UPS Topologies

Topologies for Uninterrupt ible Power Sup plies R Krishnan and S . Sriniwan Ahtract The Bradley De partmen t o f Electrical Engineering Va.Tech, Blacksburg, VA 24061, USA. e- mail [email protected] Ph: 703 231 4311 ) Fax 703 231 3362) Tbip paper reviews the development o f Uninterruptible Power Supply ( here after r eferred to P UP8 ) over the years from one with ER front end charger with isolation at h e requency to one with sinua oid d current input c -r with isolation at high frequenay (h.f.). Thip paper is concerned with the developments in the single phase low power (less than 1 kVA UPS topologies. Each scheme hi analyzed briefly and its principal merits and demerits are dentified. 1. ntroduction UPS systems are intended to improve the quality of ac power in order to provide continuos operation of ac powered equipments[l]. T o accomplish these functions a UPS takes in utility ac input and improves the power quality throu gh power processing. It also provides a redundant (Back-up) power source so that the load will be interfaced to the utility directly in the case o f fail ur e o f any subsystem. Power quality defects which may be improved by the UPS include surges, noise, sags and harmonics. A block diagram of the general UPS is shown in Figure 1 . t Figure 1 Block Diagram of a general UPS The most recent developments having a profound effect o n he config uratio n of UPS topologies include High frequency switching to minimize filter size. New semiconductor power devices resulting in a dramatic increase in the UPS power output. Digital techniques with microprocessors and DSPs to implement compl ex control algorithms. Some o f t he desira ble featu res o f a n UPS are : Sinusoi dal input curre nt drawn fi he utility mrrine. Sinusoidal output voltage regardles s o f the oad. Isolation at lo w weigh t and cost High efficiency and hence minimum power conversion stages. High reliability. Acceptance of wide variation of the input voltage with no variation of output voltage. o single topology satisfies all these requirements simultaneously. Every topo lo gy tr ies to optimize one or many of the desirable features giving rise to a wide variety of UPS configurations. The choice of a particu lar topology depends o n the application and selecti ve optimization o f t he desired features. This paper discus ses the various UPS topologies from ths view-point. The UPS is classified depending on the power flow pa th chosen. I f th e primary power fl ow is through the surge suppressor and filter to the load when the ac input is healthy, and through the battery and inverter to the oad when the ac input fails, then i t i s a standby UPS system. I f the primary power flow i s through the charger, battery, inverter and load when the ac input is healthy then it is an on-line UPS system. The path through surge suppressor, filter and transfer switch is resorted to, when the other path is out-of-operation due to failure or for maintenance. The classification o f UPS topologies is shown in Figure 2. The paper is organized as ollows. Section 2 contains standby off-line opologies. Section 3 contains on- line UPS opol ogi es. Line- interactiv e UPS topologies are described in Section 4. Conclusions are summarized inSection5. 2. Standby UPS characteristics A block diagram of this configuration is shown in Figure 1. I n the standby mode the charger size is @7803-1227-9/93/ 3.M)@ 993 I 1 22

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UPS Topologies