Electrical network

Summary

An electrical network is an interconnection of electrical components (e.g., batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e.g., voltage sources, current sources, resistances, inductances, capacitances). An electrical circuit is a network consisting of a closed loop, giving a return path for the current. Thus all circuits are networks, but not all networks are circuits (although networks without a closed loop are often imprecisely referred to as "circuits"). Linear electrical networks, a special type consisting only of sources (voltage or current), linear lumped elements (resistors, capacitors, inductors), and linear distributed elements (transmission lines), have the property that signals are linearly superimposable. They are thus more easily analyzed, using powerful frequency domain methods such as Laplace transforms, to determine DC response, AC response, and transient response. A resi

Official source

https://en.wikipedia.org/wiki/Electrical_network

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Distribution management system including dispersed generation and storage in a liberalized market environment

Elvira Kägi-Kolisnychenko

Distribution systems are now fundamentally changing. This is due to the fact that a lot of small size dispersed generation units (DG) are expected to be installed at the distribution level, that is, in the medium voltage (MV) and low voltage (LV) networks [1.1]. The problems arise from the current design features of distribution networks, which were not supposed to include energy sources. At this level, until now, the energy is bought from the wholesale market, transported through a transmission system and delivered to the customers through a distribution system. This primary objective is compatible with the arborescent operational network configuration, the conditions of protection and security assessment design, the wire characteristics, and also the type of customer service. Since the distribution grid is used strictly in open-loop mode and has no energy sources, the currents are unidirectional and consequently the top-down centralized management model is the strait-forward choice. With the introduction of dispersed energy resources, the network may need reinforcement and the grid protection schemes should be adapted to bidirectional energy flows [1.1, 1.2]. In this case, the amount of information to be treated centrally would grow considerably [1.3], due to the number of generation equipments inserted into the grid. Large amounts of data would need to be collected, treated simultaneously and provided quickly for further processing. The presence of stochastic energy sources and the reliance on the communication system could compromise the centralized grid operation and the security of supply. In this respect, the decentralized approach is adopted in this study to both energy management planning (day-ahead) and on-line operation. The decentralized energy management system is built using a multi-agent approach. This approach allows for the simultaneous integration of competition and collaboration among the actors. It is characterized by a high flexibility and robustness, while using minimum data exchange. Since generation sources at the MV/LV level are of relatively small capacity, their generation is normally consumed locally. This specificity and the arborescent topology (feeders with laterals) tend to create zones containing some local generation corresponding to local energy needs. The multi-agent approach is thus applied to clusters of active distribution networks at two levels: intrazonal and interzonal, based on an original definition of zones. This framework offers a partial autonomy to each cluster and allows for parallel optimization and for cooperation among several independent entities. The decentralized approach was successfully applied to several distribution network problems, such as: unit commitment and dispatch, voltage profile control and supply restoration after a blackout. It seems however evident from this study that an appropriate centralization degree should be preserved in order to maintain an adequate system operation.

EPFL2009

Free carriers in nanocrystalline titanium dioxide thin films

Simon Springer

This thesis analyzes the properties of heavily doped nanocrystalline titanium dioxide. Thin films were deposited by magnetron sputtering with either water vapor as reactive gas or a periodically interrupted oxygen supply. The samples were at the same time electrically conducting and transparent. They consisted of mixtures of rutile, anatase and amorphous phases. Titanium dioxide is chemically stable, hard, non-toxic, transparent, and inexpensive. Due to a high refractive index it is often found in optical applications as a thin film coating. For many purposes it is interesting to take advantage of a combination of good transparency and electrical conductivity. An even larger field of applications would open if the electric conductivity of TiO2 could be increased in a controlled manner. Conventional bulk doping is limited, however, by low solubility limits. Highly conducting nanocrystalline TiO2 thin films have been obtained recently by sputtering in presence of water vapor. The present project intends to elucidate the doping mechanisms at work in such films. In addition, an alternative process to achieve grain-boundary doping of TiO2 was explored. Spectrophotometry over a wide spectral range (0.05 to 5 eV) was the main tool in probing the mobile electrical charge carriers. The interpretation of these optical measurements required appropriate models. The models developed took many structural properties into account, such as thickness, surface roughness, density, anisotropy and crystalline phase composition. The phase and morphology of the films were analyzed by X-ray diffraction and reflection, scanning probe microscopy, and electron microscopy. The chemical analyses included electron-probe microanalyses, Rutherford backscattering and elastic recoil detection analysis. The DC electrical properties were measured between room temperature and 250 °C. Most water-deposited samples showed semiconducting behavior with an activation energy of the order of 100 meV. The optical measurements revealed a broad absorption band around 1 eV. The absorption coefficient scaled with conductivity. It was successfully modelled as a Lorentzian. Similar absorption bands have been reported for reduced rutile and anatase single crystals. Water-deposited samples containing anatase exhibited a metallic behavior. In these samples the thermal activation energy was reduced to about 30 meV and the charge carriers displayed a high mobility (3 cm2V-1s-1). The infrared absorption band was about 10 times less important than in the samples devoid of anatase. Thin films of TiO2 intercalated with the equivalent of nanometer-thick layers of Ti were prepared. By changing the period in the oxygen supply the grain size, and thereby the film properties, could be modified. The films obtained in this way had many properties in common with water-deposited films. They showed the same absorption band in the infrared, had comparable charge carrier densities and were electrically conductive (102 to 104 Sm-1 at room temperature). To our knowledge, this was the first time that such high electrical conductivities were achieved by sputtering in an oxygen plasma.

EPFL2004

This research presents a high-speed hardware platform dedicated to emulate electrical power networks for the fault location. The solution implements an algorithm based on the Electromagnetic Time-Reversal (EMTR) principle, which allows locating faults in various network types and topologies. Although the technique is highly robust and accurate, its processing is complex and time consuming if solved with classical digital approaches. Therefore, a dedicated computation platform optimized on the processing speed was developed in order to allow its real-time implementation and make it compatible with smart-grids. Two different power network modelling approaches are presented. The first one is based on a finite element representation of the distributed parameters transmission line. The lossless line, initially characterized by a per-unit length inductance and capacitance, is replaced by a series of identical ladder connected inductor-capacitor (LC) elements. The second model is based on the general solution of the telegrapher's equations describing the signals propagated along the transmission line. In this method, the travelling waves' propagation taking place in the line is simulated with cascaded discrete-time delay elements. A possible implementation by means of analog circuits is then presented for each line model. The discretized parameters LC line is simulated by transconductance-capacitor, also called gyrator-C or gm-C topologies, more suitable for microelectronic implementation. On the other hand, the discrete-time delay element of the second method is implemented by switched-capacitor (SC) circuits. The processing time associated to each method can be scaled down according to the microelectronic parameters of the LC line, or by increasing the sampling frequency of the discrete-time model. Through this time scaling, the hardware emulation allows a fault location within duration of up to a hundred times shorter than with classical digital implementations of similar accuracy. The impact of non-ideal effects associated to the microelectronic implementation, such as the CMOS active elements finite gain, offset and dynamic range, or the switched-capacitor charge injection, etc., is evaluated for each model. Associated design constraints are then derived in order to ensure a given fault location accuracy, similarly to that of classical digital methods. Since the switched-capacitor model is characterized by higher robustness and accuracy than the LC line, it is therefore preferred for a silicon implementation. Results obtained after a CMOS AMS 0.35um process implementation have shown that the discrete-time model allows a fault location within 160ms, versus 6s in a classical digital method, with similar resolution (1%). The speed improvement obtained through the presented method is essential, potentially allowing real-time fault management in power grids. Finally, the impact of the magnitude quantization on the line model, offering perspectives of full digital implementations, is evaluated. A possible extension of the model for the simulation of interconnected or multi-conductor lines is also discussed.

EPFL2017