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Class F insulation materials have usually been operated at Class B temperatures since their introduction in the early 1970s. Extensive testing and service experience appears to show that there is no reason for this reluctance to fully utilize the capability of the insulation. Experience with the latest types of generators indicates that perhaps this caution is still justified
The following topics were dealt with: electric machines; power cables; outdoor insulation; transformers; life assessment; partial discharge measurement; insulating materials; GIS; capacitors; switching phenomena; nanodielectrics; permittivity; electric field
In searching process for new, advanced technologies which could be applied for on-site diagnostics of machine insulation this paper discuses the application of damped AC voltages to detect and to analyze the PD processes. Based on laboratory investigation on a full scale stator insulation system, as well as during field inspections on large motors, a comparison of PD behavior: the PD inception conditions...
The aging due to mechanical, thermal, electrical and environmental stresses of a stator insulation system inherently involves alterations of the material properties which are detrimental to its service operation. When these properties are deteriorated to the point where the material can no longer operate safely under normal stress conditions, it implies that it has reached the end of its useful life...
The insulation of high voltage stator bars is covered with layers of tape or varnish that have different conductivity to prevent slot discharge or to spread the voltage along the end winding. The limit between these layers is visible when there's a change in the color or texture. The electrical limit is not always easy to spot, for example if an insulated tape covers the limit of the conductive layer...
The cause of high difference number of insulation failures in two old hydro power plants in Iran is investigated. Results are introduced according to the comparative electrical and combined electrical, thermal and mechanical accelerated aging tests that are carried out on the old and spare generator bars
The ongoing evaluation of material properties, including thermal performance, is part of the continuing evolution of insulation system development. In large strip-wound rotors, a coated aramid paper is used to bond the coil turns together and provide turn-to-turn insulation. Experience has shown that the thermal performance of the system is limited by the capability of the bonding material. This paper...
The electrical breakdown performance, either unaged or after ageing (laboratory or service), is often used as the basis for qualification of a device, design or material. Many of the features that affect these performance levels have been discussed in other documents; contaminants, propensity for water treeing, insulating and semiconducting materials. However the size of cable tested is rarely discussed...
Partial discharge is a useful diagnostic method for a wide range of power apparatus. In the case of shielded power cable, partial discharge can detect workmanship-induced defects in accessories, as well as cavities and electrical trees in the dielectric. Shielded power cables can be considered lossy transmission lines. The high frequency attenuation of the cable causes the peak amplitude of a PD pulse,...
Defects can be accidentally introduced into the insulation system of cable joints during manufacture, due to the manual nature of the jointing process. Any defect introduced can have an affect on the working lifetime of the cable, therefore, it is necessary to detect and remove these defects. Detection of defects can be achieved by using conventional X-ray techniques, however, there are problems with...
Flat-conductors made by high temperature superconducting filaments can be electrically insulated by a host of different materials. After selecting the best performing system, an electrical characterisation stage has been implemented through dielectric breakdown tests in air and in liquid nitrogen (LN2) and through ageing in LN2, at atmospheric pressure
Within the last decade, developments in both detection technology as well as signal processing technology have added significantly to the diagnostic knowledge which can be extracted from on-line partial discharge measurements performed on medium voltage cable systems. Consequently, the diagnostic assessments of the insulation condition of cables and cable accessories improved significantly. In this...
The paper describes a series of visual corona tests performed in accordance with the voltage gradient test method. The tests were performed on insulator assemblies designed for use on AEP's 6-conductor bundle 765-kV transmission line
The following paper shows a clear correlation between the measured tracking index and the breakdown field strength for noncoated glass fibre reinforced polymers (GFRP) with either a polyester or an epoxy based resin. 17 types of specimens have been tested according to IEC Publication 60587. The breakdown field strength of specimens cut from similar samples is determined by a new method capable of...
This paper investigates the electric field distribution along the surface of high voltage polymer insulators and its enhancement due to water droplets by means of computational simulations and laboratory experiments. Different factors affecting the electric field distribution are analyzed through finite element method simulations. Among them, the presence of a grading ring and water droplets on the...
In this study, composite materials were put to dry interfacial treatment by use of plasma technology. It has been presented that the optimum parameters for the best wettability of the samples at the time of generation of plasma were oxygen atmosphere, 0.1 torr of system pressure, 100 W of discharge power, and 3 minutes of discharge time. The decrease in surface potential of charged samples by corona...
Advantages of XLPE covered conductors in comparison with bare conductors of low voltage and medium voltage power distribution networks are introduced. The configuration, production and application of these conductors in Iran are discussed and the electromagnetic effects of them are studied through the simulation results carried out by finite element methods
This paper describes how requirements on transformer oil are reflected by different specified physical and chemical parameters of oil. Impulse breakdown is not included in many specifications but ASTMD3487 does have it. Breakdown behaviour with DC impulses and a heterogeneous gap is very different from the AC breakdown strength. It is designed to simulate lightning pulse striking during a thunderstorm...
On some units both the generator and the step-up transformer have been equipped with capacitive couplers. This has provided an opportunity to determine how the various couplers respond to common sources of partial discharge
Synthetic and natural oils are used in many items of high voltage plant, both as an electrical insulator and as a coolant. In addition, analysis of the oil may provide a convenient means of monitoring plant health-as in the case of dissolved gas analysis (DGA). Although it is well appreciated that failure of plant occurs when the insulating properties of the dielectric are reduced through ageing,...
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