From EMC Modern equipment can be sensitive to brief disturbances on the AC power mains. Electrical systems are subject to a wide variety of power quality problems which can interrupt production processes, affect sensitive equipment, and cause downtime, scrap, and capacity losses. The most common disturbance, by far, is a brief reduction in voltage, lasting for a few hundred milliseconds. These ‘voltage sags’ (in American English) or ‘voltage dips’ (in British English) are the most common power problem encountered. Besides fuse or breaker operation, motor starting, or capacitor switching that trigger voltage sags, they are also caused world-wide by short circuits on the power distribution system (snakes getting across insulators, trenching machines hitting underground cables, lightning ionizing the air around high-voltage lines, etc.).
A decade ago, the standard solution to voltage sags was to try to fix them by somehow storing up enough energy and then releasing it onto the AC mains when the voltage dropped. Some of the old solutions included UPS, flywheels, ferro-resonant transformers, etc. But more recently, engineers have realized that voltage sags really represent a compatibility problem, presenting at least two classes of solutions. You can either make the power better, or you can make the equipment tougher. The latter approach, called “voltage sag immunity,” is the basis for several compliance standards and the subject of this article.
Figure 1: Voltage sag immunity testing has been common in the semiconductor industry for years, where it has proved its economic value. New IEC standards for voltage dip immunity will expand this kind of testing and certification to any other industry.
Standards for Voltage Sag Immunity In this article, we will discuss the three main voltage sag immunity standards: IEC 61000-4-11, IEC 61000-4-34, and SEMI F47. However, there are also many other voltage sag immunity standards, including IEEE 1100, CBEMA, ITIC, Samsung Power Vaccine, international standards, MIL STD, etc.
IEC 61000-4-11 and IEC 61000-4-34 represent a closely-related pair of standards, and both address voltage dip immunity. IEC 61000-4-11 2nd Edition covers equipment rated at 16 amps per phase or less. IEC 61000-4-34 1st Edition covers equipment rated at more than 16 amps per phase. Since that standard was written after IEC 61000-4-11, it features better explanations.
SEMI F47 is the voltage sag immunity standard used in the semiconductor manufacturing industry. It is used both for semiconductor equipment, and for components and subsystems in that equipment. Enforcement is entirely customer-driven; the purchasers of semiconductor equipment know the economic consequences of sag-induced failures, and generally refuse to pay for new equipment that fails the SEMI F47 immunity requirement. Presently, SEMI F47 is going through its 5-year revision and update cycle.
All three standards specify voltage sags with certain depths and durations (for example 70% of nominal for 500 milliseconds). The percentage is the amount of voltage remaining, not the amount that is missing. These sags are applied to the equipment under test (EUT). Each standard specifies pass-fail criteria for the 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: A typical example for a voltage sag ride-through curve that is used in the process industry
Three-Phase Testing For three-phase EUTs, 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 (that is, three different phase-to-phase sags, and three different phase-to-neutral sags). If there is no neutral conductor, there are just three different sags at each depth-duration pair in the standard (that is, just three different phase-to-phase sags). In all of the standards, all three phases are never sagged at the same time.
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, a set-up that is performed automatically by sag generators designed for these standards.
Figure 3: The IEC standards require phase shifting during sags on 3-phase systems, but sags on all three phases simultaneously are not required. Required Test Equipment A voltage sag generator is a piece of test equipment that is inserted between the AC mains and the EUT. It generates voltage sags of any required depth and duration. Some voltage sag generators include pre-programmed sags for all of the IEC, SEMI or MIL standards.
Because a common EUT failure mechanism is a blown fuse or circuit breaker during the current inrush after a voltage sag, the sag generator must be specified for delivering large peak currents – typically in the hundreds of amps. This peak current requirement in the IEC standards means that electronic amplifier AC sources generally can only be used for pre-compliance testing, not certification.
Figure 4: Voltage sag generators like this one handle hundreds of amps at three-phase voltages, while still staying portable. Built-in standards help speed up testing; built-in digital oscilloscopes help the test engineer diagnose EUT problems.
Portability of sag generators is also a key consideration. It is often impossible to bring larger room-sized industrial equipment to a test lab. Instead, the test lab must come to the equipment, which requires bringing along a sag generator. In general, the largest portable sag generators can handle no more than 200 amps per phase at 480 volts. Some of the standards, such as SEMI F47, offer specific advice about how to test EUTs that require more than 200 amps by breaking them down into subsystems.
Many conformance certification labs sub-contract voltage sag testing to labs that have engineers with the training and experience to both perform the sag testing and to help diagnose EUT failures. This is an especially attractive approach when certifying large, industrial loads.
For smaller commercial and industrial loads, many labs choose to rent a voltage sag generator. Such a rental often includes over-the-phone engineering support from an experienced sag testing engineer. This can be the best way to get started on voltage sag immunity testing.
What makes voltage sag testing different? Unlike most other emissions and immunity testing, the test engineer must control and manipulate all of the power flowing into the EUT. For smaller devices such as personal computers, this is not a great challenge. But for larger industrial equipment, perhaps rated at 480 volts 3-phase at 200 amps per phase, with an expected inrush current of 600 amps or more, the test engineer must be prepared for serious performance and safety challenges.
Figure 5: The voltage sag test engineer will insert a sag generator between the AC source and the EUT. Often, high currents (200 amps) and high voltages (480 volts 3-phase) must be handled.
Certain sag immunity testing software comes with extensive safety checklists. Some of the checklist items are obvious (for example, “Who on the test team is trained in CPR?” or “Where is the closest fire extinguisher?”), and some are less obvious (“How do we get access to at least two upstream circuit breakers?” and “Where is the closest trash can?”).
This kind of testing requires a fully-functional EUT, and someone who knows how to operate it. The only way to determine if an EUT is immune to the required voltage sags is to have it fully operational during the voltage sags; in many cases, the sags will need to be applied during different stages of EUT operation. Remarkably often, the EUT is not ready on time for voltage sag testing. In some cases, development work may need to be completed, or there is no one available to operate the EUT, or the supplies to operate the EUT (raw materials, cooling water, compressed air, etc.) are not available, or the EUT software is broken. Test engineers should plan for these and other kinds of problems.
EUT failure mechanisms can be complicated as well, and the test engineer will be expected to help diagnose them. The built-in digital oscilloscopes in most sag generators will help, but the test engineer must figure out where to connect the channels to circuits inside the EUT.
Common EUT Failure Mechanisms During Voltage Sags The most common failure mechanism is also the most obvious one, that is, lack of energy. This failure can manifest itself in something as simple as voltage insufficient to keep a critical relay or contactor energized, to something as complex as an electronic sensor with a failing power supply giving an incorrect reading, thereby causing EUT software to react inappropriately.
Surprisingly, the second most common failure mechanism typically occurs just after the sag has finished. All of the bulk capacitors inside the EUT attempt to re-charge at once, causing a large increase in AC mains current. This increase can trip circuit breakers, open fuses, and even destroy solid-state rectifiers. Most design engineers correctly protect against this inrush current during power cycling, but many do not consider the similar effects of voltage sags. (EUTs that are tested with sag generators that lack sufficient current capability will incorrectly pass, if there is insufficient current available to blow a fuse or trip a circuit breaker in a half-cycle.)
Figure 6A
Figure 6B
Figure 6C
Figure 6D
Figures 6A, 6B, 6C, and 6D: Figure 6A shows a typical voltage sag. Figure 6B shows the current waveform, which was about 40 amps peak before the sag, but increases to 450 amps peak afterwards. Figure 6C shows the same current, this time as an RMS value. Before the sag, it is about 23 amps RMS (the equipment was rated at 30 amps), but after the sag the current increases to 175 RMS, typical behavior of an EUT. Figure 6D shows the output of a DC supply during this sag.
Another common EUT failure mechanism is a sensor detecting the voltage sag and deciding to shut down the EUT. In a straightforward example, a three-phase EUT might have a phase-rotation relay that incorrectly interprets an unbalanced voltage sag as a phase reversal, and consequently shuts down the EUT. In a more obscure example, an airflow sensor mounted near a fan might detect that the fan has slowed down momentarily, and the EUT software might misinterpret the message from this sensor as indicating that the EUT cooling system has failed. (In this case, a software delay is the obvious solution to improve sag immunity.)
Yet another common EUT failure mechanism involves some obscure sequence of events. For example, a voltage sag is applied to the EUT, and its main contactor opens with a bang. But further investigation reveals that a small relay wired in series with the main contactor coil actually opened, because it received an open relay contact from a stray water sensor. That sensor, in turn, opened because its small 24Vdc supply output dropped to 18V during the sag. (In this case, the solution is an inexpensive bulk capacitor across the 24 Vdc supply.)
Many other failure mechanisms can take place during voltage sags. The question to the test engineer will always be “how do we fix this problem?” Usually, there is a simple, low-cost fix, once the problem is identified. Only in extreme cases should devices that eliminate voltage sags on the AC circuit be considered, as this is the most expensive possible solution.
With the ever-increasing use of sophisticated controls and equipment in industrial, commercial, institutional, and governmental facilities, the continuity, reliability, and quality of electrical service has become extremely crucial to many power users. Power quality can hardly be expected to improve in the future, so the ultimate goal for any manufacturer is to ensure that its product is immune to voltage sags. Just as all modern cars should be able to drive through regular bumps on the road, every electrical product should be able to “ride through” any regular voltage sags that will occur. g
About the Authors Alex McEachern is the President of Power Standard Labs. He represents the United States on the IEC Working Group that wrote IEC 61000-4-11 and 61000-4-34, and is also the Secretary of the Task Force that is updating SEMI F47. He can be reached at Alex@PowerStandards.com.
Andreas Eberhard is Vice-President of Technical Services at Power Standard Labs and can be reached at aeberhard@powerstandards.com. © Copyright 2007 Conformity |
