Juan A. Martinez-Velasco

Control Strategies Based on Symmetrical Components for Grid-Connected Converters Under Voltage Dips

Low-voltage ride-through (LVRT) requirements demand wind-power plants to remain connected to the network in presence of grid-voltage dips. Most dips present positive-, negative-, and zero-sequence components. Hence, regulators based on symmetrical components are well suited to control grid-connected converters. A neutral-point-clamped topology has been considered as an active front end of a distributed power-generation system, following the trend of increasing power and voltage levels in wind-power systems. Three different current controllers based on symmetrical components and linear quadratic regulator have been considered. The performance of each controller is evaluated on LVRT requirement fulfillment, grid-current balancing, maximum grid-current value control, and oscillating power flow. Simulation and experimental results show that all three controllers meet LVRT requirements, although different system performance is found for each control approach. Therefore, controller selection depends on the system constraints and the type of preferred performance features.

Analysis of large distribution networks with distributed energy resources

espanolLa conexion de recursos energeticos distribuidos (es decir, generacion distribuida y almacenamiento de energia) y algunas demandas especiales (por ejemplo, el coche electrico) generan nuevos retos en el analisis y simulacion de redes de distribucion. Unidades con generacion intermitente, asi como demandas variables en el tiempo o asimetricas, requieren nuevas prestaciones que hasta muy recientemente no han estado disponibles en herramientas de simulacion de redes de distribucion; por ejemplo, prestaciones para llevar a cabo simulaciones probabilisticas o en funcion del tiempo. Este articulo presenta el trabajo realizado por los autores para adaptar OpenDSS, un simulador libre de analisis de redes de distribucion, y poder realizar este tipo de estudios teniendo en cuenta que el modelo de la red de distribucion es en general muy grande y complejo. El objetivo principal de este trabajo es presentar la aplicacion de OpenDSS, funcionando como herramienta autonoma o controlada desde MATLAB, en el analisis y diseno de grandes redes de distribucion. Varias aplicaciones han sido desarrolladas para obtener las curvas anuales de demanda y generacion renovable (solar, eolica) y poder ejecutar OpenDSS en un entorno de calculo distribuido; es decir, en una instalacion con multiples procesadores funcionando en paralelo. EnglishThe connection of distributed energy resources (i.e., distributed generation and energy storage) and some special loads (e.g., electrical vehicle) raises new challenges in distribution system analysis. Intermittent and non-dispatchable generation units, time-dependent or asymmetrical loads require capabilities that until very recently were not available in distribution software packages; for instance, capabilities for probabilistic and time mode simulations. This paper presents the work made by the authors to adapt OpenDSS, a freely available distribution simulator, in order to perform these types of studies taking into account that the model of an actual distribution system is usually very large and complex. A goal of the paper is to present the usage of OpenDSS as either a stand-alone tool or as a COM DLL driven from MATLAB. Several applications have been developed to obtain yearly curve shapes aimed at representing time-varying loads and renewable energy sources (sun, wind) and to run OpenDSS in a distributed computing environment (e.g., multicore installation).

Optimal design of a medium voltage hybrid fault current limiter

The connection of distributed generation increases the short circuit power which in turn might exceed the ratings of the installed circuit breakers. A solution is to limit the available short circuit power by increasing the grid impedance, but since there is a constant strive for lower losses and higher power transfer capabilities, this is not desired. The application of a fault current limiter (FCL) that can limit the current before the first peak enables a power system with high short circuit power and low short circuit current. This can increase the stability of the grid and reduce the requirements of other equipment. This work presents a simulation model to be used as an aid in the design of a hybrid FCL for a 12 kV AC system. The proposed model combines a transient analysis circuit model with an optimization module to obtain multiple sets of possible design parameters. The design is not straight forward since there is a trade-off between several of the design parameters.

MODELING OF OVERHEAD TRANSMISSION LINES FOR LIGHTNING OVERVOLTAGE CALCULATIONS

Resumen es: Un modelo de linea aerea de transmision adecuado para el calculo de sobretensiones originadas por el rayo debe incluir varias partes de la linea: conduct...

EMTP IMPLEMENTATION OF A MONTE CARLO METHOD FOR LIGHTNING PERFORMANCE ANALYSIS OF TRANSMISSION LINES

An accurate calculation of lightning overvoltages is an important issue for the analysis and design of overhead transmission lines. The different parts of a transmission line that are involved in lightning calculations must be represented taking into account the frequency ranges of transients associated to lightning. In addition, the procedures to be used in these calculations must be developed considering the random nature of lightning phenomena. Several simulation tools have been used to estimate the lightning performance of transmission lines. The most popular approaches are those based on a time-domain simulation technique for which adequate procedures and transmission line models have to be developed. This paper presents a summary of the computational efforts made by the authors for the development and implementation in an EMTP-like tool of a Monte Carlo procedure, as well as the models of some transmission line components, aimed at analyzing the lightning performance of transmission lines. An actual test line is used to illustrate the scope of this procedure and the type of studies that can be performed.

Circuito equivalente de un transformador con regulación

espanolEste articulo presenta una metodologia para la obtencion y utilizacion del circuito equivalente de un transformador con regulacion en el que es posible variar su relacion de transformacion. Este aspecto esta insuficientemente tratado en la mayoria de libros de texto actuales en los que el circuito de un transformador con regulacion se presenta de forma poco justificada. Este documento muestra como obtener el circuito equivalente del transformador y como aplicarlo utilizando tanto valores reales como valores por unidad (pu). El tratamiento con valores por unidad se puede realizar seleccionando los valores base de forma arbitraria, lo que supone una ventaja evidente cuando se trata de determinar alguna de las tensiones nominales del transformador en estudio. Aunque el objetivo del articulo es el de presentar un metodo para transformadores con regulacion, el metodo es tambien valido para transformadores con relacion de transformacion constante en los que se tenga que determinar alguna de sus tensiones nominales. El articulo incluye dos ejemplos con campos de aplicacion muy distintos que serviran para ilustrar las ventajas del metodo propuesto. EnglishThis paper presents a methodology aimed at obtaining and applying the equivalent circuit of a regulating transformer, whose turn ratio can be varied. This aspect is insufficiently treated in most current textbooks in which the equivalent circuit of a tapped transformer is not well justified. This document shows how to obtain the equivalent circuit of the transformer and how to apply it using either physical quantities or per unit (pu) quantities. The usage of per unit values can be performed by selecting the base quantities in an arbitrary manner, which is an obvious advantage when one of the rated voltages must be estimated. Although the goal of the paper is to develop a method for regulating transformers (e.g., under load tap changers, ULTC), it can be applied to transformers with a fixed turn ratio for which one of the rated voltages is unknown. The paper includes two illustrative examples with different fields of application.

A review of tools, models and techniques for long-term assessment of distribution systems using OpenDSS and parallel computing

Many distribution system studies require long-term evaluations (e.g. for one year or more): Energy loss minimization, reliability assessment, or optimal rating of distributed energy resources should be based on long-term simulations of the distribution system. This paper summarizes the work carried out by the authors to perform long-term studies of large distribution systems using an OpenDSS-MATLAB environment and parallel computing. The paper details the tools, models, and procedures used by the authors in optimal allocation of distributed resources, reliability assessment of distribution systems with and without distributed generation, optimal rating of energy storage systems, or impact analysis of the solid state transformer. Since in most cases, the developed procedures were implemented for application in a multicore installation, a summary of capabilities required for parallel computing applications is also included. The approaches chosen for carrying out those studies used the traditional Monte Carlo method, clustering techniques or genetic algorithms. Custom-made models for application with OpenDSS were required in some studies: A summary of the characteristics of those models and their implementation are also included.

MODELING PROTECTIVE DEVICES FOR DISTRIBUTION SYSTEMS WITH DISTRIBUTED GENERATION USING AN EMTP-TYPE TOOL

This paper presents the main features of a library of modules for representation of protective devices of distribution networks with distributed generation (DG). The impact of distributed generation in a distribution network depends, among other aspects, on the design of the protection system and the coordination between the different protective devices. The presence of distributed generation changes the radial nature of distribution systems and affects the performance of the protection system. This document discusses the present protection practices for distribution systems and DG interties, describes the implementation of protective devices (circuit breakers, relays, reclosers, fuses, sectionalizers) in an EMTP-type tool, and presents some simulation results whose main goal is to analyze the impact of synchronous machine-based embedded generators on the coordination between the different types of protective devices.