Understanding UPS Overload Capabilities

8/11/2019 Understanding UPS Overload Capabilities

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2012 Eaton Corporation. All rights reserved.

Understanding UPS overload capabilities

Appropriate application of UPS rating for overload and short

circuit conditions


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22 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


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Defining overload performance

UPS overload rating is itsability to temporarily provide

additional output power at nominal voltage.

The specification defines the amount of overload that canbe 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 todays market, the levels and ranges ofsupport time are:

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 ofpossible 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 acrosslive 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 oflighting 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 tobypass 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 isnt cleared, or load will fail due to under voltage


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Handling overloads

An overload that is more than just a transientevent 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 switchtransferring 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 customerperception

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 customerperception

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 islower, to enable UPS to handle higher overload

rating:

Overall inverter robustness is lower and inverter

can only handle one overload event before over-heating


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Common practices:

UPS requires battery bank to support the overload

performance:

Indicates under-sized rectifier. Overload maydeplete battery. Overload capacity not available

during power outages

UPS automatically re-rated for increased power at

lower ambient temp. eg, 100kVA becomes 110kVA at25oC:

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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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


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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 linefault 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 conversionmodules that are actually rated at a higher rating due to

design and/or manufacturing standardisation.

This additional overhead can help to dispel a customers

perception that our systems may be under-rated or less robustbecause of our overload handling principles:

Model Actual Power Conversion

module/device rating

9155 8kVA 10kVA

9155 12kVA 15kVA

9355 8/10/12kVA 15kVA

9355 20kVA 30kVA

9390 60kVA 80kVA

9390 100kVA 120kVA

Model Actual Power Conversion

module/device rating

9E 80kVA 100kVA

9E 160kVA 200kVA

9395 200/225kVA 275kVA

9395 400/450kVA 550kVA


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Support information

Overload Capabilities white paper


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Understanding UPS Overload Capabilities

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