Classification of UPS system

UPS is classified as

Offline Line UPS(VFD)
Line Interactive UPS(VI)
Online double conversion UPS(VFI)

Based on the quality of UPS output voltage and frequency, waveform of the output voltage and based on the output performance under transient loading conditions, (these conditions are defined in iec 62040-3, the international standard for testing of UPS performance)

Classification of UPS

Offline UPS(VFD)

The topology of Offline UPS is classified as vfd as per iec 62040-3 which is output voltage & frequency is dependent on input.

Offline UPS passes the AC mains supply directly to the output load if the AC mains supply is present. Only in case of power failure, it switches to inverter within few milliseconds to ensure uninterrupted power to load until mains supply returns.

Figure 1 Offline UPS

Line interactive UPS

The topology of line interactive UPS is classified as vfi as per iec 62040-3 which is output voltage is independent from input. Line interactive UPS is almost similar to Offline UPS, passes the ac mains supply directly to the output load if the ac mains supply is present. Only in the case of power failure, it switches to inverter within few milliseconds to give power to load 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

Figure 2 Line Interactive UPS

Online Double conversion UPS

The topology of online UPS is classified as vfi as per iec 62040-3 which is output voltage & frequency is independent from input. Online Double Conversion UPS is the most widely UPS topology to protect the critical loads. As shown in the below figure, the Online Double conversion UPS has

Rectifier Converts AC-DC and charges the battery
InverterConverts DC-AC and supports the loads connected to it
BypassSecondary power, normally the mains power to support in case of emergency
BatteryTo store energy

Figure 3 Online Double Conversion UPS

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

Comparison of UPS topologies