T. W. Dakin

Electrical Insulation Deterioration Treated as a Chemical Rate Phenomenon

The basis is presented for a more accurate interpretation of the results of physical type tests to measure the thermal aging of insulation together with a more accurate method of applying the results of such tests to predicting insulation deterioration in practice. Since the observed physical changes during thermal aging are the result of internal chemical changes in organic material, it is shown that the theory of chemical reaction rates can be applied to analyze experimental data on aging. The approximate 7 to 10 rule for the temperature coefficient of deterioration rate is replaced by a more accurate theoretical expression. Various examples of insulation life tests are analyzed using the graphical methods outlined in the paper. The chemical rate theory interpretation of thermal aging offers a more satisfactory method for extrapolating the results of limited aging tests of insulating materials so they can be applied to predicting amounts of thermal aging in high temperature cycles.

The voltage endurance of cast epoxy resin-II

Cast epoxy resins are being used in various app1ications such as high voltage conductor supports in compressed gas systems, high voltage bushings, and instrument transformers. In some cases the operating electric stress through the resin is sufficiently high to support partial discharges in series gas gaps or cavities, if they exist and are large enough.This paper summarizes the progress of an extensive study of the ac voltage endurance, extending from a single cycle to more than a year, for cast silica-filled epoxy resin, with recessed silver paint electrodes and cast-in aluminum electrodes, giving a nearly uniform electric field. Although the specimens were prepared to avoid gas cavities, and partial discharges were usually undetected, the ac strength declines regularly with time in a manner similar to that due to partial discharges. Therefore, it is inferred that undetected partial discharges are responsible. A derivation is given for a suggested new analytic relation for the ac volt-failure time curve. It is suggested that the distribution of ac dielectric strength values for a given test time is correlated with the distribution in failure times at a given test voltage level.

Application of Epoxy Resins in Electrical Apparatus

This summarizes the important physical and electrical characteristics of these diverse and highly useful insulating materials. Factors of chemical structure which influence the resulting resin properties are discussed. The great variety of properties which can be achieved through variations in this one class of resins is emphasized. A range of applications of these resins is mentioned and illustrated.

An Electromagnetic Probe for Detecting and Locating Discharges in Large Rotating-Machine Stators

The detection and location of discharges in rotating- machine high-voltage windings is useful in directing attention to abnormalities in the insulation structure which can lead to premature failure of the winding. In order to achieve a practically useful test it is necessary to distinguish between conditions involving many small size voids which are not severely limiting and relatively few large voids.

Effect of electric discharges on the breakdown of solid insulation

The effect of corona on the long-time dielectric strength of insulation is emphasized in this paper, and various factors influencing the breakdown of insulation with corona are discussed. Data on the corona threshold voltages on dielectric surfaces at electrode edges show that the threshold voltage varies for a wide variety of dielectrics only as the ratio of dielectric thickness to dielectric constant.

Ultrasonic Sensing of Partial Discharges within Microfarad Value AC Capacitors

As new dielectric fluids and materials are introduced, partial discharge (P.D.) testing of microfarad value capacitors in the factory is assuming greater importance. However, the large capacitance restricLs the use of the common electrical methods for detecting small (1-20 pC) discharges, and bridge techniques are not economical for production line tests. For these reasons, the use of ultrasonic sensors for detecting P.D. within microfarad value capacitors was studied, and this resulted in the development of a combined acoustic waveguide probe, ultrasonic sensor and indicating meter, suitable for factory P.D. testing of capacitors.

A Resonant-Cavity Method for Measuring Dielectric Properties at Ultrahigh Frequencies

A re-entrant cylindrical cavity has been adapted to measure the dielectric constant and power factor of small disk samples of insulating materials. The methods of measurement, mechanical details, and electric coupling and detecting circuits all are described. A single cavity can be utilized only over a frequency range of about 1.5 ratio from lowest to highest frequency. Therefore, several different-sized cavities would be required to cover a range of frequency from 50 to 1,000 megacycles. An accuracy of ±0.00005 in tan δ and ± 1 per cent in dielectric constant may be obtained in routine measurements. Because the cavity has a very high Q(>2,000), it is much more sensitive to low-power-factor dielectric samples than any conventional coil-and-capacitor resonant circuit. The chief advantages of this method are that the operation of the apparatus is simple, very rapid, and similar to the susceptance variation technique now used at lower radio frequencies. Also, the involved computations usually found in other methods operative in this frequency range are eliminated.

The Relationship Between the Picocoolomb and Microvolt for Corona Measurements on HV Transformers and Other Apparatus

Historically in North America, partial discharge or corona measurements in MV have been used for transformer quality control and customer acceptance, but in the future it is anticipated the pC unit may also be used.

Continuous Recording of Outdoor Insulator Surface Conductance

Surface conductance of outdoor insulators exposed to pollution and moisture from rain or fog is related to two aspects of insulator application: 1) possibility of insulator flashover and 2) duration and severity of surface arc discharges. This paper will discuss some of the experience with monitoring the surface conductance of porcelain and synthetic insulation under voltage stress and exposed to outdoor weathering in various locations in the U. S. Simple flat surfaces mounted at a 30° angle from the horizontal and some shed-type insulators have been followed in this study. Recordings have been made over periods of several years, comparing results with low and high voltage stress. Results of these outdoor tests are also compared with results of tests of similar surfaces in a synthetic salt-fog chamber.

Voltage Endurance Tests of Insulating Materials Under Corona Conditions

A systematic study shows how the failure times of insulating materials exposed to corona discharges are influenced by various factors, including conductor edge surface discharge conditions, test frequency, void depth, humidity, temperature, material thickness, and average electric stress. Accelerated higher frequency tests permit testing at low-voltage stresses used continuously in applications. These yield results equivalent to 60-cps (cycles per second) tests, in cycles to failure, if both tests are run at low humidity with limited dielectric heating.