- Uninterruptible power supplies (UPS) mitigate voltage sags by supplying the load using stored energy. Upon detection of voltage sag, the load is transferred from the mains supply to the UPS.
- Obviously, the capacity of load that can be supplied is directly proportional to the amount of energy storage available.
- UPS systems have the advantage that they can mitigate all voltage sags including outages for significant periods of time (depending on the size of the UPS).
- There are 2 topologies of UPS available; on-line and off-line. Figure 1 shows a schematic of an off-line UPS while Figure 2 shows a schematic of an on-line UPS.
- Comparison of the figures shows that the difference between the two systems is that for an on-line UPS the load is always supplied by the UPS, while for off-line systems; the load is transferred from the mains supply to the UPS by a static changeover switch upon detection of voltage sag.
- The lack of a changeover switch renders the on-line system more reliable as any failure of the changeover switch will result in the off-line UPS being ineffective. UPS systems have disadvantages related to energy storage components (mostly batteries) which must be maintained and replaced periodically.
Small UPS systems are relatively simple and cheap. However, large units are complex and highly expensive due to the need for large energy storage capacities.