A framework for reliability assessment in expansion planning of power distribution systems

This article proposes a framework that uses analytical assessment of reliability to guide the expansion planning of power distribution systems (PDS) considering reliability criteria. The framework allows the estimation of reliability indices with and without the execution of expansion projects, thus supporting the decision-making process on investments in expansion projects. In the analytical assessment of reliability, failure rates of zones and restoration times are calculated from past data of interruptions in the primary distribution network. In addition, the estimated reliability indices are adjusted to historical values through failure rates proportionate to the length of each zone. To test and validate the proposed framework, it was applied to the distribution network at bus 5 of the Roy Billinton Test System (RBTS) and also to a real distribution feeder located in Brazil. The results indicated that the proposed framework can help define the most attractive investments leading to improvements in reliability indices and reduction in unsupplied energy. The estimation of reliability indices and energy not supplied, considered the following expansion alternatives: (i) the installation of normally-closed sectionalizing switches, (ii) the installation of normally-open switches with interconnection to adjacent feeders, (iii) the automation of switches, and (iv) the reconductoring of zones of the primary distribution network. Nevertheless, the proposed framework allows the inclusion of other expansion alternatives. Finally, the proposed framework proved to be handy and useful for real-life applications

MILP model for volt-var optimization considering chronological operation of distribution systems containing DERs

This paper presents a mixed-integer linear programming model for volt-var optimization considering the chro-nological operation of distribution systems containing distributed energy resources (DERs). The proposed model describes the operation problem of capacitor banks (CBs) and voltage regulators (VRs), and it is further based on the steady-state operation during each time interval contained in typical scenarios of distribution systems. A procedure using a K-means clustering algorithm is used to select the scenarios, thus preserving the simultaneity and chronological combination of different loads and DERs. According to the formulation that we developed, the regulation devices become sensitive to downstream load variations, since we use explicit current variables to control automatic CBs, and since we include means to compensate voltage drops along distribution lines in the control of VRs. The model is validated by comparing the results obtained during several tests of two typical cases with those obtained through nonlinear power flow. The typical case studies presented in the paper highlight the good agreement between the results obtained with the linearized model and with power flow method; further, the practical results confirm that the use of typical scenarios allows representing different levels of loads and DERs, while keeping the validity and performance of the proposed model

Expansion planning of power distribution systems considering reliability : a comprehensive review

One of the big concerns when planning the expansion of power distribution systems (PDS) is reliability. This is defined as the ability to continuously meet the load demand of consumers in terms of quantity and quality. In a scenario in which consumers increasingly demand high supply quality, including few interruptions and continuity, it becomes essential to consider reliability indices in models used to plan PDS. The inclusion of reliability in optimization models is a challenge, given the need to estimate failure rates for the network and devices. Such failure rates depend on the specific characteristics of a feeder. In this context, this paper discusses the main reliability indices, followed by a comprehensive survey of the methods and models used to solve the optimal expansion planning of PDS considering reliability criteria. Emphasis is also placed on comparing the main features and contributions of each article, aiming to provide a handy resource for researchers. The comparison includes the decision variables and reliability indices considered in each reviewed article, which can be used as a guide to applying the most suitable method according to the requisites of the system. In addition, each paper is classified according to the optimization method, objective type (single or multiobjective), and the number of stages. Finally, we discuss future research trends concerning the inclusion of reliability in PDS expansion planning

Modelo linearizado para problemas de planejamento da expansão de sistemas de distribuição

Este trabalho apresenta um modelo linearizado para ser utilizado em problemas de planejamento da expansão de sistemas de distribuição de energia elétrica (SDEE) com geração distribuída (GD), em um horizonte de curto prazo. O ponto de operação em regime permanente é calculado através de um modelo linearizado da rede, sendo as cargas e geradores representados por injeções constantes de corrente, o que torna possível calcular as correntes nos ramos e as tensões nas barras através de expressões lineares. As alternativas de expansão consideradas são: (i) alocação de bancos de capacitores; (ii) alocação de reguladores de tensão; e (iii) recondutoramento. Ainda, o modelo considera a possibilidade de seleção do tap dos transformadores de distribuição como alternativa para a redução das violações de tensão. A flexibilidade do modelo permite obter soluções considerando a contribuição das GDs no controle de tensão e potência reativa sem a necessidade de especificar uma tensão para a barra da subestação. O modelo de otimização proposto para a solução destes problemas utiliza uma função objetivo linear, além de restrições lineares e variáveis contínuas e binárias. Dessa forma, o modelo de otimização pode ser representado como um problema de programação linear inteira mista (PLIM) A função objetivo considera a minimização dos custos de investimento (aquisição, instalação e remoção de equipamentos e aquisição de condutores) e dos custos de operação, que correspondem aos custos anuais de manutenção somados aos custos das perdas de energia e das violações dos limites de tensão. A variação da carga é representada através de curvas de duração, sendo que os custos das perdas e das violações são ponderados pela duração de cada nível de carregamento. Utilizando uma abordagem de PLIM, sabe-se que existem condições suficientes que garantem a otimalidade de uma dada solução factível, além de permitir que a solução seja obtida através de métodos de otimização clássica. O modelo proposto foi implementado na linguagem de programação OPL e resolvido utilizando o solver comercial CPLEX. O modelo foi validado através da comparação dos resultados obtidos para cinco sistemas de distribuição com os resultados obtidos utilizando um fluxo de carga convencional. Os casos analisados e os resultados obtidos demonstram a precisão do modelo proposto e seu potencial de aplicação.This work presents a linearized model to be used in short-term expansion planning problems of power distribution systems (PDS) with distributed generation (DG). The steady state operation point is calculated through a linearized model of the network, being the loads and generators modeled as constant current injections, which makes it possible to calculate the branch currents and bus voltages through linear expressions. The alternatives considered for expansion are: (i) capacitor banks placement; (ii) voltage regulators placement; and (iii) reconductoring. Furthermore, the model considers the possibility of adjusting the taps of the distribution transformers as an alternative to reduce voltage violations. The flexibility of the model enables solutions that includes the contribution of DGs in the control of voltage and reactive power without the need to specify the substation voltage. The optimization model proposed to solve these problems uses a linear objective function, along with linear constraints, binary and continuous variables. Thus, the optimization model can be represented as a mixed integer linear programming problem (MILP) The objective function considers the minimization of the investment costs (acquisition, installation and removal of equipment and acquisition of conductors) and the operation costs, which corresponds to the annual maintenance cost plus the costs related to energy losses and violation of voltage limits. The load variation is represented by discrete load duration curves and the costs of losses and voltage violations are weighted by the duration of each load level. Using a MILP approach, it is known that there are sufficient conditions that guarantee the optimality of a given feasible solution, besides allowing the solution to be obtained by classical optimization methods. The proposed model was written in the programming language OPL and solved by the commercial solver CPLEX. The model was validated through the comparison of the results obtained for five distribution systems with the results obtained through conventional load flow. The analyzed cases and the obtained results show the accuracy of the proposed model and its potential for application

Avaliação do impacto dos controladores de excitação na estabilidade transitória de geradores síncronos conectados em sistemas de distribuição

A conexão de geradores em sistemas de distribuição, usualmente chamados de geradores distribuídos, traz novos aspectos técnicos que devem ser analisados pelas empresas distribuidoras, uma vez que os dispositivos de proteção e controle, bem como as técnicas de planejamento e otimização da operação destes sistemas consideram um sistema radial, ou seja, sem a presença de geradores. Em se tratando de geradores síncronos, sabe-se que os sistemas de excitação podem ser equipados com meios para o ajuste automático de tensão, potência reativa ou fator de potência, sendo que três diferentes modos de controle podem ser aplicados a geradores síncronos: regulação de tensão, regulação de potência reativa ou fator de potência e controle de potência reativa ou fator de potência. Ainda, sabe-se que não há um consenso entre as empresas distribuidoras de energia sobre qual o melhor modo de controle destas máquinas, e na maioria dos casos a estratégia de controle de excitação é definida através de um acordo operativo entre a empresa de distribuição de energia e o produtor independente ou autoprodutor que opera o gerador distribuído. Neste contexto, esta dissertação de mestrado tem como objetivo apresentar contribuições com relação à avaliação da estabilidade transitória de geradores síncronos conectados em sistemas de distribuição, avaliando o impacto que os diferentes modos de controle de excitação podem ocasionar, em diferentes condições de operação do sistema. Como parâmetro principal de análise tem-se o tempo crítico de eliminação de falta, obtido através de exaustivas simulações dinâmicas utilizando dados de um sistema real e considerando diferentes condições de operação do sistema, sendo estas: diferentes modos de controle da geração distribuída, modificação do ganho proporcional do controlador de fator de potência, diferentes pontos de operação da geração distribuída, diferentes cenários de carregamento do sistema e diferentes potências de geração.The connection of generators in distribution systems, commonly called distributed generation, brings new technical issues that must be analyzed by the utilities, since protection and control devices, as well as techniques for system planning and operation optimizing consider radial systems, i.e. without the presence of generators. In the case of synchronous generators, it is known that the excitation systems can be equipped with means for automatic adjustment of voltage, reactive power or power factor, and three different control modes can be applied to synchronous generators: voltage regulation, reactive power or power factor regulation and reactive power or power factor control. It is also known that there is no consensus among the power distribution companies about which is the best control mode for use in these machines, and in most cases the excitation control strategy is defined by an operating agreement between the company and the energy producer that operates the distributed generator. In this context, this work aims to provide contributions about the transient stability of synchronous generators connected to distribution systems, evaluating the impacts that different control modes may result in different system operating conditions. The main analysis parameter is the critical clearing fault time, obtained through extensive simulations using real system data. The different system operating conditions considered in the simulations are: different control modes, variation of the power factor controller proportional gain, different operating points of the distributed generation, different system loadings and power generation