January 15, 2008

Protecting plant equipment from voltage sags

Andreas Eberhard, Power Standard Labs

Modern equipment can be sensitive to brief disturbances on utility power mains. Electrical systems are subject to a wide variety of power quality problems that can interrupt production processes, affect sensitive equipment, and cause downtime, scrap, and capacity losses. The most common disturbance, by far, is a sag: a brief reduction in voltage lasting a few hundred milliseconds.

Sags are commonly caused by fuse or breaker operation, motor starting, or capacitor switching, but they are also triggered by short circuits on the power distribution system caused by such events as snakes slithering across insulators, trenching machines hitting underground cables, and lightning ionizing the air around high-voltage lines. Many utilities report that 80% of electrical disturbances originate within the user’s facility.

A decade ago, the solution to voltage sags was to try to fix them by storing enough energy somehow and releasing it onto the AC mains when voltage dropped. Some of the old solutions included an uninterruptible power supply (UPS), flywheels, and ferroresonant transformers.

More recently, engineers have realized that voltage sag is really a compatibility problem with at least two classes of solutions: You can improve the power or you can make the equipment tougher. The latter approach is called “voltage sag immunity,” and equipment manufacturers have several compliance standards that you should be aware of when specifying future equipment purchases (Figure 1).

1. Immunize your plant. Voltage sag immunity testing has been common in the semiconductor industry for years and has proved its economic value. New IEC standards for voltage sag immunity will expand this kind of testing and certification to other industries. Courtesy: Power Standard Labs