In order to handle future electrical demand, a major scientific university (name withheld by request) in the Boston metropolitan area undertook an ambitious program in the last few years to upgrade every facet of its power distribution system. As one of the premier technical engineering research and educational institutions in the nation, the school is well aware of its critical need for ensuri...
In order to handle future electrical demand, a major scientific university (name withheld by request) in the Boston metropolitan area undertook an ambitious program in the last few years to upgrade every facet of its power distribution system. As one of the premier technical engineering research and educational institutions in the nation, the school is well aware of its critical need for ensuring power quality and reiiability.
Classrooms, dormitories and offices on a college campus could lose lighting temporarily and perhaps suffer a few “fried” PCs or laptops if power reliablity is not ensured. Generally speaking, a power anomaly is a mere inconvenience in the classroom or domitory. But in a laboratory, an unchecked power surge could wreak havoc on a multitude of ongoing research projects. And even a minor power surge disrupt major IT systems.
Officials at these institutions have learned to protect their systems against major—and minor—power events. The following case study offers a prime example of how authorities at one such institution took a proactive approach, focusing on ensuring reliability in critical power settings and safeguarding research labs and IT infrastructure—now and for the future.
“We're in phase three of a six- or seven-phase upgrade,” explains the university's senior electrical engineer, who is supervising the upgrade. “Only about 25% of our overall demand is met through utility hookups. For the rest, we've had our own generating plant and switchgear since 1994. Because the losses from a significant power surge or short circuit could be incalculable, we are determined to head off any problems.”
The university's biomedical engineering center provides the perfect example of the critical nature of its research. Professors and students study cell responses to molecular stimuli, protein-DNA interactions, molecular dynamics and macromolecular binding/folding kinetics. The center's new two-photon microscopy imaging facility includes a climate-controlled cell-culture room and two state-of-the-art microscopes that use a tunable titanium-sapphire laser as their light source.
Other valuable instruments at the center include a scanning probe microscope, fluorescence microscopes, a surface plasmon resonance device, a radiolabeling hood and gamma counter. Typical of the university's many research nodes, the center also has more than its share of high-performance computer equipment.
The school's on-site gas-turbine generators have an approximate output of 22 MW. But total load demand exceeds that capacity, and is around 25 MW on a typical day—and even higher on hot summer days that require air conditioning. To make up the shortfall, the system draws from the nearest utility substation. Serving as a buffer between the university and utility systems are two custom-made, dry-type, air-core, current-limiting reactors built by Phoenix Electric, Boston. and Installed side by side in May 2004. The reactors are designed to limit the effects of short circuits, thus reducing the stress on buses, insulators, circuit breakers and other high-voltage devices. All day every day, the reactors serve as a buffer between incoming current surges and campus facilities. This keeps the feed constant and predictable, school officials explain, even when there is a fault that would have damaged the university's generating equipment and other devices down the line.
“Reactors are sort of like giant surge suppressors,” says the university's supervising engineer. “Without them, current spikes could disrupt or damage the utility's equipment as well as our own.”
“Our old reactors could have lasted another 40 years, but we outgrew them,” he continues. “They were rated for 28 MVA, but with the campus still growing, we think we're going to need a capacity of around 35 MVA in a few years. The new reactors are rated for 47 MVA, so they should be good for at least 15 years to 20 years, depending on future growth.”
Because incoming current has three phases, each reactor is a stack of three coils, weighing approximately 35,000 lbs. and resting on a custom-designed aluminum support stand. The new units have an amperage rating of 6,000 amps per three-phase reactor, or 2,000 amps per coil. The reactors they replaced were each rated at 3,600 amps, or 1,200 amps per coil.
The challenge was to fit the new higher-capacity reactors into the same space, and to do without attracting attention from the 20,000 people who work or study on campus. Engineers designed and supervised the entire retrofit, from conceptual diagrams to equipment installation. The project followed procedures that ensured optimum performance while minimizing service disruption and maximizing re-use of existing components. When the old units were removed, the university kept the four-sided aluminum enclosure towers. Although the new reactors have the same diameters as the old ones, because they are taller, the roofs of the enclosure towers were raised and another section was added at the top of each enclosure for a height of approximately 24.5 ft.
The university's two new reactors, the internal copper buswork and all other equipment were manufactured by the same supplier. The reactors share a common bus with four incoming circuits (two per reactor), and each reactor system is connected to the main generator bus system separately. This provides redundancy if one of the utility systems is down.
Design of the enclosures required sophisticated calculations on magnetic fields, to make sure the exterior panels would minimize loop and eddy currents. If allowed to circulate, such currents could cause overheating and could even shock someone touching the enclosure. Proper grounding is key to preventing such scenarios and was accomplished with custom-designed straps that link each panel to the one below it. The spacing and shape of the panel louvers are also important design elements. These louvers not only provide ventilation but also break up magnetic fields, limiting their range and intensity.
The new system was an engineering acheivement worthy of an academic institution known for its scientific and technological breakthroughs.
Types of Reactors
Reactors include the following types:
Current-limiting reactors re-duce short circuit levels to meet system needs and reduce stresses on buses, insulators, circuit breakers, and other high voltage devices.
Filter reactors provide tuned series resonant LC circuits to meet specified harmonic requirements and minimize the effects of dangerous harmonics on power systems.
Neutral grounding reactors limit line-to-ground fault currents and reduce stresses on power equipment.
Transient-limiting reactors connect to capacitor banks to limit inrush currents on energization or limit resonant.