UPS (Uninterruptible Power Supply) Systems Works
UPS stands for Uninterruptible Power Supply. A UPS
system is an autonomous source of alternate power that is used to supply
sensitive electronic loads such as computer centers, telephone exchanges and many
industrial-process control and monitoring systems.
A UPS solution for sensitive
electrical loads is used to provide a power interface between the utility and
the sensitive loads, providing voltage that is:
1. Free of all disturbances present
in utility power and in compliance with the strict
tolerances required by loads.
2. Available in the event of a
utility outage, within specified tolerances
UPS systems satisfy requirements in 1
& 2 above in terms of power availability and quality by:
1. Supplying loads with voltage
complying with strict tolerances, through use of an
inverter
2. Providing an autonomous alternate
source, through use of a battery
3. Stepping in to replace utility
power with no transfer time, i.e. without any interruption in the supply of
power to the load, through use of a static switch.
These characteristics make UPS units
the ideal power supply for all sensitive applications because they ensure power
quality and availability, whatever the state of utility power.
Basic Parts of a UPS System
A UPS comprises the following main
components:
1. Rectifier/charger, which produces
DC power to charge a battery and supply an inverter
2. Inverter, which produces quality
electrical power free of all utility-power disturbances, notably micro-outages
and that is within tolerances compatible with the requirements of sensitive electronic
devices.
3. Battery, which provides sufficient
backup time to ensure the safety of life and property by replacing the utility
as required
4. Static switch, a semi-conductor
based device which transfers the load from the
inverter
to the utility and back, without any interruption in the supply of power
Types of Static UPS Systems
Types of static UPSs are defined by
standard IEC 62040. The standard distinguishes three operating modes for UPSs
which are:
1. Passive standby (also called
off-line)
2. Line interactive
3. Double conversion (also called
on-line)
These definitions concern UPS
operation with respect to the power source including the distribution system
upstream of the UPS. IEC Standard 62040 defines the following terms:
a. Primary power: power normally
continuously available which is usually supplied by an electrical utility
company, but sometimes by the user’s own generation
b. Standby power: power intended to
replace the primary power in the event of
primary-power failure
c. Bypass power: power supplied via
the bypass
UPS Operating in Passive Standby Mode
Operating Principle:
The inverter is connected in parallel with the AC input in a standby as shown below:
Normal Mode Operation
In normal mode operation, the load is
supplied by utility power via a filter which eliminates certain disturbances
and provides some degree of voltage regulation (IEC 62040 specifies some form
of power conditioning). The inverter operates in passive standby mode.
Battery Backup Mode Operation
In battery backup mode operation,
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 less than 10 ms
transfer time. The UPS continues to operate on battery power until the end of
battery backup time or the utility power returns to normal, which causes
transfer of the load back to the AC input (normal mode).
Application
This configuration is a compromise
between an acceptable level of protection against disturbances and cost. It can
be used only with low power ratings less than 2 kVA.
Limitations
This UPS 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 centers, telephone
exchanges, etc.). Furthermore, the frequency is not regulated and there is no
bypass.
UPS Operating in Line-interactive
Mode
The inverter is connected in parallel with the AC input in a standby configuration, but also charges the battery. It thus interacts with the AC input source as shown below:
Normal Mode Operation
In normal mode operation, the load is
supplied with conditioned power via a parallel connection of the AC input and
the inverter. The inverter operates to provide output-voltage conditioning
and/or charge the battery. The output frequency depends on the AC-input
frequency.
Battery Backup Mode Operation
In this mode of operation, 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 transfer without interruption using a static
switch which 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 the utility power returns to normal, which
provokes transfer of the load back to the AC input (normal mode).
Bypass Mode Operation
This type of UPS may be equipped with
a bypass. In the bypass mode, If one of the UPS functions fails, the load can
be transferred to the bypass AC input (supplied with utility or standby power,
depending on the installation).
Application and Limitation
This UPS 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.
UPS Operating in Double Conversion
(On-line) Mode
Operating Principle:
In this type of UPS, the inverter is
connected in series between the AC input and the application as shown below:
Normal Mode Operation
During normal operation, all the
power supplied to the load passes through the rectifier/charger and inverter
which together perform a double conversion (AC to DC to AC), hence the name.
Battery Backup Mode Operation
In 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 transfer without interruption using a static
switch. The UPS continues to operate on battery power until the end of battery
backup time or utility power returns to normal, which causes transfer of the
load back to the AC input (normal mode).
Bypass Mode Operation
This type of UPS is generally
equipped with a static bypass, sometimes referred to as a static switch. The
load can be transferred without interruption to the bypass AC input (supplied
with utility or standby power, depending on the installation), in the event of
UPS failure, load current transient (inrush or fault currents) or load peaks.
The presence of a bypass assumes that the input and output frequencies are
identical and if the voltage levels are not the same, a bypass transformer is
required.
For certain types of load, the UPS
must be synchronized with the bypass power to ensure load-supply continuity.
Furthermore, when the UPS is in bypass mode, a disturbance on the AC input
source may be transmitted directly to the load because the inverter no longer
steps in. Another bypass line, often called the maintenance bypass, is
available for maintenance purposes. It is closed by a manual switch.
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