EFFECT OF NANO-SiO2 PARTICLES ON PARTIAL DISCHARGE SIGNAL CHARACTERISTICS OF FR3 TRANSFORMER OIL

Liquid insulation for high voltage transformer applications based on natural esters derived from abundantly available vegetable oils are becoming popular in recent times. Since these natural ester based oils have environmental advantages and superior thermal performance, electrical utilities are slowly replacing the conventional mineral oils with natural ester based vegetable oils. FR3 oil, which is a soya based natural ester oil with superior dielectric and thermal characteristics, is becoming popular as an alternate insulating medium for high voltage transformers. With recent developments in nanotechnology field, it is possible to enhance the dielectric performance characteristics of natural ester based oils, which is a major constraint for high voltage transformer applications. However few research reports are only available in the area of nanofluids based on natural esters for high voltage insulation applications. In depth analysis and collection of large data base of insulation performance of natural ester based vegetable oils is important to improve the confidence level over nano-fluids based on natural esters. Considering these facts, in the present work, partial discharge characteristics of  nano-SiO2 modified FR3 oil at different electrode configurations are investigated at different %wt filler concentrations. Important parameters such as partial discharge inception voltage, stable PD formation voltage, partial discharge amplitude at different voltage magnitude and PD signal frequency characteristics are evaluated. From the results, it is observed that the partial discharge performance of FR3 oil is significantly improved with the addition of nano-SiO2 filler. Since in recent times FR3 oil is commercially used in many transformers, these results will be useful for enhancing the dielectric strength of high voltage transformers

Transonic flow of a fluid with positive and negative nonlinearity through a nozzle

The one-dimensional transonic flow of an inviscid fluid, which at large values of the specific heats exhibits both positive (Γ>0) and negative (Γ<0) nonlinearity regions {Γ=(1/ρ)[∂(ρa)/∂ ρ]s} and which remains in a single phase, is studied. By assuming that Γ changes its sign in the small neighborhood of the throat of the nozzle where transonic flow exists and introducing a new scaling of the independent variables, an approximate first-order partial differential equation (PDE) with a nonconvex flux function is derived. It governs both the steady transonic flows and the upstream moving waves near sonic point. The existence of continuous and discontinuous steady transonic flows when the throat area is either a maximum or a minimum is shown. The existence of standing sonic discontinuities and rarefaction shocks in the transonic flow are noted for the first time. Unlike in the classical gas flows, there are two sonic points and continuous transonic flows are possible only through one of them. The numerical evolution of those transonic waves that have both positive and negative nonlinearity in the same pulse is studied and some comments are made on the local stability of the particular steady flows

Observations of copolar correlation coefficient through a bright band at vertical incidence

Includes bibliographical references (page 52).This paper discusses an application of polarimetric measurements at vertical incidence. In particular, the correlation coefficients between linear copolar components are examined, and measurements obtained with the NSSL's and NCAR's polarimetric radars are presented. The data are from two well-defined bright bands. A sharp decrease of the correlation coefficient, confined to a height interval of a few hundred meters, marks the arttom of the bright band