UPS basics

Uninterruptible Power Supply Basics

In the sixties, if a person wanted to protect sensitive equipment from power outages he needed
several stand alone instruments to fulfill the requirement. First, he had to purchase a Battery to
supply the required energy. In addition, he had to buy an Inverter to Invert the DC battery
voltage to proper AC voltage with the proper AC waveform, like the one coming from the
mains. A Charger was also necessary to replenish, after the utility power is restored, the charge
which was driven from the battery during mains outage. And finally he had to acquire a Switch
that transfers the equipment connection from the utility to the output of the Inverter and vice
versa. All these items required a lot of space, but they did the job.

Those four items, the Battery ,Inverter, Charger ( or Rectifier) and Transfer Switch, were then
and are today the main building blocks of any static Uninterruptible Power Supply ( UPS).

The boost in Computer industry enhanced also sales of Power UPS systems. In the beginning,
the systems comprised all the necessary components, except batteries, in one compact unit.
The size of these units shrank, when fast power semiconductors appeared. At the end of the
eighties, newly introduced maintenance free batteries were also located inside the UPS
cabinet. In the ninetieth embedded microcomputers increased the complexity of the systems
adding enhanced protection, paralleling features, supervisory circuits, self-checking and
communicating functions.

UPS stands for Uninterruptible Power Supply. It is an instrument connected between the
electric grid and the consumer, comprising of electric hardware and rechargeable batteries.
The aim of the instrument is to supply continuous undisturbed and conditioned power to the
critical load. The energy for powering the load comes from the utility, or from the battery
upon mains outage.

Figure 1 describes main UPS building blocks. The Output is fed either from the incoming
power line, called Bypass or from batteries via the DC to AC Inverter. The Transfer Switch
makes the selection. The Rectifier charges the batteries when utility power is present.

It is the quality of the power supplied by the UPS, which differentiates between various UPS
types and designs. Basically there are three major UPS types: Backup, Line Interactive and
On-line.

Backup UPS

The Backup UPS, also called off-line, or Stand-by UPS feeds normally the load directly from
Mains. In this mode the Transfer switch connects the "AC Out" terminal to the "Mains In "
terminal . The rectifier is used mainly for charging the batteries. A small amount of charger's
output current energizes the Inverter, which is working without load, a state generally called
Hot Standby mode. Filtering and surge absorbing elements, (not shown in the drawing)
protect the load from mains disturbances and transients.

Upon Mains power outage, the UPS enters a Backup mode. Transfer Switch connects the
load to the output of the Inverter. The Inverter now feeds the load from the battery, and will
continue to do so until utility power is restored or until the charge capacity in the battery is
withdrawn.
If power outage is prolonged, the UPS before turning off, will warn the user by means of an
audible beep signal, to enable manual shut down of user's critical loads. Some systems initiate
a communication signal to user's computer to perform an automatic graceful shut down.
When the utility power is restored the system will restore to Standby mode. The load will be
reconnected to the mains,while the charger recharges the batteries.

The Backup UPS cannot correct mains voltage fluctuations. This inability may cause transfers
to Backup mode at mains undervoltages, thus decreasing considerably the battery lifetime due
to frequent discharges. Backup system does not correct mains frequency deviations, Its
output waveform is generally non sinusoidal which may cause un-smooth operation with
certain loads. A sinusoidal output, implemented in some backup systems and in other UPS types
is achieved by high frequency modulation of Inverter Output. Some call such power supply
systems High Frequency Power Supply, or High Frequency UPS.

Backup systems are generally used for low power installations, small offices, personal home
computers and other less critical application, where eliminating 85% to 90% of computer
malfunctions caused by power failures, is satisfactory.

Line Interactive UPS

Line Interactive UPS, is also called sometimes Interactive UPS, or Smart UPS, These systems
are also based on stand-by principle, working normally directly from mains, switching to battery
(via the inverter) upon mains outage.

The line Interactive UPS is equipped with additional transformer type Automatic Voltage
Regulator ( AVR) on the Bypass line (between Mains input and the Transfer switch). The
AVR increases the voltage to load at low mains and decreases it at high mains, thus keeping
the output voltage within an allowed tolerance band, as required for correct operation of
computers and related consumers. This feature eliminates the need of utilizing the inverter and
discharging batteries at low line voltage, thus increasing system reliability and battery lifetime.

The output voltage is generally sinusoidal, therefore, special precautions as regards to type of
load might not be required. It is usually equipped with communication port to enable control
of UPS parameters and to provide an orderly computer shutdown at prolonged power outages.
Neither Off-line nor Interactive systems are able to correct frequency. The output frequency
of any stand-by unit is the same as the input utility grid. These types should therefore be
avoided on sites equipped with small back-up generators, exhibiting generally poor frequency
regulation behavior.

The Line Interactive UPS is a professional solution for business application where the main
concern is to provide protection from most utility grid disturbances. These units are used for
small networks comprising 10-12 computers.

On-Line UPS

On-Line UPS, also called Double Conversion UPS, or True On-Line UPS, is the ultimate
solution for all applications from one, or two kilowatts up to Megawatt sized consumers.

Here, the load is constantly fed from the Inverter, providing conditioned, stabilized sinusoidal
voltage. The utility line in these systems backs up the UPS in case of UPS malfunction. The
Transfer switch will automatically transfer the load to mains in case of overload or UPS
failure.
The On-Line unit comprises two converting stages. The first stage converts the incoming ac
power to DC, thus creating a dc-buss, which is fed either from mains or from storage battery.
A second stage converts the DC power back to conditioned ac in order to feed the critical load.
Filters on the DC buss and fast regulating circuits in the converters practically isolate the load
from any abnormal utility behavior.
In normal operation, load energy comes from mains, via the Rectifier and the Inverter. Upon
mains outage the battery supplies the energy, which the load requires (by the dc/ac Inverter).
Should the mains voltage return before the battery was fully discharged, the Rectifier will
feed the load through the Inverter and start a battery recharge regime to compensate for the lost
charge.
Otherwise, the UPS will turn off when battery becomes fully discharged. The UPS will resume
automatically normal operation upon mains restoration. Also here, the Rectifier will feed the
load via the Inverter, recharging the battery. The Output voltage of an On-Line UPS is
generally stabilized within one percent tolerance. Output frequency is locked to input if it is
within preset tolerance band, otherwise a free running crystal controlled clock will dictate the
output.

The merits of Double Conversion ON -Line system make it the preferred choice for business
and industrial applications. It is the best solution in spite of the added system and electricity
costs.

It offers the best power protection, covering any and all types of mains disturbances.

There is no size limit. Standard ON-Line UPS's are available to backup any installation.

With the right system, no practical limit exists on the available back-up time. Batteries
may be added to increase backup time.

Many systems allow power extension to satisfy the needs of a growing enterprise

Units can be connected in parallel redundant configuration, for increasing reliability.

In addition, this is the best choice, considering such issues as modularity, ability to

work from generator, power factor correction, maintenance, hot swapping, fault
clearing, supervising, and communicating.

Other UPS systems

Other, less popular UPS systems are also proposed by some manufacturers. The main being
the Hybrid UPS and The Electromechanical UPS.

Hybrid UPS - Represented mainly by APC's Delta Conversion UPS, uses an AC/DC
converter between the AC input and the DC battery buss. The converter is able to add or
subtract voltage from the mains in order to stabilize output voltage and correct input power
factor. An output converter is connected between UPS output and battery. It acts as an ac/dc
rectifier to charge the batteries and as a dc/ac inverter upon mains outage.
The Delta Converter is offered from 5 kVA to 1Mwatt, it has better efficiency than the On -
Line unit. But, may suffer from drawbacks inherent to the Interactive UPS, such as inability to
correct frequency.

Electromechanical UPS - Also called Rotary UPS, Batteryless UPS, Flywheel UPS, is a
standby UPS which enables to supply short time energy by utilizing kinetic energy stored in a
flywheel. Normally load is fed from the mains, while energy is stored in a flywheel driven by
means of a motor fed from the utility. Upon mains outage the flywheel energizes a generator,
that continuous to feed the load via a proper converter.
These systems are offered in power ratings from about 50KW up to Megawatts. They
generally provide uninterruptible ride through energy until a backup generator is turned on.
High efficiency and ability to operate without a battery seem to be the main advantages.
Batteries are generally the weakest link in any UPS due to low reliability and short live times.
On the other hand these types may suffer from drawbacks associated with Off line Systems.
Also, the UPS systems and the backup generator may need periodic and relatively frequent
maintenance (strongly dependant on specific make).

See Glossary for additional information about UPS terms.

UPS selection guide see Select UPS folder.

Additional information see K nowHow folder.