Device level, systems level harmonic mitigation solutions

Rudy Wodrich of Square D explains the differences between device level and system level harmonic mitigation solutions, their characteristics, their pros and cons and why you should understand them.

By Rudy Wodrich, Schneider Electric September 5, 2007

In general, device level solutions are attractive to consulting engineers because they can force each equipment vendor (UPS, VSD, etc.) to comply with harmonic limits at the point of connection of their individual piece of equipment. However, the shortcoming of device-level solutions is that the individual vendors may comply with the specification in different ways and,

For example, on a recent institutional building project, the consultant had specified harmonic limits for individual equipment. The uninterruptible power supply (UPS) vendor for the computer lab UPS complied by providing a 5th harmonic passive filter on the input of the UPS. The variable speed drive (VSD) vendor for the elevator drives managed to convince the consultant that his small VSDs should be exempt from the requirement. The vendor for the HVAC equipment provided 6-pulse drives with 3% line reactors to comply with the harmonic limits. A third VSD vendor was selected for process equipment drives used in a laboratory and implemented broadband filters on these devices. Finally, a separate section of the specification required that there be automatic power factor correction equipment installed on the service entrance. The vendor of the
distribution equipment supplied this equipment but without taking into account the possibility of harmonics on the network. Two negative interactions took place.

First, the switching of the power factor correction capacitors had a tendency to cause voltage transients and cause the elevator drives to trip on an‘overvoltage’ alarm. This could have been avoided if the consultant had insisted on making the elevator vendor provide line reactors in front of his VSDs. In addition to reducing harmonics, line reactors provide a measure of transient protection to the drive. Second, the harmonics had been reduced by all the measures implemented at the device level (line reactors, 5th filters and broadband filters), but the capacitor bank created a sharp resonance at the 11th harmonic with the upstream transformer. As a result, the 11th harmonic current was magnified dramatically and the capacitors failed prematurely after only six months in service. During this resonance, the harmonic levels at the Point of Common Coupling (PCC) were approaching the limits outlined in the IEEE 519 guidelines.

Furthermore, the harmonic standards (such as IEEE 519 in North America or G5/4 in the UK) used by the electrical utility are enforced not at the device level, but rather at the PCC. It is possible to comply at the device level but, due to interactions between loads and even parts of the medium voltage network, not comply at the PCC. Only a detailed network study and simulation can ensure that these types of problems are avoided.

Systems level solutions
It is often far more economical to take a systems level approach versus a device level approach. A systems level approach can take into account load diversity and natural harmonic cancellation that can occur due to phase shifting within the electrical network.
The two most common examples of systems level solutions are passive filters and active filters. Systems level passive filters are quite similar to device level tuned filters. They employ stages of reactor/capacitor usually switched with electromechanical contactors and controlled by a microprocessor that energizes the stages based on reactive power requirements — in other words, power factor. Passive filter design requires accurate load data and usually involves some kind of complex and time consuming computer modeling to ensure adequate removal of harmonics (compliance with standards), and that the filter itself will not be overloaded. System models must also take into account if the network can be fed from backup generators, as passive filters will behave quite differently in this instance.

Most system level passive filters are designed to remove a portion of the dominant 5th harmonic while avoiding resonance. It should be noted that system level passive filters need to be carefully designed to ensure long-term stability of the passive components. A passive filter that is under-designed could overload the capacitors, causing them to decline prematurely. As this occurs, the 5th harmonic filter can become a 5th harmonic amplifier — exactly opposite to the desired effect. Given this limitation, it is also possible for future load changes with higher harmonic levels to catastrophically affect the passive filter and once again have end users exceeding harmonic limits at the PCC.

Passive filters also provide the side benefit of correcting system displacement power factor, which may or may not be required depending on the load conditions. For example, a load which is predominantly ac VFDs requires harmonic filtering but does not need any displacement power factor correction since ac VFDs have displacement power factor in excess of 90%. In this instance — and many others — it may be more attractive to use an active filter.

Active harmonic filters
Active harmonic filters are sometimes called active power line conditioners. Rather than block or shunt harmonic currents, active filters attempt to condition them. Active harmonic filters monitor and sense harmonic currents and generate corresponding waveforms to counter the original harmonic currents. The generated waveform is injected back into the electrical supply to cancel the harmonic current generated by the load. The advantages of active filters include:
• Simple design tools — no complex computer modeling required
• Guaranteed compliance with harmonic standards — treats all harmonics from H2 up to H50 (some brands)
• It is impossible to overload an active filter
• Multiple units can be paralleled together for larger power applications or as load increases in the future
• Active filters can be programmed to ignore or to correct displacement power factor as well as harmonics
• Active filters can be used with all types of harmonic-producing loads: UPS, VSD, rectifiers, etc.
• Active filters work equally well on utility or generator-fed networks (their function is not impedance-dependent).

There are some simple rules for safely using active filters, including the need for 3% or greater line reactors on most harmonic-producing loads. Also, there are special precautions that must be taken when applying active filters in conjunction with capacitor systems or any stray network capacitance. However, if these guidelines are followed, the active filter is by far the simplest and most effective means to attack harmonic problems.

So what is the right solution for your electrical network? The answer is — it depends. There are many factors to be considered when tackling a harmonic mitigation project. A device level approach may be right for you or it may be necessary to take a holistic approach to the entire system. The good news is that there are experts out there who can help you analyze both the technical and economic aspects of the available options and come to an educated decision.

Rudy T. Wodrich is the director of the Power Quality Group of Schneider Electric / Square D in Toronto, Ontario. He has worked in the power quality field for more than 15 years. His team has the responsibility to solve power quality problems relating to flicker, harmonics, voltage sag and power factor.