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A current transformer, in general, is a means of measuring a very large electrical current, converting it to a much smaller current, requiring low power equipment as simple as a multimeter to display it. The traditional method is based on the electromagnetic principle of an iron ring around the current conductor, supplied with a secondary transformer winding. The latter will deliver a small current directly proportional to the large current, to be displayed by a small current measuring device. An optical current transformer (OCT) is constructed by winding an optical fibre cable around the current conductor. The phenomenon where the angle of a polarized light beam propagating through the optical cable, is deflected when the magnetic flux lines generated by the large current, are appearing longitudinal in the said cable, are utilized to determine the main current. OCT's are already well developed, but lack to respond to very fast changing transient impulses. This paper describes a research and developing process to address this problem.

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This paper report on an theoretical investigation using FEA simulations on the stress distribution in medium voltage cable joints. Three different supplies namely 50 Hz sinusoidal, VLF 0.1 Hz sinusoidal and VLF 0.1 Hz cosine square were evaluated. The simulation results show that phase shifts occur in the stresses under 0.1 Hz sinusoidal voltages and that only above 10 Hz sinusoidal no phase shifts occur. With the application VLF 0.1 Hz cosine square the stresses are totally different to normal operating stresses. The reason for these differences is the influence of the volume resistivity and the interfacial polarization during low frequencies.The mechanisms involved to determine the stress distribution in the joint which as a multi layer insulation system is totally different to a cable which is a single layer insulation system. This should be taken into account when the most appropriate test voltage and frequency are selected for commissioning tests.