Voltage Sag - Maxwell Technologies - #1

PAPER

By Maxwell Technologies

Utility power in the U.S. is very reliable; we count on it to be there, expect it to be there, and it usually is. Barring natural disasters (ice storms, tornados, hurricanes are notorious for destroying local distribution) and the rare system-wide blackout (such as the Northeast Blackout on August 14, 2003), the U.S. has a continuous supply of electrical power. But what is the quality of that power? It is not a digital product; on or off. It is an analog product, with continuous variations in voltage and phase, depending on load profiles and myriad variables across the transmission and distribution network. The utilities do an admirable job controlling what they can, but with the hundreds of thousands of local loads affecting power quality, local consumers with critical loads should consider what steps they can take on their own to harden their systems against poor power quality.

When we talk about this type of power quality, we are talking about brief instances that most applications ignore. Voltage can sag to sometimes as much as half the nominal line voltage for brief periods. These are brief sags, on the order of fractions of a cycle (at 60 Hz, equivalent to 16 msec) to as long as a second.

Voltage sags can be delivered from the grid, however, in most cases, sags are generated on the facility-side of the meter. For example, in residential installations, the most common cause of voltage sag is the starting current drawn by motors in refrigerators and air conditioners. In industrial installations, the numerous motors, compressors, etc. and their large size, generate many voltage sag events every day within the facility.

Sags do not generally disturb lighting (incandescent or fluorescent), motors, or electric heaters, all of which have a relatively slow frequency response because they contain an effective energy storage element, whether the mechanical inertia (kinetic energy) of a motor, or the thermal inertia (thermal energy) of incandescent lights, or the small reactive inertia (electrical energy) in fluorescent ballast.

Voltage sags are the most common power disturbance experienced by factory equipment. It is not unusual to see several sags per year at the service entrance of a typical industrial site, and due to power fluctuations within the confines of the site, far more sags are experienced at equipment terminals.

Electronic equipment with fast response times (e.g. programmable logic controllers (PLCs), adjustable speed drives, switching power supplies) lack sufficient internal energy storage to tolerate severe sags in the supply voltage. In recognition of the fact that voltage sags do occur, the Computer and Business Equipment Manufacturers' Association (CBEMA), and Semiconductor Equipment and Materials Institute (SEMI) have both published information defining what levels of poor power quality, specifically voltage sag, equipment should be able to tolerate. The CBEMA curve (Figure 1) includes voltage surges as well as sags, while the SEMI E10 curve (Figure 2) only defines sag tolerance. In addition, other agencies have also defined various power quality tolerance criteria. (Figure 3).

These curves are helpful in defining what voltage sags equipment should be capable of tolerating, but they give no indication of how one should design the equipment to do so. Some equipment may be able to ride through very brief, deep sags, or longer but shallower sags. In any event, the key element to surviving voltage sags is the presence of enough energy storage to ride through the sag event.

Ultracapacitors (UCs) are ideally suited as an energy storage solution for hardening sensitive equipment against voltage sag. They have extremely high energy density for capacitors (typically 1000 times greater than electrolytic capacitors), they can deliver very high power instantaneously, and they can

9244 Balboa Avenue, San Diego, CA 92123 CH1728 Rossens

United States Switzerland

Phone: +1-858-503-3300 Phone: +41 (0) 26 411 85 00

Fax: +1-858-503-3301 Fax: +41 (0) 26 411 85 05

© Maxwell Technologies, Inc. | www.maxwell.com | info@maxwell.com