[Show abstract][Hide abstract] ABSTRACT: This paper focuses on the analysis of the connection of wind farms to the electric power system and their impact on the harmonic load-flow. A possibilistic harmonic load-flow methodology, previously developed by the authors, allows for modeling uncertainties related to linear and nonlinear load variations. On the other hand, it is well known that some types of wind turbines also produce harmonics, in fact, time-varying harmonics. The purpose of this paper is to present an improvement of the former method, in order to include the uncertainties due to the wind speed variations as an input related with power generated by the turbines. Simulations to test the proposal are performed in the IEEE 14-bus standard test system for harmonic analysis, but replacing the generator, at bus two, by a wind farm composed by ten FPC type wind turbines.
Preview · Article · Aug 2015 · Dyna (Medellin, Colombia)
[Show abstract][Hide abstract] ABSTRACT: Air pollution in large cities and high prices of oil (nonrenewable resource) are signs that the current mobility model is unsustainable. The expectations have been set in plugin electric vehicles (EVs). EVs are penetrating markets and, soon, will be part of the alternatives of mobility in countries of Latin America. Therefore, efforts are needed in research and development to understand how the massive connection of EVs will impact the way in which power systems operate. This paper presents a literature review about this topic. The paper has focused on three main impacts: on network planning, on power quality and on markets and tariff schemes. After a quantitative analysis of the available literature, each issue is separately developed. Finally, conclusions are presented.
[Show abstract][Hide abstract] ABSTRACT: This paper presents the second part of a research work dealing with the performance assessment of commercially available magnetometers. The aim of the article is to continue a comparative study of a magnetometer set, but now using non-sinusoidal signals. Nineteen magnetometers were tested using signals with two and more frequency components; some features of the meters were investigated: fft, rms value, peak value and wave capture functions.
[Show abstract][Hide abstract] ABSTRACT: This article presents and develops main aspects to be considered when harmonics have to be modeled in a power system including wind farms in its generator fleet. In particular, uncertainty involved in the analysis, because the wind speed variations, is considered and included in a more complete methodology, that is capable to compute the possibility distributions of the harmonic voltage distortion at all buses of the power system. Such a methodology, developed by the authors, is the possibilistic harmonic load-flow. Simulations to test the proposal are developed in the IEEE 14-bus standard test system for harmonic analysis, but replacing the generator at bus two, with a wind farm.
[Show abstract][Hide abstract] ABSTRACT: In this paper, a MATLAB model of a Digital Fourier Transform (DFT)-based digital distance relay was developed and then its behavior was analyzed when it is applied on distance protection of a real series compensated transmission system, belonging to the Chilean generation and transmission utility Colbún S.A. The complete signal processing and the impedance calculation of real relays are simulated by the model. The transmission system was modeled with ATP and various fault types at buses and diverse line locations were simulated. The DFT-model response was evaluated in each fault case from the obtained current and voltage signals. The results show how series capacitors impact negatively on the impedance calculation using DFT.
No preview · Article · Sep 2012 · IEEE Latin America Transactions
[Show abstract][Hide abstract] ABSTRACT: A Power Transformer is an efficient and reliable machine that is designed and built to ensure a long useful life under normal operation. However, among the total number of operative power transformers installed in networks around the world, there are many aged units, which are approaching to the state of final failure. This situation of simultaneous aging becomes a major problem, and its proper solution is critical for the electric utilities because their financial performance and economic viability are unavoidably dependent on the reliability of this equipment. In this regard, this paper presents an overview on the optimal management of a power transformer fleet. The review is focused on a methodological scheme oriented to the assessment of the transformer risk index and its two main components: the probability of failure index and the post-failure consequence index.
[Show abstract][Hide abstract] ABSTRACT: A computationally efficient multirate scheme was proposed to model a linear and time invariant part of an electric network for electromagnetic transient calculation. The main contribution is expected in the field of real-time calculation, where very efficient models are required to fulfil stringent time performance requirements. Two models of the equivalent sub-network based on different time steps are implemented in the low- and high-frequency channels of a basic Laplacian Pyramid unit. A detailed model based on the longest time step reproduces the low-frequency response, while a simplified (lower order) model, based on the shortest time step, reproduces the high-frequency behaviour. In this way, an accurate response of the equivalent over a wider frequency range can be achieved as compared with previous proposals based on a single low-frequency channel. Also, the proposed scheme make a much greater flexibility in choosing the decimation factor possible as compared with a previous proposal of two co-authors based on a 2-fold decimated filter banks.
No preview · Article · Aug 2012 · IET Generation Transmission & Distribution
[Show abstract][Hide abstract] ABSTRACT: In a previous study by the authors, a possibilistic harmonic load flow (PHLF) approach was presented. The PHLF was capable of modelling, through possibility distributions, uncertainties regarding magnitude and composition in both linear (LL) and non-linear loads (NLL) connected at the linear (LL) and non-linear loads(PS). However, in that approach, possibility non-interaction between parameters for modelling LL and NLL was assumed. Such assumption is quite conservative and conduces to uncertainty overestimation. In fact, there are well-defined physical relationships among model parameters that constrain the variability of these. This study presents an improved approach which overcomes the aforementioned drawback by including both linear and non-linear equality constraints into the non-linear programming problem used to compute the PHLF. These linear and non-linear equality constraints are specifically used to model the balance of active and reactive power, respectively, at each bus of the PS. The equality constrains inclusion is supported from a possibility theory point of view by means of the `information fusion concept`. Influence of the possibility interaction consideration is evaluated in the IEEE 14-bus PS for harmonic analysis. Then, the complete PHLF is tested in a real 88-bus PS, and comparisons with Monte Carlo simulation results are performed and discussed.
No preview · Article · Jun 2012 · IET Generation Transmission & Distribution
[Show abstract][Hide abstract] ABSTRACT: In the field of power transformer (PT) management, there are efforts to formulate a reliable methodology to estimate a PT risk index that usually is based on two sub-indices: the consequence factor (CF) and the probability of failure (PF). While the first sub-index is based on the fact that every PT will fail and the consequences of such failure are roughly estimated, the second one essentially depends on the monitoring and assessment of the PT condition. A PF assessing method must provide a quantitative or qualitative statement regarding the current PT condition and relies on condition monitoring tools like vibration analysis, dissolved gas analysis, degree of polymerization measurement, partial discharges analysis, frequency response analysis, loading analysis, among others. Some of these tools require special equipment, disconnecting, or opening the PT. However, a wellknown, economic and non-invasive method is to estimate the aging of the PT insulating paper by means of hot-spot temperature and load data. It is the deterioration of cellulose caused exclusively by temperature that can be assessed with some PT characteristics and a set of equations. The equations are required, first, for calculating the hot-spot temperature (HST) as a function of ambient temperature, and second, for estimating the equivalent accumulated aging as a function of the HST and historical load profile. However, in many cases of in-service PT, the available load profile data is incomplete. Therefore, to calculate the equivalent accumulated insulating paper aging for the whole period of PT operation, the historical load profile has to be estimated. In this paper a method to estimate the historical load and ambient temperature profiles by means of an artificial neural network and Monte Carlo simulations, to account for the related uncertainties, is presented and applied to a currently in-service PT. Furthermore criteria for choosing a suitable HST model are defined. Finally, the corresponding results for the assessed PT are presented.
[Show abstract][Hide abstract] ABSTRACT: An efficient expert system for the power transformer condition assessment is presented in this paper. Through the application of Duval’s triangle and the method of the gas ratios a first assessment of the transformer condition is obtained in the form of a dissolved gas analysis (DGA) diagnosis according IEC 60599. As a second step, a knowledge mining procedure is performed, by conducting surveys whose results are fed into a first Type-2 Fuzzy Logic System (T2-FLS), in order to initially evaluate the condition of the equipment taking only the results of dissolved gas analysis into account. The output of this first T2-FLS is used as the input of a second T2-FLS, which additionally weighs up the condition of the paper-oil system. The output of this last T2-FLS is given in terms of words easily understandable by the maintenance personnel. The proposed assessing methodology has been validated for several cases of transformers in service.
Full-text · Article · Jul 2011 · Expert Systems with Applications
[Show abstract][Hide abstract] ABSTRACT: Harmonics in power systems are responsible for several technical problems that justify the development of models to study them. Well-established models exist to analyse the harmonic load-flow (HLF) from a deterministic point of view. Moreover, models based on the probability theory have been developed to deal with the inherent variability and random nature of loads, network configuration etc. In the last few years, possibility theory has arisen as an alternative tool that in many cases could be better suited to describe and quantify the real nature of the uncertainty involved in harmonic studies. In this study a methodology for HLF calculation based on the possibility theory is presented. Possibility distributions instead of probabilities are the input used to describe the uncertainty in the magnitude and composition of the loads. Tests presented shows that the results of the proposed model are consistent with those obtained with a probabilistic method, and that both models lead to the same ranking of the risk that the bus harmonic voltages exceed a given level. Independent possibility distributions are assumed at the development stage reported here; research is being carried out in order to overcome this constraint.
No preview · Article · May 2011 · IET Generation Transmission & Distribution
[Show abstract][Hide abstract] ABSTRACT: Harmonic distortion is a growing problem in all power systems (PS) around the world due to the increasing use of electronic power devices and nonlinear loads (NLL). Several methods have been developed for the computational analysis of PS harmonic load-flow (HLF). These approaches allow harmonic distortion to be estimated at each PS bus when NLLs (the harmonic sources) are distributed throughout a whole network. Some widely accepted deterministic formulations are used in HLF analysis; however, harmonic distortion in PS is a timevarying phenomenon because both linear loads (LL) and NLLs change non-predictably all the time. Moreover, network configuration also varies and such considerations make HLF calculation a mathematical problem which must be able to model the uncertainty associated with input data. Some approaches based on probability theory and others using fuzzy sets and possibility theories have been proposed for modeling such uncertainty. This paper was thus aimed at providing an overview regarding these approaches. The main HLF formulations within probabilistic and possibilistic frameworks have thus been introduced and some numerical comparisons have been made to clarify some concepts raised. Index terms fuzzy set, harmonic distortion, power system harmonics, probability, statistical analysis, uncertainty.
[Show abstract][Hide abstract] ABSTRACT: This work illustrates a network equivalent multirate model based in a Laplacian Pyramid Filter bank of two channels: a low-frequency decimated channel and a high-frequency non-decimated channel. The first one is a detailed model whereas the second one is a simplified network model. The use of a Laplacian Pyramid scheme allows greater flexibility to select the decimation factors and the non-decimated channel model properly reproduces the response in the high-frequency band. Both features, contribute to overcome some limitations that former proposals, based on multirate processing, have experienced. The accuracy and the advantages of the herein proposed model are validated through simulations in a real case.
[Show abstract][Hide abstract] ABSTRACT: A new methodology for the estimation of the possible remanent life of a power transformer is presented in this paper, considering several types of uncertainties which are involved in the solution of this problem. The premise is to consider the transformer as individual equipment and not as a part of a group of transformers, so as to get its remanent life considering for this purpose the condition of the insulating paper and the location of the substation in which the transformer is installed regarding the power system network. An interval of remanent possible life is estimated, using intervals of possibility, which are obtained from experts' opinion. The proposed evidences (input variables) to be taken into account in order to obtain these intervals are the following: the risk of short circuits external to the transformer regarding the condition of the paper insulation, the diagnosis obtained from the dissolved gas analysis of the transformer tank oil and another variables that influence the condition of the transformer insulation, the risk due to the load and temperature and the loss of life of the transformer. Each one of these parameters is considered as an influence index taken as condition level evidence in obtaining a possible interval of remanent life. The external short circuit risk index considers the stochastic nature which is present in external faults, as well as the degree of polymerization of the insulating paper. The external short circuit risk index is obtained by contrasting the insulation paper condition with the probability that the transformer withstands the short-circuit current flowing along the winding during an external fault. In order to assess the risk, this probability and the value of the degree of polymerization of the insulating paper are regarded as inputs of a type-2 fuzzy logic system (T-2 FLS), which computes the fuzzy risk level. The index related to the diagnosis by means of dissolved gases analysis and another variables, evaluates the condition of the transformer insulation and the result is given using words easy to understand by the maintenance personnel. This tool is a translator that converts the diagnosis from the dissolved gas analysis of the transformer oil in words, obtaining a diagnosis expressed in a simple form which is easy to understand. These two indexes are derived using T-2 FLS which allow to take into account the uncertainty present in the words. In the same way, in the evaluation of the condition of the transformer insulation the recommended procedures given by the present international standards are used, such as IEC 60599 method and Duval's triangle. Furthermore, the risk index due to load and temperature is the one proposed for Weihui Fu et al., which is based on the history of load and temperature of the transformer in order to obtain a risk level for the near future. The methodology is tested shown useful results.
[Show abstract][Hide abstract] ABSTRACT: A novel methodology to assess the risk of power transformer failures caused by external faults, such as short-circuit, taking the paper insulation condition into account, is presented. The risk index is obtained by contrasting the insulation paper condition with the probability that the transformer withstands the short-circuit current flowing along the winding during an external fault. In order to assess the risk, this probability and the value of the degree of polymerization of the insulating paper are regarded as inputs of a type-2 fuzzy logic system (T2-FLS), which computes the fuzzy risk level. A Monte Carlo simulation has been used to find the survival function of the currents flowing through the transformer winding during a single-phase or a three-phase short-circuit. The Roy Billinton Test System and a real power system have been used to test the results.
Full-text · Article · Apr 2009 · Electric Power Systems Research
[Show abstract][Hide abstract] ABSTRACT: Harmonic distortion is a growing problem in power systems due to the increasing use of power electronic devices and loads. A possibilistic methodology for harmonic load-flow calculations based on fuzzy numbers has recently been proposed by the authors as an alternative to probabilistic approaches for modeling uncertainties in linear and nonlinear loads. This paper presents a methodology to aggregate uncertainly known bus loads, whose magnitudes and composition are described through fuzzy parameters, into a fuzzy admittance and injected harmonic currents of a possibilistic harmonic load-flow calculation. Interaction between fuzzy parameters (the possibilistic counterpart of probabilistic dependence) is properly modeled. A numerical example is given in the last section.
[Show abstract][Hide abstract] ABSTRACT: Nonlinear devices are being increasingly used in power systems and, as a consequence, harmonic distortion is a continuously growing phenomenon. Since linear and nonlinear loads are usually uncertain variables that, in addition, vary constantly, probabilistic methods for harmonic load-flow calculation have been developed to quantify the resulting random harmonic voltages. More recently, a possibilistic harmonic load-flow based on fuzzy sets theory has been proposed with the same objective. Although the possibilistic approach seems better suited to deal with the kind of uncertainty usually found in practice, the Classic Fuzzy Solution on which this methodology relies exhibits some draw-backs. This paper proposes a possibilistic harmonic load-flow based on the Marginal Joint Solution, which overcomes two major limitations of the Classic Fuzzy Solution approach: the lack of fuzzy linear loads models, and the over-and underestimation of the harmonic voltages.
No preview · Article · Jul 2008 · Latin American applied research Pesquisa aplicada latino americana = Investigación aplicada latinoamericana
[Show abstract][Hide abstract] ABSTRACT: In this work a novel algorithm to evaluate the condition of power transformers is presented. The algorithm evaluates the condition of the equipment via dissolved gas analysis. Firstly, the condition is evaluated using the gas ratio of IEC and the Triangle of Duval. Then, the vector of possible disorders is used as input of a Type-2 Fuzzy Logic System. In this way, the condition is obtained and expressed in words easy to understand by the maintenance personnel. The efficiency of the algorithm has been tested with field data obtained in the bibliography. The validity of the algorithm is shown by means of an example.