Showing papers on "Partial discharge published in 1968"

Modes of Corona Discharges in Air

Abstract: Corona discharges in air exist under several distinctive forms, either pulsative or stable. The properties of different corona modes are analyzed as they appear in a cylindrical distorted field. Differences between dc and ac excitations are emphasized. Corona discharges may produce energy loss without detectable radio noise or high noise at low energy loss, depending upon their form of appearance. The different withstands of asymmetric gaps under different polarities also find their explanation in corona mechanism.

AC Breakdown of Epoxy Resins by Partial Discharges in Voids

Abstract: The subject of this paper is the effect of partial discharges in a void on the performance of epoxy resin insulation. Theoretical and experimental investigations are reported that seek a relationship between measurable electrical quantities and a possible breakdown of the insulation. All experiments were done under controlled conditions with samples of quartz-power filled and unfilled epoxy resin containing an artificial void of known cylindical shape and dimensions. The test specimen and the measuring circuits are described. Theoretical considerations of the equivalent circuit of the test circuit indicate that the charge fed into the specimen per pulse and the discharge power are independent of the external circuit. The electric charge and power fed into the specimen are appropriate fundamental quantities for the assessment of the pulse-type process. The theory was confirmed by measurements. It was found that the power consumed by the internal partial discharges up to a breakdown could be measured by two independent methods. Experimental results of long-time aging investigations with alternating current voltage are reported. Partial discharges in voids in epoxy resin insulation subjected to long-term voltage stressing can lead to breakdown of the insulation. The period of time of voltage application to a test specimen can be divided into two characteristic intervals, which were of different duration in the filled and unfilled resins. During the first interval, all important corona quantities (i. e., corona charge, discharge current, power and radio interference voltage) remain either constant or decrease.

The Breakdown Voltage of Oil Gaps with High DC Voltage

Abstract: The electrical strength of technically clean insulating oil in a direct voltage field is lower when the high-voltage electrode is sharper and of positive polarity. In a uniform field, with distances up to 100 mm, approximately 30 percent lower withstand values are obtained with direct voltages than with alternating voltages (rms values); this is also valid when the oil is heated up to transformer operating temperature. For longer applications of the test voltage (up to 30 minutes) a reduction in the electrical strength of approximately 15 percent, compared with 1 minute values, is to be expected. With direct voltages having a superimposed alternating component, the same results apply if the peak value of the combined voltage is used.

Elektrische Eigenschaften von Xenon-Impulsplasmen

Abstract: A description is given of an experimental arrangement for studying xenon pulse discharges with high specific power dissipation. By means of time resolved measurements of discharge pressure and spatial radiation distribution the discharge has been observed and periods of time are found during which the discharge conditions are quasistationary. Electrical measurements yield the electrode drop voltages and the electric field strength as functions of pressure and current. A method for determining the discharge pressure from measurements of voltage drop across the lamp and current in closed discharge tubes is discussed. A calculation of temperature dependence of radiated power and electrical conductivity results in discharge temperatures of 10000–12000°K with an error of 10 … 15 percents.

Calibration and comparison of partial-discharge and radio-interference measuring circuits

Abstract: Methods of calibrating circuits for measuring partial discharges and radio-interference (r.i.) voltages are discussed. It is shown that discharge-measuring circuits, whether using a peak-measuring discharge detector or an r.i. meter, are best calibrated by applying a charge-quantity pulse calibrator of known repetition frequency in parallel with the test sample. The calibration of circuits for measuring r.i. voltages is more complex, and certain anomalies arise when a sine-wave signal generator is used to calibrate the circuit in terms of voltage input. It is established that, for a pulse-repetition rate of 100 pulse/s, it is possible to relate the results obtained from a conventional peak-reading discharge detector to those from a quasipeak r.i. meter; e.g. for a meter having 60? input impedance and a bandwidth of 9 kHz, the relationship is: 1 ?V is equivalent to 2.6pC. At repetition rates other than 100 pulse/s, it is shown that theoretically the maximum error involved by using the above relationship is 6 dB between 25?2000 pulse/s. Practical measurements confirm that the error does not exceed ±6dB over a wide range of pulse sizes and repetition frequencies. Comparative measurements have shown that the conventional r.i. meter is approximately an order of magnitude less sensitive than a tuned-circuit discharge detector. Methods of improving the sensitivity of the former are described. It is concluded that for many applications either a discharge detector or an r.i. meter will enable both discharge magnitude and r.i. voltage to be determined with sufficient accuracy, considering the erratic behaviour of pulses in practice.

Theory of discharges at the foil edge in capacitors

Abstract: There is a high field concentration at the electrode edge in rolled-paper or film capacitors, and if the edge is squarely cut, the field at the corner tends to infinity. Breakdown in the dielectric fluid surrounding the edge constitutes a partial discharge, but a breakdown field, and hence the discharge-inception voltage, cannot be simply derived with an infinite field at the corner and a highly divergent field. lt.is argued that a discharge occurs when the voltage gradient along a line of maximum field exceeds a certain critical value over a certain length. The applied voltage necessary to reach this condition is calculated from an approximate analytical treatment of the potential configuration. The resulting formula is verified as regards the dependence of discharge-inception voltage on the dielectric and electrode thickness, using castor-oil- impregnated paper capacitors. Capacitors constructed with longitudinally folded foil electrodes show a markedly increased inception voltage. An analysis of the field for a rounded-edge profile is carried out to a first approximation. The experimental results for the two types of edge profile enable the disposable constants in the theory to be evaluated.

Method and apparatus for electrical discharge machining using a substantially constant current density during a spark discharge

Abstract: A METHOD FOR MAINTAINING A SUBSTANTIALLY CONSTANT CURRENT DENSITY DURING A SPARK DISCHARGE IN ELECTRICAL DISCHARGE MACHINING WHEREBY THE WEAR OF THE TOOL ELECTRODE IS REDUCING SUBSTANTIALLY. ONE APPARATUS INCLUDES AN INDUCTANCE AND RESISTOR IN PARALLEL WHEREBY THE RESISTOR PRODUCES A PREDETERMINED MINIMUM CURRENT FLOW BETWEEN THE WORKPIECE AND THE TOOL DISCHARGE UPON START OF CURRENT FLOW THEREBETWEEN, AND THE INDUCTANCE REGULATES THE RATE OF INCREASE OF CURRENT FROM THE PREDETERMINED MINIMUM CURRENT TO MAINTAIN A SUBSTANTIALLY CONSTANT CURRENT DENSITY AS THE AREA OF THE DISCHARGE INCREASES.

A New Form of Needle Test for Polyethylene Insulation

Abstract: A 1962 paper [1] described the single-needle test, a method of electrically stressing an artificial fault in transparent insulating polymers for the purpose of evaluating the effects of additives in improving their dielectric strength and voltage life. This paper reviews developments in test procedure since that time. Oil has replaced air as an interelectrode medium. Other details of current technique are described, and some experimental phenomena are discussed.

Partial discharge measuring means including discharge detecting circuits and a discharge detector connected thereto and including a selector circuit

Abstract: A MEANS FOR MEASURING PARTIAL DISCHARGE OF ELECTRICAL EQUIPMETS, SUCH AS ELECTRIC CABLES AND TRANSFORMERS, HAVING A SPECIAL DISCHARGE DETECTOR, IN WHICH A DETECTION CIRCUIT IS FORMED BY TEST OBJECTS, COUPLING CONDENSERS, AND DETECTION IMPEDANCES. A DEFECT IN THE TEST OBJECT PRODUCES PARTIAL DISCHARGE, WHICH MAY BE MEASURED AS DISCHARGE PULSES IN THE DISCHARGE DETECTOR. TO KEEP EXTERNAL DISTURBANCES FROM HAMPERING MEASUREMENT OF PARTIAL DISCHARGE, PULSES FROM THE TEST OBJECT AND THE COUPLING CONDENSER, AFTER BEING SHAPED RESPECTIVELY, ARE LED INTO A DIFFERENTIAL AMPLIFIER WHERE EXTERNAL NOISES CANCEL EACH OTHER. DISCHARGE MEASUREMENT CAN THEREFORE BE MADE WITHOUT BEING HAMPERED BY EXTERNAL DISTURBANCES.

The Significance of Corona Measurements on Transformers

Abstract: Tests to show the attenuation of voltage and charge phenomena between corona sources located in power transformer windings and the terminals of the transformer are described. The attenuation of corona, or simulated corona, in both major and minor insulation is considered. A circuit developed specifically for the purpose of simulating corona is detailed and its application in the tests is described.

Corona discharges in d-c and partially rectified a-c insulation systems

Abstract: During the past several decades of rather intensive study of corona (partial) discharges and their effects on insulation systems, most of the attention has been focused on a-c discharge behavior. This has been rightly motivated by the much more serious effects of corona in most a-c insulation applications, where usually many more discharges can occur, during an average time, than with d-c. Because discharges in d-c insulation systems are much more intermittent, their rate of erosion or penetration of insulation barriers is correspondingly reduced, and it has been practical to apply much higher d-c stresses, without completely preventing corona, than with a-c cases. This characteristic of d-c stresses has been utilized effectiyely in d-c over-voltage testing of insulation systems, particularly of rotating machines.

The design of high voltage bushings

Abstract: In all electrical equipment operating at a potential over about 2000 volts the problem of the penetration of walls, normally at ground potential, by current-carrying conductors is one which must be solved. The normal solution is a bushing which will provide the necessary conductor isolation and frequently will also perform the function of a gas or oil seal. A well-designed bushing will perform its function with little measureable dielectric loss and will not distort the circuit with an objectionable discharge at operating voltage levels. While discharge inception above nominal operating voltage is usually the point of reference, it is not the sole operational criterion. The possible presence of transient overvoltages, even for short periods of time, may cause the inception of discharge. It is therefor mandatory in a bushing that the discharge extinction voltage be above the operating level in order that transient induced discharges may not be continued at the operating voltage level.