Are drives unfairly singled out as a main harmonic-generating device? Black cloud? You're probably wondering if you read that right. What could possibly be dark or disturbing about power quality? Isn't power quality just the business of delivering clean power? Rest assured, power quality is important to all of us. We depend on our power provider to deliver a sinusoidal voltage source, so we can plug in everything from radios, to computers, to industrial control equipment, and have them operate correctly.
But a segment of the market we call power quality may have unknowingly undermined the full potential of the business of drives.
At the heart of this phenomenon is ANSI/IEEE-519-1992, IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems. Arguably, (and this writer is sure there will be arguments, post-haste), this standard's basis is the need for a clean voltage source.
At the risk of oversimplifying an admittedly complex subject, consider this: IEEE drafted a recommendation in 1981 in which power providers in the United States would have to provide a voltage source without any harmonic distortion. It recommended holding total harmonic distortion of voltage to a level less than 5% of the value of the fundamental frequency. It was a sure bet we would load our electrical grid with computers and control devices having varying levels of tolerance to a distorted voltage. Therefore, limiting distortion meant protecting equipment.
You know the rest. When the utilities realized they were being held to a standard, but that load-demanded current distortion contributed to voltage distortion, the document was rewritten. The 1992 revision incorporates recommended levels for current distortion, measurable at a point of common coupling (PCC). Instantly, the users of energy were equally responsible for power quality.
There were many reactions to the publication of IEEE-519-1992, and this major responsibility swing. First, power quality "gurus" (so-called specialists that mixed math, magic, and salesmanship) appeared everywhere. They arrived at industrial facilities with meters and probes and recommended special transformers and black boxes. They charged big consulting fees and gave seminar, after seminar, after seminar. They sold tons of hardware to reluctant but fearful power users.
Then, providers of solid-state electronic equipment like drives began to see specifications requiring their equipment "meet IEEE-519." In fact, the recommendation as written does not include any equipment-specific distortion level. Drives, which provide productivity improvements and energy efficiency gains, came under heavy scrutiny. Arguments ensued as well as position papers. Specifiers and sellers pointed fingers, and customers remained confused.
It may be that the United States AC drives market, which was on pace to double in three years, with a starting value of $500 million, was in fact slowed by the hype surrounding power quality. In addition, if the power quality equipment providers sold, say, $50 million in equipment, it might be safe to speculate the drives market slowed by that same dollar amount. Imagine, variable speed drives for motor control, which show an immediate pay back to the user and allow the utility to stretch its resources, were avoided. Instead, users bought assorted facility filtering schemes and spent huge consulting dollars, without any promise of pay back.
Now hold on, you say. Aren't drives significant contributors of harmonics? Shouldn't we worry that too many harmonics in a system might lead to shutdown; or a burning transformer; or failed power factor correction?
That depends. Consider these two very different scenarios. There is a plant engineer employed by a dynamic, cutting-edge silicone wafer manufacturer. The purpose of our introduction was to discuss how he could enjoy the benefits of speed control on the dozens, if not hundreds of fans and pumps delivering clean air to clean rooms where chips are grown, all while controls and computers receive pure sine wave voltage. This writer has heard more rationale for harmonic control than he cares to admit. But this guy finally made it all come together. He understood those perfect sweeps of voltage had a direct correlation to his plant's output. Clean power, he said, ensured a high-quality product, with no downtime. He chose to incorporate 12-pulse technology into his specification for all AC drives. The greater investment in 12-pulse technology will fade as the plant churns out product and brings in profits. The equipment providers that bid for his work will face a challenging specification that stretches the technical competence of their gear and their application engineers. And the cost of chips will continue to go down, as productivity is increased.
Known to this writer is an engineer on the East Coast who designs commercial buildings. Architects, and their electrical and mechanical engineering specialists, make sure customers receive high-quality designs by maintaining and adhering to a state-of-the-art library of recommendations and standards. There is more than one engineer, this guy included, who has read IEEE-519 and determined the lowest levels listed in the standard's table 10.3 (See Table above) must, in fact, be the safest standard to write into their specifications.
The difference between these two applications is clear. The first is a perfect example of purposeful engineering, with clear objectives for performance and cost. The second is an unfortunate misinterpretation of a well-intended recommendation.
Unfortunately, when the type of limit noted in the second scenario is mandated, most good intentions are lost and costs go up. Either a single niche-oriented provider writes its own ticket, or competing hardware providers charge much more for non-standard products. The cost increases created by these mandates are ultimately carried by the end-user. And, if the job is municipal, the "end-user" is you and I; we pay the bill with tax dollars.
I have seen drive manufacturers scramble to deliver expensive alternative topologies, feign misunderstanding of the intent of the specification, or walk away from the job all together. Instead of receiving an energy-efficient building on time, the user often has to contend with construction delays and bickering before receiving the building keys. And, a bad taste about drives remains long after.
There's a balance that's not hard to achieve. This writer suggests: * You should weigh harmonic distortion control measures, considering hardware and installation costs, combined with the long-term benefits harmonic control creates. * Hold the cost of the harmonic control devices or methods to reasonable limits by smart engineering from the start of any project. Consider the following graphs (In the print version of the article). Fig. 1 shows current and voltage distortion levels using varying drive topologies or filter products versus the installed cost of the equipment (including the drive). Fig. 2 summarizes the same information and weighs the per-dollar cost of peripheral harmonic control equipment (or drive topology enhancements) versus performance. The lower the "performance cost factor" (a ratio of harmonic mitigation versus cost), the more cost-effective the option. For instance, the current distortion performance factor for an AC drive with a drive-applied harmonicfilter is 20. The same ratio for an 18-pulse drive is 39. Though the 18-pulse drive might have other performance or feature advantages not related to harmonic distortion control, it ends up nearly twice as expensive to control harmonics with an 18-pulse drive for the amount of mitigation realized.
Our friend at the wafer manufacturer went through a similar exercise before he made his choice.
What are we trying to say? We're saying remember the original intent of IEEE-519: to limit voltage distortion. Its purpose was not to slow the positive potential of the use of AC drives. AC drives either improve productivity or save energy. They are not power quality "problems," as some have portrayed them to be. Competition and improvement have brought AC drive prices to levels that make the energy savings opportunities too good to believe and impossible to ignore. Taking advantage of the benefits they bring should be your primary focus. I'm sure those that rode the post IEEE-519-1992 power quality bandwagon never intended to do damage to the potential of drives. All told, the hype around it did create a demand for education, which is, in this writer's view, the best way of addressing this complex subject.