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Principles Of Fluorescence Spectroscopy

This article discusses three aspects of inner filter effect (IFE) in fluorescence spectroscopy. (i) First, IFE as undesirable in fluorescence measurements: IFE results in non-linear fluorescence response of the analyte under study and it has been verified that IFE cannot be eliminated; it can either be minimized or corrected for intensity loss. Over the years, researchers have proposed many intensity correction methods to avoid IFE related issues. Often analysts using fluorescence spectroscopy, knowingly or unknowingly, ignore IFE or use an inappropriate intensity correction method. Herein, we have highlighted the basis and significances of various correction models that are proposed since 1970s to till date. (ii) Second, IFE mediated concentration dependent red shift (CDRS) as an analytical tool: the conventional fluorescence measurements and IFE correction strategies cannot be applied in analysis of optically dense multi-fluorophoric samples like oils, petrochemicals, biological samples and food samples etc. The strategy for exclusive inclusion of IFE and IFE induced CDRS as characteristics of the system for development of fluorescence based assay, towards maximizing fluorescence sensitivity of optically dense multi-fluorophoric systems, have been discussed. (iii) Third, IFE based sensing: when the sample contains chromophores, which absorb either at the excitation or at the emission wavelength range of the fluorophore, then the chromophores act as a filter. Thus tuning either the absorber or fluorophore concentration will lead to development of fluorescence based assay for a selective analyte. Principles and protocols are described to identify whether a sensing event is due to IFE or any other fluorescence mechanism. Additionally, a brief description is given on advanced findings and progresses made in sensing of for various classes of analytes in the recent past, using IFE concept. The second and third aspect combined together serve as a tool towards enhancing sensitivity of fluorescence measurement.

Inner filter effect in fluorescence spectroscopy: As a problem and as a solution

The dielectric modulus Μ∗(ω) is used for studying the frequency domain response and activation energy (E a ) of transformer immersed cellulose insulation. Existing literatures show that the temperature is a crucial factor affecting the Μ∗(ω) and aging rate of transformer oil-paper insulation, while the E a can be regarded as an indicator to reflect the average aging rate of the transformer immersed cellulose insulation. Therefore, the discussion of the temperature-dependence on both Μ∗(ω) and E a is of great significance. This paper attempts to report an approach for understanding the temperature impact on both Μ∗(ω) and E a . The obtained knowledge can be used to establish two available models for temperature correction on Μ∗(ω) and activation energy prediction of transform immersed cellulose insulation, respectively. These findings are expected to promote the reliability of the condition prediction result of transformer immersed cellulose insulation by using Μ∗(ω) and E a technique.

Temperature correction to dielectric modulus and activation energy prediction of oil-immersed cellulose insulation

It is widely accepted that the service life of transformer oil–paper insulation is determined by the aging condition of transformer cellulosic insulation. Therefore, it is rather meaningful to accurately and reliably evaluate the aging condition of transformer cellulose insulation. Existing studies have shown that activation energy can eliminate the temperature effect on the frequency dielectric spectroscopy, however, the traditional activation energy might not be suitable for assessing the aging condition of transformer cellulosic insulation. The aim of the work is to assess the aging condition of cellulosic insulation by using average activation energy technique. The experiment process is firstly discussed. Then, the second-order dynamic equation formed by Arrhenius equation and Ekenstam equation is strictly investigated to accurately calculate the degree of polymerization value of transformer cellulosic insulation. The experimental results preliminarily show that the average activation energy can effectively assess aging condition of transformer cellulosic insulation.

Aging condition assessment of transformer oil-immersed cellulosic insulation based upon the average activation energy method

Moisture is one of the critical factors to determine the service life of transformers. The moisture inside the transformer oil-immersed insulation could be quantified with feature parameters. This article proposes and develops a genetic algorithm support vector machine (GA-SVM) model to carry out the moisture diagnosis. Present findings reveal that these feature parameters can be obtained by using frequency-domain spectroscopy. Therefore, a novel model for predicting the frequency-domain spectroscopy curves is first reported based on a small number of samples, which could be utilized to obtain the feature parameters database to develop GA-SVM. Then, the moisture diagnosis in the lab and field conditions is presented to verify its feasibility and accuracy. The novelty of this article is in an exploration of the reported model as an intelligent based moisture diagnosis tool for power transformers.

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Moisture Diagnosis of Transformer Oil-Immersed Insulation With Intelligent Technique and Frequency-Domain Spectroscopy

Cellulosic pressboard is a key insulation material in oil filled transformers. Surface discharge or partial discharge degrades the insulation material and reduces the life time of the transformer. This study is aimed at understanding the effect of thermal aging of pressboard on the variation of its surface discharge and chemical characteristics. Thermal degradation of pressboard was carried out at 180 °C for 500 h in presence of ester oil under simulated aging conditions. The variation of salient electrical characteristics like surface discharge inception voltage (SDIV), charge measurement and optical emission spectra of the discharges were assessed at different aging periods. A significant reduction in SDIV was observed due to thermal aging. The charge measurement indicated that total charge deposition on pressboard increased with aging duration. Optical emission spectroscopy yielded valuable information regarding the elements evolved during discharge activity. Chemical characterization of the virgin and aged pressboard samples was carried out using analytical pyrolysis coupled with gas chromatograph-mass spectrometric technique (Py-GC/MS). The production of anhydrosugars, furan derivatives and oxygenates from the pressboard increased with aging time, while the esters in the oil were degraded to carboxylic acids with aging. Owing to the weakening of the hydrogen bonding network in oil impregnated pressboard with thermal ageing, high yield of end-chain depropagation products like anhydrosugars was observed during pyrolysis. The weak sites in cellulosic pressboard are expected to act as charge deposition sites.

Thermal aging of cellulosic pressboard material and its surface discharge and chemical characterization

The molecular level changes in natural ester oil due to thermal aging are studied. Accelerated thermal aging of natural ester oil at 180°C is performed and the subsequent effect on oil matrix is studied using corona inception voltage (CIV) variation under AC and DC voltage. Various physico-chemical diagnostic techniques such as UV-Vis absorbance, FT-IR, fluorescence microscope, DLS, and TEM were used to study the thermally aged natural ester oil. Thermal aging leads to the reduction in corona inception voltage as observed in the present work. The CIV is much higher under DC voltage than AC voltage but its variation is not much under positive/Negative DC voltage. Spectrum analyzer measurement indicates that burst type discharges occur under negative DC voltages. Thermal aging induces degradation of natural ester oil, resulting in the formation of carbonaceous particles. As these carbonaceous particles are fluorescent in nature, a facile fluorescence based analysis is developed to understand and monitor its degradation. 3D excitation emission matrix (EEM) fluorescence data are modeled with parallel factor analysis (PARAFAC).

Understanding the physico-chemical properties of thermally aged natural ester oil adopting fluorescent technique

Ester oils are evolving as next-generation transformer insulants, and it is essential to understand their characteristic variation upon thermal ageing. Corona inception voltage reduces with thermally aged ester oil under AC and DC voltages. The UHF signal generated due to corona discharge activity has energy content in the range 0.4–4 GHz. The magnitude and energy content of the UHF signal formed due to corona activity increases with thermally aged oil. Ionic mobility studies are carried out to understand the impact of thermal ageing on polarization and depolarization current, ion concentration, and ionic radius formed in the oil. Thermal ageing of ester oil leads to an increase in the ionic mobility of oil samples along with an increment in the ionic concentration. The conductivity of oil is observed to increase with ageing, accompanied by a reduction in ionic radii. The effect of voltage stress shows enhanced conductivity for thermally aged oil samples. Dielectric response spectroscopy exhibits higher loss and permittivity values for thermally aged oils. The physicochemical analysis shows a gain in viscosity, moisture content and turbidity, and a marginal loss in thermal conductivity due to the formation of new molecular species. UV analysis exhibits a significant redshift in the oil sample aged at a higher temperature.

Investigation on thermal ageing impact on dielectric properties of natural ester oil

Ambience during thermal ageing of ester liquid has a high impact on corona inception voltage (CIV) under AC and DC voltages. The CIV is high with ester liquid thermally aged in nitrogen ambience and is the least with air aged. CIV is high under negative DC voltage followed by positive DC and AC voltages. CIV reduces with increase in harmonic voltage and the reduction is predominant with harmonic voltages with higher total harmonic distortions. Corona activity radiates ultra-high-frequency (UHF) signal with its dominant frequency at 1 GHz. Phase-resolved partial discharge analysis carried out using UHF signal indicates that corona discharges occur at the peak of the voltage waveform. The magnitude of discharges is less in thermally aged liquid but the number of discharges increases. Fourier transform infrared spectroscopy analysis indicates the formation of carbonaceous particles with thermal ageing. Thermal ageing of ester liquid causes to form new chromophoric moieties causing red shift of absorption band in ultraviolet-visible spectra. Fluorescence spectroscopy traces the characteristic variations due to thermal ageing, which can be adopted for condition monitoring.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-smt.2018.5079

Understanding the influence of ambience on thermal ageing of natural ester liquid

An attempt is made to understand the effect of thermal ageing on natural ester oil impregnated pressboard material. Surface discharge inception voltage of natural ester oil impregnated pressboard material is reduced drastically under AC, harmonic AC voltages and with DC voltages with high ripple content. Phase-resolved partial discharge studies show an increase in the number of discharges for natural ester oil impregnated pressboard (OIP) aged at higher temperature, under AC and harmonic AC voltages with different total harmonic distortions. Surface potential measurement study complies with results obtained by surface discharge inception voltage. It indicates the presence of high-density charge traps evolved as a function of ageing time and temperature. Laser-induced breakdown spectroscopy analysis is carried out to understand the characteristic variation with thermally aged OIP material. A decrease in plasma temperature is observed for sample aged at higher temperature. Differential scanning calorimetric studies indicate reduction in peak temperature with thermally aged specimens. Ageing observed to affect mechanical strength of the cellulosic material, as indicated by tensile test. Dielectric response spectroscopy study indicates that with an increase in thermal ageing, the permittivity, tan(δ), DC conductivity and relaxation time have increased. A reduction in activation energy of the aged material is observed, which indicates the status of the OIP material.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-smt.2019.0117

Soumya Thakur

Papers

Thermal aging of cellulosic pressboard material and its surface discharge and chemical characterization

Understanding the physico-chemical properties of thermally aged natural ester oil adopting fluorescent technique

Investigation on thermal ageing impact on dielectric properties of natural ester oil

Understanding the influence of ambience on thermal ageing of natural ester liquid

Investigation on Insulation Performance of Thermally Aged Natural Ester Oil Impregnated Pressboard