Case Study - Old School Synchronization

The University of Miami's new central energy plant is designed to offer both chilled water and emergency power to the meet expansion plans for the Miller School of Medicine and the University of Miami Health System. Atlanta's Newcomb & Boyd provided the mechanical, electrical, plumbing and fire protection engineering as well as the electrical commissioning. Elcon Electric Inc. of Pompano Beach, Florida was the electrical contractor.

The 49,000 sq. ft. chiller section of the CEP can initially supply 12,000 tons of chilled water and can be expanded to provide an additional 8,000 tons when needed. The water is travels through 36” ductile iron distribution mains to cool approximately 2.8 million sq. ft. of research, administrative and healthcare facilities. The chillers and the cooling towers are housed inside a parking structure designed to survive Category 5 hurricanes, ensuring power at critical times.

To deliver emergency power for the chillers as well as the new 177,000 sq. ft. biomedical research facility, the university installed three 2.8 MW, 4160V Kohler 2800REOZDB, 20-cylinder diesel generators, along with the necessary transformers, switchgear, and a 50,000 gallon fuel storage tank. Plans include installing two more generators when the chiller plant expands. The Kohler PD4000 Series medium voltage parallel switchgear is already in place to accommodate the additional generators.

Normally, providing emergency power is a fairly straightforward operation, but this installation posed several engineering challenges. To begin with, NFPA 110: Standard for Emergency and Standby Power Systems requires that emergency power for hospital facilities be on line within ten seconds. Starting one very large generator and closing it on line within that time period is enough of a challenge. Bringing up and synchronizing three units consistently within ten seconds is nearly impossible.

“No manufacturer could guarantee multiple units on line in ten seconds utilizing today's standard paralleling technology,” said Tom Ferry, Sales Manager for Kohler, WI-based Kohler Power Systems Americas-Systems Group.

Further complicating the matter were the Environmental Protection Agency's rules for emissions control which affect fuel control for Tier 2 diesel engines, adding to the ramp time requirement.

“In the normal sequence of events, with an installation like this, you have a time delay on starting the engines so you ignore momentary utility outages,” Ferry continued. “Let's say you set that for one second, it means you are down to 9 seconds maximum response time. Then the fuel control to meet the EPA's Tier 2 regulations is around 7-8 seconds to get to 100% load. You can get the first unit on in less than ten seconds, but that gives you one second to parallel the second unit.”

The original plan was to use conventional switchgear to bring at least two of the three generators on line within 13 seconds. The university, however, insisted on a 10 second guarantee due to specific critical load requirements, so Kohler's engineers custom designed a solution that would meet that need. Impossible? Until recently. And with a lesson from history.

To bring all three units on line within ten seconds, Kohler decided to use a modern application of an older synchronization method called “close before excitation” or “dead field paralleling.” Most control systems require the gensets to be started and then synchronized against each other one-at-a-time. The first unit becomes the master and each successive generator parallels to the master set. This can take several seconds per generator.

Close before excitation is a technology that goes back a century, long before the existence of computerized controls. Old generators, often turned by hydro or steam turbines, would start the genset spinning. The field would be shorted across a resistor, so any flux lines cut by the difference in speed between the rotor and the rotating magnetic field would not produce dangerous voltages or currents in the field. The AC power circuit breaker would be closed, the field breaker shorting resistor opened and the field breaker closed. Finally, once the speed approached synchronous or the other generators, the field excitation would slowly be increased, pulling the two gensets into sync.

That technology is still the best option today for quickly bringing multiple gensets into sync. The only hardware difference is that sometimes an additional relay or contactor is required to switch on the AC power to the field excitation module. All gensets are sent a start signal at the same time and the AC power circuit breakers are closed at a speed just above idle. As soon as the minimum number of gensets has started to carry the load, the fields are all ramped up slowly. The gensets are automatically pulled into sync as the field increases.

In order to make this work on the university's power system, Kohler selected controllers with Close before excitation built in from DEIF. The generators arrived on location in spring 2008 and installation began in late summer. Elcon established the electrical connections and Kohler staff assisted its distributor, Tampa Armature Works, with the startup ensuring everything worked properly. Everything was set up and operating as expected by the project completion date. Three generators available in less than 10 seconds!

Note: Photos courtesy of Kohler Co.

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