Power outages can be disastrous for healthcare facilities. Some procedures, especially X-ray procedures that use medical imaging equipment, need uninterrupted power to ensure patient safety and successful results. Certain types of equipment, such as catheterization lab equipment, also require clean power, plus isolation from the rest of the facility. In Toronto, Canada, engineers at the Trillium Health Centre know all about the hazards associated with power interruptions. Recently, a lightning storm caused a reclosure to operate at the utility feed, creating a momentary outage that affected many of the centre's critical systems. Here's what they did to resolve the problem — and prevent future occurrences.
Trillium's engineers turned to experts from Piller Inc. and On Power Systems Inc. to spearhead the project. The team began with upfront monitoring to identify the most appropriate solution. The facility did not have the space to accommodate a battery-supported uninterruptible power supply (UPS). In fact, the only available space was located outside the building and above the boiler room. In the end, the team decided to install a rotary UPS because of its rugged, battery-free design (see photo). A permanent monitoring system also was installed to review input, output, the utility feed, and power to the medical equipment.
Nuts and Bolts
Fig. 1, on page 58, illustrates the total system configuration for the project. The setup was designed for 500kVA-rated power and a ride-through period of 38 sec during complete power outages. Engineers installed the rotary UPS between the normal power sources and the catheterization laboratory loads. When utility outages occur, the onsite emergency generator immediately begins supplying power to the system — well within the flywheel ride-through time of the UPS.
The UPS consists of a synchronous motor/generator coupled with a flywheel-based energy storage device through a redundant AC-DC-AC converter. The AC-DC-AC converter allows for power transfers in both directions. The motor/generator windings are interleaved, which permits energy transfer between the stator windings for an efficiency rating of 99%.
Motor/generator output is connected to the load through the low-impedance (1%) end of a coupling/isolating choke. The choke simultaneously de-couples the load and the utility input through its high-impedance (49%) end. In other words, conditioned power is delivered to the load through the coupling choke, but it's separated from the utility by the isolation choke. The choke and motor/generator combination helps prevent line disturbances (e.g., surges, sags, and harmonics) from affecting the load — and vice versa.
The flywheel-based storage device consists of a conventional, synchronous machine with a brushless exciter and steel flywheel that are mounted vertically on the same shaft, along with fully redundant silicon-controlled rectifier converters. The storage device can produce a maximum power output of 1.65MW for 10 sec. In addition, a helium-filled steel case helps reduce windage losses. Standby losses of <10kW equate to 99.5% peak efficiency, and normal operating speeds (ranging from 3600 rpm to 1800 rpm) use 75% of the total stored energy, or 16.5 megajoules. Finally, machine-commutated 6-pulse bridges are capable of charging and discharging 1.1MW within 15 sec.
The rotary UPS offers the advantage of complete line-to-line load isolation. This permits Trillium staff members to segregate medical equipment (harmonic-rich loads) from the rest of the facility without derating the UPS. Under sag or surge conditions, the UPS can correct load voltages to 1% of nominal for continuous utility input excursions of +15% to -20%. It also bridges voltage sags of -50% for a minimum of 10 sec and -30% for 10 min.
Of course, a major benefit of the rotary UPS is the battery-free aspect. This means that no unexpected failures due to cyclical loading can cause short-term utility outages. It also allows for periodic engine generator transfer tests, saves on space, and reduces costs associated with battery care and disposal.
Other benefits include low-output impedance, nonlinear load performance, low harmonic distortion, high power factor, high efficiency, and fully redundant control electronics and power semiconductors. These benefits combine to form a unit that provides a mean-time-between-failure rating of more than 1.38 million hours.
The engineers at Trillium quickly realized the benefit of the permanently installed monitoring system during the first equipment shutdown following the installation. The monitor recorded a utility outage, the 20-sec generator start sequence, the UPS operation, and an outage occurrence at a particular piece of X-ray equipment (indicated by a mechanical lockout). This led engineers from On Power Inc. to discover that the emergency power-off for the X-ray equipment was fed from emergency instead of UPS power.
The system installed at Trillium Health Centre has been operational for approximately one year. During that time, the permanently installed monitoring system has analyzed utility input and UPS output. Fig. 2 highlights actual UPS input and output data taken over a five-day period. As you can see, the graph shows no corresponding effects on the critical load. The utility input voltage drops to zero, but the output voltage remains within 3% of nominal. Also note the 1% swing in output voltage. Overall, the system has consistently protected critical loads against all voltage line disturbances, including utility outages and generator testing, and it has achieved excellent damping of harmonics and bidirectional flicker.
Gary Rackow is the national manager for market development at Piller Inc.'s office in Norcross, Ga. You can reach him at [email protected].