Showing papers on "Electrical network published in 2007"

Modeling, Analysis and Testing of Autonomous Operation of an Inverter-Based Microgrid

Abstract: The analysis of the small-signal stability of conventional power systems is well established, but for inverter based microgrids there is a need to establish how circuit and control features give rise to particular oscillatory modes and which of these have poor damping. This paper develops the modeling and analysis of autonomous operation of inverter-based microgrids. Each sub-module is modeled in state-space form and all are combined together on a common reference frame. The model captures the detail of the control loops of the inverter but not the switching action. Some inverter modes are found at relatively high frequency and so a full dynamic model of the network (rather than an algebraic impedance model) is used. The complete model is linearized around an operating point and the resulting system matrix is used to derive the eigenvalues. The eigenvalues (termed "modes") indicate the frequency and damping of oscillatory components in the transient response. A sensitivity analysis is also presented which helps identifying the origin of each of the modes and identify possible feedback signals for design of controllers to improve the system stability. With experience it is possible to simplify the model (reduce the order) if particular modes are not of interest as is the case with synchronous machine models. Experimental results from a microgrid of three 10-kW inverters are used to verify the results obtained from the model

Low-Voltage DC Distribution System for Commercial Power Systems With Sensitive Electronic Loads

Abstract: In this paper, the use of DC power system to supply sensitive electronic loads is treated. First, general design issues regarding DC power systems are discussed, and then the measurement results from a scaled laboratory setup are presented. The results show that it is possible to supply sensitive electronic loads through an AC/DC interface, and to keep them online during grid transients. The use of a DC power system to supply sensitive electronic loads will have lower losses compared with a conventional AC uninterruptible power-supply solution due to fewer power conversion steps.

Impact of Network Reconfiguration on Loss Allocation of Radial Distribution Systems

Abstract: This paper presents allocation of power losses to consumers connected to radial distribution networks before and after network reconfiguration in a deregulated environment. Loss allocation is made in a quadratic way and it is based on identifying the real and imaginary parts of current in each branch, and losses are allocated to consumers. The network reconfiguration algorithm is based on the fuzzy multiobjective approach and the max-min principle is adopted for the multiobjective optimization in a fuzzy framework. Multiple objectives are considered for real-power loss reduction in which nodes voltage deviation is kept within a range, and an absolute value of branch currents is not allowed to exceed their rated capacities. At the same time, a radial network structure is maintained with all loads energized. The three objectives considered are modeled with fuzzy sets to evaluate their imprecise nature and one can provide his or her anticipated value of each objective. A 69-node example is considered to demonstrate the effectiveness of the proposed method.

Overcurrent Protection on Voltage-Source-Converter-Based Multiterminal DC Distribution Systems

Abstract: This paper proposes a protection scheme which utilizes modern voltage-source converters as fast-acting current-limiting circuit breakers. This paper investigates the main challenges of detecting and localizing a fault, and interrupting it as quickly as possible in a multiterminal dc system. A system protection scheme consisting of smart relays associated with converters has been developed. The protection relays monitor local quantities to detect and isolate disturbances/faults. It is shown that overcurrent-based schemes can be adopted for these relays to meet the fast response requirements. The effectiveness of the proposed protection scheme is illustrated through simulations

Top layers of metal for high performance IC's

Abstract: A method of closely interconnecting integrated circuits contained within a semiconductor wafer to electrical circuits surrounding the semiconductor wafer. Electrical interconnects are held to a minimum in length by making efficient use of polyimide or polymer as an inter-metal dielectric thus enabling the integration of very small integrated circuits within a larger circuit environment at a minimum cost in electrical circuit performance.

Power system transmission network expansion planning using AC model

Abstract: An optimisation technique to solve transmission network expansion planning problem, using the AC model, is presented. This is a very complex mixed integer nonlinear programming problem. A constructive heuristic algorithm aimed at obtaining an excellent quality solution for this problem is presented. An interior point method is employed to solve nonlinear programming problems during the solution steps of the algorithm. Results of the tests, carried out with three electrical energy systems, show the capabilities of the method and also the viability of using the AC model to solve the problem.

A Distributed Static Series Compensator System for Realizing Active Power Flow Control on Existing Power Lines

Abstract: Flexible AC transmission systems (FACTS) devices can control power flow in the transmission system to improve asset utilization, relieve congestion, and limit loop flows. High costs and reliability concerns have restricted their use in these applications. The concept of distributed FACTS (D-FACTS) is introduced as a way to remove these barriers. A new device, the distributed static series compensator (DSSC), attaches directly to existing HV or EHV conductors and so does not require HV insulation. It can be manufactured at low cost from conventional industrial-grade components. The DSSC modules are distributed, a few per conductor mile, to achieve the desired power flow control functionality by effectively changing the line reactance. Experimental results from a prototype module are presented, along with examples of the benefits deriving from a system of DSSC devices

The p-q theory in three-phase systems under non-sinusoidal conditions

Abstract: The conventional theory of active and reactive power in single-phase or three-phase systems is based on an average value concept, thus making it impossible to define instantaneous active and reactive power in a real sense. On the basis of an instantaneous value concept, the authors have already proposed a new definition of instantaneous active and reactive power in three-phase circuits for arbitrary voltage and current waveforms without any restriction. This is called the instantaneous reactive power theory or the p-q theory, which is considered a basic theory of active power line conditioners such as reactive power compensators and active power filters for harmonic compensation

State-space model of grid-connected inverters under current control mode

Abstract: Growth of distributed generation has led to distribution systems with a mixture of rotating machine generators and inverter interfaced generators. The stability of such networks needs to be studied through the analysis of state-space models, and so suitable models of inverters are needed to complement the well-established models of rotating machines. As machine models include features such as automatic voltage regulators and wash-out functions, the inverter model also includes phase-locking functions and internal control loops. The model for voltage source inverters with an internal current control loop, an outer power regulation loop, a measurement of average power and a phase-locked loop has been developed. The model is presented in detail and is formed with a state-vector, similar to that used for rotating machines. The model includes nonlinear terms but can be linearised about an operating point. The state-space model is verified against a component-level time-step simulation in Simulink/PLECS.

Hydroacoustic modelling and numerical simulation of unsteady operation of hydroelectric systems

Abstract: Hydropower represented in 1999 19% of the world electricity production and the absolute production is expected to grow considerably during the next 30 years. Francis turbines play a major role in the hydroelectric production due to their extended range of application. Due to the deregulated energy market, hydroelectric power plants are increasingly subjecting to off design operation, start-up and shutdown and new control strategies. Consequently, the operation of Francis turbine power plants leads to transients phenomena, risk of resonance or instabilities. The understanding of these propagation phenomena is therefore paramount. This work is a contribution to the hydroacoustic modelling of Francis turbine power plants for the investigation of the aforementioned problematic. The first part of the document presents the modelling of the dynamic behavior and the transient analysis of hydroelectric power plants. Therefore, the one-dimensional model of an elementary pipe is derived from the governing equations, i.e. momentum and continuity equations. The use of appropriate numerical schemes leads to a discrete model of the pipe consisting of a T-shaped equivalent electrical circuit. The accuracy in the frequency domain of the discrete model of the pipe is determined by comparison with the analytical solution of the governing equations. The modelling approach is extended to hydraulic components such as valve, surge tanks, surge shaft, air vessels, cavitation development, etc. Then, the modelling of the Francis, Pelton and Kaplan turbines for transient analysis purposes is presented. This modelling is based on the use of the static characteristic of the turbines. The hydraulic components models are implemented in the EPFL software SIMSEN developed for the simulation of electrical installations. After validation of the hydraulic models, transient phenomena in hydroelectric power plants are investigated. It appears that standard separate studies of either the hydraulic or of the electrical part are valid only for design purposes, while full hydroelectric models are necessary for the optimization of turbine speed governors. The second part of the document deals with the modelling and analysis of possible resonance or operating instabilities in Francis turbine power plants. The review of the excitation sources inherent to Francis turbine operations indicates that the draft tube and the rotor-stator interaction pressure fluctuations are of the major concern. As the modelling of part load pressure fluctuations induced by the cavitating vortex rope that develops in the draft tube at low frequencies is well established, the focus is put on higher frequency phenomena such as higher part load pressure fluctuations and rotorstator interactions or full load instabilities. Three hydroacoustic investigations are performed. (i) Pressure fluctuations identified experimentally at higher part load on a reduced scale model Francis turbine are investigated by means of hydroacoustic simulations and high speed flow visualizations. The resonance of the test rig due to the vortex rope excitation is pointed out by the simulation while the special motion and shape of the cavitating vortex rope at the resonance frequency is highlighted by the visualization. A description of the possible excitation mechanisms is proposed. (ii) A pressure and power surge measured on a 4 × 400 MW pumped-storage plant operating at full load is investigated. The modelling of the entire system, including the hydraulic circuit, the rotating inertias and the electrical installation provides an explanation of the phenomenon and the related conditions of apparition. A non-linear model of the full load vortex rope is established and qualitatively validated. (iii) The rotor-stator interactions (RSI) are studied in the case of a reduced scale pump-turbine model. An original modelling approach of this phenomenon based on the flow distribution between the stationnary and the rotating part is presented. The model provides the RSI pressure fluctuation patterns in the vaneless gap and enables to predict standing waves in the spiral case and adduction pipe. The proposed one-dimensional modelling approach enables the simulation, analysis and optimization of the dynamic behavior of hydroelectric power plants. The approach has proven its capability of simulating properly both transient and periodic phenomena. Such investigations can be undertaken at early stages of a project to assess the possible dynamic problems and to select appropriate solutions ensuring the safest and optimal operation of the facility.

Reconfiguration and Load Balancing in the LV and MV Distribution Networks for Optimal Performance

Abstract: To get the distribution network to operate at its optimum performance in an automated distribution system reconfiguration was been proposed and researched. Considering, however, that optimum performance implies minimum loss, no overloading of transformers and cables, correct voltage profile, and absence of phase voltage and current imbalances, network reconfiguration alone is insufficient. It has to be complemented with techniques for phase rearrangement between the distribution transformer banks and the specific primary feeder with a radial structure and dynamic phase and load balancing along a feeder with a radial structure. This paper contributes such a technique at the low-voltage and medium-voltage levels of a distribution network simultaneously with reconfiguration at both levels. While the neural network is adopted for the network reconfiguration problem, this paper introduces a heuristic method for the phase balancing/loss minimization problem. A comparison of the heuristic algorithm with that of the neural network shows the former to be more robust. The approach proposed here, therefore for the combined problem, uses the neural network in conjunction with a heuristic method which enables different reconfiguration switches to be turned on/off and connected consumers to be switched between different phases to keep the phases balanced. An application example of the proposed method using real data is presented.

Electrical Circuit Theory and Technology

Abstract: Preface Section 1 Basic Electrical Engineering Principles Units associated with basic electrical quantities An introduction to electric circuits Resistance variation Batteries and alternative sources of energy Series and parallel networks Capacitors and capacitance Magnetic circuits Electromagnetism Electromagnetic induction Electrical measuring instruments and measurements Semiconductor diodes Transistors Main formulae for Part 1 Section 2 Electrical Principles and Technology Dc circuit theory Alternating voltages and currents Single-phase series ac circuits Single-phase parallel ac circuits Dc transients Operational amplifiers Three phase systems Transformers Dc machines Three-phase induction motors Main formulae for Part 2 Part 3 Advanced Circuit Theory and Technology Revision of complex numbers Application of complex numbers to series ac circuits Application of complex numbers to parallel ac circuits Power in ac circuits Ac bridges Series resonance and Q-factor Parallel resonance and Q-factor Introduction to network analysis Mesh-current and nodal analysis The superposition theorem Thevenin's and Norton's theorems Delta-star and star-delta transformations Maximum power transfer theorems and impedance matching Complex waveforms A numerical method of harmonic analysis Magnetic materials Dielectrics and dielectric loss Field theory Attenuators Filter networks Magnetically coupled circuits Transmission lines Transients and Laplace transforms Main formulae for Part 3 Part 4 General reference Standard electrical quantities - their symbols and units Greek alphabet Common prefixes Resistor colour coding and ohmic values Index

Estimation of the Lifetime of the Electrical Components in Distribution Networks

Abstract: Due to the large amount of electrical equipment and the costs of an individual diagnosis in distribution networks, a life assessment of the representative electrical components is necessary. For a thorough electro-thermo-mechanical life model of the electrical components, the individual aging phenomena of the representative electrical components as well as the general aging mechanisms of insulating materials are taken into account in this paper. On the basis of the probabilistic approach, the failure probability about possible failure time and the related statistical variables are given in terms of probabilistic failure density, failure rate and failure probability of electrical components. For such a purpose, the relationship between lifetime, electrical stress, mechanical stress and temperature as well as their effects on aging processes and on reliabilities of electrical components are studied. Thereby a new approach is developed for the life assessment of electrical components which supports the decision-making in future deregulation of the electric energy market. Furthermore, the data of historical failure events from distribution networks is collected and evaluated in a special failure statistic. Thus the models can be parameterized with the failure data from practice in distribution networks as a great deal of statistical data is available for the probabilistic assessment. It is demonstrated with the data of some electrical components like conductors, cables, transformers and circuit-breakers that the method is able to assess the lifetime and reliability of electrical components with reasonable and accurate data

Nested Fast and Simultaneous Solution for Time-Domain Simulation of Integrative Power-Electric and Electronic Systems

Abstract: As power electronics are increasingly used in power-electric networks, there is an interest in the creation of time-domain simulation techniques that can model the diversity of the integrative power-electric and electronic system while achieving high accuracy and computational speed. In the proposed method, generation of electric network equivalents (GENE), this is supported through the nested structure of the overall simulation process. One or multiple parent simulations, in which the unknown voltages are calculated using nodal analysis, launch multiple child simulations concerned with diakoptic subdivisions of the system under study. The interfaces for information exchange between parent and child levels are designed to provide encapsulation. This makes the subdivisions appearing from outside in the form of network branches compatible with the nodal analysis approach. It also facilitates the use of diverse solution methods for different child simulations, as it is shown for the simultaneous solution of equations formulated with nodal analysis and state space methods. Computational efficiency is obtained through the coordinated application of sparse-matrix methods, piecewise linear approximation of nonlinear characteristics, and precalculation of operations pertaining to recurring power-electronic switch statuses. The resulting overall solution process is simultaneous, distributed, and suitable for real-time simulation. The devised methodology is validated through simulation of the CIGRE HVdc benchmark model, comprising ac networks, twelve-pulse power-electronic converter stations, harmonic filters, and dc transmission

Micro concentrators elastically coupled with spherical photovoltaic cells

Abstract: With small dimensional optics, small photovoltaic cells have heat distribution surfaces, very high concentrations and subsequently high utilization of the semiconductors can be achieved. Discrete photodiodes can be formed as spherical and other geometric shaped, cells with high performance characteristics, precision dimensions, and low cost. This invention positions discrete photovoltaic cells by using their geometric shape, elastic electrical mounts, couples them to small optical concentrator systems of refractory and or reflective optics and makes electrical network connections to those photodiodes, reliably, adjusting for thermal expansion, and at low cost to form low cost and reliable electrical power arrays. The electrical connectors and network can form part of the reflective optics and heat removal system. The electrical interconnection system can also form a reliable network that is self-correcting and tolerant of point failures.

Minimization of Voltage Sag Costs by Optimal Reconfiguration of Distribution Network Using Genetic Algorithms

Abstract: The paper describes genetic-algorithm (GA)-based optimization software for reconfiguration of a distribution network in order to minimize financial losses due to voltage sags. The developed methodology starts with a selected number of switches which generate various topologies. Load flows are then performed to evaluate the feasibility of topologies. For each feasible topology, fault analysis is performed to first calculate voltage sags at different buses in the network and then to calculate financial losses incurred by voltage sags at buses with sensitive industrial processes. The result of the optimization is the topology which yields the lowest voltage sag cost to customers. The main features of the GA include double-point crossover and adaptive mutation. The developed software tool is applied to the 295-bus generic distribution system.

A Method for Voltage Sag State Estimation in Power Systems

Abstract: This paper presents a new technique for voltage sag state estimation. The proposed method is based on estimating the number of voltage sags occurring at nonmonitored buses from the number of sags recorded at a limited number of monitored buses. This problem, contrary to the traditional state estimation where redundant measurements are available, is formulated as an underdetermined system of equations. In this paper, linear integer programming techniques are employed to solve this estimation problem. The performance of the proposed method is tested by means of several case studies applied in the IEEE 24-bus reliability test system (RTS) and in the IEEE 118-bus test system.

Locating Switched Capacitor Using Wavelet Transform and Hybrid Principal Component Analysis Network

Abstract: The transient caused by capacitor switching is one of the important power quality (PQ) problems. In particular, switching on the capacitor may result in an overvoltage to a sensitive load, serious disturbance to an adjustable-speed drive or a resonance in the system. This paper presents a new method to locate the positions of the transient sources with the help of the PQ monitoring system. The discrete wavelet transform is used to extract the features of transients caused by capacitor switching. The fuzzy-c-means is then used to determine the placement of PQ measurement facilities. The signal energies attained by wavelet coefficients serve as inputs to the hybrid principal component analysis neural network for locating the transient sources. The simulation results obtained from an 18-bus distribution system show the applicability of the proposed method

System Studies of the Superconducting Fault Current Limiter in Electrical Distribution Grids

Abstract: A superconducting fault current limiter (SFCL) in series with a downstream circuit breaker could provide a viable solution to controlling fault current levels in electrical distribution networks. In order to integrate the SFCL into power grids, we need a way to conveniently predict the performance of the SFCL in a given scenario. In this paper, short circuit analysis based on the Electromagnetic Transient Program was used to investigate the operational behavior of the SFCL installed in an electrical distribution grid. System studies show that the SFCL can not only limit the fault current to an acceptable value, but also mitigate the voltage sag. The transient recovery voltage (TRV) could be remarkably damped and improved by the presence of the SFCL after the circuit breaker is opened to clear the fault.

A Robust One-Cycle Controlled Full-Bridge Series-Parallel Resonant Inverter for a High-Frequency AC (HFAC) Distribution System

Abstract: Resonant inverters are connected to a high-frequency AC (HFAC) bus, where power is delivered to different locations for points-of-use power management. Such a power distribution system subjects to more perturbations and load uncertainties than inverters operating with single load. A novel voltage control method is proposed in this paper for a high-frequency full-bridge resonant inverter with series-parallel resonant tank. A modified one-cycle controlled phase-shift modulation is proposed to effectively compensate the input line variations. The uncertainty model of the high frequency resonant inverter is developed and analyzed with the resonant circuit component tolerance, input line and load variations taken into design considerations. The voltage feedback controller is designed based on the Hinfin robust control theory and is implemented with analog discrete devices. The proposed control scheme has the advantages of fast response for both input line and load perturbations. It also ensures a wide range of system stability and guarantees robustness of the power converter. Both simulations and experimental results are provided to verify with the theoretical analysis through an experimental prototype of a full-bridge resonant inverter with an output power of 150-W operating at 1 MHz and an output voltage of 28 V (rms).

A generalised instantaneous non-active power theory for STATCOM

Abstract: A generalised instantaneous non-active power theory is presented. Comprehensive definitions of instantaneous active and non-active currents, as well as instantaneous, average and apparent powers, are proposed. These definitions have flexible forms that are applicable to different power systems, such as single-phase or multi-phase, periodic or non-periodic and balanced or unbalanced systems. By changing the averaging interval and the reference voltage, various non-active power theories can be derived from this theory. The definitions of instantaneous active and non-active currents provide an algorithm for a STATCOM to calculate the non-active current in the load current. The theory is implemented by the STATCOM, and four cases (three-phase balanced RL load, three-phase unbalanced RL load, diode rectifier load and single-phase load) are tested. The experimental results show that the STATCOM can perform instantaneous non-active power compensation, and both the fundamental non-active component and the harmonics are eliminated from the utility so that nearly unity power factor can be achieved. The STATCOM also has a fast dynamic response for transients.

Propulsion Drive Models for Full Electric Marine Propulsion Systems

Abstract: Integrated full electric propulsion systems are being introduced across both civil and military marine sectors. Standard power systems analysis packages cover electrical and electromagnetic components, but have limited models of mechanical subsystems and their controllers. Hence electromechanical system interactions between the prime movers, power network and driven loads are poorly understood. This paper reviews available models of the propulsion drive system components: the power converter, motor, propeller and ship. Due to the wide range of time-constants in the system, reduced order models of the power converter are required. A new model using state-averaged models of the inverter and a hybrid model of the rectifier is developed to give an effective solution combining accuracy with speed of simulation and an appropriate interface to the electrical network model. Simulation results for a typical ship manoeuvre are presented.

Distribution Line Carrier: Analysis Procedure and Applications to DG

Abstract: This paper deals with the analysis of carrier signal transmission in medium-voltage (MV) distribution networks. A multiconductor matrix procedure based on the bus admittance matrix, which accounts for the earth return currents, enables predicting the high-frequency behavior of the distribution networks and, hence, the effectiveness of transmitted signals. To this aim, suitable models have been implemented based on the Carson's theory for overhead lines with ground return and Wedepohl's theory for cable lines. A comparison with the modal analysis demonstrates the advantages of the multiconductor matrix algorithm with respect to the modal analysis. In order to validate the theoretical results, two measurement campaigns have been carried out on real MV networks. A good agreement between measured and computed values has been verified. Since distribution networks are generally unlike each other, the procedure becomes a valuable tool, which makes it easy to assess the feasibility of distribution line carrier (DLC) applications in any system. In particular, a protection system to prevent the islanding of dispersed generators is proposed. In addition, in a foreseeable hypothesis of allowed islanding operation of portions of distribution networks (microgrids), DLCs are proposed as communication control of the interface device at the point of common coupling

Data center power distribution

Abstract: A power distribution apparatus includes a housing holding a plurality of electrical outlets, a plurality of independent electrical circuits within the housing, and a plurality of cord sets serving the plurality of independent electrical circuits and extending from the housing.

Phasor-Based Synchronized Frequency Measurement in Power Systems

Abstract: Phasor-based frequency measurement techniques have very good steady-state performance, but their dynamic performance is not well documented. This paper analyzes a phasor-based frequency measurement method that considers the effect of dynamic frequency, and proposes a method to improve the dynamic performance of the phasor-based frequency measurements. Simulation results verify the effectiveness of the proposed method. Results of laboratory experiments are presented to show the performance of the proposed method compared to frequency measurements obtained from commercial phasor measurement units (PMUs).

Active and passive vibration isolation and damping via shunted transducers

Abstract: Many different active control techniques can be used to control the vibrations of a mechanical structure: they however require at least a sensitive signal amplifier (for the sensor), a power amplifier (for the actuator) and an analog or digital filter (for the controller). The use of all these electronic devices may be impractical in many applications and has motivated the use of the so-called shunt circuits, in which an electrical circuit is directly connected to a transducer embedded in the structure. The transducer acts as an energy converter: it transforms mechanical (vibrational) energy into electrical energy, which is in turn dissipated in the shunt circuit. No separate sensor is required, and only one, generally simple electronic circuit is used. The stability of the shunted structure is guaranteed if the electric circuit is passive, i.e., if it is made of passive components such as resistors and inductors.This thesis compares the performances of the electric shunt circuits with those of classical active control systems. It successively considers the use of piezoelectric transducers and that of electromagnetic (moving-coil) transducers.In a first part, the different damping techniques are applied on a benchmark truss structure equipped with a piezoelectric stack transducer. A unified formulation is found and experimentally verified for an active control law, the Integral Force Feedback (IFF), and for various passive shunt circuits (resistive and resistive-inductive). The use of an active shunt, namely the negative capacitance, is also investigated in detail. Two different implementations are discussed: they are shown to have very different stability limits and performances.In a second part, vibration isolation with electromagnetic (moving-coil) transducers is introduced. The effects of an inductive-resistive shunt circuit are studied in detail; an equivalent mechanical representation is found. The performances are compared with that of resonant shunts and with that of active isolation with IFF. Next, the construction of a six-axis isolator based on a Stewart Platform is presented: the key parameters and the main limitations of the system are highlighted.