Cellulosic paper and oil insulation in a transformer degrade at higher operating temperatures. Degradation is accelerated in the presence of oxygen and moisture. Power transformers being expensive items need to be carefully monitored throughout their operation. Well established time-based maintenance and conservative replacement planning is not feasible in a current market driven electricity industry. Condition based maintenance and online monitoring are now gaining importance. Currently there are varieties of chemical and electrical diagnostic techniques available for insulation condition monitoring of power transformers. This paper presents a description of commonly used chemical diagnostics techniques along with their interpretation schemes. A number of new chemical techniques are also described in this paper. A number of electrical diagnostic techniques have gained exceptional importance to the utility professionals. Among these techniques polarisation/depolarisation current measurement, return voltage measurement and frequency domain dielectric spectroscopy at low frequencies are the most widely used. This paper describes analyses and interpretation of these techniques for transformer insulation condition assessment.

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Review of modern diagnostic techniques for assessing insulation condition in aged transformers

Detailed procedures for analyzing gas from gas spaces or gas-collecting devices as well as gas dissolved in oil are described. The procedures cover: 1) the calibration and use of field instruments for detecting and estimating the amount of combustible gases present in gas blankets above oil, or in gas detector relays; 2) the use of fixed instruments for detecting and determining the quantity of combustible gases present in gas-blanketed equipment; 3) obtaining samples of gas and oil from the transformer for laboratory analysis; 4) laboratory methods for analyzing the gas blanket and the gases extracted from the oil; and 5) interpreting the results in terms of transformer serviceability. The intent is to provide the operator with useful information concerning the serviceability of the equipment. An extensive bibliography on gas evolution, detection, and interpretation is included.

IEEE guide for the interpretation of gases generated in oil-immersed transformers

Different partial discharge (PD) detection and measurement procedures suitable for use on cables, capacitors, transformers and rotating machines are examined and compared. Both narrow and wide bandwidth PD detectors are considered; particular attention is given in regard to their suitability to different types of electrical apparatus and cable specimens under test as well as their applicability to discharge site location and their capability to detect different forms of PD. A rather substantial portion of the discussion is devoted to the use of intelligent machines as applied to PD pattern recognition in terms of either PD pulse-height/discharge epoch (phase) distributions or discharge pulse shape attributes.

Partial discharges. Their mechanism, detection and measurement

The interest in studying electrical conduction and polarization phenomena in polymers has increased since last year. Besides the classical problems of conduction, polarization and breakdown in polymeric insulators, interesting for the electric and electronic industry, new aspects are being considered by more and more research groups in the field. One of these is the application of the dielectric method, especially the depolarization technique, for studying molecular mobilities, i.e., multiple phase transitions in polymeric systems. Combined with mechanical relaxation, thermal analysis, magnetic resonance and with some additional techniques dielectric spectroscopy is becoming a powerful tool for studying the phase-structure of such complex polymer systems as blends, filled, and reinforced materials. This way the dielectric technique is helpful in quality control even of such systems which are not used in the electrical or electronic industry.

http://ieeexplore.ieee.org/iel7/7731603/7731604/07731995.pdf

Electrical properties of polymers

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Electrical Insulation for Rotating Machines

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.

Electrical Insulation Deterioration Treated as a Chemical Rate Phenomenon

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.

Application of Epoxy Resins in Electrical Apparatus

A more sensitive and accurate circuit has been devised and tested for showing integrated corona charge transfer per cycle in insulation samples. The method can be used as a very accurate and more rapid quality-control measurement of degree of impregnation or residual voids in apparatus insulation. A rather precise measurement of the fraction of gas space to solid in insulation structures can be obtained from this technique. The estimate of internal voids obtained from this technique is believed to be more accurate than that obtained from highvoltage Schering bridge measurements, with which it can be correlated.

A Capacitance Bridge Method for Measuring Integrated Corona-Charge Transfer and Power Loss per Cycle

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

The voltage endurance of cast epoxy resin-II

A practical means of estimating the fractional volume of gas space in insulation from high-voltage capacitance and dissipation factor measurements has been suggested in this paper. This method has been evaluated by tests on models and various high-voltage insulation specimens. An analysis has been derived relating the observed a-c capacitance and dissipation factor increase with high voltages to the individual electric discharges between the surfaces of gas spaces in insulation.

Thomas W. Dakin

Papers

Electrical Insulation Deterioration Treated as a Chemical Rate Phenomenon

Application of Epoxy Resins in Electrical Apparatus

A Capacitance Bridge Method for Measuring Integrated Corona-Charge Transfer and Power Loss per Cycle

The voltage endurance of cast epoxy resin-II

The Relation of Capacitance Increase with High Voltages to Internal Electric Discharges and Discharging Void Volume