Asian Power Electronics Journal, Vol. 6, No. 1, Oct 2012 1 Novel Reduced Switches Single-Phase to Three-Phase On-line Uninterruptible Power Supply Mehrnaz Sharifian 1 Mehdi Niroomand 2 Abstract—This paper describes design considerations and performance analysis of a novel on-line single-phase to three- phase uninterruptible power supply (UPS) with reduced number of switches. The proposed topology uses only 5 active switches reducing the cost of the system compared to the traditional 10-switch topologies. The proposed UPS consists of a single-phase rectifier/charger, a two-leg/three-phase inverter connected to load, battery bank, DC link capacitor and an AC inductor. Detailed circuit operation and analysis as well as simulation results are also presented to verify its feasibility. Keywords-Uninterruptible Power Supply (UPS); Single- Phase to Three-Phase Conversion; Reduced Switches I. INTRODUCTION Uninterruptible Power Supplies (UPS’s) are nowadays indispensable equipment in supplying power to critical and sensitive loads. They protect sensitive loads against many existing disturbances in utility network like voltage variations, transients and harmonics. Applications of UPS systems include medical facilities, life supporting systems, data centers, emergency equipment, telecommunications and industrial processing systems. Ideally, a UPS should deliver clean and uninterrupted power to the load, and at the same time draw sinusoidal, near unity power factor current from input supply. Additionally, static UPS’s must be able to switch between utility and storage batteries as alternate energy sources. This switching must take place in zero or minimum time in order to avoid any malfunctioning of the supplied equipment. Other specifications like high reliability, high efficiency, low EMI and acoustic noise, electric isolation, low maintenance, low cost, weight and size must be also considered in a high performance UPS [1]. IEC-62040-3 standard classifies UPS’s as passive-standby, line-interactive and double-conversion (on-line UPS). Each topology has its own characteristics and is used based on the load requirements and the severity and type of network disturbances [2]. The main advantages of a passive-standby UPS are its design, low cost and small size. The line conditioning is passive which makes this topology fairly robust. On the other hand, rather long switching time between standby and backup modes is the main disadvantage of this topology. The paper first received 5 Jan 2012 and in revised form 26 Sep 2012. Digital Ref: A190711120 1 Department of Electrical Engineering, University of Isfahan, Isfahan, Iran, msharifianesfahan@unomaha.edu 2 Department of Electrical Engineering, University of Isfahan, Isfahan, Iran, mehdi_niroomand@eng.ui.ac.ir The line-interactive topology has simple design, fairly high reliability and lower cost, and thus is becoming more attractive as compared to the double-conversion solutions. Since there is just a single stage conversion in this topology, the efficiency is inherently higher than that of the double-conversion UPS. The main disadvantage is the fact that there is no output voltage conditioning during normal mode because the inverter is not connected in series with the load [3], [4]. The double-conversion or on-line topology is considered as the superior topology in performance, and is widely used as standard solution for protecting sensitive loads. However, it has lower efficiency as compared to other topologies due to two conversion stages in its structure. In other words, power flow through the rectifier and inverter even during the standby mode means higher power losses and lower efficiency compared to passive-standby and line-interactive UPS systems. Another important feature of the online topology is the decoupling of the input from the output, which allows converting single-phase to three-phase UPS [5]. In view of the machine efficiency, power factor, and torque ripples, a three-phase induction motor is preferable to a single-phase induction motor. Therefore, it is desirable to replace the single-phase induction motor drives by the three phase induction motor drives .Even if the distribution of electric power is typically three phase, where only a single-phase utility is available, a single- phase-to-three-phase power converter system is required to feed the three-phase induction motor drives. In rural electric systems, the cost of bringing three-phase power to a remote location is often high due to high cost for a three- phase extension. Furthermore the rate structure of a three- phase service is higher than that for single-phase service. Therefore, single-phase to three-phase power converters are excellent choices for situations where three-phase power is not available. Sometimes, a specific appliance needs three-phase power, requiring some kind of power conversion [6]. Such converters have a wide range of applications in which a three-phase motor is a main component and the available supply is single-phase. This happens in residential, light industrial, farming, low-power industrial applications and rural areas [7], [8], and [9]. Fig.1 shows a typical 1-phase to 3-phase dual bridge converter topology. The simplicity of these circuits inherently requires a simple control. However, they suffer from large number of switches and from here high cost. The problem of reduction the cost of converters has been recently attracting the attention of researchers [10]-[12].