still remains the insulation system. Oil-paper insulation system in a power transformer degrades under the electrical, thermal, and environmental stresses even ...
2011 IEEE International Conference on Dielectric Liquids
Assessing Oil Paper Insulation Conditions by Poles computed from Frequency Domain Spectroscopy Y. Hadjadj, I. Fofana, F. Meghnefi and H. Ezzaidi Canada Research Chair on Insulating Liquids and Mixed Dielectrics for Electrotechnology (ISOLIME), Université du Québec à Chicoutimi, Québec, Canada Abstract—In this contribution, the dielectric dissipation factor of oil impregnated paper were monitored along with the degree of polymerization. A series of experiments have been performed under controlled laboratory conditions. Since the dielectric parameters values are geometry dependent, poles which are independent of the geometry, calculated from resistances and capacitances, were considered to evaluate the condition of oil impregnated paper insulation. A correlation has been observed between the degree of polymerisation of the paper insulation and the poles values outputted from the frequency response. From the investigations performed on new and aged oil paper samples, it was shown that poles can be regarded as insulation aging indicator.
I. INTRODUCTION Power transformers, which are often the most valuable asset in a substation or plant, are indispensable components for power generation plants, transmission systems and large industrial plants. Despite great progresses in power equipment design in recent years, the weak link in the chain still remains the insulation system. Oil-paper insulation system in a power transformer degrades under the electrical, thermal, and environmental stresses even under normal operating conditions. The concomitant physical and chemical deterioration processes will affect conduction and polarization processes. Any increase in temperature, will contribute to cellulosic chains scission and the decomposition of hydrocarbon bounds. These degradation processes will contribute to the increase in charge carriers and dipolar concentration inside the insulation system, thus affecting the conduction and polarization phenomena. 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 [1-3]. Over the last decades, increasing requirements for appropriate tools to diagnose power systems insulation nondestructively and reliably in the field drive the development of diagnostic tools such as time domain measurement based on Polarization/Depolarization Current or Recovery Voltage and Frequency Domain Spectroscopic measurements [1-4]. A better understanding and analysis of the dielectric test results are only possible with a clear understanding of the physical behaviour of the insulation system in response to
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moisture and temperature. A circuit model, based on the principles of linear dielectric response has been derived. Since the dielectric parameters values are geometry dependent, poles, calculated from resistances and capacitances, were used as they are independent of the geometry. A correlation has been developed between the physical condition of the insulation and the equivalent model parameters that enable a clear and transparent interpretation of the dielectric test results. The feasibility of using poles to assess insulation condition is discussed. II. BACKGROUND ON FREQUENCY DOMAIN SPECTROSCOPY The frequency response of the dielectric materials is being widely used as a diagnostic tool for insulation system [4, 5]. The monitoring of complex permittivity and dissipation factor of transformer insulation, as function of frequency provide inside information concerning the state of insulation within the components. The relative complex permittivity (εr) is a dimensionless quantity, which compares the complex permittivity of a material (ε) to the permittivity of the free space (εo = 8.854.10-12 F/m). It describes the interaction of a material with the electric field and consists of a real part εr′, which represents the storage, and an imaginary part εr′′, which represents the losses. The relative ″lossiness″ or Dielectric Dissipation Factor (DDF) of the material is the ratio of the energy lost to the energy stored and it represent the dissipation factor given by: I ε′′ (1) tan δ = loss = r I ch arg e ε′r
The DDF (also known as tan δ) is propriety of an electrical insulation system; low values of it are usually regarded as proof of good quality of the insulation. The progressive increase of the DDF is closely related to the chemical degradation which accompanies thermal ageing of the insulation system [1-5]. In practice, for the expression of DDF, it is necessary to take into account conduction losses [5]. The DDF in frequency domain can therefore be defined as follows [1, 2]: σo + χ" (ω) ε" (ω) ε o ω tan δ(ω) = = ε' (ω) ε ∞ + χ' (ω)
(2)
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 [6]. Frequency Domain Spectroscopy (FDS) method has been implemented in the Insulation Diagnostic Analyzer “IDA 200” [7]. 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).
Transformer insulation ageing by-products are mostly polar in nature and will affect DDF/conductivity as well as permittivity and the capacitance. Thus, knowledge about the DDF of the oil-impregnated pressboard and the solid insulation material can be used as an important basis for the assessment of the condition of the oil-paper insulation. The frequency scan of the DDF is represented in Figure 2. 100
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III. EXPERIMENTAL PROCEDURES
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0,1
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Fig. 2. Effect of ageing on the DDF of oil impregnated pressboard samples.
Below 0.1 Hz (Figure 3) the DDF values increase with aging. Low frequency measurements appear to be very helpful for accurately monitoring the condition of insulation. Therefore, at the lowest frequencies, different aging cases can be detected. This is agreement with investigations reported by other authors [2, 8, 9]. 100
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10 tan δ (115°C)
Moisture content of the cellulose paper (pressboard), when delivered, was measured at about 6% using Karl Fisher coulometer. Pressboard samples were carefully dried under vacuum (
