The ultimate goal for any product manufacturer is to make its product immune to voltage sags.
Andreas Eberhard, Power Standards Lab Alameda, CA
Modern equipment can be sensitive to brief disturbances on the AC power mains. Unfortunately, electrical systems are subject to a wide variety of power quality problems that can result in interruption of the production process, damage to sensitive equipment, 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. Fuse or breaker operation, motor starting, or capacitor switching can trigger voltage sags. Sags occur world-wide due to such varied causes as short circuits on the power distribution system: trenching machines hitting underground cables; lightning ionizing the air around high-voltage lines; and numerous other effects.
A decade ago, the solution to voltage sags was to try to fix them—i.e., store up enough energy somehow and then release it onto the AC mains when the voltage dropped. Older solutions included UPS, flywheels, and ferro-resonant transformers. More recently, engineers have realized that a dual dilemma exists, and it has at least two classes of solutions. We can make the power better, or we can make the equipment tougher. The latter approach is called “voltage sag immunity,” and it is the basis of several compliance standards.
STANDARDS FOR VOLTAGE SAG IMMUNITY
We discuss three main voltage sag immunity standards in this article: IEC 61000-4-11, IEC 61000-4-34, and SEMI F47. There are, however, many other voltage sag immunity standards, including IEEE 1100, CBEMA, ITIC, Samsung Power Vaccine, International standards, and MIL standards.
IEC 61000-4-11 and IEC 61000-4-34 are closely related. They both cover voltage dip immunity. IEC 61000-4-11 Ed. 2 covers equipment rated at 16 amps per phase or less. IEC 61000-4-34 Ed. 1 covers equipment rated at more than 16 amps per phase, was written after IEC 61000-4-11, and contains clearer explanations of environments of use and application.
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. SEMI F47 is currently going through its 5-year revision and update cycle.
All three standards specify voltage sags with certain depths and durations—for example, 70 percent 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 1).
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, there must be six different sags at each depth-duration pair—i.e., 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. 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, the tester must create phase shifts during the phase-to-phase sags – something that sag generators designed for these standards do automatically (Figure 2).
TEST EQUIPMENT REQUIRED
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 models include pre-programmed sags for all of the IEC, SEMI , or MIL standards.
Common EUT failure mechanisms are a blown fuse or a circuit breaker activated during the current inrush after voltage sag, so 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 be used only for pre-compliance testing, not certification.
Portability of sag generators is 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 along with 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 laboratories subcontract voltage sag testing to labs with engineers who have the training and experience both to perform the sag testing and to help diagnose EUT failures. Accessing this expertise 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 comes complete with over-the-phone engineering support from an experienced sag-testing engineer. This latter approach can be the best way to get started on voltage sag immunity testing.
WHAT MAKES VOLTAGE SAG TESTING DIFFERENT?
The engineer performing sag-testing must control and manipulate all of the power flowing into the EUT—a marked difference from the techniques used in other emissions and immunity testing. For smaller devices such as personal computers, this requirement is not a great challenge; but for larger industrial equipment, perhaps rated 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.
The voltage sag test engineer will insert a sag generator between the AC source and the equipment-under-test. Often, high currents (200 amps) and high voltages (480 volts, 3-phase) must be handled.
Certain software contain extensive safety checklists. Some of the checklist items are obvious. “Which member of the test team is trained in CPR?” “Where is the closest fire extinguisher?”
Some are less obvious. “How do we get access to at least two upstream circuit breakers?” “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 subject it to voltage sags while fully operational. In many cases, the sags will need to be applied during different stages of EUT operation. Remarkably often, the EUT is not ready for timely voltage sag testing. Development work may need to be completed, or an engineer is unavailable to operate the EUT. The supplies to operate the EUT (raw materials, cooling water, or compressed air) may not be available, or the EUT software is out-of-date. Test engineers should anticipate these types of problems.
EUT failure mechanisms can be complicated, too. The test engineer will be expected to help diagnose them. A built-in digital oscilloscope in some sag generators will help; however, the test engineer must still 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 the obvious one: lack of energy (Figure 3). This can manifest itself in something as simple as insufficient voltage to keep a critical relay or contactor energized, or it may be more complex—e.g., an electronic sensor with a failing power supply produces an incorrect reading causing the EUT software to react inappropriately.
The second most common failure mechanism, surprisingly, occurs just after the sag has finished. All of the bulk capacitors inside the EUT 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 lacking sufficient current capability will seemingly pass if there is insufficient current available to blow a fuse or trip a circuit breaker in a half-cycle.)
Another common EUT failure mechanism is a sensor detecting the voltage sag and then shutting the EUT down. In a straightforward example, a three-phase EUT might have a phase-rotation relay that incorrectly interprets 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.)
Another 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. 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 24-VDC supply output has dropped to 18 V during the sag. (In this case, the solution is an inexpensive bulk capacitor across the 24 VDS supply.)
Many other failure mechanisms can take place during voltage sags. The question for the test engineer remains, “How do we fix this problem?” Usually, there is a simple, low-cost fix, once the problem has been 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. The power is unlikely to improve in the future, so the ultimate goal for any product manufacturer is to make its product immune to voltage sags. Just as all modern cars should withstand regular bumps on the road, every electrical product should be able to ride-through any regular voltage sags that will occur.
Andreas Eberhard is member of various power quality standard committees around the world and has over 10 years of experience in product compliance based on international standards. He is Vice President of Technical Services at Power Standards Lab and can be contacted at [email protected], or Tel ++1-510-522-4400.