A single phase photovoltaic inverter control for grid connected system

This paper presents a control scheme for single phase grid connected photovoltaic (PV) system operating under both grid connected and isolated grid mode. The control techniques include voltage and current control of grid-tie PV inverter. During grid connected mode, grid controls the amplitude and frequency of the PV inverter output voltage, and the inverter operates in a current controlled mode. The current controller for grid connected mode fulfills two requirements – namely, (i) during light load condition the excess energy generated from the PV inverter is fed to the grid and (ii) during an overload condition or in case of unfavorable atmospheric conditions the load demand is met by both PV inverter and the grid. In order to synchronize the PV inverter with the grid a dual transport delay based phase locked loop (PLL) is used. On the other hand, during isolated grid operation the PV inverter operates in voltage-controlled mode to maintain a constant amplitude and frequency of the voltage across the load. For the optimum use of the PV module, a modified P&O based maximum power point tracking (MPPT) controller is used which enables the maximum power extraction under varying irradiation and temperature conditions. The validity of the proposed system is verified through simulation as well as hardware implementation.     

On infinitesimal transformations and limit cycles†

A necessary condition for the existence of the limit cycle(s) of a second-order autonomous system, which can be reduced to a first-order system invariant under an infinitesimal transformation, is discussed in this paper. A method for obtaining an expression which will always include any limit cycle of the system that may exist is also shown.     

Electrical breakdown in very thin Al2O3 films

Electrical breakdown in very thin A1₂O₃ films of thickness less than 100 Å and sandwiched between Al and Au metal electrodes has been studied experimentally. Recovery of high resistance of the oxide insulator film has been observed (after breakdown) on the application of either a reverse voltage or a high current pulse or just by resting the junction for a few hours after the removal of the applied external voltage. An explanation for the effect is suggested. The breakdown field is found to depend on the insulator thickness, the temperature and the nature of the electrode metals. On the basis of experimental results the mechanisms for electrical break down at room temperatures and above, in very thin Al₂O₃ films, can be attributed to electronic modified thermal breakdown.     

Realizing NiO nanocrystals from a simple chemical method

Nanocrystalline NiO has been prepared successfully by a simple chemical route using NiCl₂·6H₂O and NaOH aqueous solution at a temperature of 70°C. The prepared material has been characterized from XRD, SEM, and M–H characteristics. It has been found that NiO nanocrystals have been formed which shows a superparamagnetic/superantiferromagnetic behaviour.     

Relationship between Landsat MSS data and forest tree parameters

Abstract

Band ratios, indices and radiance in the four channels of the Multi-spectral Scanner (MSS) on the Landsat-4 satellite for October 1984 and March 1985 were correlated with mean tree parameters of teak plantations (age, mean tree height, mean tree diameter at breast height, mean canopy diameter, mean canopy volume). The Landsat MSS data for March (when teak trees are leafless) were more suitable than the Landsat MSS data for October for categorizing tree parameters.     

Varna-based optimization:a novel method for capacitated controller placement problem in SDN

Recently,software defined networking(SDN)is a promising paradigm shift that decouples the control plane from the data plane.It can centrally monitor and control the network through softwarization,i.e.,controller.Multiple controllers are a necessity of current SDN based WAN.Placing multiple controllers in an optimum way is known as controller placement problem(CPP).Earlier,solutions of CPP only concentrated on propagation latency but overlooked the capacity of controllers and the dynamic load on switches,which is a significant factor in real networks.In this paper,we develop a novel optimization algorithm named varna-based optimization(VBO)and use it to solve CPP.To the best of our knowledge,this is the first attempt to minimize the total average latency of SDN along with the implementation of TLBO and Jaya algorithms to solve CPP for all twelve possible scenarios.Our experimental results show that TLBO outperforms PSO,and VBO outperforms TLBO and Jaya algorithms in all scenarios for all topologies.     

A theoretical study of the asymmetric excitation of conducting flanges by a primary aperture antenna and the consequent radiation pattern†

The excitation of a conducting flange, asymmetric both in length of the flange elements and orientation with respect to the axis of the primary aperture antenna, is considered theoretically. Assuming the two edges of the flanges as two linear secondary sources excited by the primary antenna, the resultant radiation pattern is derived.     

A study of the effect of spacing of probes in two-element directional couplers†

An analysis of the interaction between the two coupling elements of a waveguide directional coupler is made. The observed fact that the required separation of the elements is less than one-quarter wavelength is theoretically explained.     

Sub-4-micron full-field optical coherence tomography on a budget

We evaluate the construction and performance of an ultra-low-cost full-field optical coherence tomography (FF-OCT) instrument, also known as an optical coherence microscope. Although the cost of construction of the instrument from off-the-shelf parts is at least ten times lower than those of commercial products, sub-4-micron axial and lateral resolutions are obtained, albeit at the cost of higher acquisition times. Standard test samples are imaged and the performance of the instrument is evaluated. The device is found to be useful in measuring length, dispersion and group refractive index as well. Suggestions for bettering performance are discussed.