Data Center Handbook

27Uninterruptible Power Supply System

Chris Loeffler and Ed Spears

Eaton, Raleigh, NC, USA

27.1 Introduction

Uninterruptible power supplies (UPS) are an extremely important part of the electrical infrastructure where high levels of power quality and reliability are required. In this chapter, we will discuss basics of UPS designs, typical applications where UPS are most commonly used, considerations for UPS selection, and other components or options that are an important part of purchasing and deploying a UPS system.

27.1.1 What Is a UPS?

Put simply, a UPS is a device that provides backup power when utility power fails or becomes unusable by devices requiring regulated electricity to operate. The UPS can either provide electricity long enough for critical equipment to shut down gracefully so that no data is lost or no process is interrupted or long enough to keep required loads operational until another electrical generating source (typically a generator) comes online. Some of the different UPS topologies also provide conditioning to incoming power so that all-too-common sags and surges don’t damage sensitive electrical and electronic gear. UPS systems are designed to integrate easily into the standard electrical infrastructure, so that means smaller power requirements are typically single-phase designs, with larger power requirements being handled by three-phase systems. In North America, the typical single-phase UPS design is smaller than 25 kVA, while three-phase systems ...