Features. Power Quality: Protecting plant equipment from voltage sags

Power, Jan 2008 by Eberhard, Andreas, Labs, Power Standard

face= Italic; Immunity from voltage sags is vital for reliable operation of our ever-more-sophisticated electronic controls and equipment. Every electrical product should be able to ride through typical voltage sags, but in many cases the first sag test occurs after equipment is installed and in operation. Select the appropriate sag immunity specification and equipment compliance testing, and you'll be glad you did.face=-Italic;

By 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).

face= Bold; Standards developedface=-Bold;

Three main primary voltage sag immunity standards are discussed in the following paragraphs: IEC 61000-4-11, IEC 61000-4-34, and SEMI F47. There are others in use--such as IEEE 1100, CBEMA, ITIC, Samsung Power Vaccine, international standards, and MIL-STD--but the first three seem to have the widest acceptance in the marketplace. (IEC is the International Electrotechnical Commission, SEMI is the Semiconductor Equipment and Materials Institute, CBEMA is the Computer Business Equipment Manufacturers Association, ITIC is the Information Technology Institute Council, and MIL-STD is the U.S. Defense Department's specification.)

IEC 61000-4-11 and IEC 61000-4-34 are a closely related set of standards that cover voltage sag immunity. IEC 61000-4-11 Ed. 2 covers equipment rated at 16 amps per phase or less while IEC 61000-4-34 Ed. 1 covers equipment rated at more than 16 amps per phase. The latter was written after IEC 61000-4-11, so it seems to be more comprehensive.

SEMI F47 is the voltage sag immunity standard used in the semiconductor manufacturing industry, where a single voltage sag can result in the multi-million-dollar loss of product if a facility is not properly protected. The semiconductor industry has developed specifications for its manufacturing equipment and for components and subsystems in that equipment. Enforcement is entirely customer-driven in this industry, as semiconductor manufacturers understand the economic consequences of sag-induced failures and generally refuse to purchase new equipment that fails the SEMI F47 immunity requirement. SEMI F47 is currently going through its five-year revision and update cycle.

All three standards specify voltage sags with certain depths and durations for the equipment under test (EUT). For example, a specification may state 70% of nominal for 500 milliseconds. The percentage is the amount of voltage remaining, not the amount that is missing. Each standard specifies pass-fail criteria for EUT when a voltage sag is applied; the IEC standards have a range of pass-fail criteria, but the SEMI F47 standard is more explicit (Figure 2).

face= Bold; Three-phase testingface=-Bold;

For three-phase EUT, the sags are applied between each pair of power conductors, one pair at a time. If there is a neutral conductor, this implies that there are six different sags at each depth-duration pair: three different phase-to-phase sags and three different phase-to-neutral sags. If there is no neutral conductor, there are just three different phase-to-phase sags at each depth-duration pair in the standard.

Note that IEC 61000-4-11 and 61000-4-34 specifically forbid creating phase-to-phase sags by sagging two phase-to-neutral voltages simultaneously--an approach that is permitted in SEMI F47. Instead, you must create phase shifts during your phase-to-phase sags--something that sag generators designed for these standards do automatically (Figure 3). Typical suppliers of compliant sag generators include Keytek (www.keytek.com), Power Standards Lab (PSL, www.powerstandards.com), and Schaffner (www.schaffner.com).