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A User’s Guide

The future grid is evolving into a smart distribution network that integrates multiple distributed energy resources ensuring at the same time reliable operation and increased power quality. In recent years, many research papers have addressed the voltage violation problems that arise from the high penetration of distributed generation. In view of the transition to active network management and the increase in the quantity of collected data, distributed control schemes have been proposed that use pervasive communications to deal with the complexity of smart grid. This paper reviews the recent publications on distributed and decentralized voltage control of smart distribution networks, summarizes their control models, and classifies the solution methodologies. Moreover, it comments on issues that should be addressed in the future and the perspectives of industry applications.

Distributed and Decentralized Voltage Control of Smart Distribution Networks: Models, Methods, and Future Research

Short circuit currents or forces during transport can cause mechanical displacements of transformer windings. The transfer function method is presented as a tool to detect these displacements. In order to be able to evaluate the measurements, the correlation between the characteristics of transfer functions and possible damages must be known. Axial displacement and radial deformation of transformer windings have been studied in this research work using two test transformers. The primary winding of the first transformer for axial displacement has 31 double disk coils (1.3 MVA, 10 kV) and the secondary winding is a four layer winding. The second transformer for the study of radial deformation has 30 double disk coils (1.2 MVA, 10 kV) as primary winding and a one layer winding as secondary winding. The detailed mathematical models were developed for the test objects and a comparison was carried out between measured and calculated results. It is shown that this model can present the behavior of the transformer windings in the frequency domain in the case of sound and displaced conditions.

Transfer function method to diagnose axial displacement and radial deformation of transformer windings

A complete, three phase transformer model for the calculation of electromagnetic transients is presented. The model consists of a set of state equations solved with the trapezoidal rule of integration in order to obtain an equivalent Norton circuit at the transformer terminals. Thus the transformer model can be easily interfaced with an electromagnetic transients program. Its main features are: (a) the basic elements for the winding model are the turns; (b) the complete model includes the losses due to eddy currents in the windings and in the iron core; and (c) the solution of the state equations is obtained in decoupled iterations. For validation, the frequency response of the model is compared with tests on several transformers. Applications to the calculation of transients are given for illustration. >

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Complete transformer model for electromagnetic transients

It is expected that increasing amounts of new generation technologies will be connected to electrical power systems in the near future. Most of these technologies are of considerably smaller scale than conventional synchronous generators and are therefore connected to distribution grids. Further, many are based on technologies different from the synchronous generator, such as the squirrel cage induction generator and high or low speed generators that are grid coupled through a power electronic converter. When connected in small amounts, the impact of distributed generation on power system transient stability will be negligible. However, if its penetration level becomes higher, distributed generation may start to influence the dynamic behavior of the power system as a whole. In this paper, the impact of distributed generation technology and penetration level on the dynamics of a test system is investigated. It is found that the effects of distributed generation on the dynamics of a power system strongly depend on the technology of the distributed generators.

Impacts of distributed generation on power system transient stability

This paper presents a comprehensive analysis of the possible impacts of different penetration levels of distributed generation (DG) on voltage profiles in low-voltage secondary distribution networks. Detailed models of all system components are utilized in a study that performs hundreds of time-domain simulations of large networked distribution systems using the Electromagnetic Transients Program (EMTP). DGs are allocated in a probabilistic fashion to account for the uncertainties of future installations. The main contribution of this paper is the determination of the maximum amount of DG that secondary distribution networks can withstand without exhibiting undervoltage and overvoltage problems or unexpected load disconnections. This information is important for network planning engineers to facilitate the extension of the maximum penetration limit. The results show that depending on the location, type, and size of the installed DGs, small amounts of DG may cause overvoltage problems. However, large amounts of DG may not cause any voltage problems when properly selected.

Analysis of Voltage Profile Problems Due to the Penetration of Distributed Generation in Low-Voltage Secondary Distribution Networks

An algorithm for the optimal voltage regulation of distribution secondary networks with distributed generators (DGs) is proposed in the paper. Based on the e decomposition of the sensitivity matrix (inverse of Jacobian) obtained from the solution of the Newton-Raphson power flow problem, a large secondary network is divided into several small subnetworks. From the e decomposition, the range of influence of each DG on the voltage of the entire network is determined. When voltage at particular nodes exceeds normal operating limits, the nearest DGs can be located and commanded to control the voltage. The control action can be coordinated using communications in a small-size subnetwork. The voltage regulation is achieved by solving a small linear programming optimization problem with an objective function that makes every DG to optimize its generation. The algorithm is tested with a model of a real heavily-meshed secondary network. The results show that the algorithm proposed in this paper can effectively control the voltage in a distributed manner. It is also discussed in the paper how to choose the value of e for the system decomposition.

Optimal Distributed Voltage Regulation for Secondary Networks With DGs

Eddy current effects are included in a model of a power transformer for the study of electromagnetic transients. Existing analytical formulae for the calculation of losses in the windings are evaluated. Various equivalent circuits are fitted to represent in the time domain the damping produced by eddy currents in the windings. A frequency dependent model is derived for the iron core, based on the physical distribution of losses and magnetization effects. The parameters of this model are obtained by optimal discretization of the laminations. Simulation show the effects of eddy current in the damping of transients. >

/pdf/time-domain-modeling-of-eddy-current-effects-for-transformer-4fk9iu1nmu.pdf

Time domain modeling of eddy current effects for transformer transients

Very efficient procedures for computing elementary parameters (turn leakage inductances and capacitances) in a transformer are presented. The turns are used as a calculation base to permit modeling at very high frequencies. Turn-to-turn (or loop) leakage inductances are obtained by an image method. The charge simulation method is used for finding the capacitances between turns and from turns to ground. The new methods are very efficient compared with the use of the technique of finite elements and are also remarkably accurate. Thus, the short circuit (or test) leakage inductance can be obtained from turn-to-turn information. Examples of calculated parameters are given for illustration. For validation, the results are compared with the parameters obtained using finite elements and tests. The elementary parameters can be used to create reduced-order computational models for the calculation of transient phenomena. >

/pdf/efficient-calculation-of-elementary-parameters-of-3q3snjora4.pdf

F. de Leon

Papers

Complete transformer model for electromagnetic transients

Analysis of Voltage Profile Problems Due to the Penetration of Distributed Generation in Low-Voltage Secondary Distribution Networks

Optimal Distributed Voltage Regulation for Secondary Networks With DGs

Time domain modeling of eddy current effects for transformer transients

Efficient calculation of elementary parameters of transformers