Model Predictive Control for paralleled UPS systems with load-side neutral wire

Uninterruptible Power Supplies (UPS) play a key role in feeding a wide range of critical applications such as high-tier datacenters and medical facilities. Given the variety of loads that typically exist in such applications, UPS systems should be able to correctly operate under highly unbalanced load conditions and to simultaneously supply single- and three-phase loads. On the other hand, UPS systems are usually parallel-connected in order to improve the overall system feasibility and/or to increase the overall installed power capacity. In this paper, a system topology based on two paralleled UPS systems with neutral wire at the load-side is proposed. Moreover, a new Finite Control Set Model Predictive Control (FCS-MPC) strategy is also proposed to control all converters of the paralleled systems. The proposed technique permanently ensures: the generation of a high-quality load voltage waveform; elimination of the undesired circulating current that can flow between both UPS systems; a fully controlled load power distribution. Simulation results are presented, demonstrating the effectiveness of the proposed strategy even under highly unfavourable load conditions.

I. Introduction

Uninterruptible Power Supplies (UPS) have proven to be a necessary technology to supply a wide range of highly sensitive and important applications in electric power systems. These systems are used either on low-power applications such as domestic computers and small networks or high-power applications such as high-tier datacenters, industrial processes and medical facilities. Some of these applications only require the connection of Three-Phase Three-Wire (3P3W) loads and in this case, no neutral wire is typically necessary. However, there are several applications in which a wide range of equipment, including single-phase loads of different types, needs to be connected. In this case, the UPS systems to be used should provide a neutral connection at the load-side in order to feed either single- or three-phase loads.