Investigations of dielectric monitoring on an energised transformer oil–paper insulation system

Frequency domain spectroscopy (FDS) is being used to assess the insulation condition of oil–paper power transformers. However, it has to date only been implemented on de-energised transformers, which requires the transformers to be shut down for an extended period and may cause significant costs. To solve this issue, a newly improved monitoring method based on the FDS principle is proposed to implement the dielectric measurement on energised transformers. Moreover, a chirp waveform excitation and its novel processing method are introduced. Compared with the conventional FDS results, dielectric results from the energised insulation system have higher tanδ values because of the increased losses. To further understand the insulation ageing process, the effects of the geometric capacitance are removed from the measured imaginary admittance of the insulation system to enhance the ageing signature. The resulting imaginary admittance is then shown to correlate well with the central time constant in return voltage measurements results. The proposed methods address the issues on techniques used on energised transformers and provide a clue for on-line FDS diagnostic application.

Simulation study for analysis of online dielectric measurement problems on transformer insulation system

Determining the condition as well as the remaining life of an insulation system is essential for the reliable operation of large oil-filled power transformers. Frequency-domain spectroscopy (FDS) is one of the diagnostic techniques used to identify the dielectric status of a transformer. Currently, this technique can only be implemented on a de-energized transformer. This paper presents an initial investigation into a novel online monitoring method based on FDS dielectric measurements for transformers. The proposed technique specifically aims to address the real operational constraints of online testing. This is achieved by designing an online testing model extending the basic “extended Debye” linear dielectric model and taking unique noise issues only experienced during online measurements into account via simulations. Approaches to signal denoising and potential problems expected to be encountered during online measurements will also be discussed. Using fixed-frequency sinusoidal excitation waveforms will result in a long measurement times. The use of alternatives such as a chirp has been investigated using simulations. The results presented in the paper predict that reliable measurements should be possible during online testing.

Experimental investigations of improved dielectric condition monitoring methods on transformer oil-paper insulation system

Frequency Domain Spectroscopy (FDS) is successfully being used to assess the insulation condition of oil filled power transformers. However, it has to date only been implemented on de-energized transformers, which requires the transformers to be shut down for an extended period which can result in significant costs. To solve this issue, a method of implementing FDS under energized condition is proposed here. A chirp excitation waveform is used to replace the conventional sinusoidal waveform to reduce the measurement time in this method. Investigation of the dielectric response under the influence of a high voltage stress at power frequency is reported based on experimental results. To further understand the insulation ageing process, the geometric capacitance effect is removed to enhance the detection of the ageing signature. This enhancement enables the imaginary part of admittance to be used as a new indicator to assess the ageing status of the insulation.

Enhancements to dielectric response measurements of oil-paper insulation for online and offline applications

The research introduces a promising technique for monitoring the degradation status of oil-paper insulation systems of large power transformers in an online mode and innovative enhancements are also made on the existing offline measurements, which afford more direct understanding of the insulation degradation process. Further, these techniques benefit from a quick measurement owing to the chirp waveform signal application. The techniques are improved and developed on the basis of measuring the impedance response of insulation systems. The feasibility and validity of the techniques was supported by the extensive simulation works as well as experimental investigations.

Development of a switch mode assisted linear amplifier for use as a high fidelity voltage source

Frequency Domain Spectroscopy (FDS) is one of the major techniques used for determining the condition of the cellulose based paper and pressboard components in large oil/paper insulated power transformers. This technique typically makes use of a sinusoidal voltage source swept from 0.1 mHz to 1 kHz. The excitation test voltage source used must meet certain characteristics, such as high output voltage, high fidelity, low noise and low harmonic content. The amplifier used; in the test voltage source; must be able to drive highly capacitive loads. This paper proposes that a switch-mode assisted linear amplifier (SMALA) can be used in the test voltage source to meet these criteria. A three level SMALA prototype amplifier was built to experimentally demonstrate the effectiveness of this proposal. The developed SMALA prototype shows no discernable harmonic distortion in the output voltage waveform, or the need for output filters, and is therefore seen as a preferable option to pulse width modulated digital amplifiers. The lack of harmonic distortion and high frequency switching noise in the output voltage of this SMALA prototype demonstrates its feasibility for applications in FDS, particularly on highly capacitive test objects such as transformer insulation systems.

Atlas of frequency distribution, auto-correlation and cross-correlation of daily temperature and precipitation at stations in the U.S., 1948-1991 (in English units) /

"September 1993."

Temperature measurement of cold atoms using transient absorption of a resonant probe through an optical nanofibre

Optical nanofibres are ultrathin optical fibres with a waist diameter typically less than the wavelength of light being guided through them. Cold atoms can couple to the evanescent field of the nanofibre-guided modes and such systems are emerging as promising technologies for the development of atom-photon hybrid quantum devices. Atoms within the evanescent field region of an optical nanofibre can be probed by sending near or on-resonant light through the fibre; however, the probe light can detrimentally affect the properties of the atoms. In this paper, we report on the modification of the local temperature of laser-cooled 87Rb atoms in a magneto-optical trap centred around an optical nanofibre when near-resonant probe light propagates through it. A transient absorption technique has been used to measure the temperature of the affected atoms and temperature variations from 160 μk to 850 μk, for a probe power ranging from 0 to 50 nW, have been observed. This effect could have implications in relation to using optical nanofibres for probing and manipulating cold or ultracold atoms.

Observation of Size-Dependent Electron-Phonon Scattering and Temperature-Dependent Photoluminescence Quenching in Triangular-Shaped Silver Nanoparticles

Research studies on plasmonic properties of triangular-shaped silver nanoparticles might lead to several interesting applications. However, in this work, triangular-shaped silver nanoparticles have been synthesized by simple solvothermal technique and reported the effect of size on the electron-phonon scattering in the synthesized materials by analyzing their temperature-dependent photoluminescence (PL) emission characteristics. It has been observed that total integrated PL emission intensity is quenched by 33 % with the increase in temperature from 278 to 323 K. The observed decrease in PL emission intensity has been ascribed to the increase of electron-phonon scattering rate with the increase in temperature. The values of electron-phonon coupling strength (S) for synthesized samples have been evaluated by theoretical fitting of the experimentally obtained PL emission data. Smaller sized triangular nanoparticle has been found to exhibit stronger temperature dependence in PL emission, which strongly suggests that smaller sized triangular silver nanostructures have better electron-phonon coupling.

Single-photon-emitting diode at liquid nitrogen temperature

We report on the study of a single-photon-emitting diode at 77 K. The device is composed of InAs/GaAs quantum dots embedded in the i-region of a p-i-n diode structure. The high signal to noise ratio of the electroluminescence, as well as the small second order correlation function at zero-delay g((2))(0), implies that the device has a low multiphoton emission probability. By comparing the device performances under different excitation conditions, we have, in detail, discussed the basic parameters, such as signal to noise ratio and g((2))(0), and provided some useful information for the future application. (c) 2008 American Institute of Physics.

Evaluation of the Uncertainties Caused by the Forward Scattering in Turbidity Measurement of the Coagulation Rate

The forward scattering light (FSL) received by the detector can cause uncertainties in turbidity measurement of the coagulation rate of colloidal dispersion, and this effect becomes more significant for large particles. In this study, the effect of FSL is investigated on the basis of calculations using the T-matrix method, an exact technique for the computation of nonspherical scattering. The theoretical formulation and relevant numerical implementation for predicting the contribution of FSL in the turbidity measurement is presented. To quantitatively estimate the degree of the influence of FSL, an influence ratio comparing the contribution of FSL to the pure transmitted light in the turbidity measurement is introduced. The influence ratios evaluated under various parametric conditions and the relevant analyses provide a guideline for properly choosing particle size, measuring wavelength to minimize the effect of FSL in turbidity measurement of coagulation rate.

Sensitivity of Geraldton waxflower to ethylene-induced flower abscission is reduced at low temperature

Exposure to ethylene gas elicits flower abscission from cut stems of Geraldton waxflower (Chamelaucium uncinatum Schauer). Ethylene response rates in plants are mediated by temperature. At 20degreesC, flower abscission from waxflower 'Purple Pride' occurred upon 12 h exposure to I mu11(-1) ethylene. This ethylene treatment did not cause flower abscission at either 10 or 2degreesC. Moreover, flowers held at 2degreesC were insensitive to 48 h exposure to 1, 10 and 100 mu11(-1) ethylene. However, increasing the duration of treatment with I mu11(-1) ethylene at 10 and 2degreesC to 48 and 144 h, respectively, induced flower abscission. When flowers were held at 20degreesC in air without exogenous ethylene following continuous exposure to I mu11(-1) ethylene at 2degreesC, the duration required to elicit flower abscission was reduced from 144 to 72 It. Collectively, these responses show that maintaining harvested waxflower at low temperature (e.g. 2degreesC) is an effective means to minimise ethylene-mediated flower abscission.

Noninvasive functional optical spectroscopy of human breast tissue

Near infrared diffuse optical spectroscopy and diffuse optical imaging are promising methods that eventually may enhance or replace existing technologies for breast cancer screening and diagnosis. These techniques are based on highly sensitive, quantitative measurements of optical and functional contrast between healthy and diseased tissue. In this study, we examine whether changes in breast physiology caused by exogenous hormones, aging, and fluctuations during the menstrual cycle result in significant alterations in breast tissue optical contrast. A noninvasive quantitative diffuse optical spectroscopy technique, frequency-domain photon migration, was used. Measurements were performed on 14 volunteer subjects by using a hand-held probe. Intrinsic tissue absorption and reduced scattering parameters were calculated from frequency-domain photon migration data. Wavelength-dependent absorption (at 674, 803, 849, and 956 nm) was used to determine tissue concentration of oxyhemoglobin, deoxyhemoglobin, total hemoglobin, tissue hemoglobin oxygen saturation, and bulk water content. Results show significant and dramatic differences in optical properties between menopausal states. Average premenopausal intrinsic tissue absorption and reduced scattering values at each wavelength are 2.5- to 3-fold higher and 16–28% greater, respectively, than absorption and scattering for postmenopausal subjects. Absorption and scattering properties for women using hormone replacement therapy are intermediate between premenopausal and postmenopausal populations. Physiological properties show differences in mean total hemoglobin (7.0 μM, 11.8 μM, and 19.2 μM) and water concentration relative to pure water (10.9%, 15.3%, and 27.3%) for postmenopausal, hormone replacement therapy, and premenopausal subjects, respectively. Because of their unique, quantitative information content, diffuse optical methods may play an important role in breast diagnostics and improving our understanding of breast disease.

Measuring morphological features using light-scattering spectroscopy and Fourier-domain low-coherence interferometry.

We present measurements of morphological features in a thick turbid sample using light-scattering spectroscopy (LSS) and Fourier-domain low-coherence interferometry (fLCI) by processing with the dual-window (DW) method. A parallel frequency domain optical coherence tomography (OCT) system with a white-light source is used to image a two-layer phantom containing polystyrene beads of diameters 4.00 and 6.98 mum on the top and bottom layers, respectively. The DW method decomposes each OCT A-scan into a time-frequency distribution with simultaneously high spectral and spatial resolution. The spectral information from localized regions in the sample is used to determine scatterer structure. The results show that the two scatterer populations can be differentiated using LSS and fLCI.

Detection of Brillouin scattering temperature signal in Brillouin optical time-domain reflectometer sensing system based on instantaneous frequency measurement technology

We present a simple and practical method for the single-ended distributed fiber temperature measurements using microwave (11-GHz) coherent detection and the instantaneous frequency measurement (IFM) technique to detect spontaneous Brillouin backscattered signal in which a specially designed rf bandpass filter at 11 GHz is used as a frequency discriminator to transform frequency shift to intensity fluctuation. A Brillouin temperature signal can be obtained at 11 GHz over a sensing length of 10 km. The power sensitivity dependence on temperature induced by frequency shift is measured as 2.66%/K. (c) 2007 Society of Photo-Optical Instrumentation Engineers.

Detection of Brillouin scattering temperature signal in Brillouin optical time-domain reflectometer sensing system based on instantaneous frequency measurement technology

abstract {We present a simple and practical method for the single-ended distributed fiber temperature measurements using microwave (11-GHz) coherent detection and the instantaneous frequency measurement (IFM) technique to detect spontaneous Brillouin backscattered signal in which a specially designed rf bandpass filter at 11 GHz is used as a frequency discriminator to transform frequency shift to intensity fluctuation. A Brillouin temperature signal can be obtained at 11 GHz over a sensing length of 10 km. The power sensitivity dependence on temperature induced by frequency shift is measured as 2.66%/K. © 2007 Society of Photo-Optical Instrumentation Engineers.}