Showing papers by "Sivaji Chakravorti published in 2022"
TL;DR: In this article , a novel method for predicting frequency-domain spectroscopy (FDS) characteristics of oil-paper insulation at a temperature, which is different from the measurement temperature, through estimating activation energy.
Abstract: This article proposes a novel method for predicting frequency-domain spectroscopy (FDS) characteristics of oil–paper insulation at a temperature, which is different from the measurement temperature, through estimating activation energy. The predicted FDS characteristics curves at different temperatures show a very good agreement with each other when compared to the experimentally measured characteristics. From the FDS characteristics, two moisture-sensitive parameters are proposed for accurate estimation of moisture content of oil–paper insulation. The proposed technique is experimentally validated on several test samples with different moisture contents and also on a real-life transformer.
8 citations
TL;DR: In this article , the effect of thermal aging on Nomex-paper-based vacuum pressure impregnation (VPI) insulation is reported and a new parameter sensitive to the severity of temperature aging is introduced.
Abstract: The effect of thermal aging on Nomex-paper-based vacuum pressure impregnation (VPI) insulation is reported in this article. The frequency-dependent dielectric loss of Nomex-paper-based VPI dry insulation is affected after significant thermal aging. This variation in dielectric loss of the insulation maintains a good correlation with the duration of thermal aging. A new parameter sensitive to the severity of thermal aging is also introduced. To identify the parameter, the excitation voltage waveform and the corresponding insulation response at a given frequency are required. Determining the parameter does not require denoising of recorded waveforms, reducing the overall computational burden. Based on the newly introduced parameter, a cost-effective method is proposed to predict the dielectric loss and degree of polymerization (DP) in a thermally aged VPI-type dry insulation. The proposed method employs the intermediate frequency range and hence does not require time-consuming low-frequency measurements.
3 citations
TL;DR: In this article , the significance and influence of de-trapped charge polarity on polarization and depolarization currents are investigated on data collected from real-life in-service power transformers.
Abstract: Assessment of insulation condition by sensing and subsequent analysis of oil-impregnated paper response at periodic intervals is necessary for reliable operation of transformers. Sensing frequency domain insulation response requires the application of either a sinusoidal excitation voltage (over a wide range) or a custom excitation waveform. This not only requires a specially designed variable-frequency variable-amplitude excitation voltage source but also makes measurement practically problematic. Measurement of low-frequency data is time-consuming and the data tends to get affected by field noise. In comparison, a simpler and cheaper alternative is the analysis of time-domain insulation response. This involves the measurement of low amplitude polarization and depolarization current. A pico-ammeter or Electrometer is generally used for this purpose. The available interpretation scheme of time domain response sensed using Electrometer does not consider the de-trapping charge’s polarity. The result presented in the paper shows that such an assumption inadvertently leads to inaccurate diagnosis. This, in turn, generates doubt regarding the current sensing capability of the Electrometer involved. In the present work, a better interpretation scheme of the data sensed by the Electrometer is discussed. The significance and influence of de-trapped charge polarity on polarization and depolarization currents are investigated. The analysis is tested on data collected from real-life in-service power transformers.
2 citations
DOI•
TL;DR: In this article , a Lissajous pattern-based technique was employed to identify dissipation factor and % paper moisture in oil paper insulation, which is capable of identifying moisture within a few seconds.
Abstract: Cellulosic paper moisture is widely considered an important factor for assessing the condition of transformer insulation. An increase in paper moisture accelerates the aging of the cellulosic parts and reduces the lifespan of the transformers. The proposed paper presents a methodology that is capable of identifying moisture within a few seconds. The methodology is based on an analysis of insulation response measured at 1 Hz. The method employs a Lissajous pattern-based technique for identifying dissipation factor (tan $\delta $ ) and % paper moisture in oil paper insulation. Unlike available frequency domain spectroscopy (FDS)-based commercial equipment, the proposed method is observed to take significantly less time to operate without sacrificing accuracy.
1 citations
17 Dec 2022
TL;DR: In this paper , a multi-layered UHF PD sensor design compatible with mounting over the inspection hatch windows of the high-voltage power transformers is presented, which offers an ultra-wide bandwidth of 2572 MHz (95.26% of the UHF band) and an operating frequency range of 303 MHz to 3000 MHz.
Abstract: Monitoring of partial discharges (PD) activities within high-voltage power transformers are necessary for assessing insulation health conditions and early prediction of insulation failures. Hatch-mounted UHF PD sensors are used for non-contact monitoring of PD activities within power transformers on a continuous basis. The design of the UHF PD sensors is intended to capture electromagnetic waves emitted from the PD sources in the UHF frequency band (300–3000 MHz). Conventional hatch-mounted UHF PD sensors are mechanically robust but lag in sensitivity for their limited bandwidth. This paper presents a multi-layered UHF PD sensor design compatible with mounting over the inspection hatch windows of the high-voltage power transformers. The proposed design offers an ultra-wide bandwidth of 2572 MHz (95.26% of the UHF band) and an operating frequency range of 303 MHz to 3000 MHz.
DOI•
TL;DR: In this article , a non-invasive and effective approach to predict activation energy of oil-paper insulation using a dielectric response that is recorded for a very short span of time is reported.
Abstract: Activation energy represents the average rate of interaction between aging by-products and cellulose. Activation energy is a crucial parameter that can be used to identify the remaining life of insulation in high voltage (HV) equipment. Existing noninvasive methods take a significantly longer time to sense activation energy for given insulation. This is primarily due to the volume of data required for such analysis, which generally takes significant time to measure. This work reports a noninvasive and effective approach to predict activation energy of oil–paper insulation using a dielectric response that is recorded for a very short span of time. The proposed method requires polarization current data sensed for a few seconds (15–20 s) to operate. The initial decay rate (DR) of the sensed data is found to be sensitive to the activation energy. This feature of the initial DR is utilized to sense the value of activation energy within a short duration. The proposed technique utilizes the current sensor (present within an electrometer) more efficiently. This facilitates the measurement of a highly accurate polarization profile and ensures reliable activation energy estimation. The proposed methodology has been successfully applied to data collected from a few real-life transformers. Reported results show that the suggested method provides satisfactory results with good accuracy.
17 Dec 2022
TL;DR: In this article , a Lissajous pattern-based technique is used for condition assessment of oil-paper insulation and a methodology and preliminary research findings that can be used for achieving this temperature compensation are presented.
Abstract: Lissajous pattern-based technique can be effectively used for condition assessment of oil-paper insulation. However, such methods are reported for a specific measurement temperature. It is not always possible to record data at a pre-determined temperature, especially during field measurement. In such cases, temperature compensation needs to be made. The present paper reports a methodology and a few preliminary research findings that can be used for achieving this temperature compensation for Lissajous pattern-based condition assessment techniques.
17 Dec 2022
TL;DR: In this paper , the authors proposed a novel method to analyze the relaxation behavior of epoxy mica insulation under accelerated thermal ageing conditions using Havriliak-Negami (H-N) relaxation model.
Abstract: This paper proposes a novel method to analyze the relaxation behavior of epoxy mica insulation under accelerated thermal ageing conditions using Havriliak-Negami (H-N) relaxation model. The epoxy mica composite insulation was subjected to thermal ageing for 200 hours, 400 hours and 600 hours, respectively, at 150°C. Then, frequency domain spectroscopy (FDS) measurement was performed on the insulation samples from 0.1 mHz to 1 kHz to record the real part ε′(ω) and imaginary part ε″(ω) of complex permittivity ε*(ω), respectively. From the recorded FDS data, real part M′(ω) and imaginary part M″(ω) of complex dielectric modulus spectrum M*(ω) was computed to unmask the dielectric relaxation, which is suppressed due to conductivity effect at very low frequencies. Then M′(ω) and M″(ω) computed at each measurement frequency were plotted and their variation was modelled using H-N relaxation model. Investigations revealed that parameters of H-N model are sensitive to thermal ageing. Finally, two empirical relationships have been developed between the H-N model parameters with ageing duration to estimate the ageing state of the epoxy-mica insulation quantitatively.