Application Note – UPS Power System Design Parameters

APPLICATION
UPS POWER SYSTEM DESIGN PARAMETERS
Shri Karve
August 2012
ECI Publication No Cu0115
Available from www.leonardo-energy.org/node/156501

Publication No Cu0115
Issue Date: August 2012
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Document Issue Control Sheet
Document Title: Application Note – UPS Power System Design Parameters
Publication No: Cu0115
Issue: 01
Release: August 2012
Author(s): Shri Karve
Reviewer(s): Bruno De Wachter
Document History
Issue Date Purpose
1 August
2012
Initial publication, in the framework of the Good Practice Guide
2
3
Disclaimer
While this publication has been prepared with care, European Copper Institute and other contributors provide
no warranty with regards to the content and shall not be liable for any direct, incidental or consequential
damages that may result from the use of the information or the data contained.
Copyright© European Copper Institute.
Reproduction is authorised providing the material is unabridged and the source is acknowledged.

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CONTENTS
Summary ........................................................................................................................................................ 1
Introduction.................................................................................................................................................... 2
Basic functions of UPS systems....................................................................................................................... 2
Different types of basic UPS systems .............................................................................................................. 2
Rotary UPS systems ................................................................................................................................................3
Diesel rotary UPS systems........................................................................................................................3
Hybrid rotary UPS systems.......................................................................................................................3
UPS with mechanical flywheel .................................................................................................................4
Static UPS systems..................................................................................................................................................4
Static UPS main components .......................................................................................................................... 6
UPS System efficiency................................................................................................................................... 10
UPS Input breaker sizes ................................................................................................................................ 10
UPS size selection ......................................................................................................................................... 11
Site location.................................................................................................................................................. 11
Generic Specifications for a UPS System ....................................................................................................... 11
Generator set sizing...................................................................................................................................... 11
Conclusion .................................................................................................................................................... 12
References.................................................................................................................................................... 12
Annex: Generic Specifications for a UPS System ........................................................................................... 13

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SUMMARY
This application note is intended to be a source of guidance and to help reduce confusion pertaining to the
design, configuration, selection, sizing, and installation of Uninterruptable Power Supply (UPS) systems. This
document is a useful information source for electrical consultants, electrical engineers, facility managers, and
design and build contractors.
In the recent past, many design engineers have tried to create the perfect UPS solution for supporting critical
loads. However, these designs have generally overlooked coverage for changing load profiles (e.g. leading
power factor), sleep mode, and advanced scalability solutions. Such solutions and/or options can assist in
gaining higher system efficiency, without exposing the critical load to disruptions from the utility.
This paper presents information related to various generic types of current UPS units, complete with their
merits and demerits. It covers different topologies and various system solutions for clients. Auxiliary items,
such as the battery bank, diesel generator set, and switchgear are included in the document since they also
form an integral part of a UPS system.
To aid in the reduction of the carbon footprint, the paper has indicated achievable operational efficiency
figures for different solutions.
A typical generic UPS Specification has been included as an Appendix to this paper to support electrical
engineering professionals.

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INTRODUCTION
In our modern, digital world we all have an expectation that power failure or disturbance is not an option.
Most of us are dependent upon readily available and reliably functioning critical business, telecommunication,
banking, medical, and other applications. We expect to access information or the ability to carry out
commercial transactions on demand, 24 hours a day, 7 days a week, and every day of a year. There is an
essential need therefore to have an electrical supply that has a more than merely adequate resilience and
availability incorporated to support critical applications.
It is possible to achieve a high quality power supply by eliminating single points of failure and utilizing UPS
systems with superior availability. A suitable standby generator system can provide cover for any long-term
utility outages.
UPS power systems now form part of the value chain for most companies since power quality and availability
have direct impact on the continuity of their operations. In some cases, a major discontinuity may even
jeopardize survival of the business itself.
BASIC FUNCTIONS OF UPS SYSTEMS
A UPS ensures continuity of the power supply regardless of fluctuations or interruptions in the utility supply.
This is an essential requirement for all critical applications such as IT/data centres, stock exchanges, aerospace
applications, et cetera. Such fluctuations and interruptions can have major consequences if there is even a
momentary break in the supply.
Add to this requirement the fact that the UPS needs to provide a clean and stable power supply, free from
voltage distortion, frequency variations, electrical noise, harmonics, spikes, brownouts, and surges.
Disturbances of these sorts can damage equipment. If any of the power quality based issues listed above occur
in the mains supply at a significant level then critical loads and computer systems can fail. However, the most
basic type of UPS system (and one which is not a genuine online system) provides a very low degree of
protection from poor power quality of mains supply. Hence such basic units are not recommended for major
critical applications.
The key purpose of a UPS system is to act as a buffer between the raw electrical mains supply and sensitive
equipment such as medical scanners and radar systems, et cetera. A general survey of the UPS market
indicates that the proportion of principal users is as follows: 70% administration plus data processing and
banking; 25% telecoms, marine, and industrial applications; with the remaining 5% spread across a wide
variety of other applications.
In the event of brownout or blackout, the UPS provides necessary—albeit limited—backup from a stored
energy source incorporated within the system. If the utility fails for a prolonged period, then most critical
systems are backed up by an alternative source, for example, a standby generator.
DIFFERENT TYPES OF BASIC UPS SYSTEMS
The following three different UPS topologies are stated within the EN/IEC62040-3 Standard:
 VFI—UPS output is independent of input mains supply voltage and frequency variations
 VI—UPS output is dependent on input mains supply frequency variations but mains supply voltage
variations are conditioned
 VFD—UPS output is dependent on mains supply voltage and frequency variations

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The topology chosen for installation must provide adequate protection for the critical load and meet the
requirements of the specific application. The UPS also needs to meet the specific demands of the load profile
with regard to power quality. In some cases the load may exhibit a high inrush current or a leading power
factor due to modern Blade Servers. Typical computers can tolerate steady state slow averaged line voltage
variations between approximately +5% to +10%, depending upon the manufacturer. However, short duration
excursions outside these mains voltage limits can be tolerated. Most computers have an adequate amount of
stored energy within their power supply units to support the DC to logic circuits. Loads are now getting
greener, i.e. utilizing more active power. It is likely that in the very near future most UPSs will be rated in kW
and not in apparent power (kVA).
UPS systems can be divided between two generic types: rotary and static. These are fundamentally different in
their construction, method of operation, and protection of the load. Static UPSs account for almost 98% of the
UPS market share with rotary UPSs making up the remaining 2%. The primary reasons for the difference in
market shares are costs, topology, size, and resilience. The merits and demerits of each generic type are
discussed below.
ROTARY UPS SYSTEMS
A rotary UPS generally incorporates a motor and/or alternator unit plus a diesel engine and a kinetic energy
storage unit. Under normal operating conditions, it is powered by the mains supply and produces clean and
stable power for critical loads. Most Rotary UPSs fall outside of VFI type topology due to their inherent
operational design aspects. The way in which a rotary UPS continues to drive the alternator in the event of a
mains failure depends on the type of individual system. There are three basic types: diesel rotary UPS systems,
hybrid rotary UPS systems, and simple mechanical flywheel backed UPS units. Diesel rotary units are generally
noisy and are only available in higher (500 to 1,600 kVA) power ranges. Step load performance is rather poor in
that they can take up 100 msec. to stabilize compared to Static UPS options. Rotary units generally exhibit a
very high mechanical component count. This results in a higher rate of equipment failure than Static UPS
options. Repairs can also take longer since some of the components are rather bulky. Initial cost of rotary units
can be high (40-50% higher than similar Static UPS system) and their scalability is limited due to larger ratings.
In some cases the units can save on the space requirement for installation. Rotary units have a better fault
clearing capacity compared to other types of UPSs. However, in terms of efficiency, they do not match other
types of UPSs when compared across the entire load range. There are very few rotary unit manufacturers,
mainly because their market share is small and unpredictable from year to year.
DIESEL ROTARY UPS SYSTEMS
Diesel rotary UPS systems contain a device often referred to as an induction type coupling. This is an electro-
mechanical eddy current based flywheel that stores kinetic energy able to last for a few seconds (3 to 6). In the
event of a mains failure, the energy stored in the induction type coupling is used to maintain the required
alternator shaft speed while the diesel engine is started and brought up to speed. Once the diesel engine
speed has stabilized, it can then commence to support the load. Such units are able to provide voltage
correction to the load by means of an inline choke. To provide frequency correction, however, it has to
operate in emergency mode, i.e. to switch to diesel operation. Hence this type of unit is not classified as a true
online UPS.
HYBRID ROTARY UPS SYSTEMS
Hybrid rotary UPS systems do not incorporate a diesel engine or induction type coupling. Instead, they use a
rectifier, batteries, and an inverter to provide the ac power needed to support the motor alternator in the
event of a mains disturbance or failure. Such systems range from 300 kVA to 800 kVA as single units. A standby
diesel generator will be required for the long-term support of critical loads that require power at all times
regardless of the mains blackout period. Such units are not classified as true online UPS since there is an

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operational switching process (from direct mains to rectifier-inverter path) taking place when the mains input
is falling short.
Figure 1—Block diagram—Hybrid Rotary UPS.
UPS WITH MECHANICAL FLYWHEEL
These units are mainly designed to provide a ride through lasting only 10-15 seconds. This is sufficient to
enable the standby generator to start up and provide support for the critical load. We will not cover these
types of units in detail since their market share is extremely small. The majority of the suppliers of this type of
UPSs are small young companies with a weak financial base. Several of these manufacturers have closed down
or gone into administration in recent years. Due to their company size they are only able to provide limited
after sales support to clients. Ratings are available from 60 KVA to 250 KVA. Repair times can be very long and
rather expensive. Flywheel energy recharge time can be a risk due both to expected step loads and in the
event of consecutive brownout situations. Recovery from possible step load condition is slow and can take a
few cycles before it reaches steady state condition. The time to repair and the very high initial cost of these
units must be taken into account. If the backup generator fails to start in a timely manner, the flywheel system
does not have enough energy left to protect the load and enable an orderly shutdown to save computer data.
Given the points just mentioned, very few consultants and clients have opted for such UPS solutions. Such
units are not true online (VFI) UPS systems since they are dependent upon the switching function.
STATIC UPS SYSTEMS
Static UPSs make up almost 98% of the UPS market and their power can range from approximately 100 VA to
1,100 KVA per unit. Static systems utilize a frontend rectifier and a DC link connected to the output stage
inverter module. When the input utility mains supply is within acceptable limits, the input power is converted
from AC to DC by the rectifier. The DC link is utilized for recharging the battery bank. The majority of the DC
power is designated for the inverter unit, which converts the DC power into tightly regulated clean AC sine
wave output to supports the critical load.
We will not cover the smaller micro or mini static UPS units since most of these are for small, consumer market
applications. The main emphasis of this document is to look at Static, true online double conversion UPSs in
compliance with VFI topology. The VFI topology provides high degree of protection for the critical load when
compared with off-line or line-interactive systems. Please refer to figures depicting various topologies within
this document.

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Figure 2—UPS with the double-conversion online topology.
Figure 3—Simplified diagram of off-line UPS.
Figure 4—UPS with the passive standby topology.
Note: The Inverter is connected in parallel and acts simply to back up utility power.

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A typical online static UPS has a frontend rectifier/charger which normally carries out two functions, i.e.
simultaneously float charging the battery bank and providing stable DC power via the DC link (also called DC
bus) for the inverter. The DC link will have suitable filters to reduce AC ripple presented to the battery in order
to extend battery life expectancy. The rectifier circuit includes a current limiter device and DC overvoltage
device to protect the battery, DC filter, and the inverter. If the mains supply has failed for any reason the
rectifier shuts down and the battery bank provides DC power to the inverter without a break. The battery
begins to discharge while it is providing power to the load via the inverter. There is normally an alarm available
for alerting critical load users that the UPS is now on battery operation as well as displaying (at an appropriate
location for the customer) the precise amount of backup time remaining. Most critical loads have the UPS
system backed up by suitable diesel generator sets. The generator set back up enables sizing the UPS battery
bank for a shorter duration, typically 10 to 15 minutes. The generator set can offer backup via an automatic
changeover in the event of a long-term blackout. However, if the generator is not available or fails, then the
UPS system will shut down at the end of battery autonomy period. The length of this period is subject to the
load level. Some UPSs are designed to attempt load transfer to static bypass, prior to final imminent shutdown
of the UPS system, provided a separate Mains 2 supply is available.
The inverter is connected to the load via a fast acting thyristor static switch, and mains supply side
disturbances are blocked by the DC link and the inverter. Mains borne transient voltage excursions and noise
are thus barred and a normal tightly regulated voltage is provided from the inverter. The same static switch is
utilized to transfer the load to bypass during unit maintenance or when there is an internal UPS fault (the so-
called make before break connection principle). In such a situation, it is preferable to connect the load to the
unregulated mains rather risking total loss of power to the critical load.
STATIC UPS MAIN COMPONENTS
In deciding upon a UPS system, it is worth considering main components that exhibit a true online double
conversion UPS, since other types of UPS units do not provide full protection for critical loads.
This type of UPS is made up of a rectifier, DC link, battery bank, inverter, and static switch. Under normal
operation with the mains in a healthy state, the rectifier provides the necessary float charging to the battery.
At the same time, the DC link supports the continuous demand of power from the inverter, which in turn
protects the load. Since the battery is connected permanently to the DC link, it assumes the role of supporting
the load during any mains supply disruptions.
Rectifiers in traditional UPSs utilize full-wave phase controlled SCRs rather than diodes. They are available with
either a 6 pulse or 12 pulse bridge, depending upon UPS rating and/or manufacturer. The six pulse rectifier has
an approximately 30% Total Harmonic Current Distortion (THDI), while the twelve pulse rectifier has about
10% THDI. In each case, a suitable harmonic filter is required. This helps limit reinjection of harmonic pollution
into the upstream mains supply, and to comply with local or IEC standards. In the past, UPS manufacturers
offered passive LC filters. Recently some are providing hybrid filters that combine passive and active filters.
Passive filters are not very effective at partial loads. They may impose a leading power factor to the mains
upstream, and can have compatibility issues when operating with standby generator sets. Over the past few
years, most major manufacturers have begun to offer new transformerless UPSs with an active frontend
rectifier. Such a design utilizes a boost-converter type switched mode power supply with several benefits. The
advantages are very low harmonic distortion (3% THDI), as well as a built-in power factor correction that helps
to create a typical power factor of almost unity at full load. Such rectifiers can save space and accept a wide
range of input voltage and frequency.
Most UPS units are fitted with temperature compensated rectifiers to avoid damaging the battery at high
ambient temperature.

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The Inverter Block converts the DC link voltage into an AC output with a tight control on tolerance to suit
critical load applications. Inverter output is always synchronized to Mains 2 sine wave supply of 50 Hz (or 60 Hz
for the North American market). This is important; enabling the load can be transferred to static bypass
without a break. Over the years, there has been a major shift in inverter power component technology. It
started with SCRs, continued with the move to bi-polar transistors, and has currently begun to employ
Insulated Gate Bipolar Transistors (IGBT) with very high switching frequencies, typically 2.2 kHz/sec. These
changes have helped improve the efficiency of UPS systems and contributed to the reduction of noise levels
and ecological foot print. Output wave form is generated with the use of pulse width modulation (PWM). This
is in conjunction with an output transformer/choke combination, which provides a very clean sine wave output
voltage suitable for non-linear loads.
Most UPSs are designed to withstand overloads of between 120 to 150 % for a limited time. Under such
conditions, the inverter will operate in current-limit, and may also function with reduced output voltage. If
overload persists beyond the pre-set time, then the load gets transferred to static bypass without a break.
A static bypass switch is necessary to protect the load with a feed from inverter or utility supply, either due to
overload or UPS malfunction. The built-in control and intelligence constantly monitors the mains condition and
the phase angle. This is critical in order to achieve a transfer without a break, i.e. from inverter to mains or
from mains back to inverter. A Static UPS can offer fault clearing level of about 2.3 X In for a few milliseconds.
However if this is not adequate, then the fault is transferred to the static bypass. During the UPS selection
process, consideration should be given the fault clearing capacity of the static switch in order to avoid a
bottleneck and/or affect discrimination.
A maintenance bypass is essential and is normally built into the UPS to provide the isolation necessary during
maintenance or repair. For larger units, it is better to have an external wraparound bypass. This should provide
total isolation with suitable breakers and Castell/Kirk Key interlocks (electrical or mechanical type) to achieve a
no break transfer, as well as meet health and safety requirements.

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Figure 5

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Figure 6
Batteries (accumulators) are one of the key components of static UPS systems. They provide necessary storage
for backup energy when a utility fails or is outside the agreed tolerance level. Typical autonomy times vary
from 10 to 20 minutes. Applications dictate the backup time, but in order to reduce the battery size, due
consideration must be given the generator set. The choice of battery type is usually made by the equipment

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supplier and/or consultants. Users need to be aware of the type of battery used and the maintenance
procedures required since these parameters may influence the choice of equipment and the related TCO.
Stationary batteries are used when weight is not important, and are usually of the sealed lead acid (SLA) type
because of their lower cost. For larger UPSs, it is recommended that a battery is employed that utilizes a 10
year design life. Note that SLA batteries need to be kept within operating temperature range of 10 to 25
o
C to
achieve optimum life expectancy. Most UPS units are fitted with temperature compensated rectifiers to avoid
damaging the battery at high ambient temperatures. Vented lead acid cells can be utilized for longer life but
they are more expensive, demand more maintenance, and may require more space in addition to posing a
greater a risk of hydrogen emission. There is a trend towards the use of the Sodium Nickel (Zebra) battery for
UPS applications. The Zebra battery has more benefits compared to the standard SLA unit, but there are still
some limitations both on the technical and commercial front. SLA batteries are available for use within a UPS
cabinet, in separate cabinets, or on steel racks. Careful consideration needs to be given to the selection of the
battery monitoring system—from string monitoring right down to block monitoring—depending on UPS size
and budget. Most good UPSs have basic built-in battery monitoring system. However, clients should be aware
that basic monitoring systems do not offer life expectancy projections or cell performance trend
measurements. Under normal operation, the battery bank is on float charge but following any blackout the
battery will require full recharge from the rectifier. A typical battery bank may take several hours to reach
repeat duty charge level or full capacity.
It is worth noting the difference between design life and operational life of SLA batteries. A 10 year design life
battery may last only about 7 years, even when used and maintained in line with the manufacturer’s
recommendations for ambient temperature, routine impedance testing, et cetera.
Since batteries are particularly heavy, it is important to make sure that structural issues, such as point
loadings, are addressed beginning at the project design stage. A number considerations need to be considered,
including safety steps during installation and future regular maintenance.
UPS SYSTEM EFFICIENCY
The full load efficiency of a typical double conversion true online UPS can range from 93% to 97%. This
depends on the manufacturer and the type of UPS design, from traditional rectifier (6 or 12 pulse) to Active
front-end power factor corrected rectifier. It is important that UPS selection is NOT based simply on its
projected high efficiency; due consideration needs to be given to its operational mode as well.
There are UPSs that offer higher efficiency by operating in bypass/stand-by/ECO mode option. This ECO mode
is utilized while the mains are in a healthy state; otherwise the unit switches back to a double conversion path.
Be aware that the ECO mode does not protect the load to the same extent as a unit that operates continuously
in online mode.
System operational efficiency of parallel units can be enhanced by an automatic sleep mode. Such a mode
starts to operate only when the actual load drops to a figure that is well below the installed capacity and
without sacrificing the planned redundancy.
UPS INPUT BREAKER SIZES
Most large UPS system installations are designed with two separate breakers—Mains 1 and Mains 2. Mains 1
feeds the online path, i.e. rectifier input while Mains 2 feeds the bypass path, i.e. static switch path. Typical
sizing should be based on international and local guidelines and on information provided by the UPS
manufacturer. Other design parameters such as discrimination and expected short-circuit currents
downstream of each breaker must be considered as well.

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The Mains 1 beaker needs to deal with UPS input current at full load plus necessary battery recharge current.
In other words, allowance must be made for UPS efficiency, input power factor (across the load range), plus an
added 25% to the full load current for battery recharge needs.
The Mains 2 breaker should be sized to cover full load current, power factor, and any loads that may have
some inrush current demand.
Battery breakers need to be sized with input from the UPS manufacturer since it is based on lowest DC link
voltage and battery autonomy time.
UPS SIZE SELECTION
Apart from normal basic information—utility voltage, frequency, and load current—consideration must be
given to the load power factor across entire load range. This should include Blade Server type loads with
leading power factor. UPS sizing should take into account the load inrush current and the non-linear profile of
load currents, including crest factors. Both the apparent (kVA) and the active (kW) power load consumption—
with expected load variation over time—need to be considered. If lighting loads made up of Gas discharge and
fluorescent fittings are to be supported, then the UPS will have to be sized for very high inrush current
demand.
Allow for necessary redundancy, future expansion, scalability, and a high degree of unbalanced single phase
loads. If multiple parallel UPS units are planned, then it is better to have suitable walk-in sequential transfer
designed within the UPS system. This will avoid oversizing the stand-by generator. It may pay to select a
modular type UPS unit, since it provides better scope and flexibility for scalability without major disruption to
the critical load. It also achieves better operational efficiency.
SITE LOCATION
Ensure that the UPS location is safe from flooding, since sites selected by architects for electrical power and
UPS installation tend to be in basements. If possible, plan to have the units placed on steel plinths to help
protect from flooding. If the UPSs are mounted on plinths, this can also ease system cabling as most UPSs are
suitable only for bottom cable entry.
It is important to provide adequate access at the back (800-1,000 mm) and front (1,000-1,200 mm) of units to
ease servicing. Overhead water pipes should be avoided since any leaks may cause serious damage to the UPS
system. Battery rooms will require air conditioning to keep the environmental temperature between 10 and 25
°C. However, some UPS modules may only require clean air ventilation to avoid excess heat in the UPS plant
room. Some applications may require a special type of UPS to meet environmental demands. For example, a
marine application may need a unit that is suitable for a salty atmosphere. A UPS system for a
semiconductor/chip manufacturer will need units with special anti-vibration mountings to limit vibrations
being passed on to the production platform.
GENERIC SPECIFICATIONS FOR A UPS SYSTEM
See Annex.
GENERATOR SET SIZING
The following points should be noted in the event that the system is designed with a requirement for a
standby generator back up. A generator would provide for longer power breaks exceeding normal battery
autonomy. It may be also necessary for business critical requirements.

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It is recommended that the generator be approximately 1.5 times the size of the UPS system KVA, as this will
provide better resilience for the load. Diesel engine cooling and lubrication needs to be kept on pre-heat since
this will help achieve a quick engine start. Diesel engine must have an electronic governor rather than
mechanical type.
Generator should have following items for better compatibility:
1. Alternator that is suitable for non-linear loads
2. Battery charger with a monitor
3. Alternator that can handle leading power factor load, to cover for blade servers
4. Automatic mains failure panel
5. Engine with better step load performance
6. Switchgear panel with built-in load bank hook up facility
CONCLUSION
UPS systems to support critical loads have been in use for decades. In recent years, the utilization of UPS
systems has undergone substantial evolution and growth in innovation. Most of the early rotary UPSs have
been replaced by static units using the latest IGBT technology.
Selection of the right UPS must take into account a number of key characteristics. These include performance,
efficiency across the load range, reliability, TCO, weight, size, and ease of maintenance. While scalable UPS
systems and sleep mode options can significantly raise system efficiency, such solutions are only appropriate if
they do not sacrifice system resilience.
Since the IT industry—a major UPS customer—continues to grow at an exponential rate, UPS demand will also
continue to grow strongly.
REFERENCES
 APC by Schneider Electric—Design Guide
 IEC62040-3 UPS Topology
 EN50091-1: UPS-Safety, EN50091-2: UPS-EMC, EN50091-3: UPS—Performance
 "Three of a Kind-UPS", Shri Karve, IEE review, March 2000

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ANNEX: GENERIC SPECIFICATIONS FOR A UPS SYSTEM
1. GENERAL
1.1 DESCRIPTION
1.1.1 Supply and install a UPS system complete with integral system static bypass switch, maintenance by-
pass switch, and harmonic filter.
1.1.2 The system shall consist of a unitary or modular scalable [xxx] kVA UPS system. The UPS unit shall be
capable of meeting the required [xxx] minutes of autonomy at full load during loss of mains.
1.1.3 The completely integrated uninterruptible AC power system (UPS) shall provide regulated and
transient-free AC power from the unregulated AC mains supply, i.e. under normal conditions including
total mains failure and return of normal power.
1.1.4 The system shall automatically bypass the critical load directly to the AC mains supply in order to
maintain power to the load in the event of a UPS malfunction or major short circuit on the load side.
1.2 SYSTEM DESCRIPTION: UPS MODULE COMPONENTS
1.2.1 Rectifier/Charger
1.2.2 Static inverter
1.2.3 Input and bypass circuit isolators, battery breaker, and inverter disconnect isolator
1.2.4 Integral or external system static bypass switch
1.2.5 Microprocessor controlled logic and control panel with status indicators and alarms
1.2.6 Valve regulated sealed lead acid batteries mounted on steel stands
1.2.7 Battery circuit breakers and transition cubicles
1.2.8 Input harmonic filter required, if active front end rectifier is not utilized with PFC
1.2.9 Maintenance by-pass switch complete with necessary Castel interlocking
1.3 MODES OF OPERATION
1.3.1 The UPS shall be designed to operate as an online double conversion (VFI to IEC 62040-3) transfer
system in the following modes.
1.3.1.1 Normal: The critical load shall be continuously supported by the inverter. The rectifier shall
derive power from the AC supply and provide DC power to the inverter while simultaneously
float charging the battery. The inverter shall be synchronized with the Mains 2 line so that the
load can be transferred from the inverter to the Mains 2 path in the event of a system
overload or inverter stop, without any interruption in the power supply to the critical load.
1.3.1.2 Emergency: When the utility supply is outside the pre-set tolerance or fails totally, the critical
load shall then be protected by the battery and the inverter. A visible and an audible signal
shall alert the user of this emergency state of operation.
1.3.1.3 Restoration of primary AC Source: Upon return of the Primary AC source to within the
tolerance limits, the UPS shall start operating in normal mode again. Even if the battery is
completely discharged, the rectifier/charger shall automatically restart and assume both the
inverter and battery recharge load demands.
1.3.1.4 Static Bypass Operation: In the event of an overload exceeding system capabilities, or inverter
shutdown, the static bypass switch shall instantaneously transfer the load to the bypass AC
source without interruption on the condition that the bypass power is available and within
voltage/frequency tolerances. Transfer back to the inverter output can be automatic or manual
and without interruption or disturbance to the critical load. It is assumed that there are no
chokes and capacitor or Active Harmonic filters fitted within the static bypass path since this
may affect fault clearing capacity and discrimination for this circuit.

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1.3.1.5 UPS Maintenance: The UPS shall include a manually operated mechanical by-pass switch for
maintenance purposes. For safety during servicing or testing, this system shall be designed to
isolate the rectifier, inverter, and static switch while continuing to supply the power to the
load from the bypass AC source. Transfer to the manual bypass mode and back shall be
possible without interruption to the load. It shall also be possible to isolate the rectifier from
the normal AC source.
1.3.1.6 Battery Maintenance: To facilitate service maintenance for the battery, it will be possible to
disconnect the battery from the DC link by means of a circuit breaker. The UPS will continue to
function and support the critical load without battery.
1.3.2 UPSs with operational mode transfers switching between VFD, VI and VFI to achieve higher
efficiency, hereby exposing the critical load to risky switching, are not acceptable.
2. SIZING AND GENERAL CHARACTERISTICS
2.1 TECHNOLOGY
The UPS Inverter shall be based on IGBT technology and a free-frequency chopping mode.
2.2 POWER RATING
The UPS shall be sized to continuously support a load of [xxx] KVA, at a power factor of 0.8 lagging to 0.9
leading and crest factor of 3:1. System shall be suitable for Blade Server type loads.
2.3 BATTERY BACKUP TIME
The battery backup time in the event of a normal AC source outage shall be [xxx] minutes. Battery design life
shall be at least 10 years. Refer to battery technical specification for further details.
2.4 TYPES OF LOAD
If all of the connected loads are 100% non-linear, the UPS shall support load with crest factor of 3:1 without
the need to derate. For both linear and non-linear loads the THDU downstream shall comply with the following
limits.
 THDU downstream ph/N  5%
 THDU downstream ph/ph  3%
2.5 LIMITATION OF RE-INJECTED HARMONICS UPSTREAM OF THE UPS
It shall be possible to equip the rectifier/charger with an Active Harmonic Conditioning (Sine Wave—Schneider
type) system or IGBT active rectifier and built-in PFC to limit Total Harmonic Current Distortion (THDI)
upstream of the rectifier/charger to 4% at any load and any input power factor greater than 0.94.
2.6 EFFICIENCY
Overall efficiency shall be greater or equal to
 97% at the full rated load (In)
 95% at the half rated load (In/2)
3. MAINS SUPPLY
3.1 NORMAL AC SOURCE (MAINS 1)
The normal AC source supplying the UPS rectifier unit will have the following characteristics under normal
operating conditions:
 Voltage: 400 volts,  15%

Page 15
 Number of phases: 3 phases + earth
 Frequency: 50Hz  10%
3.2 BYPASS AC SOURCE (MAINS 2)
The bypass power supplying the UPS in the event of an inverter shutdown or an overload condition shall have
the following characteristics:
 Voltage 400 volts  10%
 Number of phases 3 + N + earth.
 Frequency 50Hz  5%.
4. ELECTRICAL CHARACTERISTICS
4.1 RECTIFIER / CHARGER
4.1.1 INRUSH CURRENT
A walk-in circuit shall eliminate overcurrent during start up by imposing a gradual increase of the rectifier input
current until the nominal conditions are reached. This walk in time will be of 15-seconds duration.
4.1.2 LIMITING CURRENT
For operational battery life greater than 5 years, the charging current shall be automatically limited to the
maximum value as specified by the battery manufacturer (0.1x C10) for a sealed lead acid battery. The current
drawn by the rectifier shall also be limited to avoid overloading the power supply line.
4.1.3 TEMPERATURE COMPENSATED CHARGER
In order to protect the battery during high ambient temperature conditions (in excess of 25 °C), the charger
shall reduce the level of recharge current as recommended by battery maker.
5. OPERATING MODES/DC VOLTAGE LEVELS
5.1 FLOAT CHARGE MODE
The battery charger output voltage shall be set to the value specified by the battery manufacturer.
5.2 AUTOMATIC CHARGING MODE
In the event of a normal AC source outage lasting longer than the battery autonomy time, a battery charging
cycle shall be automatically initiated upon restoration of the normal source. Fast charging without lowering
the battery performance shall be possible, this cycle should consist of two charging phases, the first a constant
current and the second a constant voltage. The constant voltage for the second phase shall be as specified by
the battery supplier. On completion, the DC voltage shall return to the float charge value.
5.3 MANUAL CHARGING MODE
The UPS shall also include a manually initiated 24-hour charge cycle. On completion, the DC voltage shall
return to the float charge value.
6. INPUT POWER FACTOR
The rectifier/charger shall have an input power factor greater than or equal to 0.94 for the normal AC source
rated voltage and frequency with the inverter operating at full load.
7. VOLTAGE REGULATION

Page 16
The rectifier shall ensure that the DC output voltage fluctuates by less than 0.5% irrespective of load and AC
input voltage variations.
8. BATTERY
The battery shall be sized to ensure continuity in the supply to the inverter for at least [xxx] minutes up to the
end of life of the battery, in the event of the normal AC source failure with the inverter operating at full load—
kVA at a power factor of 0.8. The batteries shall be of the VRLA type to BS6290 Pt. 4 1997 made by Exide or
Yuasa, mounted on a steel rack. Sizing calculations shall assume an ambient temperature of 15 to 20
centigrade. The batteries shall be protected against deep discharge.
9. INVERTER
9.1 The inverter shall be sized for Blade Server type loads and to supply a rated load of [xxx] kVA at 0.8pf lag
to 0.9 leading and shall comply with the specification as listed below.
9.1.1 Rated Voltage: 380/400/415 Vrms, adjustable to  3%
9.1.2 Number of phases: 3 phases + neutral + earth
9.1.3 Steady state voltage regulation: Within  1% for a balanced load between 0 and 100% of the rated
load
9.1.4 Voltage transients: Output voltage transients shall not exceed 5% of the rated voltage for 0 to 100%,
or 100 to 0% for step loads. In all cases the voltage shall return to within the steady state tolerances in
less than 20 milliseconds.
9.1.5 Phase to phase harmonic distortion: The UPS shall be designed with a system limiting the THD of the
phase to phase output voltage to 3% and the individual harmonic distortion to 1.5% irrespective of
the type of load.
9.1.6 Output frequency: 50 Hz  0.5 Hz, adjustable up to 2 Hz
9.1.7 Overload capacity: 110% of load for 1 hour, 125% of load for 10 minutes. and 150% load for 1 minute
10. SYNCHRONIZATION WITH BYPASS POWER
10.1 Bypass power is within tolerance: To enable the transfer to bypass power the inverter output voltage
shall be synchronized with the bypass source voltage. During normal operation, the synchronization system
shall automatically limit the phase deviation between the voltage to less than 3 degrees. If the bypass source is
a generator the synchronization tolerances shall be  2 Hz.
10.2 Bypass power outside tolerance: The inverter shall switch over to free running mode with internal
synchronization, regulating its own frequency to within 0.04%. When bypass supply returns to within
tolerances, the inverter shall automatically resynchronize.
11. STATIC BYPASS
11.1 Static bypass function: Facilitates the instant transfer of the load from the inverter to the bypass supply
and back without disturbance to the load. Transfer shall take place automatically in the event of a major
overload or an internal malfunction within UPS. Manual initiation of this transfer as well as transfer back
to the inverter shall also be possible.
11.2 Compliance with previous sub-paragraph 1.3.2.5 is a key requirement.
 Uninterrupted automatic transfer shall be inhibited under the following conditions
 When transfer to the bypass is activated manually or remotely
 In the event of multiple transfer-retransfer operations in the control circuitry limited to three
operations in any 10 minute period. The fourth transfer locks the load on the bypass source.
 UPS failure
12. MECHANICAL CHARACTERISTICS

Page 17
12.1 Modularity: The UPS shall be of modular design so as to allow the installed capacity to be easily increased
on site by connection of additional UPS units, either to meet new load requirements or to enhance system
reliability by introducing redundancy.
12.2 Enclosures: The UPS shall be housed in a free standing housing sufficiently strong and rigid to withstand
handling and installation operations without risk. Access to the UPS shall be through front doors equipped with
locking facilities.
12.3 Dimensions: The UPS shall require as little floor space as possible. The UPS height shall not exceed
1950mm and passage through a 900mm wide door shall be possible
12.4 Cabling: Entry for the power cables, including any auxiliary cables shall be possible from the bottom of
the UPS module.
12.5 Ventilation: The UPS shall be provided with forced-air cooling. To avoid UPS shutdown in the event of fan
failure, redundant fans shall be provided and a fan failure shall initiate an alarm.
12.6 Safety: The equipment shall comply with ingress protection of IP21, in line with IEC 60529. For the safety
of maintenance personnel, the cabinet shall be provided with a manually operated mechanical bypass
designed to isolate the rectifier/charger, inverter, and static switch while continuing to protect the load via
mains.
13. ENVIRONMENTAL CONDITIONS
13.1 UPS (excluding battery): The UPS, excluding the battery shall be capable of operating under the following
environmental conditions without loss of performance.
 Ambient temperature: 0 °C to + 40 °C
 Recommended temperature range +15 °C to +25 °C
 Maximum average temperature 35 °C for 24 hours.
 Maximum temperature: 40 °C for 8 hours
 Maximum relative humidity: 95% at 25 °C
 Maximum altitude: 1,000 meters above MSL
13.2 Battery: The battery shall be capable of operating under the following environmental conditions.
 Ambient temperature: -15 °C to +40 °C
 Recommended temperature: +15 °C to 25 °C
 Maximum relative humidity: 95% non-condensing
 Maximum altitude: 1,000 meters above MSL
14. PROTECTION
14.1 UPS: The UPS shall include protection against mains over voltages (as per standard IEC 60146), excessive
external or internal temperature rises, and vibrations during transport.
14.2 Rectifier: The rectifier shall be equipped to receive an external command to automatically shut down
under the following circumstances.
 Emergency off. In this case the shutdown will be accompanied by opening of the battery circuit
breaker.
 Battery room ventilation fault. The rectifier and the charger shall automatically shut down if the DC
voltage exceeds the maximum value as specified by the battery manufacturer.

Page 18
14.3 Discharge time shall be limited to three times the backup time at full rated load to avoid excessive
damage to the battery bank.
15. USER INTERFACE AND COMMUNICATION
15.1 Operating and start up assistance: The UPS shall be equipped with an operating and start up assistance
including.
 Display of installation parameters, configuration, operating status, alarms, and indication of operator
instructions
 Logging and an automatic or manual initiated recall of all important status changes
15.2 Controls: Two push buttons on the front panel of the UPS shall Control UPS ON/OFF status.
 Forced transfer or forced shutdown of the inverter when the bypass AC source is outside specified
load tolerances
 Equipment self-test and battery charge cycle
15.3 Indications: The following information shall be monitored by alpha-numeric display or indicating lights on
the front UPS display panel.
 Rectifier on
 Load on inverter
 Load on bypass
 General alarm
A buzzer shall warn of faults, malfunctions, or operation on battery power. The system shall have an alarm
reset button.
15.4 Display of parameters: A display panel on the front of the UPS will indicate the following parameters.
 Remaining battery back-up time.
 Internal fan fault or over temperature
 Bypass AC source outside tolerance or not available
 Battery malfunction
15.5 Measurements: The display unit shall also indicate the following:
 Inverter output, phase to phase voltages
 Inverter output currents and frequency
 UPS input voltage, current and frequency
 Voltage across battery bank
 Battery charge or discharge current
 Rectifier input currents
 Load Crest factor
 Active and apparent power
 Load Power factor
 Load as % of UPS rated output
15.6 Communication: The UPS shall be designed to enable the extension of communications without system
shutdown to:

Page 19
 A building and energy management system utilizing a RS485 serial link communication board capable
of implementing the J-Bus protocol.
 A computer network management system. The UPS shall be supplied with an SNMP communications
board for connection to an Ethernet network.
 IP address
16. MAINTENANCE
16.1 The UPS subassemblies shall be accessible from the front.
16.2 The UPS shall be equipped with a self-test function to verify the correct system operation.
16.3 The UPS control and monitoring assembly shall be fully microprocessor based. This shall allow:
 Auto compensation of component drift
 Self-adjustment of component sub-assemblies
 A socket for connection to a computer-aided diagnostics system
16.4 UPS system shall be tested annually by utilizing a load bank to check various parameters including
autonomy of the battery.
16.5 Emergency call out telephone number shall be displayed on UPS.
16.6 Date of last service by UPS manufacturer and due date for the next service shall be recorded on a chart
placed inside the UPS door.
16.7 UPS maintenance shall be carried out by trained engineers from the UPS manufacturer and not any
other third party service firms. This will help to protect the original Product Liability Insurance cover
provided by the UPS manufacturer.
17. STANDARDS AND TESTS
17.1 The UPS equipment shall be designed in accordance with the standards as listed below:
 IEC 146-4: UPS-Performance
 EN50091-1: UPS-Safety
 EN50091-2: UPS-EMC
 ENV50091-3:UPS-Performance
 IEC60950/EN 60950: safety of IT equipment, including electrical business equipment
 IEC 61000-2-2: Compatibility levels for low-frequency conducted disturbances and signalling in public
low voltage power supply systems
 IEC 61000-3-4: Limits for harmonic current emissions
 IEC 61000-4: EMC- electrical fast transient/burst immunity
 EN 55011: Limits and methods of measurements of radio interference characteristics of industrial,
scientific, and medical (ISM) radio-frequency equipment—Level A conducted and radiated emissions
 IEC 439: Low voltage switch gear and control gear assemblies
 IEC 60529: Degrees of protection provided by enclosures (IP Code)
 ISO 3746: Sound power levels
 BS 6290 Pt.4 1997
 IEC 62040-3
 CE marking

Page 20
18. TEST PROCEDURE
18.1 The UPS manufacturer shall provide for full equipment test at the place of manufacture. Final inspection
and adjustments shall be documented in a report drawn up by the supplier’s Quality Inspection department.
ISO 9001 certification of the production site is compulsory.
18.2 UPS system shall be site tested after completion of installation and cabling. Suitable load bank shall be
utilized to establish system integrity and battery autonomy.
19. COMMISSIONING
19.3 Commissioning of the UPS on the site shall be carried out by the manufacturer or an approved
representative.
19.4 It shall include on-site acceptance testing with possible requirement of system integrated testing. Basic
training for the site engineers shall be provided after successful commissioning of UPS system.
20. QUALITY SYSTEM
The UPS design procedure shall be covered by an ISO9001 quality system.
21. REPLACEMENT PARTS
The supplier undertakes to provide replacement parts for at least 10 years following the date of delivery.
22. WARRANTY
The UPS system shall be guaranteed (parts and labour on site) for one year following the start-up date.
23. SERVICES
23.1 Supply of the UPS and any accessory parts or elements.
23.2 Delivery, off load and positioning.
Optional Services to be quoted separately:
 UPS positioning and installation at site
 DC cabling between the UPS and the battery
 AC cabling between the input switchboard and the UPS unit
 AC cabling between the bypass AC source and the UPS bypass
 AC cabling of the load circuit to the UPS output
 Witness testing of UPS units at manufacturer’s factory in presence of two engineers from the
consultant

Application Note – UPS Power System Design Parameters

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Application Note – UPS Power System Design Parameters