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Originally featured on BroadcastEngineering.com
Mar 15

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3/15/2010 12:04 PM  RssIcon

Editor’s note: We continue our series on power systems, written by Don Markley. These articles appeared in previous issues of Broadcast Engineering magazine.

809be15_ktud_fw_toshiba-200.jpgAlong with the spring and summer season, comes thunder and lightning—all of which mean trouble for broadcasters. We are well aware of the problems caused by lightning strikes on or near power lines. They can cause power outages that can take the station off the air as well as doing real physical damage to the station's equipment. Such causes also may get blamed for wiping out the entire East Coast.

However, did you know that it may well be that the winter months, at least in the northern states, are more worrisome. For stations in the South, you miss out on all the ice but have hurricanes to worry about. Like a divorce, these natural disasters can take your house, your car and your tower along with the power lines. In addition, as so many stations have been reminded, the power grid is not totally safe from a massive failure. Indeed, as the infrastructure continues to age, electrical loads continue to increase, and we continue to have an irrational fear of nuclear power plants, the odds of massive power failures will continue to increase.

Power generators

Just how does one handle this problem? The subject of UPS power systems has been discussed previously in this column. Either a battery-powered or a flywheel type system can tide the station over for a brief period of time, usually measured in a few minutes. Then, an auxiliary power system must be brought online to meet that continuing power drain.

All modern power systems have automatic start and monitoring systems incorporated into their control systems. On sensing either a failure of one or more phases or the line voltage falling below a predetermined level, as in a “brown-out,” the generator will start automatically. An automatic transfer relay will then switch from the incoming mains to the output of the generator. Usually, the length of time between generator start-up and the actual switchover is adjustable. That allows the generator to warm up a bit before accepting the load. In some applications, this time delay is necessary to allow the load to stabilize before the switchover takes place.

If synchronous motors are in use, applying full line voltage out of phase with a motor's instantaneous electrical phase position can place destructive loads on the rotor. To avoid that, it is better to let the system come to a complete halt and then restart from scratch. Thankfully, that is usually not a problem with the normal induction motors found in broadcast transmitters. Even though that may be the case, letting the generator have a few seconds to stabilize before switchover is still a good idea. The start-up itself is fairly routine on newer systems. In cold parts of the country, the engine is normally heated, either by an engine block heater or by a combination of engine block and oil heating. With a good diesel engine, that means the actual start will be almost instantaneous. Of course, the motor-generator system is protected at several levels and may even be shut down if the monitors detect errors such as low oil pressure, over temperature, etc.

One standby power plant recently viewed by the author consisted of a generator bolted to the floor in the utility room of the station. A shaft with a universal joint was routed through a hole in the wall. In a lean-to type of shed attached to the back of the building was an old international model “M” farm tractor with the power take-off attached to the other end of the shaft. In the case of a power failure, the on-duty operator simply would go out into the back room and fire up the tractor, engage the power take-off and open the throttle. Once inside the building, he would operate a big manual transfer switch. While it might have been crude, it was inexpensive, and it worked very well. A little trickle charger was used to keep the battery on the tractor full and the tractor was started once a month and run for a little while to keep it ready to work.

What’s your hurry?

Once the generator has started, the major failing by stations is being in a big hurry to switch back to the mains. There is no reason for this other than to save a little fuel. When the mains come back, as everyone has experienced, the voltage often fluctuates somewhat over the next few minutes and may even fail again. The generator should be allowed to continue carrying the load until the time that the main power has returned and is fully stable. It's not a bad idea to wait five or 10 minutes before switching back to mains.

Next, give the generator some time to run without load to cool down a bit before switching off. This is particularly true with big diesel engines. They should never be switched off immediately after operating for some time at full load. The heat buildup inside the engine block can cause significant damage in such cases. In complete gen-set packages, all of these timing functions have already been included and are adjustable to the customer's preferences.

Picking a solution

Now, it's decision time. Just where do you do the switching, and how do you size the generator? The answer to these questions will vary somewhat depending on whether it is a new system or adding standby power to an existing system. For a new physical plant, it is possible to pick the loads that should stay on the UPS. Obviously, the transmitter and terminal equipment should not be interrupted. It won't hurt the air handling or HVAC systems to be interrupted. In fact, most of the mechanical portions of the station will not be bothered by a short shutdown, including tower lights, building lights, water systems, etc. Remember, this shut down is going to be measured in seconds, not hours. Therefore, everything in the plant can return to normal in a reasonable fashion without having to provide UPS capacity.

On the other hand, once the cost of the UPS system has been determined to handle the transmitter and terminal equipment, it may not add significant cost to let the whole station ride. That is especially true for an existing system where breaking off part of the systems will require a major rewiring of the transmitter plant. The cost of that rewiring, along with the additional breaker panels, new conduit, etc. may run in excess of simply adding another 10 Kw or so to the UPS.

In a separate but related area, remote sites often have a problem with three-phase power. Even when single-phase power is available, the cost of upgrading to a full three-phase feed can be totally prohibitive for a station. In the past, various types of converters have been implemented to change the single phase supply into a three phase system. The most popular has been rotary converter systems.

Those systems are fine for fixed loads such as big motors. However, they have a tendency to either create large transients in high voltage supplies or to have a third phase that varies in voltage and phase angle. That can wreak havoc in solid-state systems unless a proper regulator is used. A solution is to use a UPS to actually drive the load with the rotary converter driving the UPS. The UPS doesn't have any difficulty dealing with the “wild leg” of the incoming three phase and will provide clean service for the transmitters.

Today there are several solid-state digital phase converters available. Models for up to 30KW exist that maintain 1 percent phase balance, electronic power factor correction and produce little harmonic distortion. Typical efficiency ranges from 95 percent to 98 percent which is quite good. The units can be located in outdoor enclosures, which avoids adding any heat to the transmitter room.

To summarize, in a new system, break the system so that the necessary components are on the UPS and the rest of the equipment is only on the standby power plant. That is, those systems that can experience a short amount of down time are fed separately from the on-air critical systems. For existing systems, do an analysis of the actual cost of rewiring the plant to allow for separating the critical systems and compare that to the cost of simply providing a large enough UPS to run the whole place. You may well find that the additional cost is like putting a 100K ohm resistor in parallel with a 1K ohm resistor. It has an effect, but you won't really notice it.

Finally, remember when installing a UPS or a standby power system that it must not be possible to get the generator online at the same time as the main power supply. Unless you are trying to feed power back to the power company, the result will be large amounts of smoke, noise and flames if the systems try to self-parallel.

The next column will examine the use of environmentally-friendly flywheel UPS systems.

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