With double conversion online topology, grid power always passes through an inverter before reaching the load equipment.
In normal power conditions, AC power from the grid is converted to DC power within the UPS. The converted DC power is divided into two paths, one which charges the battery, and the other which is passes through the inverter to be converted to AC power then fed to the load. During an outage, power is still supplied through the inverter, so the UPS doesn’t need to switch power sources. This results in high-fidelity output without any breaks or voltage fluctuations. Power always passes through the inverter making it a reliable topology.
The topology gets its name because power is converted twice from AC to DC then from DC to AC. The “online” comes from the fact the inverter is always on and available.
Two advantages of the double conversion online topology are that electrical equipment is always fed clean, conditioned, high-quality inverter output and that there are no power supply breaks during outages. Unlike passive standby UPSs introduced in the previous session, there is zero worry that the operation of the load will stop due to grid voltage fluctuations.
A drawback, however, is that the inverter does not permit a large current to flow. When an inrush current or other large current flows to the inverter, the UPS switches from inverter output to bypass output.* At this time, if the waveforms of the inverter and grid power are not synchronized, a break of about 50 to 300 ms will occur until the inverter is synchronized. This momentary disruption may cause the electrical equipment to stop or fail to switch. Therefore, it is important to select the rated capacity of UPS that can accommodate the expected inrush current. In addition, since power is converted twice, power consumption increases two- or threefold compared to standby UPSs. Also, with more components and, therefore, a larger main body, online UPSs tend to be expensive.
From the above characteristics, double conversion online UPSs are mainly used to backup electrical equipment which have minimal power fluctuation but require constant and clean power. Typical examples of such equipment would be data centers and high-end servers.
* Bypass output is an operating mode where raw grid power is fed so that the electric equipment (load) can continue to receive power during maintenance and UPS failures.