Vacuum circuit breakers are known to potentially cause voltage escalation when interrupting small currents. Consequently, their installation in electrical systems may require the use of specific means aiming at reducing overvoltages. In this paper, the subject is explored in a configuration of an industrial plant. A medium-voltage transformer is connected to the electrical feeding system using a cable equipped with a breaker on the opposite terminal. The risk of voltage escalation is studied when the transformer loaded with a small inductive load is switched off. The study is conducted with an EMTP-like software program; a detailed high-frequency model of the transformer is used as well as a vacuum circuit breaker model able to represent current chopping and restriking. The protection provided by surge arresters and a filter is also studied.

Modern grid codes attempt to unify the existing standards developed by various national and international organizations, implemented from different utilities and generator manufacturers. This evolution of new grid code practices is associated with increasing interoperability of different power systems across borders. This paper compares existing regional network codes for requirements for grid connection applicable to all generators regarding their compliance with the newly formulated code by the European network of transmission system operators for electricity (ENTSO-E). Furthermore, it studies the impact of the unified ENSTO-E codes on the fault-ride-through capability of nuclear power plants (NPP) under extended operating voltage and frequency ranges. The analysis is performed on the typical 1000MW NPP under different voltage and frequency critical ranges and shows that extending the voltage fluctuation below 0.9 pu could affect NPP reliability.

This paper describes the key outputs of the INTEGRID project related to the impact of the new Grid Codes and the implementation of new technologies on the electrical systems and equipment of Nuclear Power Plants (NPPs). The project is funded by the European Commission under the NUGENIA+ umbrella as a pilot project, and carried out by EDF (leader), the University of Manchester, Areva GMBH, and TECNATOM. INTEGRID investigates the increased stresses on the equipment of NPPs that could result from the newly proposed modifications of the Grid Codes by the ENTSO-E, specifically the impacts of wider voltage and frequency ranges. In particular, this paper discusses the impact of these new frequency and voltage operating ranges, and the corresponding code modifications, on different aspects related to the regulation of machines, overheating and vibrations of the equipment, and the appearance of electrical transients in the plant distribution network. The implementation of new technologies, replacing the existing equipment, is also discussed.

This paper describes the methodology developed for the simulation of phenomena observed when connecting a 3 kW single-phase slow charger of an electric vehicle (EV) to the LV distribution network, in the case of a domestic charging. Firstly, the modeling of the up-stream network, the filter, the boost with its power electronics, and the battery have been described; some points specifically have been addressed considering the representation of the non-linearity involved. Secondly, a particular focus has been made on the simulations performed when connecting a charger to the network, for different impedance values of the upstream network; a comparison between simulations and on site tests is presented that gives very satisfactory results.

This document describes the methodology developed for the simulation of phenomena observed when energizing and de-energizing a 250 MVA step-up transformer of a pumped storage power plant connected to the 400 kV network.Firstly, the modeling of the up-stream network and the modeling of the transformer have been described in detail, and some points specifically addressed as the representation of its saturation behavior.Secondly, a particular focus has been made, for the energization and the de-energization of the step-up transformer, on the determination of the closing times ( respectively opening times) of the circuit breaker poles, which have to be accurate in order to reproduce the correct stresses on the transformer. A comparison between the tests carried out on site and the simulations performed have given very satisfactory results, both for the inrush currents when energizing the transformer and the residual fluxes when de-energizing it

This document describes the modelling of the 400 kV up-stream network for the energization of a 340 MVA transformer, using a Thevenin equivalent and also its detailed representation; after the validation of the complete network under steady-state conditions, a comparison between measurements made on site and transient simulations have also been performed, giving very satisfactory results, especially when a detailed modelling of the network is considered, as the transformer considered is highly non linear; non linear aspects are discussed.

A random energization of unloaded transformers may cause large inrush currents and overvoltages; they lead to malfunction of protection relays and possibly to mechanical damages of the transformers windings. The paper describes the reduction of those stresses by the implementation of a controller system installed in a 400kV pumped storage power plant — Transformer, circuit breaker and controller system are from independent manufacturers. Firstly, the assessment of the circuit-breaker parameters (on site) is presented. Secondly, specific operating conditions of the 400kV circuit breaker have been performed, leading to non zero residual flux values. Thirdly, several energizations of the step-up transformer have been performed, leading to a substantial reduction of inrush currents, when the synchronous controller is active. The principles of such a synchronous controller are also addressed, and a comparison with EMTP simulations is presented.

The electromagnetic transient programs (EMTP-like tools) are based on the nodal (or modified nodal) equations that enable an efficient numerical solution and, subsequently, fast time-domain simulations. The state-variable-based simulation programs, such as Simulink, are also used for studying the dynamics of electrical systems. Both the offline and real-time versions of these two types of simulation tools are widely used by the researchers and engineers in industry and academia to study the transient phenomena and dynamics in power systems with rotating electrical machines. This paper provides a summary of the interfacing techniques that are utilized to integrate the general-purpose models of electrical machines with the rest of the power system network for these studies. The interfacing methods are broadly classified as indirect and direct approaches. The paper also describes the numerical properties as well as limita- tions imposed by the interfacing of the commonly used machine models that should be considered when selecting the simulation parameters and assessing the final results.

The electromagnetic transient programs (EMTP-like tools) are based on the nodal (or modified nodal) equations that enable an efficient numerical solution and, subsequently, fast time-domain simulations. The state-variable-based simulation programs, such as Simulink, are also used for studying the dynamics of electrical systems. Both the offline and real-time versions of these two types of simulation tools are widely used by the researchers and engineers in industry and academia to study the transient phenomena and dynamics in power systems with rotating electrical machines. This paper provides a summary of the interfacing techniques that are utilized to integrate the general-purpose models of electrical machines with the rest of the power system network for these studies. The interfacing methods are broadly classified as indirect and direct approaches. The paper also describes the numerical properties as well as limitations imposed by the interfacing of the commonly used machine models that should be considered when selecting the simulation parameters and assessing the final results.

The voltage quality of the energy delivered is a major issue for wind-farms owners and also for the Distribution networks operators. The no-load energization of step-up transformers of wind-farms from the Distribution network may lead to high inrush currents which may affect this power quality and, in certain cases, lead to high overvoltages and high inrush currents. Those phenomena involved are described in the case of a 2.05 MVA 20kV/960V transformer, with a detailed description of the modeling of the network. A comparison between simulations and on site tests is firstly presented in the case of the energization of one transformer alone, secondly in the case where three others transformers are already connected, showing a good agreement between simulations and measurements.

La ferroresonance designe tous les phenomenes oscillatoires, le plus souvent harmoniques mais aussi pseudo-periodiques qui peuvent affecter les reseaux de transport et de distribution de l’electricite. Rencontres egalement en mecanique des fluides, thermique et mecanique, ces phenomenes non lineaires ont fait l’objet d’etudes mathematiques et de la mise au point de modelisation permettant l’etude et donc une meilleure comprehension de la ferroresonance. Par exemple, les phenomenes de surtensions en regime transitoire apparaissant a la mise sous tension de transformateurs sont modelises et expliques par la theorie des bifurcations.

Interfacing of disparate simulation programs is increasingly undertaken to gain a deeper understanding of their functionality and exploit their merits for modeling complex systems. In this paper, techniques for interfacing field and circuit equations of low-frequency electromagnetic apparatus are reviewed, analyzed, and classified into two main categories of direct and indirect methods according to the coupling method. Each category includes a vast array of techniques employing different methods for circuit modeling. The field analysis is restricted to 2-D finite-element method, which is a widely used numerical technique for modeling magnetic behavior of power apparatus. The main features and problems associated with each technique are summarized. Methods for coupling of mechanical equations are addressed in a separate section. A comprehensive list of references is also included at the end of this paper to provide further information to the readers.

La ferroresonance est un phenomene de resonance non lineaire qui peut affecter les reseaux de transport et de distribution de l'electricite. Elle designe tous les phenomenes oscillatoires, le plus souvent harmoniques, mais aussi pseudo-periodiques, voire chaotiques dans les cas extremes, qui se manifestent dans un circuit electrique compose, d'une part, d'une ou de plusieurs inductances non lineaires (comportant des materiaux ferromagnetiques saturables) et, d'autre part, d'un reseau comprenant au moins une capacite alimentee par une ou plusieurs sources de tension generalement sinusoidales. La propriete essentielle et caracteristique d'un tel phenomene est de presenter au moins deux regimes stables pour une meme excitation. Classiquement, en electrotechnique, on considere que les caracteristiques electriques des composants sont lineaires, ce qui implique que le regime permanent atteint est unique et independant des conditions initiales. Ici, la presence d'inductances aux caracteristiques non lineaires peut conduire a des comportements radicalement differents et meme surprenants pour les electrotechniciens. Plusieurs regimes permanents differents peuvent apparaitre dans un circuit donne en fonction des conditions initiales (flux remanent, instants d'enclenchement, etc.). Generalement, l'un d'eux est celui que l'on attend habituellement et les autres sont anormaux et parfois meme dangereux pour le materiel electrique, car ils presentent des surtensions ou des surintensites. Le phenomene a deja ete observe a plusieurs reprises dans des reseaux, et il est ainsi possible de classer les comportements que l'on rencontre le plus souvent. Des regimes periodiques de periode multiple de celle de la source et meme des regimes pseudo-periodiques peuvent apparaitre. A noter que la forme d'onde generee, qui illustre le comportement du reseau de facon globale, permet de renseigner sur la typologie du reseau considere et de son comportement plus ou moins non lineaire. Les surtensions temporaires harmoniques sur les reseaux sont de nature differente des phenomenes de ferroresonance, bien que generees par des reseaux de meme nature. Les phenomenes s'amortissent en general au bout de quelques secondes, cas le plus frequemment rencontre sur les reseaux THT. Les phenomenes non lineaires, que l'on rencontre ici en electrotechnique font egalement l'objet d'un interet croissant dans de nombreux domaines de la physique. On les rencontre aussi en mecanique des fluides, thermique, mecanique, thermodynamique, chimique, et l'exemple le plus connu est la meteorologie, avec la sensibilite aux conditions initiales (effet papillon) et la difficulte de prediction sur un horizon au dela de quelques jours. Les dossiers [D 91] et [D 92] font le point du sujet. Ce premier dossier [D 91] presente la definition, la description et les classifications des phenomenes observes. Le dossier suivant [D 92] presente la modelisation et les outils permettant l'etude de ces phenomenes, ainsi que les applications aux differentes topologies de circuits.

This document describes the determination of the saturated inductance of a transformer, which is the slope of the saturation curve Phi(I) under highly saturated conditions. This parameter, which has a strong impact on the overvoltages when energizing a transformer, has been determined from analytic formulae for a shell-type transformer. Comparisons with the values derived from an electromagnetic 3D calculation and on-site tests are also given in this paper.

This document describes the determination of the saturated inductance of a transformer, which is the slope of the saturation curve Phi(I) under highly saturated conditions. This parameter, which has a strong impact on the overvoltages when energizing a transformer, has been determined from analytic formulae for different transformer technologies. A comparison with the values derived from an electromagnetic 3D calculation is also given in this paper.

This paper presents a programmed link between the electromagnetic transients program EMTP-RV and the finite element field solver FLUX3D. The model created in FLUX3D is driven from simulation designs in EMTP-RV. The test cases presented in this paper demonstrate that the coupling method is numerically robust and with sufficient accuracy. This approach benefits from EMTP advantages in modeling large-scale networks and from field solver advantages for detailed representation of power transformer iron cores.

-The voltage quality of the energy delivered is a major issue for wind-farms owners and also for the Distribution networks operators. The no load energization of step-up transformers of wind-farms from the Distribution network may lead to high inrush currents and lead to high overvoltages which may affect, in certain cases, the power quality. The phenomena involved are described in the case of a 2.05 MVA 20kV/960V transformer, with a detailed description of the modeling of the network. A comparison between simulations and on site tests is firstly presented in the case of the energization of one transformer alone.

This document presents the energization of a transformer in a 110 kV distribution network. It shows that high discrepancies in the circuit-breaker poles closing times may lead to high overvoltages, which have been confirmed by on site tests. Phenomena involved are ferroresonance ones, after the energization of the transformer, when phases A and C saturate while the breaker pole remains open on phase B during 17 ms. Recommendations in order to avoid these overvoltages are given in this paper.

The energization of no-load power transformers may have undesirable effects on the power quality and may damage the equipments of the network, mainly the transformers. Nevertheless, an accurate knowledge of the residual fluxes present in the magnetic core when de-energizing the transformers enables the choice of the best strategy to reduce the stresses.