Showing papers by "Sivaji Chakravorti published in 2020"

Condition Assessment of Power Transformer Insulation Using Short-Duration Time-Domain Dielectric Spectroscopy Measurement Data

Abstract: Utilities prefer noninvasive methods for assessing the condition of power transformer insulation. Analysis of polarization–depolarization current (PDC) is one such popular method. One such analysis involves the estimation of trapped charge released from the interfacial region of oil–paper insulation. The literature shows that such charges can be reliably used for the diagnosis of transformer insulation. However, such analysis requires a complete profile of PDC. PDC measurement (an offline technique) takes a large amount of time (several hours) to complete. The magnitude of PDC data for a larger value of time is also sensitive to changes in environmental conditions and field noise as its magnitude is low. Hence, a reliable estimation of detrapped charge may require numerous PDC measurements. This situation is not convenient for utilities as it prolongs shut down time. In this article, a method has been proposed which is capable of estimating detrapping charge using PDC data measured for a short span of time. The proposed method is tested on data collected from several real-life in-service transformers.

Method for identifying ageing in epoxy-mica composite insulation used in rotational machines through modelling of dielectric relaxation

Abstract: Dielectric response analysis, performed in time domain and frequency domain is one of the interesting methods used for condition assessment of high voltage equipment. In this article, the degradation of epoxy-mica composite insulation which is widely used in rotating machines has been investigated through frequency domain spectroscopy measurements. Artificial ageing of the carefully prepared stator bar insulation sample was performed through simultaneous application of high temperature and electric field in a designated environmental chamber for specific durations. To study the effect of temperature, the measurements were performed at temperatures ranging from 20 to 60°C at different ageing status. Following experimental work, the measurement results were modelled using Havriliak–Negami model and ageing sensitive parameters were extracted. To study the effect of temperature, the measurements were performed at temperatures ranging from 20 to 60°C. The modelled results matched well with the experimental measurements and certain model parameters sensitive to the ageing condition were identified. The numerical relationships between the insulation status of the stator bar and the aforementioned parameters were identified. It was observed that the nature of the numerical relationship remains identical in the investigated temperature range, although the coefficients vary to some extent.

Detrapped Charge-Affected Depolarization-Current Estimation Using Short-Duration Dielectric Response for Diagnosis of Transformer Insulation

Abstract: Analysis of detrapped charge [evaluated from polarization–depolarization current (PDC)] is effective in carrying out insulation diagnosis. However, computation of detrapped charge requires PDC data for at least 1600 s (PDCs each measured for 800 s). Such a technique of obtaining detrapped charge from short-duration PDC data considers few assumptions (including modeling of detrapping current by a straight line), which might not always be true for real-life equipment. Furthermore, PDC data are susceptible to noise and other environmental factors. Hence, it may be practically difficult to allow a measurement time of 1600 s during field measurement. Therefore, it is advisable to reduce the measurement time as well as the volume of data required. In this article, a method has been proposed, which is capable of estimating detrapped charge using only polarization current measured for 600 s. In addition, some unique advantages offered by the proposed method are also discussed in this article. The proposed technique is tested on data collected from several real-life in-service transformers.

De-noising of time-domain spectroscopy data for reliable assessment of power transformer insulation

Abstract: Polarisation-depolarisation current (PDC) measurement and its analysis is a popular technique for assessing the condition of transformer insulation. Owing to the low magnitude of PDC, recording noise-free PDC data from in-situ power transformers is a challenge. Once the relaxation current data get affected by noise, it becomes difficult to formulate insulation model (as recorded data loses its characteristic shape). This further makes the data difficult to analyse and predict insulation condition. In this study, two de-noising techniques are discussed (one is based on Wavelet Transform while the other is based on Stockwell Transform) for eliminating low-frequency non-stationary noise from recorded PDC data. Comparison between these two techniques suggests de-noising using Stockwell Transform is advantageous over wavelet analysis. The proposed methodology is first tested on data recorded from the sample prepared in the laboratory and then on data measured from real-life in-service power transformer.

Estimation of de-trapped charge for diagnosis of transformer insulation using short-duration polarisation current employing detrended fluctuation analysis

Abstract: Researchers have shown that the value of charge carriers, de-trapped from the oil–paper interface of power transformer insulation, is useful in carrying out the diagnosis. However, the evaluation of the de-trapped charge requires the analysis of polarisation–depolarisation currents. Being an off-line time-consuming process, the measurement and analysis of polarisation and depolarisation current (PDC) data are not practically advantageous. The study presents a detrended fluctuation analysis-based technique to estimate the magnitude of normalised de-trapped charge using the polarisation current measured for a short duration. Using the proposed technique, the requirement of measuring the complete PDC data, for diagnosis purposes, can be eliminated. Further, the technique also eliminates the requirement of depolarisation current which in turn facilitates a reduction in equipment shutdown time. The applicability of the proposed technique is tested on the data obtained from several real-life power transformers.

Investigation related to performance parameter estimation of power transformer insulation using interfacial charge

Abstract: In the present paper, geometric capacitance is used as a normalization factor instead of DC insulation resistance. The results suggest geometric capacitance is more efficient in reducing geometry effect on the DC-trapped charge. The influence of normalizing deep charge using DC insulation resistance and geometric capacitance and their effect on estimating various insulation sensitive parameters are compared in this paper.

Effective analysis of time-domain dielectric response for reliable diagnosis of power transformer insulation using statistical parameter evaluated from time-varying model

Abstract: Various types of insulation models with time-invariant parameters are available in the literature. Depending on the aging sensitive performance parameters to be evaluated, different models need to be employed (e.g. XY model for oil and paper-conductivity, conventional Debye model (CDM) for paper-moisture and tanδ). While the XY model cannot be used for estimating paper-moisture directly, analysis based on CDM parameter becomes dependent on its branch parameters, which are non-unique. These factors lead to either incomplete or ambiguous insulation diagnosis. These problems are resolved using the proposed new insulation model containing unique time-varying branch parameters. Another major advantage of the proposed model is that it can be used to evaluate a host of performance parameters (like paper-conductivity, oil and paper-moisture, dielectric loss) thus giving a complete picture of the insulation concerned. The application of the proposed model is also tested on data collected from several real-life power transformers.

Assessing the condition of Nomex paper-based Insulation in Open Wound and VPI type Dry-type Transformer using Frequency Domain Spectroscopy Data

Abstract: Dry-type transformers are becoming increasingly popular in urban areas due to its non-flammable properties. Being air-cooled, such units require less installation space compared to their counterpart Oil Impregnated Paper (OIP) having the same power rating. Open Wound types of dry-type transformer insulation like Dip-Bake type or Vacuum Pressure Impregnation (VPI) are popular and are being used extensively for constructing distribution transformers. Nomex paper is usually considered as the main insulation for such dry-type transformer insulation. The present paper describes the effect of thermal aging of Dip -Bake type and VPI type dry-type transformer insulation samples constructed in the laboratory. At the end of thermal aging, Frequency Domain Spectroscopy (FDS) data for both the dry type construction is recorded and compared.