Modeling and analysis of an islanded, low voltage,inverter-based microgrid

Abstract

The penetration of Voltage Source Converters (VSCs) interfacing renewable energysources in today’s power system has experienced a steep incline. In microgrids, thepenetration of VSCs can reach 100% during times when the load is supplied exclu-sively through renewables in combination with battery storage, providing a researchfield regarding VSC interaction and control.In this thesis, a real low-voltage, islanded microgrid in Thailand is modelled inSimulinkwith theSimscape Power Systemslibrary. The system consists of a 40 kWPV array, 240 kWh lead-acid battery and a 50 kW diesel generator with a peak loadof 25 kW at a lagging power factor of 0.95. The diesel generator has been omittedto simulate an inverter-based microgrid and focus on VSC interaction.The battery-interfacing VSC is controlled as a grid-forming converter, whereasthe PV-interfacing VSC is operated in grid-feeding mode. Both VSCs are controlledin the(qd)-frame. During peak-load condition the simulation shows a minimumvoltage of 0.89 pu at the load bus. Therefore, a third, small-scale battery-interfacedVSC in closer proximity to the load is integrated and operated in grid-supportingmode with the objective to maintain a minimum load bus voltage of 0.95 pu.It is shown that a 21 kVA-rated VSC can provide adequate voltage regulationthrough active power injection if operated in current-control mode withPv-droopcontrol, given the resistive nature of the low voltage lines. Reactive power can beshared among the VSCs through frequency adjustments of the grid-forming con-verter. The droop control of the grid-supporting VSC is extended with a dead bandto regulate its participation to voltage control. The simulations show that the mi-crogrid remains stable during large load steps, if the droop constants of the formingconverter are below≈4%