Parallel UPS systems see several uninterruptible power supplies installed together to either increase reliability (redundant systems) or to increase load handling capability (capacity systems).
In a typical parallel redundant UPS system, the total load demand is met by operating two or more UPS in an N+X configuration.
Each unit is sized to support the full load in the event of a system failure. The UPSs share the load between them equally and if one UPS fails, there’s a seamless transfer and the remaining UPSs continue to share the load.
An alternative solution is known as cascade redundancy. This sees the output of one UPS fed into the bypass line of the second unit. The first unit feeds the load, with the second constantly ready in a “hot-standby” position.
Under a mains failure, the first unit supports the load until the batteries are exhausted. Then the load is transferred to the second unit via the static bypass.
Likewise, if there’s a system failure in the first UPS, the second seamlessly takes over the load. In the unlikely and rare event that both UPSs experience a malfunction, the load is transferred to bypass to maintain power.
Cascade redundancy provides very high reliability and maximum redundancy, as in a two UPS system there are a total of four power sources available to support the critical load:
- The AC mains supply
- The batteries in the first UPS
- The batteries in the second UPS
- The bypass source.
A crucial point to bear in mind with cascade redundant systems is that each UPS must be correctly sized to handle the entire load.