Performance analysis of parallel-processing UPS systems

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...

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“…is the rate of change of the frequency deviation in rad/sec 2 and is the rate of change of the voltage deviation in V/sec. From Fig.…”

Section: B Drooping Coefficients Selectionmentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

Line-Interactive UPS for Microgrids

Abusara

Guerrero

Sharkh

2014

IEEE Trans. Ind. Electron.

162

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“…According to (4) and (5), the small-signal model of the dc-link voltage control is presented in Fig. 7.…”

Section: Limitation Of the Traditional Model To Design The Contrmentioning

confidence: 99%

Line-Interactive UPS for Low-Voltage Microgrids

Zhang¹,

Cai²,

Zhao³

et al. 2015

Journal of Power Electronics

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Line-interactive uninterruptible power supply (UPS) systems are good candidates for providing energy storage within a microgrid. In this paper, a control scheme for a line-interactive UPS system applied in a low-voltage microgrid is presented. It is based on the Q-w and P-E droop control to achieve a seamless transition between grid-connected and stand-alone operation modes. Moreover, a new model for designing the controllers is built in the dq-frame based on the instantaneous power definition.The new-built model takes into account the dynamic performance of the output impedance of the inverter in the dq-frame and can be evaluated in the time domain. Compared to the traditional model based on the instantaneous power definition, the new-built model is more accurate to describe the dynamic performance of the system. Simulation and experimental results obtained with a microgrid consisting of two 40-kW line-interactive UPS systems are given to validate the control strategy of the line-active UPS system and the accuracy of the new-built model.

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“…Thereby, active filter topologies have been used to compensate neutral harmonic currents (Quinn et al, 1992;Quinn et al, 1993;Thomas el al., 1996). Uninterruptible power supply (UPS) systems have enabled the improvement of power source quality, providing clean and uninterruptible power to critical loads such as industrial process controls, computers, medical equipment, data communication systems, and protection against power supply disturbances or interruptions (Oliveira da Silva et al, 2001;Kamran et al, 1995;Jeon et al, 1997, Cheung et al, 1996Lin et al, 1993). In (Lin et al, 1993) a threephase parallel processing UPS has been presented with harmonic and reactive power compensation, but the output voltages and the input currents cannot be controlled simultaneously.…”

Section: Introductionmentioning

confidence: 99%

“…Uninterruptible power supply (UPS) systems have enabled the improvement of power source quality, providing clean and uninterruptible power to critical loads such as industrial process controls, computers, medical equipment, data communication systems, and protection against power supply disturbances or interruptions (Oliveira da Silva et al, 2001;Kamran et al, 1995;Jeon et al, 1997, Cheung et al, 1996Lin et al, 1993). In (Lin et al, 1993) a threephase parallel processing UPS has been presented with harmonic and reactive power compensation, but the output voltages and the input currents cannot be controlled simultaneously. Three-phase UPS systems with seriesparallel active power-line conditioning have been proposed using different control strategies (Oliveira da Silva et al, 2001;Kamran et al, 1995).…”

Section: Introductionmentioning

confidence: 99%