Data Centre Handbook

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The UPS

data centre

handbook

The essential guide to making

informed decisions around

UPS data centre systems

Highly efﬁcient & reliable

UPS systems for the data

centre industry

PowerWAVE 6000 –

Transformerless UPS

technology for best-in-class

performance up to 5 MVA

PowerWAVE 8000DPA –

Modular, three-phase UPS

providing power protection

up to 200 kVA

PowerWAVE 9000DPA –

Three-phase, modular UPS

with a high operating

efﬁciency of up to 96%

Low total cost of ownership

Whether you’re supporting a localised IT facility or an enterprise level

data centre with over 500 servers, operating a UPS system contributes

signiﬁcantly to capital and operating expenditure. So, although the UPSs

critical role demands investment in the most advanced, reliable solutions,

maximising efﬁciency and lowering total cost of ownership is also

essential; a small change can have a big impact on the bottom line.

UPSLs PowerWAVE range, including the 6000, 8000DPA and 9000DPA,

provides a complete offering of highly-efﬁcient solutions for data centres of

all sizes. The range’s class-leading efﬁciency levels, available over a wide

load spectrum, combine with near-unity power factors to ensure operating

costs are minimised, while available scalability options reduces upfront

capital expenditure.

Find out which is right for your operation today.

2 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 3

your TCO today

01256 386700

sales@upspower.co.uk

Enterprise data centre

>5,000 sq ft

>500 servers

UPS size=>750kVA

PowerWAVE 6000

High operating efﬁciency regardless

of loading:

n Up to 96% for 25–100% loads.

Near unity input power factor:

n 99% at 100% load.

Reduced installation and

upgrading costs.

Reduced system running costs:

£19,225* cost saving over ﬁve years

per 1% efﬁciency improvement.

Reduced air conditioning costs:

Less heat loss.

Low carbon footprint.

60–500kVA ‘building blocks’ scalable

to 5MVA.

* 500kVA @ 100% load / 95% efﬁciency

Mid-tier data centre

<5,000 sq ft

101–500 servers

UPS size=101kVA–750kVA

PowerWAVE 9000DPA

High operating efﬁciency regardless

of loading:

n Up to 96% for 25–100% loads.

Near unity input power factor:

n 99% at 100% load.

Reduced installation and

upgrading costs.

Reduced system running costs:

£9,520* cost saving over ﬁve years

per 1% efﬁciency improvement.

Reduced air conditioning costs.

Low carbon footprint.

10–250kVA capacity in 10, 20, 30, 40 or

50kVA modular steps.

N+1 Redundancy (up to 200 kVA N+1).

* 250kVA @ 100% load / 95.5% efﬁciency

Localised data centre

<1,000 sq ft

25–100 servers

UPS size=25kVA–100kVA

PowerWAVE 8000DPA(ST)

High operating efﬁciency regardless

of loading:

n Up to 95.5% for 25–100% loads.

Near unity input power factor:

n 99% at 100% load.

Reduced installation and

upgrading costs.

Reduced system running costs:

£4,570* cost saving over ﬁve years

per 1% efﬁciency improvement.

Reduced air conditioning costs:

Less heat loss.

Low carbon footprint.

10-200 kVA capacity in 10 kVA and

20 kVA modular steps.

N+1 Redundancy (up to 180 kVA N+1).

* 120kVA @ 100% load / 95.5% efﬁciency

4 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 5

Contents

Summary of

survey ﬁndings

Introduction

A review of

data centre tier

classiﬁcations

The impact of

UPS technology

on the design

of green data

centres

The importance

of a reliable

Service Partner

in data centre

UPS systems

The upside and

downside of Dual

bus power

The application

of ‘economy-

mode’ in ICT

UPS systems

6 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 7

Introduction

We trust that this Handbook will become a valuable asset and

reference for owners, operators and designers. In a complex

ﬁeld with many variables, it focuses on the efﬁciency and

reliability issues that matter most today to anyone planning

a data centre UPS installation or upgrade.

UPS systems have become absolutely indispensible

to every data centre though choosing the right product,

conﬁguring and installing it correctly is far from easy as

operators face conﬂicting pressures. The availability of clean,

uninterrupted power has become business-critical and now

has to be combined with maximum possible efﬁciency. Not

only are electricity costs steadily rising, but also data centres

must demonstrate effective Green policies to comply with

existing and potential legislation, and preserve their reputation

with customers, shareholders and employees.

Fortunately, balanced solutions are possible through

improved UPS technologies and topologies, ICT equipment

and, sometimes, utility mains power. This handbook explains

the issues, interactions and solutions. It discusses how UPS

“This Handbook focuses on

the efﬁciency and reliability

issues that matter most

today to anyone planning

a UPS installation or

upgrade.”

www.upspower.co.uk The UPS data centre handbook 9

efﬁciency for all load levels is improved by transformerless

design, and how both capex and opex can be minimised

through modular design and right sizing. It also covers

the increasingly popular question of ‘Eco mode’ operation.

This mode, when ﬁrst introduced by UPSL was not widely

favoured. It is now commonly used as modern ICT equipment

has better blackout ride-through capabilities, while many

utilities now offer better voltage and frequency stability.

Availability is also discussed in terms of MTBF and Mean

Down Time (MDT), and the importance of a reliable

service partner in maximising MTBF whilst minimising

MDT is explained.

The Handbook also discusses industry views and

standards on UPS efﬁciency and availability, particularly

by examining Power Usage Efﬁciency (PUE) and data

centre tier classiﬁcations. PUE has become the driving

force behind improving data centre efﬁciency, and as cooling

systems have become more efﬁcient, attention is turning to

the UPS contribution. Tier classiﬁcations, sponsored by the

Uptime Institute for nearly 20 years, describe facilities from an

availability standpoint. The Handbook shows how dual-cord

and N+1 redundancy UPS conﬁgurations can be used to

change tier level and dramatically improve availability.

The UPS data centre handbook 11

10 The UPS data centre handbook www.upspower.co.uk

75%

of respondents are already

investigating product efficiency

as a way to control/reduce

operating costs

92%

of respondents agree that

maintenance and call out

services are as important

as the hardware itself. 65%

of those questioned are

concerned that increasing

environmental legislation

is a major concern to

their business.

of those questioned

agreed with the

statement ‘Rising power

costs are a major

concern for my

business’.

of those who completed the

survey have a carbon reduction

policy in place.

Product efficiency

Maintenance

Carbon footprint Power costs

Legislation

of respondents believe that

the reliability of power in the UK

is going to be come a major

concern within the next 10 years.

believe the most

important factor to a

potential buyer when

selecting an OEM

is ‘a reputation for

quality and reliability’.

of respondents believe that

maximum reliability is the

number one product feature

they look for.

When considering reducing power consumption, what is

the primary driver for change within your organisation?

% %

Reducing their carbon footprint

Reducing operating costs

Reliability

Operations

Power Buying

Summary of

survey ﬁndings

Each year Uninterruptible Power Supplies Limited, a Kohler company, commissions a

national survey of IT professionals across the United Kingdom, allowing it to keep pace

with current thinking, trends and attitudes regarding UPS equipment, power consumption

and energy efﬁciency. Here you will ﬁnd a snapshot of the key ﬁndings:

12 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 13

“Topology has also

changed, with double-

conversion being enhanced

with eco-mode and modular

designs enabling higher

load power factors.”

The impact of UPS

technology on the design

of ‘green’ data centres

Power Usage Effectiveness (PUE) has become the driving

force behind the improvement in energy efﬁciency of data

centre M&E infrastructures and an integral part in the pursuit

of the ‘green’ data centre. PUE can be deﬁned as a measure

of how efﬁciently power is used within a data centre. It is

measured by a ratio of total amount of power used, to the

amount of power delivered to computing equipment.

PUE = Total facility power / IT equipment power

For many years, UPS efﬁciency has been gradually improving

but it has been the mechanical cooling systems that have

attracted the most attention regarding energy-overhead

reduction. As the cooling systems have themselves improved,

in some cases drastically, the focus has now returned to the

power system. This white-paper reviews the historical case,

where design PUEs of >2.0 were not uncommon, and looks

at the possibilities provided by scalable high-efﬁciency UPS

products, and the maximum impact that the UPS can make

to the PUE as it progresses below 1.4.

UPS in historical PUE context

Although the PUE metric is a relatively recent innovation

(by The Green Grid), the principle of working out what

capacity utility was required for a given ICT load in a data

centre has always been one of the ﬁrst tasks of the designer.

Indeed, as PUE is an annualised energy metric and not a

peak-power metric, that calculation still has to be done as

the ‘peak’ dictates the size of the utility and the emergency

generation system.

If we consider a typical early-90s data centre with a

critical load of around 1MVA, (the kW load was hardly ever

considered in those days as the load power factor was

not unity and harmonic currents dominated), it had an

N+1 UPS of probably 3x500kVA with an efﬁciency of 88%

and a compressor-based mechanical cooling system that

maintained tight temperature and humidity environment. The

chilled water was supplied at 6ºC and the air supplied into the

under-ﬂoor plenum was around 15ºC. No ‘free cooling’ coils

were ever considered (or available in standard equipment) and

variable speed pumps and fans were still on the application

horizon. Humidiﬁcation and de-hum consumed energy and

even the lighting was high in proportion to the load as the

power density was 350-500W/m≤ from mainly main-frame

hardware. The overall impact of this infrastructure was a

system where the utility load was constant (non-seasonal)

and the fully loaded ‘PUE’ (if that had been innovated) was in

the order of 2.5. However, the partial load performance was

very poor – with monolithic plant (no scalability planned) and

no variable speed drives – with the result that the majority of

facilities ran at a PUE equivalent of 3.5. Hence, even at partial

load, the UPS system only contributed about 0.15 to the

PUE with the mechanical cooling load dominating the power

demand. Energy was cheap and the load was sacrosanct and

so very few people, if any, worried too much about the energy

costs of data centres.

The development of UPS efﬁciency

The efﬁciency improvement of UPS has followed a

combination of component innovation, such as thyristors

being replaced by transistors in inverters (and, much later,

rectiﬁers), which enabled the removal of passive ﬁlters and

transformers. Topology has also changed, with double-

conversion being enhanced with eco-mode and modular

designs enabling higher load power factors.

“For many years, UPS

efﬁciency has been

gradually improving, but it

has been the mechanical

cooling systems that have

attracted the most attention

for energy overhead

reduction.”

14 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 15

In the early 90s a large UPS module would have an

input transformer, 12-pulse thyristor rectiﬁer, passive 11th

harmonic input ﬁlter, DC capacitors, 6-step thyristor inverter

with isolating transformer and output ﬁlter network, which

all resulted in a maximum efﬁciency at full load of 87%.

Compare that today with a transformer-less IGBT/IGBT

rectiﬁer/boost-stage/inverter model which can offer 96% in

double-conversion mode and, if the user enables it, eco-mode

operation providing up to 99%.

It is worthy of note that any system requiring an annual

shutdown of four hours for maintenance can only achieve an

availability of 99.95% (MTBF=8,760h and MDT=4h).

So, a high availability can be achieved by either a long

MTBF or a short MDT but the MDT should (but usually does

not) include the ICT system re-start time.

The introduction of ‘free-cooling’ economisers and, more

recently, the relaxation of the thermal envelope (temperature

and humidity) by ASHRAE have led the cooling system power

to be drastically reduced. Strict air-management, ensuring that

no cooled air bypasses the load, has been established as best-

practice and this has been enhanced for partial load conditions

by the widespread use of variable speed drives for fans and

pumps. Full-load cooling coefﬁcient of performance (CoP) has

improved from 1.0 (where to move 1kW of heat from within the

critical space to the external ambient takes a further 1kW of

power in the cooling system) to better than 0.1 (1kW of cooling

system power to remove 10kW of waste heat from the load).

To compliment this contribution to a target PUE of 1.2

or better the UPS is required to offer 0.05 and the other

consumers (internal & external lighting, NOC, controls and

security etc) a further 0.05. It can be seen that getting to an

annualised PUE of c1.15 requires extremely efﬁcient systems

but a full load UPS efﬁciency of >95% is essential. Partial-load

performance must also be excellent, through technology

(e.g. with >94% efﬁciency at 30–40% load) or ‘right-sizing’

to keep the UPS load >70%, including the option of using

modular UPS topology as described later.

Tier classiﬁcation and impact on PUE

With the exceptions of the largest search engines and social

media network data centres partial-load is a common feature

of data centre operations. Newly constructed ‘enterprise’

class facilities can start life carrying loads as low as 15% and

take 4–6 years to reach higher than 65%. They often never

exceed 80% load and, as we have already seen, partial load

is a barrier to high efﬁciency in data centre systems. For

single-bus ICT systems (Tier I-III), with N+1 redundancy, this

can be mitigated by scalable systems (where modules can be

disabled to keep the load factor high (e.g. 5x500kW for a 2MW

system load) or modular systems (see over).

For dual bus (Tier I-IV) scalable (and modular) UPS

architecture can help raise the efﬁciency of each bus but the

load is never likely to exceed 40% per bus, and very often will

be less than 20%. Hence the UPS contribution to the PUE will

be higher than an N+1 singlebus system unless more radical

measures are taken – such as using an eco-mode feature in

one or both buses. In the case of ecomode enablement the

usual penalty from highly partial load in dual bus systems can

be entirely overcome.

Modular UPS topology

For small and medium systems the advent of modular

systems (where rackable modules are contained within a

single infrastructure cabinet) has made the ‘right-sizing’ of

UPS to a given load easier than ever before. Expansion of

capacity is a simple matter of adding a further module and

contraction is a simple matter of turning off modules in turn.

The initial frame must, of course, be sized for the ultimate

load. The selection of the module rating should be inﬂuenced

by the load steps anticipated and the ultimate load. Hence a

100kW ultimate load may be suitable for 10kW modules and a

1MW ultimate load suitable for 200kW. Above 1MW it is usual

to engineer a multi-module scalable solution (e.g. of 500kW

modules) and provision the switchgear infrastructure for the

ultimate conﬁguration, but not necessarily installing all UPS

modules on ‘day-1’.

The aim is simply to allow the UPS to be loaded to 70–80%

at any given load – where the UPS will be able to provide its

highest efﬁciency rating.

The future

ICT loads need an effective UPS system for continuous

operation as much today as they ever have and those UPSs

have to provide operational efﬁciencies of >95% (even at

partial loads) to produce the level of infrastructure energy

efﬁciency (PUE) expected by endusers and future carbon-

reporting and possible legislation.

As energy costs rise and the reliability of eco-mode UPS

operation is proven in mature grids the UPS of the future is

likely to operate at c99% efﬁciency for >90% of the year. This

level of performance in conjunction with the most advanced

cooling systems (such as adiabatic indirect cooling with

air:air heat exchangers with CoP of 0.025) and LED motion-

controlled lighting will permit the typical PUE to be <1.10

across all of Europe.

16 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 17

Highly efﬁcient & reliable

UPS systems from

Uninterruptible Power

Supplies Ltd.

Find out which is right

for your operation.

Call 01256 386700

or email

sales@upspower.co.uk

“To move from Tier I to

Tier IV clearly increases

the potential availability

in terms of both planned

outage for maintenance

and unplanned outage

by failure.”

The upside and downside

of dual bus power

Introduction

Data centre facilities provide high-ﬁdelity power to the critical

load by the provision of Uninterruptible Power Supply systems

in various levels of redundant architectures that are well

described in the foundation work of The Uptime Institute in the

USA. When the founders of The Uptime Institute introduced

their data centre tier classiﬁcations in the early 90s they built

on their own innovation of dual-corded ICT loads. Prior to

that time ICT loads, such as enterprise servers, were single-

corded devices and there were only two possible levels of

provision in the power domain – ﬁrstly without redundant

components, which often needed a load shutdown to

carry out maintenance and where a single failure resulted in

downtime, and the second with redundant elements which

gave some opportunities for concurrent maintenance and a

degree of fault tolerance. The best example is the UPS system –

Tier I having a single module and Tier II having a redundant

system with N+1 architecture.

With the advent of the dual-corded loads, the opportunity

for concurrent maintenance expanded when Tier III introduced

the principle of an active path (containing an N+1 system) to

one cord and a separate passive path that brought power

(from the utility or generator if required) to the other cord.

For the ultimate reliability and resilience Tier IV brought

active/active to the two load cords – with an N+1 redundant

power system in each path, 2(N+1), that provided both

concurrent maintenance and fault tolerance to the single

major failure event.

It is interesting to note that once you assume that the most

basic power system (Tier I) comprises a single UPS and single

generator, there are only four possible power architectures

to support dual corded loads. Therefore it should not be

surprising to see that many ‘design’ authorities followed in

the steps of TUI and perpetuated the four tier levels, e.g.

TIA942, BICSI and the soon to be released EN50600. In the

20 years since the original tier classiﬁcations were innovated

only one change has been seen – the reduction from 2(N+1)

to 2N in Tier IV, although this has only been well described

by the originators, TUI, and not followed by such standards

as TIA942.

So what are the upsides and downsides of dual bus?

Upsides of dual bus topology

To move from Tier I to Tier IV clearly increases the potential

availability in terms of both planned outage for maintenance

and unplanned outage by failure, and the major step occurs

between Tier II and Tier III as dual-corded loads offer the

opportunity to utilise a dual bus power architecture. However

the term ‘availability’ is often misunderstood, misused and

sometimes abused deliberately for marketing purposes. At

the heart of this ‘problem’ are the original percentages that

TUI published in their original white paper: For each tier they

gave an availability percentage and expressed it as ‘X minutes

downtime per year’, e.g. Tier I offered 99.67% with 28.8

hours/year downtime compared to Tier IV, 99.99% with 53

minutes/year downtime. It should be obvious to all that one

failure per year would be unacceptable for any system (I or IV)

and the amount of downtime hardly matters when it may take

the average integrated ICT load several hours to re-engage

with the mission critical function after a loss in power.

Clearly the beneﬁt of moving up the tier layers is to extend

the Mean Time Between Failure (MTBF) although if single-

corded loads exist in the dual bus architecture then they

should be protected by point-of-use (usually rack-mounted)

static-transfer switches.

“Clearly the beneﬁt of

moving up the tier layers is

to extend the Mean Time

Between Failure (MTBF).”

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After that (single) failure the recovery time (Mean Down

Time) needs to be short as possible as, interestingly, to give

an availability ﬁgure you need to know both the MTBF and the

MDT, as follows;

MTBF

Availability = (MTBF+MDT) x100%

It is worthy of note that any system requiring an annual

shutdown of 4h for maintenance can only achieve an

availability of 99.95% (MTBF=8,760h and MDT=4h). So a high

availability can be achieved by either a long MTBF or a short

MDT but the MDT should (but usually does not) include the

ICT system re-start time.

Having pointed out the weakness in the term ‘availability’

and accepting that MDT will always be several hours, we can

better express the upside of climbing up the tier layers as a

relative MTBF of the alternative power system architecture.

Figure 1 tabulates the relative MTBF of the architectures

from N to 2(N+1) for a change in ‘N’. In this case the MTBF

describes the voltage supplied by the UPS system inside the

latest version of the ITIC/CBEMA voltage tolerance curve,

and ignores the MTBF of the downstream power distribution

systems. In the case of the dual bus active/active the MTBF

represents the event of concurrent failure of both buses.

Figure1: Relative UPS MTBF, CapEx & availability

Availability calculated with a single UPS module

MTBF=100,000 and MDT=8h

Where N= Where N=2

Architecture N=1 N=2 N=3 CapEx MTBF (y) Availability (%)

N 1 0.9 0.8 1 10 99.991112%

N+1 10 9 8 1.8 103 99.999111%

2N 800 700 600 2.3 7,991 99.999989%

2(N+1) 1000 900 800 3.6 10,274 99.999991%

So it can be seen that the MTBF of dual bus systems is

dramatically enhanced over the MTBF of a single module.

We can see in the last three columns a typical high power data

centre (where N=2) the availability based on one failure event

with a Mean Down Time of 8 hours – a 4h response on site

followed by a 4h repair or an 8h reboot time after a momentary

failure in voltage lasting longer than 20ms.

However there is an additional advantage of any dual bus

system over a single bus system:

Depending upon which analyses of data centre failure you

choose to read, you will learn that 35–70% of all data centre

failures are down to human error and most of those take place

in the electrical infrastructure. The advantage of dual bus, be

that 2N or 2(N+1), is that simultaneous human errors (i.e. one

human error in each system at the same moment) is virtually

impossible. The obvious cause of downtime in single bus

systems is inadvertent operation of the EPO and that just

can’t happen in two separate rooms. So, the chances of

human error are substantially reduced in dual bus systems.

It is worth noting that many data centre designs that are

not Tier IV per se, incorporate 2N power to enable ease of

management and maintenance without shutdown or risk.

These are often referred to as Tier III+, although TUI do not

support, in any fashion, the concept of intermediate steps in

the tier classiﬁcation hierarchy.

Historical downsides of dual bus topology

To counteract the clear advantages of dual bus there have

been penalties. Of course, if the business model of the

organisation requiring the data centre is centred only on

ultra-high availability and high fault tolerance, these

‘downsides’ are the acceptable cost of doing business.

The most obvious penalty may be the initial capital

investment in the extra redundant components, although the

relative costs are outweighed by the huge increase in relative

availability as indicated in the above table.

Additional plant-room space to house the transformers,

generators, UPS, switchgear in segregated spaces and

delivery paths, complete with environmental control, ﬁre

detection and suppression, lighting and security all add

to the initial investment. Having said that, the facility costs

represent less than 25% of the data centre 10-year Total Cost

of Ownership but the cost of power, the next downside of dual

bus architecture to be considered, is the largest element in the

TCO – in some data centre business models as much as 50%.

If we consider the original 2(N+1) architecture at partial

load, we will be able to see why the change to 2N came about

as concerns about energy efﬁciency grew, even in the USA.

Figure 2 shows the conﬁguration of 2(N+1) when N=2.

20 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 21

Let us now consider how an ‘average’ enterprise data centre

facility load develops over time:

n The critical load often starts off below 20%.

n It may climb to c40% after 2–3 years.

n Reaches a plateaux of <80% after 5 years.

n The critical load never reaches 100%.

If we consider the 2(N+1) UPS system in Figure 2 the

module load in normal operation against these indicative

20%/40%/80% load stages we can show that it is

3.3%/6.7%/13.3% respectively.

In other words, the UPS modules work at very light loads for

the vast majority of their service life. Turning one module ‘off’ in

each system to improve the load factor is only possible when

the load is below 50%, and this improves the early years to 5%

load per module at 20% facility load and 10% load per module at

40% facility load – still very low.

It was very probably this low load problem that pushed the

change to 2N, removing the (double) redundant module on each

side and relying on each system to be 100% redundant for the

other. However, we need to view this against the efﬁciency of a

typical North American legacy UPS: A typical large UPS in the

USA was thyristor based, 460V input, 208V output with input,

output and bypass transformers, often with a 6-pulse rectiﬁer

without a harmonic ﬁlter up to 600kVA. The efﬁciency at full load

was c91% but the partial load efﬁciency was poor. On Figure 3 is

shown (in red) a typical efﬁciency curve from c2005. To consider

the impact of installing this type of UPS in 2(N+1) architecture

we need to consider the efﬁciency in the sub-10% load range of

around 50%. If we add a dual bus cooling system at similar low

load it is easy to see that an operational PUE (rather than design

PUE) of >3 was very easy to achieve.

Pressures on operational costs (power and maintenance),

and the realisation that a double redundant system gave little

increased resilience for dual corded loads, resulted in Tier IV

being downgraded to 2N from 2(N+1).

Dual bus also can have an unfortunate side-effect outside of

the UPS loading – that of under-utilised plant. This led to many

forms of distributed redundancy architectures with ‘swing’

transformers or generators and often utilising static-transfer

switches. These solutions saved capital expenditure but at

the risk of increasing complexity that sometimes led to lower

reliability and the introduction of increased opportunities for

human error.

One inadvertent consequence of following TUI and TIA942

recommendations for 15 minutes of battery autonomy per

module in Tier IV systems, was that battery autonomy is not

linear with load, and the effect of very light load on the dual

bus system is to produce battery autonomies in the region of

3 hours. With power densities in the critical space gradually

increasing, any extended UPS autonomy is not usable unless

the cooling system is also continuous – since the load will shut-

down on thermal-alarm before the power is shut-off.

Overcoming the problem – the modern solution for

the legacy

There is no doubt that the extended MTBF and lower risk of

human error that dual bus architecture offers with dualcorded

loads, is as attractive now as it always was, if not more so.

However it is now possible to mitigate, if not avoid altogether,

the problems associated with partial load by utilising state-

of-the-art UPS technology and designing the system in a

scalable, modular, topology. Designing a data centre with

dual bus power and N+1 cooling, both with concurrent

maintenance capability, is generally referred to as Tier III+,

despite TUI objections.

In Figure 3 we can see the dramatic improvement in

efﬁciency, at all loads, between a legacy North American

machine and an IGBT/IGBT transformer-free design.

A B

Figure 2:

2(N+1) when N=2

Dual bus for dual-cord loads

Load = 1MW

N=2

2x 3x500 kW UPS modules

22 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 23

In the same way that modern European UPS designs have

dramatically improved in recent years, the change in part-load

efﬁciency of static-UPS has also overtaken that of rotary-UPS

in all its variants, so, for completeness, also included in Fig.3 is

the curve for a typical hybrid-rotary UPS and a typical DRUPS.

The modern solution to this legacy problem is straightforward:

n Use high-efﬁciency IGBT/IGBT transformer-free UPS that

has high partial load efﬁciency.

n Apply it in a scalable way that suits the anticipated initial

load and anticipated load growth proﬁle – with the aim of

maintaining as high a system load as is possible by turning

‘off’ any over-redundant capacity.

n In one (or even both) bus turn ‘on’ the UPS’s eco-mode

capability and virtually halve the system losses (ability to start

the UPS inverter in static bypass if mode conditions dictate).

n Try to restrict the installed battery capacity on each module

to under 10 minutes.

For most large systems this strategy could result in an overall

system efﬁciency of over 95% rather than 50% – a small price

to pay for such high availability?

UPS Efficiency

Percentage Load

15%

100

25% 50% 75% 100%

Figure 3: Comparative UPS Efﬁciency

460V/208V legacy UPS system

with transformer plus PUD

transformer for 120V load.

400V/400V IGBT/IGBT

transformer-free UPS with 230V

load.

Hybrid-Rotary UPS with battery.

Diesel Rotary UPS with battery.

Eco-Mode static-UPS.

Highly efﬁcient & reliable

UPS systems from

Uninterruptible Power

Supplies Ltd.

Find out which is right

for your operation.

Call 01256 386700

or email

sales@upspower.co.uk

24 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 25

1 The Uptime Institute, Building 100,

2904 Rodeo Park Drive East,

Santa Fe, NM 87505, USA.

www.uptimeinstitute.org

2 Industry Standard Tier

Classiﬁcations Deﬁne Site

Infrastructure Performance; Turner,

Uptime Institute, Inc

3 TIA, Standards and Technology

Department, 2500 Wilson

Boulevard, Arlington, VA 22201,

USA. www.tiaonline.org

4 BICSI, (Building Industry

Consulting Service International

Inc) 8610 Hidden River Parkway,

Tampa, FL 33637. www.bicsi.org

5 CENELEC, European Committee

for Electrotechnical Standardization

is responsible for standardization

in the electrotechnical engineering

ﬁeld. CENELEC prepares voluntary

standards, which help facilitate

trade between countries, create

new markets, cut compliance costs

and support the development of

a Single European Market. www.

cenelec.eu

A review of data centre

tier classiﬁcations

Information technology clients expect an

availability of 99.999%, ‘Five-Nines’

The substantial investment that a business makes to achieve

Five-Nines in its computer hardware and software platforms is

unlikely to be sufﬁcient unless matched by a site mechanical

and electrical infrastructure that can support their availability

goals. Data centre’s are classiﬁed by their availability which

comes down to their capability to achieve concurrent

maintenance and fault tolerance but their overall site ‘Tier’

rating is dependent upon all aspects of the site infrastructure

and will be the lowest of the individual sub system ratings

covering such aspects as power, cooling and connectivity etc.

It is important to be aware that operational issues (how the

site is operated once constructed) also plays a signiﬁcant role

in what site availability is actually achieved. All too often it is

assumed that installing a UPS is the end of any problems but,

if the overall design, installation and ongoing service support is

handled badly it could just be the beginning of the problems.

For example, it is vital to ensure that the Mean-Time-To-Repair

(MTTR) of the system is kept to a minimum if the highest

overall availability is to be achieved. Nowhere is this more

important than in the design of data centres. Each business

has a unique availability target driving the site infrastructure tier

level requirement.

After careful alignment of IT availability objectives with

site infrastructure performance expectations, an informed

client may select a site infrastructure based on one of the

tier classiﬁcations. Data centre owners/operators have the

responsibility to determine what level of functionality and

resilience is appropriate or required for their sites. As such,

it is a business decision to determine the tier classiﬁcation

necessary to support site availability objectives. Part of this

decision is to balance the IT operational practices with the

facility practices that support the IT infrastructure but once

selected the desired tier should be uniformly implemented

across all systems.

The benchmark tier standards

The Uptime Institute1 has, for nearly 20 years, sponsored

research and practical studies into data centre design,

operation and resultant resilience and developed a tier

classiﬁcation to describe and differentiate facilities from

an availability standpoint. A white paper2 from the Institute

(authors of which include the originator of dual power supplies

in IT equipment and the tier system itself) is the basis of this

review of the facility and operational concepts. The Uptime

classiﬁcation system describes four levels of availability for the

overall site, from the basic Tier I to the ultra-available Tier IV.

A later addition to TUI is a data centre ‘standard’ in ANSI/

TIA-942-2005 Telecommunications Infrastructure Standard

for Data Centers, issued by Telecommunications Industry

Association3. This follows the same Tier I-IV format and draws

heavily on The Uptime Institute publications but extends the

detail, especially in connectivity, and is more proscriptive.

It is entirely a USA centric ANSI speciﬁcation, but can be

used as a very useful guide outside of the reach of ANSI.

One point worth noting is that TIA-942 was speciﬁcally

written for telecom related data centre environments of a

power density up to 2.7kW/m2.

Another US-centric design guide was published by

BICSI4 which introduced a ‘ﬁfth’ tier but this was Tier ‘0’ and

described a data centre without UPS or generator support

that most observers would not classify as a data centre in the

ﬁrst place.

CENELEC5 is preparing a new European Standard, EN

50600, for data centre infrastructure which will also be based

on four levels (classes rather than tiers) of availability.

“It is a business decision

to determine the tier

classiﬁcation necessary

to support site availability

objectives.”

“Each business has a unique

availability target driving the

site infrastructure tier level

requirement.”

26 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 27

Why only four levels?

It was the founders of The Uptime Institute that innovated

the concept of the ‘dual-cord’ IT load and then went on

to produce a classiﬁcation system to take advantage of

that feature. Prior to the dual-cord load there was only two

options for feeding the power to a load: With a single-bus

power system that comprised a unitary string of conditioning

components that needed to be shut down for maintenance

and should a single failure occur, disconnect the load. An

improvement on this was to introduce redundancy in the

components (e.g. N+1) that gave protection from a single

failure and a degree of concurrent maintenance. Although not

described as such at the time these two options covered Tier

I and Tier II.

Adding the second power-cord to the load introduced the

concept of the dual bus power system, with an ‘active path’

including the redundant components of Tier II and a ‘passive

path’ enabling a wrap-around power connection, for truly

concurrent maintenance operations. This describes Tier III.

Tier IV, with a physically segregated ‘active-path/active-

path’ topology comprised of two independent Tier II systems,

was a very short step to very high availability, concurrent

maintenance and near total fault tolerance. It is hard, if not

impossible, to describe a ‘ﬁfth’ tier unless the load was triple-

corded, with only one out of three cords needing power for

100% compute operation.

Tier I Tier II Tier III Tier IV

Number of delivery paths Only 1 Only 1 1 Active

1 Passive

2 Active

Redundancy N N+1 N+1 S+S or

2(N+1)

Compartmentalisation No No No Yes

Concurrently Maintainable No No Yes Yes

Fault Tolerant to Worst Event None None None Yes

Getting to Five-Nines?

Concurrent maintenance and fault tolerance is the key to

the tiers and the table (above) shows the progressive level

of redundancy and resilience required and how it might be

achieved. This table refers to each of several key systems that

are identiﬁed by TUI as critical to the operation of a speciﬁc

data centre. For a facility to achieve a tier classiﬁcation it must

achieve the benchmark in all the criteria and critical power is

just one of those (sixteen) criteria.

Tier Site A% Nines MDT h/5y

I 99.670% 2 144.54

II 99.750% 2 109.50

III 99.980% 3 8.76

IV 99.990% 4 4.38

Availability – a measure of ‘goodness’?

To achieve a high-percentage availability is simple – achieve

a long MTBF (Mean Time Between Failure) and a very short

MTTR (Mean Time to Repair), the calculation simply being:

MTBF

Availability = MTBF+MTTR) x100%

TUI has assigned a target availability (A%) to each of the

tiers (table above) and sensibly recommend to measure the

downtime (MDT) over at least a ﬁve-year period, rather than

over just one year.

It will be immediately apparent to the reader that to

achieve a deﬁned overall site availability then each of the

sixteen sub-systems must achieve much higher performance

(e.g. A% raised to the power of sixteen). For the ultimate

Tier IV site this means that every sub-system (e.g. power

at the load terminals) has to achieve 99.9994% – the magic

Five-Nines.

The importance of a short MTTR

Clearly a wide range of ‘answers’ can be generated by varying

combinations of MTBF and MTTR (see right) but the reality is

that only an emergency service back-up that can minimize

travel time to site, have comprehensive spare-parts availability

and excel in ﬁrst-time ﬁx rate will achieve the sort of MTTR’s

needed to push the availability to the required level for the

higher tiers. Indeed it is quite easy to demonstrate that Tier III

cannot tolerate travel times of more than 4 hours to site if the

system is to achieve the desired availability performance –

even with MTBF’s in the 200–400,000h range.

This conclusion highlights the need for 24x7 remote

monitoring, diagnostics and tele-assisted service via data-

connectivity and a ﬁrst-class service support organization.

28 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 29

Higher tier power systems don’t come cheap

Comparing systems is rather complicated when taking into

account the type of load (single-corded or dual-corded) and

the scale of the system with its redundancy plan. In the strict

deﬁnitions Tier III & IV are only intended for dual-cord loads

without Static Transfer Switches (STS). However at the most

fundamental level we can take Tier I as the base cost and

MTBF (=1) and make relative comparisons:

Tier Load Concurrent A% MTBF COST

I Single No 99.98333% 1 1

II Single Limited 99.98547% 1 1.6

II Dual Partial 99.99965% 45 1.8

III Dual Yes 99.99983% 45 2.2

IV Dual Yes 99.99999% 2,450 3.0

Partial load problems with legacy UPS systems in

Tier IV architecture

Legacy UPS, particularly large monolithic systems, suffered

from very low efﬁciency at partial load. When this characteristic

was overlaid with a Tier IV dual bus 2(N+1) architecture

and the usual partial load of data centres the result can

be a load per UPS module of lower than 5%. Under these

circumstances it was only to be expected that very poor

power system efﬁciency was a result.

Four developments have mitigated the traditional downside

of Tier IV:

n The Uptime Institute ‘reduced’ the requirements of Tier IV

from the double-redundant 2(N+1) to 2N (where each system

is 100% redundant for the other) and raised the UPS module

load by several percentage points.

n Modern IGBT/IGBT transformer-free UPS technology has

raised the efﬁciency bar considerably – with over 96% in

double-conversion, even at 50% load.

n Modular UPS architecture has introduced the huge

opportunity to keep the load per module high and thereby

minimize the UPS power losses.

n Eco-mode technology options in UPS have enabled

efﬁciency of >98% even at 10% load – especially useful in one

of the two power-buses even when the end-user may have

reservations about eco-mode operation for all the load.

With modern technology the load can be provided with dual

bus power from high-efﬁciency double-conversion without any

of the traditional penalties of low efﬁciency.

The upside of dual bus (Tier IV) power systems

In addition to the clear advantage of several magnitudes of

increased statistical availability, Tier IV power has the potential

to raise the actual system performance if implemented

correctly: With most reports agreeing that 60–70% of all

failures in the data centre attributable to human error any

feature that protects against human intervention has the

capacity to remove instantaneous failures and including

inadvertent EPO activation.

Conclusions

Whatever tier classiﬁcation is chosen, 24x7 remote

diagnostics, tele-maintenance, spare-parts access and

sub-4 hour emergency repair performance achievement

are essential to meet the Tier III and IV availability targets.

The ﬁrst-time ﬁx rate will dictate the site availability.

Tier IV, for dual-corded loads is, by more than 1000x, the

most resilient power architecture possible. The traditional

drawbacks have been the high CapEx (typically a 35–40%

premium over Tier III), higher OpEx with partial load

inefﬁciencies and under-utilized plant that can be regarded as

wasteful of resources, However if the client needed a speciﬁc

classiﬁcation (e.g. Tier IV for a given business case) then there

was little choice but to follow TUI. For the future in Europe

the new standard, EN50600, will offer a locally applicable

Availability Class.

With modern UPS technology, modular architecture

and, optionally, eco-mode operation, all the efﬁciency

disadvantages of Tier IV are removed.

Availability: MTBF Vs

MTTR

Availability

MTTR (hours) MTBF (hours)

40,000

99.9700%

99.9750%

99.9800%

99.9850%

99.9900%

99.9950%

100.0000%

120,000

100,000

80,000

60,000

Highly efﬁcient & reliable

UPS systems from

Uninterruptible Power

Supplies Ltd.

Find out which is right

for your operation.

Call 01256 386700

or email

sales@upspower.co.uk

30 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 31

1 As deﬁned by any of The

Uptime Institute, TIA942,

BICSI or the future EN50600

The importance of a

reliable Service Partner in

data centre UPS systems

Executive Summary

Data centres are at the heart of the Digital Economy and the

ICT loads housed within them rely on UPS systems to provide

continuous service for the operator. However to achieve the

levels of availability in a data centre power system requires

not only a reliable and resilient UPS but also a rigorous and

well executed planned maintenance programme, on-site

or fast access to critical spares inventory and a rapid and

high quality intervention service to cover emergencies. This

white paper compares the required restoration time (the

combination of response, travel and repair hours) with the

data centre operators’ expectations from their design tier

classiﬁcation1, from the most basic to the highest availability

required and the role of modular UPS in the power availability

strategy is reviewed. The conclusions drawn include the

recommendation that there is no substitute for an authorised

Service Partner with experienced and factory trained ﬁeld

technicians with direct and fast access to engineering support

and spare-parts.

Planned maintenance versus emergency intervention

Data centres are continuously manned with facilities staff who

oversee the mechanical and electrical services so, in theory at

least, there are opportunities for those staff to carry out some,

but probably not all, of the routine planned maintenance

tasks if (and only if) the power system topology incorporates

sufﬁcient ‘concurrent maintenance’ capability. Those tasks

include downloading the event-logs, set-point monitoring,

ﬁlter changes, visual inspection of all connections, general

cleaning and battery cleaning/torque setting. If the concurrent

maintenance capability is dependent upon manual switching

of complete systems (including the UPS for example) then the

correct levels of system training and familiarity exercises must

be regularly carried out. Those non-routine PM work items,

probably not suitable for on-site supervisory staff to undertake,

include the following tasks:

n Battery load bank and cell impedance measurements

(annual, rising to bi-annual).

n DC capacitor changes.

n AC capacitor changes (7–9 year intervals) Swap DC with

AC on these two lines to swap. AC 8 years, DC 10 years.

The problem with these non-routine PM tasks and all UPS

failure interventions are that they are very infrequent. In the

cases of emergency interventions (actual failure of one or

more UPS functions) they are so infrequent as to be virtually

impossible to cover properly, especially 24/7 multi-shift, by

using on-site staff.

The level of documentation and constant training

requirements does not make commercial sense when

compared to an external Service Contract which provides

both capable and well-experienced technicians arriving within

a short-time on site. Natural wastage and turnover in onsite

personnel presents a huge training issue, associated with high

costs. The core skills required to rapidly fault-ﬁnd and repair a

failure in a UPS system that will usually never experience more

than one such requirement in a 10 year operational period

is what a client can expect from external technicians that do

nothing else on a day-today basis within a large installed base.

“There is no substitute

for an authorised Service

Partner with experienced

and factory trained ﬁeld

technicians with direct and

fast access to engineering

support and spare-parts.”

32 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 33

Factory upgrades

The OEM will be the only centralised repository for ﬁeld

operation and failure statistics of their particular UPS models

and where these have an effect upon performance and

reliability the end-user will beneﬁt greatly from the continuous

improvement processes that the OEM undertakes. Of course,

equipment upgrades that involve life safety will, no doubt, be

initiated by the OEM (assuming that the full traceability path

of the equipment is intact) but only by contracting with the

OEM or their authorised service agent in some form can the

end-user participate in performance related upgrades. There

are generally no processes or mechanisms by which a third-

party maintenance organisation can keep up-to-date with

such initiatives.

Emergency intervention

When emergencies arise, as they surely will during the

anticipated 12–15 years service life of most UPS systems, the

resolution will require being speedy and permanent. In UPS

terms (because a ‘power failure’ at the load can involve the

electrical cabling and switchgear infrastructure that connects

the UPS rather than the UPS itself) the failure can be described

in only four ways:

In a single-bus UPS system where a loss of redundancy

(e.g. when one module trips off-line) and there is no impact

upon the critical load: In this case the response to the failure

can be handled with a degree of calm investigation of the root

cause. If the fault has been caused by operator error then it

can usually be rectiﬁed by on-site operation staff but where the

cause cannot be identiﬁed then a qualiﬁed service technician

will be required in a matter of within 24 hours unless a service

contract is not in place.

In a single-bus system where the entire UPS system

(redundant or not) trips off-line and successfully transfers

the load to the utility supply: In this case the failure has not

impacted the load immediately but it is at immediate risk from

utility-borne interference. A manual transfer to emergency

generator supply, with controlled load and restart shutdown,

would usually be recommended although this brings additional

risk from operator error and it has to have been previously

established that the generator can support the ICT load

without the UPS in circuit.

In mature urban grids the MTBF (mean time between

failure) of the utility voltage outside of the CBEMA PQ voltage

immunity curve (embodied in IEEE466 & 1100, also known as

the ITIC Curve) is in the order of 250h but a ‘deviation’ from

the CBEMA allowable region could occur almost immediately,

or not occur for several tens of days depending upon

transmission arrangements for the location, neighbourhood

power consumers and climatic season.

Getting the UPS rapidly back on-line is of paramount

importance and the rapid intervention of an expert service

technician will be vital – usually with a response time of less

than 4 hours. To achieve that level of availability a service

contract must be in place.

In a single-bus system where the entire UPS trips off-line

and does not (for whatever reason) transfer the critical load

to the utility supply and the load is disconnected: A loss of

data centre load is a traumatic event in any business and the

operators ability to get the UPS bypass connected, probably

including a manual starting of the emergency generator

system, will not reduce the impact of the failure but only speed

up the process of recovery. All of the above comments apply

and a service engineer is required in almost every case to

diagnose, repair and reinstate the UPS system.

In a dual bus UPS system where one system is negatively

impacted but the load remains protected either by being

dual-corded or being protected by point-of-use static

transfer switches: This is where the extra investment in a

dual bus power system is rewarded and the provisioned fault

tolerance fully utilised. Clearly the failure in one of the two

buses needs to be addressed quickly but the chances of load

impact in the intervening period are negligible, if not almost

zero. This failure mode is probably the only data centre power

event where an immediate service intervention is not required,

albeit still being a desirable target.

It is important to note that the ‘failures’ we are referring

usually exclude human error but often the human error

produces exactly one of the four main scenarios listed above

and the required response is the same. Many reports have

been published that put the incidence of human operator

error causing a load-loss in a data centre as high as 70% –

so we are here talking about the UPS ‘share’ of the remaining

30%. Perhaps as low as 5% of the total failures for the power

system and <2% for the UPS system in isolation.

When considering the support requirements (as laid out in

the preceding section on Planned Maintenance) it should be

clear that an external service contract that is provisioned with

trained and experienced staff is essential for high-availability

data centre operations. Taking a statistical approach to the

problem we can easily demonstrate that, for a single-bus UPS

system, the typical system availability target of 99.997%

2 Computer Business

Equipment Manufacturers

Association

34 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 35

(e.g. a Tier III facility with 6 (Power, Cooling, Connectivity, EPO,

Fire & Security) dependent sub-systems) would permit a single

downtime event of just 8.76 hours in a ﬁve-year operational

window. This would include a 4h ‘time-to-arrival-on-site’ and

4.76h for the fault to be rectiﬁed.

Modular UPS topology

In small and medium sized single-bus data centre power

systems (for example up to c1MW, N+1) the application of

modular UPS can reduce the downtime of any individual

module drastically, even where the services of an external

on-call service technician are employed. If the on-site

staff have the spare (pre-commissioned) UPS module

and have had the training for a module-swap-out then

the downtime can be limited to less than one hour under

most circumstances.

The failed module then has to be repaired in the same

manner, but in a more relaxed and potentially error-reduced

environment. If the end-user carries redundant spare modules

then the failed unit can be returned to the authorised OEM

repair organisation for inspection, report, repair, load testing

and return – rather than repaired on site. Such a repair

service should be part of a formal support package to include

engineering support etc.

Remote monitoring connection

For many years the opportunity for remote monitoring of UPS

systems has been available but the take-up of such services

has been somewhat limited. There is little doubt that a monthly

check on set-points and generation of a health-check report

aids both the user and the service organisation, regardless of

the service arrangements.

Spare parts availability

Access to spare-parts is essential for high availability but those

spare-parts have to be of the correct generation, complete

with any upgrades, 100% compatible with the installed

machine and fully pre-tested. Only the OEM can guarantee

the compatibility and provide local inventory that reﬂects

the installed base. This inventory must support the installed

machine for at least 15 years and be accessible within 4 hours.

A rigorously maintained ‘crash-kit’ system available to the

technician engineer on a 24/7 basis for each UPS product, is

an essential part of a comprehensive service support contract.

Conclusions

The availability of power for continuous digital services from

data centres is an essential part of the modern economy

and at the heart of the data centre power system is the UPS.

To provide the highest possible availability incorporating

routine maintenance, upgrades and emergency intervention

it is recommended that the OEM is contracted as a

comprehensive service partner.

“The availability of power for

continuous digital services

from data centres is an

essential part of the modern

economy and at the heart

of the data centre power

system is the UPS.”

Highly efﬁcient & reliable

UPS systems from

Uninterruptible Power

Supplies Ltd.

Find out which is right

for your operation.

Call 01256 386700

or email

sales@upspower.co.uk

36 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 37

The application of

‘economy-mode’ in

ICT UPS systems

Executive Summary

In the pursuit of higher energy efﬁciency, particularly in the

rapid growth sector of data centres and ICT microprocessor

loads, the utilisation of high-efﬁciency, or ‘eco-mode’, UPS

is a growing topic for debate. The fundamental question

surrounds the risks, perceived or real, of continuous voltage

supply compared to the rewards of lower electricity bills.

This white paper explores the advantages to be gained from

enabling an eco-mode feature together with reviewing the

application risks.

Historical context

In historical terms, UPSL was one of the very ﬁrst companies

to introduce an ‘eco-mode’ feature in an UPS system back

in the early 90s as well as innovating the ﬁrst three-phase

transformer-less UPS on the market. In those days the

market’s appetite for such a radical feature was small – with

a background of lower power costs, higher UPS costs than

today and lower ride-through capabilities of ICT loads. After

the UPSL innovation, many OEMs have introduced eco-mode

for static UPS in one form or another.

What is eco-mode?

There is no ofﬁcial technical deﬁnition of what ‘high efﬁciency’

or ‘economy’ mode is but as most UPS are used to protect

microprocessor based ICT equipment, it is reasonable to

assume that a UPS with eco-mode enabled will provide

the critical load with sufﬁcient voltage ﬁdelity to avoid load

disruption. This means providing continuous operation within

the voltage limits set down by the ITIC/CBEMA Power Quality

curve such that the supply voltage is never zero for longer

than 20ms. Many examples of critical load can withstand

(e.g. ride-through without disruption to service) much longer

breaks in voltage, especially when at partial load and dual-

corded but in certain circumstances, especially when the ICT

equipment is single-corded and fully conﬁgured such that

the on-board power-supply is close to full load, a maximum

of 10ms is considered safer. By coincidence the pre-1997

CBEMA Curve speciﬁed a maximum of 10ms.

All static UPS that have a static-switch automatic bypass

can be equipped with an eco-mode functionality, since the

load is run on the bypass instead of the inverter – exactly the

opposite of normal operation where the bypass is waiting

to accept the load if the UPS fails or the ‘bypass’ button is

activated. The standard static-switch ﬁtted in a UPS can

sense and operate in c4ms.

In this way, eco-mode operation can be described as

a power system that operates in a high-efﬁciency state

(c98–99%) when the mains power is suitable for the critical

load, but is ready to supply the load from the inverter within

c4ms of any mains power deviation. In operation, eco-mode

is best described as simply ‘off-line’, with the inverter running

at no-load and the bypass static-switch ready and controlled

to transfer the critical load to the inverter if the mains supply

deviates from the ITIC/CBEMA limits. Thus, with eco-mode

enabled, the load is normally fed by the mains and only reverts

to the UPS when the mains power quality deviates, thus

saving energy on UPS losses. Eco-mode generally offers an

operating efﬁciency of 98–99%.

It is worth noting that some vendors prefer not to admit that

eco-mode operation is off-line operation – rather suggesting

that it is some form of line-interactive mode and energy is

saved in partially shutting down unused elements like the

cooling fans and rectiﬁer.

“The fundamental

question surrounds the

risks, perceived or real, of

continuous voltage supply

compared to the rewards of

lower electricity bills.”

38 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 39

Advantages of eco-mode

When emergencies arise, as they surely will during the anticipated

12–15 years service life of most UPS systems, the resolution will

require being speedy and permanent. In UPS terms (because a

‘power failure’ at the load can involve the electrical cabling and

switchgear infrastructure that connects the UPS rather than the

UPS itself) the failure can be described in only four ways:

1. Higher efﬁciency, lower losses, leading to a lower TCO

and improved data centre PUE.

n With the advent of ultra-high efﬁciency double-conversion

UPS (e.g. 96.5% at 50% load) the delta between normal

and eco-mode operation has narrowed to c2.5% at typical

loads. However with power costs at GB£0.09/kWh (and set

to rise 15%/year for at least the next 5 years) even that small

difference can produce a saving of £20,000 per MW of IT load

per year.

n The effect on a facilities PUE can be 0.025 which could be a

signiﬁcant improvement if the data centre PUE is below 1.20.

n If increased risk to the load is feared (see next section) then

energy saving opportunities can still exist in dual bus power

systems – where one bus can be run in double-conversion

(e.g. at 95–96% efﬁciency) and the other bus run in eco-mode

(e.g. at c98% efﬁciency).

2. Lower cooling requirements, adding to the energy

savings, equivalent to around 60,000kWh per MW of UPS

load per year.

3. Avoidance of frequency sensitive line-interactive

operation to achieve high-efﬁciency.

4. Full series-on-line double-conversion protection is

available when needed, including for when on diesel

generator operation.

Issues for consideration

As with all engineering solutions there are issues to be

considered and the enablement of eco-mode operation raises

the following:

Mains power quality

When the control system detects a mains deviation the eco-

mode operation is disabled and the UPS returns to on-line

duty. This remains the case until the mains power has been

stable for an adjustable period, usually 30–60 minutes. This

prevents the UPS from hunting between modes in times of

mains instability, such as bad weather. In grids like the UK,

where deviations from the ITIC/CBEMA Curve occur with an

MTBF of c250h intervals and last (MTTR) for c3 seconds the

eco-mode function will be enabled for 99.6% of the year. In

areas of poor power quality eco-mode is not suitable.

Transient Voltage Surge Suppression

Although it is always recommended to install a graded SPD/

TVSS surge-suppression system from the incoming mains

right down to the critical PDU, extra care should be taken

when eco-mode enablement is anticipated. In this way the

ICT will be protected from transient over-voltages (spikes)

entering the facility from the public network.

Leading power factor ICT loads

The modern trend of ICT loads is to draw current at a leading

Power Factor. It is to be ensured in the control system that

eco-mode is disabled whenever emergency generators are

feeding the system, due to their inability to export kVAR and

feed leading power factor loads with a stable voltage.

Non-linear ICT loads and current harmonics

The modern trend of ICT loads that are not fully loaded or that

are dual-corded is for the load current to be relatively high in

harmonic current. Total Harmonic Current Distortion (THCD)

as high as 35% is possible. In normal mode the UPS will shield

the incoming mains supply system from these load harmonics,

but with eco-mode enabled, these harmonics will be present

upstream of the UPS and will have to be dealt with by the

mains transformer and wiring system. An up-rated Neutral

conductor may be required.

Conclusions

High-efﬁciency, economy or eco-mode operation of modern

on-line UPS can provide considerable energy savings, whilst

providing double-conversion protection when needed and

avoiding having line-interactive partial protection just to save

energy. It can be used to great advantage in dual bus power

systems.

The perception of ‘off-line’ operation, and any risk

associated with it, must be fully weighed against the energy

savings. As energy costs rise and pressure increases on

carbon emissions, the advantages of eco-mode may come

to overwhelm the perceived disadvantages.

If eco-mode enablement is planned then the whole

system design must take into account the possible impacts –

albeit no more so than a normal-mode UPS in bypass for

maintenance purposes.

“If eco-mode enablement

is planned then the whole

system design must take

into account the possible

impacts.”

Highly efﬁcient & reliable

UPS systems from

Uninterruptible Power

Supplies Ltd.

Find out which is right

for your operation.

Call 01256 386700

or email

sales@upspower.co.uk

40 The UPS data centre handbook www.upspower.co.uk www.upspower.co.uk The UPS data centre handbook 41

Uninterruptible Power Supplies Ltd

Woodgate

Bartley Wood Business Park

Hook

Hampshire

RG27 9XA

01256 386700

sales@upspower.co.uk

www.upspower.co.uk

UPS716-01-00 Data Centre Handbook 08.02.2013

PowerWAVE 6000 –

Transformerless UPS technology for

best-in-class performance up to 5 MVA

PowerWAVE 8000DPA –

Modular, three-phase UPS providing power

protection up to 200 kVA

PowerWAVE 9000DPA –

Three-phase, modular UPS with a high operating

efﬁciency of up to 96%

Data Centre Handbook

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