UPS (Uninterruptible
Power Supplies)
UPS Applications
Outline
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Introduction
UPS Basics
UPS Applications & Specifications
Generator Sizing to Serve a UPS
UPS Maintenance Bypass
Conclusions
Introduction
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What is a UPS
Uninterruptible Power Supplies (UPSs)
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A system designed to automatically provide power, without delay or
transients, during any period when the normal power supply is
incapable of performing acceptably. IEEE Definition
Provide reliable power to critical loads
Improve reliability and power quality
UPS Basics
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Three fundamental topologies utilized for
static UPSs
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Standby power systems (off-line)
Line interactive (single conversion)
Double conversion (on-line)
UPS Basics
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Standby power systems (SPSs)
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No power conditioning during normal
operation
Load sees brief interruption when switching
to battery backup
Square wave or stepped sine wave output
under backup power
Typically small (up to 10 kVA), singlephase
Sine Wave vs Square Wave
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100
50
Sine Wave
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-50
-100
-150
0
1.5
1
Square Wave
VOLTAGE
0.5
0
-0.5
-1
-1.5
0
45
90
135
180
225
270
315
ANGLE, DEGREES
360
45
90
135
180
225
Standby Power Systems
(Off Line System)
Utility
Storage
Batteries
Transfer
Switch
Inverter
Critical
Load
UPS Basics
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Line interactive UPSs
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Power passed to the load through
magnetic components (inductors,
transformers)
Inverter interacts with magnetic
components to boost or buck input voltage
Provides some power conditioning but
frequency variations are passed to load
Typically 100 kVA or less, 1- or 3-phase
Line Interactive (Single Conversion)
Utility
Storage
Batteries
Power
Conditioner
Inverter
Critical
Load
UPS Basics
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Double conversion (on-line) UPSs
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Input voltage not passed directly to load
Rectifies AC to DC (where Storage are
connected) then inverts back to AC
Seamlessly draws power from DC storage
when input voltage lost
Independent, conditioned output power
Single-phase or three-phase, ratings can
exceed 1000 kVA, higher if paralleled
Double Conversion (on-line)
Utility
TVSS
Conditioner
DC
Voltage
Storage
Batteries
Flywheel
SMES
Inverter &
Conditioner
Critical
Load
UPS Application
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Type Load
X Computers
X Controls
PLCs
W Relays
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X Motors
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Time of Back-Up
Generator
UPS Specification
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Voltage
Load kW & kVA
Transfer Time
Transfer Voltage
Waveform Output
Type Storage & Time
Storage Life
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Batteries 4 to 6 years
Generators Serving UPSs
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Generators serving UPSs will need to be
sized for
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Harmonics
Motor starting
X Voltage
drop
X Slew rate
UPS Maintenance Bypass
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If a load is important enough to require a
UPS it is also important enough to require a
proper maintenance bypass path
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External & Internal bypass switches on
larger UPSs
Internal bypass switches (present on most
UPSs) are used for smaller UPSs
Conclusions
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UPS Basic Types
Applications
Specifications
Generator Serving a UPS
UPS Maintenance Bypass