Pulsed electroacoustic space charge measurement for power cable

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Supervisor:
Dr. Németh Bálint
Department of Electric Power Engineering

The pulsed electroacoustic (PEA) measurement method have been developed to examine space charge distributions. The electric field can be highly distorted compared to the distribution defined by the electrodes when space charges are present in the insulation under high voltage. Hence, even in normal operation the electric stress can be locally multiple times higher than the nominal value in the dielectric.

The objective of this paper is to document the design and the construction of the first specimen of such a device described above in Hungary. We use high voltage typically in electric power transmission, but this equipment can be used to examine the insulation of other HV apparatus, e.g. x-ray machines, cathode ray tubes, photomultipliers, railway accessories etc.

The basic result of the measurement is the one-dimensional distribution of charges in the insulating material. The physical basis of the detection is as follows. We put the dielectric between two electrodes and apply a fast voltage impulse across it. Force will act on the free charges and mechanical waves will be initiated. There is a piezoelectric film placed on the back of the ground electrode made of PVDF (polyvinylidene fluoride) that is able to convert the pressure waves to electric signals. After a proper signal conditioning procedure the voltage waves can be displayed on an oscilloscope. The measurement is more efficient if the piezoelectric film is followed by a backing material, which is thick enough that the reflected waves are out of the detection time. This sheet is chemically identical to the piezoelectric one (PVDF-β), thus they have the same mechanical wave impedance, hence there will be no reflection on their interface, but it is not activated, it is not piezoelectric (PVDF-α).

In the beginning PEA method was used to analyse films and plane samples, but with this technique it became easier to examine power cables, so special editions of the device were introduced with curved electrodes. Since every size of the cables needed own electrode, other methods with plane electrode have been worked out.

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