Dielectric spectroscopy in the time or frequency domain offers new opportunities for an off-line, on-site insulation condition assessment of HV electric power equipment and its predictive maintenance.

Applications of dielectric spectroscopy in time and frequency domain for HV power equipment

The role of electrical insulation is critical for the proper operation of electrical equipment. Power equipment cannot operate without energy losses, which lead to rises in temperature. It is therefore essential to dissipate the heat generated by the energy losses, especially under high load conditions. Failing to do so results in premature aging, and ultimately to failure of the equipment. Heat dissipation can be achieved by circulating certain liquids, which also ensure electrical insulation of energized conductors. The insulating-fluids market is therefore likely to be dominated by liquids, leaving to gases (such as compressed air and SF6) limited applications in power equipment such as circuit breakers and switchgear [1]-[3]. Several billion liters of insulating liquids are used worldwide in power equipment such as transformers (power, rectifier, distribution, traction, furnace, potential, current) [4], resistors [5], reactors [6], capacitors [7], cables [8], bushings [9], circuit breakers [10], tap changers [11], thyristor cooling in power electronics, etc. [12]. In addition to their main functions of protecting solid insulation, quenching arc discharges, and dissipating heat, insulating liquids can also act as acoustic dampening media in power equipment such as transformers. More importantly, they provide a convenient means of routine evaluation of the condition of electrical equipment over its service life. Indeed, liquids play a vital role in maintaining the equipment in good condition (like blood in the human body). In particular they are responsible for the functional serviceability of the dielectric (insulation) system, the condition of which can be a decisive factor in determining the life span of the equipment [13]. Testing the physicochemical and electrical properties of the liquids can provide information on incipient electrical and mechanical failures. In some equipment, liquid samples can be obtained without service interruption.

50 years in the development of insulating liquids

Knowledge about moisture content in oil impregnated paper insulation is essential when estimating remaining lifetime of power transformers. Direct evaluation of moisture content is rarely possible due to inaccessibility of the internal insulation system in transformers. Therefore, various indirect estimation techniques are utilized. Frequency domain spectroscopy (FDS) measurements of transformer insulation belong to this group. To perform high quality interpretation of results of FDS measurements a good knowledge on dielectric responses in oil impregnated pressboard and paper is required, especially as it refers to their variation with water content and temperature. The aim of this paper is to provide an open access to the frequency domain spectra of oil impregnated paper and pressboard samples, which can then be used in modeling of the results of diagnostic measurements in power transformers.

Frequency response of oil impregnated pressboard and paper samples for estimating moisture in transformer insulation

Dielectric testing techniques, in both time and frequency domains, are currently widely used by power utilities for assessment of the condition of transformer oil-paper insulation systems. However, it has been reported that results of these tests are highly influenced by the operating temperature during measurements. The distribution, migration and equilibrium of moisture between oil and paper in a complicated insulation system is highly temperature dependent. It requires adequate experience and proper understanding to interpret the dielectric response results in the presence of temperature variations and thermal instability. Proper analysis of the dielectric test result is only possible with an understanding of the physical behavior of the insulation system in response to temperature. A circuit model, which describes the dielectric behavior of the transformers main insulation system, has been investigated in this paper. The values of the parameters of the model have been identified from the dielectric tests. A correlation has been observed between the operating temperature and the equivalent model parameters that can be used as additional information for better interpretation of the dielectric test results. This paper thus reports a detailed study on the effects of temperature on dielectric measurements of a transformer under controlled laboratory conditions. Some results of practical on-site testing are also presented to demonstrate the possibility of errors that may be introduced in dielectric test results analysis unless temperature effects are taken into consideration.

Investigations of Temperature Effects on the Dielectric Response Measurements of Transformer Oil-Paper Insulation System

This paper examines the practical aspects of the usage of cellulose insulation in transformers with regard to maintaining the integrity over long periods of time and preserving and extending insulation life. The factors that relate to loss of life are examined and life plots are discussed in more detail.

Cellulose insulation in oil-filled power transformers: part II maintaining insulation integrity and life

This paper provides an overview of the classic moisture equilibrium curves and their history and provides useful information on the relationships among them and their validity.

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Moisture equilibrium in transformer paper-oil systems

An effort was made to determine whether or not static electrification production mechanisms could be present in the external oil circulating system of a power transformer. Several techniques were utilized. These were monitoring the leakage current from insulated pipe sections and conducting probes introduced into the flowing oil. The value of the charge contained in the oil was determined using a recently developed absolute charge sensor. Acoustic emission detection techniques were also used to detect any partial discharges resulting from significant charge separation buildup in any one location. Although doubts exist about the quality of some of the data obtained, sufficient evidence was collected to show that, under certain conditions, the oil circulating pump could be a significant contributor to static electrification levels in operating power transformers. >

S.R. Lindgren

Papers

Moisture equilibrium in transformer paper-oil systems

Static electrification effects in transformer oil circulating pumps