Ans. Ideally, the voltage supply by the utility system should be perfect sine wave without any
harmonics, at its nominal frequency of 50 Hz and at its nominal magnitude. In practice however
voltage can significantly depart from the ideal condition due to power line disturbances e.g.
overvoltage, under voltage (brown out), outage (blackout), voltage spikes, harmonics and
electromagnetic interference. Sustained over voltage and under voltages may cause equipments, to
trip out, while voltage spikes may cause hardware problems. Power conditioners i.e. metal oxide
varistors (MOVs), EMI filters, isolation transformers; Ferro resonant transformers provide an
effective way of suppressing some or all of these disturbances.
An Uninterruptible Power Supply system is used for supplying critical loads like computer/data
processing machines, hospitals, control and instrumentation, communication equipment and
critical processes in industries. This provides protection against power outages as well as voltage
regulation during power line over voltage and under voltage conditions.
UPS TOPOLOGIES:
The UPS system can be classified into two types.
a) On load UPS
b) Off load UPS
On load UPS
The input rectifier feeds to battery as well as the load all the time. Battery remains in
floating condition and takes over as soon as the mains goes off. This gives conditioned output
of fixed frequency, fixed voltage irrespective of line condition.
OFF load UPS
In off load UPS system, battery bank is usually not connected to the load. lt comes into
circuit only when the mains is off.
In the normal mode, the switching converter operates as rectifier, charging the battery bank.
In addition, it can draw, inductive or capacitive currents from the mains, thus providing a
fine regulation of the voltage supplied to the load. In case of a main power outage, the power is
isolated and the switching convener operates as an inverter, supplying power to the load from
the battery bank. This arrangement is also referred to as the "standby power supply'. The
response time of the OFF LINE UPS is slower compared to ON LINE UPS system.
Static Bypass switch
In on load UPS systems, the battery takes care of mains failure but there is no standby system to
take care of the failure of inverter. Hence, for additional reliability the power line itself is used as
a backup to the UPS and static bypass switch is provided to transfer the load from the UPS to the
mains in case of any problem in the UPS. Whenever a static transfer switch is used the inverter output should be synchronized to the line voltage so that transferring the load from one source to
the other results in the least amount of disturbance seen by the load.