During these winter months, especially with the dreaded onset of El Nino, storms have been causing enormous power outages in many areas of the country. No area is totally safe from power failures; even some of the most sophisticated transmitter sites are vulnerable. Therefore, standby power plants and power line filtering are highly recommended if not totally necessary.
The only exception known to the author is the Sears Building in Chicago. Two transmission lines coming in from two separate power plants feed a substation in the basement of the building. The building electrical system consists of multiple risers going up to the mechanical floors, where all the risers are connected in parallel. Failure of a riser simply causes fuses or breakers to trip, isolating the failed riser from the rest of the system. All loads can be accommodated, even with the failure of more than one riser. In the 30 years that this system has been in operation, no broadcaster has ever lost primary power. However, based on the cost of airtime in Chicago, that isn't enough. At least three of the stations still have standby power plants.
Flywheel UPS generators provide an alternative to battery-powered backups. This flywheel from Caterpillar features a 700lb. flywheel that turns at 7700 rpm.
The advent of battery-powered uninterruptible power supplies at reasonable cost has solved the problem of short-term power failures for computers. This same technology has been applied at numerous locations in many stations to keep critical components online and ready for use when primary power returns. This avoids the reboots and loss of data or setups that can occur during momentary power failures.
The same technology is applicable for the high-power end of things. The transmitting plant, while growing more efficient, has become even touchier about glitches in the primary power and, in obvious course, in the high voltage power supply. In other words, high-power IOTs are best served by a smooth, uninterrupted source of high voltage power. The concern here isn't only the situation where the power goes off for hours or days. That is a stable situation and not nearly as hard on transmitters as when the power is broken momentarily by lightning storms or glitches on the power line. A large UPS system will smoothly cover those short breaks and, when incorporated into a total system with a standby generator, will keep operations running smoothly when longer-term failures occur.
While it's hard to believe, five seconds is a long time for today's crop of standby generators. In critical applications, generators that are up to speed at all times can actually pick up the load in a few cycles, far less than several seconds. Such systems are far more than is needed or realistic for a broadcast plant with a UPS. Modern battery UPS systems will normally handle the full load of a transmitter plant for a minute or more, which is far more time than needed. Picking the right UPS is not a simple matter, and is beyond the scope of a simple article. The proper way to handle that problem is to work with a local dealer's engineering department or that of the manufacturer to select just the right system for your application.
Remember, IOT transmitters do have some unusual requirements. The crowbar circuit takes the high voltage power supply to ground to stop sensed arcs in the tube. That places an enormous load on the power supply, albeit for only a very small part of a second. Some UPS systems will handle this short-term load when properly set up by the manufacturer. The solution to the problem is simply to switch the load back to the power line for the period of the high current demand. When the current drops back to within limits, the load is again picked up by the UPS. Again, the UPS manufacturers are aware of this problem and can adjust their systems accordingly.
A great source of information on UPS systems can be found at www.powerware.com, through the Powerware three-phase UPS Engineering CD. While obviously weighted slightly toward their own products, the CD has an enormous amount of information about how the big UPS systems work and the pitfalls inherent in their use. It also shows how to select the right system and integrate it into a complete system.
So far, we've only discussed battery-operated UPS systems. Another option is an old technology that has been modernized and is now becoming popular again — a flywheel-operated UPS system. Large systems of that type were successfully used, primarily in Europe, over 40 years ago. Updated versions of these systems are currently available and offer some real advantages over battery systems, although with accompanying drawbacks.
For information on such systems, look at www.caterpillar.com. Under their products, standby power plants and UPS systems can both be found. There is a significant amount of information concerning the UPS systems as well as specification sheets on all available models and sample specifications for bid. Those specifications really tell a lot about just how the system works.
First, to the author's dismay, the flywheel isn't some giant thing threatening to destroy the transmitter site in case of failure. A reasonably sized UPS requires from 10 to 15 square feet of floor space — about the size of a large FM transmitter. Remember, that is everything for the UPS — motor, flywheel and generator — along with monitoring and control systems. The efficiency of these systems is well above 90 percent. The one drawback is that they can normally provide full rated power for about 15 seconds, although there are systems that will extend that to as much as 26 seconds. While this doesn't sound like very long, a modern standby generator is quite happy with five seconds or so, as was previously discussed.
Now, about that flywheel. The Caterpillar systems use a flywheel that is enclosed in a chamber that is evacuated. In that vacuum, the weight of the flywheel itself is 80 percent supported by a magnetic system, greatly reducing the problem of bearing wear and the associated reduction in system efficiency caused by bearing friction. Obviously, a magnetic suspension system is relatively friction-free. The only friction is in the bearings still needed to support the remainder of the weight and to keep everything in alignment. Oh yes, the flywheel operates horizontally, as opposed to some of the old systems that had a big vertical flywheel.
Despite having the disadvantage of a shorter period during which full power can be delivered, the system offers the real advantage of eliminating batteries with their associated mess. The high current demand of a crowbar trip is handled the same way as in the battery UPS — the load is simply switched back to the primary source for the few milliseconds that the unusual load exists.
The idea of Caterpillar making a flywheel system of this type conjures up a vision of a D-6 bulldozer revolving at a high rate of speed. Not so — the system is small and has full monitoring to allow the operator to keep track of its health. It is really intended to be incorporated into a complete standby power plant with an automatically synchronizing generator and automatic changeover switches. Again, this unit must be installed with a total bypass switch to take it out of the system for service or in case of failure. The front office would not be pleased if the station were to go down not because of power failure, but because the standby system blocked primary power from use.
Don Markley is president of D.L. Markley and Associates, Peoria, IL.
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