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Recently, Polarization Depolarization current (PDC) measurement is widely accepted time domain spectroscopy-based method for assessing the insulation condition. Various performance parameters like Dissipation factor (%tan $\delta $ ), Paper Moisture (%pm), Dielectric Adsorption Ratio (DAR), Polarization index (PI) etc. can be estimated by analyzing the PDC data. During field measurement various factors influences the recorded PDC data. As per existing literature, presence of low frequency noise, effect of temperature variation and influence of residual charge are common during field measurement. These factors significantly affect recorded polarization current and hence estimated performance parameters. Hence, analysis using recorded polarization current data may provide misleading information regarding insulation condition. Under such practical situation where polarization current is affected by above mentioned factors that generally observed during field measurement, depolarization current should be used for analysis of insulation condition. The depolarization current does not influence by such external factors. The present work shows the importance of depolarization current where polarization current is influenced by external low frequency noise and residual charge. The analysis firstly applied on sample prepared in the laboratory and then on data collected from real life in-situ transformers. The results obtained from the analysis shows that the data obtained from depolarization current is more reliable.

This paper presents an advanced technique for calculating activation energy of oil-impregnated pressboard through dielectric response current measurement under switching impulse voltage at different temperatures. The proposed technique analyzes the measured impulse voltage and current signals to formulate its equivalent circuit at different temperatures. The equivalent circuit parameters are utilized to determine the shifting of ac conductivity curve along frequency axis with variations in temperatures. Based on the shifted ac conductivity curves at different temperatures, the corresponding activation energy of the insulation is estimated. The effectiveness of the proposed technique is investigated through a few laboratory prepared test samples. [ABSTRACT FROM AUTHOR]

Polarisation and depolarisation current (PDC) measurement and analysis is one of the popular tools for effective diagnosis of power transformer insulation. Normally, it is assumed that polarisation current is the combination of the current due to dipole movement and conduction current. Similarly, the depolarisation current is only due to the relaxation of dipoles. However, it is found that after eliminating the effect of dc conduction from polarisation current the resulting current is not similar to that of measured depolarisation current. This shows some non‐linearity is present in the system. This non‐linearity occurs due to movement of trapped charge that resides in the interfacial region of oil–paper insulation. This study shows the effect of de‐trapping charge on various performance parameters that are used for insulation diagnosis like paper moisture and dielectric dissipation factor (tanδ). [ABSTRACT FROM AUTHOR]

This paper presents a novel method for estimating the degradation of solid insulation in power transformers, considering thermal ageing and paper moisture dynamics. Current ageing models are based on both, experimental evidence and theoretical developments; considering that all models are approximation to reality, loss of life estimation could be found in a large range depending on the evidence considered; this amplitude could lead inaccurate results to make adequate decisions in an asset-management context. These differences in results can be explained because the models consider only nominal operative temperatures ranges overlooking low and high temperatures degradation process and the influence of variations in paper moisture content. Considering the above, this document proposes a holistic methodology for solid insulation ageing assessing based on all thermal degradation process (oxidation, hydrolysis and pyrolysis) and the influence of dynamics on paper moisture. Paper moisture is estimated using as input external variables such as: load, ambient temperature, transformer technical data and measurements regarding oil moisture, in order to consider uncertain in oil moisture growing Arithmetic-Brownian-Motion algorithms are presented. The proposed methodology was tested for four power transformers, for which load and ambient temperature hourly profiles are available over a period of almost nine years. In order to compare different degradation rates, three alternatives to model the chemical environment in which cellulose is aged, are analysed. Results are presented, and conclusions are finally detailed. [ABSTRACT FROM AUTHOR]

This paper presents a novel technique to determine the optimized slope of triangular excitation in frequency domain spectroscopy that can provide better information about condition of oil-paper insulation. In conventional frequency domain spectroscopy, estimation of frictional losses due to the interactions of oscillating dipoles helps in investigating insulation condition. But, different types of dipoles in insulation cannot have similar dielectric relaxation times. Therefore, excitation having multiple frequencies is necessary for the dipoles to undergo more synchronized oscillations. This synchronized oscillation of dipoles reduces the inter-dipolar interactions and provides better information regarding insulation condition. Hence, in this work, triangular excitation is used for frequency domain spectroscopy since its harmonic contents can be varied by changing the slope. The slope of the triangular excitation is optimized to obtain more accurate information about the insulation condition. The optimized triangular excitation for insulation is determined using the proposed equivalent circuit model. To experimentally determine the effectiveness of the model, three test samples having preset moisture content in paper have been prepared in the laboratory. The optimized triangular excitations are then applied to the test samples to estimate their insulation condition. Experimental results show that the use of optimized triangular excitation provides information regarding the condition of oil-paper insulation with greater accuracy. In order to investigate the applicability of the proposed equivalent circuit model, the experiment has also been performed on real-life transformers. [ABSTRACT FROM PUBLISHER]

Frequency Domain Spectroscopy is a popular non-invasive method that is widely used for condition assessment of oil-paper insulation in real-life power transformers. Conventional Frequency Domain Spectroscopy is done using sinusoidal excitation over a wide frequency range. In this work, a different approach has been taken to investigate some more possibilities of Frequency Domain Spectroscopy using non-sinusoidal excitation for a better condition assessment of oil-paper insulation. For that, triangular waveshape was chosen as non-sinusoidal excitations whose slope can be changed to vary the harmonic content and amplitudes of constituent components. Samples with preset moisture contents that emulate the oil-paper insulation of real-life transformer have been prepared in the laboratory for the assessment of insulation condition. For a comparative study, both sinusoidal and triangular excitations having same peak as well as time period are applied to the prepared samples under controlled environmental condition and corresponding dielectric response currents are measured. From the applied voltages and corresponding dielectric response currents, dielectric dissipation factors are calculated and compared. It has been observed that use of nonsinusoidal excitation provides better results than sinusoidal excitation in the estimation of paper moisture content. It has also been observed that among the applied non-sinusoidal excitations, a particular waveform provides more accurate information about the paper moisture content. [ABSTRACT FROM AUTHOR]

Polarization-Depolarization Current (PDC) analysis is a well known technique for condition monitoring of oil-paper insulation in power transformers. However, due to variation in insulation geometry, information obtained through analysis of PDC measured for one transformer cannot be used for predicting condition of another transformer even if the loading history of the two units are similar. Furthermore, parts of solid insulation closer to winding are exposed to much higher temperature than the parts away from it. Prolonged exposure to this temperature variation leads to nonuniform aging in cellulosic parts. It is reported that Modified Debye Model (MDM) is capable of modeling this non-uniform aging. In the present work it is shown that a parameter sensitive to insulation condition can be obtained from Transfer Function of MDM. Several laboratory samples having different physical dimensions has been constructed. PDC data recorded from these samples are used to obtain the performance parameter (which is less influenced by insulation geometry) and its relation to paper moisture content. The results obtained from the proposed method have been compared with that obtained using Frequency Domain Spectroscopy (FDS) based commercial instrument (IDAX 300) in the case of real-life power transformer. [ABSTRACT FROM AUTHOR]

The need for economic, reliable, and effective delivery of electric power has lead to the search for fast, efficient, and effective methods for diagnosing the insulation of high-voltage (HV) equipment in the power industries. The recent dielectric techniques that have been carefully considered by major industries for transformer insulation condition assessment are the recovery voltage method (RVM) and the polarization and depolarization current (PDC) measurement. However, due to the complexity of the transformer insulation structure and various degradation mechanisms under multiple stresses, insulation condition assessment is usually performed by experts with special knowledge and experience. In this paper, an expert system (ES) is developed, which imitates the performance of a human expert, to make the complicated insulation condition assessment procedure accessible to plant maintenance engineers. The structure of the ES is described in detail including knowledge base, inference engine, and human-computer interface. Examples of the application of the ES are also presented to confirm that the system can provide accurate insulation diagnosis. [ABSTRACT FROM AUTHOR]