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MANY OF YOU have spent and will continue to spend significant sums on backup power systems, power filtering devices, and other power-protection components. However, most of these expenditures aren’t necessary because short-duration voltage sags are the real source of most power problems in North American industrial plants. Fortunately, equipment for shielding your plant from these voltage sags is inexpensive, easy to install, and extremely reliable, especially when compared to the elaborate, expensive, and hard-to-maintain systems required for protection against power outages.
Power outages and commercial airline plane crashes make the headlines, which give the impression that they’re common, when, in fact, they’re actually very rare.
Hard data reveals some interesting, contradictory facts. In 2005, SoftSwitching Technologies conducted a study of 30 major automotive plants located in 10 states and one Canadian province. They installed a power quality monitor for 12 months at each site and captured 472 power quality events.
Of those 472 events, 461 events (98%) were less than 2 seconds in duration. The other 11 events were all over 15 minutes, and were classified as power outages. Seven of these 11 events occurred within one week of July 4, and may have been related to planned shutdowns.
There was an average of 16 events per site per year, with an average of less than one event per year longer than 15 minutes per site (an outage). These data clearly show that most power quality events are short-term interruptions, not relatively longer term outages. The geographic distribution of the 30 automotive plants covered a wide swath of the U.S. and Canada, and there’s no reason to believe that a more widespread study would generate different conclusions.
What exactly are these short-term disturbances? In almost all cases, they’re voltage sags. A detailed study of one U.S. automotive plant by SoftSwitching showed that 94% of voltage sags were less than 2 seconds. Based on these and other data collected over the last few years, SoftSwitching reports that 98% of power quality events in the United States and Canada are, in fact, these short-term voltage sags of 2 seconds or less.
What causes voltages sags? In technical terms, a short-circuit fault on the three-phase utility power grid. When a relatively conductive material comes into contact with one of the individual three-phase lines and ground, or with two of the three-phase lines, then a short-circuit condition occurs.
When this happens, an instantaneous current demand is placed on the power source, which is the supplying power plant. Because the power plant can’t deliver current to the fault quickly enough, the voltage on the lines starts to collapse. The effect is voltage sag on the lines that can be seen for up to 400 miles.
Typical causes of voltage sags include lightning strikes or failed utility equipment. Any disturbance that connects a phase to ground or a phase to phase also can cause voltage sag. Typical accidental connections are made by small animals, wind-blown debris, and overgrown tree branches. Statistical data compiled by the Electrical Power Research Institute concludes that a typical industrial manufacturing site experiences an average of 12–14 voltage sags each year.
So, what do short-term voltage sag do to your plant’s operating equipment? SoftSwitching’s aforementioned study of 11 automotive plants showed that about eight of the average 15 short-term voltage sags per site per year would have had no effect on typical production processes.
The other seven short-term voltage sags per site per year could have caused the following effects. “Typical automation equipment such as relays, solenoids, contactors, PLCs, and HMIs will usually error out or trip off during these types of events,” says Bill Brumsickle, SoftSwitching’s engineering director.
The conclusion is clear. The most cost-effective strategy for dealing with power quality events is to protect against relatively frequent, short-term voltage sags as opposed to much rarer power outages.
Protection against outages is extremely expensive, especially since most outages are longer than 15 minutes, which is a relatively long period of time to provide backup power. The most common way to provide protection against power outages is to install and maintain on-site backup power generation, which is an expensive and complex undertaking.
Luckily, it’s a lot less expensive and easier to protect against much more common short-term voltage sags with equipment available from SoftSwitching Technologies and others. For example, SoftSwitching sells a 480 VAC, three-phase, 100 amp unit for about $35,000. This unit will protect against voltage sags down to zero volts for up to 2 seconds.
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