On the frequency domain dielectric response of oil-paper insulation at low temperatures
ABSTRACT Results of Frequency Domain Spectroscopy measurements are known to be largely influenced by environmental conditions, such as the temperature. Because field measurements, last hours after de-energizing the transformer, the ambient temperature may affect the results. Especially in cold regions of the world, extreme care are required to interpret the results when performing tests at relatively low surrounding temperatures. A better understanding and analysis of the dielectric test results are therefore only possible with a clear understanding of the physical behavior of the insulation system in response to the ambient conditions. In this contribution, the dielectric behavior of a composite oil paper insulation system has been explained from the properties of Debye basic model. A series of experiments have been performed under controlled laboratory conditions with preset moisture content inside the insulation. The equivalent circuit parameters of a laboratory made oil paper condenser bushing model were obtained using a non-linear optimization procedure. Since the dielectric parameters values are geometry dependent, poles, calculated from resistances and capacitances, were used as they are independent of the geometry. It was shown that the poles can be regarded as parameters able to be used for insulation condition assessment.
Article: Review Of Modern Diagnostic Techniques For Assessing Insulation Condition In Aged Transformers[show abstract] [hide abstract]
ABSTRACT: 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 its 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. In recent times 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.
Article: Dielectric spectroscopic measurements on transformer oil-paper insulation under controlled laboratory conditions[show abstract] [hide abstract]
ABSTRACT: For reliable operation of power transformers, the condition of the insulation system is essential. This paper reports on a detailed study of the effect of ageing, temperature and moisture on frequency and time domain spectroscopic measurements carried out on oil-impregnated pressboard samples as well as on a distribution transformer under controlled laboratory conditions. Because field measurements are generally performed after de-energizing the transformer, extreme care is required in interpreting the results due to inherent temperature instabilities. To avoid large thermal variations that may affect the results, a customized adiabatic room was built around the transformer for measurements above the ambient. Capacitance ratio and direct current conductivity deduced from the spectroscopic measurements, helped to interpret the data. Because, low frequency measurements techniques are time consuming, alternative to a transfer of time domain data into frequency domain data was investigated.IEEE Transactions on Dielectrics and Electrical Insulation 09/2008; · 1.09 Impact Factor
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ABSTRACT: 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 validityIEEE Electrical Insulation Magazine 02/1999; 15(1):11-20. · 1.33 Impact Factor