phase) and may be designed both with and without a magnetic core, the description of the background art is to a large extent relevant also to reactors.

A possible method to reduce third order voltage harmonics in generator plants is to connect the phase voltages of the generator to a Δ/Y-connected transformer (a so called "step-up transformer"). The third order voltage harmonics generated by the generator provide a third order current harmonics via the neutral point resistor to ground.

The third order voltage harmonics is superposed on the phase voltages of the Δ-winding but cannot drive any third order cur- rent harmonics. This means that one does not experience any third order current harmonics on the Y-side of the transformer, i.e. on the power network side. This requires, however, that the "step-up transformer" is dimensioned for full power and that an over voltage protection is installed on the input side of said transformer. In addition, current measuring apparatuses, breakers and disconnectors are required. This solution has not tur- ned out to be satisfactory, in particular not with respect to higher voltages. It is pointed out that in connection with the particular type of rotating electric machine which will be described more closely hereafter, conditions are created to be able to enti- rely avoid the so called "step-up transformer" so that accordingly the rotating electric machine is connected directly to an electric power network adapted for high voltage, suitable 36 kV and more, without intermediate transformer. A problem in such direct connection is, however, that the occurrence of harmonics tends to increase. Thus, it becomes even more essential to detect such harmonics.

Also with respect to transformers and reactors there will be described hereafter a new technique where the windings are formed by means of a high voltage cable including an electric conductor having a casing which is magnetically permeable but capable of substantially enclosing the electric field around the conductor. In such a transformer it is possible to increase the magnetic flux in the core up to higher levels than those which are possible in prior art transformers. It is the filter action of the high voltage cable which makes this possible. An increased magnetic flux means that the magnetic core may be made smaller while retaining the output power of the transformer. This means in turn that the transformer becomes more compact, lighter and also less expensive. A problem with increasing the magnetic flux is, however, that the core may be saturated, which means generation of harmonics. It would be desirable to be able to control the magnetic flux for avoiding these harmonics.

OBJECT OF THE INVENTION

The present invention aims at devising routes to be able to master the problems occurring with voltages harmonics on the network. In particular, the invention aims at devising routes to be able to simply and accurately supervise and measure the voltage of the electric devices where harmonics are generated. Supervising and measuring are carried out so as to be able to determine in what way the voltages harmonics should be sub- pressed or filtered away.

SUMMARY OF THE INVENTION

For fulfilling this object, the present invention proposes that the measuring apparatus should comprise at least one capacitive sensor including an inner electrode and a screen electrode, which is connected to ground or otherwise a potential different from the potential of the measured object and which screens the inner electrode from disturbing electric fields, that the sensor is adapted to provide voltage measurement by sensing , by means of the inner electrode, such part of an electric field which penet- rates into the inner electrode and that the sensor is directed towards an electric line forming the measured object and connecting the electric device with the distribution or transmission network so as to sense, at insulation distance, the voltage, including transients and harmonics, of this electric line via the electric field surrounding the line.

Such a measuring apparatus has the advantage that the voltage may be measured at a distance from the electric line, which considerably simplifies arrangement of the measuring appara- tus. The design of the measuring apparatus means, furthermore, that it becomes flexible, has a simple structure and a low production cost. Furthermore, possibilities are created to measure the voltage of the electric device within a large frequency range. It is in particular pointed out that it means not only a lower cost but also elimination of operation disturbances to be able to arrange the measuring apparatus at a distance from the electric device to be measured with respect to voltage instead of providing, according to the prior art, the measuring apparatus in contact with the electric line in question. A particular advantage with the measuring apparatus according to the invention is that it is capable of efficiently screening off undesired electric fields and easily may be directed towards the electric line to be voltage measured with respect to voltage magnitude or frequency or both.

A high-voltage conductor is surrounded by an electric field which carries information about the potential of the conductor, its variation and its frequency contents. A capacitive sensor comprising two mirror-symmetrical electrodes which are introduced into this electric field may sense these quantities. One problem, however, is that such a capacitive sensor is sensitive to changes in all directions of the electric field. Thus, also other field-generating objects may influence and sometimes completely dominate such a measurement. It is, therefore, not possible to distinguish from the measurement result which change belongs to the component selected for the measurement. Since distribution of electric power is normally carried out in three adjacently extending conductors, it is thus not possible with such a sensor to delimit the field which emanates from one of the conductors.

The invention relates to a capacitive sensor with two electrodes, adapted for sensing changes in a directed part of an electric field. This sensor is also adapted, in such an electric field, to detect transients and ionic discharge from the electric device. According to the invention, a directed part of an electric field is sensed by screening the other electrode from undesired electric fields by means of an electrode connected to ground or to some other controllable potential. A sensor intended for this purpose is arranged with one electrode predominantly surrounding the other electrode and connected to ground or to some other controllable potential. In the surrounding electrode, which hereinafter will be referred to as a screen electrode, an opening is arranged, through which a directed partial amount of the electric field reaches the surrounded electrode, which hereinafter will be referred to as an inner electrode. All other directed sub-quanti- ties of the electric field are efficiently prevented by the screen electrode.

In a preferred embodiment, the electrodes are insulated from each other by a gaseous dielectric, the capacitance formed by the electrodes thus becoming insensitive to temperature variations. Thus, the sensor may be configured and its capacitance be measured in a laboratory and then be used in other environments without needing calibration again. The sensor may also be used for a long period of time at varying temperatures, in which case no correction has to be made. For the purpose of increasing the sensitivity or reinforcing the directional effect of the sensor, the inner electrode may be divided into sub-electrodes which are insulated from each other and which may be pla- ced both in the lateral and vertical directions.

Thus, the invention relates to a measuring apparatus, including the sensor described above, for voltage measurement at insulation distance of an electric device in an electric field with a plu- rality of field-generating components. The measuring apparatus is arranged by connecting a signal converter to the sensor, whereby, when the screen electrode is connected to ground, a measuring apparatus is obtained by means of which a directional sub-quantity of an electric field may be measured. This mea- suring apparatus constitutes a simple, inexpensive and reliable device for measuring, in a contactless manner, an alternating voltage at a distance from a line. The apparatus is a broad-band apparatus which, within a large frequency range, permits measurement in a simple manner also of the occurrence and mag- nitude of harmonic components of the electric device intended to be measured.

The signal converter includes members for impedance conversion, amplification, and may also include members for filtering and digital conversion, of the measurement signal. The signal converter is placed at a short distance from the actual sensor, and in a preferred embodiment of the invention it is integrated with the sensor. The converted, analog or digital, signal may thereafter be transmitted to an analyzer via an electric or optical medium or be transmitted in a contactless manner via a trans- mitter and a receiver.

When applying the measuring apparatus to voltage measurement, the screen electrode may be connected to a controllable potential instead of to ground, in which case, by phase-locking to one of the phases, greater dynamics and higher resolution of the measurement may be obtained. In another preferred embodiment of the invention, the inner electrode is instead connected to a phase lock circuit, in which case the contribution from an unwanted field-generating source may be suppressed. The sig- nal converter is thus brought to include also a conductor adapted for signals in the opposite direction. In the same way, when using filtering of the sensed signal, the signal converter may include a plurality of conductors for transmission of different filtered signals to a multi-channel analyzer.

The measuring apparatus according to the invention has a wide field of use, especially in connection with measurement of alternating voltage in high-voltage devices including rotating electric machines and transformers/reactors. In a preferred use of the measuring apparatus, the device is placed at insulation distance from each line, belonging to the respective phase, for measurement of alternating voltage. In three-phase systems, the measuring apparatuses may be placed at a common point and be individually directed towards a respective line belonging to the electric device.

The measuring method is sensitive to variations in the distance between a measured object and the measuring apparatus. However, this is usually no problem since such variations, vi- ewed over a longer period of time, tend to become negligible. It is advantageous to place the measuring apparatus in locations where the position of the measured object is fixed, for example at points of attachment or suspension where the variation in distance is minimal.

In electric devices with one phase only, the accuracy is increased in a simple manner by placing a plurality of measuring apparatuses around the conductor. In a use with one phase only, the accuracy of the voltage measurement is increased by placing four measuring apparatuses rotationally-symmetrically around an extended part of the line. Since the measuring apparatuses are placed equidistantly from the line, the correct value of the voltage is obtained from the mean value of the four measuring apparatuses.

The sensitivity to variations in the distance between the measured object and the measuring apparatus may be utilized for detecting movement of the measured object. By placing at least three measuring apparatuses in fixed positions around a high- voltage conductor, it may be detected if the line is moved and in which direction this movement takes place.

A stable measurement distance to an electric high-voltage line is achieved by arranging the measuring apparatus and the line at either end of an insulator, which may be hollow. In this way, the length of the insulator is utilized to constitute a non-varying measurement distance. The measuring apparatus may be placed both outside and inside the insulator body. Among preferred embodiments may be mentioned suspension insulators, especially in transmission towers, and support insulators.

A measuring apparatus according to the invention is also suited, in connection with distribution networks, to allow to control relay protection functionalities, in which case the apparatus, by its low investment cost, may concentrate the measuring points and hence increase the selectivity. The measuring apparatus is also suited for measuring voltage in a line in connection with debiting of energy consumption. For this purpose, the measurement is to be combined with a differently measured current through the line, whereby the electric energy which passes may be determined.

As already has been established above, the measuring apparatus according to the invention is very well suited to measure the voltage, including harmonics, in connection with such an electric device where the high-voltage winding comprises a flexible con- ductor having a casing which is magnetically permeable but capable of operating in an enclosing manner with regard to the electric field generated by the conductor. Thus, said casing comprises an insulation system.

In a wide sense, it is established that the design according to the invention reduces the occurring losses such that the device, accordingly, may operate with a higher efficiency as a consequence of the fact that the invention makes it possible to substantially enclose the electric field occurring due to said electric conductor in the insulation system. The reduction of losses results, in turn, in a lower temperature in the device, which reduces the need for cooling and allows possibly occurring cooling devices to be designed in a more simple way than without the invention.

The conductor/insulation system according to the invention may be realized as a flexible cable, which means substantial advantages with respect to production and mounting as compared to the prefabricated, rigid windings which have been conventional up to now. The insulation system used according to the invention results in absence of gaseous and liquid insulation materials.

As to the aspect of the invention as a rotating electric machine it is thus possible to operate the machine with such a high voltage that the "step-up" transformer mentioned above can be omitted. That is, the machine can be operated with a considerably higher voltage than machines according to the state of the art to be able to perform direct connection to power networks. This means considerably lower investment costs for systems with a ro- tating electric machine and the total efficiency of the system can be increased. The invention eliminates the need for particular field control measures at certain areas of the winding, such field control measures having been necessary according to the prior art. A further advantage is that the invention makes it more simple to obtain under- and overmagnetization for the purpose of reducing reactive effects as a result of voltage and current being out of phase with each other.

As to the aspect of the invention as a power transformer/ reac- tor, the invention, first of all, eliminates the need for oil filling of the power transformers and the problems and disadvantages associated thereto.

The design of the winding so that it comprises, along at least a part of its length, an insulation formed by a solid insulating material, inwardly of this insulation an inner layer and outwardly of the insulation an outer layer with these layers made of a semi conducting material makes it possible to enclose the electric field in the entire device within the winding. The term "solid insu- lating material" used herein means that the winding is to lack liquid or gaseous insulation, for instance in the form of oil. Instead the insulation is intended to be formed by a polymeric material. Also the inner and outer layers are formed by a polymeric material, though a semiconducting such.

The inner layer and the solid insulation are rigidly connected to each other over substantially the entire interface with such adhesion that a temperature change does not result in separation of the layer and the insulation. Also the outer layer and the solid insulation are rigidly connected to each other over substantially the entire interface therebetween with such adhesion. The inner layer operates equalizing with respect to potential and, accordingly, equalizing with respect to the electric field outwardly of the inner layer as a consequence of the semiconducting properties thereof. The outer layer is also intended to be made of a semiconducting material and it has at least an electrical conductivity being higher than that of the insulation so as to cause the outer layer, by connection to earth or otherwise a relatively low potential, to function equalizing with regard to potential and to substantially enclose the electrical field resulting due to said electrical conductor inwardly of the outer layer. On the other hand, the outer layer should have a resistivity which is sufficient to minimize electric losses in said outer layer.

The rigid interconnection between the insulating material and the inner and outer semiconducting layers should be uniform over substantially the entire interface such that no cavities, pores or similar occur. With the high voltage levels contemplated according to the invention, the electric and thermal loads which may arise will impose extreme demands on the insulation mate- rial. It is known that so-called partial discharges, PD, generally constitute a serious problem for the insulating material in high- voltage installations. If cavities, pores or the like arise, internal corona discharges may arise at high electric voltages, whereby the insulating material is gradually degraded and the result could be electric breakdown through the insulation. This may lead to serious breakdown of the electric device. Thus, the insulation should be homogenous.

The inner layer inwardly of the insulation should have an electric conductivity which is lower than that of the electric conductor but sufficient for the inner layer to function equalizing with regard to potential and, accordingly, equalizing with respect to the electric field externally of the inner layer. This in combination with the rigid interconnection of the inner layer and the insula- tion over substantially the entire interface, i.e. the absence of cavities etc, means a substantially uniform electric field externally of the inner layer and a minimum of risk for PD.

It is preferred that the inner layer and the solid insulation are formed by materials having substantially equal thermal coefficients of expansion. The same is preferred as far as the outer layer and the solid insulation is concerned. This means that the inner and outer layers and the solid insulation will form an insulation system which on temperature changes expands and con- tracts uniformly as a monolithic part without those temperature changes giving rise to any destruction or disintegration in the interfaces. Thus, intimacy in the contact surface between the inner and outer layers and the solid insulation is ensured and conditions are created to maintain this intimacy during pro- longed operation periods.

Furthermore, it is pointed out that it is essential that the materials in the inner and outer layers and in the solid insulation have a high elasticity so that the materials may endure the strains oc- curring when the cable is bent and when the cable during operation is subjected to thermal strains. A good adhesion between the solid insulation and the inner and outer layers and a high elasticity of these layers and the solid insulation respectively are particularly important in case the materials in the layers and the solid insulation would not have substantially equal thermal coefficients of expansion. Furthermore, it is preferable that the materials in the inner and outer layers and in the solid insulation have substantially equal elasticity (E-modules), which will counteract occurrence of shear stresses in the border zone bet- ween the layers and the solid insulation.

In order to be able to form windings by means of the cable, it is essential that flexibility thereof is good. It is preferred that the cable should be capable of being subjected to bending, without negative influence on the function, with a radius of curvature which is 25 times the cable diameter or less. Preferably, the ra- dius of curvature is 15 times the cable diameter or less. The most preferable value on the radius of curvature is 8 times the cable diameter or less.

The electric load on the insulation system decreases as a consequence of the fact that the inner and the outer layers of semiconducting material around the insulation will tend to form substantially equipotential surfaces and in this way the electric field in the insulation proper will be distributed relatively uniformly over the thickness of the insulation.

It is known that high voltage cables for transmission of electric energy may be constructed as conductors with an insulation of a solid insulation material with inner and outer layers of semicon- ducting material. In transmission of electric energy, it has since long been realized that the insulation should be free from defects. However, in high voltage cables for transmission, the electric potential does not change along the length of the cable but the potential is basically at the same level. However, also in high voltage cables for transmission purposes, instantaneous potential differences may occur due to transient occurrences, such as lightning. According to the present invention a flexible cable according to the enclosed claims is used as a winding in the electric device.

An additional improvement may be achieved by constructing the electric conductor in the winding from smaller, so-called strands, at least some of which are insulated from each other. By making these strands to have a relatively small cross section, preferably approximately circular, the magnetic field across the strands will exhibit a constant geometry in relation to the field and the occurrence of eddy currents is minimized.

According to the invention, the winding is thus preferably made in the form of a cable comprising the conductor and the previously described insulation system, the inner layer of which extends about the strands of the conductor. Outside of this inner semiconducting layer is the main insulation of the cable in the form of a solid insulation material.

The outer semiconducting layer shall according to the invention exhibit such electrical properties that a potential equalization along the conductor is ensured. The outer layer may, however, not exhibit such conductivity properties that current will flow along the surface, which could cause losses which in turn may create an unwanted thermal load. For the inner and outer layers the resistance statements (at 20°C) defined in the enclosed claims 20 and 21 are valid. With respect to the inner semiconducting layer, it must have a sufficient electric conductivity to ensure potential equalization for the electric field but at the same time this layer must have such a resistivity that the enclosing of the electric field is ensured.

It is important that the inner layer equalizes irregularities in the surface of the conductor and forms an equipotential surface with a high surface finish at the interface with the solid insulation. The inner layer may be formed with a varying thickness but to ensure an even surface with respect to the conductor and the solid insulation, the thickness is suitably between 0.5 and 1 mm.

Such a flexible winding cable which is used according to the invention in the electric device thereof is an improvement of a XLPE (cross-linked poly ethylene) cable or a cable with EP (ethylene-propylene) rubber insulation used per se for transmission purposes. The improvement comprises, inter alia, a new design both as regards the strands of the conductors and in that the cable, at least in some embodiments, has no outer casing for mechanical protection of the cable. However, it is possible according to the invention to arrange a conducting metal shield and an outer mantel externally of the outer semiconducting layer. The metal shield will then have the character of an outer mechanical and electrical protection, for instance to lightning. It is preferred that the inner semiconducting layer will lie on the potential of the electrical conductor. For this purpose at least one of the strands of the electrical conductor will be uninsulated and arranged so that a good electrical contact is obtained to the inner semiconducting layer. Alternatively, different strands may be alternatingly brought into electrical contact with the inner semiconducting layer.

Manufacturing transformer or reactor windings of a cable accor- ding to the above entails drastic differences as regards the electric field distribution between conventional power transformers/reactors and a power transformer/reactor according to the invention. The decisive advantage with a cable-formed winding according to the invention is that the electric field is enclosed in the winding and that there is thus no electric field outside the outer semiconducting layer. The electric field achieved by the current-carrying conductor occurs only in the solid main insulation. Both from the design point of view and the manufacturing point of view this means considerable advantages:

- the windings of the transformer may be formed without having to consider any electric field distribution and the transposition of strands, mentioned under the background art, is omitted;

- the core design of the transformer may be formed without having to consider any electric field distribution;

- no oil is needed for electrical insulation of the winding, that is, the medium surrounding the winding may be air;

no special connections are required for electrical connection between the outer connections of the transformer and the immediately connected coils/windings, since the elect- rical connection, contrary to conventional plants, is integrated with the winding; - the manufacturing and testing technology which is needed for a power transformer according to the invention is considerably simpler than for a conventional power transformer/reactor since the impregnation, drying and vacuum treatments described under the description of the background art are not needed.

In application of the invention as a rotating electric machine a substantially reduced thermal load on the stator is obtained. Temporary overloads of the machine will, thus, be less critical and it will be possible to drive the machine at overload for a longer period of time without running the risk of damage arising. This means considerable advantages for owners of power generating plants who are forced today, in case of operational distur- bances, to rapidly switch to other equipment in order to ensure the delivery requirements laid down by law.

With a rotating electric machine according to the invention, the maintenance costs can be significantly reduced because trans- formers and circuit breakers do not have to be included in the system for connecting the machine to the power network.

Above it has already been described that the outer semiconducting layer of the winding cable is intended to be connected to ground potential. The purpose is that the layer should be kept substantially on ground potential along the entire length of the winding cable. It is possible to divide the outer semiconducting layer by cutting the same into a number of parts distributed along the length of the winding cable, each individual layer part being connectable directly to ground potential. In this way a better uniformity along the length of the winding cable is achieved.

Above it has been mentioned that the solid insulation and the inner and outer layers may be achieved by, for instance, extrusion. Other techniques are, however, also well possible, for in- stance formation of these inner and outer layers and the insulation respectively by means of spraying of the material in question.

It is preferred that the winding cable is designed with a circular cross section. However, also other cross sections may be used in cases where it is desired to achieve a better packing density.

To build up a voltage in the rotating electric machine, the cable is disposed in several consecutive turns in slots in the magnetic core. The winding can be designed as a multi-layer concentric cable winding to reduce the number of coil-end crossings. The cable may be made with tapered insulation to utilize the magnetic core in a better way, in which case the shape of the slots may be adapted to the tapered insulation of the winding.

A significant advantage with a rotating electric machine according to the invention is that the E field is near zero in the coil- end region outside the outer semiconductor and that with the outer casing at ground potential, the electric field need not be controlled. This means that no field concentrations can be obtained, neither within sheets, in coil-end regions nor in the transition therebetween.

In a method for manufacturing a magnetic circuit, a flexible cable, which is threaded into openings in slots in a magnetic core of the rotating electrical machine, is used as a winding. Since the cable is flexible, it can be bent and this permits a cable length to be disposed in several turns in a coil. The coil ends will then consist of bending zones in the cables. The cable may also be joined in such a way that its properties remain constant over the cable length. This method entails considerable simplifications compared with the state of the art. The so-called Roebel bars are not flexible but must be preformed into the desi- red shape. Insulation winding and impregnation of the coils are also an exceedingly complicated and expensive technique when manufacturing rotating electric machines today.

To sum up, thus, an electric device in the form of a rotating electric machine according to the invention means a conside- rable number of important advantages in relation to corresponding prior art machines. First of all, it can be connected directly to a power network at all types of high voltage. Another important advantage is that ground potential has been consistently conducted along at least a part of and preferably along the whole winding, which means that the coil-end region can be made compact and that bracing means at the coil-end region can be applied at practically ground potential. Still another important advantage is that oil-based insulation and cooling systems disappear also in rotating electric machines as already has been pointed out above with regard to power transformers/reactors. This means that no sealing problems may arise and that the dielectric ring previously mentioned is not needed. One advantage is also that all forced cooling can be made at ground potential.

Furthermore, the invention is related to a method for supervising, in an electric plant comprising an electric device connected to an electric distribution or transmission network, said electric device having a magnetic circuit and at least one win- ding, the electric device by means of a measuring apparatus in accordance with claim 36. Such a measuring apparatus comprising at least one capacitive sensor has turned out to be very suitable for sensing, at a distance from an electric line connecting the electric device to the distribution or transmission net- work, the voltage of this electric line via the electric field surrounding the line. This presupposes of course that the measuring apparatus is designed to sense the voltage of the electric line at a place where this line does not present any insulation system which substantially entirely encloses the electric field. Accor- dingly, this means that the flexible cable forming the winding in the electric device must transfer into a line portion at least sub- stantially in absence of an insulation system, either by the electric conductor in the winding being liberated from its insulation system at the sensed portion in question or by the electric conductor in a cable termination transferring into said electric line. The place of sensing proper is suitably located comparatively close to the electric device so that harmonics etc generated in the device may be efficiently sensed so as to be able to be eliminated by filtration/compensation.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the enclosed drawings, a more specific description of embodiment examples of the invention will follow hereinafter.

In the drawings:

Fig 1 is a view, partly in section, of a measuring apparatus including a capacitive sensor and a signal converter for directed voltage measurement according to the invention;

Fig 2 is a view, partly in section, of an alternative embodiment of the measuring apparatus;

Fig 3 is a calculated distribution of an electric field penetrating through the opening of the screen electrode;

Fig 4 is a view, partly in section, of an insulator with a mea- suring apparatus according to the invention applied thereto;

Fig 5 is an explanatory sketch of an electric device with three lines formed as busbars with a measuring appa- ratus according to the invention associated with each of the lines; is an explanatory sketch of an electric device for a phase with four measuring apparatuses according to the invention;

is a view, partly in section, of a preferred embodiment of a measuring apparatus with the inner electrode divided into sub-electrodes insulated from one another;

is a partly cut view showing the parts included in the current modified standard cable;

is an axial end view of a sector/pole pitch of a magnetic circuit according to the invention;

is a view showing the electric field distribution around a winding of a conventional power transformer/reactor;

is a perspective view showing an embodiment of a power transformer according to the invention;

is a cross section illustrating a cable structure modified relative to Fig 1 and having several electrical conductors;

is a cross section of a further cable structure comprising several electric conductors but in another arrangement than that in Fig 5;

is a diagrammatical view illustrating an electric plant comprising a measuring apparatus as used for a generator; and

is a diagrammatical view illustrating the measuring apparatus at a transformer. DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a measuring apparatus 10 for directed voltage measurement according to the present invention. The measuring apparatus 10 is intended to measure alternating voltage at insulation distance from a line 22. The line 22 is in absence of insulation which substantially screens the electric field. The measuring apparatus 10 comprises a capacitive sensor 1 1 and a signal converter 13. The capacitive sensor has an inner elect- rode 12 and a screen electrode 14 which surrounds the inner electrode. The screen electrode is provided with an opening 16 intended, during measurement, to be directed towards the line 22. In the example, the two electrodes are made from a conducting material. However, the electrodes may be made as bodies of an arbitrary material as long as their limiting surfaces are conducting. For example, an electrode may be made of a nonconducting material but with a surrounding conducting layer, for example a body of plastic, on which is applied a conducting coating.

In the example, the screen electrode has the shape of a bucket, that is, it has a bottom from which extends a cylindrical or slightly conical border. In the example, the inner electrode has a planar extent in a plane parallel to the opening of the screen electrode. The inner electrode is insulated from the screen electrode and adjustably fixed thereto such that the distance between the inner electrode 12 and the opening 16 of the screen electrode 14 may be adjusted, as indicated by arrow A in Fig 1. In the example, this is made possible by means of an insulating tube 17 which extends through the screen electrode and to which the inner electrode 12 is fixed.

The signal converter 13 is placed near the screen electrode 14 and comprises members for impedance conversion as well as amplification. The signal converter may comprise also members for filtering and digital conversion of the analog signal from the sensor 1 1 . Primarily, the signal is adapted to condition the signal from the sensor, which is very disturbance-prone, into an analog signal or a digital pulse train adapted for transmission of the measured information. Advantageously, the signal converter is provided with a screen connected to ground or, as in the example, enclosed in a space 21 surrounded by a screen 19. The sensor 1 1 and the signal converter 13 are interconnected by means of a conductor 18, which may be screened, running in the tube 17. For evaluation of the signal, the signal converter is connected to an analyzer 15, which may be located at a distance from the measuring apparatus 10. The transmission of the signal may be arranged both electrically and optically, but also in a contactless manner via a transmitter and a receiver.

The capacitive sensor 1 1 is capable of sensing the electric field with a large bandwidth with respect to frequency, usually between just over zero and several thousand Hertz. It is, therefore, advantageous to arrange a broad-band signal converter 13 to the measuring apparatus. The members for impedance conver- sion and amplification comprised in the signal converter are thus preferably arranged by a so-called video amplifier. A plurality of filters with different filter characteristics may also be arranged in the signal converter. Such filters may be bandpass filters or low- or high-pass filters. During measurement, these may each deli- ver a signal, or be sequentially connected.

For special applications, it is advantageous to connect the screen electrode, instead of to ground, to a potential which may be locked in relation to, for example, a phase for increased dy- namics and resolution of the desired measurement quantity. The signal converter 13 may therefore comprise a phase lock circuit (not shown) connected to the screen electrode 14. This circuit, a so-called PLL circuit (Phase Locked Loop), makes it possible, for example in a three-phase system, to lock the screen elect- rode 14 of the measuring apparatus 10 to a potential which varies with the phase intended to be measured. In this way, the effect from the other phases may be suppressed in the output signal from the signal converter 13, which results in increased measurement accuracy.

During measurement with the measuring apparatus 10 for measuring alternating voltage at insulation distance from a high- voltage conductor 22, the measuring apparatus 10 is directed towards the conductor 22. For the inner electrode to be able to sense a drawn part of the electric field, the desired part of the field must be able to fall through the opening in the screen electrode. For this purpose, a conceived axis which passes through the center of the inner electrode and through the mid-point of the opening is directed towards the measured object. Before the actual measurement is started, the measuring apparatus 10 is calibrated in situ. The calibration is carried out by applying a known voltage, whereupon measurement with the measuring apparatus is performed. The measuring apparatus is thus calibrated by adjusting the measured value to correspond to the known voltage. When this has been completed, the actual mea- surement may be started.

One advantage of the measuring apparatus 10 according to the invention is that the measuring apparatus need not be brought into galvanic contact, or even in contact with the line, the vol- tage of which is to be measured. The measurement may instead be performed at insulation distance from the line 22, which means that the measuring apparatus is safe for all those who get into contact therewith. This also implies that no installations need be made in the immediate vicinity of the conductor, and therefore no operational disturbances need be caused by the measuring method. The measuring apparatus has an exceedingly simple design and is therefore very inexpensive to manufacture and is also reliable. The measuring apparatus may be advantageously arranged to constitute a detector, a so-called PD (Partial Discharge) detector, for transients and ionic discharge of a measured object. Because of its reliability, its broad-band design and slight investment cost, the measuring apparatus is exceedingly well suited in connection with energy measurement for debiting of consumed electric energy and when functioning as relay protection but first of all in the applications discussed with assistance of Figs 14 and 15.

Figure 2 shows an alternative embodiment of the measuring apparatus 10 according to the invention. In the same way as in the preceding example, the measuring apparatus comprises a sen- sor 1 1 and a signal converter 13. A screen electrode 14 surrounds an inner electrode 12, only leaving an opening 16 which, during measurement, is directed towards a measured object, i.e. the previously mentioned line 22. In this embodiment, the screen electrode is globular whereas the inner electrode is cup-shaped with the concave side facing the opening. The screen electrode may, however, have an arbitrary shape and be formed of an arbitrary, dense or perforated, conducting material. Likewise, the inner electrode may have an arbitrary shape. It is preferable, however, to arrange the inner electrode with a planar extent which is substantially parallel to the plane of the opening. The sensor 1 1 is not limited, as indicted in the examples, to exhibit a circular shape. A narrower opening gives a fainter signal but greater directional sensitivity. In case of a field-generating line source in the form of a line, it may therefore be advantageous to design the sensor with the opening and the inner electrode, respectively, elongated in a direction coinciding with the line source.

Figure 3 shows a calculation of the distribution of an electric fi- eld penetrating through the opening of a ground-connected screen electrode into an inner electrode 12. The figure shows only part of such a field in the vicinity of the edge of the inner electrode, which is cup-shaped in the figure. The calculation, which has been verified by experiments, shows that that part of the electric field which penetrates through the opening initially has a direction parallel to the normal to the opening. Further in- side the screen electrode, the field lines diverge out towards the inside of the screen electrode and are finally absorbed by the inner electrode 12. The inner electrode 12 has its concave surface directed towards the opening in the screen electrode. The advantage of this geometry is that the field distribution inside the screen electrode becomes more uniform. The design of the cup-shaped inner electrode 12 as a spherically curved plate causes all the field lines to become incident perpendicularly to the plate. This provides additional advantages with a stronger output signal from the sensor while at the same time its screening properties are maintained.

Figure 4 shows an advantageous use of a measuring apparatus 10 according to the invention. In the example shown, the mea- suring apparatus is applied at the end of a hollow insulator 25. The insulator comprises an insulant 26 of porcelain or other insulating material, as well as a first pole 27 and a second pole 28. The first pole is connected to the previously discussed line whereas the second pole is connected to ground. The measuring apparatus is applied to the second pole 28 with its screen electrode 14 connected to the pole and with its inner electrode 12 insulated and adjustably fixed to the screen electrode 14. By the described use, an exact, non-varying distance between the measured object, i.e. the line and the measuring apparatus is ob- tained. This is advantageous since a variation of the distance jeopardizes the accuracy in the voltage measurement.

To further screen unwanted electric fields, the two poles may be provided with screens (not shown) in the form of plates of con- ducting material, extended in the transverse direction of the insulator, which are connected to the respective pole. In the case of hollow insulators with protective gas of the SF₆ type, the smaller insulation distance on the inside of the insulator, which is caused by the gas, may be utilized such that the measuring apparatus 10 in its entirety is housed inside the insulator. An insulator including an apparatus for voltage measurement may thus be manufactured in a simple manner as a finished product.

Figure 5 shows an explanatory sketch of lines formed as bus- bars R, S, and T for three-phase alternating voltage surrounded by an enclosure 29. According to the figure there is arranged a measuring apparatus 10_R, 10_s, 10_τ for each one of the busbars for directed voltage measurement according to the invention. The three measuring apparatuses have been brought together into a common position to achieve a simple installation and wiring, respectively, and each measuring apparatus is directed towards its respective busbar. However, each measuring apparatus may be placed at insulation distance in an arbitrary position within the plant. The common location, however, is an ad- vantage since the greatest possible angle between the sensitivity directions of the measuring apparatuses may then be achieved. With this type of use of the measuring apparatus, among other things, mounting time and space in the enclosure are saved.

It has been indicated above that a voltage measurement with a measuring apparatus according to the invention is sensitive to variations in the distance between the measuring apparatus and the measured object. This fact may be utilized for the purpose of studying a movement of a measured object. A method for measuring voltage from a measured object moving stochastically around a mid-point may thus be achieved in a simple manner. A measurement arrangement which permits, on the one hand, a method for studying the movement of a conductor and, on the other hand, a method for increasing measurement accuracy is shown in Figure 6. Like the previous figure, Figure 6 shows an explanatory sketch of an electric device with one phase only, surrounded by an enclosure 29. Around a centrally placed busbar 33, four measuring apparatuses 10a, 10b, 10c, 10d accor- ding to the invention are each arranged in a corner of a cross section of the enclosure. The converted signal from each one of the measuring apparatuses is preferably analyzed by a four- channel analyzer (not shown). To study the movement, the signals are compared, and to increase the measurement accuracy, the average value of the signals is formed.

By geometrical calculations of standard type from the measurement signals, the position of the busbar is determined, whereby a measurement of the voltage is corrected for the change in position such that a correct measurement value may be arrived at by calculation. The method may also be used for detecting and correcting the measurement for changes in the electric background field, which provides unsymmetrical changes in the measured signals. A greater measurement accuracy is obtained by introducing a plurality of measuring apparatuses which carry out measurement on the same object. The measuring methods described are not limited to be applied to enclosed electric devices only, but may also be applied to free conductors and to non- enclosed devices. In a non-enclosed device, it is particularly valuable to be able to correct the measurement value for changes in the background field.

An alternative embodiment of a sensor 1 1 included in a measuring apparatus according to the invention is shown in Figure 7. The sensor 1 1 comprises a screen electrode 14 which is pro- vided with an opening 16 and which surrounds a first inner sub- electrode 12a and a second inner sub-electrode 12b, which are insulated from each other and which are each adjustably fixed to the screen electrode. In the shown case, the opening 16 is limited by a ring 34 of a conducting material, connected to the screen electrode, the task of which is to equalize the electric field so as to prevent corona. In the example shown, the inner sub-electrodes are equally large and preferably have the shape of half a round plate, such that the sub-electrodes together exhibit the shape of a full round plate. The inner sub-electrodes are each connected to a respective signal converter (not shown), and, in the same way as in example 1 , the measure- ment signals are each transmitted to a respective analyzer, or to a common multi-channel analyzer.

By dividing the inner electrode into sub-electrodes, the above- mentioned sensitivity to distance dependence may be further utilized. The electrodes are preferably adjusted to exhibit the same capacitance, whereby, by means of a bridge circuit, a comparison between the converted signals from each one of the sub-electrodes may be determined, on the one hand, whether the measured object is in front of the sensor and, on the other hand, whether the measured object moves during the measurement. An electric field generated from another unwanted object is detected by dividing the inner electrode by comparisons of the measurements in an analyzer and may thus be eliminated from the measurement result. According to the invention, the inner electrode is divided into an arbitrary number of sub-electrodes. For example in case of the elongated sensor described above, it is advantageous to form the sub-electrodes as a plurality of plates arranged side-by-side.

An important condition for being able to manufacture a magnetic circuit primarily intended with the invention, is to use for the winding a conductor cable with a solid electrical insulation with an inner semiconducting layer between the insulation and one or more electrical conductors located inwardly thereof and with an outer semiconducting layer located outwardly of the insulation. Such cables are available as standard cables for other power engineering fields of use, namely power transmission. To be able to describe an embodiment, initially a short description of a standard cable will be made. The inner current-carrying conductor comprises a number of non-insulated strands. Around the strands there is a semiconducting inner layer or casing. Around this semiconducting inner layer, there is an insulating layer of solid insulation. The solid insulation is formed by a polymeric material with low electrical losses and a high breakthrough strength. As concrete examples polyethylene (PE) and then par- ticularly cross-linked polyethylene (XLPE) and ethylene-propylene (EP) may be mentioned. Around the outer semiconducting layer a metal shield and an outer insulation casing may be provided. The semiconducting layers consist of a polymeric mate- rial, for example ethylene-copolymer, with an electrically conducting constituent, e. g. conductive soot or carbon black. Such a cable will be referred to hereunder as a power cable.

A preferred embodiment of a cable intended for a winding in a rotating electrical machine appears from Fig 8. The cable 41 is described in the figure as comprising a current-carrying conductor 42 which comprises transposed both non-insulated and insulated strands. Electromechanically transposed, solidly insulated strands are also possible. These strands may be stran- ded/transposed in a plurality of layers. Around the conductor there is an inner semiconducting layer 43 which, in turn, is surrounded by a homogenous layer of a solid insulation material. The insulation 44 is entirely without insulation material of liquid or gaseous type. This layer 44 is surrounded by an outer semi- conducting layer 45. The cable used as a winding in the preferred embodiment may be provided with metal shield and external sheath but must not be so.

To avoid induced currents and losses associated therewith in the outer semiconducting layer 45, this is cut off, preferably in the coil end, that is, in the transitions from the sheet stack to the end windings. The cut-off is carried out such that the outer semiconducting layer 45 will be divided into several parts distributed along the cable and being electrically entirely or partly separated from each other. Each cut-off part is then connected to ground, whereby the outer semiconducting layer 45 will be maintained at, or near, ground potential in the whole cable length. This means that, around the solidly insulated winding at the coil ends, the contactable surfaces, and the surfaces which are dirty after some time of use, only have negligible potentials to ground, and they also cause negligible electric fields. To optimize a rotating electric machine, the design of the magnetic circuit as regards the slots and the teeth, respectively, are of decisive importance. As mentioned above, the slots should connect as closely as possible to the casing of the coil sides. It is also desirable that the teeth at each radial level are as wide as possible. This is important to minimize the losses, the magnetization requirement, etc., of the machine.

With access to a conductor for the winding such as for example, the cable described above, there are great possibilities of being able to optimize the magnetic core from several points of view. In the following, a magnetic circuit in the stator of the rotating electric machine is referred to. Figure 9 shows an embodiment of an axial end view of a sector/pole pitch 46 of a machine ac- cording to the invention. The rotor with the rotor pole is designated 47. In conventional manner, the stator is composed of a laminated core of electric sheets successively composed of sector-shaped sheets. From a back portion 48 of the core, located at the radially outermost end, a number of teeth 49 ex- tend radially inwards towards the rotor. Between the teeth there are a corresponding number of slots 50. The use of cables 51 according to the above among other things permits the depth of the slots for high-voltage machines to be made larger than what is possible according to the state of the art. The slots have a cross section tapering towards the rotor since the need of cable insulation becomes lower for each winding layer towards the rotor. As is clear from the figure, the slot substantially consists of a circular cross section 52 around each layer of the winding with narrower waist portions 53 between the layers. With some justification, such a slot cross section may be referred to as a "cycle chain slot". Since there will be required, in such a high voltage machine, a relatively large number of layers and the access to relevant cable dimensions concerning insulation and outer semiconductor are concerned is restricted, it may in prac- tice be difficult to achieve a desired continues reduction of the cable insulation and the stator groove respectively. In the em- bodiment shown in Figure 9, cables with three different dimensions of the cable insulation are used, arranged in three correspondingly dimensioned sections 54, 55 and 56, that is, in practice a modified cycle chain slot will be obtained. The figure also shows that the stator tooth 49 can be shaped with a practically constant radial width along the depth of the whole slot.

In an alternative embodiment, the cable which is used as a winding may be a conventional power cable as the one described above. The grounding of the outer semiconducting layer 45 then takes place by stripping the metal shield and the sheath of the cable at suitable locations.

The scope of the invention accommodates a large number of al- ternative embodiments, depending on the available cable dimensions as far as insulation and the outer semiconductor layer etc. are concerned. Also embodiments with so-called cycle chain slots can be modified in excess of what has been described here.

As mentioned above, the magnetic circuit may be located in the stator and/or the rotor of the rotating electric machine. However, the design of the magnetic circuit will largely correspond to the above description independently of whether the magnetic circuit is located in the stator and/or the rotor.

As winding, a winding is preferably used which may be described as a multilayer, concentric cable winding. Such a winding means that the number of crossings at the coil ends has been minimized by placing all the coils within the same group radially outside one another. This also permits a simpler method for the manufacture and the threading of the stator winding in the different slots. Since the cable used according to the invention is relatively easily flexible, the winding may be obtained by a com- paratively simple threading operation, in which the flexible cable is threaded into the openings 52 present in the slots 50. Figure 10 shows a simplified and fundamental view of the electric field distribution around a winding of a conventional power transformer/reactor, where 57 is a winding and 58 a core and 59 illustrates equipotential lines, that is, lines where the electric field has the same magnitude. The lower part of the winding is assumed to be at ground potential.

The potential distribution determines the composition of the insulation system since it is necessary to have sufficient insulation both between adjacent turns of the winding and between each turn and ground. The figure thus shows that the upper part of the winding is subjected to the highest insulation loads. The design and location of a winding relative to the core are in this way determined substantially by the electric field distribution in the core window.

The cable which can be used in the windings contained in power transformers/reactors according to the invention have been described with assistance of Fig 8. The cable may, as stated before, be provided with other, additional outer layers for special purposes, for instance to prevent excessive electrical strains on other areas of the transformer/reactor. From the point of view of geometrical dimension, the cables in question will have a conductor area which is between 2 and 3000 mm2 and an outer cable diameter which is between 20 and 250 mm.

The windings of a dry power transformer/reactor manufactured from the cable described under the summary of the invention may be used both for single-phase, three-phase and polyphase transformers/reactors independently of how the core is shaped. One embodiment is shown in Figure 1 1 which shows a three- phase laminated core transformer. The core comprises, in conventional manner, three core limbs 60, 61 and 62 and the retaining yokes 63 and 64. In the embodiment shown, both the core limbs and the yokes have a tapering cross section. Concentrically around the core limbs, the windings formed with the cable are disposed. As is clear, the embodiment shown in Figure 1 1 has three concentric winding turns 65, 66 and 67. The innermost winding turn 65 may represent the primary winding and the other two winding turns 66 and 67 may represent secondary windings. In order not to overload the figure with too many details, the connections of the windings are not shown. Otherwise the figure shows that, in the embodiment shown, spacing bars 68 and 69 with several different functions are disposed at certain points around the windings. The spacing bars may be formed of insulating material intended to provide a certain space between the concentric winding turns for cooling, bracing, etc. They may also be formed of electrically conducting material in order to form part of the grounding system of the windings.

ALTERNATIVE CABLE DESIGNS

In the cable variant illustrated in Fig 12, the same reference characters as before are used, only with the addition of the letter a characteristic for the embodiment. In this embodiment the cable comprises several electric conductors 42a, which are mutually separated by means of insulation 44a. Expressed in other words, the insulation 44a serves both for insulation bet- ween individual adjacent electrical conductors 42a and between the same and the surrounding. The different electrical conductors 42a may be disposed in different manners, which may provide for varying cross-sectional shapes of the cable in its entirety. In the embodiment according to Fig 13 it is illustrated that the conductors 42a are disposed on a straight line, which involves a relatively flat cross-sectional shape of the cable. From this it can be concluded that the cross-sectional shape of the cable may vary within wide limits.

In Fig 12 there is supposed to exist, between adjacent electrical conductors, a voltage smaller than phase voltage. More specifi- cally, the electrical conductors 42a in Fig 12 are supposed to be formed by different revolutions in the winding, which means that the voltage between these adjacent conductors is comparatively low.

As before, there is a semiconducting outer layer 45a exteriorly of the insulation 44a obtained by a solid electrical insulation material. An inner layer 43a of a semiconducting material is arranged about each of said electrical conductors 42a, i.e. each of these conductors has a surrounding inner semiconducting layer 43a of its own. This layer 43a will, accordingly, serve potential equalizing as far as the individual electrical conductor is concerned.

The variant in Fig 13 uses the same reference characters as before only with addition of the letter b specific for the embodiment. Also in this case there are several, more specifically three, electrical conductors 42b. Phase voltage is supposed to be present between these conductors, i.e. a substantially higher voltage than the one occurring between conductors 42a in the embodiment according to Fig 12. In Fig 13 there is an inner semiconducting layer 43b inwardly of which the electrical conductors 42b are arranged. Each of the electrical conductors 42b is, however, enclosed by a further layer 70 of its own, with proper- ties corresponding to the properties discussed hereinabove with regard to the inner layer 43b. Between each further layer 70 and the layer 43b arranged thereabout, there is insulation material. Accordingly, the layer 43b will occur as a potential equalizing layer outside the further layers 70 of semiconducting material belonging to the electrical conductors, said further layers 70 being connected to the respective electrical conductor 42b to be placed on the same potential as the conductor.

Fig 14 illustrates diagrammatically how a generator 71 is con- nected to a distribution or transmission network denoted 77. The generator 71 is designed in accordance with the invention in the sense that it comprises a winding formed as a flexible cable. This cable extends from the generator to a cable termination 73, in which the conductor of the cable transfers into what is here denominated "electric line". The measuring apparatus according to the invention and denoted 74 is adapted to sense the voltage of this line denoted 78, which may have the character of a flexible conductor or rail. In the measuring area of the measuring apparatus 74, the line 78 must be in absence of such an insulation system which entirely of substantially entirely screens off the electric field around the line. It is pointed out that the line 78 also could be formed by such a portion of the flexible conductor which is liberated from insulation system and which is comprised in the cable forming the winding of the generator 71 . A surge arrester 75 for diverting overvoltages is introduced into the con- nection between the generator 71 and the network 77. Furthermore, there is provided a breaker 76 in said connection, said breaker preferably including or being supplemented with a disconnector function.

It also appears from Fig 14 that the measuring apparatus 74 is connected to a control unit 79. Thus, the measuring apparatus 74 delivers to the control unit 79 information about the actual voltage, including harmonics and possible transients. A current sensing component, e.g. a current transformer, is coupled into the connection between the generator and the network 77 and is connected to the control unit 79 for delivering thereto information about the actual current magnitude. The control unit 79 is connected to one or more constituents 80 comprised in the plant for elimination or at least reduction of occurring voltage related deficiencies, such as transients and harmonics. Said constituents 80 are suitably formed by one or more filters having the ability of filtering off harmonics and other transients. Since the control unit 79 also obtain information about the actual current, the control unit may also be caused to eliminate or at least re- duce current related deficiencies. In the variant illustrated in Fig 15, the difference is mainly only that instead a transformer 81 is connected to the distribution or transmission network 77. As before the high voltage cable forming the secondary winding of the transformer 81 transfers at the cable termination 73 into a line 78 which in any case is not entirely screened as far as electric field is concerned. Suitably, the line 78 is entirely or substantially entirely without insulation. Harmonics arising in the transformer are detected by means of the measuring apparatus 74. The occurrence of harmonics indi- cates for instance that the magnetic flux in the core of the transformer is so great that the core has been saturated or that there is a risk therefor. Accordingly, the control unit 79 is adapted to control, based upon the measuring signals of the measuring apparatus 74, one or more constituents comprised in the plant for elimination or at least reduction of occurring voltage related deficiencies, such as harmonics, when there is a need therefor. This control may for instance occur via a component 82 adapted to control operation of the plant so that the transformer 81 obtains reduced magnetic flux and, thus, so that the harmo- nics generated are eliminated or at least reduced.

POSSIBLE MODIFICATIONS

It is evident that the invention is not only limited to the embodi- ments discussed above. Thus, the man skilled within this art will realize that a number of detailed modifications are possible when the basic concept of the invention has been presented without deviating from this concept as it is defined in the enclosed claims. As an example, it is pointed out that the invention is not only restricted to the specific material selections exemplified above. Functionally equal materials may, accordingly, be used instead. As far as the manufacturing of the insulation system according to the invention is concerned, it is pointed out that also other techniques than extrusion and spraying are possible as long as intimacy between the various layers is achieved. Furthermore, it is pointed out that additional equipotential layers could be arranged in the insulation between those layers designated as "inner" and "outer" hereinabove. As already pointed out above, the measuring apparatus according to the invention may be subjected to numerous modifications within the scope of the enclosed claims as long as the apparatus is capable of fulfilling the basic requirements imposed on it.

Claims

1 . An electric plant comprising an electric device (71 , 81 ) connected to an electric distribution or transmission network (77), said electric device having a magnetic circuit and at least one windinOg, and a measuring apparatus (10)adapted to supervise the electric device, characterized in that the measuring apparatus (10) comprises at least one capacitive sensor (1 1 ) including an inner electrode (12) and a screen elect- rode (14), which is connected to ground or a potential different from a potential of a measured object and which screens the inner electrode (12) from disturbing electric fields, that the sensor (1 1 ) is adapted to execute, by means of the inner electrode (12), voltage measurement by sensing such a part of an electric field which penetrates to the inner electrode, and that the sensor is directed towards an electric line (78) forming the measured object and connecting the electric device (71 , 81 ) with the distribution or transmission network (77) so as to sense, at insulation distance, the voltage, including transients and harmonics, of this electric line (78) via the electric field surrounding the line.

2. A plant according to claim 1 , characterized in that the potential of the screen electrode (14) is variable.

3. A plant according to claim 1 or 2, characterized in that the screen electrode (14) and the inner electrode (12) are insulated from each other by a gaseous dielectric.

4. A plant according to any of claims 1 -3, characterized in that the screen electrode (14) surrounds the inner electrode (12) and is provided with an opening (16), through which the inner electrode is exposed to the electric field to be measured.

5. A plant according to any of claims 1 -4, characterized in that the inner electrode (12) is adjustably fixed in the screen electrode (14).

6. A plant according to claim 4, characterized in that the inner electrode (12) has a substantially planar extent, the plane of extent of which is substantially perpendicular to a normal to the opening (16) in the screen electrode (14).

7. A plant according to any of the preceding claims, characterized in that the inner electrode (12) is cup-shaped.

8. A sensor according to any of the preceding claims, characterized in that the inner electrode (12) comprises a plurality of sub-electrodes (12a, 12b) electrically insulated from each other.

9. A plant according to any preceding claim, characterized in that the measuring apparatus comprises a signal converter (13) which is arranged close to the sensor (1 1 ) and comprises members for amplification and impedance conversion of the measurement signal.

10. A plant according to claim 9, characterized in that the signal converter (13) includes members for phase locking the screen electrode (14) and/or the inner electrode (12).

1 1. A measuring apparatus according to any preceding claim, characterized in that the measuring apparatus (10) is ar- ranged at the grounded part of an insulator.

12. A plant according to any preceding claim, characterized in that a control unit (79) is connected to the measuring apparatus and adapted to control, based upon the measurement signals of the measuring apparatus, one or more constituents

(80, 82) comprised in the plant for eliminating or at least re- ducing occurring voltage related deficiencies, such as transients and harmonics, when there is a need therefor.

13. A plant according to claim 12, characterized in that said one or more consituents controlled by the control unit (79) comprise at least one filter (80) for filtrating away harmonics and/or transients.

14. A plant according to any preceding claim, characterized in that the winding, or a part thereof, of the electric device (71 ,

81 ) comprises at least one flexible electric conductor (42) having a casing (43, 44, 45) which is magnetically permeable but capable of substantially enclosing the electric field arising around the conductor (42).

15. A plant according to claim 14, the casing (43, 44, 45) comprising an insulation system, characterized in that the insulation system comprises an insulation (44) formed by a solid insulation material and exteriorly of the insulation an outer layer (45) which has an electric conductivity which is higher than that of the insulation to make the outer layer capable, by connection to earth or otherwise a relatively low potential, of operating to equalize potential and substantially enclose the electric field, arising as a consequence of said electric conductor (42), inwardly of the outer layer (45).

16. A plant according to claim 14, the casing (43, 44, 45) comprising an insulation system, characterized in that the insulation system comprises an electrical insulation (44) formed by a solid insulation material, that interiorly of the insulation

(44) there is arranged an inner layer (43), that said at least one electric conductor (42) is located interiorly of the inner layer (43) and that the inner layer has an electric conductivity which is lower than the conductivity of the electric con- ductor but sufficient for the inner layer to operate for equali- zation of potential and accordingly for equalization with respect to the electric field exteriorly of the inner layer (43).

17. A plant according to any of claims 15 or 16, characterized in that the inner and outer layers (43, 45) and the solid insulation present substantially similar thermal properties.

18. A plant according to any of claims 14-17, characterized in that said at least one conductor (42) forms at least one in- duction turn.

19. A plant according to any of claims 15-18, characterized in that the inner and/or outer layer (43, 45) comprises a semiconducting material.

20. A plant according to any of claims 15-19, characterized in that the inner layer (43) and/or the outer layer (45) has a resistivity in the range 10"⁶ ╬⌐cm-100 k╬⌐cm, suitably 10"³-1000 ╬⌐cm, preferably 1 -500 ╬⌐cm.

21 . A plant according to any of claims 15-20, characterized in that the inner layer (43) and/or the outer layer (45) has a resistance which per length meter of the conductor/insulation system is in the range 50 ╬╝╬⌐ - 5 M╬⌐.

22. A plant according to any of claims 15-21 , characterized in that the solid insulation (44) and the inner layer (43) and/or the outer layer (45) are formed by polymeric materials.

23. A plant according to any of claims 15-22, characterized in that the inner layer (43) and/or the outer layer (45) and the solid insulation (44) are rigidly connected to each other over substantially the entire interface to maintain adhesion between the respective layers and the solid insulation on tempe- rature changes and flexing of the conductor (42) and its insulation system.

24. A plant according to any preceding claim, characterized in that the inner layer (43) and/or the outer layer (45) and the solid insulation (44) are formed by materials presenting substantially equal thermal coefficients of expansion.

25. A plant according to any of claims 15-24, characterized in that the conductor (42) and its insulation system constitutes a winding formed by means of a flexible cable (41 ).

26. A plant according to any of claims 15-25, characterized in that the inner layer (43) is in electric contact with the at least one electric conductor (42).

27. A plant according to claim 26, characterized in that said at least one electric conductor (42) comprises a number of strands and that at least one strand of the electric conductor

(42) is at least in part uninsulated and arranged in electric contact with the inner layer (43).

28. A plant according to any preceding claim, characterized in that the conductor (42) and its insulation system is designed for high voltage, suitably in excess of 10 kV, in particular in excess of 36 kV and preferably more than 72,5 kV.

29. A plant according to any preceding claim, characterized in that it is constituted of a rotating electric machine.

30. A plant according to claim 29, characterized in that the magnetic circuit thereof is arranged in the stator and/or rotor of the machine.

31 . A plant according to any of claims 29-30, characterized in that the magnetic circuit comprises one or more magnetic cores (48) having slots (50) for the winding (41 ).

32. A plant according to any of claims 29-31 , characterized in that it is constituted of a generator, motor or synchronous compensator.

33. A machine according to any of claims 29-32, characterized in that it is directly connected to a power network for high voltage, suitably 36 kV and more, without intermediate transformer.

34. A plant according to any of claims 1 -28, characterized in that it is constituted by a power transformer/reactor.

35. A plant according to any of claims 1 and 15-34, characterized in that the casing/insulation system of the winding (41 ), which comprises at least one flexible conductor (42), of the electric device (71 , 81 ) is terminated, in a direction towards the distribution or transmission network, in a cable termination (73) and that the electric line (78), to which the measuring apparatus (10) is directed, is formed by a line bet- ween the cable termination (73) and the network (77).

36. A method for supervising, in an electric plant comprising an electric device (71 , 81 ) connected to an electric distribution or transmission network (77), said electric device having a magnetic circuit and at least one winding (41 ), the electric device by means of a measuring apparatus (10), characterized in that said supervising is carried out by means of said measuring apparatus (10) comprising at least one capacitive sensor (1 1 ) including an inner electrode (12) and a screen electrode (14), which screens the inner electrode from disturbing electric fields, that voltage measurement, by means of the inner electrode (12) of the sensor, is carried out by sensing such a part of the electric field which penetrates into the inner electrode, and that the sensor is directed tow- ards an electric line (78) connecting the electric device (71 ,

81 ) with the distribution or transmission network (77) so as to sense, at insulation distance, the voltage, including transients and harmonics, of this electric line (78) via the electric field surrounding the line.

37. A method according to claim 36, characterized in that a member for phase locking is connected to the sensor (1 1 ), the potential of the screen electrode (14) and/or the inner electrode (12) being locked relative to the line by means of said member so that influence from disturbing electric fields is subpressed.

38. A method according to claims 36 or 37, characterized in that the inner electrode (12) is caused to comprise a plurality of sub-electrodes (12a, 12b), and that a change of the incident electric field caused by a movement of the line is detected by comparing the signals from the sub-electrodes.