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App n5 (rework) Power Quality
Testing Network
Sizing Single-Phase Uninterruptible Power Supplies
Power Quality
Testing Network
Published by the EPRI Power Electronics Applications Center Application No. 5 September 1995
APPLICATION
Power Conditioning
Uninterruptible Power Supplies
Sizing Power Conditioners
PQTN
Application
Traditionally, the uninterruptible power supply, or UPS, has been used to protect critical comput-
ing and data processing equipment. However, in recent years the UPS has been applied to
everything from industrial process controllers to telephone switches to medical equipment.
Because the loads supplied by a UPS can be either linear or nonlinear, be cycling or non-cycling,
and have various levels of inrush current, sizing a UPS to match its load is not a simple task. This
PQTN Application provides a method of sizing a single-phase UPS to match its intended loads.
Most loads have inrush currents that may be up to twenty times their steady-state currents.
Therefore, the size of the UPS relative to its load is one of the most important considerations
when specifying a UPS. Additionally, some loads such as laser printers and medical laboratory
equipment can have intermittently operated heater elements or other internal devices that cycle
on and off. If start-up and cycling effects are not addressed during the initial load analysis, an
undersized UPS may be specified, resulting in load dropouts.
The three main types of single-phase UPS are the standby, line-interactive, and rectifier/charger.
Each type has a normal ac line operation mode and a battery-power operation mode. As
discussed in the next section, the various features of the three types can complicate the sizing of a
UPS. UPSs are often sized by adding the nameplate current of each load and then multiplying the
total current by the UPS output voltage to yield a VA rating required to support the loads.
However, this method is unreliable because nameplate current values can be inaccurate or
missing and do not include inrush and cycling currents. To precisely determine the power
demands of a load, its input current must be monitored during normal operation, including start-
up and cycling. This measurement method for sizing a UPS considers steady-state, inrush, and
cycling currents.
Sizing a UPS: The Measurement Method
Step 1
Get the proper measurement device (either an ammeter or a multimeter). For this
procedure, use only a true RMS meter with peak hold detection circuitry. The use of peak
detection RMS calibrated meters or average responding RMS calibrated meters will result in
erroneous current and voltage readings if harmonics are present.
Step 2
Set the meter to measure the steady-state ac RMS current of each load to be connected to
the UPS. Steady-state current is the current drawn by the load over a long period. For example, a
computer draws steady-state current any time it is on. A laser printer draws steady-state current
when it is on but idle. Steady-state current should not be measured when a load cycles (heater
elements of a printer turn on) or when it is first turned on (inrush current). Enter all steady-state
What To
Look For
How To Size a UPS
To measure current, use a cheater cord with the three
individually insulated conductors exposed. A clamp-on
current probe attached to a true RMS meter can be
used to measure the current flowing through the hot
conductor (brown or black insulation).
Hot
Conductor
Clamp-On
Current
Probe
current measurements into the UPS Sizing
Worksheet.
Step 3
Make sure the load is turned off. Then,
set the meter to record maximum peak ac
current (not RMS current). Turn the load on and
let it operate for one hour. Be certain that the
load has performed its main functions during
that hour. For instance, make sure that a printer
has printed several pages or control relays are
all on. The meter will record the highest peak
current drawn by the load. Enter all peak current
measurements into the Worksheet.
Step 4
Fill out the rest of the Worksheet. Add
all the steady-state currents together and then
add the single highest peak current of the loads
to get the total current requirement. The size of
the UPS should be at least the total current times Copyright © 1996 Electric Power Research Institute
Printed with soya inks on recycled paper (at least 50% recycled fiber,
including at least 10% postconsumer waste) in the United States of America.
TA-105721
PQTN APPLICATION No. 5
Inrush Issues
Sum of Steady-State RMS Current
Highest Peak Current of All Loads
+
Total Current
Output Voltage Required by Loads
x
Required VA Rating of UPS
UPS Sizing Worksheet
UPS Loads To Be Protected
Measured
Steady-State
RMS Current
Measured Peak
Current Over
One Hour
Bring down
highest peak
current.
s Save time and money by installing the right size of
UPS.
s Ensure UPS reliability by selecting the right type
and features for the job.
s UPS dealers and manufacturers
s Your local utility
PEAC thanks the sponsors of Task 9 of the EPRI
System Compatibility Research Project: Uninterrupt-
ible Power Supplies.
the UPS output voltage (VA). If the UPS is intended to
support more than one cycling load or you anticipate
that some or all of the loads will be turned on at the
same time, then the size of the UPS may have to be
increased, depending upon the type of UPS. For
instance, line-interactive UPSs with ferro-resonant
transformers must be sized properly to support high
inrush current, as must rectifier/charger UPSs. How-
ever, nearly all standby and some line-interactive UPSs
will support inrush currents beyond their rated capacity
because they are normally connected to the utility
power (see Inrush Issues, above). In fact, you may be
able to size those same UPSs by multiplying only the
steady-state current total by the UPS output voltage.
But ask the UPS dealer or manufacturer to verify that
the UPS can handle high momentary inrush currents.
Once you have completed the UPS Sizing Worksheet,
you may still need the advice of a UPS dealer or
manufacturer. Although the Worksheet yields a good
estimate for UPS size, it does not factor in UPS type
and features. If you are sizing a UPS for unique loads,
such as blood analyzers or mass spectrometers, consult
the UPS manufacturer, who will have size recommen-
dations for those particular loads. Additionally, line or
source impedances vary and can affect inrush currents,
so the measurement method of sizing a UPS may not
apply if the measured loads and the UPS are going to
be used in a different location.
BENEFITS
WHERE TO FIND HELP
ACKNOWLEDGMENTS
Steve Rector, EPRI Knoxville, (423) 974-8307, or
Gene Sitzlar, PEAC, (423) 974-8288
The
EPRI
Power Electronics Applications Center
10521 Research Drive, Suite 400
Knoxville, TN 37932
Fax: (423) 974-8289
Power Quality Hotline: (800) 832-7322
FOR INFORMATION ABOUT PQTN
APPLICATIONS, CONTACT:
UPS Type
Inrush Notes
Options
Standby
Typically handles high inrush. Consult dealer or manufacturer.
Line-Interactive with Ferro-
Resonant Transformer
Output voltage may sag
during high inrush.
Oversize this type or add inrush-limiting thermistors to the
UPS output.
Line-Interactive without Ferro-
Resonant Transformer
Typically handles high inrush. Consult dealer or manufacturer.
Rectifier/Charger
Output voltage may sag
during high inrush.
Oversize this type, get this type with an auto-bypass feature,
or add inrush-limiting thermistors to the UPS output.
®
For ordering information, call PEAC (423) 974-8288.