of the dielectric test results are therefore only possible with a clear understanding of the physical behaviour of the insulation system in response to the ambient ...
2009 IEEE Electrical Insulation Conference, Montreal, QC, Canada, 31 May - 3 June 2009
Low Temperature and Moisture Effects on Oil-Paper Insulation Dielectric Response in Frequency Domain I. Fofana1,2, H. Hemmatjou1,2, F. Meghnefi1,2, M. Farzaneh2, A. Setayeshmehr3, H. Borsi3 and E. Gockenbach3
1
Canada Research Chair on Insulating Liquids and Mixed Dielectrics for Electrotechnology (ISOLIME), University of Quebec at Chicoutimi, Qc, Canada 2 International Research Centre on Atmospheric Icing and Power Network Engineering (CenGivre), University of Quebec at Chicoutimi, Qc, Canada 3 Institute of Electric Power Systems, Division of High Voltage Engineering, Schering- Institute; University of Hanover, Germany
Abstract- Results of Frequency Domain Spectroscopy measurements are known to be largely influenced by environmental conditions, such as the temperature. From fields and laboratory investigations this technique was found to be highly operating conditions (external and internal factors) dependant. Because field measurements, (generally performed after de-energizing the equipment), 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 behaviour of the insulation system in response to the ambient conditions. The purpose of this contribution is to report the effects of low temperature on the dielectric response of oil impregnated paper insulation in frequency domain. Moisture content inside the solid insulation acted as parameter.
I. INTRODUCTION Nowadays, a large number of power transformers around the world are approaching towards the end of their design life. Replacing them with new ones - only because of their age - is clearly uneconomic, since some of these transformers are still in good condition and could be used for many more years. For these reasons, transformer life management gained an ever increasing interest over the past decade, due to both economic and technical reasons [1-7]. Because the lifetime of a transformer is directly related to the quality of the insulation, condition monitoring of the insulation of transformers appeared to be an important issue. Indeed, condition monitoring can be utilized to attempt the prediction of the insulation condition and the remaining lifetime of a transformer. In this context, the adequacy of existing and the application of new diagnostic tools and monitoring techniques gain increasing importance [3-7]. Increasing requirements for appropriate tools to diagnose power systems insulation non-destructively and reliably in the field drive the development of diagnostic tools like Time domain measurement based on Polarization/Depolarization Current measurements and Frequency Domain Spectroscopic measurements, over the last decades [4-7]. This is facilitated
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significantly by the availability of modern computer controlled instrumentation. Frequency Domain Spectroscopy (FDS) measurement techniques provide indication of the general ageing status and moisture content of the oil-paper insulation of transformer. However, the results of these tests are severely influenced by several environmental factors, predominantly the temperature [8]. This temperature effect is more prominent in outdoor substations where the external environmental conditions are hardly predictable and controllable. In cold regions of the world, the annual average temperature can be as low as 0°C (even lower in some regions) with only few summer weeks. Maintenance engineers perform measurements under low outdoor temperatures. For accurate interpretation of the measurement results in such situations, it is essential to understand the variations of FDS measurement results under low temperatures. This contribution reports laboratory tests results on oil impregnated paper condenser bushing model with controlled variations of temperature with moisture content in the paper as a parameter. II. BACKGROUND ON FREQUENCY DOMAIN SPECTROSCOPY This technique allows studying the slow polarization processes by measuring current due to a sinusoidal excitation U(ω). It offers an alternative method to study of the dielectric response in the frequency domain. An analytical transition from time to frequency domain can be made using the Laplace- or Fourier transform by rewriting polarization equations [6]. Since single frequency component is considered at a time, resultant current can be written as follows: ⎡ ⎤ ⎢ ⎛σ ⎞⎥ I (ω ) = jωCo ⎢ε ∞ + χ ' (ω ) − j ⎜⎜ o + χ " (ω )⎟⎟⎥U (ω ) ⎢ ⎝ ε oω ⎥ ⎠⎥ ⎢ ε '(ω ) ε "(ω ) ⎣ ⎦ (1) = jω [C ' (ω ) − jC ' ' (ω )]U (ω ) = jωC (ω )U (ω ) where: Co is the geometrical capacitance of the test object, computed to 20.136 nF.
mbar, 48 hours at 105 °C) before impregnation. Then, impregnation with degassed and dried commercial grade mineral oil (moisture content < 5 ppm) was performed. In order to access low temperatures effect on FDS measurements, an adiabatic climate chamber with ± 0.1°C accuracy was used to keep the temperature at -10, 5 or 20°C.
Û(ω) is the applied voltage. C’(w) and C”(w) are real and imaginary components of the complex capacitance Ĉ( ω). χ( ω) =χ’( ω) - jχ”(ω) is the Fourier transform of the dielectric response function f(t) and defined as the complex dielectric susceptibility. Given that ε( ω) = ε’(ω) - jε’’( ω), the loss factor tan δ in frequency domain can be defined as follows [6, 9]:
IV. MEASUREMENT RESULTS
σo + χ " (ω ) ε " (ω ) ε oω tan δ (ω ) = = (2) ε ' (ω ) ε ∞ + χ ' (ω ) Both quantities C and tanδ depend on frequency. As aging
IDA 200 was used to evaluate frequency scan of insulation material properties in a large frequency range, starting from 1 mHz to 1 kHz. The frequency scan of the Capacitance C is represented in Fig. 2.
effects will change these quantities in quite different and specific frequency ranges, new diagnostic tools will monitor and detect this effect. Frequency Domain Spectroscopy (FDS) method has been implemented in the Insulation Diagnostic Analyzer “IDA 200” [10]. This instrument allows the frequency scan, of the capacitance, power factor, dielectric constants and dielectric losses over essential frequency ranges, that is from 0.1 mHz to 1 kHz (typically 1 mHz to 1 kHz). It should be emphasized that all dielectric quantities are more or less temperature dependent. Any comparison or measurement of these quantities must take this into account. III.
EXPERIMENTAL SETUP
A laboratory oil paper condenser model (Fig. 1) has been designed to study the effect of low temperature on the frequency domain spectroscopic measurement results.
Fig. 2. Effect of paper moisture content and temperature on the frequency scan of capacitance.
Terminals Box containing paper samples for Karl Fisher titrations OIP condenser
Fig. 1. Overview of the OIP condenser Model.
The object model was constructed by wrapping a conductor with cellulose paper and aluminium foils. Cellulose paper used was a Diamond Pattern Paper (DPP), manufactured by Weidmann [11] having a thickness dlayer = 0.125 mm and a dielectric strength VB,layer = 8.5 kV, performed according to ASTM D-202, Section 143. The moisture content of the cellulose paper, when delivered, was measured to 6% using Karl Fisher coulometer. The oil paper condenser model was carefully dried under vacuum (
