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

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.

Sliding Mode Control with Minimization of the Regulation Time in the Presence of Control Signal and Velocity Constraints

“…t f ≤ t 0 . In this case, the state reaches the desired point along the non-stationary switching curve and the regulation time is given by Equation (23). Calculating the derivative of this function with respect to ι, we obtain:…”

“…Substituting the above value into Equation (23), we obtain the minimum value of the regulation time:…”

“…On the other hand, it is well known that varying the switching frequency in this manner can have unpleasant auditory effects. When dealing with control systems in which maintaining constraints is one of the main factors, model predictive control (MPC) [23,24] is one of the main solutions. However, it has some drawbacks, as it requires online optimization, which makes it difficult to apply to complex systems with short time constants (as the computational power required to perform the optimization can become prohibitively expensive in these cases).…”

“…Let us observe that for t = 0, the constraint on ι is of the form (33). If t = t f , then by substituting Equation (23) into inequality (29), we obtain that the control signal is limited, when:…”

A Linear Matrix Inequality Approach to Optimal Voltage Control of a Three-Phase UPS Inverter under Unbalanced Loads

“…The purpose of UPS inverters is to safeguard vital equipment and appliances from the negative effects of power outages by maintaining a constant power supply to them. To ensure that the voltage quality is well regulated, a lot of research has been conducted, such as reducing the output voltage distortion [1], improving transient performance [2][3][4], and increasing robustness [5,6]. However, the above-mentioned methods have not taken the unbalanced load conditions into account.…”