Abstract-Line interactive Uninterruptable Power Supply (UPS) systems are good candidates for providing energy storage within a microgrid to help improve its reliability, economy and efficiency. In grid-connected mode, power can be imported from the grid by the UPS to charge its battery. Power can also be exported when required, e.g., when the tariffs are advantageous. In stand-alone mode, the UPS supplies local distributed loads in parallel with other sources. In this paper, a line interactive UPS and its control system are presented and discussed. Power flow is controlled using the frequency and voltage drooping technique to ensure seamless transfer between grid-connected and stand-alone parallel modes of operation. The drooping coefficients are chosen to limit the energy imported by the USP when re-connecting to the grid and to give good transient response. Experimental results of a microgrid consisting of two 60kW line interactive UPS systems are provided to validate the design. Index Terms-Line interactive UPS, microgird, distributed generation. I. INTRODUCTIONO INCREASE reliability, energy storage systems within a microgrid are essential. Energy is stored while in grid-connected mode, when the microgrid's Distributed Generation (DG) systems produce excess power, to be used later to supply critical loads during power outages. In stand-alone mode, they can be used to boost the power supplied by the microgrid if the DG systems cannot meet the expected level of power. To meet these demands, the energy storage system needs to be able to work in grid-connected and stand-alone modes. In the latter mode of operation, the system needs to operate in parallel with other DG systems to meet the variable power demand of the load. More importantly, it needs to switch seamlessly between the two modes.Line interactive UPS systems are good candidates for providing energy storage within microgrids as they can be connected in parallel with both the main grid and local load. The classical topology of line interactive UPS systems [1], [2] is simpler, cheaper, more efficient and more reliable than the on-line double conversion UPS. This topology, however, does not provide voltage regulation to the load. Voltage regulation is M. A. Abusara was with Bowman Power Group, UK and now he is with the Renewable Energy Research Group, University of Exeter, Cornwall Campus, Penryn, TR10 9EZ, U.K. (e-mail: m.abusara@exeter.ac.uk).J. M. Guerrero is with the Department of Energy Technology, Aalborg University, 9220 Aalborg, Denmark (e-mail: joz@et.aau.dk).S. M. Sharkh is with the Electro-Mechanical Engineering Research Group, University of Southampton, Highfield Campus, Southampton, SO17 1BJ, U.K. (e-mail: suleiman@soton.ac.uk).possible as in series-parallel or delta conversion line-interactive UPS [3]-[6] at the expense of lower efficiency and extra size and cost due to the use of extra inverter and bulky transformer. However, this topology is still more efficient than classical on-line double conversion UPS because the complementary inverte...
In inverter-based microgrids, the paralleled inverters need to work in grid-connected mode and stand-alone mode and to transfer seamlessly between the two modes. In grid-connected mode, the inverters control the amount of power injected into the grid. In stand-alone mode, however, the inverters control the island voltage while the output power is dictated by the load. This can be achieved using the droop control. Inverters can have different power set-points during grid-connected mode but in stand-alone mode they all need their power set-points to be adjusted according to their power ratings. However, during sudden unintentional islanding (due to loss of mains), transient power can flow from inverters with high power set-points to inverters with low power set-points, which can raise the DC link voltage of the inverters causing them to shut down. This paper investigates the transient circulating power between paralleled inverters during unintentional islanding and proposes a controller to limit it. The controller monitors the DC link voltage and adjusts the power set-point in proportion to the rise in the voltage. A small signal model of an islanded microgrid is developed and used to design the controller. Simulation and experimental results are presented to validate the design. Index Terms-microgrid, droop control, power sharing, unintentional islanding.
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