A DEVICE FOR SENSING OF ELECTRIC DISCHARGES IN A TEST OBJECT

Abstract: A device for sensing electric discharges in a test object (1) with at least two electric connection conductors (1a, 1b, 1c, 1d) comprises transducer equipment and evaluation equipment (PA, ABU, LU). The transducer equipment comprises at least one transducer (6a, 6b, 6c, 6d) for direction-sensitive sensing of current pulses through the connection conductors by sensing a magnetic field, generated by the current pulses, and the polarity thereof. The transducer delivers a transducer signal (v1", v2", v3", v4", v11", v21") to the evaluation equipment in dependence on said current pulses and their direction. The evaluation equipment generates, in dependence on received transducer signals, an indicating signal (IND) indicating an electric discharge in the test object.

Description: A device for sensing of electric discharges in a test object TECHNICAL FIELD The present invention relates to a device according to the preamble to claim 1 for sensing of electric discharges in a test object. BACKGROUND ART The presence of local electric discharges, so-called partial discharges, in the insulation system for electric installa¬ tion components or electric high-voltage apparatus, such as, for example, instrument transformers, switching devices, generators, power transformers, cables, etc. , constitutes an important parameter when assessing the quality, condition, and expected service life of the component. Therefore, the acceptance tests for components for an electric high-voltage installation normally comprise a testing with respect to the occurrence of partial discharges under well defined condi¬ tionε, for example according to IEC Publ. 270 "Partial Discharge Measurements". In addition to this, different methods and devices have been developed for testing at the site of the installation for the purpose of estimating, after some time in operation, the condition of components and hence, for example, creating a more reliable basis for decision on preventive maintenance. For power generators, for example, measurement equipment is also available which allows a continuous monitoring during operation. In installations and/or components which comprise several electric circuits, it is also important, in addition to being able to distinguish partial discharges from external noise, to be able to locate a detected partial discharge to a definite electric circuit, for example a certain generator coil in a power generator. A partial discharge in a component gives rise to current pulses through the conductors by which the component is connected to the surroundings and these current pulses can be sensed either by measuring the voltage caused by the current pulses when passing through a measuring impedance connected to one of the conductors, or by a direct or indirect current measurement. During voltage measurement, the measurement equipment comprises a coupling capacitor, which must be free from partial discharges under the condi- tions under which the measurement is carried out, and the test object, the coupling capacitor and the measuring impe¬ dance are galvanically interconnected, in a manner known per se, and connected to the surrounding components and testing equipment, respectively. One problem when measuring partial discharges is to dis¬ tinguish measurement signals emanating from partial dis¬ charges in the test object from disturbances emanating from partial discharges in the surrounding components or in the testing equipment or which have another origin but a frequency and amplitude spectrum resembling the spectrum of the partial discharges. Such disturbances may be passed to the test object through the conductors by which they are galvanically connected to the surroundings but may also, in the frequency ranges used, be captured by the measurement device by electromagnetic radiation from the surroundings. During testing in a test room environment, the noise level may be reduced by screening the test room and filtering the voltage supply, but this is normally not possible when testing at the site of an installation. Disturbances of the kind mentioned may also be suppressed by so-called balanced test circuits where the component to be tested is balanced against a reference impedance, in which case disturbances occur as cophasal signals across measuring impedances series-connected to the test object and to the reference impedance. The reference impedance may consist of a com¬ ponent similar to the test object or of an impedance imita¬ ting this, usually a reference capacitor, which is then to be free from partial discharges during the testing. The balanced test circuits may also be designed as complete bridge connections. The above-mentioned coupling capacitors and the reference impedances and measuring impedances, respectively, must be galvanically connected to the component during the testing and hence also to the high- voltage source to which the test object is connected. Voltage measurement across a measuring impedance has the fundamental disadvantage that the measurement sensitivity decreases with increasing capacitance of the test object. When sensing the partial discharge by current measurement, only one transducer is required, sensitive to the magnetic field associated with the current pulse. The measurement signal obtained from such transducers may thus be kept galvanically separated from the test object and the high- voltage circuit connected to the test object, thus, inter alia, eliminating problems such as the influence from loops in the ground circuits. Current-measuring transducers eliminate the need of coupling capacitors and measuring impedances and can be designed with very small dimensions, which makes it simple to arrange them at components of varying dimensions and shapes. German patent document DE 37 08 731 describes an electric switching device for detecting interference pulses, in particular partial discharges, in a high-voltage installa¬ tion. Between a live part of the installation and ground potential, a voltage divider of a capacitive nature is connected. The voltage from the medium voltage terminal thereof is supplied, via impedance networks adapted for the purpose, to a circuit for evaluating partial discharges. Such a device cannot usually distinguish pulses emanating from partial discharges in the component from interference pulses originating from outside the component in question, and the device shown in the patent document suggests a method of dealing with this problem. A current transformer is connected with its primary winding to a conductor which. at high-voltage level, connects the component to the surroundings, and its secondary winding is divided into two parts by means of a centre tap, each part being loaded by a resistance. The secondary winding is wound in such a way that the voltages between the centre tap and the respective terminal are equal in magnitude but of opposite phase posi¬ tion. The medium-voltage terminals of the voltage divider are connected to the centre tap on the secondary winding of the current transformer such that the voltage at the res- pective terminal of the secondary winding consists of the vectoral sum of the voltage from the voltage divider and the voltage across the corresponding part of the secondary winding. Depending on the current direction through the primary winding of the current transformer, different magni- tudes of these voltages are thus obtained. A partial dis¬ charge within the component gives rise to a current pulse through the primary winding of the current transformer in a direction away from the component towards its surroundings whereas an externally arriving interference pulse gives rise to an interference pulse through the primary winding of the current transformer in the opposite direction. By evaluating the voltages sensed at the terminals of the secondary win¬ ding, it can thus be determined whether a detected pulses emanates from a partial discharge in the component in question. The current transformer may advantageously be designed as a bifilarly wound Rogowski coil. In components with a plurality of voltage terminals, a more accurate location of a partial discharge may be achieved by arranging the switching device at more than one voltage terminal. The measurement principle is thus based on a voltage propor¬ tional to the voltage across the component being available and therefore comprises, in addition to a device for direc¬ tion-dependent current sensing, also a voltage divider of a high-pass character. In the event that such a voltage divi- der cannot be galvanically connected at the component in question, the use of a field probe or an aerial for capaci¬ tive sensing of the voltage on the component is proposed. European patent document EP 0 061 254 B describes a device for monitoring of partial discharges in high-voltage equip¬ ment comprising more than two components and exhibiting more than two discharge paths for a partial discharge, for example a transformer substation, in which case it should be possible to individually and continuously monitor each selected component and to distinguish partial discharges from noise and signals emanating from externally arriving voltage transients. The described device is designed for monitoring a number of components in the form of high- voltage devices, connected between a common high-voltage busbar and ground potential or a low-voltage conductor. At their connection to ground potential and to the low-voltage divider, respectively, the components to be monitored have a discharge detection unit inductively connected to the connection by means of a high-frequency current transformer. A partial discharge in a component gives rise to a current pulse through the current transformer belonging to the component but, since the components, the high-voltage busbar and ground potential and the low-voltage divider, respec¬ tively, form a network for the partial discharge paths, also through the other current transformers, but with reversed polarity therein. A voltage transient or other disturbance on the high-voltage busbar, on the other hand, gives rise to current pulses of the same polarity through all the current transformers. Each one of the discharge detectors delivers a coded signal, the pulse width of which corresponds to the polarity of the detected current pulse, to a decoder. The decoder comprises a number of demultiplexers, which generate and store a signal pattern corresponding to the detected state of polarities for each of the components. This signal pattern is decoded and evaluated, whereby a deviating pola¬ rity of a detected current pulse from one component indi¬ cates a partial discharge therein. The device requires an extensive electronic system for coding and evaluation, comprising monostable multivibrators, memory and decoding circuits, and the system in its entirety is necessarily given a large physical extension. A pulse emanating from a partial discharge in a component is damped during its propagation along the busbar and through the other components, which may result in not all the discharge detection units delivering a signal to the demultiplexers. The device described above will thus not function as inten¬ ded and a special electronic circuit has been introduced to indicate and signal this state. It is also conceivable that more than one of the discharge detection units deliver a signal with a polarity deviating from that of the remainder, which may be due to correctly or incorrectly detected par¬ tial discharged. Also this results in the device not func¬ tioning in the intended way and also this state is sensed and signalled by means of a special electronic circuit. SUMMARY OF THE INVENTION The object of the invention is to provide a device of the kind stated in the introduction, which, by means of trans¬ ducers which can be applied in a simple manner to components of a varying size and shape also during continuous monito¬ ring under regular operating conditions, allows a simple and reliable distinction between, on the one hand, partial discharges in a predetermined component and, on the other hand, external interference and partial discharges in connected components or test equipment. In this way, a safe and reliable location of a possible partial discharge to the predetermined component can be carried out. What characterizes a device according to the invention will become clear from the appended claims. Advantageous improvements of the invention will become clear from the following description and claims. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in more detail by descrip¬ tion of embodiments with reference to the accompanying drawings, wherein Figure 1 shows a device according to the invention in a test object with two connection conductors and one transducer at each of these, Figure 2A shows a device according to the invention in a test object with four connection conductors and one transducer at each of these, Figures 2B-2C show transducer signals in a device according to Figure 2A during a discharge in, and outside, the test object, respectively, Figures 3A-3B show embodiments of evaluation equipment in a further development of the device according to Figure 1, Figures 3C-3D show transducer signals in a device according to Figures 3A-3B during a discharge in, and outside, the test object, respectively, Figure 4 shows embodiments of transducer equipment in a further development of the device according to Figure 1, Figure 5 shows an embodiment of evaluation equipment in a further development of the device according to Figure 1, comprising a model of the test object, Figures 6A-6D show embodiments of a transducer in a device according to the invention, Figures 7A-7C show further embodiments of a transducer in a device according to the invention, Figures 8A-8D show further embodiments of a transducer in a device according to the invention, Figures 9A-9C show further embodiments of a transducer in a device according to the invention, Figure 10 shows a further embodiment of a transducer in a device according to the invention. Figures 11A-11B show embodiments of transducer equipment when the test object consists of a cable joint, Figure 12 shows embodiments of transducer equipment when the test object consists of a cable termination. Figure 13 shows embodiments of transducer equipment when the test object consists of a voltage transformer. Figure 14A shows embodiments of transducer equipment when the test object consists of a three-phase power trans- former, Figure 14B shows embodiments of transducer equipment when the test object consists of a tap-changer for a power transformer, Figures 15A-15B, Figure 16, and Figures 17A-17B show embodiments of transducer equipment when the test object consists of a high-voltage bushing, Figure 18 shows an embodiment of movable transducer equipment when the test object consists of a high- voltage bushing, Figure 19 shows an embodiment of the invention when the test object consists of a high-voltage generator, Figure 20 shows embodiments of transducer equipment when the test object consists of a switchgear unit, Figure 21 shows embodiments of transducer equipment when the test object consists of a gas-insulated switchgear unit, Figures 22A-22B show a further embodiment of transducer equipment when the test object consists of a power transformer, Figure 23 shows an embodiment of evaluation equipment in transducer equipment according to Figures 22A-22B, Figures 24A-24B show embodiments of transducer equipment when the test object consists of a current transformer, Figure 25 shows an embodiment of transducer equipment when the test object consists of a movable path, Figure 26 shows an embodiment of transducer equipment com¬ prising a superconducting sensor, and Figure 27 shows an embodiment of transducer equipment com- prising series-connected Rogowski coils. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a test object 1 with two electric connection conductors la and lb. The test object consists of an elec¬ tric component which is insulated against its surroundings, or of an electric high-voltage apparatus, such as, for example, a cable, a power transformer, a switchgear cubicle, or a high-voltage generator, but is shown generally in the figure as an impedance element. The test object is energized by power frequency, that is, usually 50 Hz or 60 Hz, which is marked in the figure by a generator 2a, connected to the test object via an external impedance element 2b, which impedance element usually com¬ prises an inductive element, in a series-connection with the generator. Across the test objects there are stray capaci- tances and interference voltages, marked in the figure by capacitive elements 3a, 3b and an interference voltage generator 3c. Additional components, marked by 4 in the figure, are connected to the generator 2a by way of a con¬ ductor 5 and to the test object 1 by way of the conductor lb. The conductor 5 is connected to ground by way of a ground conductor 5' . An electric discharge in the insulation system of the com¬ ponent, that is, a partial discharge, gives rise to a current pulse which, by current splitting, is distributed through the connection conductors of the component to the circuit in which the component is included. If such a discharge of a certain polarity occurs in the component 4, the current pulse through the connection conductors of the test object will have the direction marked by dashed arrows in the figure, that is, the current pulse passes into the test object through the connection conductor lb and passes out of this through the connection conductor la. If, on the other hand, such a discharge occurs in the test object, the current pulses through the connection conductors of the test object will have the direction which is marked by unbroken arrows in the figure, that is, the discharge generates current pulses which essentially simultaneously flow through the two connection conductors, either in a direction out from the test object or in a direction into this. Whether the current direction is out from or into the test object is dependent on the polarity of the discharge. By "essentially simultaneously" is meant in this context that the current pulses in the two connection conductors have their common origin in one and the same discharge in the test object. It is to be understood, however, that since at least certain kinds of test objects exhibit an impedance which, in the frequency range used for sensing of current pulses, damps the amplitude of and/or phase-shifts currents which pass through the test object through their connection conductors, generally no absolute simultaneousness occurs. If, for example, discharges occur in the component 4, current pulses through the connection conductor la, because of deviation and phase shift in the test object, will occur delayed in phase and damped in relation to current pulses through the connection conductor lb. Transducer equipment comprises a transducer 6a arranged at the connection conductor la and a transducer 6b arranged at the connection conductor lb. The sensors sense the magnetic field which is generated by the current pulses through the connection conductors and are direction-sensitive with res- peet to the polarity of the magnetic field. Each one of the transducers delivers a transducer signal vl" and v2", respectively, when a current pulse flows through the respective connection conductor, whereby the polarity of the transducer signal is dependent on the polarity of the magnetic field and hence on the direction of the current pulse. In this embodiment, the transducers consist of Rogowski coils 61a and 61b, respectively, which surround the respective connection conductor. However, for the sake of clarity, the coils are shown in the figure as elongated coils along the respective connection conductor. In this connection, a Rogowski coil means a coil arranged in such a way in an electric conductor that the axis of the coil is directed at least substantially in the same direc- tion as the field lines in the magnetic field that an electric current in the conductor generates around the conductor. The coils are arranged such that voltages of the same polarity are generated at the terminals on the coils which are marked in the figure by a point, when current pulses flow through the connection conductors in a direction so as to pass through the coils from that end of the coils marked by a point to the unmarked end. The transducer can also be based on other principle, known per se, which give a direction-sensitive sensing of the mag- netic field generated by the current pulses, for example comprising Hall elements, magnetoresistive elements, or superconducting elements, so-called SQUIDs. Each one of the transducer signals is supplied to evaluation equipment comprising a filter unit PA, associated with the respective transducer, a signal-mixing unit ABU and a logic unit LU. Each one of the filter units comprises resistive and capaci- tive impedance elements R, C for tuning the transducer to a resonance frequency which is within the frequency range of interest to the sensing, which typically is in the range 0.1 - 1 MHz. Further, each one of the filter units comprises an amplifier AMP and a bandpass filter BP, the bandpass of which lies within the frequency range of interest to the sensing. The filter units PA, the resistive and capacitive impedance elements R, C, the amplifiers, and the bandpass filters are in the figure marked PAa, PAb, Ra, Ca, Rb,Cb, AMPa, AMPb and BPa, BPb, respectively. The output signals from the bandpass filters are designated vl' and v2 ' , respectively. The signal-mixing unit ABU comprises an adding member ADD, which is supplied with the output signals vl' and v2 ' from the bandpass filters via respective weighting members BM, schematically marked in the figure as potentiometers BMa and BMb, respectively. The output signals vl and v2, respecti¬ vely, from the weighting members, that is, from the movable terminals of the potentiometers, are supplied to the adding member, the output signal S of which constitutes a sum signal formed as the sum of the signals vl and v2. The sum signal S is supplied to a discriminator DC which is comprised in the logic unit and which delivers an indicating signal IND when its input signal in magnitude exceeds a preselected comparison value. The device described with reference to Figure 1 operates as follows. When a discharge occurs outside the test object, for example in the component 4, the output signals vl' and v2 ' from the bandpass filters will be of different polarity, since the current pulse associated with the discharge passes the transducer 6b in a direction from that end of the coil marked by a point to the unmarked end of the coil, but passes the transducer 6a in a direction away from the un¬ marked end of the coil to that end which is marked by a point. For example, by sensing of an interference pulse externally applied in some manner known per se, the weighting members may be adjusted such that the output signals vl and v2 from them will have the same amplitude. This means that the sum signal S under these circumstances will deviate insignificantly from zero, and therefore the discriminator DC, with a suitably selected comparison value, will not deliver any indicating signal. If a discharge occurs in the test object, the output signals vl ' and v2' from the bandpass filters will be of the same polarity, since the current pulses associated with the dis- charge pass both transducers either in a direction from that end of the coil marked by a point to the unmarked end or in the reverse direction. Under these conditionε, the sum signal S will deviate from zero, and therefore the discri¬ minator DC, with a suitably selected comparison value, will deliver an indicating signal. The indicating signal is forwarded to monitoring equipment SUEQ, in some manner known per se, possibly via a signal interface unit BUF. In particular in test objects with an inductive impedance and exhibiting a high capacitance to ground, it is advan¬ tageous to choose the passband of the bandpass filters such that the natural resonance frequency of the test object falls outside the passband thereof. Figure 2A shows a test object with four connection conduc¬ tors la, lb, lc, ld, which within the test object are in galvanic connection with each other. Each one of the connection conductors is associated with a transducer 6a, 6b, 6c, 6d and a filter unit PAa, PAb, PAc, PAd, respec¬ tively, of the same kind and arranged in the same way as those described with reference to Figure 1. The output signals from the respective bandpass filters, designated vl' , v2' , v3' , v4' , are supplied, in a manner analogous to that described for the device with reference to Figure 1, to weighting means BMa, BMb, BMc, BMd, and the output signals vl, v2, v3, v4 from these are supplied to the adding member. The sum signal S from the adding member, which thus in this embodiment consists of the sum of the signals vl, v2, v3 and v4, is supplied to the discriminator DC which delivers the indicating signal IND when the sum signal exceeds a preselected comparison value. Figure 2B shows a typical appearance of the signals vl, v2, v3, v4 and S, plotted on the vertical axis in the diagrams, as a function of the time, plotted on the horizontal axis, in case of an electric discharge in the test object, when transducers sense current pulses which essentially simul- taneously flow through all the connection conductors, either out from or into the test object. All the signals vl, v2, v3, v4, which substantially have the shape of a damped sinusoidal oscillation, are then essentially in the εame phase position, and therefore the maximum amplitude of the sum signal consists at least approximately of the sum of the maximum amplitudes of the signals vl, v2, v3 and v4. Figure 2C shows a typical appearance of the signals vl, v2, v3 , v4 and S in case of an electric discharge outside the test object. The current pulses associated with the dis¬ charge reach the test object via the connection conductor lb and pass through the test object in order to leave this through the connection conductors la, lc and ld. The signals vl, v2 and v4 will have essentially the same phase position whereas the signal v3 is essentially in reverse phase, and therefore the maximum amplitude of the sum signal consists of the sum of the maximum amplitudes of the signals vl, v2, v4 reduced by the maximum amplitude of the signal v3. An advantageous improvement of the invention, as described with reference to Figure 1, is shown in Figure 3A. The signal-mixing unit ABU comprises, in addition to the adding member ADD, also a subtracting member SUB. The output signals vl, v2, respectively, from the weighting members are supplied to the subtracting member, the output signal D of which constitutes a difference signal formed as the diffe¬ rence of the signals vl and v2. The sum signal S and the difference signal D are supplied to a comparing member Q in the form of a quotient generator which is arranged in the logic unit LU and which forms the quotient of the sum and difference signals. The output signal SQ from the comparing member is supplied to the discriminator DC which delivers an indicating signal when the signal SQ exceeds a preselected comparison value. It is realized from the foregoing descrip¬ tion that the amplitudes of the respective sum and diffe¬ rence signals will be in reverse proportion to each other depending on whether the transducers indicate external interference pulses, associated with discharges outside the test object, or interference pulses associated with dis¬ charges in the test object. In the former case, the maximum amplitude of the sum signal will be comparatively low, whereas the amplitude of the difference signal will be comparatively high, whereas in the latter case the condi- tions are the reverse. The comparison between the sum and difference signals thus results in an increased sensitivity in the determination of the origin of the discharges. The comparing member Q may in another embodiment, illu¬ strated in Figure 3b, comprise a multiplier M and a summator SUM. In the summator there is formed as output signal SQ the difference of the sum signal S and a signal D', where the signal D* is formed of the difference signal D, multiplied in the multiplier by an optional constant K. The constant K may be chosen to be of the typical order of magnitude of 2 to 3. Figure 3C shows a typical appearance of the signals vl, v2, the sum signal S = vl+v2 and the difference signal D - vl- v2, plotted on the vertical axis in the diagrams, as a function of time, plotted on the horizontal axis. In case of an electric discharge in the test object, the transducers sense current pulses which essentially simultaneously flow through all the connection conductors, either out from or into the test object. The signals vl, v2 then have essen¬ tially the same phase position, and therefore the maximum amplitude of the sum signal consists at least approximately of the sum of the maximum amplitudes of the signals vl and v2, whereas the maximum amplitude of the difference signal becomes at least approximately zero. Figure 3D shows a typical appearance of the signals vl, v2, the sum signal S = vl+v2 and the difference signal D = vl-v2 in case of an electric discharge outside the test object. The signals vl and v2 are essentially in reverse phase, and therefore the maximum amplitude of the difference signal consists at least approximately of the sum of the amounts of the maximum amplitudes of the signals vl and v2, whereas the maximum amplitude of the sum signal becomes at least approximately zero. A corresponding increase of the sensitivity of the device in the determination of the origin of the discharges may be achieved with an embodiment of the device as illustrated in Figure 4. Each one of the connection conductors of the test object is associated with two transducers 6a, 6c and 6b, 6d, respectively, each one with an associated filter unit and an associated weighting member PAa, PAc, Ba, Be and PAb, PAd, BMb, BMd, respectively. Transducers for sensing of current pulses flowing through the same connection conductor are adapted so as to deliver transducer signals of the same polarity when a current pulse flows through the connection conductor. The sum signal S is formed in the same way as has been described with reference to Figure 1 as a sum formed in dependence on transducer signals originating from the trans¬ ducers 6a and 6b, whereas the difference signal is formed in the subtracting member SUB in dependence on transducer signals originating from the transducers 6c and 6d. It should be realized that sum and difference signals will have similar appearances both in the embodiment according to Figure 3A and in the embodiment according to Figure 4. Figure 5 illustrates a further improvement of the invention, which is especially advantageous when the test object exhibits an impedance which, in the frequency range used for sensing of current pulses, in a manner disturbing the sen- sing damps the amplitude of and/or phase-shifts currents passing through the test object via the connection conduc¬ tors thereof. The test object 1 is shown in this figure depicted as a Fl-network comprising three impedance elements 101, 102 and 103, of which the first one is connected between the connection conductors la and lb, whereas the last two ones are connected between the conductor 5 and the connection conductors la and lb, respectively. When the evaluation equipment is designed, for example, as described with reference to Figure 1, the weighting members BMa and BMb cannot under these conditions be set such that the sum signal becomes zero or very near zero in case of discharges outside the test object. If the discharge is conceived to occur in the component 4, the transducer signal emanating from the transducer 6a, because of deflection and phase shift in the impedance elements representing the test object, will appear delayed in phase and damped in relation to the transducer signal emanating from the transducer 6b. Inversely, if the discharge is conceived to occur in a component connected to the connection conductor la of the test object, the transducer signal emanating from the trans¬ ducer 6b will occur delayed in phase and damped in relation to the transducer signal emanating from the transducer 6a. A satisfactory extinction of the signals vl and v2 is thus made difficult when the test object in the frequency range in question exhibits properties which can be illustrated in a model as illustrated in Figure 5. A compensation for these properties can, however, be achieved by comprising in the signal-mixing equipment ABU a model of the test object which damps and/or phase-shifts the signals from which the sum signal is formed in a manner which imitates the amplitude damping and/or the phase shift undergone by a current pulse passing through the connection conductors of the test object. An example of such a model is shown in Figure 5. It comprises three impedance eleents ZI, Z2, Z3, each one with a characteristic similar to that of the impedance elements 101, 102, 103 in the test object and mutually connected in a manner analogous to these at connection points Pl, P2. Further, two impedance elements Z4, Z5 are connected to the model at connection points P3, P4, which impedance elements, in the manner described above, are adjusted for tuning the transducer to a resonance frequency which is within the frequency range of interest to the sensing. Each one of the transducers is connected, with one of its terminals, to the connection points Pl and P2, respectively, and with its other terminal to the connection points P5 and P6, respec¬ tively, at the impedance elements Z4 and Z5. Signals vl and v2 are sensed at the connection points P3 and P4 and are supplied to the adding member ADD. For example, by sensing of an interference pulse, externally applied in some manner known per se, the impedance elements ZI, Z2, Z3 may be set such that the output signals vl and v2 from the model will have the same amplitude and phase position, whereby the sum signal S becomes zero for discharges outside the test object. Figures 6A-6D show an embodiment of a transducer 7, which is particularly advantageous in those cases where the test object consists of a coil for a stator winding for an elec- trie power generator and this coil is located in a winding slot in the stator of the generator in which a further coil is located. Figure 6A shows the transducer in a side view. A thin insulated conductor is wound with a plurality of turns around a thin rectangular disc 72 of magnetically conducting material, for example iron, with low hysteresis losses, so as to form a coil 71 with two terminals 73, 74. The coil has its axis parallel to a short side of the rectangular disc and is surrounded by a screen 75 of an electrically conduc- tive material, for example aluminium foil. The transducer is shown in Figure 6A with the screen cut-away to the left of a section B-B. Figure 6B shows the transducer as viewed from the section B-B. Figure 6D shows the transducer embedded in a disc 76 of laminated plastic and with the terminals of the coil withdrawn from the disc in a slot 761 therein via a coaxial cable 77. Figure 6C shows a cross section of part of a stator for a generator with a winding slot 8. Two coils 81, 82 are located in the winding slot and the transducer is placed between two adjacent coil sides of the two coils. Figures 7A-7C show an embodiment of a transducer 7 of a composition similar to that described with reference to Figures 6A-6D, but with the difference that the coil has its axis parallel to a long side of the rectangular disc. By, for example, winding the coil around a disc of amorphous iron, the transducer can be made flexible in the longitu¬ dinal direction of the disc and, for example, be applied around a cable 9 as illustrated in Figure 7C. The transducers described with reference to Figures 6A-6D and 7A-7C are suitable to use in a frequency range of typi¬ cally 0.1 - 5 MHz. The coils comprised in the transducers can also be wound around a disc of a non-magnetic material. Figures 8A-8D show an embodiment of a transducer 7, com¬ prising a Rogowski coil 71, which may advantageously be applied around the connection conductor of the test object. The test object, in Figure 8A exemplified by a coil 81 for a stator winding for an electric power generator, has trans- ducers 7a and 7b, respectively, arranged at the respective connection conductors la, lb. Figure 8B shows a cross section through the connection conductor la and Figure 8C shows a section C-C through the coil and the connection conductor. The coil 71 is wound with typically 10 to 30 turns around a core of epoxy and embedded in a layer 78, also of epoxy. The embedded coil is surrounded by an outer screen 75 of electrically conductive material, for example aluminium sheet. Figure 8D shows a section C-C through the connection conductor with two transducers 7 and 7 * within a common screen 75. Figures 9A-9C show an additional embodiment of a transducer 7. A coil 71 is wound around a U-shaped core 72 of a dielec- trie material, alternatively of amosphous iron, and the coil is surrounded by a screen 75 of an electrically conductive material, for example aluminium foil, covered by a plastic film (not shown in the figure) . Figure 9A shows transducers of this kind applied to the connection conductors la, lb of a coil 81 of a stator winding for an electric power gene¬ rator. Figures 9B and 9C show a view of the transducer as viewed in the longitudinal direction of the connection conductor and perpendicular thereto, respectively. A capa¬ citor (not shown in the figure) may be arranged at the two terminals 73, 74 of the coil for tuning of the resonance frequency of the transducer. Figure 10 shows an improvement of the transducer according to Figures 9A-9C, which may advantageously be used when the test object consists of a coil in a stator winding for an electric generator in those cases where an additional coil is disposed in the same winding slot. Figure 10 shows a cross section through two coils 81, 82, disposed in a common stator slot (not shown in the figure) . Around the two coils a transducer is arranged which is composed of a U-shaped core of the same kind as that described with reference to Figures 9A-9C. On condition that the test object consists of a coil 81, in this case and with a coil wound as described with reference to Figures 9A-9C, because of the connection between coil 81 and coil 82, also a discharge in coil 82 would give rise to a transducer signal. By dividing the coil into two mutually series-connected parts 71' and 71", res¬ pectively, wound in opposite directions around the core, and by a suitable choice of the ratio between the number of turns in part 71' and part 71" and by arranging part 71" at the coil 81 and part 71' at the coil 82, the effect of dis¬ charges in the coil 82 can be reduced. Figures 11A-11B show schematically, in a longitudinal sec¬ tion, a location of a transducer in case the test object consists of a cable joint. Two cable ends 91, 92 are joined to each other, in a manner known per se, by means of a sleeve 93, surrounded by a grounded screen 94. Figure IIA illustrates the case where the screen is separated from the sleeve, in which case it is advantageous to arrange the transducer 6a, 6b, exemplified in the figure by Rogowski coils, at the cable ends in the longitudinal direction of the cable between the sleeve and the screen. Figure IIB illustrates the case where the screen is integrated with the sleeve, in which case the transducer can be embedded in the sleeve at the locations indicated in the figure. Figure 12 shows schematically a location of a transducer in case the test object consists of a cable termination. A cable 91 is terminated in a manner known per se by a termi¬ nation 95. In this case it is advantageous, as marked in the figure, to arrange transducers 6a, 6b, exemplified in the figure by Rogowski coils, at the cable near the cable termi- nation and around the terminating flange of the cable termi¬ nation, respectively. A current pulse associated with a discharge outside the cable termination thereby passes through both coil, causing the sum signal to become at least approximately zero. In case of a discharge in the cable termination, a current pulse will flow to the grounding screen surrounded by the transducer 6b and arranged in the cable termination. This current pulse consists of the sum of the current pulses passing through the connection conductors of the cable termination and is sensed by the transducer 6b whereas, under these conditions, the transducer 6a only senses the current pulse which passes through the cable 91. Figure 13 shows schematically a location of a transducer in case the test object consists of an instrument transformer for measuring voltage. A single-phase voltage transformer 11 comprises a high-voltage bushing 111 and a box 112 shown in cross section. A high-voltage winding 113 and a low-voltage winding 114, shown in cross section, are arranged on a common core 115 in the box. The high- and low-voltage windings and the core are connected to ground potential by means of a conductor 116. In this case, it is advantageous, as marked in the figure, to arrange transducers 6a, 6b, exemplified in the figure by Rogowski coils, around the lower part of the high-voltage bushing near the box and at the conductor 116 outside the box. Figure 14A-14B schematically show a location of a transducer in case the test object consists of a high-voltage winding for a power transformer and a tap-changer, respectively. A three-phase power transformer 12 comprises a box 121 and high-voltage windings WR, WS, WT, each one passed out of the box via bushings BRI, BR2, BSl, BS2, BT1, BT2, respectively (Figure 14A) . In this case, it is advantageous, as marked in the figure, to arrange transducers 6aR, 6bR, 6aS, 6bS, 6aT, 6bT, exemplified in the figure by Rogowski coils, around the lower part of the respective bushing near the box. A tap- changer, of which one phase is schematically shown in Figure 14B, has a connection conductor 123 secured to the winding 122 and the other connection conductor 124 connected to the contact 125, which is movable along the winding, and trans¬ ducers 6a, 6b are arranged at the respective connection con¬ ductor. In case the test consists, for example, of a three-phase reactor, transducers can placed in a manner completely analogous to that described with reference to Figure 14A. In case the test object consists, for example, of a power capacitor, depending on its connection to the other parts of the power network in which it is included, a transducer can be placed in a manner analogous to that described with reference to Figures 13-14, that is, at the high-voltage bushing of the power capacitor and at a conductor to ground potential outside the enclosure of the power capacitor, or at a pair of the high-voltage bushings of the power capa¬ citor. Figures 15A-15B, 16, 17A-17B and 18 show schematically examples of the location of transducers in case the test object consists of a high-voltage bushing for an electric high-voltage apparatus. A high-voltage bushing 13, arranged on a cover 133 of an electric high-voltage apparatus, com¬ prises an external connection conductor 131, arranged in an insulator 132, for example of porcelain. The external connection conductor passes through a flange 134 down into the lower part 135 of the bushing, located in the interior of the high-voltage apparatus. Figure 15A shows a transducer 6a, exemplified in the figure by a Rogowski coil, arranged around the outer connection conductor of the high-voltage bushing above the insulator. Figure 15B, in which the insulator is shown cut up, shows a transducer 6a, exemplified in the figure by a Rogowski coil, arranged around the outer connection conductor of the high- voltage bushing inside the insulator and at the upper part thereof. Figure 16 shows a number of alternative locations of a transducer 6a', 6a", 6a" 1 , 61"", exemplified in the figure by a Rogowski coil, arranged around the lower part of the high-voltage bushing. Figure 17A shows a transducer 6a, exemplified in the figure by a Rogowski coil, arranged around the lower part of the insulator in the vicinity of the flange. Figure 17B shows a number of alternative locations of trans¬ ducers 6a', 6a", exemplified in the figure by Rogowski coils, arranged around the outer connection conductors of the high-voltage bushing inside the insulator and at the lower part thereof and below the cover, respectively. Figure 18 shows a transducer 6a, exemplified in the figure by a Rogowski coil, arranged around the insulator and arranged movable along this . Such a transducer may advan- tageously be used in combination with a transducer 6b, placed, for example, as illustrated in Figure 16. It is clear from the foregoing that by summing signals originating from transducer placed according to any of the ways shown in Figures 15A-15B, 16, 17A-17B and 18, the location of electric discharges in a desired part of the elongated test object is made possible. By combining, for example, the movable transducer 6a, as shown in Figure 18, with a fixed transducer 6a', as shown in Figure 16, by moving the movable transducer a discharge may be located in a defined part of the test object, and by arranging two movable transducers along this, the location can take place completely freely along the test object. Figure 19 shows an application of the invention when the test object consists of a plurality of mutually series- connected coils in a phase of the stator winding of an electric ac generator. A number of coils 81-87 are mutually series-connected via their terminals 81', 81"-87', 87". The iron part of the stator is symbolically marked in the figure by two lines 141, 142. At each of the terminals 81"-87", a transducer 6al-6a7 is arranged, for example in an embodiment according to the description with reference to any of Figures 7-10. The respective transducer signals are supplied to filter units PA1-PA7, the output signals therefrom, designated vl'-v7", are supplied to a signal-mixing unit of an embodiment similar to that signal-mixing unit described with reference to Figure 3. To simplify Figure 19, however, the weighting members comprised in the signal-mixing unit are not shown in the figure. The signal-mixing unit compri¬ ses an adding member ADD1-ADD7, associated with each trans¬ ducer, and a subtracting member SUB1-SUB7, associated with each transducer, and is furthermore adapted such that each one of the adding and subtracting members is supplied with signals originating from a pair of transducers arranged at terminals to two coils arranged in a series connection adjacent to each other. Thus, for example, the adding member ADD2 and the subtracting member SUB2, respectively, are supplied with signals originating from the transducer pair 6a2, 6a3 and form sum and difference signals S23 = v2 + v3 and D23 » v2 - v3, respectively, whereas the adding member ADD3 and the subtracting member SUB3, respectively, are supped with signals originating from the transducer pair 6a3, 6a4 and form sum and difference signals S34 = v3 + v4 and D34 = v3 - v4, respectively. All the sum and difference signals are supplied to a logic unit LU of the same kind as the logic unit described with reference to Figure 3 and form, by combinations of associated sum and difference signals, indicating signals IND2-IND7, where thus the signal IND2 indicates an electric discharge in coil 82, IND3 indicates an electric discharge in coil 83, and so on. The indicating signals are forwarded to central monitoring equipment of a kind known per se via a databus, marked 15 in the figure, and to an indicating panel 16, comprising indi¬ cating means, for example in the form of light-emitting diodes 162-167 which light up when indicating a discharge in the respective coil. It is realized from the foregoing and from Figure 19 that by arranging the signal-mixing unit such that the adding and subtracting members are supplied with signals originating from a pair of transducers arranged at terminals between which are placed a number of coils arranged in a series connection adjacent to each other, discharges in an optional group of mutually series-connected coils can be sensed. Figure 20 shows an application of the invention when the test object consists of a switchgear cubicle, for example in enclosed indoor medium-voltage switchgear or in gas- insulated switchgear. Figure 20 shows parts of a switchgear unit in the form of a schematic single-line diagram. The switchgear unit comprises a continuous busbar 17 and the test object consists of a cubicle 171, comprising a circuit breaker 172 and an output line 173. Transducers 6a, 6b, 6c, exemplified in the figure by Rogowski coils, are arranged around the busbar on both sides of the switchgear cubicle 171 between this and it surrounding cubicle, and around the output line 173. The transducer signals are supplied, in a manner similar to that described with reference to Figure 3, via filter units PAa, PAb, PAc, to a signal-mixing unit ABU for forming a sum signal S - vl+v2+v3. If only signals ori¬ ginating from the transducers 6a, 6b are supplied to the signal-mixing unit, the test object will comprise the switchgear cubicle 171 and the output line 173 with equip¬ ment connected thereto. If signals originating from the transducers 6a, 6b and 6c are supplied to the signal-mixing unit, the test object is limited to comprising only the switchgear cubicle 171. Figure 21 shows an application of the invention when the test object is in the form of gas-insulated switchgear. The figure shows in the form of a single-line diagram a switchgear unit 18 with an enclosure 181, comprising an insulating gas 182, such as SF6. The switchgear unit is of three-phase design, but in the figure only one phase is marked with reference numerals, it being understood that all the phases are identical. The switchgear unit comprises a busbar 183, a grounding disconnector 184, a bushing 185, a circuit breaker 186, an additional bushing 187, an addi¬ tional grounding disconnector 188 and an outdoor bushing 189. Transducers 6a, 6b, 6c, 6c', 6c", 6c" 1 , exemplified in the figure by Rogowski coils, are arranged respectively around the busbar on both sides of the connection to the grounding disconnector 184, around the connection between the grounding disconnector 184 and the bushing 185, around the connection between the bushing 187 and the grounding disconnector 188, around the connection between the grounding disconnector 188 and the lower part of the outdoor bushing 189, and around the conductor through the outdoor bushing 189 at the upper part thereof. As an example it is shown how transducer signals originating from the trans- ducers 6a, 6b, 6c, via filter units PAa, PAb, PAc are supplied to the signal-mixing unit ABU for forming a sum signal S = vl+v2+v3. The test object consists in this case of the grounding disconnector 184 and parts of the busbar located between the transducers and the connection of the grounding disconnector thereto. It is realized from the foregoing and from Figure 21 that by supplying to the signal-mixing unit signals originating from suitably chosen transducers, the test object can be limited to comprising a chosen part of the switchgear unit. By, for example, desig- ning the signal-mixing unit as described with reference to Figure 3 and supplying it with signals originating from the transducers 6c" and 6c"' , a discharge can be located with high reliability at the outdoor bushing 189. Of course, all the phases in the switchgear may be equipped with trans- ducers arranged in the manner described above. Figures 24A-24B show an application of the invention when the test object is in the form of a current transformer. Figure 24A shows a single-phase current transformer 19 comprising a high-voltage bushing 191 and a box 192, both shown in cross section. A conductor 193, carrying the current to be measured, is passed down through the high- voltage bushing, through a core 194 arranged in the box, and again up through the high-voltage bushing. Around the core 194 also a secondary winding 195 is arranged. The core and the secondary winding are connected to ground potential by means of a conductor 196 and a measured value of the current through the conductor 193 is forwarded via a conductor 197. In this case, it is advantageous, as marked in the figure, to arrange a transducer 6a, exemplified in the figure by a Rogowski coil, around the lower part of the high-voltage bushing near the box. A discharge in those parts of the current transformer which are located in the box gives rise to current pulses which, viewed from the transducer, pass through the downwardly- and upwardly-extending parts of the conductor 193 in the same direction, whereas a discharge outside of these parts of the current transformer gives rise to current pulses which, viewed from the transducer, pass through the downwardly- and upwardly-extending parts of the conductor 193 in different directions. The magnetic field generated by the current pulses are therefore in the former case different from zero, whereas in the latter case it is essentially zero, in this case, thus, the test object con¬ sists of those parts of the current transformer which are located in the box. Figure 24B shows a single-phase current transformer 19 of a so-called top-core model, comprising a high-voltage bushing 191 and a box 192, both shown in cross section. A toroidal core 194, which is connected to ground potential in a manner not shown in the figure, is arranged at the upper part of the high-voltage bushing. A conductor 193, carrying the current to be measured, is passed down through the high- voltage bushing, through the core and again up through the high-voltage bushing. A secondary winding 195 is wound around the core and is brought with both its terminals 196, 197 through the high-voltage bushing to the box. The termi- nal 196 is connected to ground potential and a measured value of the current through the conductor 193 is forwarded via a conductor 197. Also in this case, it is advantageous, as marked in the figure, to arrange a transducer 6a, exem¬ plified in the figure by a Rogowski coil, around the lower part of the high-voltage bushing near the box. Discharges in those parts of the current transformer which are located above the transducer give rise to current pulses which, viewed from the transducer, pass through the downwardly- and upwardly-extending parts 196 and 197, respectively, of the conductor 195 in the same direction, whereas discharges outside of these parts of the current transformer give rise to current pulses which, viewed from the transducer, pass through the downwardly- and upwardly-extending parts of the conductor 195 in different directions. In the former case, therefore, the magnetic field generated by the current pulses becomes different from zero whereas in the latter case it becomes essentially zero. In this case, thus, the test object consists of those parts of the current trans- former which are located above the transducer. Figure 25 shows an application of the invention when the test object is in the form of a continuos web, for example an insulated electric conductor for a transformer winding which is under manufacture. Conductors of the above kind are manufactured by casting a number of sub-conductors of copper in epoxy into a conductor in the form of a tape. To increase the insulating resistance of the conductor, a paper strip is wound around the tape so that the paper strip, viewed from a cross section through the conductor, becomes C-shaped. These steps in the manufac¬ ture take place while the conductor is fed forward in a continuous web, at a rate typically of the order of magni- tude of 0.2 m/s, and it is thus advantageous to arrange a continuous inspection of the insulation of the conductor. The figure shows a number of sub-conductors 201 which are fed forward between two roll pairs 202 and 203 in a direc- tion of movement, marked by arrows, from left to right in the figure. A paper web 204 is supplied and folded around the sub-conductors in a rotating feed device 205. Two mutually galvanically connected electrodes 206a, 206b, which are elongated in the direction of movement of the web and which cover the entire width of the web, are arranged between the roll pairs 202 and 203. The electrode pair is fed with a high alternating voltage from a generator 207 and thus generates, in a section X-X of the web, an electric field perpendicular to the longitudinal direction of the web. Transducers 6a, 6b are arranged at the web, one at either end of the section X-X, and transducer signals vl", v2" are supplied via filter units PAa, PAb to a signal- mixing unit ABU. In this case, thus, the test object con¬ sists of the section X-X of the continuous web and the connection conductors of the test object of those two parts of the web which are connected to this section at the res¬ pective ends of the section. A device 208, only roughly indicated in the figure, comprises two metal brushes in electrical contact with the web. The brushes are connected to a voltage source and a current-sensing member senses if an electric current flows through the brushes, which indi¬ cates the absence of insulation. Figures 22A-22B show an improvement of the invention, which is especially advantageous for monitoring during normal operation in those cases where the test object exhibits a very high impedance, for example consists of a power trans¬ former, and in particular in a difficult noise environment. The high impedance of the test object results in a current pulse, caused by an electric discharge outside of this, being greatly damped during its passage through the test object, and therefore a sensing via the magnetic field, associated with the current pulse, at the connection conduc- tor where it flows out from the test object can be rendered considerably difficult. Figure 22A shows a part of a power transformer 12 with a box 121 and a high-voltage bushing BRI. A capacitive measure- ment terminal 126, arranged in a manner known per se, is provided at the high-voltage bushing. Around the lower part of the high-voltage bushing, just above a flange 128, through which the bushing passes into the transformer, a transducer 6aR is arranged, comprising two Rogowski coils 6aR', 6aR", mutually series-connected by means of a conduc¬ tor 73' (Fig. 22B) , each one being designed, for example, as described with reference to Figures 7A-7C. Figure 22B shows the transducer viewed from a plane perpen- dicular to the longitudinal axis of the bushing and illu¬ strates that each one of the Rogowski coils comprises less than half the circumference of the bushing. The coils can be fixed to the bushing, for example by gluing, and the design of this transducer facilitates above all its mounting on bushings of various dimensions. From the interference point of view, it is advantageous to place the coils so as to be rotationally symmetrical around the bushing. The output signal vl from the transducer is supplied via conductors 73, 74 to evaluation equipment 127. The output signal Vc from the capacitive measurement terminal is likewise supplied to the evaluation equipment via a conductor 75. In this improvement of the invention, both the magnetic and the electric field, which are generated by a current pulse associated with a electric discharge in the test object, are sensed. By means of multiplication of signals caused by these fields, the power flux through a connection conductor which is associated with the discharge can be determined in magnitude and direction. The electric discharge may have either a positive or a negative polarity, by which is meant that the current pulses through the connection conductors of the test object associated with the discharge are either directed in a direction towards the test object of away from this. This polarity is determined by sensing the polarity of the output signal Vc from the capacitive measurement termi¬ nal whereas the direction of the current pulses is deter¬ mined by sensing the polarity of the output signal vl from the transducer 6aR. An embodiment of the evaluation equipment 127 is shown in Figure 23. The output signal vl from the transducer 6aR and the output signal Vc from the capacitive measurement ter¬ minal are each supplied to a respective interface unit 21', 21", respectively, in order to be adapted to each other and to a suitable signal level, in a manner known per se, by means of resistive, capacitive, inductive and amplifying circuit elements. The output signals from the respective interface units are each supplied to a respective bandpass filter 22', 22". The bandpass filter 22', associated with the output signal from the transducer 6aR, has its passband around the natural resonance frequency of the transducer, which in turn is selected to obtain the best signal/noise ratio in the measurement system. The passband for the band- pass filter 22" may suitably be given the same characteris¬ tic as the bandpass filter 22'. The output signals from the respective bandpass filter are each supplied for level adaptation to an amplification member 23', 23", respectively, and the output signals from these are supplied to a multiplying member 24, in which the two signals are multiplied by each other. The output signal from the multiplier is now either positive or negative, depending on the origin of the discharge . In this embodi¬ ment, it is assumed that a positive output signal indicates a discharge in the test object, which can be achieved by selection of the winding direction for the coils comprised in the transducer 6aR. The output signal Uind from the multiplying member is supplied to a peak value detector 25, which to an envelope circuit 26 only forwards signals with a positive polarity. The envelope circuit reproduces its input signal with an extended decay time. The amplitude of the output signal from the envelope circuit is related to the level of the sensed discharges in the test object such that an increasing level of internal discharge results in an increasing value of the output signal of the envelope circuit. The output signal thereof is supplied to a signal-converting member 27, which converts the output signal from the envelope circuit to a corresponding direct current IIND. The amplitude of this direct-current signal is dependent on the amplitude of the input signal to the signal-converting member and thus dependent on the magnitude of the discharge. The signal IIND is forwarded to monitoring equipment (not shown in the figure) to be compared in a manner known per se with a chosen alarm level. It may be noted that, in this improvement of the invention, an indicating signal for indicating the presence of a dis¬ charge in the test object is obtained with a transducer signal for sensing the magnetic field, associated with the current pulse, at only one of the connection conductors of the test object. In the event that no capacitive measurement terminal exists at the bushing of the transformer, some other type of transducer for sensing the electric field generated by the current pulse may be used. Still another advantageous embodiment of transducer equip¬ ment is shown in Figure 26. Two transducers 6a, 6b, respec¬ tively, in the form of Rogowski coils 61a, 61b, respecti¬ vely, are arranged one on each side of a test object 1. Each one of the Rogowski coils is connected to a superconducting sensor of a kind known per se, a so-called SQUID. The output signals from the respective superconducting sensor are supplied to bandpass filters BPa, BPb to form signals vl' and v2' , which can then be processed further, for example as described above with reference to Figure 3A. Figure 27 shows an alternative embodiment of transducer equipment. Four transducers 6a, 6c and 6b, 6d, respectively, in the form of Rogowski coils, are arranged two-by-two on each side of a test object 1 at the connection conductors la and lb, respectively, thereof. Coils arranged on both sides of the test object are series-connected to each other such that the coils 6a and 6b are wound in opposite directions to each other and the coils 6c and 6d are wound in the same direction with respect to each other. The output signal from the series connection of the negative-feedback coils is supplied to a filter unit PAa and the output signal from the series connection of the positive-feedback coils is supplied to a filter unit PAc, which filter units are of the same kind as those described with reference to Figure 1. It is realized that, by the connection of the coils, the amplifiers, in Figure 27 designated AMPa and AMPc, respectively, comprised in the filter units will form signals corresponding to the sum and difference signals S and D described with reference to Figure 4. These signals are supplied to a comparing member Q, for example of the same kind as that described with reference to Figure 3B. The invention is not limited to the embodiments shown but the inventive concept also comprises other combinations of the transducers described, of the principles of signal mixing and of measurement objects. The invention is based on the use of transducers which can be manufactured in shapes and dimensions adapted to test objects of varying kinds and which are galvanically separa¬ ted therefrom. The evaluation equipment for locating the discharge at the test object is based on simple and reliable principles and is therefore insensitive to external inter¬ ference.

Claims: CLAIMS 1. A device for sensing electric discharges in a test object (1) with at least two electric connection conductors (la, lb, lc, ld) , comprising transducer equipment and evaluation equipment (PA, ABU, LU) , the transducer equipment comprising at least one transducer (6a, 6b, 6c, 6d) for direction- sensitive sensing of current pulses through the connection conductors by sensing a magnetic field, generated by the current pulses, and the polarity thereof, said at least one transducer delivering a transducer signal (vl", v2", v3", v4", vll", v21") to the evaluation equipment in dependence on said current pulses and their direction, characterized in that the evaluation equipment in dependence on received transducer signals generates an indicating signal (IND), indicating an electric discharge in the test object, when the transducer equipment senses current pulses which essen¬ tially simultaneously flow either in a direction out of the test object through all the connection conductors or in a direction into the test object through all the connection conductors. 2. A device according to claim 1, characterized in that the transducer equipment comprises transducers such that at least one transducer is associated with each one of at least two connection conductors for sensing of current pulses through the respective connection conductor. 3. A device according to claim 2, characterized in that it comprises transducers such that two transducers are associa¬ ted with each one of at least two connection conductors for sensing of current pulses through the respective connection conductor. 4. A device according to claim 2 or 3, characterized in that the evaluation equipment comprises an adding member (ADD) which forms a sum signal (S) in dependence on a sum of transducer signals originating from transducers associated with mutually different connection conductors and that the indicating signal is formed in dependence on said sum signal. 5. A device according to claim 4, characterized in that the evaluation equipment comprises a subtracting member (SUB) and a comparing member (Q, SUM) , whereby the subtrac¬ ting member forms a difference signal (D) in dependence on a difference of transducer signals originating from trans¬ ducers associated with mutually different connection con- ductors, and that the comparing member forms a comparison signal (SQ) in dependence on said sum signal and said difference signal and that the indicating signal is formed in dependence on said comparison signal. 6. A device according to claim 5, characterized in that the difference signal is formed in dependence on transducer signals (vl", v2 M ) originating from the same transducers (6a, 6b) as those which deliver those transducer signals (vl", v2" ) from which the sum signal is formed. 7. A device according to any of the preceding claims, characterized in that the evaluation equipment comprises, for each transducer, a filter unit (PAa, PAb, PAc, PAd) associated with it and which filter unit comprises a tuning circuit (Ra, Ca, Rb, Cb, Rc, Cc, Rd, Cd) for tuning the natural frequency of the transducer as well as a bandpass filter (BPa, BPb, BPc, BPd) , the transducer signals being supplied to the filter unit and said sum and difference signals being formed in dependence on output signals (vl 1 , v2' , v3 ' , v4' , vll', v21') from the filter unit. 8. A device according to claim 7, characterized in that said bandpass filters are tuned to the series resonance frequency of the test object. 9. A device according to any of claims 4-8, characterized in that the evaluation equipment comprises weighting members (BMa, BMb, BMc, BMd) for weighting the signals from which the sum signal is formed. 10. A device according to any of claims 4-9, characterized in that the evaluation equipment comprises a model (ZI, Z2, Z3, Z4, Z5) of the test object which damps and/or phase- shifts the signals from which the sum signal is formed in a way which imitates the amplitude damping and/or phase shift which a current pulse flowing through the connection conduc¬ tors of the test object undergoes. 11. A device according to any of claims 1-10, wherein the test object consists of a section (X-X) of a continuous web (201, 204), such as, for example, an insulated conductor for a transformer winding which is in the process of being manufactured, and the connection conductors consist of those parts of the web which are connected to said section, characterized in that it comprises at least two electrodes (206a, 206b) which are energized by alternating voltage and extended in the direction of movement of the web and which are each arranged on a respective side of the web so as to cover said section of the web, and the transducer equipment comprises at least one transducer arranged at each of the connection conductors. 12. A device according to any of claims 1-10, wherein the test object consists of a switchgear cubicle (171) in an electric switchgear unit with a busbar (17) and an output line (173), such as, for example, medium-voltage switchgear or gas-insulated switchgear, characterized in that the transducer equipment comprises transducers (6a, 6b) arranged at the busbar on both sides of the switchgear cubicle. 13. A device according to claim 12, characterized in that the transducer equipment further comprises transducers (6c) arranged at the output line viewed from the switchgear cubicle. 14. A device according to any of claims 4-10, wherein the test object consists of a plurality of mutually series- connected coils (81, 82, ... 86, 87) in a phase of a stator winding for an electric ac generator, characterized in that the transducer equipment comprises a transducer (6al, 6a2,..., 6a6, 6a7, respectively) arranged at a terminal (81', 81", 82', 82", ..., 86", 86", 87', 87", respectively) of each one of said coils and that the evaluation equipment comprises adding members (ADD1, ADD2, ..., ADD6, ADD7, respectively) which form sum signals (S12, S23, ..., S67) in dependence on a sum of transducer signals originating from an arbitrarily chosen pair of said transducers. 15. A device according to claim 14, characterized in that the evaluation equipment comprises adding members which form sum signals in dependence on a sum of transducer signals originating from a pair of said transducers arranged at terminals of coils arranged adjacent to each other in a mutual series connection. 16. A device according to any of the preceding claims, characterized in that the transducer comprises at least one Rogowski coil (7, 7a, 7b) arranged around a connection conductor. 17. A device according to claim 16, characterized in that said Rogowski coil is arranged around a U-shaped core (72) of magnetic material, which core is arranged around a connection conductor. 18. A device according to any of claims 1-10, wherein the test object consists of a coil (81) in a stator winding for an electric generator, said coil and an additional coil (82) being disposed in one winding slot in the stator of the generator, characterized in that the transducer comprises a series connection of two Rogowski coils (71', 71") mutually wound in opposite directions and arranged around a common U-shaped core (72) of magnetic material, such that one (71") of the coils is arranged at that coil (81) which constitutes the test object and the other (71') of the coils is arranged at the other (82) of the coils in the slot of the stator winding. 19. A device according to any of claims 1-10, wherein the test object consists of a coil (81) in a stator winding for an electric generator, said coil and an additional coil (82) being disposed in one winding slot (8) in the stator of the generator, characterized in that the transducer is in the form of a substantially flat Rogowski coil (7) arranged in said winding slot in the stator and between said two coils. 20. A device according to any of claims 1-10, wherein the test object has an elongated extension, such as, for example, a bushing (13) for an electric high-voltage appara¬ tus, characterized in that the transducer equipment com¬ prises at least one Rogowski coil (6a) arranged movable along the test object. 21. A device according to any of claims 1-10, wherein the test object is in the form of a power transformer (12) with at least one high-voltage bushing (BRI) and a capacitive measurement terminal (126) arranged at said bushing, characterized in that said transducer comprises at least one Rogowski coil (6aR', 6aR") arranged around said high- voltage bushing and that the evaluation equipment is supplied with a measurement signal (Vc) from said capacitive measurement terminal. 22. A device according to claim 21, characterized in that said transducer comprises two separate (6aR', 6aR", respec¬ tively) , mutually series-connected Rogowski coils, arranged opposite to each other around said high-voltage bushing. 23. A device according to claim 1, wherein the test object is in the form of a current transformer (19) arranged at a current-carrying conductor (193), said transformer compri¬ sing a high-voltage bushing (191) and a secondary winding (195), characterized in that the transducer consists of a Rogowski coil (6a) which surrounds downwardly and upwardly extending parts of either of said current-carrying conductor and said secondary winding, said parts being arranged in said high-voltage bushing and constituting connection conductors for the test object. 24. A device according to any of the preceding claims, characterized in that said transducers comprise at least one Rogowski coil (61a, 61b), arranged at a connection con¬ ductor to the test object, and a superconductor sensor (SQUID) connected to said Rogowski coil.

Inventors: BENGTSSON TORD, DAHLBERG LARS-GUNNAR, ERIKSSON THOMAS, KHEIRMAND AKBAR, LEIJON MATS, RUDOLFSSON DICK, SIBERG HENRY, WINKLER DAG, BENGTSSON, TORD, DAHLBERG, LARS-GUNNAR, ERIKSSON, THOMAS, KHEIRMAND, AKBAR, LEIJON, MATS, RUDOLFSSON, DICK, SIBERG, HENRY, WINKLER, DAG
Applicants: ABB RESEARCH LTD, BENGTSSON TORD, DAHLBERG LARS GUNNAR, ERIKSSON THOMAS, KHEIRMAND AKBAR, LEIJON MATS, RUDOLFSSON DICK, SIBERG HENRY, WINKLER DAG, ABB RESEARCH LTD., BENGTSSON, TORD, DAHLBERG, LARS-GUNNAR, ERIKSSON, THOMAS, KHEIRMAND, AKBAR, LEIJON, MATS, RUDOLFSSON, DICK, SIBERG, HENRY, WINKLER, DAG
Application year: 1996
Main classification: G01R31
IPC codes: G01R15, G01R31