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Principles Of Fluorescence Spectroscopy

Optical sensors for power transformer monitoring: A review

This paper reports the experimental investigation of the corona discharge activity of punga oil under the influence of a local magnetic field and different voltage profiles (AC and DC voltages) using fluorescent fiber sensor as well as UHF sensor technique. The corona inception voltage (CIV) of punga oil is higher under negative DC voltage followed by positive DC and AC voltage, with a marginal reduction in its CIV magnitude observed on the impact of external magnetic field. The dominant frequency of the UHF signal obtained under AC voltage shifted towards lower frequency (0.6 GHz) with the influence of magnetic flux density to about 85 mT. The rise time, pulse width and energy content of the fluorescent signal formed due to corona activity under AC voltage is found to vary under high magnetic fields confirming the inception results. The fluorescent signals formed due to corona discharge under both AC and DC voltages have its dominant frequency at 1 MHz with no shift observed in the presence of magnetic field. Also, there is no variation in the phase resolved partial discharge (PRPD) pattern observed due to corona discharge (with and without magnetic field) signal using both UHF sensor and the fluorescent sensor. The fluorescent fiber-based technique provides a better accuracy on detecting the corona discharges in punga oil at an early stage compared to conventional UHF sensor. The breakdown voltage of punga oil under different voltage profiles with and without the effect of magnetic field follows normal distribution. The dielectric dissipation factor and electrostatic charging tendency (ECT) of punga oil is observed to be higher than the limit set for insulating fluids towards power transformer operation.

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Investigation on Impact of Magnetic Field on the Corona Discharge Activity in Punga Oil Using Fluorescent Fiber and UHF Sensor Techniques

Phosphor materials with broad spectral range and an average emission lifetime (20 μs) have been achieved from carbon dots (CDs)-NaCl crystals. A one-pot synthesis pathway has been developed for CDs-NaCl crystals formation at room temperature. Precursor for CDs materials was screened at room temperature by oxidation methodology from different simple sugar molecules. CDs (size less than 10 nm) prepared from the fructose sugar exhibit most intense emission. Utilizing ripe banana peel (contains ∼27% of fructose) as a precursor for the carbon dot formation, white-light emission with a CIE index of (0.29, 0.34) has been achieved from the single source with CDs-NaCl crystals upon excitation at 430 nm. The crystals also function as Fabry-Perot (F-P) mode resonator for lasing, with a laser threshold value of 0.9 mW and a resonating Q-factor of 207. These results outline a new approach for realizing F-P lasing and white light emission from a non-toxic green source with a quick, facile and low-cost synthesis procedure.

Carbon Dot-NaCl Crystals for White-Light Generation and Fabry-Perot Lasing.

Raman spectroscopy, with its specific ability to generate a unique fingerprint-like spectrum of certain substances, has attracted much attention in diagnosing the ageing degree of oil–paper insulation. In this study, the feature extraction and ageing diagnosis methods of oil–paper insulation Raman spectroscopy data are further studied. Based on the non-linear analysis of Raman spectra of different ageing samples, kernel principal component analysis was applied to extract the spectral features, and the back-propagation neural network was used to build a diagnosis model with high diagnostic accuracy. The results show that Raman spectroscopy combined with kernel principal component analysis and the back-propagation neural network can diagnose the ageing state of oil–paper insulation, with a diagnostic accuracy of 91.43% (64/70). The proposed method provides an effective and feasible method for the ageing assessment of oil-immersed electrical equipment.

Raman spectrum feature extraction and diagnosis of oil–paper insulation ageing based on kernel principal component analysis

Thermal aging of natural ester oil shows drastic reduction in partial discharge inception voltage (PDIV) and a significant variation is observed only above a certain aging time, under AC, DC, high frequency AC voltages and with harmonic voltages with different total harmonic distortion. Weibull distribution studies on PDIV measurements indicate a reduction in scale parameter (
 α 
) with increase in thermal aging temperature. A characteristic reduction in breakdown voltage was observed with the thermally aged ester oil, under AC, DC and standard lightning impulse voltage. The breakdown voltage variation with aged ester oil follows normal distribution. Ultraviolet (UV) analysis of ester oil thermally aged at 160°C has revealed a regular shift of the derived absorbance parameter to longer wavelengths. The interfacial tension and turbidity exhibits an inverse relationship with the thermally aged ester oil. Gas chromatography/mass spectrometric analysis of the thermally aged ester oil predicted the formation of more carboxylic acids and ketones with aging duration. The steady-state fluorescence on thermally aged ester oil exhibits a shift in its emission profile, which is in tandem with the UV absorption spectroscopic analysis. Fluorescence analysis can be adopted as a real-time monitoring tool in transformers, to understand the condition of liquid insulation. The viscosity dependence on the wavelength of derivative absorption maxima follows a direct relationship with the thermally aged natural ester oil.

Investigation on thermally aged natural ester oil for real-time monitoring and analysis of transformer insulation

Fecal matter is considered to be one of the primary sources of water pollution. Understanding the aggregation behavior of the fecal pigments (FPs) could play a critical role in their detection and analysis. This work shows that in aqueous media, the fluorescence of FPs indicates the presence of multiple emitting species, which have been assigned to monomers, lower-order H-aggregates (dimers), and higher-order H-aggregates. Steady-state absorbance, fluorescence and time-resolved fluorescence decay studies conclude that the emission of FPs in aqueous medium indicates H-type of aggregation, even up to nanomolar and sub-nanomolar concentrations. Four sets of independent experiments involving the variation of (i) concentration of FPs, (ii) temperature, (iii) pH, and (iv) ethanol/water composition as solvent media suggest the presence of monomer (540 nm), dimer (516 nm), and higher-order aggregates (500 nm) of FPs in aqueous solutions. The dimeric FP species appear to be present in the entire concentration range of 1 pM to 1 μM. Fluorescence lifetimes of H-aggregates are relatively longer as compared to the corresponding monomers. Hydrogen bonding appears to play an important role in forming H-aggregates in the aqueous phase of FPs as observed in the IR spectra of the FPs in dichloromethane. Density functional theory (DFT) calculations using the B3LYP functional and the LANL2DZ basis set show the contributions of π-π stacking and hydrogen-bonding interactions toward the formation of H-aggregated dimer of FPs in aqueous media.

Swayam Prakash

Papers

Investigation on thermally aged natural ester oil for real-time monitoring and analysis of transformer insulation

Carbon Dot-NaCl Crystals for White-Light Generation and Fabry-Perot Lasing.

Stercobilin and Urobilin in Aqueous Media: Existence of Specific H-Aggregates and Nonspecific Higher Aggregates at Different Concentrations

Understanding the photophysics of stercobilin-Zn(II) and urobilin-Zn(II) complexes towards faecal pigment analysis.

The structure and stability of faecal pigment-Zinc(II) complexes