A Seventeen-level Inverter With a Single DC-link For Motor Drives

In the present paper, a novel topology for generating a 17-level inverter using three-level flying capacitor inverter and cascaded H-bridge modules with floating capacitors. The proposed circuit is analyzed and various aspects of it are presented in the paper. This circuit is experimentally verified and the results are shown. The stability of the capacitor balancing algorithm has been verified during sudden acceleration. This circuit has many pole voltage redundancies. This circuit has an advantage of balancing all the capacitor voltages instantaneously by switching through the redundancies. Another advantage of this topology is its ability to generate all the 17 pole voltages from a single DC link which enables back to back converter operation. Also, the proposed inverter can be operated at all load power factors and modulation indices. Another advantage is, if one of the H-bridges fail, the inverter can still be operated at full load with reduced number of levels.

Moisture Aided Degradation of Oil Impregnated Paper Insulation in Power Transformers

The presence of moisture in oil impregnated paper insulation (OIP) is detrimental to its long time performance. Until recently, it was thought insulation ageing was only a function of temperature and electrical stress. It has now been realized that moisture in all its forms causes rapid degradation of the electrical and mechanical properties with time. In this study, insulation paper samples were conditioned for desired level of moisture and were impregnated with premium quality transformer oil. The oil impregnated samples with 1 to 3 % moisture content were aged at 90 to 130 C. The indices for determining the extent of ageing considered in this work are degree of polymerization (DP), furan, carbon monoxide and carbon dioxide content. These quantities were monitored throughout the ageing experimental run. End-of-life (EOL) criterion used here is the reduction in the value of DP. Phenomenological models for estimating the service life of insulation are proposed and are validated against actual experimental data.

Effect of post-deposition annealing on composition and electrical properties of dc reactive magnetron sputtered Al2O3 thin films

We have investigated the effect of post- deposition annealing on the composition and electrical properties of alumina (Al2O3) thin films. Al2O3 were deposited on n-type Si < 100 >. substrates by dc reactive magnetron sputtering. The films were subjected to post- deposition annealing at 623, 823 and 1023 K in vacuum. X-ray photoelectron spectroscopy results revealed that the composition improved with post- deposition annealing, and the film annealed at 1023 K became stoichiometric with an O/Al atomic ratio of 1.49. Al/Al2O3/Si metal-oxide-semiconductor (MOS) structures were then fabricated, and a correlation between the dielectric constant epsilon(r) and interface charge density Q(i) with annealing conditions were studied. The dielectric constant of the Al2O3 thin films increased to 9.8 with post- deposition annealing matching the bulk value, whereas the oxide charge density decreased to 3.11 x 10(11) cm(-2.) Studies on current-voltage IV characteristics indicated ohmic and Schottky type of conduction at lower electric fields (<0.16 MV cm(-1)) and space charge limited conduction at higher electric fields.

Surface Degradation of Silicone Rubber Nanocomposites Due to DC Corona Discharge

Corona discharge is recognized as one of the mechanisms that can influence the surface hydrophobicity of Silicone Rubber (SR) because of the chemical changes that occur on its surface. In this study SR samples were exposed to positive and negative DC corona for 25 and 50 hours using a needle-plane electrode system. Hydrophobicity changes were monitored using a sessile drop contact angle measurement facility. The physical changes on the surface were studied using Scanning Electron Microscopy (SEM) and surface roughness measurements. The effect of positive dc corona was found to be different from that of negative dc corona. Significant surface degradation and loss of hydrophobicity was found in the case of negative dc corona exposed samples. Significant improvement in the above mentioned properties were obtained by adding small quantities of nSIL into the SR matrix.

DC-Bias-Superposition Characteristics of Ni0.4Zn0.2Mn0.4Fe2O4 Nanopowders Synthesized by Auto-Combustion

Ni0.4Zn0.2Mn0.4Fe2O4 nanopowders were prepared by auto-combustion method. The as-synthesized powders were characterized using X-ray diffraction (XRD) and thermo-gravimetric-differential thermal analysis (TG-DTA), and the powders were densified at different temperatures 400 degrees C, 500 degrees C, 600 degrees C and 700 degrees C/4 hrs using conventional sintering method. The sintered samples were characterized by XRD and transmission electron microscope (TEM). The bulk densities of the samples were increased with an increase of sintering temperature. The grain sizes of all the samples vary in between 18 nm to 30 nm. The hysteresis loops show high saturation magnetization and low coercivity, indicates that it is a soft material. The incremental permeability (permeability with magnetic field superposition) was influenced by both Delta M and H-c. A sample with higher initial permeability and favoured the attainment of a higher incremental permeability. The Q-factor was mainly determined by the sintered density and microstructure. To summarize, a uniform and dense microstructure with relatively small average grain size is favourable for obtaining better dc-bias-superposition characteristics, including permeability and Q-factor.

A critical review of the definition of FRA resonance frequency of transformers as per IEEE Std C57.149-2012

The explanation of resonance given in IEEE Std C57.149-2012 to define resonance during frequency response analysis (FRA) measurements on transformers implicitly uses the conditions prevalent during resonance in a series R-L-C circuit. This dependence is evident from the two assertions made in the definition, viz., resulting in zero net reactive impedance, and, accompanied by a zero value appearing in the phase angle of the frequency response function. These two conditions are satisfied (at resonance) only in a series R-L-C circuit and certainly not in a transformer, as has been assumed in the Standard. This can be proved by considering a ladder-network model. Circuit analysis of this ladder network reveals the origin of this fallacy and proves that, at resonance, neither is the ladder network purely resistive and nor is the phase angle (between input voltage and input current) always zero. Also, during FRA measurements, it is often seen that phase angle does not traverse the conventional cyclic path from +90 degrees to -90 degrees (or vice versa) at all resonant frequencies. This peculiar feature can also be explained using pole-zero maps. Simple derivations, simulations and experimental results on an actual winding are presented. In summary, authors believe that this study dispels existing misconceptions about definition of FRA resonance and provides material for its correction in IEEE Std C57.149-2012. (C) 2014 Elsevier B.V. All rights reserved.

Low-frequency dc bus ripple cancellation in single phase pulse-width modulation inverters

This study presents a topology for a single-phase pulse-width modulation (PWM) converter which achieves low-frequency ripple reduction in the dc bus even when there are grid frequency variations. A hybrid filter is introduced to absorb the low-frequency current ripple in the dc bus. The control strategy for the proposed filter does not require the measurement of the dc bus ripple current. The design criteria for selecting the filter components are also presented in this study. The effectiveness of the proposed circuit has been tested and validated experimentally. A smaller dc-link capacitor is sufficient to keep the low-frequency bus ripple to an acceptable range in the proposed topology.

Experimental results on low cost microcontroller based DC drive implementation

This paper presents the experimental results for an attractive control scheme implementation using an 8 bit microcontroller. The power converter involved is a 3 phase full controlled bridge rectifier. A single quadrant DC drive has been realized and results have been presented for both open and closed loop implementations.

Low-Order Harmonic Suppression for Open-End Winding IM With Dodecagonal Space Vector Using a Single DC-Link Supply

This paper proposes a technique to suppress low-order harmonics for an open-end winding induction motor drive for a full modulation range. One side of the machine is connected to a main inverter with a dc power supply, whereas the other inverter is connected to a capacitor from the other side. Harmonic suppression (with complete elimination of fifth- and seventh-order harmonics) is achieved by realizing dodecagonal space vectors using a combined pulsewidth modulation (PWM) control for the two inverters. The floating capacitor voltage is inherently controlled during the PWM operation. The proposed PWM technique is shown to be valid for the entire modulation range, including overmodulation and six-step mode of operation of the main inverter. Experimental results have been presented to validate the proposed technique.

Spectrophotometric determination of molybdenum with Syzygium jambolanum DC leaf extracts

A new spectrophotometric method for the determination of molybdenum in industrial materials has been developed using the leaf extract of Syzygium jambolanum DC based on the reaction of Mo (VI) at pH 7.0 to produce an orange-yellow complex with an absorption maximum at 426 nm. The molar absorptivity of the complex is 4.27 x 10(4) l mol(-1) cm(-1) and the absorbance, is linear in the range 0.05-0.8 ppm. Sandell sensitivity coefficient was found to be 2.25 x 10(-3) mu g/cm(2). The method is ten times more sensitive than the aqueous thiocyanate system. It has been applied successfully in micronutrient fertilizer, artificial freshwater and sea-water analyses.

Computation of power released during corona treatment on polymeric insulators under ac and dc excitation

With increasing energy demand, it necessitates to generate and transmit the electrical power with minimal losses. High voltage power transmission is the most economical way of transmitting bulk power over long distances. Transmission insulator is one of the main components used as a mechanical support and to electrically isolate the conductor from the tower. Corona from the hardware and conductors can significantly affect the performance of the polymeric insulators. In the present investigation a methodology is presented to evaluate the corona performance of the polymeric shed material under different environment conditions for both ac and dc excitation. The results of the comprehensive analysis on various polymeric samples and the power released from the corona electrode for both the ac and dc excitation are presented. Some interesting results obtained from the chemical analysis confirmed the presence of nitric acid species on the treated sample which in long term will affect the strength of the insulator, also the morphological changes were found to be varying for different experimental conditions. (C) 2015 The Authors. Published by Elsevier Ltd.

Harmonic analysis of DC capacitor current in sinusoidal and space-vector modulated neutral-point-clamped inverters

The voltage ripple and power loss in the DC-capacitor of a voltage source inverter depend on the harmonic currents flowing through the capacitor. This paper presents a double Fourier series based analysis of the harmonic contents of the DC capacitor current in a three-level neutral-point clamped (NPC) inverter, modulated with sine-triangle pulse-width modulation (SPWM) or conventional space vector pulse-width modulation (CSVPWM) schemes. The analytical results are validated experimentally on a 3-kVA three-level inverter prototype. The capacitor current in an NPC inverter has a periodicity of 120(a similar to) at the fundamental or modulation frequency. Hence, this current contains third-harmonic and triplen-frequency components, apart from switching frequency components. The harmonic components vary with modulation index and power factor for both PWM schemes. The third harmonic current decreases with increase in modulation index and also decreases with increase in power factor in case of both PWM methods. In general, the third harmonic content is higher with SPWM than with CSVPWM at a given operating condition. Also, power loss and voltage ripple in the DC capacitor are estimated for both the schemes using the current harmonic spectrum and equivalent series resistance (ESR) of the capacitor.

Study on Low Temperature DC Electrical Conductivity of SnO2 Nanomaterial Synthesized by Simple Gel Combustion Method

Nanocrystalline tin oxide (SnO2) material of different particle size was synthesized using gel combustion method by varying oxidizer (HNO3) and keeping fuel as a constant. The prepared samples were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and Energy Dispersive Analysis X-ray Spectroscope (EDAX). The effect of oxidizer in the gel combustion method was investigated by inspecting the particle size of nano SnO2 powder. The particle size was found to be increases with the increase of oxidizer from 8 to 12 moles. The X-ray diffraction patterns of the calcined product showed the formation of high purity tetragonal tin (IV) oxide with the particle size in the range of 17 to 31 nm which was calculated by Scherer's formula. The particles and temperature dependence of direct (DC) electrical conductivity of SnO2 nanomaterial was studied using Keithley source meter. The DC electrical conductivity of SnO2 nanomaterial increases with the temperature from 80 to 300K and decrease with the particle size at constant temperature.

Computationally efficient models for simulation of non-ideal DC-DC converters operating in continuous and discontinuous conduction modes

This paper discusses dynamic modeling of non-isolated DC-DC converters (buck, boost and buck-boost) under continuous and discontinuous modes of operation. Three types of models are presented for each converter, namely, switching model, average model and harmonic model. These models include significant non-idealities of the converters. The switching model gives the instantaneous currents and voltages of the converter. The average model provides the ripple-free currents and voltages, averaged over a switching cycle. The harmonic model gives the peak to peak values of ripple in currents and voltages. The validity of all these models is established by comparing the simulation results with the experimental results from laboratory prototypes, at different steady state and transient conditions. Simulation based on a combination of average and harmonic models is shown to provide all relevant information as obtained from the switching model, while consuming less computation time than the latter.

Design Optimization of Thin-Film Transistors Based on a Metal-Substrate-Semiconductor Architecture for High DC Voltage Sensing

We discuss the potential application of high dc voltage sensing using thin-film transistors (TFTs) on flexible substrates. High voltage sensing has potential applications for power transmission instrumentation. For this, we consider a gate metal-substrate-semiconductor architecture for TFTs. In this architecture, the flexible substrate not only provides mechanical support but also plays the role of the gate dielectric of the TFT. Hence, the thickness of the substrate needs to be optimized for maximizing transconductance, minimizing mechanical stress, and minimizing gate leakage currents. We discuss this optimization, and develop n-type and p-type organic TFTs using polyvinyldene fluoride as the substrate-gate insulator. Circuits are also realized to achieve level shifting, amplification, and high drain voltage operation.