Selectivity of damped AC (DAC) and VLF voltages in after-laying tests of extruded MV cable systems

The purpose of HV after-laying tests on cable systems on-site is to check the quality of installation. The test on extruded MV cable systems is usually a voltage test. However, in order to enhance the quality of after installation many researchers have proposed performance of diagnosis tests such as detection, location and identification of partial discharges (PD) and tan /spl delta/ measurements. Damped AC voltage (DAC) also called oscillating voltage waves (OVW) is used for PD measurement in after-laying tests of new cables and in diagnostic test of old cables. Continuous AC voltage of very low frequency (VLF) is used for withstand voltage tests as well as for diagnostic tests with PD and tan /spl delta/ measurements. Review on the DAC and VLF tests to detect defects during on-site after-laying tests of extruded MV cable systems is presented. Selectivity of DAC and VLF voltages in after-laying testing depends on different test parameters. PD process depends on type and frequency of the test voltage and hence, the breakdown voltage is different. The withstand voltage of XLPE cable insulation decreases linearly with increasing frequency in log scale. Experimental studies with artificial XLPE cable model indicate that detection of defects with DAC or VLF voltage can be done at a lower voltage than with DC. DAC voltage is sensitive in detecting defects that cause a breakdown due to void discharge, while VLF is sensitive in detecting defects that cause breakdown directly led by inception of electrical trees.     

Condition assessment of XLPE cable insulation using short-time polarisation and depolarisation current measurements

The authors describe how a commercially available test set has been used to make preliminary determinations of condition of medium voltage underground power cables with extended cross-linked polyethylene cable insulation at remote sites in north-east Queensland. Polarisation and depolarisation currents are measured with an injection voltage of 1 and 5 kV with observed polarisation and depolarisation periods of 600 and 60 s, respectively. The condition of cables is estimated on the basis of a newly defined apparent conductivity, which is based on difference between polarisation and depolarisation currents in the first 20 and 60 s, and on a degree of nonlinearity factor which is the ratio of apparent conductivity at different voltages. Procedures needed to obtain consistent field test results are recommended as are diagnostic criteria to provide an indication of cable insulation condition. It is concluded that while the technique can provide evidence of poor insulation quality, it is also possible to distinguish between water trees in insulation and poor joint insulation.     

Application of dielectric response measurement on power cable systems

Water treeing is one of the factors leading to failure of medium voltage XLPE cables in long-term service. Increased moisture content inside oil-paper insulated cable is not desirable. To identify water tree degraded XLPE cables or oil-paper cables with high moisture content, diagnostic tests based on dielectric response (DR) measurement in time and frequency domain are used. Review of individual DR measurement techniques in the time and frequency domains indicates that measurement of one parameter in either domain may not be sufficient to reveal the status of the cable insulation. But a combination of several DR parameters can improve diagnostic results with respect to water trees present in XLPE cables or increased moisture content in oil-paper cables. DR measurement is a very useful tool that reveals average condition of cable systems. However, it is unlikely that DR measurement will detect few, but long water trees. In addition, DR cannot locate the defect or water tree site within the cable system. Combination of DR and partial discharge (PD) measurements can improve diagnostic results with respect to global and local defects. However, it is doubtful whether PD test can identify the presence of water trees inside a cable in a nondestructive manner. Further research is needed for more detailed conclusions regarding the status of a particular insulation and for predicting the remaining life of the insulation system.     

Development and Application of the Specialist Task Organization Procurement Approach

The study focuses on the development of an innovative procurement method in order to improve owners’ contracting strategies. The aim of this paper is to develop a procurement system that improves the overall project implementation process, in order to meet the project objectives of time, cost, and quality. The Specialist Task Organization (STO) approach is suggested based on a constructive research approach, where extensive theoretical development of the STO concept is developed. The analogical validation was carried out through the review of four case study examples in order to find the analogies between the positive outcomes of four case studies and the elements of the STO route. The proposed task-oriented approach utilizes integrated product (task) development, integrated management, and fragmented execution via task organizations. The merits of the STO approach include, the shifting of competition from only single design and cost based to multiple designs, life-cycle costs, alternative materials, and maintainability implications. The STO approach assumes a modular approach to design, and allows experts knowledge in design and construction through their involvement from preconstruction through project starts up. The suggested STO route is a potential tool in realizing complete sustainability in construction in terms of environmental and social sustainability, as well as economic sustainability.     

Voltage distribution in the stator winding of an induction motor following a voltage surge

Contents  This paper presents results of voltage transient tests. The primary aim of these tests is to develop an experimental understanding of what happens after a voltage transient arrives at the stator winding of an electrical machine. The test was performed on the stator of a 6 kV induction motor. The present measurements are performed with a 42 V input voltage step transient. The results show that, among the stator coils, the highest voltage peak appears across the coil next to the input; thus, this first terminal-end coil is subjected to the highest voltage stress. It was found, additionally, that the surge voltage distribution within the first coil is independent of the terminating condition at the neutral end of the winding, i.e. either grounded or ungrounded. The direct measurement shows that the inter-turn voltage is non-linearly distributed and the amplitude of the inter-turn voltage of the first coil depends quite considerably on the features (e.g. length) of the cable feeding the winding. The present experimental findings should be of help to those concerned with the turn insulation in motors fed by a frequency converter and surge protection problems of large AC motors.

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übersicht  In diesem Artikel werden die Ergebnisse eines Testes pr?sentiert, welcher sich auf transiente Spannung bezieht. Das Hauptziel dieser Tests besteht in der Schaffung eines experimentellen Verst?ndnisses darüber, was passiert, wenn eine transiente Spannung an der Statorwindung einer elektrischen Maschine auftritt. Der Test wurde an einem Stator eines inodernen 6 kV Induktionsmotors ausgeführt. Bei den aktuellen Messungen betrug der Eingangswert der transienten Spannung 42 V. Diese war stufenf?rmig ausgebildet. Die Resultate zeigen, da? die h?chste Spannungsspitze in jener Statorspule auftritt, welche am n?chsten zum Spannungseingang angeordnet ist; daher ist diese Spule des Terminalendes der h?chsten Spannungsbelastung ausgesetzt. Weiter wurde herausgefunden, da? die Sto?spannungsverteilung in der ersten Spule unabh?ngig davon ist, welche Endbedingung am neutralen Ende der Wicklung vorliegt, d.h.ob Erdung vor liegt oder nicht. Die unmittelbare Messung zeigt, da? die Spannung zwischen den jeweiligen Windungen nichtlinear verteilt ist und die Spannung zwischen den jeweiligen Windungen der ersten Spule relativ stark davon abh?ngt, welche Eigenschaften der Draht der Wicklung aufweist (z.B. L?nge). Die erzielten experimentellen Erkenntnisse sollten all jenen von Hilfe sein, welche mit der Windungisolierung von Motoren mit Frequenzkonvertern und Problemen des Spannungsschutzes von gro?en AC-Motoren befa?t sind.

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Received: 6 December 1999