White Paper UPS Dimensioning
White Paper
UPS Dimensioning
This w hite paper is about the right dim ensioning of an uninterruptible pow er supply
system w hich protects critical applications against pow er surges , failures and
blackouts.
UPS topologies & classification
Mains voltage variations occurs more often than expected. Consequences are crashes, loss
of data and cost intensive dow n times. Protection offers only an uninterruptible pow er
supply (UPS). There three UPS topologies that offers different protection levels against all
10 mains disruptions and variations.
UPS
Uninterruptible pow er supplies are combinations of pow er converters, sw itches and energy
storage media, e.g. batters, that form a pow er supply system guaranteeing ongoing supply
to the load in case of a supply voltage failure. At the same time, voltage and frequency
remain w ithin the static and dynamic limits defined for the load. The international product
standard IEC 62040-3 puts UPS units into classes 1, 2 or 3 depending on these limits.
VFD (offline) UPS Topology
Designations / classification
The term VFD comes from "Voltage and Frequency Dependent from mains supply". The
term offline UPS is also used, but is outmoded. The UPS output is dependent on changes in
the mains voltage and mains frequency if the UPS does not have any improvement
measures by tapping transformers, ENC filters or varistors. Otherw ise, almost all mains
faults are passed on to the loads in normal "mains operation". In particular in UPS units of
the VFD type, the voltage curve of the UPS output can be square-w ave or trapezoidal in
battery mode, meaning that it differs significantly from the sine w ave. By no means all loads
are suitable for this.
VFD dynamics: If a pow er cut is detected, a mechanical sw itch is used for changing over to
the inverter and thereby to battery mode – w ith a typical gap of 4 – 8 ms. Therefore, it is
clear that this UPS type can only achieve the tolerance range of class 3.
Advantages of VFD technology
VFD UPS units protect against 3 (of 9) voltage problems:
 Pow er failures
 Voltage dips

Voltage spikes
Compared to devices in the VFI/VI class, VFD UPS units have low er purchase and
operating costs, w hich means they represent the ideal choice w herever non-critical
applications or loads have to be protected.
Disadvantages of VFD technology
If a pow er cut is detected, a mechanical sw itch is used for changing over to the inverter and
thereby to battery mode – w ith a typical gap of 4 – 8 ms.
How VFD technology w orks (in an AEG PS UPS)
The UPS is connected to a shockproof socket betw een the public utility's mains and the
loads to be protected.
Under normal operating conditions, i.e. if the UPS is supplied w ith mains voltage, the
battery charger w ill ensure that the battery is alw ays completely charged.
In a UPS of this design, the current is directly passed on from the mains to the connected
devices during normal operation. If the mains supply fails then the UPS sw itches over to
battery mode.
VI (line-interactive) UPS Topology
Designations / classification
The term VI comes from "Voltage Independent from mains supply". Line-interactive UPS is
also used, but this terminology is outmoded. In accordance w ith industrial standard IEC
62040-3, w hen a UPS is classified in class 2, this means w ithin a time period from 100 ìs to
5 ìs, the output voltage is not allow ed to deviate from the tolerance range of +/- 30% under
any conditions. This requirement is met w ith VFI technology. VI UPS units are therefore in
class 2.
Advantages of VI technology
Line-interactive UPS units protect against 5 (of 9) voltage problems:
 Pow er failures
 Voltage dips
 Surge voltages
 Undervoltage
 Overvoltage
Compared to devices in the VFI class, VI UPS units have low er purchase and operating
costs, w hich means they represent the ideal choice w herever non-critical applications or
loads have to be protected.
Disadvantages of VI technology
There may be a brief supply gap, of up to 6 milliseconds, if there is a low -resistance mains
short circuit, w hich is due to a sw itching operation w ithin the UPS during the changeover
from mains to battery mode.
How VI technology w orks (in an AEG PS UPS)
The UPS is connected to a shockproof socket betw een the public utility's mains and the
loads to be protected.
The UPS is connected to a fused socket betw een the mains and the loads to be protected.
Under normal operating conditions, in w hich the UPS is pow ered w ith mains voltage, the
battery charger keeps the battery fully charged. The loads connected to the UPS are
supplied w ith voltage during this operating status via line filters that offer effective protection
against mains surge voltage spikes and high frequency interference. If there is continuous
mains undervoltage or mains overvoltage w ithin def ined ranges, the automatic voltage
regulation (AVR) provides additional stabilisation of the load voltage. As a result, voltage
fluctuations in the public utility's mains are reduced to a level w hich is acceptable for the
loads. This is performed w ithout recourse to the internal energy reserve, w hich in turn has a
positive effect on battery availability. The static bypass sw itch is activated in case of a
pow er failure. The inverter then takes over the voltage supply of the connected loads, in
order to prevent the risk of data loss or damage to the loads. The UPS continues to supply
voltage until the battery is discharged or an IT system is shut dow n and sw itched off
correctly. This standby time chiefly depends on the connected load. If the mains pow er
supply is back to normal values, the UPS w ill sw itch back the loads to mains supply. The
battery charger w ill then recharge the battery. For safety reasons (as required by German
standards, VDE), the mains input in the unit w ill be disconnected by a tw o-pole sw itch in the
event of a mains failure. Energy backfeed to the mains and voltage supply to the pins of the
mains connector are thus reliably avoided. For safety reasons (as required by German
standards, VDE), the mains input in the unit w ill be disconnected by a tw o-pole sw itch in the
event of a mains failure. Energy backfeed to the mains and voltage supply to the pins of the
mains connector are thus reliably avoided.
Operating statuses in detail
Norm al operation
During normal operation, i.e. w hen mains voltage is available, the built-in battery charger
keeps the battery fully charged and the mains voltage monitoring system sw itches the
inverter to standby mode. The connected loads are pow ered using the monitored and
filtered mains voltage, w hich is additionally stabilised by the integrated A.V.R. regulator. The
UPS capacity utilisation can be read off the bar chart of the operating panel.
Battery operation / autonom ous operation
When there is a mains failure or if the input voltage moves outside the toleranc e range, the
inverter automatically sw itches over to autonomous mode and supplies the loads w ith
voltage from the battery. This drains the capacity of the battery and it is discharged. This
status is signalled by the yellow LED BAT.MODE flashing as w ell as an intermittent acoustic
signal. During the discharge process as the battery capacity consistently drops, the LED
BAT.MODE flashes, accompanied by an intermittent acoustic signal (once every 4
seconds). When the battery undervoltage limit is reached (acoustic signal sounds every
second before this), the electronics of the UPS sw itch off the voltage supply of the loads.
VFI (online) UPS Topology
Designations / classification
The term VFI comes from "Voltage and Frequency Independent from mains supply". Online
continuous converter UPS is also used but is an outmoded term. In accordance w ith
industrial standard IEC 62040-3, w hen a UPS is classified in class 1, this means w ithin a
time period from 100 ìs to 5 ìs, the output voltage is not allow ed to deviate from the
tolerance range of +/- 30% under any conditions. This requirement can only be achieved
using VFI technology. VFI UPSs are therefore in class 1.
Advantages of VFI technology
Online continuous converter UPS systems offer optimum protection against almost all
possible kinds of mains fault. The permanent and double conversion of mains voltage from
the input to the output of the UPS means that even frequency fluctuations, oscillations and
voltage spikes are filtered out w ith an excellent level of effectiveness. The sinusoidal out put
voltage means that all kinds of load can be connected to the output. There are no internal
sw itching times or supply gaps in the UPS during the changeover from mains to battery
mode! A VFI UPS therefore guarantees complete shielding from current malfunctions as
w ell as protection against all 9 voltage problems:









Pow er failures
Voltage dips
Surge voltages
Undervoltage
Overvoltage
Sw itching spikes
Interference voltages
Frequency modifications
Harmonic distortion
VFI UPS units are clearly the first choice for critical or very sensitive applications (significant
damage potential, sensitive hardw are) and if the pow er system is subject to severe
disruptions.
Disadvantages of VFI technology
How ever, this first-class protection is offset by some disadvantages. Due to their high
electronic sw itching complexity, these systems are more expensive than offline or lineinteractive UPS systems. Also, this electronic sw itching complexity reduces the efficiency,
and also results in increased heat generation w ithin the unit, meaning that the heat has to
be dissipated w ith fans (noise).
How VFI technology w orks (in an AEG PS UPS)
The UPS is connected to a shockproof socket betw een the public utility's mains and the
loads to be protected.
Explanation of the circuit diagram:
• Mains filter w ith overvoltage protection (appliance protection / class D) and mains energy
backfeed protection
• Rectifier section w ith PFC logic (pow er factor correction unit)
• Separate battery charger w ith sw itch mode pow er supply technology
• Sealed, zero-maintenance battery system as energy storage medium w ith dow nstream
DC/DC converter unit
• IGBT inverter for continuous supply of connected loads w ith sinusoidal AC voltage
• Automatic bypass as additional passive redundancy
• Microprocessor-controlled control unit
The current flow s permanently through the UPS. The pow er section of the rectifier converts
the mains voltage to DC voltage for supplying the inverter. The circuit technology used
(PFC) enables sinusoidal current consumption and therefore operation w ith little system
disturbance. A separate, second rectifier (charging REC set up using sw itch mode pow er
supply technology) is responsible for charging or trickle-charging the battery connected in
the intermediate circuit. The configuration of this charging REC means the harmonic content
of the charging current for the battery is almost zero, w hich increases the service life of the
battery even more. The inverter is responsible for converting the DC voltage into a
sinusoidal output voltage. A microprocessor-driven control based on a pulse-w idth
modulation (PWM) guarantees, in conjunction w ith a digital signal processor system and
extremely fast pulsating IGBT pow er semiconductors of the inverter, a voltage system of the
highest quality and availability on the secured busbar.
In the event of mains faults (e.g. current failures), the voltage continues to be supplied from
the inverter to the load w ithout any interruption. From this point onw ards, the inverter draw s
its pow er from the battery instead of the rectifier. Since no sw itching operations are
necessary, there is no interruption in the supply to the load.
For safety reasons (as required by German standards, VDE), the mains input in the unit w ill
be disconnected by a tw o-pole sw itch in the event of a mains failure. Energy backfeed to
the mains and voltage supply to the pins of the mains connector are thus reliably avoided.
The automatic bypass serves to increase the reliability of the supply further. It sw itches the
public mains directly through to the load if there is an inverter malfunction. As a result, the
automatic bypass represents an extra passive redundancy for the load.
Operating statuses in detail
Norm al operation
Once you have connected the UPS to a suitable mains connection, you can start operation
using the UPS main sw itch: Normally, the UPS operates continuously. The UPS now
supplies the output w ith voltage, w hich is signalled by the mains symbol (Line LED) and the
inverter symbol (INV LED) being lit.
This is often referred to as online mode. It offers the greatest protection, in particular w hen
there are mains fluctuations and mains failures, because the loads are supplied w ith voltage
w ith no interruptions in this operating mode.
The rectifier is pow ered from the mains and converts the AC voltage statically into a
stabilised DC voltage. The inverter converts this DC voltage into a stabilised sinusoidal AC
voltage and pow ers the connected loads. The battery is automatically charged or
maintained at full charge by the battery charger. Battery charging is electronically controlled
and monitored. Malfunctions are detected and result in charging being interrupted. A signal
is also generated immediately.
Battery Operation / Autonom ous Operation
The mains is not w ithin the required tolerance range or has failed. In this case, pow er is
supplied to the inverter from the charged battery w ithout interruption. The pow er supply to
the loads is therefore also ensured in the event of a mains failure. This drains the capacity
of the battery and it is discharged. This status is signalled by the "BATTERY" symbol
lighting up, as w ell as an intermittent acoustic signal. Depending on the expansion level,
age and condition of the battery and in particular on the load to be supplied, the standby
time can vary from a few minutes to several hours.
The inverter is sw itched off if the battery voltage drops below a factory -set minimum voltage
value. Never store the unit in this condition! The discharged battery system should be
recharged w ithin a w eek at the latest.
When the voltage and frequency are w ithin the tolerance range once more, the rectifier and
the battery charger sw itch back on automatically. The rectifier then continues supplying the
inverter and the battery charger takes over charging the battery.
Bypass Operation
If the inverter is overloaded or if overtemperature is detected, e.g. also if an inverter defect
is detected, voltage is supplied to the load via the bypass that sw itches on automatically.
This is signalled by the "BYPASS" symbol. This so-called passive redundancy protects
against the total failure of the voltage supply on the secured busbar; how ever, in this
operating mode, mains faults w ould directly affect the load. As a result, the electronics
continuously attempt to sw itch back to "online" / normal operating mode (e.g. w hen the
overload or overtemperature goes aw ay).
The bypass is a mechanical link that sw itches extremely rapidly. It is located betw een the
load and the mains. The associated synchronisation unit in the bypass ensures that the
frequency and phase of the inverter voltage is synchronised w ith the mains.
Bypass mode is also adopted for a short period w hen the unit is starting up during the
synchronisation phase betw een the inverter and the mains prior to the transition into normal
operating status. This does not represent a system malfunction, how ever. Bypass operation
is exclusively initiated fully automatically by the electronics in the UPS; it cannot be forced
by manual sw itching operations.
UPS dimensioning
The difference betw een w atts and VA
Introduction
This description is intended to help you clarify the difference betw een w att and VA, and
explains how the terms in the specification of the overload protection are used correctly or
incorrectly.
Background
The pow er consumption by computers is expressed in w atts or volt amperes (VA). The
value in w atts designates the current that is actually consumed by the equipment. Volt
ampere is referred to as the apparent pow er, and is the produce of the voltage and the
r.m.s. value of the current that is received by the load. Both the w att and VA number have a
specific purpose. The value in w atts defines the actual c urrent that is draw n from the energy
supply, and the heat that the unit generates. The VA value is used for establishing the
dimensions of cables and cut-outs. The VA and w att numbers are the same for certain
electrical products such as incandescent bulbs. In computer equipment, how ever, the w att
and VA values can differ w idely, w ith the VA number being at least as large as the w att
number. The ratio betw een w att and VA is referred to as the "pow er factor" and expressed
either as a number (i.e. 0.7) or as a percentage (i.e. 70%).
The w att num ber of a com puter does not have to correspond to the VA num ber
All IT devices including computers operate w ith electronic sw itch mode pow er supply units.
There are tw o basic types of sw itch mode pow er supply units for computers, w hich are
referred to as 1) PFC pow er supply units (Pow er Factor Corrected, i.e. pow er supply units
w ith pow er factor correction) or 2) Capacitor pow er supply units. Even by checking the
device, it is not possible to say w hich pow er supply unit it is using, and also the specification
of the device does not normally provide any information in this regard. PFC pow er supply
units w ere introduced in the mid-1990s and have the peculiarity that their w att and VA
numbers are equal (pow er factor betw een 0.99 and 1.0). The w att value of capacitor pow er
supply units is betw een 0.55 and 0.75 of the VA value (pow er factor of 0.55 to 0.75). All
loads such as routers, sw itches, drive arrays and servers manufactured around 1996 or
later use a PFC pow er supply unit, meaning that the pow er factor of these units is 1. PCs,
small hubs and accessories for PCs typically operate w ith capacitor pow er supply units,
w hich means the pow er factor here is less than 1. Normally, it is about 0.65. Larger devices
manufactured before 1996 are also typically equipped w ith these pow er supply units, and
therefore have a pow er factor of less than 1.
Rating of the UPS
UPS units are designed for maximum w att and VA values. Neither the w att number nor the
VA number of the UPS is allow ed to be exceeded on a continuous basis. It is in line w ith the
industry standard for the w att number of smaller UPS systems to be about 60% to 70% of
the VA number. The ratio is expressed as the pow er factor cos phi = P/S (see also Physical
principles).
Exam ple of a design problem :
Consider a typical UPS w ith 1000 VA. The user w ants to supply a unit w ith 800 W using the
UPS. This means the load has 800 VA and a pow er factor of 1. Although the load is 800
VA, w hich is w ithin the VA value of the UPS unit, the UPS is not suitable for a load of this
type because the 900 W load exceeds the w att number of the UPS w hich is typically 60% of
1000 VA, therefore about 600 W.
Avoiding rating errors
The information on the nameplate is often given in VA. This means it is difficult to find out
the w att number. Simply by looking at the nameplate of a unit alone might mean that the
user w ill develop a system w hich is correctly rated in accordance w ith the VA number but
w hich actually exceeds the w att number of the UPS. When selecting the UPS, use the
available real pow er, i.e. the W value as the basis. Although this cautious approach to rating
means that the UPS w ill normally be rated slightly too highly, how ever this offers the
advantage of consumption reserves, better overload capability and, in particular, a longer
standby time compared to that in new operation.
Conclusion
Information about current consumption is often not specified in a w ay that allow s the UPS to
be configured straightforw ardly. This possibly leads to systems being developed w hich
appear to be correctly rated, but w hich in effect overload the UPS. A UPS that is slightly
over-dimensioned w ith regard to the data on the nameplate of the device ensures correct
operation of the system. In addition, over-dimensioning offers the advantage that there is
more time available for a backup.
Glossary of technical terms
AVR (Automatic Voltage Regulation)
Automatic voltage regulation to prevent mains voltage deviations
DC/DC Booster
Circuit technology for increasing a DC voltage to a higher voltage level
EPO (Emergency Power Off)
Emergency switch-off device
PFC (Power Factor Correction)
Switching technology for minimising system disturbances (particularly important when connecting
non-linear loads)
Appliance protection
Term from surge voltage technology. Classic mains overvoltage protection consists of a lightning
surge arrester (Class B), overvoltage protection (Class C) and, finally, what is referred to as
appliance protection (Class D)
IGBT (Insulated Gate Bipolar Transistor)
High-performance transistors of the latest design with an ultra -low control power requirement
(MOSFET structure) and ultra-low losses on the output side (structure of a bipolar transistor)
Class D
See: Appliance protection
LED (Light Emitting Diode)
Electronic semiconductor component, usually referred to by its acronym, used for optical signalling
PWM (PulsWeitenModulation)
Here: circuit technology for generating a sinusoidal voltage of the highest quality from an existing
DC voltage
SNMP (Simple Network Management Protocol)
Frequently encountered protocol in networks for managing / handling components
VFD (Output Voltage and Frequency Dependent from mains supply)
The UPS output is dependent on mains voltage and frequency fluctuations. Earlier designation:
OFFLINE
VI (Output Voltage Independent from mains supply)
The UPS output is dependent on mains frequency fluctuations, but the mains voltage is prepared
by electronic / passive voltage control units. Earlier designation: LINE-INTERACTIVE
VFI (Output Voltage and Frequency Independent from mains supply)
The UPS output is independent of mains voltage and frequency fluctuations. Earlier
designation: ONLINE
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