This paper attempts to present the state of art of research work carried out on the optimal planning of distributed generation (DG) systems under different aspects. There are number of important issues to be considered while carrying out studies related to the planning and operational aspects of DG. The planning of the electric system with the presence of DG requires the definition of several factors, such as: the best technology to be used, the number and the capacity of the units, the best location, the type of network connection, etc. The impact of DG in system operating characteristics, such as electric losses, voltage profile, stability and reliability needs to be appropriately evaluated. For that reason, the use of an optimization method capable of indicating the best solution for a given distribution network can be very useful for the system planning engineer, when dealing with the increase of DG penetration that is happening nowadays. The selection of the best places for installation and the preferable size of the DG units in large distribution systems is a complex combinatorial optimization problem. This paper aims at providing a review of the relevant aspects related to DG and its impact that DG might have on the operation of distributed networks. This paper covers the review of basics of DG, DG definition, current status of DG technologies, potential advantages and disadvantages, review for optimal placement of DG systems, optimizations techniques/methodologies used in optimal planning of DG in distribution systems. An attempt has been made to judge that which methodologies/techniques are suitable for optimal placement of DG systems based on the available literature and detail comparison(s) of each one.

Optimal planning of distributed generation systems in distribution system: A review

This paper shows that the current an inverter interfaced distributed generator (IIDG) contributes to a fault varies considerably, due mainly to fast response of its controller. This paper proposes a method to extend the conventional fault analysis methods so that IIDG contribution can be estimated in the fault analysis. The proposed method gives rms profiles of the fault currents of interest (IIDG contribution and the fault currents the protective device will see). Test results, based on a prototype feeder, show that the proposed approach can estimate the fault current's contributions under both balanced and unbalanced fault conditions.

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Fault analysis on distribution feeders with distributed generators

Radial power distribution systems, common in North America, typically use coordinated relay protection for fault protection. However, the rising interest in distributed generation (DG) poses a problem, as DG causes such systems to lose their radial nature, disrupting the coordinated relay protection. The use of a fault current limiter (FCL) is proposed to limit the effect of the DG on the coordinated relay protection scheme in a radial system during a fault. This paper shows that the FCL enhances the stability of and limits the transient stresses on the DG. Such a device is only recently more plausible with the ongoing developments of a new hybrid mechanical/electrical fault current limiter. To determine the effectiveness of the FCL for the proposed application, test systems are introduced, the effects of DG on the relay protection system are examined, and the effectiveness of FCL to mitigate these effects are determined.

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Application of a fault current limiter to minimize distributed generation impact on coordinated relay protection

A study of a distribution system is presented where the effect of distributed generation (DG) presence is analyzed. The system is typical of a rural, industrial, and low-power (weak) distribution scheme. The concept of specific energy for voltage sag immunity of sensitive equipment (SE) under the new scenario created by the DG presence is applied, especially studying the double magnitude/duration sag that is caused by the change in source impedance due to the response of dissimilar protection devices. The new protection coordination concepts of the not-anymore-radial distribution systems are described. A new methodology for the study of SE behavior under voltage sag when connected to a weak distribution system with DG is proposed.

Coordination of voltage sag and overcurrent protection in DG systems

As a result of deregulation, distributed generation (DG) systems with distributed generators installed in the middle or low voltage networks become popular in distribution systems. Traditional protections developed in radial systems are difficult in coordination and in protecting against high impedance fault (HIF), sometimes it may cause nuisance tripping in the DG systems. A decentralized multi-agent based protection with capability of HIF detection, fault location and load shedding for the DG systems is proposed in the paper. Digital relay is designed as a relay agent which is capable of searching for information from other relay agents, interacting with other relay agents and performing tasks of protection with autonomy and cooperating. The agent based protection scheme is also presented. EMTP simulation results show that the proposed agent based protection scheme can remove the influence of load switching operations, protect against HIF, electric shock. The prototype of relay agent is developed, the research is now in progress on testing the prototype in dynamic power system.

Multi-agents based protection for distributed generation systems

S ensitive equipment protection against voltage sags can be given for overcurrent protective devices, provided that careful coordination has been carried out. The inclusion of distributed resources (DR) in a distribution system creates the possibility of an important change in the coordination procedure. Overcurrent protection and voltage sag coordination studies are based on the upstream impedance of the point of common coupling (PCC) or system impedance (Z1). Voltage sag magnitude is obtained taking into account the voltage drop across (ZI). Sag duration depends on the operating time of the overcurrent protective device. The normal coordination procedure is obtained by comparing the sensitive equipment (SE) dropout curve with the time-voltage characteristic (TVC) of the protective device. TVC is obtained from the time-current characteristic using (ZI) as the the conversion factor. Under normal system conditions, (Zi) is approximately constant, but, in the case of distributed generation (DG), this impedance can change from a low value (utility plus distributed generators) to a high value when the nonutility generators are the only supply (emergency). The new scenario implies that the protective device TVC is changed into a zone that modifies previous methodologies and increases the SE dropout susceptibility. The new situation should be considered, since SE will be kept operating under emergency conditions. An example using fuses as protective device is illustrated in this article.

J. C. Gomez

Papers

Coordinating Overcurrent Protection and Voltage Sag in Distributed Generation Systems