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Measurement and analysis of dielectric response in oil-paper insulation systems /

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ABSTRACT Diss. no. 12832 techn. sc. SFIT Zurich. Literaturverz.

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    ABSTRACT: This article deals with monitoring of electrical properties in natural ester fluids and method dielectric spectroscopy. The paper describes properties such as the dielectric permittivity ε r and dissipation factor tanδ of different types of oils. Dielectric spectroscopy has been developed towards a powerful tool for diagnosis of insulating systems and used two methods: the analysis of relaxation current (PDC, Polarisation and Depolarisation Current), analysis of the dissipation factor tanδ over a wide frequency range (FDS, Frequency Domain Spectroscopy). Test objects were measured in different temperature.
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    ABSTRACT: Transformer failures are often related to bushings with oil-impregnated paper (OIP) insulation. There-fore a strong need for reliable condition assessment exists, but problems are to be solved: (1) Traditional diagnosis consists of C/ tan δ measurements at the potential tap of the bushing, but relations between insulation conditions (e.g. moisture and ageing) and diagnostic quantities are uncer-tain. Tan δ at service temperatures is unknown (although possibly high and dangerous). A significant improvement is achieved by polarisation/ depolarisation current measurements (PDC measurements), even at ambient temperatures. A new evaluation method is proposed, the so called charge difference method CDM. It allows calculation of d.c. conductivities very quickly within some minutes. (2) Laboratory tests on wetted OIP samples show the dependence of dielectric properties (ex-pressed by PDC) on temperature, moisture and oil conductivity. D.c. conductivity in OIP is strongly related to moisture content, and -to a smaller extent -to oil conductivity and ageing. Ageing mainly influences short term polarisation currents because of increased oil conductivity. (3) Strongly aged 420 kV OIP bushings could be identified with PDC analysis at room tempera-ture, although there was no indication of defects from C/ tan δ values. Measurements at service tem-perature revealed the danger of thermal instabilities, partial discharges and breakdown. PDC analysis is the first known dielectric method for identification of significantly aged OIP at room temperature. (4) It is not always correct to relate measurements to the core of the bushing only. Signals can be influenced by parasitic currents with access to grading foils. Theory, measurements and simulations show that tan δ and PDC can (apparently) be increased, reduced or reversed in polarity. Influences are weak for aged and strong for new bushings. Measurements with conductive bandages give lower and upper limits and guarded signals. Therefore it is proposed to measure with "worst case" and "guard ring bandages" in order to make improved estimations of currents related to the insulation core. It is proposed to distinguish influences of ageing, moisture and environment by PDC analysis: Ageing can be detected from short term currents even at ambient temperature. OIP d.c. conductivity contains information both about moisture and ageing, it can be calculated by a new charge difference method CDM. External influences have to be considered by means of bandages on the bushing surface.
    01/2006;
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    ABSTRACT: This paper presents a study on the characterization of aging of transformer oil–pressboard insulation. Traditional and modern diagnostic tools and techniques were used to investigate aging of oil–paper insulation under controlled laboratory conditions. Insulation condition testing includes common chemical diagnostics such as acidity tests and new chemical techniques: determination of relative content of dissolved decay products by spectrophotometry, solid suspension by turbidimetry, Fourier Transform Infrared (FTIR) spectroscopy and electrical diagnostic techniques that have gained exceptional importance for utility professionals: polarisation/depolarisation current, recovery voltage measurement and frequency domain dielectric spectroscopy measurements. From the obtained results it is found that chemical tests depicted more sensitivity to insulation degradation. Also, different oil–pressboard insulation ratios were investigated to assess the influence of the relative amount of cellulosic materials on aged mineral oil decay products and vice versa. Emphasize is laid on the potentialities/avenues offered by FTIR to assess oil insulation condition. Notable experimental results have been reported, which could be valuable to researchers and practicing engineers in this field.
    European Transactions on Electrical Power 12/2010; 11(1):1110–1127. · 0.63 Impact Factor

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