AU719566B2 - Method, device and sensor for capacitive detecting of field and voltage and use thereof - Google Patents

Description

WO 98/05974 WO 9805974PCT/SE97/01289 METHOD, DEVICE AND SENSOR FOR CAPACITIVE DETECTING OF FIELD AND VOLTAGE AND USE THEREOF TECHNICAL FIELD The present invention relates to a* capacitive sensor for sensing variations in an electric field. The invention also relates to a measuring device including such a sensor f or measuring voltage at a distance from high-voltage conductors, and to a method for such measurement. The other quantities which are detected by thie measuring device are transients and ionic discharge. The measuring device is particularly adapted to measure voltage in electric power networks, which may comprise one or more phases, for control of transmission or distribution of electric power, or as a basis for debiting of consumed energy.

BACKGROUND ART There is often a need of measuring the voltage of a highvoltage line at a distance. This may, for example, be in connection with distribution of electric power with several conductors, where particularly the voltage of one of the conductors is t~o be determined. Such voltage measurement is used for control of a voltage drop along the distribution chain or for controlling that limit values are not exceeded.

It may also be a qu..estion of calculating the electric energy passing, based on the measured voltage. In both cases there is a great need of a simple and inexpensi.ve measuring device, which is flexible and with the aid of which it is economically possible to achieve a densification of the number of measuring points in a distribution network.

The detectors and measuring devices which are currently available are costly and complicated devices which must often be brought into contact with the high-voltage line, for example a voltage transformer. These solutions entail considerable investments and therefore imply a great departure from the low investment and flexibility which is desirable.

WO 98/05974 WO 9805974PCT/SE97/01289 2 From EP-B1-0 181 054, an apparatus for measuring the potential difference between ground and a power conductor, placed above the ground, is previously known. The apparatus shown is intended to be mounted on the conductor and generally has annular shape. one limitation with this apparatus is that it has to be applied onto the conductor, which, in addition to a great investment cost, also involves operational disturbances.

In an article by P. Sarma Maruvada, R.D. Dallaire and R.

Pedneault, "Development of field-mill instruments for ground-level and, above-ground level electric measurement under HVDC transmission lines", IEEE Transactions on Power Apparatus and systems, Vol. PAS-102, No. 3, pp. 738-744, March 1983, there is shown a device, popularly named "field mill' for measuring, at ground level, an electric field under a high-voltage conductor. The device comprises essentially an electrode, fixed to ground potential, in the form of a cylindrical box with, for example, six sectorshaped openings on the upper s ide thereof, and a rotating electrode inside the box but electrically insulated from the same. The rotating electrode comprises sixc identical lobes formed as a Bernoulli leynniscate, arranged uniformly around a circle. The device may be used either at the same level as ground-level by placing it inl a hole dug in the ground, covered with a metal -protective plate with an opening for the device. The device may also be placed so as to project above the ground.

The above-mentioned article also shows a device for measurement above ground level of the electric field under a high-voltage conductor. The device comprises two metal cylinders, each being divided lengthwise into two halves insulated from each other. The cylinders have equal or different radii, but different lengths. The cylinders rotate at different speeds and from a mechanical point of view it is desirable that the cylinders rotate in directions opposite to each other.

WO 98/05974 PCT/SE97/01289 3 One problem with the devices described above is that both are sensitive to disturbing electric fiel~ds, which therefore may gr~eatly distort the measurement result. Another problem is that both have a relatively complicated design.

From US-4,328,461, an apparatus fQr measuring electric fields is previously known. The task of the apparatus is to investigate electrical charac teris tics in the atmosphere during a thunderstorm. The apparatus is shown in three different embodiments, its basic design comprising two hemispherical electrodes, between which a device for measuring and generating measured data is enclosed. In a second embodiment, the mneasuring electrodes are in the form of two circular metal plates, whereby the associated measurement electronics is arranged between the Plates and thus at least partly screened thereby. In a third embodimerit, the measuring electrodes are in the form of two semicylindrical electrodes, insulated from each other, where the electronics, in the same way as above, is located between the electrodes.

The known device for measuring electric fields in case of a stroke of lightning has a certain directional sensitivity because of its two uniform plates. However, it is not capable of screening undesired electric fields and is therefore not useful in the application sought, where the electric field generated by only one of several high-voltage parts is to be measured.

SUMMARY OF THE INVENTION The object of the present invention is to achieve a measuring device, by which may be measured the voltage in a high-voltage part in an electric power system, which high-voltage part is selected from a plurality of such parts and is located at a distance. The measuring device shall have a simple design, be flexible and have a low production cost. The measuring device shall be capable of screening undesired electric fields arid be capable of WO 98/05974 PCT/SE97/01289 4 being directed, in a simple manner, towards a high-voltage part for measuring the magnitude and the frequency spectrum of the voltage therein. The measuring device shall also detect the presence of transients or ionic discharge of a high-voltage part. This is achieved according to the invention by a measuring device comprising a sensor which detects changes in a directed part of an electric field and a signal converter, which sensor and measuring device, respectively, exhibit the characteristic features described in the independent claims. The invention also relates to a method of measuring the vol1tage in a high-voltage part, selected from a plurality of such parts and located at a distance, in an electric power system.

is 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 fieldgenerating objects may influence and sometimes completely dominate such a measurement. it is, therefore, not possible to distinguish from the measuremenit 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.

According to a first aspect, 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 a high-voltage apparatus. According to the invention, a directed part of WO 98/05974 WO 9805974PCTISE97/01289 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 Lo somne 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-quantities of the.

electric field are efficiently prevented by the screen electrode.

is in a preferred embodiment, the electrodes are insulated f romi 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 ma~de. For the purpose of increasing the sensitivity or reinforcing the directional effect of the sensor, the inner electrode may be divided into subelectrodes which are insulated with each other and which may be placed both in the lateral and vertical directions.

According to a second aspect, the invention relates to a measuring device, including the sensor described above, for voltage measurement at insulation distance of a highvoltage part in an electric field with a plurality of field-generating components. The measuring device is arranged by connecting a signal converter to the sensor, whereby, when the screen electrode is connected to ground, a measuring device is obtained by means of which a directional sub-quantity of an electric field may be measured. This measuring device constitutes a simple, WO 98/05974 WO 9805974PCT/SE97/01289 6 inexpensive and reliable device for measuring, in a contactiess manner, an alternating voltage at a distance from a high-voltage conductor. The device is a broad-band device which, within a large frequency range, permits measurement in a simple manner also of the occurrence and magnitude of harmonic components of the object intended to be measured.

The signal converter includes members for impedance conversion, amplification, and may also include members f or filtering and digital con-version, 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 transmitter and a receiver..

When applying the measuring device to voltage measurement, the screen electrode may be connected to a controllable potential instead of to ground, in which case, by phaselocking 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 f ield-generating source may be suppressed. The signal converter is thus brought to include also a conductor adapted for signals in the opposite direction. XIn 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.

3S The measuring device according to the invention has a wide field of use, especially in connection with measurement of alternating voltage in high-voltage equipment. one such field of use is enclosed or non-enclosed switchgear in electric power-related distribution systems. In a WO 98/05974 PCT/8E97/01289 7 preferred use of the measuring device, the device is Placed in an enclosed switchgear unit at insulation distance from each busbar, belonging to the respective phase, for measurement of alternating voltage. In threephase systems, the measuring devices may be placed at a common point and be individually directed towards a respective busbar in the switchge ar.

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

In distribution systemr, with onie phase only, the accuracy is increased in a simple manner by placing a plurality of measuring devices around the conductor. In a preferred use, for example in enclosed switchgear with one phase only, the accuracy of the voltage measurement is increased by placing four measuring devices rotationallysymmetrically around an extended part of the conductor.

Since the measuring devices are placed equidistantly from the conductor, the correct value of the voltage is obtained from the mean value of the four measuring devices.

The sensitivity to variations in the distance between the measuring object and the measuring device may b~e utilized for detecting movement of the measuring object. By placing at least three measuring devices in fixed Positions around a high-voltage conductor, it may be detected if the conductor is moved and in which direction this movement takes place.

WO 98/05974 PCT/SE97/01289 A method for obtaining a stable measurement distance to a high-voltage part is achieved by arranging the measuring device and the measuring object 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 device 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.

An additional preferred use of the measuring device is measurement of direct voltage in a high-voltage apparatus.

This is made possible by creating, in the incident rectified electric field entering through the opening in the screen electrode, a known variation which may be detected in a simple manner. Such a variation of the incident electric field ia achievjed by covering the opening in the screen electrode with a stable frequency by a conducting screen connected to ground. This can be performed in the simplest manner by applying a plate or a plurality of plates to a rotating shaft connected to ground. Through this rhythmic screening of the opening of the surrounding electrode, a varying electric field is created, from which the magnitude of a directed part of the direct voltage of a high-voltage part can be measured.

Another way of achieving a varying field from which the sought magnitude can be measured is to arrange the inner electrode to oscillate with a known frequency.

A measuring device according to the invention is also especially suited, in connection with distribution networks, to allow to control relay protection functionalities, in which case the device, by its low investment cost, may concentrate the measuring points and hence increase the selectivity. The measuring device is also especially suited for measuring voltage in a conductor in connection with debiting of energy consumption. For this purpose, the measurement is to be combined with a diffe- WO 98/05974 PCT/SE97/01289 9 rently measured current through the conductor, whereby the electric energy which passes may be determined.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in greater detail by description of an embodiment with preferred embodiments thereof with reference to the accompanying drawings, wherein Figure 1 Figure 2 Figure 3 Figure 4 shows a view, partly in section, of a measuring device including a capacitive sensor and a signal converter for directed voltage measurement according to the invention; shows a view, partly in section, of an alternative embodiment of the measuring device; shows a calculated.distribution of an electric field penetrating through the opening of the screen electrode; shows a view, partly in section, of an insulator with a measuring device according to the invention applied thereto; shows an explanatory sketch of an enclosed switchgear unit with three busbars with a measuring device according to the invention associated with each of the busbars; shows an explanatory sketch of a transmission tower for transmission of three-phase alternating current, on which a measuring device according to the invention is mounted for voltage measurement; shows an explanatory sketch of an enclosed switchgear unit for a phase with a measuring device according to the invention; Figure 5 Figure 6 Figure 7 WO 98/05974 WO 9805974PCT/SE97/01289 Figure 8 Figure 9 Figure 10 shows an explanatory sketch of an enclosed switchgear unit for a phase with four measuring devices according to the invention; shows a view, partly in section, of a preferred emibodiment of a measuring device with the inner electrode divided into sub-electrodes insulated from one another; and shows a view, partly in section, of a preferred embodiment of the measuring device, which for measurement of direct voltage comprises a rotating wing which rhythmically covers the opening in the screen electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a measuring device 10 for directed voltage measurement according to the present invention. The measuring device 10 is intended to measure alternating voltage at insulation distance from a conductor 22. The measuring device 10 comprises a capacitive sensor 11 and a signal converter 13, The capacitive sensor has an inner electrode 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 conductor 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 s their limiting surfaces are conducting. For example, an electrode may be made of a non-conducting 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 plane extent in a plane parallel to the opening of the screen electrode. The inner electrode is WO 98/05974 WO 9805974PCT1SE97/01289 11 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 Figure 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 11. Primarily, the signal is adapted to condition the signal from the sensor, which is very dis turbanc e-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 11 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 device 10. The transmission of the signal may be arranged both electrically and optically, but also in a contactiess manner via a transmitter and a receiver.

The capacitive sensor 11 is capable of sensing the electric field with a large bandwidth with respect to frequency, usually between zero and several thousand Hertz. it is, therefore, advantageous to arrange a broad-band signal converter 13 to the measuring device. The members for impedance conversion 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 WO 98/05974 WO 9805974PCT/SE97/01289 12 high-pass filters. During measurement, these may each deliver 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 dynamics and resolution of the desired measurement qu~antity. 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 (pulse Locked Loop), makes it possible, for example in a threephase system, to lock the screen electrode 14 of the measuring device 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 device 10 for measuring alternating voltage at insulation distance from a high-voltage conductor 22, the measuring devic e 10 is directed towards the conductor 22. For the inner electrode to be able to sense a drawn part of the electric f ield, 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 centre of the inner electrode and through the mid-point of the opening is directed towards the measuring object. Before the actual measurement is started, the measuring device 10 is calibrated in situ. The calibration is carried out by applying a known voltage, whereupon measurement with the measuring device is performed. The measuring device is thus calibrated by adjusting the measured value to correspond to the known voltage. When this has been completed, the actual measurement may be started.

One advantage of the measuring device 10 according to the invention is that the measuring device need not be brought into galvanic contact, or even in Contact with the conduc- WO 98/05974 WO 9805974PCT/SE97/01289 13 tor, the voltage of which is to be measured. The measurement may instead be performed at insulation distance from the conductor 22, which means that the measuring device is semiprotected 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 device has an exceedingly simple design and is therefore very inexpensive to manufacture and is also reliable. The measuring device may be advantageously arranged to constitute a detector, a-so-called PD (Partial Discharge) detector, for transients and ionic discharge, of a measuring object. Because of its reliability, its broad-band design and slight investment cost, the measuring device is' exceedingly well suited in connection with energy measurement for debiting of consumed electric energy and when functioning as relay protection.

Figure 2 shows an alternative embodiment of the measuring device 10 according to the invention. in the same way as in the preceding example, the measuring device comprises a sensor 11 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 measuring object. 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 plane extent which is substantially parallel to the plane of the opening.

The sensor 11 is not limited, as in dicted 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, 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.

WO 98/05974 WO 9805974PCT/SE97/01289 14 Figure 3'shows a calculation of the distribution of an electric f ield penetrating through the opening of a groundconnected 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 f ield which penetrates through the opening initially has a direction parallel to the normal to the opening. Further inside 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 gsame time its screening properties are maintained.

Figure 4 shows an advantageous use of a measuring device according to the invention. In the examnple shown, the measuring device is applied at the end of a hollow insulator 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 a highvoltage apparaturs (not shown) whereas the second pole is connected to ground. The measuring device is applied to the second pole 28 with its screen electrode 14 connected to the pole and with its inner electrode 12 insulated and adjustaibly fixed to the screen electrode 14. By the described use, an exact, non-varying distance between the measuring object and the measuring device is obtained. This is advantageous since a variation of the distance jeopardizes the accuracy in the voltage measurement.

WO 98/05974 PCT/SE97/01289 To further screen unwanted electric fields, the two poles may be provided with screens (not shown) in the form of plates of conducting 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 SF5 type, the smaller insulation distance on the inside of the insulator, which is caused by the gas, may be utilized such that the measuring device in its entirety is housed inside the insulator. An insulator including a device for voltage measurement may thus be manufactured in a simple manner as a finished product.

Figure 5 shows an explanatory sketch of a switchear unit with busbars R, S, and T for three-phase alternating voltage surrounded by an enclosure 29. According to the figure, in the upper part of the sWitchgear and connected to the enclosure 29, there is arranged a measuring device 10., intended for each one of the busbars, for directed voltage measurement according to the invention. 'The three measuring devices have been brought together into a common position to achieve a simple installation and wiring, respectively, and each measuring device is directed towards its respective busbar. However, each measuring device may be placed at insulation distance in an arbitrary position -within the switchgear. The common location, however, is an advantage since the greatest possible angle between the sensitivity directions of the measuring devices may then be achieved. With this type of use of the measuring device, among other things, mounting time and space in the enclosure are saved.

An explanatory sketch of a transmission tower. with measuring devices 10., 105 10T according to the invention is shown in Figure 6. The transmission tower comprises a framework beam 31 supported by two framework columns 30., 3 0 and three suspension insulators 32,, 329, 32?, which are arranged in the framework beam and each of which supports a high-voltage conductor 22., 22B, 227. An enlarged picture shows, in two side views, how a measuring device 10T iS applied to the WO 98/05974 WO 9805974PCU/SE97/01289 16 framework beam 31. close to the suspension insulator 3 2T, which supports the high-voltage line The positioning of the measuring device at such a suspension point for the line implies that the measurement distance becomes defined and riot significantly changed when the. line moves caused by wind or any other force. The measuring device may advantageously be integrated also with the insulator, as Shown in Figure 4.

The converted signal delivered from the measuring device may be transmitted to an analyzer both electrically, optically and in a wireless manner with the aid of telephony. With a measuring method according to- the example, by collection of measurement values from a plurality of fixedly installed measuring devices along a network, the transmission or distribution may be checked in an advantageous manner.

An explanatory sketch of a switchgear unit with only one phase surrounded by an enclosure 29 is shown in Figure 7. A measuring device 10 according to the invention for measuring the alternating voltage from one single busbar 33 may here, in the same way as in the examp le in Figure 5, be arranged at insulation distance in an arbitrary position within the enclosure. one advantage with this application is that no other electric fields are generated in the enclosure, which permits the positioning of the measuring device to be controlled for other reasons. A use of a measuring device according to the invention in enclosed switchgear with one phase only constitutes an exceedingly simple and cost-saving installation.

It has been indicated above 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 fact may be utilized for the purpose of studying a movement of a measuring object- A method for measuring voltage from a measuring 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 WO 98/05974 WO 9805974PCT/SE97/01289 17 measurement accuracy is shown in Figure S. Like the previous f igure, Figure 8 shows an explanatory sketch of switchgear with one phase only, surrounded by an enclosure 29. Around a centrally placed busbar 33, four measuring devices 10a, l0b, 10c, 10d according to the inventiOXI are each arranged in a corner of a cross section of the switchgear. The converted signal from each one of the measuring devices 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 are formed.

By geometrical calculations of standard type f rom 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 devices which carry out measurement on the same object. The measuring methods described are not limited to be applied to enclosed switchgear only, but may also be applied to free conductors and to non-enclosed switchgear. In non-enclosed switchgear, it is particularly valuable to, be able to correct the measurement value for changes in the background field.

An alternative embodiment of a sensor 11 included in a measuring device according tp the invention is shown in Figure 9. The sensor 11 comprises a screen electrode 14 which is provided with an opening 16 and which surrounds a first inner sub-electrode 12a and a second inner subelectrode 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 WO 98/05974 WO 9805974PCT/SE97/01289 1s prevent corona in the example shown, the inner subelectrodes 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 subelectrodes are each connected to a. respective signal converter- (not shown) and, in the samne way as in example 1, the measurement 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 determine, on the one hand, whether by measuring object is in front of the sensor and, on the other hand, whether the measuring object moves during the measurement. An electric f ield 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. Especially in connection with point-shaped high-voltage objects, the inner electrode is formed into a plurality of uniform sectors of a circle. in other embodiments, for example in case of the elongated sensor described above, it is instead advantageous to form the sub-electrodes as a plurality of plates arranged side-by-side.

An additional alternative embodiment of a measuring device according to the invention is shown in Figure 10. This example shows a measuring device which may comprise one of the embodiments according to the examples in Figure 2 or 9. supplemented by a device for creating, at the inner electrode with a known frequency, a variation of a rectified electric field. A measuring device 10 includes a sensor 11 and a signal converter 13. The sensor comprises an inner electrode 12 and a screen electrode 14 surrounding the inner WO 98/05974 WO 9805974PCT/SE97/01289 19 electrode and provided with an opening 16. The inner electrode is fixed to an insulating tube 17, which is adjustably fixed to the screen electrode, such that the inner electrode 12 may change in a direction designated A.

The inner electrode is connected to the signal converter 13 by means of a conductor IS drawn through the tube 17, the signal converter being arranged in a space 21 surrounded by a screen 19.

A shaft 36, which is rotated by a drive means (not shown), is rotatably fixed to the measuring device, and on this shaft 36 a disc .35, which is parallel to the opening 16, is fixed. The shaft 36 and the disc 35 are connected to the same potential as the screen electrode 14. During rotation of the shaft 36, the disc 35, which extends, for example, along a semi-circle, will screen the opening with a known frequency. of the rectified electric field, a directed part will in this way, with variation between zero and full field strength, penetrate through the opening in the screen electrode. From this variation, the magnitude of the directed part of the rectified field, which falls through the opening, may be measured.

The disc 35, which is preferably made of metal, will, during rotation, alternately cover and thus screen the opening and leave the opening unscreened such that a variation of the penetrating stationary field is created. The invention is not limited to only comprise a disc but may also include a plurality of discs, preferably evenly divided on an axi.s.

Also, the discs may be formed to completely cover the opening 16, or only a part thereof. The disc or discs may also be arranged, with the same result, to screen the whole sensor or parts thereof.

The invention is not limited to the embodiments shown, but a plurality of modifications are possible within the scope of the appended claims. Thus, the screen electrode may, for example, comprise a plurality of openings and, in certain applications, be designed from net or a perforated material.

Claims (12)

1. A capacitive Sensor (11) for detection of a directed part of an electric field comprising an inner electrode (12) and a screen electrode characterized in that the screen electrode surrounds the inner electrode (12) leaving an opening whereby, when the sensor is directed towards a field-generating object, the part of the electric field penetrating through the opening is detected by the irmer electrode.

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

3. A sensor according to claim 1 or 2, cheaaterized in that the inner electrode (12) is adjustably fixed to the screen electrode (14).

4. A sensor according to any of the preceding claims, characterized in that the inner electrode (12) has a substantially plane extent, the plane of extent of which being substantially perpendicular to a normal to the opening (16) in the screen electrode (14). A sensor according to any of the preceding claims, characterized in that the inner electrode (12) is cup- shaped.

6. 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.

7. A measuring device (10) for measuring of voltage in a high-voltage part (22) surrounded by an electric field, characterized in that the measuring device (10) comprises a capacitive sensor including an inner electrode (1.2) and a screen electrode (14) connected to a controllable WO 98/05974 WO 9805974PCT/SE97/01289

21. reference potential, the s'creen electrode being adapted to screen off the inner electrode from a disturbing electric field, whereby the sensor, by sensing a directed part of the electric field penetrating into the inner electrode, at insulation distance measures the voltage of the high-voltage part. 8. A measuring device according to claim 7, characterized in that the screen electrode (14) and the inner electrode (12) are insulated from each other by a gaseous dielectric. 9. A measuring device according to claim 7 br 8, characterized in that the screen electrode (14) surrounds the inner electrode (12) and is arranged with an opening through which the inner electrode senses a directed part of an electric field. A measuring device according to claims 7 9, characterized in that the inner electrode (12) includes a plurality of sub-electrodes (12a, 12b). 11. A measuring device according to claims 7 characteriz.ed in that the measuring device comprises a signal converter (13) which is arranged close to the sensor 2S (11) and comprises members for am~plification and impedance conversion of the measurement signal. 12. A measuring device according to claim 11, characterized in that the signal converter (13) includes members for phase locking the screen electrode (14) and/or the inner electrode (12). 2.3. A measuring device according to any of claims 7 12, characterized in that the measuring device (10) is arranged at the grounded part of an insulator. 14. Use of a measuring device (10) according to claims 7 13 in switchgear. WO 98/05974 PCT/SE97/01289 22 Use of a measuring device (10) according to claims 7 13 in electric power networks for debiting of Consumed energy. 16. Use of a measuring device according to claims 7 13 in electric power networks for establishing relay functionalities. 17. A method for measuring voltage of a high-voltage part (22) surrounded by an electric field, characterized in that a directed part of the electric field is sensed at insulation distance with at least one capacitive sensor (211). 18. A method according to claim 17, characterized in that the sensor (11) is adapted to comprise an inner electrode (12) and a screen electrode (14) connected to a controllable potential, the screen electrode being adapted to screen off the inner electrode from disturbing electric fields, such that the electric field generated from the high-voltage part is sensed by the inner electrode. 19. A method according to claim 17 or 18, characterized in that the screen electrode (14) is adapted to be insulated from the inner electrode (12) by a gaseous dielectric. A method according to claims 17 19, characterized in that the screen electrode (14) is adapted to surroundi the inner electrode leaving an opening (16) through which a directed part of an electric field is sensed by the inner electrode. 21. A method according to claims 17 20, characterized in that a member for phase locking is connected to the sensor by means of which the potential of the screen elec- trode (14) and/or the inner electrode (12) is locked in relation to the high-voltage part, such that the effect of disturbing electric fields is suppressed. WO 98/05974 PCTISE97/012S9 23

22. A method according to claims 17 21, characterised in that the inner electrode (12) is brought to include a plurality of sub-electrodes (12a, 12b), whereby, by a comparison of the signals from the sub-electrodes, a change in the incident electric field caused by a transfer of the high-voltage part is detected.

23. A method according to claims 17 22, characterized in that the sensor (11) is arranged in relation to the high- voltage part (22) at a fixed distance.

24. A method according to claims 17 23, characterized in that the sensor (11) is brought to be alternately screened off, with a known frequency, from a directed static electric field, whereby a varying electric field is created in the sensor, from which field the direct voltage of a high- voltage part (22) is measured. Use of a method according to claims 17 24, in switch- gear.

26. Use of a method according to claims 17 24, in an elec- tric power network for debiting consumed energy.

27. Use of a method according to claims 17 24. in an elec- tric power network for establishing relay functionalities.