Theoretical review
Contents
Supplying sensitive loads .............................................. 2
Types of electrical disturbances ...........................................................2
Main disturbances in low-voltage electrical power................................3
UPSs ................................................................................. 4
The UPS solution .................................................................................4
UPS applications ..................................................................................5
Types of UPSs ................................................................. 7
Static or rotary UPS ..............................................................................7
Types of static UPSs ............................................................................9
UPS components and operation .................................... 16
Components of a UPS ..........................................................................16
Main characteristics of UPS components .............................................19
Summary diagram for main characteristics ..........................................24
UPS operating modes ..........................................................................25
UPS configurations ...............................................................................26
Technology ...................................................................... 28
Transformerless UPSs .........................................................................28
Electromagnetic compatibility (EMC) ............................ 34
Electromagnetic disturbances ..............................................................34
EMC standards and recommendations ................................................35
UPS standards................................................................. 36
Scope and observance of standards ....................................................36
Main standards governing UPSs ..........................................................36
Energy storage ................................................................ 39
Possible technologies ...........................................................................39
Batteries ...............................................................................................39
Flywheels .............................................................................................43
UPS / generator-set combination ................................... 46
Use of a generator................................................................................46
UPS / generator-set combination .........................................................46
Transient load conditions............................................... 48
Review of inrush currents .....................................................................48
Harmonics........................................................................ 49
Harmonics ............................................................................................49
Characteristic harmonic values ............................................................51
Non-linear loads and PWM technology ......................... 54
Non-linear load performance of UPSs using PWM technology ............54
Comparison of different sources...........................................................57
Free-frequency chopping .....................................................................58
PFC rectifier ..................................................................... 60
APC by Schneider Electric
01/2012 edition
p. 1
Supplying sensitive loads
Types of electrical
disturbances
Power distribution systems, both public and private, theoretically supply electrical
equipment with a sinusoidal voltage of fixed amplitude and frequency (e.g. 400 volts
rms, 50 Hz, on low-voltage systems).
In real-life conditions however, utilities indicate the degree of fluctuation around the
rated values. Standard EN 50160 defines the normal fluctuations in the LV supply
voltage on European distribution systems as follows:
• Voltage +10% to -15% (average rms values over 10-minute intervals),
of which 95% must be in the +10% range each week.
• Frequency +4 to 6% over one year with ±1% for 99.5% of the time (synchronous
connections in an interconnected system).
Practically speaking, however, in addition to the indicated fluctuations, the voltage
sine-wave is always distorted to some degree by various disturbances that occur on
the system.
Î See White Paper WP 18 “The Seven Types of Power Problems”
See WP 18 Î
Origins of disturbances
Utility power
Utility power can be disturbed or even cut by the following phenomena:
• Atmospheric phenomena affecting overhead lines or buried cables:
- lightning which can produce a sudden voltage surge in the system,
- frost which can accumulate on overhead lines and cause them to break,
• Accidents:
- a branch falling on a line, which may produce a short-circuit or break the line,
- cutting of a cable, for example during trench digging or other construction work,
- a fault on the utility power system,
• Phase unbalance,
• Switching of protection or control devices in the utility power system, for load
shedding or maintenance purposes.
User equipment
Some equipment can disturb the utility power system, e.g.:
• Industrial equipment:
- motors, which can cause voltage drops due to inrush currents when starting,
- equipment such as arc furnaces and welding machines, which can cause voltage
drops and high-frequency interference,
• Power electronics equipment (switch-mode power supplies, variable speed drives,
electronic ballasts, etc.), which often cause harmonics,
• Building facilities such as lifts which provoke inrush currents or fluorescent lighting
which causes harmonics.
Types of disturbances
Disturbances that are due to the above causes are summed up in the following table,
according to the definitions contained in standards EN 50160 and ANSI 1100-1992.
APC by Schneider Electric
01/2012 edition
p. 2
Supplying sensitive loads (Cont.)
Disturbances
Power outages
Micro-outages
Outages
Voltage variations
Voltage sags
Overvoltage
Characteristics
Main causes
Main consequences
Total absence of voltage ≤ 10 ms.
Atmospheric conditions, switching,
faults, work on the utility.
Faulty operation and loss of data
(computer systems) or interrupted
production (continuous
processes).
Total absence of voltage for more
than one period:
- short outage: ≤ 3 minutes
(70% of outages last less than 1 s)
- long outage: > 3 minutes
Atmospheric conditions, switching,
faults, incidents, line breaks, work
on the utility.
Depending on the duration,
shutdown of machines and risks
for people (e.g. lifts), loss of data
(computer systems) or interrupted
production (continuous processes).
Reduction in the rms value of
voltage to less than 90% of the
rated value (but greater than 0%),
with return to a value greater than
90% within 10 ms to 1 minute.
Temporary increase to more than
10% over the rated voltage, for a
duration of 10 ms to a few
seconds.
Atmospheric phenomena, load
fluctuations, short-circuit on a
neighbouring circuit.
Shutdown of machines,
malfunctions, damage to
equipment and loss of data.
- Quality of utility generators and
transmission systems.
- Interaction between generators
and load fluctuations on the utility
power system.
- Switching on the utility power system.
- Stopping of high-power loads
(e.g. motors, capacitor banks).
Peak in consumption, when the
utility cannot meet demand and
must reduce its voltage to limit
power.
- For computer systems:
corruption of data, processing
errors, system shutdown, stress
on components.
- Temperature rise and premature
aging of equipment.
Undervoltage
Drop in voltage lasting from a few
minutes to days.
Voltage spike
Sudden, major jump in voltage
(e.g. 6 kV).
Close lightning strikes, static
discharges.
Processing errors, corruption of
data, system shutdown.
Damage to computers, electronic
boards.
Voltage unbalance
(in three-phase systems)
Condition where the rms value of
the phase voltages or the
unbalances between phases are
not equal.
- Induction furnaces.
- Unbalanced single-phase loads.
- Temperature rise.
- Disconnection of a phase.
Instability in the frequency.
Typically +5%, - 6% (average for
ten-second time intervals).
- Regulation of generators.
- Irregular operation of generators.
- Unstable frequency source.
Flicker in lighting systems due to a
drop in voltage and frequency
(< 35 Hz).
Welding machines, motors, arc
furnaces, X-ray machines, lasers,
capacitor banks.
These variations exceed the
tolerances of certain instruments
and computer hardware (often ±
1%) and can therefore result in the
loss or corruption of data.
Physiological disturbances.
Sudden, major and very short
jump in voltage.
Similar to a voltage spike.
Atmospheric phenomena
(lightning) and switching.
< 1 µs
Amplitude < 1 to 2 kV at
frequencies of several tens of MHz.
> 1 µs and ≤ 100 µs
Peak value 8 to 10 times higher than
the rated value up to several MHz.
> 100 µs
Peak value 5 to 6 times higher than
the rated value up to several
hundred MHz.
Distortion of the current and voltage
sine-waves due to the harmonic
currents drawn by non-linear loads.
The effect of harmonics above the
25th order is negligible.
Electromagnetic or electrostatic
conducted or radiated disturbances.
The goal is to ensure low emission
and high immunity levels.
Starting of small inductive loads,
repeated opening and closing of
low-voltage relays and contactors.
Faults (lightning) or high-voltage
switching transmitted to the lowvoltage by electromagnetic coupling.
Stopping of inductive loads or
high-voltage faults transmitted to
the low-voltage system by
electromagnetic coupling.
Electric machines with magnetic
cores (motors, off-load
transformers, etc.), switch-mode
power supplies, arc furnaces,
variable speed drives.
Switching of electronic components
(transistors, thyristors, diodes),
electrostatic discharges.
Frequency variations
Frequency fluctuations
Flicker
Other disturbances
HF transients
Short duration
Medium duration
Long duration
Harmonic distortion
Electromagnetic compatibility (EMC)
APC by Schneider Electric
01/2012 edition
Shutdown of computer systems.
Corruption or loss of data.
Temperature rise.
Premature ageing of equipment.
Destruction of equipment,
accelerated aging, breakdown of
components or insulators.
Oversizing of equipment,
temperature rise, resonance
phenomena with capacitors,
destruction of equipment
(transformers).
Malfunctions of sensitive electronic
devices.
p. 3
UPSs
The UPS solution
Modern economic activities are increasingly dependent on digital technologies which
are very sensitive to electrical disturbances.
As a result, many applications require a backed up supply of power to protect against
the risk of disturbances in utility power:
• Industrial processes and their control/monitoring systems - risks of production
losses,
• Airports and hospitals - risks for the safety of people,
• Information and communication technologies related to the internet - risks of
processing shutdowns with very high hourly downtime costs due to the interruption in
the exchange of vital data, required by global companies.
UPSs
A UPS (uninterruptible power system) is used to supply sensitive applications with
secure power.
A UPS is an electric device positioned between the utility and the sensitive loads that
supplies voltage offering:
• High quality: the output sine-wave is free of any and all disturbances in utility
power and within strict amplitude and frequency tolerances,
• High availability: the continuous supply of voltage, within the specified tolerances,
is ensured by a backup supply of power. The backup supply is generally a battery
that, if necessary, steps in without a break in the supply to replace utility power and
provide the backup time required by the application.
These characteristics make UPSs the ideal power supply for all sensitive
applications because they ensure power quality and availability, whatever the state
of utility power.
Components of a UPS
A UPS generally comprises the main components listed below.
Rectifier/charger
It draws utility power and produces a DC current to supply the inverter and charge or
recharge the battery.
Inverter
It completely regenerates a high-quality voltage output sine-wave:
• Free of all utility-power disturbances, notably micro-outages,
• Within tolerances compatible with the requirements of sensitive electronic devices
(e.g. tolerances in amplitude ± 0.5% and frequency ± 1%, compared to ± 10% and ±
5% in utility power systems, which correspond to improvement factors of 20 and 5,
respectively.
Note. The term inverter is sometimes used to designate a UPS, when in reality it is
only a part of the UPS.
Battery
The battery provides sufficient operating backup time (6 minutes to a number of
hours) by stepping in to replace utility power as needed.
Static bypass
The static bypass ensures no-break transfer of the load from the inverter to direct
utility power and back. No-break transfer is carried out by a device implementing
SCRs (sometimes called a static switch).
The static bypass makes it possible to continue supplying the load even if an internal
fault occurs or during maintenance on the rectifier/charger and inverter modules. It
can also serve for transfers to call on the full power available upstream in the event
of overloads (e.g. short circuits) exceeding UPS capacity.
During operation on the static bypass, the load is supplied directly by utility power
and is no longer protected (operation in downgraded mode).
Maintenance bypass
This bypass may be used to supply the load directly with utility power, without calling
on the inverter or the static switch. Transfer to the maintenance bypass is user
initiated with switches. By actuating the necessary switches, it is the means to isolate
the static bypass and the inverter for maintenance, while continuing to supply the
load in downgraded mode.
APC by Schneider Electric
01/2012 edition
p. 4
UPSs (Cont.)
HV system
HV/LV
transformer
Normal utility power
(disturbances and
system tolerances)
UPS
Non-sensitive loads
Rectifier/
charger
Battery
Inverter
Maintenance
bypass
Static
bypass
Reliable power
(no disturbances, within
strict tolerances
and available due to
battery backup power)
Sensitive loads
Fig. 5.1. The UPS solution.
UPS applications
APC by Schneider Electric
UPSs are used for a wide range of applications requiring electrical power that is
available at all times and not affected by disturbances on the utility power system.
The table below presents a number of applications.
For each, it indicates the sensitivity of the application to disturbances and the type of
UPS that is suitable for protection.
The applications requiring this type of installation are:
• Computer systems,
• Telecommunications,
• Industry and instruments,
• Other applications.
The required UPS typologies are presented on page 9, "Types of static UPSs".
They include static UPSs implementing the following typologies:
• Passive standby,
• Interaction with the distribution system,
• Double conversion.
01/2012 edition
p. 5
UPSs (Cont.)
UPS applications
Application
Protected devices
Computer systems
Data centres
- Large bays for rack-mounted servers
- Internet data centres
Company networks
- Sets of computers with terminals and
peripheral devices (tape storage units,
disk drives, etc.)
Small networks and
- Networks made up of PCs or
servers
workstations, server networks (WAN,
LAN)
Stand-alone computers - PCs, workstations
- Peripheral devices: printers, plotters,
voice mail
Telecommunications
Telecommunications
- Digital PABXs
Industry and instruments
Industrial processes
- Process control
- PLCs
- Numerical control systems
- Control systems
- Robot control/monitoring systems
- Automatic machines
Medical and laboratories - Instrumentation
- Scanners (60 Hz)
Industrial equipment
- Machine-tools
- Welding robots
- Plastic-injection presses
- Precise regulation devices (textile,
paper, etc.)
- Heating equipment for manufacture of
semi-conductors, glass, pure materials
Lighting systems
- Public buildings (elevators, safety
equipment)
- Tunnels
- Runway lighting in airports
Other applications
Special frequencies
- Frequency conversion
- Power supplies for aircraft (400 Hz)
*
low sensitivity to disturbances.
*****
high sensitivity to disturbances.
APC by Schneider Electric
Protection required against
MicroOutages
Voltage
outages
variations
Frequency Other
variations
*****
*****
*****
*****
*****
Double conversion
*****
*****
*****
*****
*****
Double conversion
****
****
***
***
**
Interaction with the
distribution system
**
**
*
*
**
Passive standby
*****
*****
*****
*****
*****
Double conversion
***
*****
***
***
****
Double conversion
****
*****
****
****
***
Double conversion
***
****
***
***
***
Double conversion
**
****
***
***
**
Double conversion
Interaction with the
distribution system
****
****
****
*****
***
Double conversion
01/2012 edition
UPS type
(see p. 8)
p. 6
Types of UPSs
Static or rotary UPS
See WP 92 Î
Static or rotary UPS solutions
There are two main types of UPSs (figure 5.2 and details in Î White Paper WP 92 "Comparison of Static and Rotary UPS") which basically differ in the way the UPS
inverter function is implemented.
Static solution
These UPSs use only electronic components to perform the inverter function. A
"static-inverter function" is obtained.
Rotary solution
These UPSs use rotary machines to perform the inverter function.
A "rotary-inverter function" is obtained.
These UPSs in fact combine a motor and a generator with a highly simplified static
inverter.
The inverter filters out utility-power disturbances and regulates only the frequency of
its output voltage (generally in "square-wave" form) which supplies a regulated
motor/generator set that is sometimes combined with a flywheel.
The motor/generator set generates an output voltage sine-wave, taking the inverter
output frequency as the reference.
Fig. 5.2. Static and rotary UPSs.
Comparison
Rotary solution
The arguments often put forward in favour of this solution are as follows:
• High generator short-circuit current on the order of 10 In (ten times the rated
current) that makes setting of protection devices easier,
• 150% overload capacity (of the rated current) over a longer period (two minutes
instead of one),
• Downstream installation galvanically isolated from upstream AC source due to the
motor/generator set,
• Internal impedance providing high tolerance to the non-linear loads frequently
encountered with the switch-mode power supplies used by computer systems.
APC by Schneider Electric
05/2012 edition
p. 7
Types of UPSs (Cont.)
Static solution
Compared to the advantages of rotary solutions
The static UPSs from APC by Schneider Electric offer the advantages listed below.
• Operation in current-limiting mode (e.g. up to 2.33 In for MGE Galaxy 5000) with
discrimination ensured for circuits rated up to In/2.
These features, which are more than sufficient in practice, prevent the disadvantages
of rotary systems:
- overheating of cables,
- the effects of an excessive short-circuit current and the corresponding voltage drop
on sensitive devices, during the time taken by protective devices to clear the fault.
• 150% overload capacity (of the rated current) for one minute.
The two-minute overload capacity is of no practical use because most overloads are
very short (less than one second, e.g. in-rush currents of motors, transformers and
power electronics).
• Galvanic isolation, when required, by means of an isolating transformer.
• Double-conversion operation which completely isolates the load from utility power
and regenerates the output voltage with precise regulation of the voltage amplitude
and the frequency.
• Very low internal impedance for higher performance with non-linear loads due to
the use of power-transistor technologies.
Other advantages
) Static solutions provide many other advantages as well, due to power-transistor
technology combined with a PWM chopping technique.
• Simplified overall design, with a reduction in the number of parts and connections,
and in the number of possible causes of failure.
• Capacity to react instantaneously to utility-power amplitude and frequency
fluctuations by means of microprocessor-controlled switching regulation based on
digital sampling techniques. The voltage amplitude returns to regulated conditions (±
0.5% or ± 1% depending on the model) in less than 10 milliseconds for load step
changes up to 100%. Within the indicated time interval, such a load step change
produces a load voltage variation of less than for example ± 2% for MGE Galaxy PW
and Galaxy 5000.
• High, constant efficiency whatever the percent load, which is a major advantage
for redundant UPS units with low percent loads. A static UPS unit with a 50% load
maintains high efficiency (94%), whereas the efficiency of a rotary UPS drops to the
88-90% range (typical value), which directly impacts on operating costs.
• Redundant configurations providing high availability in the framework of ultrareliable supply systems (e.g. for data centres).
• Possible integration in redundant architectures with separate functions that
facilitate maintenance by isolating parts of the installation.
Rotary systems integrate the UPS, the backup power and the generator as a single
component, thus making it impossible to separate the functions.
• No single points of failure. Rotary systems incorporating flywheels depend on the
capacity of the motor to start quickly (typically in less than 12 seconds). This means
the motor must be in perfect condition and rigorously maintained. If it does not start,
there is no time to shut down the critical loads in an orderly manner.
) Consider also the following non-negligible advantages:
• reduced dimensions and weight,
• no wear on rotating parts, hence easier and faster maintenance. For example,
rotary systems require checks on the alignment of the rotating parts and the
replacement of the bearings after 2 to 6 years is a major operation (lifting equipment,
heating and cooling of the bearings during the replacement).
Conclusion
Given the advantages presented above, static UPSs are used in the vast majority of
cases, and for high-power applications in particular.
) In the following pages, the term uninterruptible power supply (UPS) is taken to
mean the static solution.
APC by Schneider Electric
05/2012 edition
p. 8
Types of UPSs (Cont.)
Types of static UPSs
Standards
UPSs
Due to the vast increase in the number of sensitive loads, the term "UPS" now
includes devices ranging from a few hundred VA for desktop computers up to several
MVA for data centres and telecommunications sites.
At the same time, different typologies have been developed and the names used for
the products on the market are not always clear (or even misleading) for end users.
That is why the IEC (International Electrotechnical Commission) established
standards governing the types of UPSs and the techniques used to measure their
performance levels, and those criteria were adopted by Cenelec (European
standardisation commission).
Standard IEC 62040-3 and its European equivalent EN 62040-3 define three
standard types (topologies) of UPS and their performance levels.
UPS technologies include:
● Passive standby,
● Line interactive,
● Double conversion.
AC input power
These definitions concern UPS operation with respect to the power source including
the distribution system upstream of the UPS.
The standards define the following terms:
• Primary power: power normally continuously available which is usually supplied
by an electrical utility company, but sometimes by the user's own generation,
• Standby power: power intended to replace the primary power in the event of
primary-power failure,
Practically speaking, a UPS has one or two inputs:
• Normal AC input (or Mains 1), supplied by primary power,
• Bypass AC input (or Mains 2), supplied by standby power (generally speaking via
a separate cable from the same main low-voltage switchboard (MLVS).
UPS operating in passive-standby mode
) The UPS is installed in parallel to the utility and backs it up. The battery is
charged by a charger that is separate from the inverter.
Operating principle
• Normal mode
- The inverter operates in passive standby mode.
- The load is supplied by utility power via a filter which eliminates certain
disturbances and provides some degree of voltage regulation.
- The standards do not mention this filter and speak simply of a "UPS switch". They
also indicate that "additional devices may be incorporated to provide power
conditioning, e.g. ferroresonant transformer or automatic tap-changing transformer".
• Battery backup mode
- When the AC input voltage is outside specified tolerances for the UPS or the utility
power fails, the inverter and the battery step in to ensure a continuous supply of
power to the load following a very short transfer time (generally less than 10 ms).
The standards do not stipulate a time, but do indicate that "the load [is] transferred to
the inverter directly or via the UPS switch (which may be electronic or
electromechanical)".
- The UPS continues to operate on battery power until the end of battery backup time
or utility power returns to normal, which provokes transfer of the load back to the AC
input (normal mode).
APC by Schneider Electric
05/2012 edition
p. 9
Types of UPSs (Cont.)
Fig. 5.3. UPS operating in passive-standby mode.
Advantages
• Simple diagram.
• Reduced cost.
Disadvantages
• No real isolation of the load with respect to the upstream distribution system.
• Transfer time. It operates without a real static switch, so a certain time is required
to transfer the load to the inverter. This time is acceptable for certain individual
applications, but incompatible with the performance required by more sophisticated,
sensitive systems (large computer centres, telephone exchanges, etc.).
• No regulation of the output frequency, which is simply that of the utility power.
Usage
This configuration is in fact a compromise between an acceptable level of protection
against disturbances and cost.
The mentioned disadvantages mean that, practically speaking, this type of UPS can
be used only for low power ratings (< 2 kVA) and cannot be used as a frequency
converter.
UPS operating in line-interactive mode
) The inverter is connected in parallel with the AC input in a standby
configuration, and also charges the battery. It thus interacts (reversible operation)
with the AC-input source.
Operating principle
• Normal mode
The load is supplied with conditioned power via a parallel connection of the AC input
and the inverter. As long as the utility power is within tolerances, the inverter
regulates fluctuations in the input voltage. Otherwise (reversible operation), it
charges the battery. The output frequency depends on the AC-input frequency.
• Battery backup mode
- When the AC input voltage is outside specified tolerances for the UPS or the utility
power fails, the inverter and the battery step in to ensure a continuous supply of
power to the load. The power switch (e.g. static switch) also disconnects the AC
input to prevent power from the inverter from flowing upstream.
- The UPS continues to operate on battery power until the end of battery backup time
or utility power returns to normal, which provokes transfer of the load back to the AC
input (normal mode).
APC by Schneider Electric
05/2012 edition
p. 10
Types of UPSs (Cont.)
• Bypass mode
This type of UPS may be equipped with a bypass. If one of the UPS functions fails,
the load can be transferred to the bypass AC input via the maintenance bypass.
Fig. 5.4. UPS operating in line-interactive mode.
Advantages
• The cost can be less than that for a double-conversion UPS with an equivalent
power rating because the inverter does not operate continuously.
Disadvantages
• No real isolation of the load with respect to the upstream distribution system, thus:
- sensitivity to variations in the utility voltage and frequent demands placed on the
inverter,
- influence of downstream non-linear loads on the upstream input voltage.
• No regulation of the output frequency, which is simply that of the utility power.
• Mediocre conditioning of the output voltage because the inverter is not installed in
series with the AC input. The standard speaks of "conditioned power" given the
parallel connection of the AC input and the inverter. Conditioning is, however, limited
by the sensitivity to upstream and downstream voltage fluctuations and the reversible
operating mode of the inverter.
• Efficiency depends on:
- the type of load. With non-linear loads, the current drawn comprises harmonics that
alter the fundamental. The harmonic currents are supplied by the reversible inverter
which regulates the voltage and efficiency is sharply reduced.
- the percent load. The power required to charge the battery becomes increasingly
significant as the percent load decreases.
• A single point of failure exists due to the absence of a static bypass, i.e. if a
malfunction occurs, the UPS shuts down.
Usage
This configuration is not well suited to regulation of sensitive loads in the medium to
high-power range because frequency regulation is not possible. For this reason, it
is rarely used other than for low power ratings.
APC by Schneider Electric
05/2012 edition
p. 11
Types of UPSs (Cont.)
Double-conversion UPSs
) The inverter is connected in series between the AC input and the application.
The power supplied to the load continuously flows through the inverter.
Operating principle
• Normal mode
During normal operation, all the power supplied to the load passes through the
rectifier/charger and inverter which together perform a double conversion (AC-DCAC), hence the name. The voltage is continuously regenerated and regulated.
• Battery backup mode
- When the AC-input voltage is outside specified tolerances for the UPS or the utility
power fails, the inverter and the battery step in to ensure a continuous supply of
power to the load.
- The UPS continues to operate on battery power until the end of battery backup time
or utility power returns to normal, which provokes transfer of the load back to the AC
input (normal mode).
• Bypass mode
This type of UPS comprises a static bypass (sometimes called a static switch) that
ensures no-break transfer of the load from the inverter to direct utility power and
back.
The load is transferred to the static bypass in the event of the following:
- UPS failure,
- load-current transients (inrush or fault currents),
- overloads,
- end of battery backup time.
The presence of a static bypass assumes that the input and output frequencies are
identical, which means it cannot be used as a frequency converter. If the voltage
levels are not the same, a bypass transformer is required.
The UPS is synchronised with the bypass AC input to ensure no-break transfers from
the inverter to the bypass line.
Note. Another bypass line, often called the maintenance bypass, is available for
maintenance purposes. It is closed by a manual switch.
Fig. 5.5. Double-conversion UPSs.
APC by Schneider Electric
05/2012 edition
p. 12
Types of UPSs (Cont.)
Advantages
• Complete regeneration of the output power, whether it comes from the utility or the
battery.
• Total isolation of the load from the distribution system and its disturbances.
• Very wide input-voltage range, yet precise regulation of the output voltage.
• Independence of the input and output frequencies, thus ensuring an output
frequency within strict tolerances. Capacity to operate as a frequency converter (if
planned as such), by disabling the static switch.
• Much higher performance levels under steady-state and transient conditions.
• Instantaneous shift to battery backup mode if utility power fails.
• No-break transfer to a bypass line (bypass mode).
• Manual bypass (generally standard) to facilitate maintenance.
Disadvantages
• Higher price, but compensated by the many advantages.
Usage
This configuration is the most complete in terms of load protection, regulation
possibilities and performance levels. It notably ensures independence of the output
voltage and frequency with respect to the input voltage and frequency.
Its many advantages mean that it is virtually the only configuration used for
medium and high power ratings (from 10 kVA upwards).
Conclusion
Double-conversion UPSs represent the vast majority of the medium to highpower systems sold (95% starting from a few kVA and 98% for 10 kVA and higher).
This is due to their numerous strong points in meeting the needs of sensitive loads
at these power ratings and is largely the result of the inverter positioned in series
with the AC input.
What is more, they have very few weak points except their high cost that is
required to offer a level of performance that is often indispensable given the critical
nature of the protected loads. A further weak point is slightly higher losses (a few
percent).
In the power ranges under consideration, the other technologies are marginal, in
spite of a significantly lower cost.
They have the disadvantages listed below.
• No voltage regulation for passive-standby UPSs.
• No frequency regulation for passive-standby UPSs and line-interactive UPSs.
• Mediocre isolation (often a surge arrestor) from the AC input due to the parallel
configuration of the inverter.
Conclusion
) For low power ratings (< 2 kVA), the three standardised technologies
coexist.
It is the cost effectiveness of the protection functions with respect to the
requirements of the loads and the risks run (for people, production, etc.) that
determines selection of one of the three typologies.
) Double-conversion UPSs are used almost exclusively for higher ratings.
APC by Schneider Electric
05/2012 edition
p. 13
Types of UPSs (Cont.)
The delta conversion on-line UPSs
This UPS design, illustrated in Figure 5.6, is a newer, 10 year old technology
introduced to eliminate the drawbacks of the double conversion on-line design and is
available in sizes ranging from 5 kVA to 1.6 MW. Similar to the double conversion
on-line design, the delta conversion on-line UPS always has the inverter supplying
the load voltage. However, the additional delta converter also contributes power to
the inverter output. Under conditions of AC failure or disturbances, this design
exhibits behavior identical to the double conversion on-line.
STATIC BYPASS
SWITCH
DELTA
TRANSFORMER
AC
AC
DC
DC
MAIN
INVERTER
DELTA
CONVERTER
BATTERY
Figure 5.6: Delta conversion on-line UPS
A simple way to understand the energy efficiency of the delta conversion topology is
to consider the energy required to deliver a package from the 4th floor to the 5th floor
of a building as shown in Figure 5.7. Delta conversion technology saves energy by
carrying the package only the difference (delta) between the starting and ending
points. The double conversion on-line UPS converts the power to the battery and
back again whereas the delta converter moves components of the power from input
to the output.
DOUBLE CONVERSION
DELTA CONVERSION
X
4th
Floor
5th
Floor
X
4th
Floor
5th
Floor
Figure 5.7: Analogy of double conversion vs. Delta conversion
APC by Schneider Electric
05/2012 edition
p. 14
Types of UPSs (Cont.)
In the delta conversion on-line design, the delta converter acts with dual purposes.
The first is to control the input power characteristics. This active front end draws
power in a sinusoidal manner, minimizing harmonics reflected onto the utility. This
ensures optimal utility and generator system compatibility, reducing heating and
system wear in the power distribution system. The second function of the delta
converter is to control input current in order to regulate charging of the battery
system.
The delta conversion on-line UPS provides the same output characteristics as the
double conversion on-line design. However, the input characteristics are often
different. Delta conversion on-line designs provide dynamically-controlled, power
factor corrected input, without the inefficient use of filter banks associated with
traditional solutions. The most important benefit is a significant reduction in energy
losses. The input power control also makes the UPS compatible with all generator
sets and reduces the need for wiring and generator over sizing. Delta conversion online technology is the only core UPS technology today protected by patents and is
therefore not likely to be available from a broad range of UPS suppliers.
During steady state conditions the delta converter allows the UPS to deliver power to
the load with much greater efficiency than the double conversion design.
APC by Schneider Electric
05/2012 edition
p. 15
UPS components and operation
Components of a UPS
The information presented below concerns double-conversion UPSs, the
technology most commonly used by APC by Schneider Electric for power ratings
greater than 10 kVA.
General diagram of a UPS
The various items in the diagram below have been assigned numbers that
correspond to the sections on the following pages.
Fig. 5.6. Components of a UPS.
Power sources and UPS inputs
Practically speaking, a UPS has one or two inputs:
• Normal AC input (or Mains 1), supplied by primary power,
• Bypass AC input (or Mains 2), supplied by standby power (generally speaking via
a separate cable from the same main low-voltage switchboard (MLVS).
Î AC sources, see p. 9.
UPS connection to both the primary and standby-power sources (UPS inputs
supplied by two separate circuits from the MLVS) is recommended because overall
system reliability is increased. However, if two separate circuits from the MLVS are
not available, it is possible to have both AC inputs (normal and bypass) supplied by
primary power (second cable).
Management of transfers between the two input lines is organised as follows.
• The UPS synchronises the inverter output voltage with that of the bypass line as
long as the latter is within tolerances. It is thus possible, if necessary, for the static
switch to transfer the load to the bypass AC input, without a break (because the two
voltages are synchronised and in phase) or disturbances (because the standby
power is within tolerances) for the load.
• When standby power is not within tolerances, the inverter desynchronises and
transfer is disabled. It can, however, by carried out manually.
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01/2012 edition
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UPS components and operation (Cont.)
Components of a UPS
Rectifier/charger (1)
Transforms the AC power from the primary-power source into DC voltage and
current used to:
• Supply the inverter,
• Charge and float charge the battery.
Inverter (2)
Using the DC power supplied by the:
• Rectifier during normal operation,
• Battery during autonomous operation,
the inverter completely regenerates a sinusoidal output signal, within strict amplitude
and frequency tolerances.
Battery (3)
Makes the UPS autonomous with respect to the utility in the event of:
• A utility outage,
• Utility-power characteristics outside specified tolerances for the UPS.
Battery backup times range from 6 to 30 minutes as standard and can be extended
on request. Depending on the duration of the backup time, the battery is housed in
the UPS cabinet or in a separate cabinet.
Static bypass (4)
A static switch is used to transfer the load from the inverter to the bypass without any
interruption* in the supply of power to the load (no break because the transfer is
performed by electronic rather than mechanical components). The switch is possible
when the frequencies upstream and downstream of the UPS are identical.
Transfer takes place automatically for any of the following reasons:
• Voluntary shutdown of the UPS,
• An overload exceeding the limiting capacity of the inverter (this transfer can be
disabled),
• An internal fault.
It can also be carried out manually.
* No-break transfer is possible when the voltages at the inverter output and on the bypass AC
input are synchronised. The UPS maintains synchronisation as long as the standby power is
within tolerances.
Manual bypass (5)
A manual switch is used to transfer the load to the bypass for maintenance
purposes. The switch is possible when the frequencies upstream and downstream of
the UPS are identical.
The shift to manual-bypass mode is carried out using manual switches.
Manual switches (6, 7, 8)
These devices isolate the rectifier/charger and inverter modules and/or the bypass
line for servicing or maintenance.
Battery circuit breaker (9)
The battery circuit breaker protects the battery against excessive discharge, and the
rectifier/charger and inverter against a battery short-circuit.
Upstream isolating transformer (10)
(optional equipment)
Provides UPS input/output isolation when the downstream installation is supplied via
the bypass.
It is particularly useful when the upstream and downstream system earthing
arrangements are different. May be installed in the UPS cabinet in the MGE Galaxy
PW range.
Voltage-matching transformer (11)
(optional equipment)
Adapts the voltage to the desired value.
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UPS components and operation (Cont.)
Filters (12)
(optional equipment)
• Upstream of the rectifier/charger, when it is of the thyristor-based Graetz bridge
type (the case for MGE Galaxy PW and 9000 UPSs), a harmonic filter (see “Key
factors in UPS installation” p. 24) reduces the current harmonics resulting from the
switching of the rectifier thyristors. This reduces the voltage distortion on the
upstream busbars resulting from the flow of harmonic currents (the level required is
generally <5%). What is more, these UPSs from APC by Schneider Electric are
equipped with an oversized neutral conductor installed as standard to overcome the
consequences of third-order harmonics and their multiples which flow in the neutral
conductor.
• All the other UPSs of the MGE Galaxy and Symmetra ranges are equipped with a
PFC-type rectifier that eliminates the need for a filter (see “Key factors in UPS
installation” p. 24).
• Downstream, UPSs implementing new PWM-chopping techniques may be directly
connected to non-linear loads. This technique makes it possible for UPSs from APC
by Schneider Electric to maintain the THDU below 3%.
Built-in communication (13) (14)
In addition to the need for a user-friendly human/machine interface for effective
monitoring of UPS operation, it is today increasingly important for UPSs to
communicate with their electrical and computing environment (supervision systems,
building management systems (BMS), computer management systems, etc.).
UPSs from APC by Schneider Electric are designed with built-in capacity for total
communication and include:
• A user-friendly human/machine interface (HMI) with an advanced graphic display
and mimic panel. The interface is built up around self-monitoring and self-diagnostic
systems that continuously indicate the status of the various UPS components, in
particular the batteries.
For example, for the MGE Galaxy ranges:
- the Digibat system continuously monitors the status of the battery with full battery
management features,
- the B2000 or Cellwatch battery-monitoring system immediately detects and locates
battery faults and provides predictive monitoring.
For the Symmetra ranges:
- The rack-mountable (1U) APC battery management system, accessible via a web
browser, combines battery monitoring and testing with individual boost charging for
peak battery performance.
• A large selection of communication cards compatible with market standards:
- Network Management Card (Ethernet)
- Modbus – Jbus card (RS232 & RS485)
- Relay card (dry contacts) for indications
- Teleservice modem card
These cards can be used to implement supervision, notification, controlled shutdown
and Teleservice functions.
Î Human-machine interface and Communication: see “Key factors in UPS
installation” p. 49.
Upstream and/or downstream distribution and protection devices (15) (16)
(optional equipment)
The UPS can be supplied with the following equipment:
• Upstream LV circuit-breakers for the AC inputs (normal and bypass),
• Upstream LV switchboard with circuit-breaker protection for the AC inputs (normal
and bypass),
• Downstream LV switchboard with circuit-breaker protection for the different
outgoing circuits.
APC by Schneider Electric can offer a selection of UPSs and protection devices that
are perfectly coordinated in terms of ratings and performance.
Complete solutions
APC by Schneider Electric can provide complete solutions comprising all the
components listed above, including air-conditioning solutions for data centers, in
conjunction with Schneider Electric. For users, the result is a single partner and an
installation that offers optimum performance and reliability.
APC by Schneider Electric
01/2012 edition
p. 18
UPS components and operation (Cont.)
Main characteristics of UPS
components
These characteristics are based on the main technical specifications presented in the
IEC 62040-3 / EN 62040-3 standards on UPS performance requirements.
Certain terms used here differ from the common jargon and a number of new
features have not yet been assimilated by manufacturers. New terms or
characteristics used by the standard are indicated between parentheses and
preceded by an asterisk.
For example, the title of a section "input current during battery float charging", a
commonly used term, is followed by (*rated input current), the term used in the
standard.
Note that a number of numerical values are indicated as examples.
They are, for the most part, drawn from the technical characteristics of the
corresponding UPSs, indicated in chapter 4, or indicated simply for the purposes of
the example.
AC input power
Number of phases and system earthing arrangement
The AC-input supply (primary power) is three-phase + neutral. Single-phase inputs
are not used for the power levels dealt with here.
The system earthing arrangement is generally imposed by standards (IT, TT, TNS or
TNC).
Normal AC input
The normal AC input is supplied with utility power for the rectifier/charger, within the
specified tolerances.
• Example: 400 V rms ± 15% at a frequency of 50 or 60 Hz ± 5%, three-phase.
Bypass AC input
The bypass AC input is supplied with standby power. Practically speaking, this a
cable connected to a utility feeder in the MLVS other than the one supplying the
normal AC input.
In general, it supplies voltage with the same characteristics as that of the primary
power.
• Example: 400 V rms ± 15% at a frequency of 50 or 60 Hz ± 5%, and a short-circuit
current Isc2 = 12.5 kA. The short-circuit current is important information for the
downstream protection devices in the event of operation via the static or
maintenance bypass.
Supply of separate primary and standby power is recommended because it
increases overall system reliability, but is not mandatory. However, if two separate
circuits from the MLVS are not available, it is possible to have both AC inputs
(normal and bypass) supplied by primary power (second cable).
Rectifier/charger
Floating voltage
This is the voltage supplied by the rectifier/charger which keeps the battery fully
charged.
It depends on the batteries used and the manufacturer's recommendations.
Input current during battery float charging (* rated input current)
This is the current, under normal operating conditions, required to supply the inverter
at its rated power while float charging the battery.
) Example: for a 100 kVA MGE Galaxy PW with a battery backup time of 10
minutes, this current is I input float = 166 A while float charging the battery.
Input current during battery charging
This corresponds to the current required to supply the inverter at its rated power
while charging the battery. It is consequently higher than the previous current and is
used to size the charger input cables.
) Example: for the same UPS as above, the input current is I input float = 182 A, i.e.
higher than above because it is necessary to charge the battery.
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UPS components and operation (Cont.)
Maximum input current
This is the input current with the UPS operating under worst-case conditions of
permitted overload, with the battery discharged. It is higher than the above input
current during battery charging (due to the overload current) but is limited in time (as
is the overload).
) Example: for the same UPS as above, the MGE Galaxy PW can accept a 25%
overload for ten minutes and a 50% overload for one minute. In the worst-case
situation with the battery charging, the input current can reach:
I input max. = 182 A x 1.25 = 227.5 A for ten minutes,
I input max. = 182 A x 1.5 = 273 A for one minute.
Beyond the above limits, the UPS initiates no-break transfer of the load to the bypass
line and automatically transfers back when the overload has ended or been cleared
by the corresponding protection devices.
Battery (* energy storage means)
Type
A battery is characterised by its type (vented or sealed lead acid, or nickel/cadmium)
and how it is installed. APC by Schneider Electric proposes sealed lead-acid
batteries mounted in cabinets.
Service life
This is defined as the operating period, under normal usage conditions, for which the
battery supplies at least 50% of the initial backup time.
) For example, MGE Galaxy PW is supplied as standard with sealed lead-acid
batteries with a service life of ten years or more. This type of battery, rated for 30
minutes of backup time, will contractually supply only 15 minutes at the end of the
specified service life.
It may supply more if it has been used under optimum conditions (notably concerning
the temperature). However, it is contractually guaranteed not to supply less, unless
used improperly.
Operating modes
The battery may be:
• Charging. It draws a charge current (I1 charge) supplied by the rectifier/charger.
• Float charging. The battery draws a low, so-called floating current (I1 floating),
supplied by the rectifier/charger, which maintains its charge by compensating for
open-circuit losses.
• Discharging. The battery supplies the inverter until its shutdown voltage is
reached.
When this voltage, set by the battery manufacturer, is reached, the battery is
automatically disconnected (UPSs from APC by Schneider Electric) to avoid damage
by deep discharge.
Rated voltage
This is the DC output voltage that the battery supplies to the inverter.
) Example: 450 V DC for the MGE Galaxy PW range.
Capacity
Battery capacity is expressed in ampere/hours.
) Example: for a 100 kVA MGE Galaxy PW equipped with a battery offering ten
minutes of backup time and a service life of five years, the capacity is 85 A/h.
Number of cells
Number of single battery cells making up the entire battery string.
) Example: the battery of a 100 kVA MGE Galaxy PW comprises, for a given type of
battery, 33 cells providing 13.6 V each, for a backup time of ten minutes.
Floating voltage
This is the DC voltage used to maintain the battery charge, supplied by the
rectifier/charger.
) Example: for a MGE Galaxy PW, the floating voltage is between 423 and 463 V
DC.
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UPS components and operation (Cont.)
Backup time (* stored energy time)
This is the time, specified at the beginning of the battery service life, that the battery
can supply the inverter operating at full rated load, in the absence of the AC-input
supply.
) Example: MGE Galaxy PW offers as standard backup times of 8, 10, 15, 20, 30
and 60 minutes.
This time depends on the UPS percent load.
• For a UPS operating at full rated load (100% of rated power), the end of the battery
backup time is reached when the battery voltage drops to the shutdown voltage
specified by the manufacturer. This provokes automatic shutdown of UPSs from APC
by Schneider Electric.
• For a UPS operating at a lower percent load (e.g. 75%), the actual backup time
may be longer. However, it always ends when the battery shutdown voltage is
reached.
Recharge time (* rated restored energy time)
This is the time required by the battery to recover 80% of its backup time (90% of its
capacity), starting from the battery shutdown voltage. The rectifier/charger supplies
the power.
) Example: for a MGE Galaxy 5500 UPS, the recharge time is eight to ten hours,
depending on the battery and the backup time. Note that the probability of the battery
being called on to supply power twice within such a short period is low. This means
the indicated recharge time is representative of actual performance.
Maximum battery current (Ib)
When discharging, the battery supplies the inverter with a current Ib which reaches
its maximum value at the end of discharging. This value determines battery
protection and cable dimensions.
) Example: for a 100 kVA MGE Galaxy 5500, this current is Ib max = 257 A.
Inverter
Rated power (Sn)
(* rated output apparent power)
This is the maximum apparent power Sn (kVA) that the inverter can deliver to a
linear load at a power factor of 0.8, during normal operation under steady-state
conditions.
The standards also define this parameter for operation on battery power.
Theoretically speaking, it is the same if the battery is correctly sized.
) Example: a MGE Galaxy 5500 with a rated power (Sn) of 100 kVA.
Active output power (Pa)
(* rated output active power for linear or reference non-linear load)
This is the active power Pa (kW) corresponding to the apparent output power Sn
(kVA), under the measurement conditions mentioned above. This value may also be
indicated for a standardised reference non-linear load.
) Example: the previous UPS, a MGE Galaxy 5500 with a rated power of 100 kVA
supplies an active power of Pa = Sn x 0.8 = 80 kW.
Rated current (In)
This is the current corresponding to the rated power.
) Example: again for a 100 kVA MGE Galaxy 5500 UPS and an output voltage of
400 V, this current is:
100000
Sn
In =
=
= 144.3 A
400 x 1732
,
Un 3
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UPS components and operation (Cont.)
Apparent load power (Su) and percent load
This is the apparent power Sn (kVA) actually supplied by the inverter to the load,
under the selected operating conditions.
This value is a fraction of the rated power, depending on the percent load.
.Su ≤ Sn. and .Tc = Percent load (%) = Su / Sn..
) Example: for the UPS mentioned above, if the inverter supplies 3/4 of its rated
power (75% load), it delivers an apparent power of 75 kVA, which under standard
operating conditions (PF = 0.8) corresponds to an active load power of
Pa = Su x PF = 75 x 0.8 = 60 kW.
Load current (Iu)
This is the current corresponding to the load power, that is, to the percent load in
question. It is calculated from Pu as for the rated current, where the voltage is the
rated voltage Un (value regulated by the inverter).
) Example: for the UPS mentioned above (75% load)
75000
Su
Iu =
=
= 108.2 A
400 x 1732
,
Un 3
which is the same as:
.Iu = In x Tc. = 144.3 x 0.75 = 108.2 A
Efficiency (η)
This is the ratio of active power Pu (kW) supplied by the UPS to the load to the
power Pin (kW) that it draws at its input, either by the rectifier or from the battery.
.η= Pu / Pin.
For most UPSs, efficiency is optimum at full rated load and drops sharply with lower
percent loads. Due to their low output impedance and no-load losses, the efficiency
of MGE Galaxy UPSs is virtually stable for loads from 25 to 100%. The MGE Galaxy
range offers efficiency greater than 90% starting at 25% load, up to 93% at full rated
load, as well as an ECO mode which increases efficiency by 4%, i.e. up to 97%.
Practically speaking, for MGE Galaxy UPSs, an efficiency value of 0.93 can be used
for all input-power calculations for loads from 30 to 100%.
) Example: for a 100 kVA MGE Galaxy at 75% load, 0.93 efficiency corresponds to
a UPS active input power of
Pin = Pu / η = 60/0.93 = 64.5 kW.
Output voltage Un
Number of phases
The output can be three-phase (3ph-3ph UPS) or single-phase (3ph-1ph UPS),
depending on the situation. Note that the upstream and downstream system earthing
arrangements may be different.
Rated output voltage
In general, it is the same as that of the AC input. However, a voltage-matching
transformer may be installed.
Static characteristics
These are the tolerances (maximum permissible variations) for the amplitude and
frequency of the output voltage under steady-state conditions. Stricter than those
applying to utility power, they are measured for normal operation on AC-input power
and for operation in battery backup mode.
• Output voltage variation
The amplitude tolerance is expressed as a percentage of the nominal rms value and
may be adjustable.
) Example: for a MGE Galaxy, the voltage 400 V rms ± 1% may be adjusted to ±
3%.
The standards also stipulate a rated peak output voltage and the tolerance with
respect to the rated value.
• Output frequency variation
The tolerance is expressed as a percentage of the rated frequency.
) Example: for a MGE Galaxy, 50 or 60 Hz ± 0.1% during normal operation on
primary power and ± 0.5% in battery backup mode.
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UPS components and operation (Cont.)
Frequency synchronisation with primary power
The inverter supplies an output voltage within the above tolerances, regardless of
the disturbances affecting the upstream power.
To that end, the UPS:
• Monitors the voltage parameters (amplitude, frequency, phase) for the primarypower source to determine whether they are within specified tolerances,
• Reacts to any drift in parameters so as to:
- readjust the inverter (phase and frequency) to the standby power, as long as the
drift remains within tolerances, in view of load transfer, if necessary,
- transfer the load to battery power as soon as the drift goes outside tolerances.
The new IGBT and PWM chopping technologies used in UPSs from APC by
Schneider Electric allow an excellent adaptation to these variations.
) Example: for MGE Galaxy UPSs, the maximum variation in frequency
corresponding to the tolerance is 50 Hz x 0.5% = 0.25 Hz.
Frequency synchronisation with bypass AC power is possible from 0.25 to 2 Hz, in
0.25 Hz steps. Practically speaking, this signifies that frequency variations may be
monitored at dF/dt = 0.25 Hz/s and readjustment carried out within 0.25 to 1 second.
Dynamic characteristics
These are the tolerances under transient load conditions.
MGE Galaxy UPSs are capable of withstanding the following conditions.
• Load unbalance
For unbalance in the load voltage (phase-to-neutral or phase-to-phase) of:
- 30%, the output voltage variation is less than 0.1%,
- 100% (one phase at Pn and the others at 0), the output voltage does not vary more
than 0.2%.
• Load step changes (voltage transients)
For load steps from 0 to 100% or from 100 to 0% of the rated load, the voltage does
not vary more than:
± 2% on utility power;
+ 2% to -4 % on battery power.
Overload and short-circuit capacity
• Overloads
- 1.1 In for 2 hours,
- 1.5 In for 1 minute,
with no change in the output tolerances.
• Short-circuits
Beyond 1.65 In, MGE Galaxy inverters operate in current-limiting mode up to 2.33 In
for 1 second, corresponding to:
I peak max. = √2 x 1.65 In = 2.33 In.
Beyond this value, the inverter transfers the load to standby power or performs a
static shutdown (self-protection feature).
Total output-voltage distortion
UPSs must guarantee performance levels for all types of loads, including non-linear
loads.
) Example: MGE Galaxy UPSs limit the voltage total harmonic distortion (THDU) in
output power to the following levels for:
• 100% linear loads:
- THDU ph/ph < 1.5 %,
- THDU ph/N < 2%,
• 100% non-linear loads:
- THDU ph/ph < 2 %,
- THDU ph/N < 3%.
MGE Galaxy UPSs operate in compliance with the specified characteristics for all
types of loads.
General note. The standard specifies certain of the previously mentioned
performance levels for output power during normal operation and operation on
battery power. In general, they are identical.
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UPS components and operation (Cont.)
Summary diagram for main
characteristics
Fig. 5.7. Diagram showing the main characteristics (see the list below).
Normal AC input
● Voltage Un + 10% to - 15%
● Frequency f + 4% to - 6%
Bypass AC input
● Voltage Un + 10% to - 15%
● Frequency f + 4% to - 6%
● Short-circuit current Isc2 (withstand capacity of the static bypass)
Rectifier/charger
● Floating voltage
● Input currents
- rated (battery float charging)
- maximum (battery charging)
Battery
● Backup time: standard 5, 6, 8, 10, 15, 20, 30, 60 minutes, longer times on request)
● Service life: 10 years or longer
● Maximum current Ib max.
Inverter
● Apparent output power:
- rated: Sn (kVA)
- load power: Su (kVA) = Sn x Tc%
● UPS percent load Tc% = Su / Sn
● Active output power:
- rated: Pn (kW) = Sn (kVA) x 0.8
- load power: Pu (kW) = Su (kVA) x PF = Sn x Tc% x PF = Un Iu PF
● Efficiency: η Pu / Pn = 93% (97% in ECO mode).
● Static characteristics (output-voltage tolerances under steady-state conditions)
- amplitude: Un ± 1% adjustable to ± 3%
- frequency: f ± 1% during normal operation, f ± 0.5% in battery backup mode
- inverter output voltage synchronised (frequency and phase) with that of the standby
power as long as the latter is within tolerances.
● Dynamic characteristics (tolerances under transient conditions)
- maximum voltage and frequency variations for load step changes from 0% to 100%
or 100% to 0%: Un ± 2%, f ± 0.5%
● Output voltage distortion
- 100% non-linear loads THDU < 2%
● Overload and short circuit capacity:
- overloads: 1.5 In for 1 minute
- short-circuits: current limiting to 2.33 In for 1 second
Load
● Load current (Iu)
● Power factor PF
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UPS components and operation (Cont.)
UPS operating modes
Normal mode (on utility power, see fig. 5.8 on left-hand side)
The UPS draws the AC utility power required to operate via the rectifier/charger
which provides DC current.
Part of the utility power drawn is used to charge or float charge the battery:
• I1 floating, if the battery is already fully charged,
• I1 charge if the battery is not fully charged (i.e. charging following a recent
discharge).
The remaining current is supplied to the inverter with generates an output-voltage
sine-wave within the specified amplitude and frequency tolerances.
Battery backup mode (on battery power, see fig. 5.8 on right-hand side)
The battery steps in to replace primary power and supplies the power required by the
inverter for the load, with the same tolerances as in normal mode.
This takes place through immediate transfer (the battery is parallel connected) in the
event of:
• Normal AC-input failure (utility-power outage),
• Normal AC input outside tolerances (degradation of utility-power voltage).
Normal mode.
Battery backup mode.
Fig. 5.8. Normal mode and battery backup mode.
Bypass mode (on static-bypass line, see fig. 5.9 on left-hand side)
A static switch (SS) ensures no-break transfer of the load to the bypass AC input for
direct supply of the load by standby power.
Transfer is automatic in the event of:
• An overload downstream of the UPS exceeding its overload capacity,
• An internal fault in the rectifier/charger and inverter modules.
Transfer always takes place for internal faults, but otherwise is possible only if the
voltage of the standby power is within tolerances and in phase with the inverter.
To that end:
• The UPS synchronises the inverter output voltage with that of the bypass line as
long as the latter is within tolerances. Transfer is then possible:
- without a break in the supply of power. Because the voltages are in phase, the
SCRs on the two channels of the static switch have zero voltage at the same time,
- without disturbing the load. The load is transferred to a bypass line that is within
tolerances.
• When standby power is not within tolerances, the inverter desynchronises and
operates autonomously with its own frequency. Transfer is disabled.
It can, however, by carried out manually.
Note 1. This function greatly increases reliability due to the very small probability of a
downstream overload and a standby-power failure occurring at the same time.
Note 2. To ensure correct operation of the bypass line, discrimination must be
ensured between the protection device upstream of the bypass AC input (on the
MLVS outgoer) and those on the UPS outgoing circuits (see information on
discrimination below).
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UPS components and operation (Cont.)
Maintenance mode (on maintenance bypass, see fig. 5.9 right-hand side)
Maintenance is possible without interrupting load operation. The load is supplied with
standby power via the maintenance bypass. Transfer to the maintenance bypass is
carried out using manual switches.
The rectifier/charger, inverter and static switch are shut down and isolated from
power sources. The battery is isolated by its protection circuit breaker.
Bypass mode (static bypass).
Maintenance mode (maintenance bypass).
Fig. 5.9. Bypass mode and maintenance mode.
UPS configurations
Parallel UPS with redundancy
Chapter two is entirely devoted to a presentation of the various configurations. Below
is some additional information on parallel connection for redundancy.
It concerns MGETM GalaxyTM UPSs in particular. The modular SymmetraTM UPSs
also use parallel connection.
Î Configurations, see “Selection of the UPS configuration”
Types of parallel configurations
There are two types of parallel configurations.
• Integrated parallel UPS units
This upgradeable configuration can be started using a single UPS unit with an
integrated static bypass and manual maintenance bypass. For configurations with
more than two UPS units, a common maintenance bypass is housed in an external
cubicle (see fig. 5.10).
• Parallel UPS units with a centralised static-switch cubicle (SSC)
The static-switch cubicle comprises an automatic bypass and a maintenance bypass
that are common for a number of UPS units without a bypass (see fig. 5.11).
This configuration, less upgradeable than the previous due to the rating of the
bypass, offers greater reliability (SSC and UPS units are independent).
• Modular UPSs
UPSs of the SymmetraTM range are made up of dedicated and redundant modules
(power, intelligence, battery and bypass).
Modular design with plug-in power modules improves dependability, in particular
maintainability and availability, as well the upgradeability of the installation.
Redundancy
Redundancy in parallel configurations can be N+1, N+2, etc.
This means that N UPS units are required to supply the load, but N+1 or N+2 are
installed and they all share the load.
See the example below.
APC by Schneider Electric
01/2012 edition
p. 26
UPS components and operation (Cont.)
Example
• Consider a critical load with a 100 kVA rating.
• 2+1 redundancy
- 2 UPS units must be capable of fully supplying the load if redundancy is lost.
- Each UPS unit must therefore have a 50 kVA rating.
- 3 UPS units normally share the 100 kVA load, i.e. each supplies 33.3 kVA.
- The 3 UPS units normally operate at a percent load of 33.3 / 50 = 66.6%.
- Integrated parallel UPS units are each equipped with a static bypass. Transfer is
managed such that the three UPS units transfer to the bypass simultaneously, if
necessary.
Fig. 5.10. Integrated parallel UPS units with common maintenance bypass and 2+1
redundancy. Operation with all units OK (redundancy available).
• Loss of redundancy
- One UPS unit shuts down, the two remaining units operate at 100%.
- The faulty UPS unit can be serviced due to the maintenance bypass.
Fig. 5.11. Integrated parallel UPS units with common maintenance bypass and 2+1
redundancy. Operation following loss of redundancy.
APC by Schneider Electric
01/2012 edition
p. 27
Technology: Transformerless UPSs
Transformerless UPS
technology
Principle
Originally all UPSs included an output transformer that was used to adjust the output
voltage to the required value, recreate a neutral and ensure galvanic isolation
between the upstream and downstream power systems (Fig. 5.12).
Today, technological progress and lower IGBT semi-conductor costs makes it
possible to eliminate this transformer (Fig. 5.13).
Bypass
AC input
Normal
AC input
UPS
Q1
Q1
Q4S
Rectifier
charger
Battery
Bypass
AC input
Normal
AC input
UPS
Battery
QF1
Static
bypass
Q4S
Rectifier
charger
QF1
Static
bypass
Manual
bypass Q3BP
Manual
bypass Q3BP
Inverter
Inverter
K3N
K3N
Q5N
Q5N
Loads
Loads
Fig. 5.12. UPS with output transformer.
Fig. 5.13. Transformerless UPS.
Advantages
This technology offers users a number of key advantages.
● Smaller footprint: less space required with no transformer
● Less weight: weight reduction by eliminating the transformer
● Higher efficiency: elimination of transformer losses
● Voltage regulation by signal modulation for better matching with the load. The
electronics act directly on the output voltage for a for faster and more precise voltage
regulation.
The trend
The use of transformerless UPSs began in the early 1990s for ratings up to a few
hundred kVA. Given their many advantages, they are now widely used up to higher
ratings, as shown in figure 5.14. The average power rating using the transformerless
technique has increased by a factor of 50 over the past 15 years.
P(kVA)
500
400
300
200
100
5
1990
years
1995
2000
2005
2010
Fig. 5.14. Average power ratings of transformerless UPSs.
APC by Schneider Electric
01/2012 Edition
p. 28
Technology: Transformerless UPSs
Galvanic isolation
One of the reasons cited for using output transformers is to provide galvanic
isolation.
However, three-phase UPSs above a certain power rating are equipped with a
bypass to ensure continuity of power. The presence of a bypass means that a UPS,
with or without an output transformer, cannot provide galvanic isolation between the
source and the loads. For this reason, transformerless UPS technology is gradually
becoming the preferred solution for high ratings.
This aspect will be discussed below by comparing the use of the two technologies
depending on the system earthing arrangement encountered.
Use with computer
loads
Review of system earthing arrangements
Earthing arrangements refer to the earthing of:
● the neutral point of the distribution system,
● the exposed conductive parts (ECPs) of the loads.
These ECPs are always interconnected, either all together or in groups. Each
interconnected group is connected to an earthing terminal by a protective conductor
(PE or PEN depending on whether or not it is combined with the neutral conductor or
separate).
Standard IEC 60364(1) uses 2 letters to identify the different earthing arrangements.
● The 1st letter describes the earthing of the transformer neutral point:
- T: earthed,
- I: not earthed.
● The 2nd letter describes the earthing of the ECPs of load equipment:
- T: earthed,
- N: connected to the neutral which is earthed.
In this case (N), a 3rd letter indicates the relationship between the neutral (N) and
protective (PE) conductors:
- C: single conductor used for both functions,
- S: separate conductors.
(1) Replaced by the Power Transformer Loading Guide IECI 60076-7 Ed. 1.
The standard thus defines the following systems:
● IT: Isolated neutral
● TT: Earthed neutral
● TN-C: Combined protective earthing and neutral conductor (PEN)
● TN-S: Separate earthed neutral (N) and protective earthing (PE) conductors.
Earthing arrangement for computer rooms
Systematic use of the TN-S system
The TN-S system is the earthing arrangement recommended by manufacturers and
standards for computer systems. This is because it provides single-phase distribution
while ensuring a reference potential for the ECPs with the protective conductor.
L1
L2
L3
N
PE
ECPs
3-ph loads
ECPs
Phases: L1, L2, L3
Neutral: N
Protective conductor: PE
Circuit breaker pole: x
Separate N and PE
ph-N loads
Fig. 5.15. TN-S system for computer rooms.
IT and TT systems are poorly suited to computer systems
● The IT system requires competent operating personnel and sophisticated
insulating monitoring to locate and clear insulation faults before a second fault with a
high tripping current can cause disturbances.
APC by Schneider Electric
01/2012 Edition
p. 29
Technology: Transformerless UPSs
● The TT system is too sensitive to lightning overvoltages for sensitive computer
devices.
● The TN-C system(1) (combined earthed neutral and PE conductor) does not offer a
reliable reference potential like the TN-S system.
Single-phase loads, frequent in computer systems, cause H3 harmonics and their
multiples of 3 (H6, H9, etc.) in the neutral. The harmonics then flow in the PEN
conductor where they can cause:
- loss of PEN equipotentiality which spreads through the shielding and can affect
computer-system operation.
- high unbalance currents in cable ways and the building structure due to frequent
PEN connections to earth. The resulting electromagnetic radiation in the cable ways
can disturb sensitive devices.
(1)
The TN-C system may be used upstream of a TN-S system, but the contrary is not allowed
because it can result in upstream interruption of the protective conductor, thus creating a safety
hazard for people downstream.
Computer manufacturer recommendations: Recreate a network with an
earthed neutral at the entry to the computer room
Computer manufacturers recommend that the TN-S system with earthed neutral be
created as close as possible to the loads. This is generally done at the entry to the
computer room).
Use of the TN-S system without this measure, i.e. with the earthed neutral created
far upstream) can create a potential difference between earth and neutral due to the
upstream distribution.
) In conclusion, it is advised to create a TN-S system at the entry to the computer
room with the neutral earthed at this point to ensure clean and suitable electrical
distribution to the computer system.
This is generally done using PDUs (Power Distribution Units) that include an input
transformer, making it possible to obtain a reliable neutral reference potential and
ensuring galvanic isolation in all UPS operating modes (on normal AC input or
bypass).
In addition, this solution uses standard transformers that offer very high reliability,
exceeding that of UPS output transformers. This solution with an input transformers
is used widely in the USA where a 3-phase 480 V distribution system is brought to
the computer room entry to supply 480 V/208 transformer (fig. 5.16).
UPS A
UPS B
.
.
PDU A
x
Isolating
transformers
used to recreate
a TN-S system
with neutral
PDU A
x
x
x
Blade server
Fig. 5.16. Example of transformers used at the PDU input to create a TN-S distribution system
with a neutral.
APC by Schneider Electric
01/2012 Edition
p. 30
Technology: Transformerless UPSs
IT or TT system upstream
Comparison for
different upstream
earthing arrangements
In this case, the system earthing arrangement must be changed to TN-S
downstream of the UPS. Because the neutral cannot have two different references to
earth, galvanic isolation is required for all UPS operating modes (normal or bypass).
● For UPSs with an output transformer, a transformer is generally added at the input
to the bypass (see fig. 5.17).
This solution has two drawbacks:
- 4-pole protection devices must be used to wire and interrupt the neutral on the
bypass,
- the distance D2 from the UPS neutral lout and the loads can affect the neutral
potential because the isolating transformers are not located near the loads.
● Transformerless UPSs from APC by Schneider Electric can operate on 3 phases
without a neutral. This makes it possible to use a 3-phase, 3-wire distribution system
up to the PDU or equivalent and recreate the TN-S system as close as possible to
the application (see right side of fig. 5.17). This arrangement ensures a "clean"
reference potential for the PE.
) In addition to its advantages in terms of efficiency, footprint, weight and voltage
matching, transformerless technology is simple and economical.
Solution with output transformer
IT or TT upstream - TN-S downstream
Transformerless solution
IT or TT upstream - TN-S downstream
IT
IT
TT
TT
LVMS
LVMS
L1
L2
L3
N
L1
L2
L3
N
PE
PE
Normal AC
input
Bypass
transformer
UPS
Bypass AC
input
Q1
BypassAC
input
Normal AC
input
UPS
Q4S
Rectifier
charger
D1
Q1
Battery
QF1
Static
bypass
Q4S
Rectifier
charger
Q3BP
Inverter
Battery
Q3BP
QF1
Static
bypass
K3N
Inverter
Output
transformer
Q5N
K3N
Fixed and clean
reference for
Neutral
Q5N
Earthing
terminal
PE
LVS
D2
PE
L1
L2
L3
N
Power Distribution Unit
transformer
LVS
L1
L2
L3
N
Fig. 5.17. IT or TT upstream and TN-S downstream.
APC by Schneider Electric
01/2012 Edition
p. 31
Technology: Transformerless UPSs (Cont.)
TN-C or TN-S system upstream
These two situations may be dealt with in the same manner. With a TNH-C system
upstream, it is possible to separate the neutral and the PE upstream of the UPS (by
separating the wires) and thus create the situation with TN-S both upstream and
downstream. In the diagrams below, the upstream TN-C simplifies the distribution.
Fir. 5.18 illustrates the only case for an upstream TN-C system.
To provide a reference potential, it is necessary to create a "clean" distribution
system by installing a transformer at the entry to the computer room (generally using
a PDU or equivalent). The greater the distance D1 between the upstream
transformer and the UPS output, the more this solution is required because the
neutral potential can be affected by the upstream distribution
) In this case, solutions using UPSs with or without a transformer are identical,
however transformerless technology offers advantages in terms of efficiency,
footprint, weight and voltage regulation accuracy.
Solution with output transformer
TN-C upstream and TN-S downstream
Transformerless solution
TN-C upstream and TN-S downstream
LVMS
LVMS
L1
L2
L3
N
L1
L2
L3
N
Normal AC
input
UPS
D1
UPS
Q1
Q4S
Rectifier/
charger
Battery
QF1
Normal AC
input
Bypass AC
input
D1
Bypass AC
input
Q1
Q4S
Rectifier/
charger
Battery
Static
bypass
QF1
Q3BP
Static
bypass
Q3BP
Inverter
Inverter
K3N
K3N
Q5N
Q5N
Fixed and clean
reference for
Neutral
PE
Fixed and clean
reference for
Neutral
Power Distribution Unit
transformer
PE
LVS
L1
L2
L3
N
Power Distribution Unit
transformer
LVS
L1
L2
L3
N
Fig. 5.18. TN upstream and downstream.
APC by Schneider Electric
01/2012 Edition
p. 32
Technology: Transformerless UPSs (Cont.)
Results of the comparison
Solutions with an output transformer
● The transformer at the UPS output is of a specific type, more expensive and
requires more space.
● It is necessary to add a transformer at the bypass input, i.e. the installation
requires four-pole devices and a neutral cable, or an output transformer must be
installed.
● The added transformer is not located as close as possible to the loads.
Transformerless solutions
● The constraints caused by the UPS output transformer are avoided.
● A transformer is installed at the entry to the computer room, generally in a PDU.
There is no need for four-pole devices on the bypass or for upstream distribution of
the neutral.
A transformer must still be added, but there are advantages in terms of:
● UPS cost, i.e. no specific output transformer and no four-pole devices and neutral
on the bypass line,
● reduced footprint and weight,
● better output regulation for rapid load fluctuations.
) Given its many advantages, transformerless technology is rapidly becoming the
preferred solution for UPSs.
APC by Schneider Electric
01/2012 Edition
p. 33
Electromagnetic compatibility (EMC)
Electromagnetic
disturbances
Electromagnetic disturbances
All electromagnetic disturbances involve three elements.
A source
A natural source (atmosphere, earth, sun, etc.) or, more often, an industrial source
(electrical and electronic devices).
The source generates disturbances through sudden (pulse) variations in electrical
values (voltage or current), defined by:
• A wave form,
• A wave amplitude (peak value),
• A spectrum of frequencies,
• A level of energy.
A coupling mode
Coupling enables transmission of disturbances and may be:
• Capacitive (or galvanic), for example via transformer windings,
• Inductive, by a radiating magnetic field,
• Conducted, by a common impedance, via an earthing connection.
A victim
This is any device likely to be disturbed, and which malfunctions due to the presence
of the disturbances.
Examples
Sources
In low-voltage installations, sources include suddenly varying currents resulting from:
• Faults or short-circuits,
• Electronic switching,
• High-order harmonics,
• Lightning or transformer breakdown.
Frequencies may be low (< 1 MHz) for power frequencies and their harmonics or
high (> 1 MHz) for lightning.
Coupling
• Capacitive: transmission of a lightning wave via a transformer.
• Inductive: radiation of a magnetic field created by one of the above currents.
Radiation creates an induced electromotive force, that is an induced disturbing
current, in the loops of conductors made up of the cables supplying devices and the
earthing conductors of the devices.
As in indication, a radiation of 0.7 A/m can disturb a video monitor.
That corresponds to the field created 2.2 m around a conductor carrying a current of
10 A.
• Conducted (common impedance): increase in the potential of an earthing
connection.
EMC standards and
recommendations
Disturbances
Emission, immunity, susceptibility
An electric device is installed in an environment that may be more or less disturbed
electromagnetically. It must be seen as both a source and possible victim of
electromagnetic disturbances.
Depending on the point of view, on may speak of:
• The emission level for a source,
• The compatibility level for an environment,
• The immunity and susceptibility levels for a victim.
These notions are discussed on the next page in the section on disturbance levels
defined by the standards.
APC by Schneider Electric
01/2012 edition
p. 34
Electromagnetic compatibility (EMC) (Cont.)
Disturbance levels
Standard IEC 6100-2-4 defines a number of disturbance levels for EMC:
• Level 0: no disturbance,
• Emission level: maximum level authorised for a user on a public utility or for a
device,
• Compatibility level: maximum disturbance level expected in a given environment,
• Immunity level: level of disturbance that a device can withstand,
• Susceptibility level: level starting at which a device or system malfunctions.
Consequently, for devices and equipment that are considered:
• Sources, limits (emission levels) must be set for disturbances emitted by devices
to avoid reaching compatibility levels,
• Victims, they must also withstand disturbance levels higher than the compatibility
levels, if they are exceeded, which is permissible on a transient basis. These higher
levels are the immunity levels.
EMC standards set these levels.
Î List of EMC standards, see the section on page 34 on EMC standards.
Fig. 5.19 EMC disturbance levels for disturbing/disturbed devices.
Measured values
Devices are subjected to tests.
Five major values are measured:
• CE - conducted emissions,
• RE - radiated emissions,
• ESD - electrostatic discharges,
• CS - conducted susceptibility,
• RS - radiated susceptibility.
The tests require major resources, namely a Faraday cage for conducted emissions
and susceptibility and an anechoic chamber for radiated emissions.
APC by Schneider Electric has a certified anechoic test chambers.
Fig. 5.20 Five major measurement values.
APC by Schneider Electric
01/2012 edition
p. 35
UPS standards
Scope and observance of
standards
Scope of standards
Standards cover the following aspects:
• UPS design,
• Safety of persons,
• Performance levels,
• Electrical environment (notably harmonic disturbances and EMC),
• Ecological environment.
Standards on UPSs have become much more precise, notably with the creation of
the European EN standards and their harmonisation with a part of the previously
existing IEC standards.
Observance of standards and certification
Observance of standards guarantees the reliability and the quality of a UPS, its
compatibility with the loads supplied as well as with the technical, human and natural
environment.
Statement by a manufacturer of conformity with standards is not, in itself, a sufficient
indication of quality. Only certification by recognised organisations is a true
guarantee of conformity.
To that end, performance levels of UPSs from APC by Schneider Electric with
respect to standards are certified by organisations such as TÜV and Veritas.
CE marking
CE marking was created by European legislation.
It is mandatory for free circulation of goods in the EU.
Its purpose is to guarantee, through respect of the corresponding European
directives:
• That the product is not dangerous (Low-voltage Directive),
• That it does not pollute (Environment Directive) and its electromagnetic
compatibility (EMC Directive).
Before placing the CE marking on a product, the manufacturer must run or have run
checks and tests which ensure conformity of the product with the requirements in the
applicable directive(s).
It is NOT a certification standard or mark of conformity.
It does not signify that the product complies with national or international standards.
It is not a certification as defined by French law (law dated 3 June 1994).
What is more, the CE marking is placed on a product under the exclusive
responsibility of the manufacturer or the importer. It does not imply inspection by a
certified external organisation.
) Not all labels carry the same implications for manufacturers.
Conformity with standards and specified levels of performance must be certifiable by
an organisation. This is not the case for CE marking which authorises selfcertification.
Main standards governing
UPSs
UPSs from APC by Schneider Electric comply (certified by TÜV and Veritas) with the
main applicable international standards.
Safety
• IEC 60950-1 / EN 60950-1
Information technology equipment - Safety - Part: General requirements
• IEC 62040-1/ EN 62040-1
Uninterruptible power systems (UPS) - General and safety requirements for UPS.
• IEC 62040-3 / EN 1000-3
Uninterruptible power systems (UPS) - Method of specifying the test and
performance requirements.
• IEC 60439
Low-voltage switchgear and controlgear assemblies.
• LV directive: 2006/95/EC
APC by Schneider Electric
01/2012 edition
p. 36
UPS standards (Cont.)
Electrical environment, harmonics and electromagnetic
compatibility (EMC)
Harmonics
• IEC 61000-2-2 / EN 61000-2-2
Compatibility levels for low-frequency conducted disturbances and signalling in
public low-voltage power supply systems.
(see Table 5-A on the next page)
• IEC 61000-3-2 / EN 61000-3-2
Limits for harmonic current emissions (equipment input current ≤ 16 A/ph).
• IEC 61000-3-4 / EN 61000-3-4
Limits for harmonic current emissions (equipment input current > 16 A/ph).
• IEC 61000-3-5 / EN 61000-3-5
Limitation of voltage fluctuations and flicker.
• EN 50160
Voltage characteristics of public networks.
(see Table 5-B on the next page).
• IEEE 519
Recommended practices and requirements for harmonic control in electrical power
systems.
EMC
• EN 50091-2
UPS - EMC.
• IEC 62040-2/ EN 62040-2
Uninterruptible power systems (UPS) - Electromagnetic compatibility (EMC)
requirements.
• EMC Directive 2004/108/EC
For equipment liable to cause or be affected by electromagnetic disturbances.
Quality
• Design , production and servicing in compliance with standard ISO 9001 - quality
organisation.
Ecological environment
• Manufacturing in compliance with standard ISO 14001.
APC by Schneider Electric
01/2012 edition
p. 37
UPS standards (Cont.)
Acoustic noise
• ISO 3746
Sound power levels.
• ISO 7779 / EN 27779
Measurement of airborne noise emitted by computer and business equipment.
Tables on harmonic-compatibility levels
Table 5-A. Compatibility levels for individual harmonic voltages in low voltage networks as
indicated in standards IEC 61000-2-2 / EN 61000-2-2.
Odd harmonics non multiple Odd harmonics multiple of 3 Even harmonics
of 3
Harmonic
Harmonic
Harmonic
Harmonic
Harmonic
Harmonic
order n
voltage as a % order n
voltage as a % order n
voltage as a %
of fundamental
of fundamental
of fundamental
5
6
3
5
2
2
7
5
9
1.5
4
1
11
3.5
15
0.3
6
0.5
13
3
21
0.2
8
0.5
17
2
>21
0.2
10
0.5
19
1.5
12
0.5
23
1.5
>12
0.2
25
1.5
0.2
>25
0.2+0.5x25/n
Resulting THDU < 8% (for all harmonics encountered among those indicated).
Table 5-B. Compatibility levels for harmonic voltages according to the type of equipment as
indicated in standard EN 50160.
(1)
Order of the voltage
Class 1
Class 2
Class 3
harmonic generated
(sensitive systems and (industrial and public (for connection of
equipment) % of
networks) % of
major polluters) % of
fundamental
fundamental
fundamental
2
2
2
3
3
3
5
6
4
1
1
1.5
5
3
6
8
6
0.5
0.5
1
7
3
5
7
8
0.5
0.5
1
9
1.5
1.5
2.5
10
0.5
0.5
1
11
3
3.5
5
12
0.2
0.2
1
13
3
3
4.5
TDHU
5%
8%
10%
(1)
Class 2 corresponds to the limits of Table A of standards IEC 61000-2-2 / EN 61000-2-2.
APC by Schneider Electric
01/2012 edition
p. 38
Energy storage
Possible technologies
Energy storage in UPSs
The energy-storage systems used by UPSs to backup the primary source must have
the following characteristics:
•Immediate availability of electrical power,
•Sufficient power rating to supply the load,
•Sufficient backup time and/or compatibility with systems providing long backup
times (e.g. an engine generator set or fuel cells).
Evaluation of the available technologies
See WP 65 Î
The technical watch established by APC by Schneider Electric resulted in in-depth
examination of the following technologies:
•Batteries,
•Supercapacitors (ultracapacitors),
•Flywheels,
•Superconducting magnetic energy storage (SMES).
Î For more information, see WP 65 (White Paper 65: "Comparing Data Center
Batteries, Flywheels, and Ultracapacitors".
Battery and flywheel solutions are discussed below.
Batteries
The battery solution
Batteries are by far the most commonly employed solution today for energy storage
in UPSs. They are the dominant solution due to low cost, proven effectiveness and
storage capacity, but nonetheless have a number of disadvantages in terms of size,
maintenance and the environment. At the power ratings under consideration, they
offer backup times in the ten-minute range, enough to ride through long outages and
wait for start-up of an engine generator set for extended runtime.
For its SymmetraTM PX range, APC by Schneider Electric also offers extended
runtime solutions based on fuel cells, with the FCXR (Fuel Cell eXtended Run)
product range. This solution significantly reduces the environmental impact and
floorspace requirements with respect to solutions combining batteries and an engine
generator set.
) Electrochemical energy storage using batteries, where applicable backed up by a
thermal engine generator set, is the commonly used solution to protect critical loads
using UPSs.
Fig. 5.21. Energy storage using a battery and an engine generator set for long backup times.
63APC by Schneider Electric
01/2012 edition
p. 39
Energy storage (Cont.)
Types of industrial batteries
Battery families
A battery is a set of interconnected cells.
Depending on the type of cell, there are two main families of batteries:
• Lead-acid batteries,
• Nickel cadmium batteries.
Cells may also be of the :
• Recombination type Æ sealed batteries
The gas recombination rate is at least 95% and they therefore do not require water to
be added during service life (hence the term "sealed"),
• Vented type Æ vented batteries
They are equipped with ports to:
- release to the atmosphere the oxygen and hydrogen produced during the different
chemical reactions,
- top off the electrolyte by adding distilled or demineralised water.
Batteries used in UPSs
The main types of batteries used in conjunction with UPSs are:
• Sealed lead-acid batteries, used 95% of the time because they are easy to
maintain and do not require a special room, these batteries can be installed in office
environments and in any position,
• Vented lead-acid batteries,
• Vented nickel-cadmium batteries.
Vented batteries impose greater constraints in terms of maintenance (checks on the
electrolyte level) and their position (only in the vertical position).
Lithium-polymer batteries are currently being studied for use in UPSs.
For use in conjunction with its UPS ranges, APC by Schneider Electric recommends
sealed lead-acid batteries, but nonetheless offers a wide selection of other types.
It offers all three types of battery for all the available service lives.
Capacity levels and backup times may be adapted to suit the user's needs.
The proposed batteries are also perfectly suited to UPS applications in that they are
the result of collaboration with leading battery manufacturers.
Î Battery selection, see ” Key factors in UPS installation” p. 46.
Installation modes
Depending on the UPS range, the battery capacity and backup time, the battery is:
• Sealed type and housed in the UPS cabinet,
• Sealed type and housed in one to three cabinets,
• Vented or sealed type and rack-mounted.
Cabinet mounting
This installation method (see fig. 5.15) is suitable for sealed batteries. It is easy to
implement and offers maximum safety.
Batteries installed on racks
• On shelves (figure 5.16)
This installation method is possible for sealed batteries or maintenance-free vented
batteries which do not require topping up of their electrolyte.
• Tier mounting (figure 5.17)
This installation method is suitable for all types of batteries and for vented batteries
in particular, as level checking and filling are made easy.
Fig. 5.22. Cabinet mounting.
63APC by Schneider Electric
01/2012 edition
Fig. 5.23. Mounting on shelves. Fig. 5.24. Tier mounting.
p. 40
Energy storage (Cont.)
Constraints on batteries
Atmospheric constraints
The batteries supplied with UPSs from APC by Schneider Electric are designed to
operate under the following conditions:
• Optimum temperature range: 15°C to 25°C,
• Optimum relative humidity range: 5% to 95%,
• Atmospheric pressure: 700 to 1060 hPa (0.7 to 1.06 bars).
For other operating conditions, please consult us.
Access
Access must be provided for testing operations.
• Battery installed in UPS cabinet or other cabinet: comply with the clearances
indicated in the "Dimensions and weights" in chapter 4.
• Battery installed on racks: select an installation method suited to the type of
battery.
• Preliminary work: this aspect is important as it involves safety. It is discussed in ”
Key factors in UPS installation” p. 49.
Main battery parameters
Backup time
For a given battery, the backup time depends on:
• The power that must be supplied, a low value increases the available autonomy,
• The discharge conditions, a high discharge rate makes possible a lower shutdown
voltage and thus increases the backup time,
• Temperature, within the recommended operating limits, the backup time increases
with increasing temperature. Note, however, that a high temperature adversely
affects battery service life,
• Ageing, battery backup time decreases with the age of the battery.
APC by Schneider Electric offers a range of standard backup times (5, 6, 8, 10, 15 or
30 minutes and service lives (5 or 10 years or higher) and also caters to all specific
requirements.
Service life
A battery is considered to reach the end of its service life when its real backup time
has fallen to 50% of the specified backup time.
The service life of a battery is basically enhanced by:
• Providing protection against deep discharge,
• Correct charger settings, in particular the ripple factor of the charge or float current,
• An optimum operating temperature, maintained between 15°C and 25°C.
Recharge mode
The charge cycle takes place in two steps:
• Step 1, a constant current limited to 0.1 C10 (one tenth of the battery capacity for a
ten-hour discharge),
• Step 2, a constant voltage, at the maximum permissible value. The charge current
regularly decreases and reaches the floating value.
Fig. 5.25. Battery charge cycle.
63APC by Schneider Electric
01/2012 edition
p. 41
Energy storage (Cont.)
Battery management for MGETM GalaxyTM ranges
DigibatTM
TM
TM
To manage the above parameters, all MGE Galaxy UPSs from APC by
Schneider Electric come as standard with the microprocessor-based DigibatTM
battery-monitoring system (dedicated DSP for real-time processing).
DigibatTM, an easy-to-use system, offers advanced and flexible functions as well as
physical and computer-aided protection for the battery. It provides a high level of
safety, true measurement of the backup time and optimises battery service life. For
example, for an MGE Galaxy 5000 UPS, the functions include:
• Automatic entry of battery parameters,
• Measurement of the real backup time remaining, taking into account the age of the
battery, the temperature and the load level,
• Estimate of remaining battery life (1),
• Battery test to preventively detect battery-function faults (1),
• Regulation of battery voltage with respect to the temperature to optimise battery
life,
• Automatic battery-discharge test at adjustable time intervals.
Protection includes:
• Protection against deep discharge (depending on the discharge rate) and battery
isolation using a circuit breaker which automatically opens when the backup time,
multiplied by two plus two hours, has elapsed,
• Limiting of the recharge current in the battery (0.05 C10 to 0.1 C10),
• Progressive audio alarm signalling the end of the backup time,
• Numerous automatic tests.
(1) APC by Schneider Electric exclusive patents.
Fig. 5.26. Digibat
TM
Temperature monitoring
TM
TM
MGE Galaxy UPSs can also be equipped with the Temperature Monitoring
module used to:
• Optimise the charger voltage depending on the temperature in the battery room,
• Warn the user if preset permissible temperature limits are exceeded,
• Refine the estimate on battery backup time carried out by the standard system.
Natural ventilation of battery cabinets avoids battery temperature rise.
Environment Sensor is also a simple means to monitor temperature and humidity. It
can be used to launch shutdown when combined with software running the module.
Battery monitoring
APC by Schneider Electric also offers the B2000 and Cellwatch autonomous and
communicating battery-monitoring systems which immediately detect and locate all
battery faults. These systems monitor each battery block or cell and make possible
predictive maintenance.
63APC by Schneider Electric
01/2012 edition
p. 42
Energy storage (Cont.)
Flywheels
Flywheel energy storage
Operating principle
A flywheel energy storage system is a “mechanical battery” that stores energy
kinetically in the form of a rotating mass. When required during a utility outage, the
energy stored by the rotating mass is converted to electrical energy through the
flywheel’s integrated electric generator.
The amount of energy stored in a flywheel is given by:
E = kMω2
where k depends on the shape of the rotating mass, M is the mass of the flywheel
and ω its angular velocity.
) Note that the energy stored is proportional to the square of the angular velocity.
This is one of the reasons that APC by Schneider Electric proposes flywheels
spinning at relatively high speeds. This reduces both the weight and the footprint of
the energy storage system.
UPS applications
Flywheel units can replace traditional UPS batteries or work in tandem with batteries
to provide highly reliable, instantaneous backup power for today’s mission-critical
applications (data centres, hospitals, broadcast studios, casinos airports and
manufacturing plants). They interface with the DC bus of the UPS, just like a battery,
receiving charging current from the UPS and providing DC current to the UPS
inverter during discharge.
UPS
Critical
loads
AC input
Rectifier
Flywheel
Inverter
Battery
Fig. 5.27. Simplified diagram of a UPS with a flywheel energy storage connected in parallel with
a battery.
Flywheel energy storage systems have two applications depending on whether or
not the installation includes a genset.
Battery hardening for installations without gensets
For installations without gensets, a flywheel energy storage system can operate in
parallel with batteries. This flywheel application is often referred to as "battery
hardening".
In such a configuration, the flywheel is the first line of defence against power
anomalies – offering higher availability and saving the batteries for prolonged power
outages. By being first to provide the necessary energy to ride-through power
glitches, the flywheel system significantly increases battery life by absorbing over
98% of the discharges that would normally be supplied by batteries. Battery
hardening with flywheels offers a number of benefits.
• Fewer battery charge-discharge cycles, thereby extending battery life
• Less frequent battery replacement and associated lead acid disposal
• Higher availability of critical DC bus
63APC by Schneider Electric
01/2012 edition
p. 43
Energy storage (Cont.)
Battery replacement for installations with gensets
Gensets are generally able to take on the load within 10 seconds of a utility failure.
While UPS batteries can provide power during this transition, their reliability is always
in question. Are they fully charged? Has a cell gone bad in the battery string? When
was the last time they were checked?
By contrast, flywheel systems provide reliable energy storage instantaneously to
assure a predictable transition to the standby genset, all in a compact footprint.
A flywheel system providing 10 or 20 seconds of energy offer a number of
advantages over batteries for installations with gensets.
• Highly reliable and predictable energy storage:
- estimated 54,000 hour MTBF
- continuous monitoring gives highly predictable performance
• Environment-friendly alternative to batteries:
- no lead, no acid, small carbon footprint
• Lower TCO:
- 20 years useful life time
- low maintenance
- small size and light weight
- capable of operating at temperatures up to 40°C
Types of flywheels
UPS flywheels can be divided into several types depending on their speed, flywheel
material and motor generator configuration.
Flywheel speed
• Low speed flywheels
- Angular velocity <10 000 rpm
- Energy for high power requires heavy steel flywheels (heavy and bulky)
- Periodic maintenance and replacement to the mechanical bearings
- High amount of parasite energy losses
- Requires special concrete slab specifications for installation
• High speed flywheels
- 30 000 to 60 000 rpm (potentially up to 100 000 rpm)
- Much lighter for high power needs (energy stored through higher spinning velocity)
- Full magnetic levitation
- Lower periodic maintenance
- Smaller footprint and lighter weight
- Easy commissioning, start up and shutdown
As already mentioned, flywheels supplied with APC by Schneider Electric UPSs
operate at relatively high speeds (36 000 rpm when fully charge) and offer all the
corresponding advantages.
Flywheel materials
• Carbon fibre flywheels
Carbon fibber flywheels are typically made by winding great lengths of carbon-fibber
on a spindle. They are held together by an epoxy resin.
Imperfections in the process and gaps between the fibbers can lead to unbalancing
of the wheel over time due to the stresses applied as the wheel is spun from high
speed to low speed and back again, which occurs during every discharge event.
Once the carbon fibber flywheel becomes unbalanced, the entire flywheel module
much be replaced, a very costly and time consuming processes.
• Steel flywheels
The flywheels supplied with APC by Schneider Electric UPSs are made of aerospace
grade 4340 steel. The material properties are very well known, available from
numerous suppliers and this material is used in many high speed rotating
applications. Most important is the integrity of the material can be measured through
core samples and ultrasound to make sure it complies with the application
specifications. The same flywheel has been used not only in UPS applications, but
also in high-cycling, regenerating applications like in electric motors for cranes and
electric rail. These applications call on the flywheel to be charged and discharged up
to 20 times per hour. These applications prove the robustness of utilising aerospace
grade steel as the preferred flywheel material.
63APC by Schneider Electric
01/2012 edition
p. 44
Energy storage (Cont.)
Motor generator configuration
The other difference found in flywheel energy storage systems lies in the motor
generator configuration.
• Flywheels systems supplied by APC by Schneider Electric use a permanent
magnet type motor generator. The benefit of this is twofold:
- Higher efficiency of the motor generator when charging and discharging, providing
the high cycling capability of the flywheel
- The flywheel can generate its own power to maintain the flywheel levitation even if
control power is lost or a failure occurs in the power electronics.
• Other flywheel manufacturers use a synchronous reluctance motor that cannot
self generate power if a failure occurs in the power electronics.
- The unit therefore requires a back-up supply from a small UPS to provide backup
power to the magnetic bearings.
Installation
Flywheel cabinets
Flywheel energy storage systems are supplied in separate cabinets that connect to
the DC bus just like battery cabinets. Multiple flywheel cabinets can be installed in
parallel for higher power, longer run-time or redundancy.
Site preparation
Minimal site preparation is required for installation of flywheel cabinets. Before
installation, consideration must be given to a certain aspects.
• Wiring and cabling to UPS and other equipment
• Service access
• Clearances for cooling
• Floor mounting
Constraints on flywheels
Atmospheric constraints
The flywheel energy storage systems supplied with UPSs from APC by Schneider
Electric are designed to operate under the following conditions:
• Operating temperature: -20°C to 40°C (without derating)
• Minimum cold start temperature: 0°C
• Relative humidity: up to 95% (non-condensing)
For other operating conditions, please consult us.
Main flywheel parameters
Output power and backup time
The flywheel energy storage systems supplied with UPSs from APC by Schneider
Electric offer flexibility in selecting the best power level and runtime combination to
meet application requirements.
• Single units are available in 215kW and 300kW ratings.
• The 300kW model can deliver 160kW for ~18.75 seconds or 220kW for ~10
seconds, generally sufficient for battery hardening or genset startup applications.
• Multiple flywheel units can be paralleled for higher capacity, redundancy or
runtime.
Service life
• The service life of a flywheel energy storage time is typically much longer than that
of lead-acid batteries.
• The flywheel energy storage systems supplied with UPSs from APC by Schneider
Electric has a service of life of 20 years for operating temperatures up to 40°C and
frequent charge-discharge cycles.
63APC by Schneider Electric
01/2012 edition
p. 45
UPS / generator-set combination
Use of a generator
Long backup times
An engine generator set is made up of an internal-combustion engine driving a
generator that supplies the distribution system. The backup time of an engine
generator set depends on the quantity of fuel available.
In some installations, the required backup time in the event of a utility outage is such
that it is preferable to use an engine generator set to back up utility power (figure
5.28).
This solution avoids using large batteries with very long backup times.
Though there is no general rule in the matter, a generator is often used for backup
times exceeding 30 minutes. Critical installations requiring very high availability
levels and with high down-time costs (e.g. data centres) systematically combine
UPSs and engine generator sets.
The battery backup time of the UPS must be sufficient for generator start-up and
connection to the electrical installation. Connection is generally carried out on the
main LV switchboard using an automatic source-changeover system. The time
required for changeover depends on the specific characteristics of each installation,
notably the start-up sequence, load shedding, etc.
Fig. 5.28. UPS / generator-set combination.
UPS / generator-set
combination
UPS / generator-set compatibility
A number of factors must be taken into account when using an engine generator set
to provide long backup-time power to UPSs.
Load step changes
In the event of emergency conditions requiring connection of the installation to the
generator set, heavy loads can result in high inrush currents which can cause
serious generator-set operating problems. To avoid such phenomena, UPSs from
APC by Schneider Electric are equipped with a system ensuring gradual start-up of
the charger. The walk-in lasts approximately ten seconds. What is more, when utility
power returns, the charger may be stopped gradually via an auxiliary switch in order
to avoid disturbing the other loads.
APC by Schneider Electric
01/2012 edition
p. 46
UPS / generator-set combination (Cont.)
Fig. 5.29. Gradual start of the UPS rectifier during operation on generator power.
Capacitive currents
The generator can supply only relatively low capacitive currents (10 to 30 % of In).
When an LC filter is installed, the main difficulty lies in the gradual start-up of the
rectifier on generator power, when active power is equal to zero and the generator
supplies only the capacitive current for the filter. Consequently, the use of LC filters
must be correctly analysed to ensure that operation complies with manufacturer
specifications.
Use of compensated LC filters with a contactor solves this problem. For UPSs with a
PFC rectifier, compatibility is total.
Î LC filters and generators, see Ch. 1 p. 26.
Respective UPS and generator power ratings
A UPS equipped with a PFC rectifier has a high input power factor (greater than 0.9).
The engine generator set can therefore be used to maximum effectiveness.
For LC filters, compensated filters with a contactor solve the problem concerning
capacitive currents.
) Compatibility of power ratings between modern UPSs and engine generator sets
avoids all problems of derating.
Stability of generator frequency
During operation on engine generator set power, fluctuation in the generator
frequency may occur due to variations in the speed of the thermal motor for which
the regulation functions are not instantaneous. These variations are due to changes
in the load. Examples are start-up of the engine generator set itself (until it reaches
its rated speed), start-up of other loads supplied by the engine generator set
(elevators, air-conditioning systems), or shedding of loads.
This may create problems with line-interactive UPSs whose output frequency is
identical to that of the input. Generator frequency variations may lead to multiple
transfers to battery power (frequency outside tolerances) and returns to utility power
(when the inverter has stabilised the frequency, but the generator itself has not yet
stabilised), resulting in "hunting" phenomena (instability around the frequency setpoint).
With double-conversion UPSs, the regulation of the output power by the inverter
avoids this problem.
) Double-conversion UPSs are totally compatible with the frequency fluctuations of
engine generator sets. This is not the case for line-interactive UPSs.
Harmonics
The subtransient reactance X"d of a generator is generally higher than the shortcircuit voltage Uscx of a transformer (two to four times higher). Any harmonic
currents drawn by the UPS rectifier may have greater impact on the voltage
harmonic distortion on the upstream busbars. With PFC rectifier technology, the
absence of upstream harmonics avoids this problem.
APC by Schneider Electric
01/2012 edition
p. 47
UPS / generator-set combination (Cont.)
Review of inrush currents
On start-up, a number of loads cause major inrush currents (switching surges, startup peaks), which last a certain time.
For the UPS, these currents represent an apparent load Sa (kVA) that is greater than
Sn (kVA), which can be supplied under steady-state conditions.
The value of Sa to be taken into account in sizing UPS power is calculated on the
basis of these inrush currents.
Below are indications on these currents caused by common load devices.
Motors
Motors are generally of the three-phase asynchronous type (95% of all motors). The
additional power requirement corresponds to the start-up current defined by (fig. 5.30):
• Id (5 to 8 In, rated rms value) for a time td (1 to 10 seconds),
• Imax = 8 to 12 In, for 20 to 30 milliseconds.
The power drawn that must be taken into account (neglecting the peak effect of
Imax) is:
Sa (kVA) = Un Id 3 during td.
LV/LV transformers
Transformer switching produces current peaks with amplitudes that are damped
according to an exponential decay with a time constant (see fig. 5.31).
• i = I1st peak exp -t/τ where τ is a few cycles (30 to 300 ms).
• I1st peak = k In (where k is given, generally 10 to 20).
Indications generally include the number of cycles the phenomenon lasts and the
value of the various peaks as a percentage of I1st peak.
The corresponding inrush current is generally calculated on the basis of (see
example):
• Sa (kVA) = Un I1st peak 3 , i.e. Sa (kVA) = k Un In 3 during the number of cycles.
• Example of an inrush current damped in four cycles with:
1st peak (100%): k In (k from 10 to 20),
2nd peak 30 %: 0.3 k In,
3rd peak 15 %: 0.15 k In.
The total of the rms values of the currents corresponding to the various peaks (Ipeak
/ 2 ) (1) is:
k In (1 + 0,3 + 0,15 ) K In 1,45
=
≈ k In
2
2
This is roughly equivalent to the value of the first peak alone.
(1)
Considering the current peaks as sine waves; note that some manufacturers indicate an rms
value of Ipeak / 2.
Computer loads
Switch-mode power supplies are non-linear loads. The current for a single-phase
load has a wave form similar to that shown in figure 5.32. There can be a peak
during the first half wave of approximately 2 In. However, it is generally much lower
than this and can be neglected.
Fig. 5.30. Curve for direct on- Fig. 5.31. LV/LV transformer
line starting of a three-phase switching current.
asynchronous motor.
APC by Schneider Electric
01/2012 edition
Fig. 5.32. Computer load
starting current.
p. 48
Harmonics
Harmonics
Origin of harmonics
The increasing use of computing, telecommunications and power-electronics devices
have multiplied the number of non-linear loads connected to power systems.
These applications require switch-mode power supplies which transform the voltage
sine wave into periodic signals of different wave forms. All these periodic signals of
frequency f are the product of superimposed sinusoidal signals with frequencies that
are multiples of f, known as harmonics (see the section "Characteristic harmonic
values" dealing with the Fourier theorem below, on page 40). Figure 5.32 illustrates
this showing the initial current (the fundamental) and the third-order harmonic.
This figure shows what happens when a thirdorder harmonic (150/180 Hz) is superimposed
on the fundamental frequency (50/60 Hz). The
frequency of the resulting periodic signal is
that of the fundamental, but the waveform is
distorted.
Fig. 5.33. Example of harmonics.
The increased presence of harmonics is a phenomenon that concerns all electrical
installations, commercial and industrial, as well as residential. No modern electrical
environment is exempt from these disturbances caused by devices such as PCs,
servers, fluorescent tubes, air-conditioners, variable-speed drives, discharge lamps,
rectifiers, static power supplies, microwave ovens, televisions, halogen lamps, etc.
All these loads are termed "non-linear".
Consequences of harmonics
Harmonics disturb, increasingly severely, all sorts of activities, ranging from factories
producing electronic components and data-processing systems to pumping stations,
telecommunications systems, television studios, etc., because they represent a
significant part of the current drawn.
There are three types of negative consequences for users:
Impact on the electrical installation
Harmonics increase the value of the rms current with respect to that of the rated
sinusoidal current. The result is temperature rise (sometimes significant) in lines,
transformers, generators, capacitors, cables, etc. The hidden costs of accelerated
aging in such devices can be very high.
Impact on applications
Harmonic currents circulate in the source and line impedances, thus generating
voltage harmonics which lead to voltage distortion on the busbars upstream of the
non-linear loads (figure 5.34).
The distortion of the supply voltage (upstream THDU - Total harmonic distortion in
voltage) may disturb the operation of certain sensitive devices connected to the
these busbars.
What is more, for TNC systems where N and PE conductors are combined to form a
PEN conductor, the zero-sequence third-order harmonics cumulate in the neutral
conductor. This unbalance current in the neutral can disturb circuits interconnecting
low-current devices and may require oversizing of the neutral.
APC by Schneider Electric
01/2012 edition
p. 49
Harmonics (Cont.)
Fig. 5.34. Voltage distortion due to reinjection of harmonic currents by non-linear loads.
Impact on the available electrical power
Harmonics represent an outright loss of current (up to 30% more current consumed).
The user must pay more for less available power.
Precautions
General
There are a number of traditional solutions to limit harmonics:
• installation of tuned passive filters,
• installation in parallel of several cables with medium-sized cross sections,
• separation of non-linear loads and sensitive loads behind isolating transformers.
However, these solutions have two major disadvantages:
• limitation of harmonics is effective only in the existing installation (the addition or
removal of loads can render it ineffective),
• implementation is difficult in existing installations.
SineWave active harmonic conditioners (see chapter 3) avoid these disadvantages.
Much more effective than other solutions, they may be used with all types of loads
and can selectively eliminate harmonics ranging from the 2nd to the 25th order.
Î Elimination of harmonics, see “eliminate harmonic currents”
UPSs
• Due to the rectifier/charger, a UPS is a non-linear load for its power source. UPSs
from APC by Schneider Electric offer perfect control over upstream harmonics by
using "clean" PFC rectifiers or filters (MGE Galaxy PW and 9000).
Upstream of the UPS, the total voltage distortion remains within limits that are
acceptable for the other devices connected to the same busbars.
APC by Schneider Electric
01/2012 edition
p. 50
Harmonics (Cont.)
Characteristic harmonic
values
Current values
Harmonic expansion of a periodic current
The Fourier theorem indicates that any periodic function with a frequency f may be
represented as the sum of terms (series) composed of:
• a sinusoidal term with frequency f, called the fundamental frequency,
• sinusoidal terms with frequencies that are whole multiples of the fundamental
frequency, i.e. the harmonics,
• a DC component, where applicable.
Application of the Fourier theorem to the currents of non-linear loads indicates that a
periodic current I(t), of whatever form at frequency f (50 or 60 Hz), is the sum of
harmonic sinusoidal currents defined by:
∞
I( t) = IH1 2 sin(ωt + ϕ1) +
∑ IHn
2 sin(nωt + ϕn)
n= 2
where
• IH1 is the rms value of the fundamental current at frequency f (50 or 60 Hz),
• ω = 2 π f is the angular frequency of the fundamental,
• ϕ1 is the phase displacement between the fundamental current and the voltage,
• IHn is the rms value of the nth harmonic, at frequency nf,
• ϕn is the phase displacement between the nth harmonic current and the voltage.
It is important to evaluate the harmonics (n ≥ 2) with regards to the fundamental (n =
1) to determine to what degree the function differs from the fundamental.
To that end, the values below are taken into account.
Current individual harmonic content
This value expresses the ratio in percent between of the rms value of the given
harmonic and that of the fundamental.
IHn
Ihn % = 100
IH1
All the harmonics present in a given current with the indication of their relative
importance (Ihn values) constitute the harmonic spectrum of the current. Generally
speaking, the influence of the orders above the 25th is negligible.
Current total harmonic distortion
This distortion is called THDI (Total Harmonic Distortion where I is for the current). It
expresses the ratio between the rms value of all harmonics (n ≥ 2) and that of the
fundamental. The THDI is also expressed in terms of the individual harmonics.
∞
∑ IH
n
n= 2
THDI% = 100
IH1
2
∞
= 100
⎡ IHn ⎤
∑ ⎢⎣ IH ⎥⎦
n= 2
∞
2
=
1
∑ (Ih %)
2
n
n= 2
Note. Harmonic contents are sometimes expressed with respect to the complete signal Irms,
and not the fundamental (IEC documents). Here, we use the definition of the CIGREE, which
uses the fundamental.
For the low harmonic contents analysed in the following pages, the two definitions produce
virtually identical results.
Rms value of a current with harmonics
The rms value of an alternating current with a period T is:
Irms =
1
T
∫
T
0
I( t) dt
2
After calculation and using harmonic representation, this can be expressed as:
∞
Irms =
∑ IH
n
2
n =1
where IHn = rms value of the nth harmonic.
APC by Schneider Electric
01/2012 edition
p. 51
Harmonics (Cont.)
The rms value is also expressed as:
∞
∑ IH
Irms = IH12 +
n
2
or:
n= 2
∞
Ieff = IH1 1 +
∑
n=2
⎡ IHn ⎤
⎥
⎢
⎢⎣ IH1 ⎥⎦
2
hence:
∞
Irms = IH1 1 +
∑ Ih
n
2
= IH1 1 + THDI2
n= 2
• Ihn = Ihn% / 100 (individual level expressed as a value and not as a percentage).
• THDI = THDI% / 100 (distortion expressed as a value and not as a percentage).
The rms value of the current is that of the fundamental, multiplied by a coefficient
which is due to the harmonics and is a function of the distortion.
) One effect of harmonics is therefore to increase the rms value of the current,
which can lead to temperature rise and therefore require oversizing of conductors.
The lower the distortion, the less need for oversizing.
Example
Input current of a three-phase rectifier.
Harmonic distortion levels
Ih5 = 33%
Ih7 = 2.7%
Ih11 = 7.3%
Ih13 = 1.6%
Ih17 = 2.6%
Ih19 = 1.1%
Ih23 = 1.5%
Ih25 = 1.3%
THDI = 35%
Fig 5.35. Example of the spectrum of a harmonic current.
∞
THDI% =
∑ (Ih %)
2
n
n= 2
The value under the square root sign is:
332 + 2.72 + 7.32 + 1.62 +2.62 + 1.12 + 1.52 +1.32 = 1164
consequently THDI% ≈ 34% and THDI = 0.34.
Ieff = IH1 1+ THDI2 = IH1 1 + 0.34 2 = 1.056 x I1
The rms value of this current is therefore 5.6% higher than the rms value of the
fundamental, i.e. than the rated current containing no harmonics, with a
corresponding temperature rise.
Voltage values
At the terminals of a non-linear load, through which a distorted periodic AC current
flows, the voltage is also periodic with a frequency f and it is also distorted with
respect to the theoretical sinusoidal wave. The relation between voltage and current
is no longer governed by Ohm's linear law, because it is applicable only for
sinusoidal voltage and current. It is possible, however, to use a Fourier expansion for
the voltage and to define, similar to the current and with the same results, the
following values:
Voltage individual harmonic content
UHn
Uhn % = 100
UH1
The harmonic spectrum can also be calculated for the voltage.
APC by Schneider Electric
01/2012 edition
p. 52
Harmonics (Cont.)
Voltage total harmonic distortion
∞
∑ UH
n
2
n= 2
THDU% = 100
∞
= 100
UH1
⎡ UHn ⎤
⎢
⎥
UH1 ⎦
n= 2 ⎣
∑
∞
2
=
∑ (Uh )
2
n
n= 2
THDU for Total Harmonic Distortion, where U is for the voltage.
Rms value of a voltage with harmonics
∞
Irms =
∑ IH
n
2
n =1
Which, similar to the current, can also be expressed as:
∞
Urms = UH1 1+
∑Uh
n
2
= IH1 1+ THDU2
n=2
) The rms value of the voltage is that of the fundamental, multiplied by a coefficient
which is due to the harmonics.
Power values
Power factor in the presence of harmonics
On the basis of the active power at the terminals of a non-linear load P (kW) and the
apparent power supplied S (kVA), the power factor is defined by:
P (kW )
λ=
S (kVA )
This power factor does not express the phase displacement between the voltage and
the current because they are not sinusoidal. However, it is possible to define the
displacement between the voltage fundamental and the current fundamental (both
sinusoidal), by:
P1(KW )
cos ϕ1 =
S1(kVA )
where P1 and S1 are the active and reactive power, respectively, corresponding to
the fundamentals.
Standard IEC 146-1 defines the distortion factor:
λ
ν=
cos ϕ1
When there are no harmonics, this factor is equal to 1 and the power factor is simply
the cos ϕ.
Power in the presence of harmonics
• Across the terminals of a balanced, three-phase linear load, supplied with a
phase-to-phase voltage u(t) and a current I(t), where the displacement between u
and i is ϕ, the apparent power in kVA, depending on the rms values U and I, is:
S = UI 3
The active power in kW is: P = S cos ϕ
The reactive power in kvar is: Q = S sin ϕ
Where:
S=
P2 + Q2
• At the terminals of a non-linear load, the mathematical definition of P is much
more complex because U and I contain harmonics. It can however be expressed
simply as:
.P = S λ. (λ = power factor)
If U1 and I1 are the fundamentals displaced by ϕ1, it is possible to calculate the
corresponding apparent, active and reactive power by:
S1= U1 I1 3
P1 = S1 cos ϕ1 and Q1 = S1 sin ϕ1. The total apparent power is:
S = P12 + Q12 + D2
where D is the distortion power, due to the harmonics.
APC by Schneider Electric
01/2012 edition
p. 53
Non-linear loads and PWM technology
Non-linear load
performance of UPSs using
PWM technology
Importance of the UPS output impedance
Equivalent diagram of an inverter output
With respect to the load, an inverter is a perfect source of sinusoidal voltage V0 in
series with an output impedance Zs. Figure 5.36 shows the equivalent diagram of the
inverter output when a load is present.
The inverter output is a perfect voltage source Vc = impedance across the load terminals.
V0 in series with an output impedance Zs. Vs = impedance at the inverter output.
ZL = line impedance.
Zc = load impedance.
Fig. 5.36. Equivalent diagram of an inverter output.
Effects of different load types
• For a linear load, the impedances Zs, ZL, Zc are considered at the angular
frequency ω = 2 π f corresponding to the distribution frequency (f = 50 or 60 Hz),
giving
V0 = (Zs + ZL + Zc) I
• For a non-linear load, the harmonic currents drawn by the load flow through the
impedances. For the fundamental and each individual harmonic, the rms values of
the current and the voltage are related similarly and can be expressed as:
- for the fundamental: U1 = (Zs + ZL + Zc) I1
- for each harmonic order k: UK = [Zs(kf) + ZL(kf) + Zc(kf)] IK
The impedance values are considered at the frequency kf of the given order.
Voltage distortion decreases with the individual levels of the voltage harmonics UK /
U1.
These levels are related to those of the harmonic currents IK/ I1 by the equation:
[Zs(kf) + ZL(kf) + Zc(kf)] / (Zs + ZL + Zc).
) Consequently, for a given load current spectrum, the individual voltage harmonic
levels and the total distortion (THDU) decrease with the impedance of the source
and the cables at the given frequencies.
Consequences of non-linear loads
To reduce the effects of the harmonic currents (THDU at B and C), it is necessary, to
the greatest extent possible, to:
• reduce the line impedance,
• ensure a low source impedance at the various harmonic frequencies.
) Good behaviour on the part of a UPS supplying non-linear loads requires a low
output impedance at the various harmonic frequencies.
Below is a presentation of the advantages of the PWM (pulse width modulation)
chopping technique in this respect.
APC by Schneider Electric
01/2012 edition
p. 54
Non-linear loads and PWM technology (Cont.)
UPS operating principle
Chopping of the DC voltage by the inverter with filtering
An inverter is made up of a converter that transforms the DC power supplied by the
rectifier/charger or the battery into AC power. For example, on a single-phase UPS,
there are two ways to convert the DC power, using either a half bridge (see fig. 5.37)
or a full bridge (see fig. 5.38).
The square-wave voltage obtained between A and B is then filtered to produce a
sinusoidal voltage with a low level of distortion at the output.
The switches represented here to illustrate the principle are controlled IGBTs.
Fig. 5.37. Half-bridge DC/AC converter.
Fig. 5.38. Full-bridge DC/AC converter.
Practically speaking, the switches shown in figures 5.37 and 5.38 are IGBTs for
which it is possible to control the relative on and off times.
By controlling the on and off times, it is possible to "distribute" the voltage over the
reference sinusoidal wave. This principle is known as PWM (pulse width modulation).
It is shown in a simplified manner, with five square-wave pulses, in figure 5.39. The
area of the voltage sinusoidal wave is equal to that of the square-wave pulses used
to generate it. These areas represent the power supplied by the inverter to the load
over a given time, i.e.
T
∫ VIdt
0
The higher the chopping frequency (the higher the number of square-wave pulses),
the better the regulation with respect to the reference wave. Chopping also reduces
the size of the internal filter required on the LC output (see fig. 5.40).
Fig. 5.39. DC/AC converter output voltage
with five square-wave pulses per half-wave.
APC by Schneider Electric
01/2012 edition
Fig. 5.40. Inverter output filter.
p. 55
Non-linear loads and PWM technology (Cont.)
PWM inverters
PWM chopping
The PWM (pulse width modulation) chopping technique combines high-frequency
chopping (a few kHz) of the DC voltage by the inverter and regulation of the pulse
width for the inverter output, to comply with a reference sinusoidal wave.
This technique uses IGBTs (insulated gate bipolar transistors) offering the
advantages of voltage control and very short commutation times. Due to the high
frequency, the regulation system can react quickly (e.g. 333 nanoseconds for a
frequency of 3 kHz) to modify the pulse widths within a given period.
) Comparison with the reference voltage wave makes it possible to maintain the
inverter output voltage within strict distortion tolerances, even for highly distorted
currents.
Functional diagram of a PWM inverter
Figure 5.41 shows the functional diagram of a PWM inverter.
The output voltage is continuously compared to the reference voltage Vref which is a
sinusoidal wave with a very low level of distortion (< 1%).
The difference in the voltage ε is processed by a corrector, according to a transfer
function C(p), intended to ensure the performance and stability of control.
The voltage from the corrector is then amplified by the DC/AC converter and its
control system with a gain A. The Vm voltage supplied by the converter is filtered by
the LC filter to supply the output voltage Vs.
Practically speaking, it is necessary to take into account the impedance of the output
transformer when it exists, to obtain the total inductance L. Often, the inductance is
built into the transformer, which is why it is not included in diagrams.
Fig. 5.41. Functional diagram of a PWM inverter.
Output impedance of a PWM inverter
It is possible to represent the above DC/AC converter and filter as a series
impedance Z1 and a parallel impedance Z2 (see the left-hand side of fig. 5.42).
The diagram can be modified to display the output impedance Zs.
The equivalent diagram (right-hand side of fig. 42) shows:
Z2
• V'm = voltage measured under no-load conditions, i.e. V'm = Vm
Z1 + Z2
• Zs = impedance measured at the output with V'm short-circuited, i.e.:
Zs =
Z1 Z 2
Z1 + Z 2
Fig. 5.42. Equivalent diagram of an inverter as seen from the output.
APC by Schneider Electric
01/2012 edition
p. 56
Non-linear loads and PWM technology (Cont.)
Z2
is the transfer function of the filter, noted H(p).
Z1 + Z 2
To simplify, C(p) x A is replaced by µ(p) which represents the transfer function of the
correction and amplification.
It is thus possible to replace fig. 5.41 by the functional diagram in fig. 5.43.
The ratio
Fig. 5.43. Transformed functional diagram of a PWM-chopping inverter equipped with an
output-voltage regulation system with modulated chopping frequency.
It is possible to show that the inverter output impedance Zs in this case is equal to:
Z1
Z' s ≈
µ (p)
(for further information, consult Schneider Electric Cahier Technique document no.
159).
This means that in the regulation pass band, the inverter output impedance is equal
to the filter series impedance divided by the correction and amplification gain.
Given the high gain in the regulation pass band, the output impedance is significantly
reduced compared to impedance Z1 of an inverter without this type of regulation.
Outside the regulation pass band, the inverter output impedance is equal to that of
the filter, but remains low because it corresponds to the impedance of a highfrequency capacitor.
Consequently, the output impedance is a function of the frequency (see fig. 5.44).
) The free-frequency PWM (pulse width modulation) technique considerably limits
the output impedance.
Comparison of different
sources
Output impedance of various sources
The curves in figure 5.44 show the output impedances for various sources with equal
output ratings as a function of the AC frequency. The impedances are plotted as a
percent of the load impedance Zc.
• Transformers and generators - the curve is a straight line corresponding to the
effect of the inductance L (the term which rapidly becomes dominant in the reactance
with respect to the resistance and which increases linearly as a function of the
frequency).
• Modern inverters implementing the PWM chopping technique with modulated
chopping frequency - at all harmonic frequencies, the Zs/Zc ratio is:
- less than that noted for other sources,
- low and virtually constant.
Conclusion
The PWM inverter is the source offering by far the lowest output impedance in the
presence of harmonics. It is clearly the best source on the market in terms of its
aptitude to minimise the voltage distortion caused by non-linear loads. It is five to six
times better than a transformer with an identical power rating.
) The new generation of UPSs implementing IGBTs and the PWM chopping
technique with frequency modulation are the best sources of sinusoidal voltage,
whatever the type of current drawn by the load.
APC by Schneider Electric
01/2012 edition
p. 57
Non-linear loads and PWM technology (Cont.)
Fig. 5.44. Output impedance of different sources depending on the frequency.
Free-frequency chopping
Free-frequency chopping
Free frequency is an improvement to the PWM technique.
PWM chopping can use either of two techniques (fig. 5.45).
Fixed-frequency chopping
The chopping fronts occur at fixed, regular intervals corresponding to the chopping
frequency over one period.
The width of the pulses (square-wave pulses) can be modulated to conform to the
reference within the fixed time interval.
The two sine waves shown in the diagram correspond to the tolerance (< 1%)
around the reference sine wave.
Free-frequency chopping
The chopping fronts do not necessarily occur at fixed intervals. Chopping adapts to
the requirements of the regulation, i.e. the rate of change of the reference. The width
of the commutation fronts decreases (the chopping frequency increases) as the rate
of change of the reference sine wave increases. Conversely, the width of the
commutation fronts increases (the chopping frequency decreases) as the rate of
change of the reference decreases. On the whole, the average chopping frequency
is the same as that for the fixed-frequency technique (approximately 3 kHz). But
regulation is better because the commutation accelerates in the zones where the
rate of change is high (see fig. 5.46).
It can reach eight commutations per millisecond, i.e. a regulation time as low as 125
nanoseconds (compared to 300 ns for the fixed-frequency technique).
) The free-frequency technique increases the precision of the voltage regulation in
PWM inverters compared to the fixed-frequency technique.
APC by Schneider Electric
01/2012 edition
p. 58
Non-linear loads and PWM technology (Cont.)
The chopping frequency is fixed.
Modulation takes place within fixed intervals,
whatever the rate of change of the reference
sine wave.
Fixed frequency.
The free chopping frequency increases where
the rate of change of the reference is high.
Modulation therefore takes place within
intervals that are shorter when the rate of
change of the reference sine wave increases.
Free frequency.
Fig. 5.45. PWM chopping with fixed-frequency and free-frequency regulation.
Free-frequency
switching
Quality
band with
variations
< 1%
Output voltage
curve
Up to 8 commutations
per millisecond
Fig. 5.46. Regulation employing free-frequency commutation.
APC by Schneider Electric
01/2012 edition
p. 59
PFC Rectifiers
Standard and PFC rectifiers
UPS units draw power from the AC distribution system via a rectifier/charger. With
respect to the upstream system, the rectifier is a non-linear load drawing harmonics.
In terms of harmonics, there are two types of rectifiers.
Standard rectifiers
These are three-phase rectifiers incorporating SCRs and using a six-phase bridge
with standard chopping of the current.
This type of bridge draws harmonic currents with orders of n = 6 k ± 1 (where k is a
whole number), mainly H5 and H7, and to a lesser degree H11 and H13.
Harmonics are controlled by using a filter.
"Clean" PFC (Power Factor Correction) rectifier
This type of rectifier comprises built-in IGBTs and a regulation system that adjusts
the input voltage and current to a reference sine wave. This technique ensures an
input voltage and current that are:
• perfectly sinusoidal, i.e. free of harmonics,
• in phase, i.e. an input power factor close to 1.
With this type of rectifier, no filters are required.
PFC rectifiers
Operating principle
The principle behind PFC rectifiers consists in forcing the current drawn to remain
sinusoidal. To that end, they use the PWM technique presented above.
The principle is that of a "voltage source" converter (see fig. 5.47), whereas the
SineWave active harmonic conditioner uses a "current source" converter.
The converter acts as a back-electromotive force (a "sinusoidal voltage generator")
on the distribution system and the sinusoidal current is obtained by inserting an
inductor between the utility power and the voltage source.
Even if other non-linear loads increase the voltage distortion on the distribution
system, the regulation can adapt to draw a sinusoidal current.
The frequency of low residual harmonic currents is the frequency of the modulation
and of its multiples. Frequency depends on the possibilities of the semiconductors
used.
Fig. 5.47. Operating principle of a clean "voltage generator" converter.
Implementation
Single-phase rectifier
Figure 5.48 shows the operation of a single-phase rectifier.
Voltage modulation is obtained by a controller that forces the current to follow a
sinusoidal current reference.
Transistor T and diode D make up the voltage modulator. The voltage u thus
changes between 0 and Vs according to whether transistor T is in the on or off state.
When transistor T conducts, the current in inductor L can only increase as the
voltage is positive and u = 0.
Therefore:
di e
= >0
dt L
PC by Schneider Electric
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p. 60
PFC Rectifiers (Cont.)
When transistor T is off, the current in L decreases, provided that Vs is greater than
V, so that:
di e − Vs
=
>0
dt
L
For this condition to be fulfilled, voltage Vs must be greater than the peak voltage of
V, i.e. the rms value of the AC voltage multiplied by 2
If this condition is fulfilled, the current in L can be increased or decreased at any
time. The variation of the current in L with time can be forced by monitoring the
respective on and off times of transistor T. Figure 5.49 shows the evolution of current
IL with respect to a reference value.
From the source viewpoint, the converter must
act like a resistance, i.e. current i must be
sinusoidal and in phase with e (cos ϕ = 1).
By controlling transistor T, the controller forces
IL to follow a sinusoidal current reference with
full-wave rectification. The shape of I is thus
necessarily sinusoidal and in phase with e.
What is more, to keep voltage Vs at its
nominal value at the output, the controller
adjusts the mean value of IL.
Fig. 5.48. Diagram of a clean, single-phase rectifier drawing a sinusoidal signal.
Fig. 5.49. Evolution of current IL with respect to the reference.
PC by Schneider Electric
01/2012 edition
p. 61
PFC Rectifiers (Cont.)
Three-phase rectifier/charger
The basic circuit arrangement is shown in fig. 5.50. It is similar to that in fig. 5.48,
with the inductor placed upstream of the rectifiers; the operating principle is also the
same. The monitoring system controls each power leg and forces the current drawn
on each phase to follow the sinusoidal reference.
Fig. 5.50. Diagram of a clean, three-phase rectifier drawing a sinusoidal signal.
PC by Schneider Electric
01/2012 edition
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PFC Rectifiers (Cont.)
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