Switching frequency reduction for efficiency optimization in two paralleled UPS systems

In this paper, the switching frequency reduction in two paralleled Uninterruptible Power Supplies (UPS) is studied in order to obtain maximum global system efficiency. Typically, by reducing the switching frequency of a converter, its efficiency increases. However, when connected in parallel, UPSs are subjected to a Zero Sequence Circulating Current (ZSCC) that compromises a correct system operation and increases the losses of the global system. Experimental results show that to achieve the maximum system efficiency, the ZSCC must be effectively eliminated. Finite Control Set Model Predictive Control (FCSMPC) is used as the control scheme.

I. Introduction

Over the last decades, Model Predictive Control has been the object of intense research. MPC is typically divided into two main control types: Continuous Control Set Model Predictive Control (CCS-MPC) and Finite Control Set Model Predictive Control (FCS-MPC). In CCS-MPC a continuous control signal is computed. This type of control implies the use of a modulator to generate the computed voltage. FCSMPC has a simpler implementation since it takes advantage of the discrete nature of the power electronics converter. In this type of control, the control set is formed only by the possible converter switching states. The chosen control action is kept during the entire sampling period. Therefore, no modulator is needed.