Dielectric Power Factor Research Articles

Method for Optimized Analysis of Electrical Tests of Insulation Power Factor in Power Transformers

In this paper is presented a method for the classification of electrical tests of insulation power factor in power transformers, which performs the assignment of individual scores and ratings to each one of the measured variables through these tests and a final rating. To do this, it uses injective functions with which, according to the value of measured quantities, appropriate scores and ratings are determined. The equations of these functions are defined using maintenance engineering criteria in conjunction with computational optimization techniques - the Hill Climbing algorithm and the 1/5-th Success Rule. It's also presented the equations for the correction of the power factor in function of the temperature of the tests, in view of the adequate classification of these. Furthermore, the proposed method, due to its efficiency, corroborated by the high percentage of correct answers provided by it, is already being successfully implemented in the company CELG Distribuição S.A. (CELG D - currently Enel Distribuiçäo Goiás), helping the decision-making and, thus, contributing to an efficient maintenance of the company's equipments.

Analysis of Transformer Insulation by Tan Delta Testing Method

This paper presents analysis of power transformer insulation by one fundamental insulation power factor test, also known as Tan Delta. It is a routine test conducted at site to know the healthiness of insulation in transformers. Out of 108 transformers tested for research work, case studies were chosen for analysis purpose. Experimental data shows our experience on the measurement of Tan Delta techniques of earthing systems and dryness of insulation in transformers. Result shows that the Tan Delta testing method is very effi cient method.

On-line capacitance and dissipation factor monitoring of AC stator insulation

A new on-line technique for monitoring the insulation condition of ac motor and generator stator windings is proposed. The approach uses a newly developed High-Sensitivity Current Transformer (HSCT) to precisely and non-invasively measure the differential current (e.g., the insulation current) of each phase winding from the motor junction box. Conventional differential current transformers (CT) used for fault protection can be replaced with the new HSCT to measure the winding insulation current with higher sensitivity and accuracy. The HSCT can serve both motor health monitoring and motor protection functions. Presently, indicators for insulation condition such as capacitance (C), dissipation factor (DF), or insulation power factor (PF) are only obtainable off-line. The new approach can provide a low-cost solution for on-line motor insulation condition assessment. Validation of the new HSCT technology was carried out during an accelerated life testing of a 460 V, 100 HP, 1200 RPM form wound induction motor. The motor discussed in this paper was aged at high temperature (255 °C) as the load cycled between 0 % and 200 % every 5 minutes. Although this highly accelerated life test does not represent how a motor ages in service under real operating conditions precisely, the principal goal was to prove the capability of the new HSCT to accurately detect the insulation current and quantitatively monitor motor insulation gradual aging and health. On-line data from the HSCT correlated well with off-line data from a commercial capacitance and DF bridge. It is hoped that the benefits of the on-line motor health monitoring are fully realized and the method extended to other electrical assets as well.

Transformation of Adsorbed Water on Solid Surfaces and its Dielectric Losses and Conductivities

The adsorption isotherm of water vapor on nonalkali glass cloth was obtained at 25°C, and its dielectric power factor was measured also in the frequency range of 80 kc/sec∼11 mc/sec. The power factor of glass cloth and surface dc conductivities of nonalkali glass, soda glass, fused silica, Teflon, and polymethylmethacrylate plates increased rapidly to maximum peaks, when they were exposed to water vapor, and then gradually decreased to certain constant values. These phenomena may be interpreted as physical picture of phase changes in the adsorbed water phase from the amorphous state to the crystalline state, as well as the deposit films of evaporated metals on solid surfaces. But the surface conductivity of polycapramide increases as the moisture content increases with time, suggesting a rapid absorption of water molecules into the solid. An analysis of this process of phase change of adsorbed water is outlined.

On the Relation between Dielectric Power Factor and Small Moisture Content of lnsulating Paper and Oil Impregnated Paper for High Voltage Cables

On the Relation between Dielectric Power Factor and Small Moisture Content of lnsulating Paper and Oil Impregnated Paper for High Voltage Cables

Measurements on new transformer insulation

MEASUREMENTS OF THE power factor and resistance characteristics of transformer insulation are being used increasingly for the control of quality in the factory. The usefulness of these measurements is based on the fact that they are affected by any ionizable impurity such as water, low molecular weight acids from oil deterioration, soluble salts, and other contaminants. In the factory, the principal use for these measurements is the detection of moisture. This moisture has the effect of increasing the insulation power factor, decreasing the insulation resistance, and decreasing the dielectric absorption effect.

Field Studies of Generator Windings [includes discussion

Armature windings of a-c rotating machinery are expendable components, being subject to damaging influences of various natures which are becoming better understood. Tests of windings, which, because of failure or aging, become available for this purpose, contribute significant information for the design of the replacement windings or for new windings. An armature coil may fail because of some fault in the insulation between conductor and core; there are a number of different methods of investigating this insulation. Nondestructive tests include dielectric absorption at low direct-current voltages and insulation power factor at a-c potentials up to 10 kv which are commonly being applied. These methods are discussed. High-voltage d-c tests are becoming of increasing interest as possible nondestructive tests; these are discussed with special attention being made to correlate the d-c tests with high-voltage a-c proof tests. An armature coil may also fail because of turn-to-turn short circuit within a coil having good insulation over-all to the core. This type of incipient failure is not detectable by standard American Standards Association (ASA) dielectric tests. Destructive tests were made on the old generator windings being discussed to determine how well the turn-to-turn dielectric strength had been preserved.

The measurement of permittivity and power factor of dielectrics at frequencies from 300 to 600 Mc/s

The measurement of permittivity and power factor of dielectrics at frequencies from 300 to 600 Mc/s

The measurement of permittivity and power factor of dielectrics at frequencies from 300 to 600 Mc/s

The measurement of permittivity and power factor of dielectrics at frequencies from 300 to 600 Mc/s

The measurement of permitivity and power factor of dielectrics at frequencies from 300 to 600 Mc/s

The paper describes a resonance method of measuring permittivity and power factor which is essentially a development of the Hartshorn and Ward apparatus, suitable for use at the higher frequencies. The method is one of capacitance variation, and the disc form of sample, the circular plate electrodes and the cylindrical form of micrometer capacitor are retained but reduced in linear dimensions. The capacitive elements are mounted in a re-entrant cavity, and the micrometer capacitor is so placed that the voltage across it is only a fraction of that across the plate electrodes; this increases the fineness of adjustment. The micrometer capacitor can be calibrated only by using the cavity as a wavemeter; the capacitance settings were therefore expressed in terms of the corresponding resonance frequencies, and the calculations of power factor then became identical in form with those of the frequency-change method, although the measurements were made at a constant frequency. After applying the appropriate corrections the permittivity can be determined to within ± 1% over most of the range. The high Q-factor of the cavity and the precision of adjustment of the micrometer capacitor make the power-factor determinations highly accurate. For values less than about 0.01 the error is estimated as being less than ± 2 × 10−5 (for measurements on solids). For higher values a superheterodyne receiver incorporating a piston attenuator is used as the detector, and the estimated limits of error for such measurements are ± 5 × 10−4.

Dielectric-loss-measuring equipment for field and works testing

The paper describes two new direct-reading methods which have been developed for the measurement of dielectric power factor, with particular reference to the field and works testing of high-voltage insulation. The two methods to some extent resemble the Schering bridge in that they incorporate a low-loss condenser of known capacitance and negligible loss angle, as the standard with which the insulator under test is compared, but differ radically from the bridge in the manner in which the comparison is effected.The essential feature of the first method is the production of a series of unidirectional voltage pulses of rectangular waveform and constant amplitude, and of the same frequency as the supply. The circuit is so arranged that the duration of each pulse, and, in consequence, the mean value of the resulting voltage, is directly proportional to the insulator loss angle δ. The final indication of loss angle (or, what is virtually the same thing since δ is small, the power factor sin δ), is either given directly on a calibrated voltmeter, or estimated approximately by visual examination of the pulse waveform on an oscillograph screen.The second method consists essentially in using the difference between two voltages, of equal amplitude but with a mutual phase difference equal to the insulator loss angle, as a measure of the power factor, the latter being indicated directly on a calibrated voltmeter. The particular point of interest about this method lies in the means adopted for the preliminary adjustment, which must be made to ensure that the amplitudes of the two voltages are exactly equal.

The measurement of cable characteristics at ultra-high frequencies

The paper describes the two main methods of impedance measurement at frequencies above 100 Mc/s which are in general use, and their application to the determination of cable characteristics, with particular attention to the work of the authors.A preliminary account is given of the various conditions in which cables can be measured, and which are applicable to both of the methods described.An outline of the theory of the standing wave method is given, together with a general description of the equipment required. Attention is paid to coned connectors for the attachment of the cable to the measuring line, and the errors liable to be incurred by their use.The theory and equipment for the resonance line method are described, and an account is given of several investigations that have been conducted' in order to extend its usefulness. These include the measurement of twin cables, the effect on cable attenuation values of reactive discontinuities at the junction of measuring line and cable and at internal supports in the line, the direct determination of the characteristic impedance of measuring lines, and the radiation and reactive effects which occur at their open ends.The application of the resonance line method to the determination of dielectric power factor is discussed, and a method is described which permits greater accuracy than has been obtained up to the present.

Bridged-T and Parallel-T Null Circuits for Measurements at Radio Frequencies

Bridged-T and parallel-T null circuits may, under some circumstances, be preferable to bridge circuits for radio-frequency measurements as no transformer is required and the generator and detector can have a common grounded terminal. An analysis of the circuits can be made in terms of the transfer impedances of the various possible component T networks so that the nature of possible null conditions becomes evident by inspection. The circuits considered include arrangements suitable for the measurement of resistance, reactance, and frequency, and of the power factor of dielectrics. Some of the circuits, particularly suited to high-frequency work, employ neither coils nor variable resistors.

The measurement of the permittivity and power factor of dielectrics at frequencies from 104to 108cycles per second

The paper describes apparatus devised for the measurement of the permittivity and power factor of insulating materials over a wide range of radio frequencies. The range covered is from 10 kc to 100 Mc, and the same calibration of the apparatus holds good over the entire range without the use of any troublesome corrections. The only changes necessary in order to proceed from one part of the range to another are changes of tuning coils.The method is that of capacitance-variation in a tuned circuit, with a thermionic voltmeter as a detector of resonance. The adjustments are made by means of two micrometer condensers, one being a plate condenser in which the sample is inserted, and the other a cylindrical condenser of linear law and of very small range, which serves to measure the sharpness of resonance. Both permittivity and power factor may be obtained as the ratio of capacitance readings: frequency is not involved in their calculation, and it is this property which gives the apparatus its very large frequency-range.The errors due to residual inductance and resistance at the highest frequencies are discussed, and it is shown that these errors, together with those associated with imperfect contact of electrodes, may be eliminated, partly by very careful attention to details in the construction of the apparatus, and partly by the procedure followed in making the measurements.The apparatus is also applicable to measurements on resistors over the same range of frequencies.

The measurement of the permittivity and power factor of dielectrics at frequencies from 104to 108cycles per second

The paper describes apparatus devised for the measurement of the permittivity and power factor of insulating materials over a wide range of radio frequencies. The range covered is from 10 kc to 100 Me, and the same calibration the apparatus holds good over the entire range without the use of any troublesome corrections. The only changes necessary in order to proceed from one part of the range to another are changes of tuning coils. The method is that of capacitance-variation in a tuned circuit, with a thermionic voltmeter as a detector of resonance. The adjustments are made by means of two micrometer condensers, one being a plate condenser in which the sample inserted, and the other a cylindrical condenser of linear law and of very small range, which serves to measure the sharpness of resonance. Both permittivity and power factor may be obtained as the ratio of capacitance readings: frequency is not involved in their calculation, and it is this property which gives the apparatus its very large frequencyrange. The errors due to residual inductance and resistance at the highest frequencies are discussed, and it is shown that these errors, together with those associated with imperfect contact electrodes, may be eliminated, partly by very careful attention to details in the construction of the apparatus, and partly by the procedure followed in making the measurements. The apparatus is also applicable to measurements on resistors over the same range of frequencies.

Dielectric Power Factor Measurements at Audio- and Radiofrequencies

The paper presents a description of sensitive and accurate apparatus for measuring the power factor of dielectrics in the frequency range, 60–7,200,000 c.p.s. A Wien bridge with Wagner ground for the direct measurement of both power factor and frequency in the range, 60–10,000 c.p.s. is described. Double shielding of the series resistance arm eliminates errors otherwise inherent in this bridge. The operation of the resonance-substitution method is discussed for the frequency range, 10,000–7,200,000 c.p.s. Errors due to eddy current losses are eliminated through measurements on loss free liquids. A measuring cell for use with these circuits and also with d.c. measuring circuits is described. Some experimental power factor-frequency curves illustrating the results obtainable with this apparatus are presented. It is shown that power factor-frequency data furnish a delicate method of tracing out dispersion phenomena and are useful where the concentration or electric moment of material producing dispersion is small.

Standards for measuring the power factor of dielectrics at high voltage and low frequency

Standards for measuring the power factor of dielectrics at high voltage and low frequency

Accuracy required in the measurement of dielectric power factor of impregnated paper-insulated cables

This paper deals with the effect of errors in the measurement of power factor upon the usefulness of impregnated paper-insulated cables. It points to error in the knowledge of the thermal properties of the cable and the cable duct. The latter error affects the usefulness of a cable to such an extent as to permit a limited error in the power factor without materially reducing the efficiency of the cable. This limited error defines the required power-factor accuracy. The required power-factor accuracy in general is found to vary directly with the frequency and the specific inductive capacity of the insulation, and to increase with the number of cables in a duct bank and the ratio of E <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /G, where E is the operating voltage of the cable in kilovolts and G is the geometric factor. For very high-voltage single-conductor cables the power-factor accuracy should be within the order of 0.002.

II---Standards for Measuring the Power Factor of Dielectrics at High Voltage and Low Frequency

This paper points out the need which exists in the electrical industry, particularly in connection with the testing of high-voltage cables, for convenient standards for use in the measurement of dielectric loss. At present most laboratories make use of air condensers. In the paper these are classified and certain sources of error which must be guarded against are mentioned. Condensers with solid dielectrics would be much more convenient, more portable, and cheaper, but so far none have been produced which have satisfactory constancy for use as standards at high voltage.

III---Accuracy Required in the Measurement of Dielectric Power Factor of Impregnated Paper-Insulated Cables

This paper deals with the effect of errors in the measurement of power factor upon the usefulness of impregnated paper-insulated cables. It points to error in the knowledge of the thermal properties of the cable and the cable duct. The latter error affects the usefulness of a cable to such an extent as to permit a limited error in the power factor without materially reducing the efficiency of the cable. This limited error defines the required power-factor accuracy. The required power-factor accuracy in general is found to vary directly with the frequency and the specific inductive capacity of the insulation, and to increase with the number of cables in a duct bank and the ratio of E2/G, where E is the operating voltage of the cable in kilovolts and G is the geometric factor. For very high-voltage single-conductor cables the power-factor accuracy should be within the order of 0.002.