Out of sight doesn’t mean out of mind:
Probably few industrial/commercial segments are as power-quality conscious as the financial, banking, credit and insurance companies. These “consumers-of-electrons” in the United States generate nearly 10 times the revenue that the “producers-of-electrons” do according to the 2002 U.S. Economic Census (www.census.gov/prod/ec02/ec0252a1ust.pdf).
And they don’t just use any electron stream; rather, their data centers often have double or triple redundant power sources to make sure that the computers and other information technology equipment, along with the communication devices, aren’t interrupted due to issues with the quality of the supply—keeping the revenue streams flowing that exceed $100k per second.
But just having multiple backup power sources does not ensure constant connectivity. While most facilities are well-managed, there are some cases where those in charge of maintaining such systems don’t really know if they will work properly when called on.
There are several types of backup power sources that can be used to keep computers and related equipment running smoothly, at least from a power source perspective. The amount of equipment and the complexity of the interconnection scheme is based on the susceptibility of the loads, the type of power quality disturbance or phenomena that they would experience, and the length of uninterrupted operation required. Perhaps the simplest situation is a laptop computer, which already has a backup system since it runs off an internal battery. The external charger converts AC voltage to DC (usually 12V DC) to recharge the battery, though it also can power the laptop. The laptop battery, like others, has a finite life dependent on type of battery and charge/discharge history. There may be a false sense of security about ride-through for sags and interruptions unless the battery is replaced periodically.
Planning, maintenance and follow up
Another potential PQ-related problem can be hidden behind the sales pitch of some transient voltage surge suppression (TVSS) outlet strips. Transients come in a variety of types, and as a whole, typically constitute a small percentage of the power quality problems a computer might experience. A properly designed and maintained strip will be effective at clamping the peak voltage of transients at the level specified by the manufacturer. If the voltage transient exceeds the rating of the suppression devices inside the strip (such as MOVs—metal oxide varistors), the clamping voltage level can increase and/or eventually lead to catastrophic failure of the device. Such devices often fail in a short-circuit mode, which then activates the fuse on the outlet strip. Do not use outlet strips that don’t have fuse or other overcurrent protection. If the fuse is blown, don’t just replace the fuse, unless you are confident the other devices in the strip are not damaged.
In addition, TVSS strips don’t provide protection against negative transients (such as notches) or variations (such as sags and interruptions). They have no energy storage capabilities, so they can’t fill in the voltage waveform when the voltage decreases, which is the most typical type of PQ phenomena in most locations. And don’t forget to protect the communication systems such as the telephone line.
The batteries for medium and large uninterruptible power supply (UPS) systems require periodic and regular maintenance. The manufacturer’s maintenance instructions should be followed. In addition, documents such as those produced by the National Fire Protection Association, Quincy, Mass., including NFPA 70B, Recommended Practice for Electrical Equipment, can be useful in helping maintain such systems. In the 2006 edition, Chapter 25 provides a wealth of information on the maintenance and testing of both static (battery) and rotary (motor-generator) UPS systems. Failure to properly maintain the generator carries similar consequences for rotary systems as do batteries for static ones. Few people would expect a car that has been sitting in a field for several years to just start up the first time the ignition key is turned. The same is true here.
There have been cases where it wasn’t known that a particular critical load wouldn’t be powered during the emergency situations until a power failure occurred on the utility side and the load went down. Many backup systems aren’t rated to power the entire facility load. Overloading the backup generator can compromise the quality of the supply for the mission-critical loads.
Just because the source of the quality of supply needed to keep the dollars flowing sits hidden behind a wall somewhere in your facility doesn’t mean you should take it for granted that it will be ready to go when the time comes to put it into service. Be sure that the system was properly installed, maintained and periodically tested under the required loading. Your facility will be ready to go when the time comes, and your revenues will keep flowing.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.