An adaptive overcurrent protection system applied to distribution systems

Abstract

In this paper, an innovative adaptive directional overcurrent protection system for electric power distribution systems with respect to distributed generation is proposed. The proposed system supervises network topology based on the monitoring functionality of numerical relays. The system detects any changes in the configuration and recalculates the directional overcurrent protection settings by using a microgenetic algorithm. The proposed system was evaluated for several operating scenarios and insertion levels of distributed generation, and then compared with both conventional and adaptive protection systems by means of a traditional genetic algorithm. The results showed that the performance of the proposed system is superior to the other two methods in terms of both speed and selectivity. This shows that this is a promising proposal for the protection of modern electric power distribution systems.

Introduction

The supply of electricity is undoubtedly a great worldwide concern given our ever-increasing dependence on electricity. To ensure the quality of the electric power provided by utility companies, electricity regulatory agencies monitor several parameters. One of these parameters is the âǣcontinuity of serviceâǥ, which can be impaired by many kinds of disturbances; fortunately, the electrical systems have protection systems whose function is to ensure the continuity of electricity supply to users. In the case of the electrical power distribution systems (EPDS), the protection task is mainly performed by overcurrent devices which must be programmed to act in a coordinated manner; however, in systems containing numerous protection devices, their coordination becomes difficult. This problem has become increasingly complex due to the increasing prominence of distributed generation (DG), which is a common term that refers to the plant which is directly connected to EPDS and is not centrally planned and dispatched [1].

Despite its many advantages, the insertion of generating units in EPDS can cause several problems, for example, changes in the classical protection schemes and bidirectional power flow. To mitigate these problems, several strategies have been studied, among which the so-called adaptive protection strategy stands out, which can be understood as a protection strategy that aims to automatically adjust the protection functions to the prevailing operating conditions of the system. In practice, this strategy can be implemented in several ways, for example, by using:

• Protection based on communication schemes [2], [3]: this enables high performance in terms of the operation speed; however, it calls for massive investments in communication technology.

• Automatic selection of setting groups [4], [5], [6]: shows good performance in situations for which the settings have been programmed, however, due to the limited number of setting groups that can be stored in digital relays, only a few operating scenarios can be contemplated.

• Online monitoring and calculation of relay settings [7], [8], [9]: comprises one of the most widespread methods, for there are no scenario limitations, but does not guarantee the best possible settings, as it does not usually use optimization methods.

• Optimization methods to perform real-time coordination [10], [11], [12] : this derives from iii, with the additional assurance of optimal settings for all possible scenarios; however, it requires high processing because it uses optimization methods, which usually require high computational effort.

In addition to these, several other techniques have been proposed to solve this problem, such as usage of fault current limiters, optimal relay coordination, etc. The state of the art of (iv) was analyzed and it has revealed that, to date: a) the use of optimization methods to perform the coordination of directional overcurrent relays (DOCR) in EPDS in the presence of DG is a relatively incipient line of research; b) the works involving optimization methods present the most efficient solutions; however, one of the disadvantages of the methods used in this category is low processing speed. Nevertheless, developments in processing technology are enabling the production of processors with increasingly better performance; therefore, this drawback will be overcome in time.

The current work fits into this context and proposes an adaptive protection system for primary EPDS in presence of DG which was based on the monitoring of circuit breaker status and active power at monitoring points near DG centers. These signals can be easily obtained from the protection system and can be processed for decision making. A microgenetic algorithm (µGA) was used to perform the optimal coordination of DOCR. This choice was due to its robustness and speed [13], which are essential characteristics for solving the protection problem.

To validate the proposed adaptive protection system, that will be referred to as APSµg, a database with several fault cases was simulated using the Simulink/MATLAB^Ⓡ platform. Subsequently, the proposed system was compared with the conventional protection system, that will be referred to as CPS, and an adaptive protection system with the traditional genetic algorithm, that will be referred to as APSga.

The article is organized as follows: Section 2 has a brief description of the coordination of directional overcurrent relay; In Section 3, we discuss the optimal coordination of DOCR; Section 4 contains the proposal for an adaptive protection system; In Sections 5 and 6, the methodology used, and results obtained are discussed; Finally, Section 7 contains the conclusions.