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Microgrid is a distribution system composed of a set of micro-generators which are added to a network. One serious challenge facing a microgrid network is designing a proper protection scheme. This is because the fault current in a microgrid is constantly changing due to the presence of distributed generation (DG) resources at all levels of the distribution network and to the fact that they can operate in two islanding and normal modes. This makes the conventional methods inappropriate for microgrid protection. That is, new schemes should be developed for this purpose. This paper is a summary of studies recently carried out in the field of microgrid protection. Along this line, the structure and topology of microgrids are reviewed first. Afterwards, the protective challenges facing DG-equipped distribution networks are discussed. Then, the innovative methods proposed to solve these problems are analyzed. These methods can be divided up into six main categories: changing settings of protective devices at presence of DG units, disconnecting pertinent DG units when faults occur, creating a balance among different DG technologies, using fault current limiter, using smart transformers, and adaptive protection. In so doing, we consider the factors involved in selecting each method. The factors identified are microgrid type and topology, DG type, communication type and delay time, method of fault detection and analysis, relay type, fault type, method of grounding, and use of smart transformers in microgrid. It is hoped that this work will be useful to the researchers in the field of microgrid protection in finding relevant references and designing appropriate methods. [ABSTRACT FROM AUTHOR]

Abstract: Among the several components of distribution systems, protection devices present a fundamental importance, since they aim at keeping the physical integrity not only of the system equipment, but also of the electricians team and the population in general. One of the protective devices playing a vital role in overhead distribution lines is fused cutout. At the era of privatized utilities, the protection devices should be allocated and coordinated optimally to reduce capital investments and the system outage costs. Amid this situation, the mentioned type of the protection device (fuse cutout) has not been studied for economical allocation up to now. In this paper, responding to this need, an accurate reliability model of fuse cutout containing hidden failures is figured out in the shape of a new Markov model. This model is used for economical allocation of the fuse cutouts. On the basis of the proposed model, a methodology for economic allocation of fuse cutouts is presented. This methodology involves the worth of energy not supplied (ENS) of the network and makes a balance between the cost of fuse cutout installation and the benefit of ENS decrease because of the minimizing the faulty zone by using more fuse cutouts. The methodology is tested on a sample distribution network as well as on IEEE 6-bus distribution test system (RBTS). Moreover, its capability on decision making about the fuse cutout placement and its simplicity for implementing on MV overhead lines are displayed. [Copyright &y& Elsevier]

Abstract: Protection system failure is the main cause of cascading outages. It has been shown that the hidden failures lead to cascading outages. However, the effects of the coordination method as well as the hidden failure of the protection systems have not been investigated yet. In this paper, an algorithm based on sequential Monte Carlo simulation is proposed to find the reliability indices for different overcurrent relay coordination methods. The proposed algorithm is applied to the results of different relay coordination methods of a real network. The indices SAIFI, CAIDI, CAIFI and ENS for the results of different relay coordination methods are obtained strictly based on the contribution of the protection failure issues. Then, a comparison is made between the results of the different relay coordination methods in order to show how the coordination methods affect the reliability indices. [Copyright &y& Elsevier]

Abstract: For optimal co-ordination of overcurrent relays, linear programming techniques such as simplex, two-phase simplex and dual simplex are used. Another way of optimal coordination program is using artificial intelligent system such as genetic algorithm (GA). In this paper, a powerful optimal coordination method based on GA is introduced. The objective function (OF) is developed to solve the problems of miscoordination and continuous or discrete time setting multiplier (TSM) or time dial setting (TDS). In other words; the novelty of the paper is the modification of the existing objective function of GA, by introducing a new parameter and adding a new term to OF, to handle miscoordination problems both for continues and discrete TSM or TDS. The method is applied to two different power system networks and from the obtained results it is revealed that the new method is efficient, accurate and flexible. [Copyright &y& Elsevier]

Abstract: Interconnected power systems are multi-loop structured. In such networks, the determination of settings for all overcurrent relays can be carried out in different forms and may be quite complicated. The main problem for coordination is the determination of starting points, i.e. the location of starting relays in the procedure for settings, which is referred to as break points. In this paper, a powerful approach based on expert system is applied. The rules of the expert system include network configuration, protection systems, fault levels, etc. The method is applied to two networks with different configurations, pilot protection and other protection systems. From the obtained results, it is reviled that the new method is efficient, accurate, comprehensive and more optimal than the previously used graph theory. [Copyright &y& Elsevier]

Abstract: This paper presents a method to reduce the number of constraints and detect those that make the optimization of overcurrent relay settings to be infeasible. By reducing the number of constraints, the feasibility of the optimization is increased and the run-time of the program is decreased. It should be noted that, in the optimal coordination methods, the operating times of the relays are minimized in the frame of the objective function subjects to the so-called coordination constraints. However, a new method for optimum coordination of protective relays based on only constraints has been developed by authors of the paper. The pre-processing reduction constraints method introduced in this paper can be used for any optimal coordination of overcurrent relays. The new method is supported by results obtained from a typical test network and a real power system network. [Copyright &y& Elsevier]

Accurate models of overcurrent (OC) with inverse time relay characteristics play an important role for the coordination of power system protection schemes. This paper proposes a new method for modelling OC relays curves. The model is based on fuzzy logic and artificial neural network. The feedforward multilayer perceptron neural network is used to calculate the operating times of OC relays for various time dial settings (TDS) or time multiplier settings (TMS). The new model is more accurate than traditional models. The model has been validated by comparing the results obtained from the new method with linear and nonlinear analytical models. [Copyright &y& Elsevier]