OPTIMAL COORDINATION OF OVERCURRENT RELAYS FOR ENHANCED PROTECTION IN AC MICROGRIDS

Abstract

Protection of microgrid is one of major technical challenges for its deployment into the power system. The protection must work properly in all operating conditions such as radial and mesh configurations along with grid connected and islanded modes of microgrid. The frequent variations in operating conditions of microgrid cause a substantial change in magnitude and direction of fault current. For any fault in grid connected mode, both external grid and distributed generations (DGs) supply fault current. Whereas, in islanded mode, fault current is supplied by only DGs available in microgrid. These changes in fault current adversely affect some basic requirements of protection system such as sensitivity (blinding of protection), selectivity (false tripping), reliability (operation of backup relay if primary relay fails), speed (delay in tripping) and it leads to miscoordination between primary and backup relays. In microgrids, directional overcurrent relays (DOCRs) are used to protect feeders due to bi-directional fault currents. For reliable operation of microgrid, primary relay must be operated as quickly as possible to isolate faulted conditions in the network. However, if primary relay fails to clear fault then the backup relay should operate after some specific time interval known as coordination time interval (CTI). For proper coordination of primary and backup relay pairs, the CTI must be greater than some predefined time known as minimum coordination time (MCT). This can be achieved only by providing optimum settings such as pickup current setting (IP ) and time dial setting (TDS) to DOCRs. In general, the DOCRs coordination problem is formulated as an optimization problem and is solved using classical approaches, heuristic techniques and hybrid methods to obtain optimal settings (IP and TDS) of DOCRs. The main objective of DOCRs optimization coordination problem is to minimize the operating times of both primary and backup relays while maintaining coordination constraints. The DOCRs problem can be formulated as a linear programming problem (fixed IP , continuous TDS), a nonlinear programming problem (continuous IP , continuous TDS), and a mixed integer nonlinear programming problem (discrete IP , continuous TDS). In the linear programming (LP) approach, the operating time of DOCR is considered as a linear function, where the optimal value of TDS is determined by fixing the value of IP . Though the LP approach gives a solution quickly, obtaining optimal TDS by i fixing IP cannot guarantee a global optimal solution. In the nonlinear programming (NLP) problem, DOCR operating time is expressed as nonlinear functions of IP and TDS, which is solved to obtain optimal values of both IP and TDS. In mixed integer NLP (MINLP), the coordination problem is solved to obtain discrete values of IP and continuous values of TDS. The concept of a traditional microgrid with only a point of common coupling (PCC) with the main grid is well identified and documented in the literature. However, these traditional microgrids are not guaranteed to provide reliable power to most sensitive loads such as hospitals and airports during emergency conditions. Also, the maximum load handling capacity of stand-alone microgrids is limited to 10 MVA. An alternative options are microgrids with multiple PCCs and multi-microgrids. Microgrids with multiple PCCs have the ability to provide more reliable and stable power to most sensitive loads such as hospitals and airports during emergency conditions. Whereas, multi-microgrids (MMGs) can provide more continuous power to large capacity loads by combining two or more microgrids, which eliminates the need for energy storage and backup power. However, both microgrids with multiple PCCs and multi-microgrids are often operated in various operating modes to enable a reliable power supply. As a result, there is a continuous variation in the level and direction of fault current seen by DOCRs. These fault characteristics of MMG (excessive fault currents, continuous variations in level, and direction of fault currents) should impose several issues of DOCRs protection coordination. In order to exploit the various advantages of these microgrids, it is necessary to develop effective protection coordination schemes. Hence, in this thesis, effective protection coordination schemes are developed for both microgrids with multiple PCCs and multi-microgrids. Also, a hardware-in-loop (HIL) setup is developed to validate DOCRs settings obtained from proposed protection coordination schemes. The HIL setup is developed using a real time digital simulator, CMS-356 (voltage and current amplifier), and digital relay.