Understanding UPS overload capabilities
Appropriate application of UPS rating for overload and short
circuit conditions
© 2012 Eaton Corporation. All rights reserved.
Agenda
• Defining Overload performance & conditions
• Typical load profiles
• Handling fault currents & overloads
• Customer perception
• Competitor practices
• Summary
• Eaton salient features
• Support info
© 2012 Eaton Corporation. All rights reserved.
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Defining overload performance
UPS overload rating is its ability to temporarily provide
additional output power at nominal voltage.
The specification defines the amount of overload that can
be supported and the time delay before transfer to
bypass (or shutdown if bypass is not available).
Output overload performance may be limited by factors
such as temperature, input supply rating, availability of
battery, frequency of overloads etc.
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Defining overload performance
For UPSs in today’s market, the levels and ranges of
support time are:
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103% overload 10 minutes to continuous
125% overload Between 30 sec and 10 minutes
150% overload Between 10 sec and 60 sec
200% overload 10 to 20 cycles (current limit)
Since the above specifications contain a wide range of
possible overload support times, designers must
determine if a longer overload time limit provides useful
protection for the load.
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Defining overload conditions
• Short term transient or fault conditions
• Short circuit loads (eg: computer power supply
failure)
• Maintenance faults (eg: service tool dropped across
live terminals)
• Motor starts, transformer inrush, STS load transfer
• Long term overloads
• Inadvertent addition of load
• Progressively failing load device (insulation
breakdown etc.)
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Load profiles – Data Centre/IT
• Loads are typically made up of multiple small single
phase transformer-free devices.
Large 3 phase loads, tape drives etc. are now very rare.
Cooling loads (fans or CRAC units) are occasionally run on
UPS.
• Data Centres with redundant UPS systems feature
significant UPS over-capacity
The key consideration for provisioning overload capacity
in Data Centre/IT applications is the UPS’ ability to
properly handle fault currents
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Load profiles – Industrial loads
• Loads can include:
• Motors – starting currents ( the variables are
amplitude, duration and frequency).
• Lighting – Inrush currents depending on type of
lighting and application (eg: emergency lighting)
Key considerations are understanding amplitude,
duration and frequency of load steps – usually requires
UPS oversizing, regardless of overload specifications
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Handling fault currents
• In the event of a load fault, UPS must transfer to
bypass to provide enough fault clearing current to trip
the load circuit breaker
• Long duration inverter overload capability does not help –
inverter does not have sufficient fault current to clear the fault
• Trying to sustain a fault on inverter (waiting for O/L timer) will
stress or damage inverter and result in output voltage drop
• If bypass is unavailable – UPS will eventually trip off if the
fault isn’t cleared, or load will fail due to under voltage
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Handling overloads
• An overload that is more than just a transient
event will generally last a lot longer than any
UPS overload time period. Examples:
• An inadvertent addition of load – human reaction time to an
overload alarm is too slow
• A progressively failing load lasts hours or days, not seconds
or minutes
The overload timer simply allows the UPS to verify that a sudden load
increase is not due to a transient event like a motor starting or static switch
transferring load. In these very brief events, if the UPS determines that it can
handle the overload, we want the UPS to remain online, without producing
nuisance transfers to bypass. These transients are over in a few cycles, so a
“longer” overload timer is unnecessary and adds risk.
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Overload performance – customer
perception
Designers may consider overload performance
as a defining measure of UPS robustness!
Potential Pitfalls:
• Simply considering only overload percentages
and duration is not sufficient to determine if
overload rating is indicative of good design
and robustness
• Improper handling of load faults will reduce
overall system reliability and availability
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Overload performance – customer
perception
Additional considerations required to determine
robustness and suitability of a UPS system:
• Maximum continuous temperature rating of
UPS running at full load
• Cooling system employed within UPS:
• Placement of fans for increased life and elimination
of hot spots in event of fan failure
• Redundancy of fans and fan failure alarms
• Proper handling of fault currents to reduce
stress on UPS and loads
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Competitor overload performance
• Common practices:
• Output voltage drops during overload timer period:
Loads may not function with reduced voltage
• Maximum continuous ambient temperature rating is
lower, to enable UPS to handle higher overload
rating:
Overall inverter robustness is lower and inverter
can only handle one overload event before overheating
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Competitor overload performance
• Common practices:
• UPS requires battery bank to support the overload
performance:
Indicates under-sized rectifier. Overload may
deplete battery. Overload capacity not available
during power outages
• UPS automatically “re-rated” for increased power at
lower ambient temp. eg, 100kVA becomes 110kVA at
25oC:
If UPS room is not cooled, or if cooling fails, UPS
will shut down if load is greater than its “real” rating
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Competitor overload performance
• Questions to ask:
• Is overload current provided at nominal voltage?
• Can overload performance be provided at maximum
continuous ambient temperature?
• What is the maximum continuous ambient
temperature?
• Is the battery required to support overload capability?
• How many overload events can the UPS handle
within a 24 hour period?
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Summary
• Overload rating is not a pure indicator of UPS
robustness
• Overload ratings should not be used to cover up
bad design or operating practices
• Unnecessary stressing of inverter places load at
risk and reduces longevity and long term
reliability of UPS
• Beware of competitor specification conditions
used to “enhance” overload performance
© 2012 Eaton Corporation. All rights reserved.
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Eaton salient features
• 40oC continuous ratings
• Advanced DSP architecture and firmware
algorithmns protect load and UPS with fast detection
load changes and faster reactions (eg: Selective
Tripping)
• Prioritised cooling
• Energy Saver System (9390 & 9395):
• Can detect if voltage transient is created by load of line
fault and makes the correct transfer decision
• Inverter adds overload capacity to mains supply when in
ESS mode
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Nameplate rating Vs Actual rating
Some Eaton UPS models feature power conversion
modules that are actually rated at a higher rating due to
design and/or manufacturing standardisation.
• This additional “overhead” can help to dispel a customer’s
perception that our systems may be under-rated or less robust
because of our overload handling principles:
Model
Actual Power Conversion
module/device rating
Model
Actual Power Conversion
module/device rating
9155 8kVA
10kVA
9E 80kVA
100kVA
9155 12kVA
15kVA
9E 160kVA
200kVA
9355 8/10/12kVA
15kVA
9395 200/225kVA
275kVA
9355 20kVA
30kVA
9395 400/450kVA
550kVA
9390 60kVA
80kVA
9390 100kVA
120kVA
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Support information
• Overload Capabilities white paper
© 2012 Eaton Corporation. All rights reserved.
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© 2012 Eaton Corporation. All rights reserved.
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