Appendix C General Guidelines for UPS Selection

DOE-SPEC-3021-97

APPENDIX C

GENERAL

A UPS is, by definition, a system that maintains continuity of electric supply to critical loads for a

specified period of time upon loss of primary ac input power. Many different types of UPS

configurations are available, but this document defines the simplest configuration, which is called

a "Single UPS Unit" (see NEMA PE 1).

The single UPS unit used as a base system for the guide specification is typical for most

applications. There are two other types of systems that use step-wave inverters, or are rotational

(motor-generator) systems that will have differences in some parts of the specification.

UPS SYSTEM DESCRIPTION

The typical UPS system is shown in block diagram form in figure 1. Note that the UPS system

to be provided by Seller includes a battery system that will dictate the stored energy time

available. The battery provided by the Seller must be specified using either DOE Standard

"Sealed Valve-Regulated Type Lead-Acid Storage Batteries" or "Flooded-Type Lead-Acid

Storage Batteries."

It should also be noted that the overall system is designed for dual ac input sources; a normal

source which feeds the input of the rectifier charger, and a bypass source which is routed to the

critical load via either the static transfer switch or the maintenance bypass switch. These two

input sources are usually derived from either a common or two separate commercial power

sources. If the bypass source voltage does not match the output voltage of the UPS inverter, it

will be necessary to use a transformer in the bypass feed. This transformer can usually be

supplied by the UPS manufacturer and should be specified if required.

A standby generator may be used as an alternate input source, should the load reliability

requirement dictate the need for it. If a standby generator is used as an alternate input source, it

would automatically be selected when commercial power is lost. The selection of the input

source and how it should be connected to the UPS inputs is the facility engineer's responsibility.

The following building blocks make up the typical single unit UPS system:

Isolation Transformer

Attenuates the passage of noise or transients from

its primary winding into its secondary winding.

Input Filter

Attenuates the effects of high-order harmonics and

transients generated by the UPS and keeps them

from feeding back into the input power source.

C-1