Finite Control Set Model Predictive Control for Paralleled Uninterruptible Power Supplies

Abstract

<jats:p>Nowadays, uninterruptible power supplies (UPS) play an important role in feeding critical loads in the electric power systems such as data centers or large communication hubs. Due to the increasing power of these loads and frequent need for expansion or redundancy, UPS systems are frequently connected in parallel. However, when UPS systems are parallel-connected, two fundamental requirements must be verified: potential circulating currents between the systems must be eliminated and the load power must be distributed between the systems according to UPS systems availability. Moreover, a high-quality load voltage waveform must be permanently ensured. In this paper innovative control strategies are proposed for paralleled UPS systems based on Finite Control Set Model Predictive Control (FCS-MPC). The proposed strategies simultaneously provide: controlled load power distribution, circulating current suppression and a high-quality load voltage waveform. A new dynamic converters deactivation mechanism is proposed. This new technique provides improved overall system efficiency and reduced power switches stress. In this paper, two multilevel based UPS systems are parallel-connected. Each UPS contains two three-level Neutral Point-Clamped-Converters (3LNPC) and a three-level DC-DC converter. The presented experimental results demonstrate the effectiveness of the proposed control strategies in several operating conditions.</jats:p>

Model Predictive Control for Paralleled Uninterruptible Power Supplies with an Additional Inverter Leg for Load-Side Neutral Connection

“…In some of these applications, only Three-Phase Three-Wire (3P3W) loads are required to be supplied. In this case, no neutral wire in the UPS systems is required [1][2][3]. However, there are several applications in which both three-phase and single-phase loads need to be supplied [4][5][6].…”

“…Moreover, for the same DC bus voltage, the semiconductors are subjected to a lower voltage value. The 3-Level Neutral Point Clamped Converter (3LNPC) corresponds to a multilevel topology that has been largely used in the industry and significantly investigated by the scientific community in a wide range of applications, including UPS systems [2,3]. Thus, in this work, all used UPS converters consist of a 3LNPC topology.…”

“…UPS systems are connected in parallel basically due to two reasons: expansion of the installed power capacity (to meet the increase of critical load power in a facility) and increase in the global system resilience (to protect the critical load from possible internal UPS system failures). However, when power converters are parallel-connected an undesirable Zero Sequence Circulating Current (ZSCC) can appear and as shown in [3,15] this current highly distorts UPS systems phase currents, rises their magnitudes to dangerous values, and increases the global active power losses. Given that a fundamental requirement for the correct operation of paralleled UPS systems is the elimination of this current, the main strategies for circulating current elimination in paralleled UPS systems are mainly based on conventional PI control approaches, which typically use a dedicated control loop to eliminate the circulating current [16][17][18].…”

Fault Analysis and Non-Redundant Fault Tolerance in 3-Level Double Conversion UPS Systems Using Finite-Control-Set Model Predictive Control

“…In [74] a full 3-level UPS system is studied, considering all UPS converters (rectifier, inverter and DC-DC) and all operation modes. In [87,88] FCS-MPC techniques are proposed to control full UPS modules (rectifier+inverter) connected in parallel. These studies use both 2-level [87] and 3-level [88] topologies.…”

“…In [87,88] FCS-MPC techniques are proposed to control full UPS modules (rectifier+inverter) connected in parallel. These studies use both 2-level [87] and 3-level [88] topologies. However, the DC-DC converter and UPS battery bank are overlooked.…”