A line-interactive UPS maintains the inverter in line and redirects the battery's DC current path from the normal charging mode
to supplying current when power is lost.
In this design, the battery-to-AC power inverter is always connected to the output of the UPS. When the input AC power is
normal, the inverter of the UPS is in reverse operation and provides battery charging. Once the input power fails, the transfer
switch will open and the power will flow from the battery to the UPS output. This design offers additional filtering and yields
reduced switching transients since the inverter is always on and connected to the output.
Line interactive UPS systems are a cheaper option than online double conversion technology and will protect a critical load
from power failures, power sags, power surges, under-voltage and over-voltage. Unlike the online double conversion topology,
it will not protect against electrical line noise, frequency variation, switching transient and harmonic distortion.
An online UPS uses a "double conversion" method of accepting AC input, rectifying to DC for passing through the
rechargeable battery then inverting back to 120 V/230 V AC for powering the protected equipment.
In an online UPS, the input AC is charging the backup battery source which provides power to the output inverter, so the failure
of the input AC won't cause activation of the transfer switch. That is to say, if a power loss occurs, the rectifier will simply drop
out of the circuit and the batteries will keep the power steady and unchanged. No transfer time during the failure. When power
is restored, the rectifier will resume carrying most of the load and begin charging the batteries, though the charging current
may be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte.
Although the initial cost of an online UPS system is greater than a line interactive system, the total cost of ownership (TCO) is generally lower as the batteries tend to last longer.