S. Usa

Bio: S. Usa is an academic researcher from Anna University. The author has contributed to research in topics: Overvoltage & Lightning arrester. The author has an hindex of 9, co-authored 25 publications receiving 281 citations. Previous affiliations of S. Usa include College of Engineering, Guindy.

Electrical Insulation Characteristics of Silicone and EPDM Polymeric Blends. I

Abstract: The typical parts of a polymeric insulator are core, metal end fittings and polymeric housing material. The housing is intended to protect the fibre glass rod from the environment and electrical surface discharges. Since the housing materials are made of organic polymeric material, its insulation characteristics need to be studied. Amongst the many different polymers available, this work focuses on silicone rubber and ethylene propylene diene monomer (EPDM). Blends of EPDM and silicone rubber are prepared in a two roll mixing mill. Dicumyl peroxide is used as vulcanizing agent. The blends consisting of various proportions of component polymers are prepared, compression moulded into sheets, and post cured. The blends are tested for their insulation characteristics as per IEC and ASTM standards. Volume and surface resistivity, dielectric strength, dielectric constant, tan delta, tracking resistance, arc resistance, comparative tracking index, tensile strength, and percentage elongation at break of the blends are studied and discussed. The test results show that the increasing proportion of silicone enhances the electrical insulation properties whereas increasing weight percentage of EPDM improves the mechanical strength of the blends.

Effect of Repeated Impulses on Transformer Insulation

Abstract: Power transformer plays a vital role in electrical power system. Many of the transformer failures reported are due to insulation failure in windings. The sources of failures are mainly due to impulse overvoltages of different waveshapes and also due to repeated application of impulses. In this paper an attempt has been made to analyze the degradation of transformer insulation for repeated applications of different impulse voltages. As oil impregnated paper (OIP) constitute major insulation of transformer winding, detailed experimental analyses are carried out for OIP insulations using standard test cell for impulse waves of varying front times from 0.064 μs to 1.4 μs and tail times from 6.0 μs to 67 μs. Exponential mathematical models for the voltage-number (V-N) characteristics are derived and used for predicting the number of impulses that OIP can withstand for any wavefront and wavetail. The accuracy of the models are checked with the experimental values.

Frequency domain correlation technique for PD location in transformer winding

Abstract: Partial discharge (PD) is one of the sources of insulation failures in Power transformers. Knowledge of the PD source locations are important for transformer maintenance and repair. Due to the complex structure of the transformer, it is very difficult to locate the PD accurately in transformer winding. Correlation methods are applied to locate the PD source across the winding. The PD across parts of the winding are simulated by applying voltage pulse across the sections. The winding response for a reference PD impulse across transformer sections are taken as base for correlation and correlated with winding response of PD impulse across any section with different PD pulse widths. In this paper the limitation of time domain correlation method in locating PD in transformer windings is found out. To overcome the limitation of time domain correlation method, frequency domain correlation with Gram Schmidt orthogonalization is proposed. Simulation and experiments are performed on layer, continuous disc and interleaved windings to prove the feasibility of the proposed method.

Modelling of metal oxide arrester for very fast transients

Abstract: At present metal oxide arresters find limited applications against very fast transient overvoltages (VFTOs). To enable these arresters to function effectively against VFTOs, it becomes necessary to study its design aspects and improve it. With this in view, modelling of arrester under VFTO has been performed. Experimental studies have been conducted to determine the behaviour of one 12 kV arrester assembly under VFTO. It is observed that there is a delay in the initial response of the arrester, which increases with steepness of the current surge because of stray capacitance effect. Based on this study, an innovative arrester model (198 kV) with distributed parameters for VFTO is proposed by incorporating the block and stray capacitance values, which are calculated using finite element method (FEM). The dynamic performance of arrester is successfully obtained using electromagnetic transient program (EMTP) for current surges of different front times. A dynamic hysteresis curve is constructed using proposed arrester model to verify the relative delays between the residual voltage and current surge. A few modifications in the arrester have been suggested to improve the dynamic performance and also for the new testing procedure of arrester under VFTO.

Prediction of impulse voltage-time characteristics of air and oil insulation for different wavefronts

Abstract: The insulation of all the power apparatus in the power system are tested with 1.2/50 ?s standard lightning impulse voltage and this waveshape plays an important role in the insulation coordination of the power system. But in practice all the equipment in the power system are exposed to impulses of varying front times from nano seconds to micro seconds. Hence, it is essential to obtain the voltage-time characteristics of impulses of varying front times. But it is impractical to estimate the insulation strength for all possible impulses. To overcome this difficulty a few impulses of different front times from nano seconds to microseconds are generated and their voltage-time characteristics are obtained for air and oil. From these voltage-time characteristics a generalized mathematical model is obtained by regression analysis to predict the impulse voltage-time characteristics of any wavefront and this mathematical model is experimentally validated.

Performance of coconut oil as an alternative transformer liquid insulation

Abstract: Transformer mineral oil has been replaced by alternative oils such as synthetic oil and natural esters due to their biodegradability and environmentally friendly nature. This paper presents performance of coconut oil as such an alternative. Generally, as the other alternative oils, coconut oil has high conductivity due to the presence of free fatty acids. In this study, three different types of coconut oil samples consisting of virgin, copra and RBD (refined, bleached and deodorized) were initially tested by frequency dielectric spectroscopy (FDS) measurements to see how the conductivity was improved by dehydration and neutralization. The FDS results were fitted by inverse power dependence and Cole-Cole models to estimate the conductivity and response functions. Afterwards, a set of new coconut oil samples extracted from copra were thermally aged at 120°C under sealed conditions and compared with that of mineral oil. The performances of oil samples were evaluated using the test results of breakdown voltage, acidity, interfacial tension and FDS measurements under different aging periods such as 2, 5 and 7 weeks. Another 4 sets of new coconut and mineral oil samples were subjected to simulated thermal faults and electrical faults which include aging for 12 hours at 160°C, one hour at 200°C, exposing to partial discharges for four hours and subjecting to 20 low energy breakdowns. The performance comparisons were done by FDS measurements and dissolved gas analysis. In parallel, field-aged coconut oil samples collected from a sealed distribution transformer with 11 years of service were also tested by FDS measurements. In general, coconut oil shows its suitability as an alternative to the mineral oils for transformers, despite limitations found in some of their physical properties. It was found that the FDS results were in good agreement with chemical test results and with the estimated conductivity values.

Optimal design of corona ring on HV composite insulator using PSO approach with dynamic population size

Abstract: This paper deals with the use of corona ring at the HV end fitting for improving the electric field and potential distributions and then for minimizing the corona discharges on 230 kV AC transmission line composite insulator. A single corona ring is installed at the energized end side. Three-dimensional finite element method (FEM) software is employed to compute the electric field. As the performance of high voltage insulator strings closely depends on designs and locations of corona ring, the effects of the corona ring radius, the ring tube radius and the ring vertical position are examined. The minimization of the electric field necessitates the optimization of corona ring. For this purpose, new nonlinear mathematical objective function linking the electric field strength to the corona ring structure parameters is established. The optimization problem is achieved by minimizing the objective function using a modified particles swarm optimization (PSO) algorithm with a dynamic population size. The algorithm adjusts the size of population for each iteration. Based on the average value and the best solution of the objective function, we propose a new mathematical model to update the population size. This algorithm enables the population size reduction leading to computing time decrease. According to the results, FEM-PSO hybridization technique could be very helpful in optimization of corona ring design.

Comparison of the erosion resistance of silicone rubber and EPDM composites filled with micro silica and ATH

Abstract: The paper presents the results of inclined plane tests on ATH and silica composites of Silicone rubber (SiR) and EPDM to evaluate the effect of filler type and loading on the tracking and erosion resistance of the composites formed. The effect of test voltage on the outcome of the incline plane was also examined. A laser ablation test was also used to evaluate the various composites. A critical test voltage of 4.5 kV was shown to cause failures in samples which emphasizes the importance of testing composites at voltages above this critical level. In comparing the two test methods, both the inclined plane and laser ablation test methods showed the same order for EPDM composites of ATH and silica while the laser method showed silica SiR composites to perform somewhat better than in the inclined plane test which showed similar results.