SE512698C2 - Electric system with capacitive contact-free voltage measurement and control unit for reducing harmonics - Google Patents

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

lillll 10 15 20 25 30 35 512 698 Harmonics in a power grid can cause a number of problems, partly for the equipment connected to the power grid, partly for the harmonic generating objects themselves. These problems are transmission losses, resonances and overheating.

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

The voltage measuring devices currently available are often expensive and complicated devices that must be placed in contact with the live device. Examples of such solutions are voltage transformers. These solutions involve large investments and thus mean a large deviation from the low investment that is desirable. A further problem with known voltage measuring devices is that they are often limited to measuring at the system frequency. Thus, these known devices are not intended to detect harmonic voltages present on the electric power grid. The harmonic problem is often directly related to how grounded devices are grounded.

According to a first aspect of the invention, this refers to a rotating electric machine. Such electrical machines include synchronous machines which are mainly used as generators for connection to distribution and transmission networks, hereinafter collectively referred to as power networks. The synchronous machines are also used as motors as well as for phase compensation and voltage regulation, then as mechanically idling machines. The technical field also includes double-feed machines, machines of the asynchronous converter cascade type, outer pole machines, synchronous flow machines and asynchronous machines.

According to another aspect of the invention, said electrical device is constituted by a power transformer or reactor. Transformers include all transmission and distribution of electrical energy. Their task is to allow the exchange of electrical energy between two or more electrical systems and for this electromagnetic induction is used in a manner well known per se. The transformers primarily referred to in the present invention belong to the so-called power transformers with a rated power from a few hundred kVA up to over 1000 MVA with a rated voltage of 3-4 kilovolts and up to very high transmission voltages, 400 kilovolts to 800 kilovolts or higher.

Although the following description of the state of the art with regard to the second aspect mainly relates to power transformers, the invention is also intended to be applicable to reactors which can be made single-phase and three-phase. In the case of insulation and cooling, there are in principle the same embodiments as for transformers.

There are thus air-insulated and oil-insulated, self-cooled, pressure-oil-cooled, etc. reactors. Although reactors have a winding (per phase) and can be designed both with and without a magnetic core, the description of the state of the art is largely relevant for reactors as well.

One possible method of limiting thirteenth voltages at generator plants is to connect the phase voltages of the generator to an A / Y-0-connected transformer (a so-called "step-up transformer"). The thirteenth voltages generated by the generator provide a thirteen-tone current via the zero point resistor to earth.

The third tone voltage is superimposed on the phase voltages of the A-winding but cannot drive any third tone current. This means that no third-degree current is detected on the Y-side of the transformer, ie on the power grid side. However, this requires that the “step-up transformer” is dimensioned for full power and that an overvoltage protection is installed on the input side of the said transformer.

In addition, current measuring devices, switches and disconnectors are required.

This solution has proved unsatisfactory, especially with regard to higher voltages. It is pointed out here that in the case of the special type of rotating electric machine which will be described in more detail below, conditions are created for completely omitting the so-called "step-up transformer" so that consequently the The rotating electric machine is connected directly to an excessively high voltage, suitably 36 kV and above, of designed electric power grid without an intermediate transformer. A problem with such a direct connection, however, is that the occurrence of harmonics tends to increase. It thus becomes even more essential to detect such harmonics. Also in the case of transformers and reactors, a new technology will be described in the following in which the windings are formed by means of a high-voltage cable including an electrical conductor with a housing which is magnetically permeable but suitable for substantially enclosing the electric field around the conductor. In such a transformer there are possibilities to increase the magnetic flux in the core up to higher levels than those possible in conventional transformers.

It is the filter action of the high voltage cable that makes this possible. An increased magnetic flux means that the magnetic core can be made smaller with maintained power out of the transformer. This in turn means that the transformer becomes more compact, lighter and also cheaper. A problem with increasing the magnetic flux, however, is that the core can saturate, which means that harmonics are generated. It would be desirable to be able to control the magnetic flux to avoid these harmonics.

OBJECT OF THE INVENTION The present invention aims to provide ways of overcoming the problems which arise with harmonic voltages on the mains.

The invention relates in particular to the provision of paths to be able to easily and accurately monitor and measure the voltage at the electrical devices where harmonics are generated. The monitoring and measurement are performed to be able to determine in which way the harmonic voltages can be suppressed or filtered out.

SUMMARY OF THE INVENTION To fulfill this object, the present invention provides that the measuring device comprises at least one capacitive sensor including an inner electrode and a shield electrode which is connected to ground or otherwise a potential different from the potential of the measuring object. and which shields the inner electrode from interfering electric fields, that the sensor is arranged to perform voltage measurement by means of the inner electrode by sensing a part of an electric field penetrating into the inner electrode and that the sensor is directed towards an electric wire forming the measuring object which connects it the electrical device with the distribution or transmission network to detect the voltage, including transients and harmonics, of this electrical line via the surrounding electric field at isolation distances.

Such a measuring device has the advantage that the voltage can be measured at a distance from the electrical line, which considerably facilitates the arrangement of the measuring device. The design of the measuring device also means that it is flexible, has a simple structure and low production cost. Furthermore, conditions are created for measuring the voltage of the electrical device within a large frequency range. It is especially pointed out that it is not only a lower cost but also elimination of operational disturbances to be able to arrange the measuring device at a distance from the electrical device to be voltage measured instead of placing the measuring device in contact with the electrical line in prior art. question.

A special advantage of the measuring device according to the invention is that it is able to effectively shield unwanted electric fields and can easily be directed towards the electrical line which is to be measured in terms of voltage magnitude or frequency or both.

A high voltage conductor is surrounded by an electric field that carries information about the potential of the conductor, its change and its frequency content. A capacitive sensor comprising two mirror-symmetrical electrodes inserted into this electric field can sense these quantities. One problem, however, is that such a capacitive sensor is sensitive to changes in all directions of the electric field. Thus, other field-generating objects can also 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. As the distribution of electric power is usually carried out in three adjacent conductors, it is thus not possible with such a sensor to delimit the field originating from one of the conductors.

The invention relates to a capacitive sensor with two electrodes arranged for sensing changes in a directional part of an electric field. This sensor is also arranged to detect transients and ionic discharge from the electrical device in such an electric field. According to the invention, a directed part of an electric field is sensed by shielding the other electrode from unwanted electric fields with an electrode connected to ground or other controllable potential. A sensor intended for this purpose is arranged with one electrode predominantly enclosing the other and connected to earth or to another controllable potential. In the enclosing electrode, which in the following text is called a shield electrode, an opening is arranged, through which a directed subset of the electric field reaches the enclosed electrode, which in the following text is called an inner electrode. All other directed subsets of the electric field are effectively obstructed by the shield electrode. In a preferred embodiment, the electrodes are insulated from each other with a gaseous dielectric, the capacitance formed by the electrodes becoming insensitive to temperature variations. Thus, the sensor can be configured and its capacitance measured in the laboratory and then used in other environments without having to recalibrate. The sensor can also be used for a long time at varying temperatures, whereby no correction needs to be made. In order to increase the sensitivity or enhance the directional action of the sensor, the inner electrode can be divided into mutually insulated sub-electrodes, which can be placed both laterally and vertically.

The invention thus relates to a measuring apparatus, including the sensor described above, for measuring voltage at the insulation distance of an electric device in an electric field with several field-generating components. The measuring device is set up by connecting a signal converter to the sensor, whereby, when the shield electrode is connected to earth, a measuring device is obtained, with which a directed subset of an electric field can be measured. This measuring device is a simple, inexpensive and reliable equipment for measuring AC voltage at a distance from a line. The device is wide-band, which allows that within a large frequency range even the presence and size of tone components of the electrical device intended for measurement can be easily measured.

The signal converter includes means for impedance conversion, amplification, and may also include means for filtering and digital conversion of the measurement signal. The signal converter is placed at a short distance from the sensor itself and in a preferred embodiment of the invention it is integrated with the sensor. The converted, analog or digital, signal can then be transmitted to an analyzer via an electrical or optical medium or transmitted contactlessly via transmitter and receiver.

When applying voltage measurement with the measuring device, instead of to earth, the shield electrode can be connected to a controllable potential, whereby by phase welding to one of the phases a greater dynamics and resolution of the measurement can be obtained. In another preferred embodiment of the invention, the inner electrode is instead connected to a phase welding circuit, whereby the contribution from an undesired field-generating source can be suppressed. The signal converter is then also brought to include a conductor arranged for signals in the opposite direction. Likewise, when using filtering of the sensed signal, the signal converter may include a plurality of conductors for transmitting different filtered signals to a multi-channel analyzer.

The measuring apparatus according to the invention has a wide range of applications, especially in connection with AC voltage measurement in high-voltage devices including rotary electrical machines and transformers / reactors. In a preferred use of the measuring device, it is placed at an insulating distance from each line belonging to the respective phase for measuring alternating voltage. In the case of three-phase systems, the measuring devices can be placed at a common point and individually directed towards separate wires belonging to the electrical device.

The measuring method is sensitive to variations in the distance between a measuring object and the measuring device. Usually, however, this is not a problem as such variations, seen over a longer period of time, tend to become negligible. It is advantageous to place the measuring device at places where the position of the measuring object is fixed, for example at attachments or suspension points where the distance variation is minimal.

In the case of electrical devices with only one phase, the accuracy is easily increased by placing a number of measuring devices around the conductor.

In a use with only one phase, the accuracy of the voltage measurement can be increased by placing four measuring devices rotationally symmetrically around an extended part of the line. Since the measuring devices are placed at equal distances from the line, the correct value of the voltage is obtained from the average value of the four measuring devices.

The sensitivity to variations in the distance between the measuring object and the measuring device can be used to detect movement of the measuring object.

By placing at least three measuring devices in fixed positions around a high-voltage line, it can be detected whether the line is being moved and in which direction this movement is taking place. 10 15 20 25 30 35 512 698 A stable measuring distance to a high-voltage electrical line is achieved by arranging the measuring device and the line at each end of an insulator, which can be hollow. In this way, the length of the insulator is used to constitute a non-varying measuring distance. The measuring device can be placed both outside and inside the insulator body.

Preferred areas of use include hanging insulators, especially for pipe poles, and support insulators.

A measuring device according to the invention is also suitable for having relay protection functionalities controlled in connection with distribution networks, whereby the device can condense the measuring points and thereby increase the selectivity due to its low investment cost. The measuring device is also suitable for measuring voltage at a line in connection with charging for energy consumption. For this, the measurement must be combined with a current measured in another way through the line, whereby the electrical energy that passes can be determined.

As already stated above, the measuring apparatus according to the invention is very well suited for measuring the voltage, including harmonics, in connection with such an electrical device where the high voltage winding comprises a flexible conductor with a housing which is magnetically permeable but capable of functioning enclosing with respect to the electric field generated by the conductor.

Said casing thus comprises an insulation system.

In a broad sense, it is stated that the design according to the invention by creating the possibility to contain mainly the electric field arising due to said electric conductor in the insulation system reduces losses which occur so that consequently the device can operate with a higher efficiency.

The reduction of the losses in turn gives rise to a lower temperature in the device, which reduces the need for cooling and means that any existing cooling devices can be designed more easily than in the absence of the invention. 10 15 20 25 30 35 512 698 10 The conductor / insulation system according to the invention can be realized as a flexible cable, which entails significant advantages in terms of manufacture and assembly compared with hitherto conventional rigid windings in prefabricated form. The insulation system used according to the invention also means a lack of gaseous and liquid insulation media.

With regard to the invention in its form of a rotating electric machine, conditions are thus created for operating the machine with such a high voltage that the previously mentioned "step up" transformer can be excluded. The machine can thus be operated with significantly higher voltage than machines according to the state of the art for making direct connection to electric power grids. This entails significantly lower investment costs for systems with a rotating electric machine and the overall efficiency of the system can be increased. The invention eliminates the need for special field control measures at certain areas of the winding, which field control measures have been necessary according to prior art. A further advantage is that the invention makes it easier to achieve under- and over-magnetization in order to reduce reactive effects occurring when voltage and current are out of phase with each other.

As far as the invention aspect as a power transformer / reactor is concerned, the invention eliminates above all the need for oil filling of the power transformers and the consequent problems and disadvantages.

The design of the winding so that it along at least a part of its length comprises an insulation formed of a solid insulating material, inside this insulation an inner layer and outside the insulation an outer layer with these inner and outer layers of semiconducting material creates the possibility to contain the electric field in the entire device within the winding. By the term "solid insulating material" as used herein is meant that the winding should lack liquid or gaseous insulation, for example in the form of oil. Instead, the insulation is intended to be formed of a polymeric material. The inner and outer layers are also formed of a polymeric material, however a semiconducting one.

The inner layer and the solid insulation are firmly connected to each other over substantially the entire interface with such an adhesion that a change in temperature is not able to separate the layer and the insulation. The outer layer and the solid insulation are also firmly connected to each other over substantially the entire interface therebetween with such adhesion. The inner layer functions as a potential equalizer and thus as a result of the electric field outside the inner layer as a consequence of its semiconducting properties. The outer layer is likewise intended to be formed of a semiconducting material and has at least an electrical conductivity which is higher than that of the insulation so that the outer layer, by connection to earth or otherwise relatively low potential, is able to function as a potential equalizer and to contain the electric field arising due to said electric conductor inside the outer layer. On the other hand, the outer layer should have a resistivity sufficient to minimize the electrical losses therein.

The solid connection between the insulation material and the inner and outer semiconducting layers must be so uniform over substantially the entire interface that no cavities, pores or the like occur.

At the high voltage levels referred to in the invention, the electrical and thermal load that can arise will place extremely high demands on the insulation material. It is known that so-called partial discharges, PD, generally pose a serious problem for insulation materials at high voltage installations. If cavities, pores or the like occur, internal corona discharges can occur at high electrical voltages, whereby the insulation material gradually breaks down and this can eventually lead to electrical penetration through the insulation. This can lead to serious breakdowns of the electrical device. the insulation should thus be homogeneous. 10 15 20 25 30 35 512 698 12 The inner layer inside the insulation shall have an electrical conductivity which is lower than that of the electrical conductor but sufficient for the inner layer to function as a potential equalizer and thus equalizer with respect to the electric field outside the inner layer. This in combination with the fixed connection of the inner layer and the insulation over substantially the entire interface, i.e. the absence of cavities, etc., means a substantially uniform electric field outside the inner layer and minimal risk of PD.

It is preferred that the inner layer and the solid insulation consist of materials with substantially equal coefficients of thermal expansion.

The same is preferred with respect to the outer layer and the solid insulation. This means that the inner and outer layers and the solid insulation will form an insulation system which, in the event of temperature changes, expands or contracts uniformly as a monolithic part without these temperature changes giving rise to any destruction or fragmentation in the interfaces. Thus, intimacy in the contact surface between the inner and outer layers and the solid insulation and conditions for maintaining this intimacy for long periods of operation are ensured.

It is further pointed out that it is essential that the materials in the inner and outer layers and in the solid insulation have high elasticity so that the materials are able to withstand the stresses that arise when the cable is bent and when the cable is subjected to thermal stresses during operation. Of particular importance is 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, in the event that the material in the layers and the solid insulation would not have substantially equal coefficients of thermal expansion. In addition, it is advantageous that the materials in the inner and outer layers and in the solid insulation have substantially equal elasticity (E-modulus), which will counteract the formation of shear stresses in the boundary zone between the layers and the solid insulation. 10 15 20 25 30 35 512 698 13 In order to be able to form windings by means of the cable, it is essential that its flexibility is good. It is preferred that the cable be capable of being subjected to bending, without adversely affecting the function, with a radius of curvature which is 25 times the cable diameter or less.

Preferably, the radius of curvature is 15 times the cable diameter or less. The most advantageous radius of curvature value is 8 times the cable diameter or less.

The electric load on the insulation system decreases as a result of the fact that the inner and outer layers of the insulation consisting of semiconducting materials will tend to constitute substantially equipotential surfaces and that the electric field in the insulation itself will be distributed relatively evenly over the thickness of the insulation.

It is known that high voltage cables for the transmission of electrical energy can be built up of conductors with an insulation of a solid insulating material with inner and outer layers of semiconducting material. When transferring electrical energy, it has long been established that the insulation must be free of defects. In the case of high-voltage cables for transmission, however, the electrical potential does not change along the length of the cable, but the potential is in principle at the same level. However, even with high-voltage cables for transmission use, momentary potential differences can arise due to transient processes, such as in the event of lightning strikes. According to the present invention, in the electrical device, a flexible cable designed according to the following claims is used as winding.

A further improvement can be achieved in that the electrical conductor in the winding is built up of smaller so-called card parts, at least some of which are insulated from each other. By making these parts with a relatively small cross-section, preferably approximately round, the magnetic field over the parties will have a constant geometry in relation to the field and the occurrence of eddy currents will thus be minimized. According to the invention, the winding thus preferably consists of a cable comprising the electrical conductor and the previously described insulation system, the inner layer of which surrounds the parts of the conductor. Outside this inner semiconductor layer is the main insulation of the cable in the form of a solid insulation material.

According to the invention, the outer semiconductor layer must have such electrical properties that a potential equalization along the conductor is ensured. However, the outer layer must not exhibit such conductive properties that a current will be conducted along the surface, which would give rise to losses which in turn can cause undesired thermal load. For the inner and outer layers, the resistance data (at 20 ° C) defined in the following claims 20 and 21 apply. For the inner semiconductor layer, it must have sufficient electrical conductivity to ensure equipotential bonding for the electric field, but at the same time this layer must exhibit such resistivity that the confinement of the electric field is ensured.

It is important that the inner layer evens out 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 can be formed with varying thickness, but to ensure a smooth surface with respect to the conductor and the solid insulation, the thickness is suitably between 0.5 and 1 mm.

One such flexible winding cable which is used according to the present invention in its electrical device is a further development of the PEX cable used per se for transmission purposes or a cable with EP rubber insulation. The further development includes, among other things, a new design both as far as the parts of the electrical conductor are concerned and also that the cable, at least in certain designs, is not intended to have any outer casing for mechanical protection of the cable. However, it is possible according to the invention that a conductive metal shield and an outer jacket are arranged outside the outer semiconducting layer. The metal screen will thereby acquire the character of external mechanical and electrical protection, for example against ash impact. It is preferred that the inner semiconductor layer be at the potential of the electrical conductor. For this purpose, at least one of the strands of the electrical conductor is intended to be uninsulated and arranged so that good electrical contact is achieved with the inner semiconductor layer. Alternatively, different strands may be alternately conductive with electrical contact with the inner semiconductor layer.

Manufacturing transformer or reactor windings of a flexible cable as above involves drastic differences in the electric field distribution between conventional power transformers / reactors and a power transformer / reactor according to the invention.

The decisive advantage of a cable-shaped 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 semiconductor layer. The electric field provided by the live conductor appears only in the fixed main insulation.

From both a design and manufacturing point of view, this means significant advantages: - The transformer windings can be designed without having to take into account any electric field distribution and the transformation of parts mentioned under the state of the art is eliminated.

- The core construction of the transformer can be designed without having to take into account any electric field distribution.

- No oil is needed for electrical insulation of the winding, ie the surrounding medium of the winding can be air. no special connections are required for electrical connection between the external connections of the transformer and the nearest connected coils / windings, since the electrical connection, unlike conventional systems, is integrated with the winding.

The manufacturing and testing technology required for a power transformer according to the invention is significantly simpler than for a conventional power transformer / reactor because the impregnation, drying and vacuum treatments described below in the prior art are no longer needed.

When applying the invention as a rotating electric machine, a significantly reduced thermal stress occurs on the stator.

Temporary overloads of the machine thus become less critical and it becomes possible to operate the machine in case of overload for a longer period of time without risking damage. This means great benefits for power plant owners who, in the event of disruptions today, are quickly forced to switch to other equipment to ensure statutory delivery requirements.

With a rotating electric machine according to the invention, maintenance costs can be significantly reduced thanks to the fact that the transformer and switch do not have to be included in the system to connect the machine to the power grid.

It has already been described above whether the outer semiconducting layer of the winding cable is intended to be connected to earth potential. The intention is then that the layer along the entire length of the winding cable should be kept at substantially ground potential. It is possible to divide the outer semiconducting layer by cutting into a number of sections distributed along the length of the winding cable, whereby each individual layer section can be connected directly to earth potential. This creates the conditions for greater uniformity along the length of the winding cable.

It has been mentioned above how the solid insulation and the inner and outer layers can be achieved by, for example, extrusion, suitably simultaneous extrusion. However, other techniques are also quite possible, for example the formation of these inner and outer layers or the insulation by means of spraying on the material in question.

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

To build up voltage in the rotating electrical machine, the winding cable is laid in several successive turns in grooves in the magnetic core. The winding can be designed as a multi-layered concentric cable winding to reduce the number of end-to-end junctions.

The cable can be made with stepped insulation to make better use of the magnetic core, whereby the design of the grooves can also be adapted to the stepped insulation of the winding.

A significant advantage in the application of the invention to a rotating electric machine is that the electric field is close to zero in the reversal region outside the outer semiconductor and that with earth potential on the outer semiconductor layer, the electric field does not need to be controlled. This means that you can not get any field concentrations either within the core, in regions of origin or in the transition between these.

In a method of manufacturing a magnetic circuit, a flexible cable is used as winding which is inserted into openings formed in the groove of a magnetic core of the rotating electrical machine. The flexibility of the cable means that a cable length can be laid in several turns in a coil. The inverters will then consist of bending zones of the cables. The cable can also be spliced in such a way that its properties remain constant over the cable length. This procedure entails significant simplifications compared with the state of the art. The so-called rod rods are not flexible but must be preformed to the desired shape. Solar winding and impregnation of the coils is also extremely complicated and expensive technology in the production of rotating electric machines of today. In summary, it thus applies that an electrical device in the form of a rotating electrical machine according to the invention entails a significant number of important advantages relative to corresponding machines according to the prior art. First, the machine according to the invention can be connected directly to a power grid at all types of high voltage. Another significant advantage is that earth potential has been consistently carried along at least part of and preferably along the entire winding, which means that the turning region can be made compact and that bracing devices in the turning region can be applied to, in the near future, earth potential. As already pointed out above with regard to power transformers / reactors, oil-based insulation and cooling systems also disappear with rotating electrical machines. This means that no sealing problems arise and that the previously mentioned dielectric ring is no longer needed.

It is also important that all forced cooling can take place on earth potential.

The invention further relates to a method for monitoring an electrical device with an magnetic device and at least one winding in an electrical installation comprising an electrical device connected to an electrical distribution or transmission network by means of a measuring device according to claim 36. Such a measuring device comprising at least one capacitive sensor has proved very suitable for sensing at a distance from an electric line connecting the electrical device to the distribution or transmission network, the voltage on this electric line via the surrounding electric field. This presupposes, of course, that the measuring device is designed to sense the voltage of the electric line at a place where this line does not have an insulation system which substantially completely encloses the electric field.

This means that the flexible cable which forms the winding in the electrical device must be transferred to an at least substantially insulating system without a conduit section, either by releasing the electrical conductor in the winding at the current sensing section from its insulation system or by the electrical conductor in a cable termination passes into the said electrical 10 15 20 25 30 35 512 698 19 line. The sensing point itself is suitably located relatively close to the electrical device so that in the same generated harmonics etc. can be effectively sensed so that these can be eliminated by filtering out / compensating.

Brief Description of the Drawings With reference to the accompanying drawings, a more detailed description of exemplary embodiments of the invention follows.

In the drawings: Fig. 1 is a view, partly in section, of a measuring apparatus including a capacitive sensor and a signal converter for directional voltage measurement according to the invention; Fig. 2 is a view, partly in section, of an alternative design of the measuring apparatus; Fig. 3 is a calculated distribution of an electric field penetrating through the opening of the shield electrode; Fig. 4 is a view, partly in section, of an insulator with a measuring apparatus according to the invention arranged thereto; Fig. 5 is a schematic diagram of an electrical device with three wires formed with rails with a measuring device according to the invention arranged for each of them; Fig. 6 is a schematic diagram of an electrical device for a phase with four measuring devices according to the invention; Fig. 7 is a view, partly in section, of a preferred embodiment of the measuring apparatus with the inner electrode divided into sub-electrodes isolated from each other; Fig. 8 is a partially cut-away view showing the parts included in the current modified standard cabin, Fig. 9 is an axial end view of a sector / pole division of a magnetic circuit according to the invention, Fig. 10 is a view showing the electric field distribution around a winding of a conventional power transformer / reactor, Fig. 11 a perspective view illustrating an embodiment of a power transformer according to the invention, Fig. 12 a cross-sectional view illustrating a relatively modified cable structure with several electrical conductors, Fig. 13 a cross-section of a further cable structure comprising several electrical conductors but in a device other than that of Fig. 5, Fig. 14 is a schematic view illustrating an electrical plant having a measuring apparatus applied to a generator, and Fig. 15 is a schematic view illustrating the measuring apparatus of a transformer.

Description of Exemplary Examples Figure 1 shows a measuring apparatus 10 for directional voltage measurement according to the present invention. The measuring device 10 is intended to measure alternating voltage at a solution distance from a line 22. The line 22 lacks insulation which substantially shields the electric field.

The measuring apparatus 10 comprises a capacitive sensor 11 and a signal converter 13. The capacitive sensor has an inner electrode 12 and a shield electrode 14, which encloses the inner electrode. An opening 16 is arranged in the shield electrode, intended to be directed towards the lead 22 when measuring. In the example, the two electrodes are made of a conductive material. However, the electrodes can be made as bodies of any material as long as their confinement surfaces are conductive. For example, an electrode may be made of a non-conductive material but with an enclosing conductive layer, for example a body of plastic, to which a conductive coating has been applied.

In the example, the shield electrode has the shape of a bucket, i.e. it has a bottom from which extends a cylindrical or slightly conical rim. In the example, the inner electrode has a planar extent in a plane parallel to the opening of the shield electrode. The inner electrode is insulated from the shield electrode and adjustably fixed thereto so that the distance between the inner electrode 12 and the opening 16 of the shield electrode 14 can be adjusted, as indicated by the arrow A in Figure 1. In the example this can be done by means of a shield electrode. continuous insulating tube 17, on which the inner electrode 12 is fixed.

The signal converter 13 is located in close proximity to the shield electrode 14 and comprises means for impedance conversion and amplification. The signal converter may also comprise means for filtering and digital conversion of the analog signal from the sensor 11. In particular, the signal converter is arranged to condition the signal very prone to interference from the sensor to an analog signal or digital pulse train arranged for transmitting 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 enclosed by screen 19. The sensor 11 and the signal converter 13 are connected by means of a conductor 18 running in the tube 17 which may be shielded. For signal evaluation, the signal converter is connected to an analyzer 15, which can be located at a distance from the measuring device 10. The transmission of the signal can be arranged by both electrical and optical means but also contactlessly via a transmitter and a receiver. .ra 10 15 20 25 30 35 512 698 22 The capacitive sensor 11 is capable of sensing the high bandwidth electric field in terms of frequency, usually between just over zero to several thousand Hertz. It is therefore advantageous to set up a broadband signal converter 13 for the measuring device. The means for impedance conversion and amplification included in the signal converter are thus arranged with the advantage of a so-called video amplifier. A number of filters with different filter characteristics can also be arranged in the signal converter. Such filters can be bandpass filters or low or high pass filters. When measuring, these can each leave their own signal or be connected sequentially.

In special applications, it is advantageous to connect the shield electrode to earth to a potential that can be locked in relation to, for example, a phase for increased dynamics and resolution of the desired measuring variable. The signal converter 13 may for this purpose comprise a phase locking circuit (not shown) connected to the shield electrode 14. This circuit, a so-called PLL (Phase Locked Loop) circuit, makes it possible to lock the measuring electrode 14 of the measuring device 10 in a three-phase system, for example, to a potential that varies with the phase to be measured. In this way, the influence of the other phases can be suppressed in the output signal from the signal converter 13, which results in an increased measurement accuracy.

When measuring with the measuring device 10 to measure alternating voltage at an insulating distance from a high-voltage conductor 22, the measuring device 10 is directed towards the conductor 22. In order for the inner electrode to be able to detect a drawn part of the electric field, the desired part of the field incident through the opening in the screen electrode. An imaginary axis that passes through the center of the inner electrode and through the center of the opening is directed towards the measuring object. Before the actual measurement begins, the measuring device 10 is calibrated on site.

The calibration takes place by applying a known voltage, after which measurement with the measuring device takes place. The measuring device is thus calibrated by adjusting the measured value to correspond to the known voltage. When this is done, the actual measurement can be started. An advantage of the measuring device 10 according to the present invention is that the measuring device does not have to come into galvanic contact, or even into contact with the line, the voltage of which is to be measured. The measurement can instead take place at an isolation distance from the line 22, which means that the measuring device is safe to touch for everyone who comes into contact with it. This further means that no installations need to be made in direct connection with the conductor, whereby no operational disturbances need be caused by the measuring procedure. The measuring device has an extremely simple construction and is thus very cheap to manufacture and is also reliable. Advantageously, the measuring apparatus can be arranged to constitute a detector, a so-called PD detector (Partial Discharge), for transients and ionic discharge of a measuring object. Due to its reliability, broadband and small investment, the measuring device is particularly suitable in connection with energy measurement for charging for consumed electrical energy and for relay protection functionality, but above all in the applications discussed with reference to Figs. 14 and 15. Fig. 2 shows a alternative design of the measuring apparatus 10 according to the invention. In the same way as in the previous example, the measuring device comprises a sensor 11 and a signal converter 13. A shield electrode 14 enclosing an inner electrode 12, leaving only an opening 16 which when measured is directed towards the measuring object, i.e. the previously mentioned line 22. The shield electrode is in this example is spherical while the inner electrode is cup-shaped with the concave side facing the aperture. However, the shield electrode can have any shape and be made of any, dense or perforated, conductive material. Likewise, the inner electrode can have any shape. However, it is preferred to provide the inner electrode with a planar extent which is substantially parallel to the plane of the opening. The sensor 11 is not limited to, as indicated in the examples, have a circular shape. A narrower aperture gives a weaker signal but greater directional sensitivity. In the case of a field-generating line source in the form of a wire, it can therefore be advantageous to design the sensor with the opening and the inner electrode, respectively, elongated in a direction coinciding with the line source.

Fig. 3 shows a calculation of the distribution of an electric field penetrating through the opening of a grounded shield electrode into an inner electrode 12. The figure shows only a part of such a field in the vicinity of the edge of the inner electrode, which in the example is cup-shaped. The calculation confirmed by experiments shows that the part of the electric field that penetrates through the opening initially has a direction parallel to the normal to the opening.

Further into the shield electrode, the field lines diverge towards the inside of the shield electrode and are finally absorbed by the inner electrode 12. The inner electrode 12 has its concave surface directed towards the opening in the shield electrode. The advantage of this geometry is that the field distribution inside the shield electrode becomes smoother. The design of the cup-shaped inner electrode 12 as a spherically curved plate causes all field lines to fall perpendicular to the plate. This provides additional benefits with a stronger output signal from the sensor while maintaining its shielding properties.

Fig. 4 shows an advantageous use of a measuring device 10 according to the invention. In the example shown, the measuring device is attached to the end of a hollow insulator 25. The insulator comprises an insulator 26 of porcelain or other insulating material and a first pole 27 and a second pole 28. The first pole is connected to the previously discussed line while the second the pole is connected to ground.

The measuring device is mounted at the second pole 28 with its shield electrode 14 connected to the pole and with its inner electrode 12 insulated and adjustably fixed to the shield electrode 14. Through the described use an exact, non-varying distance is obtained between the measuring object, i.e. the wire, and the measuring device. This is advantageous as a variation of the distance jeopardizes the accuracy of the ice stress measurement.

To further shield unwanted electric fields, the two poles can be provided with screens (not shown) in the form of plates of conductive material extending in the transverse direction of the insulator 10, which are connected to the respective pole. In the case of hollow insulators with shielding gas of the SFß type, the smaller insulating distance caused by the gas on the inside of the insulator can be used so that the measuring device 10 is housed in its entirety inside the insulator. An insulator including a device for voltage measurement can thus be easily manufactured as a finished product.

Figure 5 shows a schematic diagram of wires designed as rails R, S and T for 3-phase alternating voltage and enclosed by a housing 29. According to the figure, a measuring device 1OR, 103, 1OT is arranged for each of the rails for directional voltage measurement. according to the invention. The three measuring devices have been brought together in a common position to achieve a simple installation and wiring, respectively, and each is directed towards each rail. However, each measuring device can be placed at an insulating distance in an arbitrary position within the admission. However, the common location is an advantage as the largest possible angle between the sensitivity directions of the measuring device can thus be achieved. With this type of use of the measuring device, assembly time and space in the enclosure are saved, among other things.

It has previously been mentioned that a voltage measurement with a measuring device according to the invention is sensitive to variations in the distance between the measuring device and the measuring object. This relationship can be used for the purpose of studying a movement of a measuring object. Thus, a method of measuring voltage from a measuring object moving stochastically about a center point can be easily accomplished. A measuring setup which allows, partly a method for studying a conductor's movement, partly a method for increasing the measuring accuracy is shown in Fig. 6. Here, like the previous figure, a principle sketch of an electrical device with only one phase enclosed by a housing is shown. 29. Around a centrally located rail 33, four measuring devices 10a, 10b, 10c, 10d according to the invention are arranged in each corner of a cross section of the housing. The converted signal from each of the measuring devices is preferably analyzed by a four-channel analyzer (not shown). To study the motion, the signals are compared and to increase the measurement accuracy, the signals are averaged.

By means of standard geometric calculations from the measuring signals, the position of the rail is determined, whereby a measurement of the voltage is corrected for the position change, so that a correct measured value can be calculated. The method can also be used to detect and correct the measurement for changes in the electric background field, which give asymmetric changes in the measured signals.

Greater measurement accuracy is obtained by the introduction of a plurality of measuring devices which measure on the same object. The described measuring methods are not limited to application only to encapsulated electrical devices but can also be applied to free conductors and to unencapsulated devices. With an unencapsulated device, it is especially valuable to be able to correct the measured value for changes in the background field.

An alternative embodiment of a sensor 11 included in a measuring apparatus according to the invention is shown in Fig. 7. The sensor 11 comprises a shield electrode 14 provided with an opening 16, which encloses a first inner sub-electrode 12a and a second inner sub-electrode 12b, which are isolated from each other and each adjustably fixed to the shield electrode. In the example shown, the aperture 16 is limited by a ring 34 connected to the shield electrode of a conductive material, the task of which is to level the electric field so that glitter is avoided. The inner sub-electrodes in the example shown are equal in size and preferably have the shape of a half-round plate, so that the sub-electrodes together have the shape of a whole round plate. The internal sub-electrodes are each connected to a signal converter (not shown) and the measurement signal is transmitted in the same way as in Example 1 to each analyzer, or to a common multichannel analyzer.

By dividing the inner electrode into sub-electrodes, the previously mentioned sensitivity to distance dependence can be further utilized. Preferably, the electrodes are adjusted to have the same capacitance, whereby by a bridge connection a comparison between the converted signals from each of the sub-electrodes can be ascertained, partly if the measuring object is located in front of the sensor, partly if the measuring object moves during the measurement. An electric field generated from another undesirable object is detected during a division of the inner electrode by comparisons of the measurements in an analyzer and can thereby be eliminated from the measurement result. The inner electrode is divided according to the invention into an arbitrary number of sub-electrodes. In the case of the previously mentioned elongate sensor, for example, it is advantageous to design the sub-electrodes as a plurality of plates arranged next to one another.

An important prerequisite for being able to manufacture such a magnetic circuit as primarily referred to in the invention is to use a conductor cable with solid electrical insulation for the winding, with an inner semiconducting layer between the insulation and one or more internal electrical conductors and with an outer semiconducting layer located outside the insulation. Such cables are available as standard cables for other power engineering applications, namely power transmission. In order to be able to account for an embodiment, a brief description of a standard cable must initially be given. The internal current conductor consists of a number of strands. There is a semiconducting inner layer or casing around the card parts. Outside this semiconducting inner layer is an insulating layer of solid insulation.

The solid insulation is formed from a polymeric material with low electrical losses and high breakdown strength. As concrete examples may be mentioned polyethylene (PE) and then especially cross-linked polyethylene (PEX) and ethylene propylene (EP). A metal shield and an outer insulating casing can be arranged around the outer semiconducting layer. The semiconducting layers consist of a polymeric material, for example ethylene copolymer, with an electrically conductive component, for example conductive soot. Such a cable will hereinafter be referred to as a power cable. A preferred embodiment of a cable intended for winding in the rotating electric machine is shown in Figure 8. The cable 41 is shown in the figure as comprising a live conductor 42 which comprises transposed both uninsulated and insulated. kardeler. Electrically machined, permanently insulated strands are also conceivable. These strands can be beaten / transposed in a number of layers. Around the conductor is an inner semiconducting layer 43 which in turn is surrounded by a homogeneous layer of a solid insulating material. The insulation 44 thus completely lacks insulation material of the liquid and gas type. This layer 44 is surrounded by an outer semiconducting layer 45.

The cable used as winding in the preferred embodiment may be provided with a metal shield and outer sheath but need not be so.

In order to avoid induced currents and associated losses in the outer semiconductor layer 45, this is cut off, preferably in the head-end alignment, ie in the transitions from sheet metal package to tangle basket. The cut-off is made so that the outer semiconductor layer 45 will be divided into several parts distributed along the cable, electrically completely or partially separated from each other. Each cut-off part is then connected to earth, whereby the outer semiconducting layer 45 will be held at or almost at ground potential throughout the cable length. This means that around the permanently insulated winding at the end faces, the touchable and, after a certain period of use, dirty surfaces only have negligible potentials to earth and that they also cause negligible electric fields.

In order to optimize a rotating electric machine, the design of the magnetic circuit with regard to the grooves and the teeth, respectively, is important.

The grooves should be connected as close to the casing side casing as possible. It is also desirable that the teeth at each radial level be as wide as possible. This is important to minimize the machine's losses, magnetization needs, etc.

With access to a conductor for the winding such as the cable mentioned above, there are great opportunities to be able to optimize the magnetic core from the mentioned points of view. In the following, reference is made to a magnetic circuit in the stator of the rotary electric machine. Figure 9 shows an embodiment of an axial end view of a sector / pole division 46 of a machine according to the invention. Rotor with rotor pole is denoted by 47. The stator is in a conventional manner composed of a laminated core of electrical sheet successively composed of sector-shaped sheets.

From a radially outermost back portion 48 of the core, a number of teeth 49 extend radially toward the rotor. Between the teeth there is a corresponding number of grooves 50. The use of cables 51 as above allows, among other things, that the depth of the grooves for high-voltage machines can be made greater than has been possible according to the state of the art.

The groove has a stepped cross-section towards the rotor because the need for cable insulation is lower for each winding layer towards the rotor. As can be seen from the figure, the groove consists essentially of a circular cross-section 52 around each layer of the winding with narrower waist portions 53 between the layers. Such a barred cross-section can with some right be referred to as a "cycle | chain track". Since such a high-voltage machine will require a relatively large number of layers and the availability of current cable dimensions in terms of insulation and outer semiconductors is limited, it may in practice be difficult to achieve a desired continuous phasing of cable insulation and the stator groove, respectively. In the exemplary embodiment shown in Figure 9, cables with three different dimensions are used for cable insulation, arranged in three sections 54, 55 and 56 dimensioned accordingly, ie in practice there will be a modified cycle chain groove. The figure also shows that the stator teeth '49 can be designed with a practically constant radial width along the entire depth of the groove.

In an alternative embodiment, the cable used as winding may be a conventional power cable such as the one mentioned above. The grounding of the outer semiconducting layer 45 then takes place by peeling off the metal shield and sheath of the cable in suitable places.

Within the scope of the invention, a large number of alternative embodiments can be accommodated, depending on the available cable dimensions as far as the insulation layer and the outer semiconductor layer, etc. are concerned. Embodiments with so-called bicycle chain tracks can also be modified in addition to what is described here.

As discussed above, the magnetic circuit may be located in the stator and / or rotor of the rotary electric machine. However, the design of the magnetic circuit will broadly correspond to the above description, regardless of whether the magnetic circuit is located in the stator and / or the rotor.

As winding, a winding which can be described as a multilayer concentric cable winding is preferably used. such a winding means that the number of intersections at the ends of the hairs has been minimized by all the hairs within the same group being placed radially outside each other. This also allows a simpler procedure in the manufacture and threading of the stator winding in the different grooves.

Because the cable used according to the invention is relatively easily flexible, the winding can be achieved by a relatively simple threading operation, in which the flexible cable is inserted into the openings 52 located in the grooves 50. Figure 10 shows in simplified form and in principle the electric field distribution. the winding around a winding of a conventional power transformer / reactor, where 57 is a winding and 58 a core and 59 indicates equipotential lines, i.e. lines where the electric field is the same size. The lower part of the winding is assumed to be at ground potential.

The potential distribution determines the structure of the insulation system because you must have sufficient insulation both between adjacent turns of the winding and between each turn and earth. The figure thus shows that the upper part of the winding is exposed to the highest insulation technical loads. The design and location of a winding relative to the core is determined in this way mainly by the electric field distribution in the core window. 10 15 20 25 30 35 512 698 31 The cable which can be used in the windings which are included in power transformers / reactors according to the invention has been described with reference to Fig. 8. As mentioned earlier, the cable can be provided with other additional outer layers intended for special purposes, for example to prevent excessive electrical stresses in other areas of the transformer / reactor. From a geometric dimension point of view, the cables in question will generally have a conductor area of between 2 and 3000 mm2 and an outer cable diameter of between 20 and 250 mm.

Windings of a dry power transformer / reactor manufactured by the cable reported in the description of the invention can be used in both single-phase, three-phase and multiphase transformers / reactors, regardless of how the core is designed. An embodiment is shown in Figure 11 which shows a three-phase laminated nuclear transformer. The core consists in a conventional manner of three core legs 60, 61 and 62 and the cohesive yokes 63 and 64. In the embodiment shown, both the core legs and the yoke have tapered cross-sections.

Concentrically around the core legs are the winding cables. The embodiment shown in Figure 11 has, as can be seen, three concentric winding turns 65, 66 and 67. The innermost winding turn 65 can represent the primary winding and the other two winding turns 66 and 67 can represent the secondary winding. In order not to burden the figure with too many details, the connections of the windings are not shown. The figure in general shows that in the embodiment shown, there are spacer rails 68 and 69 with several different functions in certain places around the windings. The spacer rails can be formed of insulating material intended to provide a certain space between the concentric winding turns for cooling, bracing, etc. They can also be designed of electrically conductive material to be included in the grounding system of the windings.

Alternative cable extensions .iri lin. In the cable variant illustrated in Fig. 12, the same reference numerals are used as before, only with the addition of the embodiment characteristic letter a. In this embodiment, the cable comprises several electrical conductors 42a, which are mutually separated. by means of the insulation 44a. In other words, the insulation 44a serves both for insulation between individual adjacent electrical conductors 42a and between these and the environment. The different electrical conductors 42a can be laid in different ways, which causes varying cross-sectional shape of the cable as a whole. The example of Fig. 13 illustrates how the conductors 42a are laid in a straight line, which causes a relatively flat cross-sectional shape of the cable. From this it can be concluded that the cross-sectional shape of the cable can vary within wide limits.

In Fig. 12 it is assumed that there is a voltage between adjacent electrical conductors which is less than the phase voltage. More specifically, the electrical conductors 42a in Fig. 12 are thought to be formed by different turns in the winding itself, which means that the voltage between these adjacent conductors is relatively moderate.

As before, a semiconducting outer layer 45a is present outside the insulation 44a provided by a solid insulating material. An inner layer 43a of a semiconductor material is arranged around each of said electrical conductors 42a, i.e. each of them has its own surrounding inner semiconductor layer 43a. This layer 43a will thus function as a potential equalizer with respect to the individual electrical conductor.

The variant in Fig. 13 uses the same reference numerals as before, only with the addition of the embodiment-specific letter b. Here, too, there are several, more specifically 3, electrical conductors 42b. Between these, in the example, phase voltage is thought to exist, ie a substantially higher voltage than that which exists between conductors 42a in the embodiment according to Fig. 12. In Fig. 13 there is an inner semiconducting layer 43b, inside which the electrical conductors 42b are arranged. However, each of the electrical conductors 42b is enclosed by its own outer layer 70 with properties corresponding to the properties of the inner layer 43b discussed above. Insulation material is present between each additional layer 70 and the layer 43b arranged around them. Consequently, the layer 43b will be present as a potential equalizing layer outside the electrical conductors' own additional layers 70 of semiconducting material, these additional layers 70 being adjacent to the respective electrical conductors 42b to be placed on equal potential therewith.

Fig. 14 schematically illustrates whether a generator 71 is connected to a distribution or transmission network designated 77.

The generator 71 is designed according to the invention in the sense that it comprises a winding designed as a flexible cable. This cable extends from the generator to a cable termination 73, in which the conductor of the cable passes into what is here referred to as an "electrical wire". The measuring device designated 74 according to the invention is arranged to sense the voltage of this wire designated by 78, which In the measuring range of the measuring device 74, the wire 78 must lack such an insulation system which completely or substantially completely shields the electric field around the wire. It is pointed out that the wire 78 could also be a part of the flexible conductor free from the insulation system. A valve diverter 75 for diverting overvoltages is connected in the connection between the generator 71 and the mains 77. Furthermore, in said connection a switch 76 is arranged, preferably including or supplemented with a disconnecting function.

Fig. 14 also shows how the measuring device 74 is connected to a control unit 79. From the measuring device 74 information is thus supplied to the control unit 79 about the prevailing voltage, including harmonics and possibly transients. A current sensing component connected in the connection between the generator and the mains 77, for example a current transformer, is connected to the control unit 79 in order to supply it with information on the current current value. The control unit 79 is connected to one or more components included in the system 10 15 20 25 30 35 512 698 34 80 in order to eliminate or at least reduce any voltage-related deficiencies such as transients and harmonics. Said constituents 80 suitably consist of one or more filters capable of filtering out harmonics and other transients. Because the control unit 79 also receives information about the current current, the control unit can also be made to eliminate or at least reduce current-related deficiencies. In the variant illustrated in Fig. 15, the only difference is that instead a transformer 81 is connected to the distribution or transmission network 77. As before, the high-voltage cable forming the secondary winding in the transformer 81 passes at the cable termination 73 in a line 78, which in any case is not completely shielded in terms of electric field. Preferably, the conduit 78 is completely or substantially completely without insulation. Upstanding harmonics in the transformer are detected by means of the measuring device 74.

The presence of harmonics indicates, for example, that the magnetic flux in the core of the transformer is so large that the core has gone or is at risk of saturation. Accordingly, the control unit 79 is arranged to control, on the basis of the measuring signals of the measuring device 74, if necessary, one or more components included in the system in order to eliminate or at least reduce any voltage-related deficiencies, such as harmonics. This control can for instance take place via a component 82 arranged to control the operation of the plant so that the transformer 81 receives a reduced magnetic flux and thus the generated harmonics are eliminated or at least reduced.

POSSIBLE MODIFICATIONS It is a given that the invention is not only limited to the embodiments described above. Thus, those skilled in the art will recognize that a variety of detailed modifications are possible once knowledge of the basic inventive concept has been obtained without departing from the inventive concept as defined in the appended claims. By way of example, it is pointed out that the invention is not limited to the specific material choices exemplified above. Functionally equivalent materials can thus be used instead. With regard to the manufacture of the insulation system according to the invention, it is pointed out that techniques other than extrusion and spraying are also possible as long as intimacy and adhesion between the different layers is achieved. It is further pointed out that more equipotential layers could be arranged. For example, one or more equipotential layers of semiconductor material could be applied in the insulation between those referred to above as "inner" and "outer". As already pointed out above, the measuring device according to the invention can also be subjected to a number of modifications within the scope of the appended claims as long as the device is able to meet those of the set, basic requirements.

Claims (33)

ha 10 15 20 25 30 35 512 698 36 Patent claim Electrical installation comprising an electrical device (71, 81) connected to an electrical distribution or transmission network (77) with a magnetic circuit and at least one winding and a measuring device (10) arranged to monitor the electrical device, characterized in that: the measuring apparatus (10) comprises at least one capacitive sensor (11) including an inner electrode (12) and a shield electrode (14), which is connected to ground or a potential separated from a potential of the measuring object and which shields the inner electrode (12) from interfering electric fields, that the sensor (11) is arranged to perform voltage measurement by means of the inner electrode (12) by sensing a part of an electric field penetrating into the inner electrode, that the sensor is directed towards a measuring object forming electrical conduit (78) connecting the electrical device (71, 81) to the distribution or transmission network (77) for sensing the voltage at insulating distances, including transients and harmonics, of this electric line (78) via the surrounding electric field, that the winding of the electric device (71, 81) comprises an electric conductor (42) having a housing (43, 44, 45) comprising a insulation system, that the insulation system comprises an insulation (44) formed of a solid insulation material and outside the insulation an outer layer (45) having an electrical conductivity higher than that of the insulation so that the outer layer by connection to earth or otherwise relatively low potential must be able to function as a potential equalizer and to contain mainly the electric field arising due to said electric conductor (42) inside the outer layer (45), that an inner layer (43) is arranged inside the insulation (44) , that said at least one electrical conductor (42) is arranged inside the inner layer (43) and that the inner layer has an electrical conductivity which is lower than that of the electrical conductor but sufficient for the inner layer to function as potential equalization and thus equalization with respect to the electric field outside the inner layer (43). 36 10 15 20 25 30 35 512 698 37 Plant according to Claim 1, characterized in that the potential of the shield electrode (14) is variable. Plant according to Claim 1 or 2, characterized in that the shield electrode (14) and the inner electrode (12) are insulated from one another by means of a gaseous dielectric. Plant according to one of Claims 1 to 3, characterized in that the shield electrode (14) encloses 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. Plant according to one of Claims 1 to 4, characterized in that the inner electrode (12) is adjustably fixed in the shield electrode (14). Plant according to claim 4, characterized in that the inner electrode (12) has a substantially planar propagation, the propagation plane of which is substantially perpendicular to a normal to the opening (16) in the shield electrode (14). Plant according to one of the preceding claims, characterized in that the inner electrode (12) is cup-shaped. Plant according to one of the preceding claims, characterized in that the inner electrode (12) comprises a plurality of electrically insulated sub-electrodes (12a, 12b) from each other. Plant according to one of the preceding claims, characterized in that the measuring device comprises a signal converter (13), which is arranged in the vicinity of the sensor (11) and comprises means for amplifying and impedance converting the measuring signal. 37 10 15 20 25 30 35 512 698 38 Plant according to claim 9, characterized in that the signal converter (13) includes means for phase locking the shield electrode (14) and / or the inner electrode (12). Plant according to one of the preceding claims, characterized in that the measuring device (10) is arranged at the earthed part of an insulator. Plant according to one of the preceding claims, characterized in that a control unit (79) is connected to the measuring device (74), arranged to control, if necessary, one or more components included in the system on the basis of the measuring signals of the measuring device (74). 80, 82) in order to eliminate or at least reduce existing voltage-related deficiencies such as transients and harmonics. Plant according to claim 12, characterized in that said one or more components controlled by the control unit (79) comprise at least one filter (80) for filtering out harmonics and / or transients. Plant according to one of the preceding claims, characterized in that the electrical conductor (42) is flexible and in that its housing is magnetically permeable. Plant according to one of the preceding claims, characterized in that the inner and outer layers (43, 45) and the solid insulation have substantially the same thermal properties. Plant according to any one of the preceding claims, characterized in that said at least one conductor (42) constitutes at least one induction revolution. Plant according to one of the preceding claims, characterized in that the inner and / or outer layer (43, 45) comprises a semiconducting material. 38 10 15 20 25 30 35 512 698 39 Plant according to any one of the preceding claims, characterized in that the inner layer (43) and / or the outer layer (45) has a resistivity in the range 10'5 Qcm - 100 kQcm, preferably 10'3-100O Qom, preferably 1 -500 Qcm. Plant according to one of the preceding claims, characterized in that the inner layer (43) and / or the outer layer (45) has a resistance which per meter of conductor / insulation system is in the range 50 pQ - 5 MQ. Plant according to one of the preceding claims, characterized in that the solid insulation (44) and the inner layer (43) and / or the outer layer (45) consist of polymeric material. Plant according to any one of the preceding claims, characterized in that the inner layer (43) and / or the outer layer (45) and the solid insulation (44) are firmly connected to each other over substantially the entire green surface in order to maintain adhesion between respective layers and the solid insulation in the event of temperature changes and bending of the conductor (42) and its insulation system. Plant according to one of the preceding claims, characterized in that the inner layer (43) and / or the outer layer (45) and the solid insulation (44) are made of materials with substantially equal coefficients of thermal expansion. Plant according to one of the preceding claims, characterized in that the conductor (42) and its insulation system form a winding formed by means of a flexible cable (41). Plant according to one of the preceding claims, characterized in that the inner layer (43) is in electrical contact with the at least one electrical conductor (42). 39 10 15 20 25 30 35 512 698 40 Plant according to claim 24, characterized in that said at least one electrical conductor (42) comprises a number of strands and that at least one strut of the electrical conductor (42) is at least partially uninsulated and arranged in electrical contact with the inner layer. (43). Plant according to one of the preceding claims, characterized in that the conductor (42) and its insulation system are designed for high voltage, suitably above 10 kV, in particular above 36 kV and preferably above 72.5 kV. Plant according to one of the preceding claims, characterized in that the electrical device constitutes a rotating electrical machine. Plant according to Claim 27, characterized in that the magnetic circuit of the electrical device is arranged in the stator and / or rotor of the machine. Plant according to one of Claims 27 to 28, characterized in that the magnetic circuit comprises one or more magnetic cores (48) with grooves (50) for the winding (41). Plant according to one of Claims 27 to 29, characterized in that the machine consists of a generator, motor or synchronous compensator. Plant according to one of Claims 27 to 30, characterized in that the machine is connected directly to an excessively high voltage network, suitably 36 kV and above, without an intermediate transformer. Plant according to one of Claims 1 to 26, characterized in that the electrical device consists of a power transformer / reactor. 40 512 698 41 Plant according to one of the preceding claims, characterized in that the casing / insulation system of the at least one flexible conductor (42) having the winding (41) of the electrical device (71, 81) ceases, in the direction of the distribution or transmission network, in a cable termination (73) and that the electrical line (78), towards which the measuring device (10) is directed, consists of a line between the cable termination (73) and the mains (77). 41