An Uninterruptible Power Supply (UPS) is an Electrical
device or equipment which serves as an emergency or backup power source when
the input power supply, usually the utility mains, fails. In addition to
serving as a backup power source, -the UPS also serves a “power conditioner” by
ensuring that the electricity being supplied to sensitive instruments and
devices in your home or office like computer, TV and telecommunication
equipment is free from common utility power problems like voltage sag, surge,
noise, frequency instability and spikes.
A UPS, unlike an emergency power system or standby
generator, provides almost instantaneous protection from interruptions in power
supplied to the electrical load or device from utility mains. For low power
electrical devices or loads the UPS provides this protection through one or
more attached batteries and electronic circuitry. While for high power
electrical loads this protection is provided by the UPS by means of diesel generators
and flywheels.
Broadly speaking there are three common types of
UPS: On-line UPS, Line Interactive UPS and Standby UPS. However, many
manufacturers categorize UPS according to the design approaches used to
implement UPS Systems. According to the design approaches used to implement UPS
Systems, as described by Schneider Electric,the following are the main types of UPS:- Standby
- Line Interactive
- Standby-Ferro
- Double Conversion On-line
- Delta Conversion On-line
To keep
discussion simple and only three common types of UPS, mentioned earlier, will
be discussed in detail.
The Standby UPS
The standby UPS is the most common type of UPS
that serves as a backup power source for desktop computers. The block diagram
of standby UPS is shown in Figure 1.In normal mode of operation the transfer
switch of UPS is set to feed the filtered AC input from utility mains to the load,
this path is shown by solid line in Figure 1.In case of failure of power being
supplied from the primary source i.e., utility mains, the transfer switch operates to switch the load from primary source of
power to backup power source, which is
usually a battery and an inverter for low power loads or a diesel generator and
flywheel for high power loads. The path from backup power source to the load is
shown by dashed line.
Figure 1 : Standby UPS
During normal mode of operation the battery is charged
from the utility mains through the battery charger drawing electrical power
from main power source also the inverter stays switched off during normal mode
of operation. The inverter starts to operate only when the power being supplied
from utility mains fails; this is why this UPS is called “standby UPS”. This
UPS, despite being small in size, delivers high performance at low cost. With
properly designed filter and surge suppression circuitry, the Standby UPS can deliver
an output to the load that is adequately free of noise and voltage surges.
The Line Interactive UPS
The Line Interactive UPS is shown in Figure 2.It is the
most commonly used type of UPS for small business firms, departmental and web servers.
In this design the inverter always stay connected to the electrical load.
During the normal mode of operation when the power is being delivered to the
electrical load from utility mains, the inverter operates in reverse for
charging the battery.
When the utility mains fail, the transfer switch opens
and the power is supplied by the battery to the output after passing through
the inverter. Since the inverter always stays connected to the output, so as
compared with Standby UPS, the output voltage delivered by this UPS is better
filtered with less switching transients.
Figure 2 : Line Interactive UPS
In addition, this design also includes a tap changing
transformer for adding voltage regulation by adjusting transformer taps with
variation of input voltage. Under low voltage conditions, the voltage
regulation is a feature of UPS that is of great significance. Without voltage
regulation feature, under low voltage conditions, the UPS would transfer to
battery and eventually down the load. This frequent use of battery will
significantly reduce the life of battery. A better inverter design which
permits power flow from the AC input to the output could eliminate the
potential of single point of failure by providing two independent power paths.
Its high efficiency, small size, low cost and high
reliability, in addition to its ability to correct low or high line voltage
problems makes it the most suitable UPS in 0.5-5KVA range.
The Double Conversion On-line UPS
The block diagram of Double Conversion On-line UPS is
shown in Figure 3.Above 10kVA, it is the most common type of UPS. Its design is
quite similar to that of Standby UPS, except that the primary path of power to
the output is the inverter instead of the AC main. In this design, the failure of
AC main does not activate the transfer switch as the main AC input is charging
the battery connected to output inverter. Therefore during failure of power
from utility mains, the on-line operation results in almost no transfer time.
Both the battery charger and the inverter convert the entire load power flow in
this design.
Figure 3 : Double Conversion On-line UPS
The electrical output performance of this type of UPS is
nearly ideal but due to constant electrical stress on power components, the
reliability of this design is less than other UPS design topologies. Also due
to probable non-linearity in the input power drawn by the large battery
charger, disturbances may be caused to the building power wiring or standby
generators.
Conclusion
A comparison of three above
mentioned types of UPS is given in Table 1.
Table 1 : Comparison Table
Each of these three types of UPS, depending upon design
and topology, has its own unique characteristics that makes it suitable for a
particular type of application. No single UPS is ideal for all applications. Energy
efficiency, cost and power range are some of the important factors that should
be kept in mind while choosing a particular type of UPS for your application.
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