Variable speed drives: a question of harmonics
11 January 2015
Automation component manufacturers aim for IEEE 519 compliance to guarantee that their products do not create harmful harmonics. But what does the IEEE 519 actually mean for buyers, particularly buyers of used, obsolete equipment? Jonathan Wilkins reports.
IEEE 519 was introduced in 1981 (and later revised in 1992) as a point of reference on the types of harmonics introduced by static power converters and other nonlinear loads. The objective was to create universally applicable power recommendations so that the negative effect of some harmonics could be easily avoided.
The standard basically applies to anything that moves; pumps, fans and exhausters, compressors, mixers, agitators, conveyors, cement kills, mills, refiners, banburys, extruders and many other types of manufacturing equipment.
However, there are two issues that concern engineers when applying the IEEE standard to variable speed drives which are not entirely clear in the guidelines. The first challenge is determining an appropriate point of common coupling. The second issue is how to accurately establish a demand current at the design stage.
In the last decade, VSDs have become crucial for almost all industry sectors. For instance, in the petrochemical sector, they play an essential part in saving energy for pumping applications. In the built environment they are the foundation on which all air handling rests and in food processing they control much of the equipment essential for manufacturing. But their benefits don’t stop there; they are also responsible for process control and reliability issues.
And, just as with any good money saving technology available, there is a downside to integrating VSDs.
VSDs draw current in a non-sinusoidal manner, rich in harmonic components which in turn introduce harmonic distortion to the power system. The dangerous harmonics introduced via the VSD can distort the supply, overload transformers and resonate with power factor correction capacitors.
The IEEE 519 standard, also known as the ‘Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems’ – is used as guidance when trying to prevent harmonics from impacting the power supply.
Broadly speaking, the standard limits the voltage and current harmonic and is meant to be applied holistically to systems, irrespective of the type of loading - linear or non-linear.
Some engineers believe that the current harmonic limits need to be met at the terminals of the non-linear equipment in order to comply with the standard. Unfortunately, this approach demands that costly treatment equipment be retrofitted and many drives manufacturers ignore this recommendation for fear of rising overall application costs.
In the petrochemical industry, the system limits indicate that no single harmonic should be greater than 3 percent. It is advisable that these limits be met at the point of common coupling. However, even with such precautions, voltage distortions can still occur downstream and connected equipment could suffer from it.
And since voltage distortion is the result of harmonic currents passing through the impedance of the power system, voltage distortion will always be higher downstream.
Crucially, if you need to specify non-linear equipment for a medical application, bear in mind that patient safety may be at risk, as indicated in section 6.6 of the standard:
“Computers and allied equipment, such as programmable controllers, frequently require AC sources that have no more than a 5 percent harmonic voltage distortion factor, with the largest single harmonic being no more than 3 percent of the fundamental voltage.
Higher levels of harmonics result in erratic, sometimes subtle, malfunctions of the equipment that can, in some cases, have serious consequences. Instruments can be affected similarly, giving erroneous data or otherwise performing unpredictably. Perhaps the most serious of these are malfunctions in medical instruments.”
When analysing performance and reliability in the context of IEEE 519, it is not only the VSDs that need attention. Good engineering practice suggests that the entire power system needs to be looked at. Onsite performance testing can be part of the maintenance programme for mission critical systems such as petrochemicals processing or life-support equipment.
Harmonic distortion is a complex issue and it’s made yet more difficult when the device you are fitting is no longer manufactured - obsolete equipment in other words. In such instances, you need to be certain of the provenance of the equipment and of its repair history. A drive acquired in this way must precisely replicate the performance of the part it’s replacing – something a reputable supplier can certainly help you with.
It’s not uncommon for grey or black market products to be passed off as the real thing, even in Europe’s highly regulated and tightly managed engineering market. Again a good supplier will be able to provide adequate paperwork to prove this isn’t the case.
Equally, with obsolete inverters, any associated motor would have to comply with the European Design Directive as well as IEEE 519; which means that products that have been extensively repaired and re-sold may create problems. They could be considered to be new introductions to the market and thus non-compliant unless they were energy efficient at the point of manufacture.
Returning to IEEE 519, harmonic distortion issues can be expensive and take up a lot of time to fix retrospectively. As a result, plant managers who want to comply with IEEE 519 should start the conversation earlier, at the design stages. This way they will benefit from the energy savings achieved by implementing a VSD without incurring power quality and safety issues further down the line. It is a delicate balance, but IEEE 519 can help guide plant managers in their quest for reliable energy saving.
Jonathan Wilkins is with European Automation, a specialist supplier of used, refurbished and obsolete industrial automation equipment
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