The best description of a True On-Line UPS system would be a UPS system that did not require any switching action to take place to allow it to continue supplying the load during a utility mains failure.
An example of a system that is not On-Line would be an emergency lighting system; this system uses a contactor to supply power directly to the lights while the utility mains supply is present, and when the mains supply fails the contactor changes over and supplies power to the lights from a battery-backed inverter. There is a short interruption to the load due to the switching action of the contactor.
A True On-Line UPS has a rectifier at its input to provide a DC source to keep the battery on charge and also providing a power source for the inverter. Therefore in the event of a utility mains failure it is only the rectifier that stops working while the DC source for the inverter is derived from the battery instead of the rectifier.
A further advantage of the True On-Line UPS system is that it provides isolation of the load from the supply source. This can be advantageous in the event of a utility mains failure where a generator provides the mains power. The On-Line UPS system provides a known harmonic load to the generator; with a non On-Line UPS the actual load harmonics can be seen by the generator, which could cause it to fail when the UPS returns to normal operation.
Due to the fact that the On-Line UPS is constantly supplying the load, any additions or changes to the load by the client will be highlighted immediately. This is in contrast to a non On-Line UPS system, where the first indication of any load problem would be during a utility mains failure where the generator refuses to accept the load and shuts down, resulting in loss of site load.
Therefore, in conclusion, a True On-Line UPS system provides seamless transfer between normal operation from the utility mains supply to battery operation in the event of mains failure.
In addition, it provides isolation between the ‘Dirty’ mains supply and the load which requires ‘Clean’ power.
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