OVERVIEW

UPS systems are designed to provide clean AC power for critical loads. In normal operation the utility input is “double converted” and supplied to the load, harmonic and transient free. There are also redundant systems in case of utility failure: a battery source can supply energy temporarily, and a backup generator can support long term outages. Combined, these ensure a reliable energy source. To achieve very high availability the inverter itself also requires redundancy, usually accomplished through paralleling systems or modules, depending on whether it is a monolithic or modular system. And finally, some systems have static bypass circuits which can directly connect the utility to the load in case of inverter failure.

Despite these efforts, there are still single points of failure that can cause the load to be dropped. For example: power surges (lightning strikes) can cause widespread damage, inverter control systems can malfunction, inter-module communication can fail, or safety and protection mechanism can experience nuisance trips. But most failures occur when transferring between power sources, especially operator error during maintenance operations.

When engineers at Alpha Technologies (an EnerSys company) analyzed the reliability and availability of a typical AMPs UPS installation, the opportunity for the biggest improvement was determined to be the maintenance bypass component. The ideal system includes an internal bypass switch for synchronized source transfer, an external static bypass switch in case of inverter failure, and an external bypass switch to isolate the inverter system for maintenance.