Diesel rotary UPS (DRUPS)
A backup machine combining a flywheel, an alternator and a diesel engine in one unit, to carry the load without batteries through a grid outage.
A diesel rotary UPS puts on a single shaft a permanently spinning alternator, a flywheel that stores kinetic energy, and a diesel engine that can be clutched in. While the grid is present, the machine spins and conditions the power; when the grid drops, the flywheel supplies a few seconds of energy while the diesel starts and engages, with no battery rooms involved. The appeal is a smaller footprint, no batteries to replace and monitor, and a high unit rating that suits large sites. The limit lies in that ride-through of a few seconds: the margin for error is very thin, the machine stays tied to the grid and absorbs its disturbances, and a mechanical failure or a fault on its high-voltage breaker drops the whole load it carries at once. That is why the architecture around it, the number of units and whether each piece of equipment is fed by two separate paths, weighs more in a risk assessment than the technology itself.
On Saturday, September 10, 2016, at around 11 a.m., a fault in the high-voltage breaker of one of the diesel rotary UPS units at the Global Switch 2 site in London cut power to three floors for 222 milliseconds. According to the trade press, the hosting provider Claranet, whose two feeds both depended on the same chain, had restored only 85 percent of its services by around 5 p.m., and the site ran for two days on utility power alone, with no backup.
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