Showing papers by "Sivaji Chakravorti published in 2019"

Application of Cole–Cole model to transformer oil-paper insulation considering distributed dielectric relaxation

Abstract: Researchers have been exploring dielectric testing techniques both in time and frequency domain for insulation condition assessment of oil-paper insulated transformers. In a practical dielectric system, dipoles are found to behave according to a distribution of elementary Debye relaxation properties. Suitable distribution density functions have been proposed to characterise such many-body interaction processes. Cole–Cole diagrams can be one of the methods for studying the nature of frequency dependency of dielectric materials of complex structure. Cole–Cole plots are commonly used for characterising different materials such as dielectric mixtures, ionic liquids, cable insulating oil, polar liquids etc. The scope of its application for assessing transformer oil-paper insulation considering distributed relaxation process has not been explored yet. The present contribution discusses mathematical formulations used for transforming the experimentally obtained time domain dielectric response test data to distribution domain and further to frequency domain for obtaining the Cole–Cole plots. Findings about the influence of various operating conditions and insulation status on the Cole–Cole diagram have been reported in this contribution. Results of tests on field transformers are also presented. This paper attempts to employ the features of Cole–Cole diagrams as potential indicators for analysing condition of the oil-paper insulation considering distributed relaxation process.

Use of Interfacial Charge for Diagnosis and Activation Energy Prediction of Oil-Paper Insulation Used in Power Transformer

Abstract: Activation energy is popularly used for the estimation of remaining life of transformer insulation. It is defined as the average rate of all reactions that happen with cellulose. Existing literature shows that the activation energy of oil–paper insulation can be obtained from polarization depolarization current (PDC) and return voltage measurement (RVM) data that are measured at a specific temperature. It is practically difficult to ensure the same measurement temperature for both PDC and RVM data. On the other hand, PDC data and its analysis get influenced by de-trapping current. This de-trapping current is generated by ionic charge carriers that get freed from trap sites during PDC measurement process. Formation of these trap sites is related to physical, chemical reactions that happen at oil–paper interface. This paper proposes a methodology which uses de-trapped charge, dislodged from deep and shallow traps, to assess insulation condition and for the prediction of activation energy. Thus, eliminating the need of RVM data. The proposed method is tested using data collected from various real-life in-service transformers.

Detection of PD Activities in XLPE Cable Using UHF Antennas

Abstract: Underground cables are widely used in urban areas for electrical power transfer, and some of those cables are very old. These cable insulations are often subjected to partial discharges due to aging which, gradually degrades dielectric strength of the insulation and ultimately results in complete failure. Therefore, detection of partial discharge (PD) within the insulation of cross-linked polyethylene (XLPE) cable may help to get an early warning about the fault condition. This paper reports about initial results obtained from the measurement of PD activities within XLPE cable using standard electrical method and UHF method.

Leakage Current Monitoring of Suspension Insulator for Effective Determination of ESDD

Abstract: Leakage current contains significant characteristics of a polluted insulator. With the increase in pollution the magnitude of leakage current increases. Equivalent Salt Deposit Density (ESDD) is a popular parameter for the estimation of the severity of pollutants. However, the measurement of ESDD involves costly equipment and requires samples directly from the surface of suspension insulator. It is a tedious task to collect a sample from the surface of the suspension insulator under operation. In the present work, a methodology has been proposed to estimate the contamination level severity parameter, ESDD using Detrended Fluctuation Analysis (DFA) on recorded leakage current data. In the present work, NaCl, Kaolin, and water are used to emulate the artificial pollutant layer on an 11 kV suspension insulator disc. It is observed that the DFA parameter maintains a well-defined relationship with ESDD and conductivity.

Time-varying model for the effective diagnosis of oil-paper insulation used in power transformers

Abstract: The analysis of dielectric response function obtained from the power transformer is a well-accepted technique in insulation diagnosis. A convenient way of analysing time-domain dielectric response φ(t) is the formulation of simple resistance−capacitance-based circuits like the conventional Debye model (CDM) or the modified Debye model (MDM) that are capable of modelling φ(t). However, available techniques do not guarantee unique branch parameters of such circuits for a given φ(t). In fact, the branch parameters are known to depend on the curve-fitting procedure opted during model formulation. This makes accuracy of CDM and MDM parameter-based diagnosis techniques less reliable. Furthermore, it seems logical to analyse the time-domain dielectric response of oil-paper insulation using a model containing time-varying parameters. In this study, a methodology is proposed using which such an insulation model (containing time-varying parameters) is formulated using time-domain insulation response. Related analysis presented suggests that the proposed model is immune to problems associated with available insulation models (containing time-invariant parameters). The performance of the proposed model in indicating insulation condition is tested on data obtained from several real-life power transformers.

Estimation of paper conductivity from short duration polarisation–depolarisation current for diagnosis of power transformer

Abstract: The value of paper conductivity provides quantitative evaluation of transformer insulation health. However, proper identification of paper conductivity requires complete profile of polarisation–depolarisation current (PDC). PDC measurement being a time-consuming offline process generally takes several hours to complete. Furthermore, magnitude of PDC becomes very low at larger value of time, which makes it sensitive to changes in environmental conditions and field noise. Hence, accuracy of paper conductivity identification can be ensured by conducting multiple measurements. This in-turn prolongs shutdown time of equipment and become less advantageous to utilities. Here, a method is proposed which is capable of estimating paper conductivity using PDC data recorded for only 800 s. The proposed technique is tested on data collected from several real-life in-service transformers. In order to illustrate the accuracy of the proposed technique, paper conductivities (calculated from short duration PDC) were compared with those computed using PDC measured for 10,000 s.

Influence of temperature on interfacial charge of power transformer insulation

Abstract: One of the popular methods for insulation diagnosis is measurement and analysis of polarisation-depolarisation current (PDC). During normal operation, charges get confined at the interface of oil-paper insulation. A part of these accumulated charges get absorbed in depolarisation current and contribute to the overall PDC data. The process through which charges are released from their confinement is known as charge de-trapping, which is highly influenced by thermal energy content of the insulation and hence by measurement temperature. In the current work, an effort has been made to investigate the effect of measurement temperature on de-trapped charge. Two samples are prepared and analysed in the laboratory at different temperatures for this purpose. This is followed by analysis of data collected from several real-life power transformers. Related analysis presented here suggests that measurement temperature of the system plays an important role in determining the amount of de-trapped interfacial charge during PDC measurement.

A Non-Linear Model for Sensing Moisture Content in Transformers at Reduced Time

Abstract: Moisture content in transformer is a prime indicator of its insulation condition. The presence of moisture accelerates the aging of cellulosic paper insulation thereby reducing the dielectric as well as the mechanical strength of the insulation. In the conventional method for sensing moisture content in transformers, sinusoidal excitation is generally applied on the oil–paper insulation system for two cycles from 0.1 mHz to 1 kHz. However, the application of sinusoidal excitations at discrete measurement frequencies leads to long measurement time during low frequency measurement (from 0.1 mHz to 1 Hz), which is a practical problem. Besides, as the moisture content increases, the transformer insulation no longer behaves as a linear time-invariant system. Therefore, non-linear modeling of insulation is necessary for accurate moisture sensing in transformers. Considering these two facts, in this study a non-linear Wiener model of transformer insulation using sine sweep excitation is proposed for reliable sensing of moisture content at reduced measurement time. It was observed that the proposed non-linear Wiener model has better moisture-sensing capability, thereby providing more accurate information regarding the insulation condition in comparison with the linear model. Moreover, an appreciable reduction in the total measurement duration was observed, enabling transformer diagnosis to be conducted quickly compared to the conventional method.

A Method to Predict Degree of Polymerization Value of Oil-paper Insulation Using Interfacial Charge

Abstract: Analysis based on Degree of Polymerization (DP) value is assumed to provide reliable regarding insulation condition. However, due to the invasive nature of testing, DP value based-analysis is not feasible in practical cases as it requires a paper sample from the in-service unit. Here, an attempt is made to develop a methodology to predict DP value based on non-invasive testing of oil-paper insulation. In this paper, it is found that DP value maintains a well-known relation with interfacial charge (estimated using Polarization Depolarization Current data) as both these parameters are related to the cellulosic condition. The proposed method successfully applied on various laboratory prepared samples containing different moisture content.

Estimation of Performance Parameters Using Charge Freed from Deep Traps Located at Interfacial Region of Oil-Paper Insulation

Abstract: De-trapped charge dislodged from interfacial region of oil-paper insulation can be used as an effective insulation sensitive parameter. Over time, various physiochemical reactions takes place at interfacial region and consequently results in the formation of different trap sites (deep and shallow). Charges which resides at these traps sites are de- trapped after gaining sufficient energy. In present analysis, it is found that charge dislocated from deep traps maintains some specific type of relationship with different insulation sensitive parameters. Before using de-trapped (dislodge from deep traps) as an effective insulation sensitive parameter the effect of geometry must be reduced, as amount of de-trapped charge depends on the area of interfacial region which is not identical for all units. Result presented in this paper shows that use of geometric capacitance for normalization purposes significantly reduces the effects of insulation physical dimensions on de- trapped charge. The capability of deep charge (normalized using dc insulation resistance and geometrical capacitance) is also compared in the present work.

Substrate Temperature Optimization for Diamond Thin Film Synthesis using Hot-Filament Chemical Vapor Deposition

Abstract: Chemical vapor deposition is a versatile process for large area good quality thin film synthesis. Hot-filament Chemical Vapor Deposition (HFCVD) is one among the cost effective methods that can produce good quality highly faceted polycrystalline diamond thin films. The physical parameters of the filaments, which are used to provide the activation energy, have a significant role in determining the temperature distribution and the gas density inside the reaction chamber. In the present work, the Finite Volume Method (FVM) is used to study the effect of the filament parameters of different grid filament configurations in optimizing the substrate temperature and determining the optimized filament parameters.

Influence of Presence of Acids in Transformer Insulation: A study using Cole-Cole Plots

Abstract: Failure of paper insulation is the end of design life of a transformer. Researchers have already stated that paper itself is the storehouse of moisture. Different operating variations over the service life in a transformer initiate degradation and breaking down of cellulosic chain of the paper. Degradation not only weakens the dielectric strength of solid insulation electrically but also produces different carboxylic acids. Organic acids that are produced as by product play a major role in insulation degradation. Among the organic acids produced, low molecular weight acids are found to be absorbed by paper. Over the past few years, attention has been focused to look into the causes of the detrimental effects of insulation ageing, but no such serious efforts have been made to look for the detrimental effects and to understand the influence on insulation ageing due to acid formation. Hence, an attempt has been made to look into the degrading effects caused by the acids by analyzing the dielectric response data obtained in time domain. To understand the physical behavior going on inside the dielectric material due to the influence of acid, time domain data is converted to frequency domain. The frequency domain parameters obtained are used to construct the Cole-Cole diagram to draw conclusions about the condition of ageing status of transformer oil-paper insulation.