Francisco González-Molina

Surge protection of underground distribution cables

This paper summarizes the main results of a study aimed at determining the maximum lightning overvoltages in branchless underground feeders protected by metal-oxide arresters. The study has two parts. Initially, it is performed assuming an ideal behavior of all components; the goal is to gain insight into the physical phenomenon. This will be useful to understand the main results obtained in the second part, where an advanced model is used for representing arresters.

Tools for Characterization and Assessment of Ferroresonance Using 3-D Bifurcation Diagrams

Ferroresonance is one of the most destructive transient phenomena in power systems. This complex phenomenon involves the association of nonlinear magnetizing inductances and capacitances, and may be initiated in many different ways, which makes its characterization very difficult. This work links EMTP and MATLAB and presents some computer tools that have been developed for assessing ferroresonance behavior. These tools allow users to visualize 2-D and 3-D bifurcation diagrams which can be used for performing sensitivity analysis, predicting the phenomenon at different stages, and finding safety zones for power system parameter design.

Effects of voltage unbalance on torque and current of the induction motors

In order to analyze the effects of voltage unbalance on torque and current of the induction motors, there have been defined 32 unbalanced situations, with 8 different types of voltage unbalance, and four levels for the Voltage Unbalance Factor, measured with the Symmetrical Components method. For each unbalanced situation, both the Torque Ripple Factor and the Current Unbalance Factor are calculated to show the effects of voltage unbalance.

Statistical evaluation of lightning overvoltages on overhead distribution lines using neural networks

The evaluation of the lightning performance of overhead distribution lines, usually characterized by a flashover rate, can be a hard and long task due to the random nature of the lightning phenomenon, this evaluation is generally based on statistical calculations. The application of a neural network can shorten the flashover rate calculation. This paper presents a summary of the work performed by the authors to analyze the lightning performance of unshielded overhead distribution lines using neural networks. The document includes a discussion on the advantages and limitations of the developed approach.

Single-phase transformer model validation for ferroresonance analysis including hysteresis

Although several built-in models are currently available in transient tools for transformer representation, users of these tools can develop their own custom-made models. A very common approach when developing new transformer models is to apply the principle of duality. It has been proved that a topologically-correct model of a single-phase transformer when applying duality is the so-called π model. This work is aimed at validating the π model, including hysteresis representation, for modeling single-phase transformers in ferroresonance studies by comparing simulation results and laboratory measurements.

Parameters calculation of single- and double-cage models for induction motors from manufacturer data

This paper uses an easy technique to calculate directly the parameters for induction motors, using single- and double-cage models. This technique has been applied in a wide sample of 2167 induction motors, where the parameters have been calculated from manufacturer data. The results show that this technique provides large errors in the start and maximum torque points, but on the other hand, it provides small error in the full-load point, for both single- and double-cage models. For this reason, when the induction motor works near the full-load point, for example when there are some disturbances in the electric supply, this technique could be applied with good results.

Ferroresonance Analysis on Power Transformers Interconnected to Self-excited Induction Generators

Abstract This research intends to prove by experimental work how ferroresonance can occur between generating systems based on a self-excited induction generator and power transformers. Up to four different paths have been tested under different commutation maneuvers. Ferroresonance appearance can unfold into a dangerous state for machinery and personnel. Several over-voltages can be present in the system by a simple delay in one of the poles responsible for grid connection/disconnection.

Ferroresonance analysis using 3D bifurcation diagrams

Nowadays the term ferroresonance is firmly established in the power system engineers vocabulary as one of the most destructive transient phenomena, which may cause failures in power equipment such transformers, cables, or arresters. Its complex non-linear behavior makes difficult its characterization. This work links EMTP and MATLAB to presents some techniques for the assessment of ferroresonance behavior. The aim is to visualize 2D bifurcation diagrams which can be used for performing a deep sensitive analysis, and 3D bifurcation diagrams to predict the phenomenon and assist finding safety zones for power system parameter design.

Analysis of Current Unbalance and Torque Ripple Generated by Simulations of Voltage Unbalance in Induction Motors

In this paper the effects of voltage unbalance in both the Complex Current Unbalance Factor (CCUF) and the Torque Ripple Factor (TRF) of a three-phase induction motor have been discussed. The effects of 13060 points of voltage unbalance taken around the limit of standards (VUF = 2%) have been analyzed, since the largest number of international standards use this value as the limit. For the analysis, the single cage model of three-phase induction motor was used. The voltage unbalance has been characterized through four parameters: type, module of Complex Voltage Unbalance Factor (VUF), angle of Complex Voltage Unbalance Factor (θ CVUF) and positive sequence voltage (V 1). The influence of each parameter in both module and angle of CCUF (CUF and θ CCUF), and the Torque Ripple Factor (TRF) have been analyzed on those 13060 points. Results show that only VUF and V 1 parameters have a big influence in CUF and TRF.

Three-phase transformer model validation for ferroresonance analysis

Ferroresonance is a complex nonlinear phenomenon that may be initiated in many different ways and involves the association of capacitances and non-linear magnetizing inductances. Although ferroresonance has been present in power systems for more than a century, its behavior and characterization still remain widely unknown This work illustrates a proper guideline for transformer modeling starting from laboratory test measurements. The paper also proposes a procedure for performing a ferroresonance essay under a controlled environment.