In the last two decades, a significant effort has been directed to understand the internal charge distributions in dielectrics. This has resulted in the development of a number of methods which give detailed information about space charge distributions. This review presents the evolution of several experimental approaches for the determination of spatial charge and field distributions in dielectrics, including the nondestructive methods of direct probing such as the thermal-pulse methods and the methods using pressure waves that propagate through the charged sample. Detailed information is also provided on the principles used in each technique, typical resolution, limitations and advantages associated with each method. The applications of the methods to several insulating materials charged with various mechanisms are described. The paper concludes with a discussion on a number of quite interesting techniques which seem most promising.

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This paper describes an on-line PD (partial discharge) detection technique applicable to power cables. The method can be applied to both wire screen and solid shielded cables. In wire screen shielded cables, PD components with frequencies t l O O MHz can be detected at distances over 150 m from the source. PD activity with levels as low as 20 pC can be detected within 100 m. However, in the case of solid shielded cables, PD levels of 3 5 0 pC can be measured within 60 m in the frequency range t50 MHz. This method also can be utilized to check cable accessories such as splices and terminations. PD pulses with frequencies to -300 MHz can be measured near the joints and terminations. The suitability and the sensitivity of the VHF technique were checked against two other alternative PD detecting techniques (pulse phase analyzer and high speed digital oscilloscope) by performing PD measurements in standard air gap and defects induced in Ern-insulated cable under laboratory-controlled conditions. The suitability and sensitivity of the VHF method were as good as the two alternative PD detection techniques.1070-9878/98/ $3.00 0 1998 IEEE

This paper describes on-line and off-line partial discharge (PO) measuring techniques applicable to transmission cable terminations. The on-line technique can be utilized for both extruded and pipe-type cable terminations. In pipe-type cable terminations, the on-line technique is only applicable to newly installed terminations, as it requires an internal inductive sensor. In the case of extruded cable terminations, on-line testing is made possible by using an external inductive coupler around the ground connection at the base of termination.Off-line PD measurements utilize high-frequency capacitive couplers. The coupler is attached to the overhead line within 4-feet from the termination under test. The off-line testing is applicable to new and existing cable terminations. I: IntroductionTransmission-class cable terminations are classified in two groups depending on the type of cables that terminated to namely, extruded-dielectric cable terminations and pipe-type cable terminations. Terminations are required where cables are connected to overhead lines or electrical apparatus. Terminations are designed to possess the same integrity as their associated cables. Cable terminations are designed to eliminate stress concentration resulting from the ending of cable insulation and shield. The utilization of a stress cone effectively separates the shield away from the insulation surface and distributes the stress. All cable terminations, independently of operation voltage utilize a stress control cone. However, for sub-transmission and transmission type terminations, 60 kV and up, in addition to a stress cone, a secondary stress relief control unit is necessary to distribute dielectric stress over the length of the porcelain insulator. A porcelain stress control unit is utilized in conventional, nongraded type terminations for 69 to 161 kV. This cannot be utilized in higher voltage due to excessive internal diameter requirements. In these cases capacitance graded termination are used. These can be coaxial type and doughnut type. The former consists -of a series of cylindrical electrodes coaxial to the cable formed by intercalating aluminum foils between the paper layers, while the latter consist of a stack of toroidal capacitors connected in parallel. 0-7803-5515-6/99/$10.00 0 1999 IEEE 2 Terminations are the second weakest component in underground transmission cables, cable splices are the weakest cable component. Cable terminations normally failed as the result of aging or improper installations. Failure of terminations, results in chattering of the porcelain insulator with pieces scattered over a wide area in addition to fluid leaks. All high voltage terminations are filled with dielectric fluids with the exception of SF6 indoor terminations and terminations in HPGF cables.Condition assessment of terminations is a difficult task.Since all transmission type terminations contain a dielectric fluid, sampling of this fluid for dissolved gas analysis presents an effective approach to termination diagnosis. However, only HPFF...

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Two approaches are available to detect PD from cables, online and off-line detection system. During off-line testing, the cable system is normally tested under no-load conditions while the system loading during on-line testing varies depend on the time of testing. For some insulation defects, PD intensity is load dependent. In some other, PD occurs only at certain load conditions. The types of defect that is affected by the cable loading are presented in this paper. Laboratory and field PD data of testing cables at different loading conditions are also presented.