Uninterruptible Power Supply (UPS)

UPS - Topologies/Classifications

Offline Line UPS (VFD)

• output voltage and frequency is dependent on input
• passes the AC mains supply directly to the output load if the AC mains supply is present
• in case of power failure, it switches to inverter within few milliseconds to ensure uninterrupted power until mains supply returns

Line Interactive UPS (VI)

• output voltage is independent from input
• passes the AC mains supply directly to the output load
• in case of power failure, it switches to inverter within few milliseconds to ensure uninterrupted power until mains supply returns
• when AC input is present, the “power interface” block in below figure filters the AC power, suppresses voltage spikes, and provides sufficient voltage regulation to operate well within the specifications

Online Double Conversion UPS (VFI)

• output voltage and frequency is INDEPENDENT from input
• most widely used UPS topology
• has:
  • rectifier converts AC-DC and charges the battery
  • inverter converts DC-AC and supports the loads connected to it
  • bypass secondary power, normally the mains power to support in case of emergency
  • battery to store energy

Under normal conditions, the mains power with all the impurities like voltage variation, frequency variation etc, are converted to DC by the rectifier and from the DC source, an ac voltage is generated by the inverters.

The battery is connected to the DC bus of UPS and in the event of power failure, the battery takes over the load immediately and there is no change over or transfer time.

As there is two conversions of power AC-DC and DC-AC, this topology is widely called as double conversions UPS

Operating Principle of Double Conversion UPS

Normal Mode

This is the most frequent operating condition: the energy is drawn from the primary mains power supply and is converted and used by the inverter to generate the output voltage to power the loads connected. The inverter is constantly synchronised with the auxiliary mains to enable load transfer to by pass (due to an overcurrent or inverter shutdown) without any break in the power supply to the load. The battery charger/rectifier supplies the energy required to maintain or recharge the battery.

Bypass Mode

In case of inverter failure, the load is automatically transferred onto the auxiliary mains without any interruption in the power supply, and may occur in the following situations:

• In the event of a temporary overload, the inverter continue to power the load. If the condition persists, output is switched onto the auxiliary mains via the automatic bypass.
• when the voltage generated by the inverter goes out of tolerance due to a major overload or a fault on the inverter
• when the internal temperature exceeds the maximum value allowed.

Battery Mode

In the event of a mains failure (micro interruptions or extended black-outs), UPS continues to power the load using the energy stored in the battery. The UPS system keeps the user constantly informed on the status of battery and on the remaining back-up time according to the battery capacity and based on connected loads in the UPS.

Online double conversion UPS is the only type of UPS used widely as they have a lot of advantages over the other Topologies as listed below

The output voltage & frequency is completely independent from the mains supply
No break transfer to battery mode or bypass mode

Resources

• https://www.india.fujielectric.com/resources/technical-guide/classification-of-ups