Twin Generators Deliver Backup Power to a Growing Hospital
The time had come to replace the aging emergency generator at Community Hospital in Watervliet, a small southwest Michigan town.
After almost a quarter-century of service, the generator was a source of growing concern to Don Englehardt, the hospital's facilities manager.
"There were serviceability issues, and parts were unobtainable," he explains. "I was told that if the governor or the voltage regulator were to fail, I would have to replace the entire unit. Since I couldn't fix it, I started trying to replace it before I needed to do so."
Englehardt consulted several vendors, including Wolverine Power Systems Inc. Located in Holland, MI, Wolverine is the southern Michigan dealer for Generac Power Systems Inc., of Waukesha, WI. Generac specializes in modular power systems, including the 750-kilowatt Gemini Twin Pack diesel generator set.
Housed within a single enclosure, the Gemini Twin Pack's two 375-kilowatt generators run in parallel. Under normal operating conditions, integrated controls combine their output, providing enough electricity to power the entire hospital if utility power becomes unavailable. That wasn't possible with the previous system, consisting of a single Detroit Diesel engine from Detroit Diesel Corporation (a DaimlerChrysler AG subsidiary) with a Cebra generator rated at 500 kilowatts.
"We're not obligated by the National Fire Protection Association [NFPA] code to run the entire facility, but it's a nice option," Englehardt says. "In a long-term power failure, it alleviates a lot of temporary wiring issues—running extension cords to operate other machines, tripping hazards, and general safety concerns."
Alone, either Gemini Twin Pack generator can back up the other to carry the most crucial aspects of Community Hospital's load, meeting all NFPA requirements for acute-care hospitals to provide backup power for life-safety, critical-care, and fire-pump applications within 10 seconds of a utility-power outage.
"Don likes redundancy. He doesn't like to be stuck without power. The Gemini Twin Pack practically guarantees that won't happen," says Glenn Emmert, Wolverine's president.
The Gemini Twin Pack employs two startup timers: a time-delay timer to screen for nuisance blips in the flow of utility power, and a transfer-delay neutral timer that prevents large motors from connecting out of phase with the backup generator.
Most applications use a 5-second setting for each timer, Emmert says, "but we can adjust them down to practically zero. When utility power goes out, both units in the enclosure get the start signal. The first one comes up in about 6 seconds. At that point, we can switch the life-safety, critical-care, and fire-pump loads to the first one. The second parallels to the first immediately after, in less than 10 seconds overall."
Switch to Transparency
Englehardt can eliminate even this brief hiatus by a discretionary switchover to backup power. The hospital's utility provider, American Electric Power Company Inc., of Columbus, OH, "is pretty reliable, but we have issues with acts of God," he says. "Being in a rural area, we get a lot of storms that bring trees down on power lines. If I see a bad storm rolling in, I can turn the generators on, and then if we do lose utility power, it will be transparent to the end users."
Such transparency comes courtesy of an optional feature included in the hospital's system, the closed transition transfer switch (CTTS). Generac's CTTS is the same physical size as a normal transfer switch, but costs about 30% more.
A normal transfer switch has an open transition from utility to generator power, which means an open relay in the transfer switch has to sense a failure of utility power before it will close, launching the generators' startup sequence. With a CTTS, a discretionary switchover avoids any interruption because, after the generators come online, the relay won't close until it's in phase with the utility power. A CTTS also provides a transparent switchover when utility power returns and the generator shuts down.
The CTTSs enable Englehardt to perform weekly and monthly tests of the emergency power system without having to announce deliberately induced power glitches in advance.
Another Generac option that Community Hospital selected is the PowerManager System Controller (PM-SC), a programmable logic controller with remote monitoring and management capability. It maintains two-way communication with the transfer switches monitoring utility power. If those transfer switches sense a utility loss, they signal the PM-SC, which alerts the generators. As the first generator starts, the PM-SC detects that event and signals the crucial load elements to switch to the online generator. After detecting the second generator's online presence, it signals the rest of the hospital's load to switch over.
The two units operate in parallel by comparing voltage and speed. Isochronous governors count the teeth on the flywheels and turn that count into a reference voltage, within a specified window of acceptability, close enough to 60 hertz for the synchronizer to recognize the similarity and close the contact for the first generator set. Then the second unit parallels with the first. Sometimes one unit connects first, sometimes the other.
Mounted in a panel on the electrical-room wall, the PM-SC communicates with the transfer switches and generators through standard RS-485 communications cables. The PM-SC also is tied to a telephone line with a modem, allowing Englehardt or Wolverine's staff to monitor the system through a remote computer.
"We've used it to manage the system sometimes when Don has a question," Emmert says. "We can do updates on the software via the modem from our own office 50 miles north of Watervliet. We could be in Hawaii and still do it. We have all the normal security you would with any system tied into a modem, including passwords and other security mechanisms.
"We also have the additional capability of load-shedding transfer switches using the PM-SC," Emmert says. "Say you're running, everything is fat and happy, and all of a sudden one of the generators goes offline. The PM-SC senses that and immediately sheds the transfer switch with the least critical load. This is user-defined. It's part of the setup of the whole system."
Testing and Reality
Although the backup system could operate silently and efficiently without human intervention, the NFPA code requires installation of a remote annunciator panel to ensure that humans know when something is amiss. Community Hospital placed this warning mechanism in the admitting department, which is staffed around the clock. Connected to the backup system through RS-485 communications cables, the annunciator panel sounds a horn and flashes a series of lights whenever utility power fails or a generator malfunction occurs, alerting the admitting clerk to notify a member of the maintenance staff.
Possible generator-malfunction conditions could include failure to start, overheating, engine oil pressure above or below specified limits, and the engine operating over or under its appropriate speed—but none of these has occurred since Community Hospital installed the new system.
"We manually opened an oil-pressure sensor once to test the warning system," Englehardt says, "and it also worked during a real outage. In July, a tree limb fell on the utility line. Both generators came online, synchronized, and operated the entire hospital for about three hours. When utility power was restored, the generators went offline, cooled down, and shut down just as they were supposed to."
The shutdown sequence employs three timers: a return-to-utility timer that delays the shutdown long enough to ensure the stability of utility power; a minimum-run timer that keeps the generators running long enough for their temperature to stabilize, even if utility power already has been restored; and a cool-down timer that allows the engine to run at full rpm without a load to avoid damage due to overheating and low oil pressure.
An Economical Alternative
Emmert says the Gemini Twin Pack is an economical alternative to a single large generator of about the same output for emergency power applications. Its capital cost is 20%–30% less, he says, "because we're using smaller engines—two 12-liter engines from Mitsubishi Heavy Industries Ltd. They're an over-the-road type, not a specialty engine. That allows us a better price point per kilowatt-hour, and the smaller the engine, the quicker it's capable of starting.
"Compared to a single 750-kilowatt unit, the two 375-kilowatt Mitsubishis are not just small big units. They are different. Everything is smaller. Parts are readily available, usually at truck equipment parts stores."
From a service standpoint, the Mitsubishi engines don't require the skills of a specialized large-engine mechanic, for whom Wolverine bills $120 to $150 an hour. They can be serviced by a technician, at a cost of $90 an hour.
The system itself is designed to expedite service, Emmert says. "A 375-kilowatt alternator made by Generac is connected to each engine, and both engines are on a single frame inside a single enclosure. With all of the maintenance items to the outside of the engine, you don't have to tear it apart to get into it."
Also, because each generator has its own paralleling switchgear on the generator frame, a separate room for conventional paralleling switchgear isn't required. "That means you aren't taking up real estate somewhere within a building, at exorbitant cost," Emmert says. "That's a huge saving. It can be hundreds of thousands of dollars."
The Mitsubishi engines can be available on short notice, Emmert says. "It's a high-volume engine, which reduces lead time. We can get down as low as eight weeks from the date of order. With a regular 750-kilowatt engine, you're talking 12 to 16 weeks."
Still another benefit of the Gemini Twin Pack is its ability to accommodate growth. This is especially vital for Community Hospital, which is considering an expansion within the next five to 10 years. "The Gemini modular power system is scalable, so Don can add system capacity if and when he needs to," Emmert notes. "He could add a second Gemini Twin Pack or a single 375-kilowatt, 400-kilowatt, or 500-kilowatt unit in a separate enclosure, and he has flexibility in where to put it. There is room adjacent to the existing Gemini pad, or he could mount another unit up to 3,000 feet away if he wants to run the wires."
A Complex Installation
Englehardt says deciding to replace the old backup system and choosing a new one took about a year, and installation took six months, including testing and load banking. "We began in the fall of 2003 and wrapped it up early in 2004," he reports.
The installation took so long because it involved removal of the old equipment and retrofitting the new equipment into a previously constructed space. Installation of a Gemini Twin Pack during construction of a new building is much less arduous, costly, and time-consuming, typically involving anywhere from a few days to a couple of weeks.
Parkway Electric & Communications LLC, of Holland, MI, engineered and performed Community Hospital's Gemini Twin Pack installation. "First we poured a concrete pad 22 feet long, 10 feet wide, and 18 inches thick," explains Jim Achterhof, Parkway's project manager. "Then we set the new Gemini Twin Pack on the pad, and temporarily tied it into the hospital before removing the old generator."
The Gemini Twin Pack sits behind the building near the loading and shipping dock, about 30 feet from the boiler-room wall and 3 feet from the edge of a parking lot. Its enclosure is about 17 feet long, 7.8 feet wide, and slightly over 9 feet tall (plus an additional 2 feet of height for the fuel tank). Without fuel, the unit weighs 17,316 pounds.
Emmert says the enclosure's footprint is 20% smaller than that of a single 750-kilowatt generator. Its patented design incorporates internal baffles and corners that optimize air flow and attenuate sound. At a distance of 7 meters (23 feet), the measured 60-hertz sound level averages just 82 decibel-amps.
"The old generator was 6 feet wide, 6 feet high, and 15 feet long," Achterhof says. "We had to remove a wall and pull it with cranes and forklifts through that opening past the new generator. We made the empty space our new main electrical-service room, where the main distribution panel is located."
Wires in three underground conduits 30 feet long and 4 inches in diameter link the Gemini Twin Pack to the main distribution panel. "We drilled through the foundation walls and cut the floors to run the new feeders into the hospital, then replaced the floors," Achterhof says.
Each conduit contains four 600 mcm [1,000 circular mils—a wire-gauge measurement] cables with a total rating of 400 amps, for a grand total of 1,200 amps. The wire is three-quarters of an inch in diameter and weighs slightly over a pound per foot.
Four Transfer Switches
The main distribution panel feeds two 800-amp circuit breakers. One is for the three transfer switches supplying the code-mandated loads (400 amps for critical care, 200 amps each for life safety and the fire pump). The other feeds a 2,000-amp main-service transfer switch for everything else. It's the last to come online during a utility-power outage, and the first to fall offline if one of the Gemini twins isn't available to carry the hospital's entire load.
Parkway installed and fed wire to new life-safety, critical-care, and fire-pump transfer switches—each 2 feet, 8 inches wide, 4 feet high, and a foot deep—in the old electrical room. The new 2,000-amp transfer switch, which is 5 feet high, 5 feet wide, and 4 feet deep, went into the new electrical room.
When the time came to complete the final hookups, "we did it in six hours with eight people," Achterhof says. "That was eerie, while we had the automatic transfer switches out of the circuit in a hospital. We picked a nice night with no storms predicted, and we had a backup generator in case of an outage, with wires ready to go and people standing by. In case we would have had to start it, everything would have been on the generator within minutes."
The hospital paid about $70,000 for the Gemini Twin Pack. The installation cost an additional $97,000, of which 45% is attributable to materials (including concrete, wire, the new main service, crane rental, and fuel lines) and 55% to labor. In operation, it will consume about 0.36 pound of fuel per horsepower-hour. Assuming the cost of diesel fuel at $1 per gallon, that translates to 8¢ per kilowatt-hour. Annual maintenance costs, for routine oil changes, oil sampling, coolant maintenance, filter changes, turbocharger overhaul, and the labor costs for these procedures should range from $500 to $700 a year.
Author's Bio: George Leposky is a science and technology writer based in Miami, FL.