Considerations in integral engine rotary UPS designs

Diesel Progress North American Edition, Nov, 1997 by Henry C. Lengefeld

Experienced critical power users have long recognized the strong performance characteristics of hybrid rotary Uninterruptible Power Supplies (UPS) in not only protecting the critical load from power aberrations, but also isolating sensitive loads from damaging power transients. Several manufacturers have recently introduced UPS products with rotary components and integrated an engine directly into the UPS. These various products enjoy mixed success in their respective emulation of the acknowledged performance benefits of rotary UPS. The following examines the evolution of the hybrid rotary UPS and contains an overview of various integral engine rotary UPS currently offered.

The rotary only UPS was one of the first developed, consisting of a synchronous motor powered by the utility. The synchronous motor rotates and is mechanically coupled to the shaft of a synchronous generator. The output of the synchronous generator produces a.c. power for the critical load. The opposite end of the synchronous generator shaft is coupled to a d.c. generator. The d.c. generator also provides d.c. charging power to a storage battery.

During a utility power outage, the storage battery discharges through the d.c. generator, which naturally behaves as a d.c. motor. The battery provides power to the d.c. motor that is converted to mechanical power to drive the a.c. synchronous generator that maintains uninterrupted a.c. power to the critical load for the duration of the battery capacity. Rotary only UPS, however, was not compatible with a standby engine generator since it lacked the ability to make up for frequency fluctuations that result from engine-driven generators.

A single path hybrid rotary UPS was developed to overcome the compatibility problems of the rotary only UPS. This design added a rectifier (a.c. to d.c.) series connected to an inverter (d.c. to a.c.) driving the synchronous motor generator (MG) of the rotary only UPS. The inverter section of the single path hybrid rotary UPS uses an internal clock to maintain output frequency at 60 Hz when input frequency deviates outside acceptable limits. This ability makes for compatibility with a standby engine generator. One problem with this approach, however, is that four power conversions must take place. Each of these power conversions takes a toll on efficiency and creates continuous harmonic heating problems

Batteries in this single power path design are also subjected to stress from inverter switching and load dictated d.c. voltage. The a.c. ripple, from the inverter that runs continuously, adds to battery heating that reduces life and causes heating in the distribution system feeding the UPS.

Adding a second power path, including a subcycle static switch to the single power path, the hybrid rotary UPS improves efficiency, isolates the battery and prevents backfeeding the utility. The Russelectric RHS, for example, is a dual power path hybrid rotary UPS that eliminates two power conversions in normal operation, which helps improve efficiency. Harmonics are reduced to less than 20 percent that of a single path hybrid rotary UPS or static only UPS. The utility or engine generator power source sees a low harmonic linear load from the RHS input. By directly feeding the MG set from the utility, the inverter demands are eliminated and thus enabling optimum battery charging voltages free of inverter switching.

A phase controlled static switch prevents the synchronous motor from becoming a generator and backfeeding the utility or engine generator. This is essential if an input fault develops. It is also important in extending the ridethrough time of the MG. Without the subcycle static switch, the synchronous motor would electrically brake the MG, thereby cutting ridethrough time. Yet another benefit is to protect the synchronous motor from out of phase return of the utility after a momentary power outage.

The dual power path hybrid rotary UPS offers unique application flexibility to improve system reliability, reduce or eliminate maintenance disruption, and enables seamless load transfer. The sine wave a.c. output enables seamless load transfer to utility or engine generator. The hybrid rotary UPS has the capacity to handle 100 percent step loads and unloads. This capability can be used with a "break before make" subcycle static automatic transfer switch in isolated redundant configurations.

The Russelectric RHS rotary UPS was developed after a five-year evaluation of large installed static and rotary UPS. A common problem found in single path UPS configurations (static and rotary) was the lower reliability of the battery-based energy storage system resulting indirectly and directly from the UPS inverter. These batteries experienced several basic problems, including mechanical stresses caused by frequent millisecond discharges and increased maintenance demands -- watering, cleaning, etc. Another problem category can be attributed directly to the UPS design. These include high cycling, frequent surging, and internal heating due to charging ripple currents. The addition of the dual power path isolates the battery from inverter switching and enables optimum battery charging voltages.