Abstract
The optimization of overcurrent relays’ operation is a topic associated with protection coordination of distribution networks. Usually, this refers to medium-voltage networks, since they are protected by numerical relay devices, as opposed to low-voltage networks, where utility operators allocate fuses. Correct setting of relays and optimal coordination is becoming a serious challenge to Distribution Network Operators around the world, since their networks’ passive operation has been greatly altered in the past two decades. Distributed generation units, a growing liberalized electricity market and more stringent legislation for distribution network planning and operation by state regulatory bodies have all indirectly affected the evolving of protection philosophy for distribution networks. In this paper the traditional optimization problem of overcurrent relay operation will be addressed and critically examined from both a theoretical and practical point of view. Optimization function, constraints and relay parameters will all be observed and compared with solutions used in distribution networks, and their modifications and improvements will be proposed and elaborated in detail.
Highlights
Background Due to the accelerated expansion of the distribution electric power grid caused by increased consumer demand and the need for increasingly reliable service that complies with regulatory requirements, strong and robust protection of the distribution electric power system is necessary
Different power system configurations with their own protection coordination philosophies have unique advantages and disadvantages [1], which means that each part of the distribution power system must be properly protected to avoid a power outage and its consequences regarding equipment malfunction and failure [2]
Definite-time Overcurrent relays (OCR) are usually used for protection of lines that are not characterized with high inrush currents and whose impedance does not change very much with their length
Summary
Overcurrent relays can be divided roughly into two types, judging by their time– current characteristic: definite-time OCRs and inverse-time OCRs [9,10]. Definite-time OCRs are usually used for protection of lines that are not characterized with high inrush currents and whose impedance does not change very much with their length The latter is indicated for relatively short networks that contain lines with larger cross-sections. Inverse-time OCRs are usually used for protection of elements with high st3aortf-20 up currents (asynchronous motors) or magnetizing currents (transformers) and lines whose impedance changes significantly with their length (relatively long networks). A 2-phase fault needs to be calculated on the LV busbars of the associated MV/LV substation 1 and at the end of the 2nd line section The lower of those 2-phase currents will be used to set Ipick-up of relay R1, since it has to detect the smallest short-circuit current in its reserve zone of protection. As stated, definite-time relays cannot be included in the optimization problem since using them nullifies Equation (3) in the subsequent section, which is central for the optimization problem
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