Abstract
Uninterruptible Power Supplies (UPS) have been demonstrated to be the key technology in feeding either single- and three-phase loads in a wide range of critical applications, such as high-tier datacenters and medical facilities. To increase the overall system power capacity and resilience, UPS systems are usually connected in parallel. When UPS systems are parallel connected, a circulating current can rise, inhibiting correct system operation. Moreover, having a controlled load power distribution is another fundamental requirement in paralleled UPS systems. However, strategies to ensure these two topics have not been explored to date for UPS systems with a load-side neutral connection. This paper proposes an innovative Finite Control Set Model Predictive Control (FCS-MPC) strategy that ensures circulating current elimination and controlled load power distribution for paralleled UPS systems that use an additional inverter leg for load neutral point connection. Additionally, a system topology based on two parallel-connected UPS systems that can simultaneously supply single- and three-phase critical loads is proposed. Experimental results show the effectiveness and robustness of the proposed control techniques even when different types of loads are connected to the UPS systems.
Highlights
Nowadays, Uninterruptible Power Supplies (UPS) represent a key technology feeding a wide range of highly sensitive and critical applications, which are increasingly growing in power grids
− R L i LX + v XM − vO0 M, dt with X = { A, B, C }. In this equation vloadX corresponds to the measured load phase voltage; the term i LX denotes the load-side converter (LSC) phase currents; and the term v XM corresponds to the pole voltage of phase X and vO0 M to the Common Mode Voltage (CMV) generated by the converter, O’ in this case being the neutral point of the load
This paper presents a system topology based on two double-conversion paralleled
Summary
Uninterruptible Power Supplies (UPS) represent a key technology feeding a wide range of highly sensitive and critical applications, which are increasingly growing in power grids. One can immediately note that in the first solution, practically no additional hardware is required This can, in some situations, be a significant advantage since, in terms of UPS configuration, only the connection of the neutral wire to the already-existing DC bus capacitors’ mid-point needs to be made, with no complicated changes in the hardware being required (e.g., rearranging a UPS that only supplies 3-phase loads into a UPS that can supply single-phase loads) and a lower cost. Proposal of a system topology that consists of two double-conversion paralleled UPS systems based on a multilevel topology, with a load-side neutral wire that allows the connection of multiple single/three-phase balanced/unbalanced loads; Proposal of an innovative control scheme that simultaneously ensures a high-quality load voltage waveform, circulating current suppression, and a controlled load power distribution between UPS systems. This paper is organized as follows: Section 2 presents all converters’ mathematical models and the circulating current dynamics; Section 3 demonstrates the control references calculation as well as the proposed predictive control strategy; experimental results are shown and discussed in Section 4; in Section 5 the main conclusions of the work are presented
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