Frequency-domain Spectroscopy Research Articles

Maximizing the Reliability and Precision of Measures of Prefrontal Cortical Oxygenation Using Frequency-Domain Near-Infrared Spectroscopy.

Frequency-domain near-infrared spectroscopy (FD-NIRS) has been used for non-invasive assessment of cortical oxygenation since the late 1990s. However, there is limited research demonstrating clinical validity and general reproducibility. To address this limitation, recording duration for adequate validity and within- and between-day reproducibility of prefrontal cortical oxygenation was evaluated. To assess validity, a reverse analysis of 10-min-long measurements (n = 52) at different recording durations (1-10-min) was quantified via coefficients of variation and Bland-Altman plots. To assess within- and between-day within-subject reproducibility, participants (n = 15) completed 2-min measurements twice a day (morning/afternoon) for five consecutive days. While 1-min recordings demonstrated sufficient validity for the assessment of oxygen saturation (StO2) and total hemoglobin concentration (THb), recordings ≥4 min revealed greater clinical utility for oxy- (HbO) and deoxyhemoglobin (HHb) concentration. Females had lower StO2, THb, HbO, and HHb values than males, but variability was approximately equal between sexes. Intraclass correlation coefficients ranged from 0.50-0.96. The minimal detectable change for StO2 was 1.15% (95% CI: 0.336-1.96%) and 3.12 µM for THb (95% CI: 0.915-5.33 µM) for females and 2.75% (95%CI: 0.807-4.70%) for StO2 and 5.51 µM (95%CI: 1.62-9.42 µM) for THb in males. Overall, FD-NIRS demonstrated good levels of between-day reliability. These findings support the application of FD-NIRS in field-based settings and indicate a recording duration of 1 min allows for valid measures; however, data recordings of ≥4 min are recommended when feasible.

Microscopic mechanisms analysis of various dielectric response processes in oil‐paper insulation with different insulating states

AbstractVarious dielectric response processes in the oil‐paper insulation are sensitively affected by the insulating states (ageing degrees and moisture contents). However, the existing research is still incomplete in revealing the microscopic mechanisms of various dielectric response processes in oil‐paper insulation with different insulating states. Given this issue, the genetic algorithm is first adopted to extract the Dissado–Hill (D–H) model parameters by simulating the frequency domain spectroscopy (FDS) of oil‐paper insulation. Then, the change laws of the extracted D–H model parameters are adopted to reveal the microscopic mechanisms of various dielectric response processes. Microscopic mechanisms of four dielectric response processes are studied, which are quasi‐dc relaxation, loss peak relaxation, optical frequency relaxation, and DC conductance. Meanwhile, due to the dielectric response processes being dominated by various polar particles (methanol, ethanol, furfural, and water molecule), the contents of various ageing by‐products dissolved in the insulating oil are measured to support the above analysis. In this respect, a dielectric theoretical reference for the FDS technique to research the insulating states of oil‐paper insulation is provided.

A Highly Frequency-Selective 3D-Printed Dielectric Structure for the Terahertz Range

AbstractIn this paper, we present a terahertz transmission frequency-selective quasi surface (FSQS) that exhibits strong absorption lines and a periodic band-pass characteristic. The FSQS structure is created by laterally combining Fabry-Pérot resonators with different thicknesses. The transfer function of the FSQS can serve as a broadband reference for testing the signal integrity of the transmission path for broadband terahertz systems. The transfer function achieves a combination of band-pass characteristics and sharp resonances with a theoretical attenuation of over 80 dB and with quality factors of more than 40,000 for a combination of 36 resonators. A single FSQS made up of four resonators is 3D printed by fused deposition modeling using a low-loss cyclic olefin copolymer (COC) filament. Finally, the 3D-printed FSQS is characterized using both frequency-domain and time-domain terahertz spectroscopy. The results show an attenuation of over 42 dB and a quality factor above 100.

The influence of voxelotor on cerebral blood flow and oxygen extraction in pediatric sickle cell disease

AbstractVoxelotor is an inhibitor of sickle hemoglobin polymerization that is used to treat sickle cell disease. Although voxelotor has been shown to improve anemia, the clinical benefit on the brain remains to be determined. This study quantified the cerebral hemodynamic effects of voxelotor in children with sickle cell anemia (SCA) using noninvasive diffuse optical spectroscopies. Specifically, frequency-domain near-infrared spectroscopy combined with diffuse correlation spectroscopy were used to noninvasively assess regional oxygen extraction fraction (OEF), cerebral blood volume, and an index of cerebral blood flow (CBFi). Estimates of CBFi were first validated against arterial spin–labeled magnetic resonance imaging (ASL-MRI) in 8 children with SCA aged 8 to 18 years. CBFi was significantly positively correlated with ASL-MRI–measured blood flow (R2 = 0.651; P = .015). Next, a single-center, open-label pilot study was completed in 8 children with SCA aged 4 to 17 years on voxelotor, monitored before treatment initiation and at 4, 8, and 12 weeks (NCT05018728). By 4 weeks, both OEF and CBFi significantly decreased, and these decreases persisted to 12 weeks (both P < .05). Decreases in CBFi were significantly correlated with increases in blood hemoglobin (Hb) concentration (P = .025), whereas the correlation between decreases in OEF and increases in Hb trended toward significance (P = .12). Given that previous work has shown that oxygen extraction and blood flow are elevated in pediatric SCA compared with controls, these results suggest that voxelotor may reduce cerebral hemodynamic impairments.

Standing Wave Pattern Suppression in the Terahertz Frequency Domain Spectroscopy by Means of Windowed Fourier Filtering

Standing Wave Pattern Suppression in the Terahertz Frequency Domain Spectroscopy by Means of Windowed Fourier Filtering

A comprehensive evaluation method for moisture and aging state of oil-immersed paper bushings based on dielectric modulus and K-nearest neighbor algorithm

The insulation performance of oil-immersed paper bushings is prone to deteriorate, primarily due to moisture intrusion and thermal aging. The frequency domain spectroscopy (FDS) method is commonly employed to assess the insulation condition of the bushing. However, identifying and extracting relaxation polarization information from the low-frequency region of the FDS curve can be challenging, and there is little research about the condition evaluation under the combined effects of aging states and moisture content. To address this issue, this article uses the Taylor formula mathematical model to extract characteristic parameters from the dielectric modulus curve of OIP bushings and uses the KNN algorithm to achieve the evaluation of aging and moisture status. Then, the effectiveness and accuracy of the proposed method are validated on three field OIP bushings. The results demonstrate that the evaluation accuracy of the proposed method exceeds 83%, which has significant advantages compared to other classification algorithms. The innovation of this article lies in extracting new feature parameters and combining them with intelligent classification algorithms to evaluate the moisture and aging state of the bushing.

Condition Assessment of Thermally-Aged Polypropylene Cable Insulation Based on Frequency Domain Spectroscopy

Condition Assessment of Thermally-Aged Polypropylene Cable Insulation Based on Frequency Domain Spectroscopy

Two Models for Predicting Frequency-Domain Spectroscopy of Nonuniformly Damped OIP Bushings Based on FEM and Dielectric Theory

Two Models for Predicting Frequency-Domain Spectroscopy of Nonuniformly Damped OIP Bushings Based on FEM and Dielectric Theory

High resolution terahertz ATR frequency-domain spectroscopy for monitoring spinal cord injury in rats.

Traumatic spinal cord injury (SCI) can lead to permanent neurological impairment, underscoring the urgency of regular therapeutic intervention and monitoring. In this study, we propose a new strategy for monitoring spinal cord injury through serum based on high-resolution THz attenuated total reflection frequency domain spectroscopy (THz-ATR-FDS). We demonstrated serum spectral differences at different time points after experimental SCI in rats. We also studied the relationship between serum lipid concentration and the time of SCI, which revealed the potential of lipid molecules as biomarkers of SCI. In addition, based on the principal component analysis (PCA) and least squares regression (LSR) models, the quantitative relationship between the refractive index spectrum and lipid concentration in serum was automatically analyzed. This work highlights terahertz spectroscopy as a promising tool for label-free, periodic, and efficient monitoring of SCI.

Single-distance and dual-slope frequency-domain near-infrared spectroscopy to assess skeletal muscle hemodynamics.

Non-invasive optical measurements of deep tissue (e.g., muscle) need to take into account confounding contributions from baseline and dynamic optical properties of superficial tissue (adipose tissue). Discriminate superficial and deep tissue hemodynamics using data collected with frequency-domain (FD) near-infrared spectroscopy (NIRS) in a dual-slope (DS) configuration. Experimental data were collected in vivo on the forearm of three human subjects during a 3-min arterial occlusion or 1-min venous occlusion. Theoretical data were generated using diffusion theory for two-layered media with varying values of the reduced scattering coefficient () (range: 0.5 to ) and absorption coefficient () (range: ) of the two layers, and top layer thickness (range: 2 to 8mm). Data were analyzed using diffusion theory for a homogeneous semi-infinite medium. Experimental data in vivo were consistent with simulated data for a two-layered medium with a larger in the top layer, comparable absorption changes in the top and bottom layers during venous occlusion, and smaller absorption changes in the top vs. bottom layers during arterial occlusion. The dataset generated by DS FD-NIRS may allow for discrimination of superficial and deep absorption changes in two-layered media, thus lending itself to individual measurements of hemodynamics in adipose and muscle tissue.

Cumulative brain desaturation: Time to consider brain derived parameters to measure daytime sleepiness in obstructive sleep apnea

Objective/Background: The change in cerebral hemodynamics induced by sleep apneas and hypopneas may contribute to the daytime sleepiness in patients with obstructive sleep apnea (OSA). However, previous studies failed to discovery their relationship. We propose and test a new parameter, the cumulative brain oxygen desaturation, which may contribute to OSA patient's daytime sleepiness. Patients/methods22 patients with severe OSA (apnea-hypopnea index (AHI) at diagnosis [mean ± standard deviation, std.]: 52.1 ± 21.6/h, median: 45.1/h, interquartile range: 34.4–60.2/h) were monitored by polysomnography during routine continuous positive airway pressure titration. The reductions of brain tissue oxygen saturation (StO2) in all respiratory events at baseline sleep were measured by frequency-domain near-infrared spectroscopy (NIRS). The cumulative brain desaturation was calculated as AHI times the mean StO2 desaturation (i.e., AHI×ΔStO2‾). Similarly, cumulative peripheral desaturation was also calculated, i.e., AHI×ΔSpO2‾ where ΔSpO2‾ was the mean reduction of peripheral arterial oxygen saturation (SpO2). The correlations between Epworth sleepiness scale (ESS) and AHI, ΔStO2‾, AHI×ΔStO2‾, and AHI×ΔSpO2‾ were tested, respectively. Linear regression was applied to predict ESS using AHI×ΔStO2‾ and AHI×ΔSpO2‾, with age and BMI as covariates. ResultsESS significantly correlates to the cumulative brain desaturation (Pearson's correlation coefficient: 0.68, p = 0.00056), not the other parameters. Regression analysis only finds significant association between ESS and the cumulative cerebral desaturation (p = 0.00195) but not the cumulative peripheral desaturation (p = 0.71). ConclusionsThe cumulative brain oxygen desaturation, which comprehensively combines total sleep time, the frequency of apnea and hypopnea events, and the severity of cerebral oxygen desaturation, is a new indicator for daytime sleepiness in severe OSA.

Research on Changes in the Phase Shift Angle and Admittance of the Cellulose–Bio-Oil Composite under the Influence of Increasing Moisture during the Long-Term Operation of Power Transformers

In this study, the temperature–frequency dependencies of the pressboard–bio-oil–water nanoparticle composite’s fundamental parameters—phase shift angle and admittance—were examined using the Frequency Domain Spectroscopy method. Measurements were conducted in a frequency range of 10−4 Hz to 5 × 103 Hz at temperatures ranging from 293.15 K to 343.15 K, with a step of 10 K. The temperature stabilization accuracy was less than ±0.05 K. A total of 15 Arrhenius plots were determined for various phase shift angle values, from which 15 values and the average activation energy of relaxation time were determined. Similarly, the values of the activation energy of admittance relaxation time and the activation energy of admittance were determined. It was established that all three average values are identical within the bounds of uncertainty. Based on 45 values, a generalized activation energy was determined, with a value of ΔW ≈ (1.032 ± 0.0196) eV. Using the generalized activation energy value, the phase shift angle curves determined for all temperatures were recalculated to a temperature of 293.15 K. It was found that after conversion, all curves perfectly overlap. A similar operation was carried out for the frequency dependencies of admittance. In this case, too, the recalculated dependencies perfectly match. This means that the shape of the frequency dependencies of the phase shift angle and admittance depends solely on the moisture content in the pressboard–bio-oil–water nanodrop composite. The position of the curves in doubly logarithmic coordinates depends only on temperature and is determined by the generalized activation energy Using the generalized activation energy determined in this study will allow for the development of accurate methods for estimating moisture content in cellulose insulation of power transformers containing bio-oil. This will contribute to the detection of critical moisture content, which is about 5% by weight, and to the elimination of the risk, associated with such a high moisture content, of catastrophic failure of power transformers.

Cerebral Hemodynamics in Infants with Sickle Cell Disease Indicate Presence of Early Hemodynamic Stress

Introduction: One of the most severe complications of sickle cell disease (SCD) is ischemic brain injury, caused by a combination of decreased oxygen transport delivery related to intravascular vaso-occlusion of sickle erythrocytes and/or cerebral vasculopathy. While newborn screening programs diagnose SCD shortly after birth, routine risk assessment for stroke typically starts with transcranial ultrasound doppler (TCD) at two years of age or later. Yet, little is known about how SCD affects cerebral hemodynamics during infancy, a crucial period for brain development. To address this gap, we used advanced quantitative near-infrared spectroscopy (qNIRS) to non-invasively monitor cerebral tissue hemoglobin concentration (HbT), oxygenation (SO 2), oxygen extraction fraction (cFTOE), blood flow (CBF), and oxygen metabolism (CMRO 2) in infants with SCD. Abnormal CBF and cFTOE indicate hemodynamic stress that may increase stroke risk. Our study aims to investigate the effects of SCD on cerebral oxygen transport in infancy and its associations with age and blood hemoglobin concentration (HgB). Methods: This is a subgroup analysis of a prospective, observational neuroimaging study of SCD at Boston Children's Hospital (NCT 04166526). Inclusion criteria of this subgroup are SCD patients age less than 24 months old at the time of enrollment who are not receiving chronic transfusions. Families read and signed informed consent before participating in research activities. Measurements were conducted with the patients' normally scheduled hematology appointments at Boston Children's Hospital. During each measurement session, bilateral frontal regions of the head were measured up to six times for 30 seconds each. qNIRS variables were compared with literature values obtained with the same optical technologies from a cohort of typically developed infants around 1 year old, equivalent to the median age in this study 1. Correlation analysis was used to analyze qNIRS variables with age and HgB from clinical lab tests. Results: To date, 8 (F/M: 4/4; HbSS/HbS/B +-thalassemia/HbSC: 6/1/1) patients were enrolled with a total of 16 measurements. The meanSD age at measurement was 11.5 5.23 months and the median (IQR) HgB was 9.4 (8.9-10) g/dL. Seven (44%) measurements were performed while patients were receiving treatment with hydroxyurea. There was no difference in HgB in the groups with and without hydroxyurea, though the treatment group was older (mean age: 15.4 vs 8.4 months, P=0.002). Group averages of qNIRS variables are summarized Table 1. Average cFTOE was 0.53 ± 0.07, significantly higher than the reference value of 0.32 (mean difference: 0.21, P&amp;lt;0.001), while the cohort's mean SO 2 of 45.78 ± 7.13% was significantly lower than the reference value of 68% (mean difference: -22.2, P&amp;lt;0.001). Furthermore, cFTOE inversely correlated with HgB (r= -0.63, P=0.04) while SO 2 increased with HgB (r= 0.63, P=0.04). HbT and cerebral blood volume (CBV) were also significantly lower than the reference values (mean difference of -24.8 μM and -0.75 ml/100g, P&amp;lt;0.001 and P&amp;lt;0.02, respectively) and positively correlated with age (r=0.51 and 0.57, P=0.04). CBF did not correlate with either age or HgB. Hemodynamic variables did not differ between groups with and without hydroxyurea, except for HbT and CBV likely secondary to the cohort age difference. Taken together, this suggests that CBF compensatory mechanisms were insufficient to prevent elevated oxygen extraction. Summary: These preliminary results suggest that inadequate oxygen transport to the brain secondary to SCD evokes a compensatory increase in cFTOE starting in early infancy. The progressive hemodynamic stress to brain health during the period of the brain's greatest growth and metabolic demand for oxygen is concerning because of the potential for injury as compensatory mechanisms become exhausted. Hydroxyurea did not reduce this stress, but may have prevented further declines. Recruitment is ongoing to increase the size of the cohort and to continue following patients' trajectories with age. We demonstrated the potential of qNIRS as biomarkers of cerebral hemodynamic compensation to determine when intervention is necessary and evaluate the effectiveness of a chosen intervention in individual patients. 1. Franceschini, M. A. et al. Assessment of infant brain development with frequency-domain near-infrared spectroscopy. Pediatr Res 61, 546-551, 2007.

Diffuse Optical Monitoring of Cerebral Hemodynamics and Oxygen Metabolism during and after Cardiopulmonary Bypass: Hematocrit Correction and Neurological Vulnerability.

Cardiopulmonary bypass (CPB) provides cerebral oxygenation and blood flow (CBF) during neonatal congenital heart surgery, but the impacts of CPB on brain oxygen supply and metabolic demands are generally unknown. To elucidate this physiology, we used diffuse correlation spectroscopy and frequency-domain diffuse optical spectroscopy to continuously measure CBF, oxygen extraction fraction (OEF), and oxygen metabolism (CMRO2) in 27 neonatal swine before, during, and up to 24 h after CPB. Concurrently, we sampled cerebral microdialysis biomarkers of metabolic distress (lactate-pyruvate ratio) and injury (glycerol). We applied a novel theoretical approach to correct for hematocrit variation during optical quantification of CBF in vivo. Without correction, a mean (95% CI) +53% (42, 63) increase in hematocrit resulted in a physiologically improbable +58% (27, 90) increase in CMRO2 relative to baseline at CPB initiation; following correction, CMRO2 did not differ from baseline at this timepoint. After CPB initiation, OEF increased but CBF and CMRO2 decreased with CPB time; these temporal trends persisted for 0-8 h following CPB and coincided with a 48% (7, 90) elevation of glycerol. The temporal trends and glycerol elevation resolved by 8-24 h. The hematocrit correction improved quantification of cerebral physiologic trends that precede and coincide with neurological injury following CPB.

Insulation aging condition assessment of transformer in the visual domain based on SE-CNN

The aging of transformer oil–paper insulation (OPI) contributes significantly to transformer failure and reduced service life. Therefore, reliable and precise evaluation of the aging status of transformer insulation condition is crucial in averting transformer failure. This article proposes a novel deep learning-based aging condition assessment technique of transformers by combining FDS (Frequency domain spectroscopy) and squeeze-and-excitation-enabled convolutional neural network (SE-CNN). An intrinsic product-based transformation using a Gramian angular field is employed to dimensionally expand the 1-D FDS signals acquired from OPI samples into 2D images. Each feature extraction channel in the SE-CNN-based aging diagnosis framework is assigned a specific weight, enabling the model to concentrate on the most relevant features while discarding the rest. Furthermore, the effect of moisture variation on the model is also considered under three different conditions to ensure the consequence of compensation for the effect of moisture on FDS. A comparison with traditional machine learning models has also been made to ensure the viability of the proposed methodology. Additionally, the model is interpreted by creating explanation weight plots of the feature coefficients using a local interpretable model-agnostic explanations (LIME) method in Python. The findings demonstrate that the novel aging diagnosis framework visually assesses the transformer’s OPI system with the highest accuracy of 99.12% in the test set. Furthermore, it facilitates the automatic extraction of deep aging features, thus providing better generalization and convergence capability.

Accurate Assessment of Moisture Content and Degree of Polymerization in Power Transformers via Dielectric Response Sensing.

Power transformers are essential apparatuses used to transfer electrical energy from one voltage-level circuit to another. For reliable systems, preventive maintenance of the transformers is required to ensure good services of all mechanical, electrical, and insulation parts. Oil-immersed paper is most often used for transformer insulation. To ensure such good insulation performance and for assessing insulation conditions, advanced transformer sensing, monitoring, and effective assessment techniques are required. This paper introduces an effective technique for assessing the insulation conditions in power transformers, which are crucial for ensuring reliable energy transfer. The method utilizes advanced transformer sensing and monitoring, focusing on oil-immersed paper insulation commonly used in transformers. The technique employs dielectric response sensing, obtained from frequency-domain spectroscopy tests, to estimate degrees of polymerization (DP) and percentages of moisture content (PMCs) in the oil-immersed paper insulation. These parameters are well-known indicators of insulation performance. The approach is based on the weighted k-nearest neighbor regression, using a database of dielectric loss factors at low frequency and oil conductivities. To overcome limited data availability, linear interpolation and extrapolation techniques are applied to enlarge the database. Experimental verification and comparison with a previously developed method demonstrate the proposed technique's superiority in accuracy and complexity. The maximum deviations of DP and PMC in the validation cases are 6.2% and 18.7%, respectively. In addition, to evaluate the validity of our proposed method in the case of a real power transformer, a comparative analysis of the DP and PMC values determined by the proposed method with those obtained through a previously developed and complicated approach was performed. The predicted results indicate that the DP and PMC values of the oil-immersed insulation fall within the ranges of 800 to 1000 and 1.5 to 2.0, respectively, which agree with the results determined by the complicated approach and closely align with real conditions. By offering a reliable and advanced means of assessing insulation conditions, this technique contributes to the preventive maintenance and overall efficiency of power transformers.

Thermal and Dielectric Characteristics of Aluminum Nitride/HTV Silicone Composites

The study describes the use of nano aluminum nitride in developing thermally conductive and electrically insulating materials. An experimental approach is followed, starting with the hydrophobic surface treatment of nanofillers using HMDS. TEM and XRD analysis are used to confirm the nanofiller presence and gain insight regarding the filler shape, dimensions, and phase. EDX, FTIR and XRD analysis revealed the co-existence of aluminum oxide in traces along with aluminum nitride. XRD patterns revealed no change after surface modification and confirmed the presence of an amorphous functionalized layer. The protocol for composite compounding is described in detail. SEM analysis of composites revealed a moderate degree of dispersion with localized pockets of agglomeration. Further, the TEM analysis performed gave insight into the dispersion of nanofillers. Finally, DSC analysis is performed to understand the effects of nano aluminum nitride on glass transition, melting point, and crystallization temperatures. An interesting pattern of increase in glass transition temperatures with nanofiller loading levels is discussed, drawing attention to the aggregated filler effect and the disentangling of silicone chains with increasing interfacial interaction. TGA analysis revealed an early onset of degradation with increase in nano aluminum nitride loading levels. Thermal conductivity increased with nanofiller loading levels, peaking at 3 phr by weight. Dielectric studies of breakdown strength and frequency domain spectroscopy from 1 mHz to 1 KHz is studied under different thermal points. The results are presented and discussed here.

Precise measurement of refractive index in the ambient environment using continuous-wave terahertz frequency-domain spectroscopy (THz-FDS)

In this paper, we present the application of THz frequency-domain spectroscopy (THz-FDS) for determining the refractive index in an ambient environment. The signal phase is extracted from the periodically oscillated photocurrent by a magnitude normalization method. The proposed method is demonstrated using experimental data of polytetrafluoroethylene and α-lactose monohydrate as reference materials collected at a relative humidity of around 17.0% ± 2.0%. A detailed comparison to the existing algorithms, including the Hilbert transform and extreme points analysis, reveals the remarkable reliability of our proposed method. This study expands the characterization capabilities of THz-FDS and furthers the development of practical terahertz spectroscopy applications.

Frequency domain spectroscopy analysis of superconducting cable during pre-cooling

A 1.2 km, 35 kV superconducting cable project has been built in Shanghai, China. In order to monitor the internal condition of the cable in real time, during the 15-day pre-cooling process, the Frequency Domain Spectroscopy (FDS) test at 0.1 Hz–1000 Hz was carried out, and the dielectric loss angle tangent (tanδ)-frequency curves at different temperatures were obtained. A special phenomenon is found: in the early stage of pre-cooling, tanδ is the largest at the lowest frequency and decreases with the increase of frequency; in the late stage of pre-cooling, it becomes that tanδ is the largest at the highest frequency and decreases with the decrease of frequency. According to the Dielectric Polarization Theory in the field of dielectric physics, it is believed that the above phenomenon occurs because the tanδ-frequency curve shifts to the low frequency direction with the decrease of temperature. The evaluation method of the pre-cooling process based on FDS is proposed. In the early stage of cooling, it is suitable to use high frequency tanδ to monitor, and in the late cooling stage, low frequency tanδ is suitable.

Health assessment of converter transformer pressboard insulation based on FDS and digital image processing

The stable operation of the converter transformer is an essential task for power system operation reliability and security. Nowadays, Frequency domain spectroscopy (FDS) technology is prominently utilized for assessing oil-impregnated pressboard insulation. The present study examines the effect of the pressboard insulation material as a function of frequency and elevated temperature. The experimental analysis on oil-impregnated pressboard insulation is carried out at temperatures from 30 °C to 130 °C with an incremental rise of 20 °C intervals with frequency variation from 1 Hz to 10 MHz. The frequency-dependent permittivity, conductivity and loss tangent angle studies also confirm the deterioration of oil-impregnated pressboard insulation. The surface morphological changes inside the pressboard insulation are recorded with the help of scanning electron microscopy (SEM). The synergistic effect generated on pressboard insulation is examined by fiber width changeand image processing approach by randomly selecting the average of three local areas of SEM image. A canny operator is selected to extract the exact boundaries of images and more change is recorded in the edge detection count after 90 °C.The porosity and pore size distribution can be increased with elevated temperatures. A single-phase, 315 MVA valve side star winding with 60 discs of single-phase converter transformers model is developed in MATLAB Simulink. An impulse of 100 kV, 1.2/50μsec is applied across the star winding to identify the pressboard insulation degradation derived from FDS data with the help of mathematical morphology and wavelet transform technique. The energy of the wavelet coefficient on the neutral current capture during the impulse test adds a significant contribution to analyzing pressboard insulation degradation. The results presented are in good agreement with the published work.