MICROGRID OPERATION AND CONTROL

Abstract

Due to everlasting electricity demand, the worldwide power system networks are under tremendous pressure and there is very strong need of strengthening our infrastructurerelatedtopowergenerationandtransmissioncapabilities. However, withrisingconcernabouttheenvironmentalissuesofconventionalgenerationand theirlimitedresources,alotofemphasisisbeinggiventorenewableenergysources (RES) based distributed generation (DG). As per the nomenclature the DG’S are to be integrated to main grid at distribution level and this eliminate the need of installing newer centralized power generation facilities and corresponding transmission line infrastructure. The integration of RES at distribution level has its own demerits primarily related to their intermittency and control. Recently, the conceptofMicrogridisgaininglotofattentionworldwide,asitcanbeveryhelpful in controlling the RES both in grid tie mode as well as in island mode. Thus, the microgrid may have its own generation and load, where it can remain grid tie in case of normal operation while it can isolate itself and keeps on feeding the localized load in case of any fault on grid side. However, microgrid has many issues regardingtheiroperation, stability, powerqualityandprotection. Moreover, most ofthemicrogridtechnologyinvolvespowerelectronicsbasedinterfacewhichmakes it almost inertia less system. Thus, the inherent advantage of transient stability by virtue of inertia is lost in case of microgrid. Therefore, to achieve stable operation of microgrid with enhanced power quality under intermittent generation and dynamic load conditions are one of the major issues. In modern distribution system, most of the loads are non-linear in nature and draw harmonic current. The variable load with the non-linear characteristics may distort the output voltage and degrade the power quality. Hence, major requirements of microgrid is to keep on feeding its connected load at regulated voltage and frequency with stable operation in transient condition, which are difficult to achieve especially in inverter based microgrid. The various control methodologies and control layers (Primary, secondary and tertiary etc.) have been used in past literature to overcome these issues. Therefore, the proposed research mainly deals with the two problems, first oneisimprovementofpowerqualityunderharmonicandunbalanceloadcondition v and second is the stable operation of microgrid having reconfigurable architecture which leads to huge variation in grid parameters. For the first problem, a novel repetitivecontrolhasbeenusedtoenhancepowerqualityofvoltagesourceinvertersindroopbasedmicrogridwhileforsecondproblema L1 baseddroopcontroller has been proposed. It is well known that variation in grid parameters may not be easily handled by conventional droop controllers which are mainly designed while assuming fixed grid configuration. However, these assumptions become invalid for a microgrid having small mesh network with reconfigurable structure. Therefore, it is most important for a microgrid to remain stable not only during various changes in droop characteristics but also during dynamic topological changes. For validation of methods and control strategy, real world hardware prototype has been developed. The hardware consists of two inverters (3-ph) with battery sources connected via variable transmission line impendences. Various types of load are connected to output of inverters and common points. These inverters are controlled by FPGA development board.