Method and a device for space-charge measurement in cables using a pulsed electroacoustic method

[0002] The present invention relates to a method and a device for measurement of space charges in an electrical insulation of a cable using a pulsed electroacoustic method, where space charges in the insulation of the cable are set in motion by an electric pulse and where this motion generates acoustic waves which are registered and evaluated with respect to the space-charge distribution in the insulation of the cable.

[0003] When using polymeric insulating material with a high specific resistance, for example polyethylene, space charges may form within the material. These space charges distort the electric field in the insulating material and this causes electrical breakdown at a lower average field strength than without these space charges. This problem becomes especially pronounced for electrical insulation which is subjected to high direct voltage, in particular cables for transmission of high-voltage direct current.

[0004] To ascertain the generation and distribution of the space charge within the insulating material, a need has arisen to provide a non-destructive method which may provide a picture of the space-charge distribution within the insulating material in a manner similar to that in which, for example, the emission of X-rays or ultrasonics is used to demonstrate variations in the material composition of a test specimen.

[0005] Two methods which may be used for non-destructive space-charge measurement are the pressure-wave propagation method, PWP, and the pulsed electroacoustic method, PEA.

[0006] In the PWP method, the material is subjected to a mechanical pulse, which generates an acoustic wave which during the propagation “shakes” space charges. This generates a current signal between short-circuited electrodes or a voltage signal over open electrodes which surround the insulating material, which signal is then evaluated with respect to the space-charge distribution.

[0007] In the PEA method, the material is subjected to a short electric voltage pulse between two electrodes which surround the insulating material, and the force exerted by the thus caused electric field on the space charge “shakes” the insulating material. This mechanical displacement propagates through the material as an acoustic wave which is detected, for example with a piezoelectric transducer, and is then evaluated with respect to the space-charge distribution.

[0008] A good summary of the prior art relating to the PEA method is given in the publication by N. Hozumi, T. Okamoto and T. Imajo: “Space charge distribution measurement in a long size XLPE cable using the pulsed electroacoustic method”, Proc. of IEEE Symposium on EL. Ins. 1992, pp. 294-297, Baltimore, Jun. 7-10, 1992.

[0009] The original method makes use of parallel plates as electrodes with the insulating material to be tested between these. One of the electrodes is in mechanical contact with a plate of piezoelectric material which converts the acoustic signal into an electrical signal. When changing to a cylindrical geometry, with an inner electrode which is concentrically surrounded by insulating material, which in turn is surrounded by an outer conducting layer, short pieces of cable may be tested using the PEA method. The inner electrode consists of the inner conductor of the cable, the outer electrode of a sector-shaped aluminium block in mechanical contact with the semiconducting layer which is applied onto the cable insulation. The aluminium block is mechanically connected to a piezoelectric PVDF film which, in a manner previously described, generates an electrical signal when hit by the acoustic wave.

[0010] To achieve a sufficiently high resolution during the space-charge measurement, the pulse length of the electric pulse must be short compared with the time needed by a acoustic pulse to propagate through the insulating material over the electrode distance. This entails pulses with a duration of a few 10 ns, in which case the frequency contents of the pulse will primarily be within the range of 10-100 MHz. Since the space-charge measurement is to be carried out during simultaneous application of a high direct voltage to the inner conductor of the cable, both the coupling capacitor for the pulse application and the end termination of the cable become large in their linear dimensions and therefore constitute a relatively great inductance, which greatly influences the pulse shape. To be able to carry out space-charge measurements on longer cable pieces, the outer conductor (metal foil or screen cover) of the cable is removed at the location where the cable is to be tested. In the middle of this zone, without an outer conductor, an annular outer electrode of metal foil is applied, in mechanical contact with an aluminium block as above. The semiconducting layer, which in this context has a high electric resistance, is left intact. The voltage pulse is applied between the annular outer electrode and the outer conductor of the cable and is connected capacitively to the inner conductor through the impedance of the cable. The fact that part of the pulse makes its way through the semiconducting layer reduces its voltage but has otherwise no disturbing influence.

[0011] The above method solves the problem of measuring on a long cable. However, this is done at the cost of a new problem:

[0012] During the application of the pulse, the annular outer electrode is energized. This pulse voltage pulse must not reach measurement and evaluation equipment located at ground potential. One way of solving the problem is to apply an electrically insulating layer, for example a plastic foil, between the outer electrode and the aluminium block. However, the foil disturbs the propagation of the acoustic signal which now has to pass through the insulating layer. It may also be charged and thereby generate disturbance. Another method, also suggested in the publication, is to allow the measurement equipment to be in electrical contact with the outer electrode and to transmit the measurement signals from there in a potential-free manner with the aid of optical fibres to the evaluation equipment. However, this solution is complicated, resource-demanding and more sensitive to disturbance.

[0013] The invention solves the problem of applying an electric pulse between the inner conductor and the outer conductor of a coaxial cable for pulsed electroacoustic space-charge measurement, independently of the length of the cable and without having to energize the outer conductor which is in contact with the measurement equipment.

[0014] This is achieved by applying the outer electrode in the measurement region, in a manner similar to that of the previously described method for measurements of long cables, to the semiconducting layer on the insulation of the cable after the outer conductor has been removed at the measurement region. Then, the electric pulse is applied between the outer conductor of the cable outside the measurement region. The pulse is then connected via the impedance of the cable to the inner conductor of the cable, such that a pulse-shaped voltage will also lie between the inner conductor of the cable and the outer electrode. To prevent the pulse from propagating along the outer conductor of the cable, it is disconnected with the aid of two transformers, one on each side of the measurement region. The transformer is in the form of a metal foil or a metal cover around the cable and an annular magnetizable core, preferably a ferrite toroid, which surrounds the metal foil/cover and the cable. Those parts of the metal foil/cover which are facing away from the measurement region are grounded. The magnetic coupling through the magnetizable core between the metal foil/cover, the outer conductor of the cable and the inner conductor of the cable causes the outer conductor and inner conductor of the cable not to be energized by the voltage pulse outside those parts of the magnetizable cores which are facing away from the measurement region.

[0015] FIG. 1 shows a device for space-charge measurement in a cable according to the invention.

[0016] FIG. 2 shows a preferred embodiment of the invention.

[0017] FIG. 1 shows a device according to the invention for space-charge measurement on a cable 1 which comprises an inner conductor 2, a cable screen/outer conductor 3 and an electric insulation 4, preferably made of an organic polymer. The inner conductor may be energized with the aid of a high-voltage source 5. The cable screen is removed on the cable sections on both sides of a measurement region, such that a semiconducting layer 6a, 6bappears which is situated between the insulation of the cable and the cable screen. The measurement region is surrounded by an annular section 7 of the cable screen, which is connected to ground potential. In direct mechanical contact with the section of the cable screen around the measurement region, there is a sound-recording device 8 for recording and amplifying acoustic signals. The device 8 is also in electrical contact with the section 7 and hence at ground potential. On both sides of the measurement region, outside those sections where the cable screen is removed, tubular electrical conductors 9a, 9bare arranged around the cable, and on those ends of the conductors which are facing away from the measurement region, these conductors are connected to ground potential. Those ends of the conductors 9a, 9band of the cable screen 3a, 3b, which are facing the measurement region, are connected to a pulse generator 10 for generating electric pulses with a short rise time. Around the tubular conductors 9a, 9b, which preferably consist of a piece of screen cover or of metal foil, toroids 11a, 11bof magnetizable material, preferably ferrite, are arranged.

[0018] If an electric pulse is applied to the tubular electrical conductors 9a, 9b, it is connected via the impedance of the cable to the inner conductor. The pulse-shaped voltage interacts with those space charges in the insulation between the annular section 7 of the cable screen and the inner conductor which are generated with the high-voltage source 5 through energization of the inner conductor, and generates acoustic pulses which are received by the sound-recording device 8 for evaluation with respect to the magnitude and distribution of the space charge. Through the toroids 11a, 11b, the tubular electrical conductors 9a, 9b, the cable screen 3a, 3band the inner conductor constitute transformers with the transformation ratio 1:1:1. The voltage drop across the “primary winding”, the conductors 9a, 9bbetween their ends becomes the same as the voltage drop along the cable screen 3a, 3band along the inner conductor within the magnetic “reach” of the toroid. This means that the pulse does not propagate along the cable outside the “reach” of the toroids. The pulse cannot cause disturbances or be reflected outside the immediate surroundings of the measurement range. The sound-recording device and the annular section 7 of the cable screen are at ground potential and thus need not be insulated electrically, neither from ground nor from each other.

[0019] A preferred embodiment according to FIG. 2 comprises annular shielding electrodes 12a, 12b, connected to ground potential, on both sides of the annular section 7, separated from the section 7 through narrow zones 13aand 13bof semiconducting material. This eliminates acoustic disturbances which arise in a device according to FIG. 1 at the boundary between the semiconducting layer 6a, 6band the annular section 7.

[0020] The dashed line 14 in FIG. 2 symbolizes a connection between the sound-recording device 8 and the high-voltage source 5. This connection provides a possibility, during the measurement of the space charge, which may be time-consuming and hence often automated and without supervision, of detecting the sound from major electrical flashovers or breakdowns in the measurement arrangement and, thereafter, of closing the high-voltage source and generating an alarm signal.