Dielectric analysis of high voltage power transformers (original) (raw)
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On the frequency domain dielectric response of oil-paper insulation at low temperatures
IEEE Transactions on Dielectrics and Electrical Insulation, 2000
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.
2021
In this paper, the oil-paper insulation system of the power transformer is analyzed in the frequency domain. Different factors that influence the dielectric insulation of oilpaper, such as the temperature, moisture in cellulose, conductivity, and oil aging, are described. The analysis for the dielectric response is carried out by analyzing the dissipation factors and the complex capacitance through the Frequency Domain Spectroscopy (FDS) method. This tool analyzes the dielectric responses of the oil-paper insulation system, by measuring a series of dissipation factors at different frequencies, typically distributed over the range of 0.001 Hz to 1000 Hz. The result of the experiments shows that the temperatures, moisture in cellulose, conductivity, and oil aging have a significant influence, in the low-frequency range than in the high-frequency range.
IEEE Transactions on Dielectrics and Electrical Insulation, 2000
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.
Assessment of oil impregnated paper condition using the dielectric response function
The general methods used to assess the condition of electrical equipments insulation systems are based on determination of the main electrical properties of the insulating materials in their composition. In this paper, a study concerning the assessment of mineral oil impregnated paper condition using the dielectric response function is presented. In this way, the values of complex permittivity components and loss factor for oil impregnated paper samples subjected to accelerated thermal stresses were analytically determined. The obtained results are compared to the experimental results measured using a dielectric spectrometer.
A Comparative Test and Consequent Improvements on Dielectric Response Methods
This paper starts with a systematic benchmark test of the three commercialised dielectric diagnostic methods RVM (recovery voltage method), PDC (polarisation and depolarisation currents) and FDS (frequency domain spectroscopy), performed on a large transformer model. The water content as analysed by the specific software was compared under three parame-ters: insulation geometry, insulation temperature and oil conductivity. It was found, that for the RV method the interpretation scheme based on the central time constant is too simplistic. The other two methods PDC and FDS compensate much better for the mentioned parameters but nevertheless analyse a drier cellulose with increas-ing temperature and a wetter cellulose for increasing oil conductivity. Actually the moisture in cellulose re-mained constant at 1 % during all the tests. Starting from these weaknesses a more sophisticated software was developed. The new software called "Dirana" bases on a new data pool, measured at...
IEEE Transactions on Dielectrics and Electrical Insulation, 2016
Dielectric response measurements in either time or frequency domain are being assessed in modern times for condition monitoring of transformer oil-paper insulation. Proper interpretation of the dielectric test results requires understanding of the physical processes taking place in the insulation structure during such tests. It is thus necessary to establish proper mathematical and analytical formulations of the insulation behavior. Dielectric response functions of various forms based on the classical "single relaxation time" Debye model have been proposed by various researchers to model the experimental data. In complex dielectrics, it seems however, that the overall response be represented by a distribution of relaxation times (or frequencies), rather than a single "average" response characteristic. The present contribution provides in brief, the mathematical background for estimating the dielectric response functions used so far for modeling dielectric response measurement data. Function representing distribution of relaxation frequencies has also been estimated. Experimental results on laboratory test samples and field transformers have been presented to study and understand the effects of insulation status and operating conditions on test data.
In this contribution, dielectric measurements in the frequency domain spectroscopy (FDS) were undertaken on oil impregnated paper samples in order to assess, with accuracy, the state of the solid insulation used in power transformers. The dielectric dissipation factor (tanĪ“) and the angle (phase shift) of the impedance of new and aged oil impregnated paper samples were measured; the moisture content served as parameter. These investigations aim at improving our understanding onto the effects of aging and moisture on the quality of the solid insulation (paper and paperboard) used in power transformers. Such investigations might be helpful in the separation of these two effects when doing FDS measurements.
Effect of temperature, water content and aging on the dielectric response of oil-impregnated paper
2008 IEEE International Conference on Dielectric Liquids, 2008
In this contribution, Fourier transform of dielectric responses from Polarization and Depolarization Current measurements were investigated to evaluate temperature and aging effects on dielectric response of oil paper insulation system. Measurements of the dielectric response were performed at three temperatures and two aging conditions. To provide quantitative analysis from measurements, the current curves were decomposed into basic exponential functions. Attempts have been performed to correlate the physical condition of the insulation and the equivalent model parameters. The obtained results indicate the feasibility of using the relaxation time constants and coefficients to depict the temperature, water content and aging effects.
The use of water activity probes immersed in the vegetable oil dielectric of a transformer requires a fresh look at the equations, and assumptions, behind the conversion of water activity to concentration of dissolved water. This is important because water negatively affects the effectiveness of insulation. A large volume of work has been performed on mineral oil based dielectrics to determine mathematical relationships. However, there is a lack of data on whether these relationships hold for vegetable oil. One example is the appropriateness of using the Henry's law coefficient of proportionality to calculate the concentration of water dissolved in the fluid from its water activity. This paper presents research investigating the water activity-concentration relationship for one vegetable oil dielectric. It was found that, at room temperature under the prescribed conditions, using Henry's law is appropriate.