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.

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The Electricity Supply Commission of South Afiica (ESKOM)and Electricit6 de France (EDF) BIT currently involved in a technical exchange agnement whereby the effectiveness of the EDF method for ststor winding insulation condition assessment, This method basically entails off-line electrical quadratic rate partial discharge mcasurrments, using relatively low center-&equcncy nmowbandfilters. Initial experience gained through "cnts on huo turbo generntm of Eskom in South Afiica, has shown the ability of this method to .acmately indicate the insulation wndition of statorwindings.On one of the generators tested, excellent comlation was found between the indications of test results and the tindmgs of a stator bar visual inspection and dissection. Dissipation factor and conventional peak-value partial discharge mensuremen& have in the past failed to indicate the extend of insulation damage (discovered through visual inspections) of this specific winding. World-wide, financial and environmental factors are necessitating refurbishment and life extension (or life management) of power generating facilities nearing the end of their design life. Plant managers can therefore expect an increasing incidence of in-seMce equipment failure. in years to come. The integrity of stator winding insulation in high voltage generators is a crucial parameter in the estimated total reliability of a power generating unit.Detection of partial discharges (PD) is widely seen as an effective method by which the condition of high voltage insulation systems can be assessed, i.e. [Campbell, 11. Across the world, many different methods for the detection of PD pulses in the stator winding insulation of high voltage generators, are k i n g employed. specifically on hvbo generaton. will be investigated.