Tuning Out Harmonics
It isn't infrequent that the best laid power schemes are foiled by power-quality problems, namely harmonics. This month's panel—two electrical engineers and two manufacturers—discuss the origins of harmonics, how to proactively prevent these problems and how to mitigate their effects. CONSULTING-SPECIFYING ENGINEER (CSE): How serious is the harmonic distortion problem in electrical ...
It isn’t infrequent that the best laid power schemes are foiled by power-quality problems, namely harmonics. This month’s panel—two electrical engineers and two manufacturers—discuss the origins of harmonics, how to proactively prevent these problems and how to mitigate their effects.
CONSULTING-SPECIFYING ENGINEER (CSE): How serious is the harmonic distortion problem in electrical systems and equipment these days?
FLICKINGER : The utilization of non-linear power conversion devices within buildings has not diminished, and is still the primary source for harmonic distortion. The proliferation of variable-frequency drives (VFDs), uninterruptible power supplies (UPS), PCs, laser printers, electronic ballasts and other devices that require DC power derived from an AC source—such as switched pulse-mode power supplies—all generate harmonics.
CSE: What promising technologies are addressing this?
VANDYNE : The 12- and 18-pulse drives, instead of 6-pulse, and the installation of passive, as well as active filters.
CSE: Can you expound on that?
HOUDEK : Active filters, which are constructed using electronic components such as insulated gate bipolar transistors (IGBT), monitor and analyze harmonic content and then electronically generate a counter, or inverse harmonic. Due to their relatively high cost, they are often installed at the service entrance, transformer secondary or distribution panel, rather than at individual loads.
Passive filters, on the other hand, use inductors, capacitors and resistors to form a filter network that either blocks the flow of harmonics or creates an alternative path.
CSE: How is this different from the way filters were used previously?
HOUDEK : In the past, harmonic filters were typically tuned for a specific harmonic frequency and multiple units were applied to reduce harmonics to acceptable levels, but only after a fairly complex analysis to determine the magnitude of harmonic and system natural frequency. Advances in both low-pass filters and IGBT-based active filters have simplified the application of filters to building electrical systems. Today, low-pass filters attenuate every harmonic frequency without the need for extensive analysis, even to the extent of offering guaranteed results.
VANDYNE : And at the plant level, where power-factor correction capacitors have been installed, we are now recognizing the implication of the tuned circuit that is created. Though no significant non-linear loads exist within the facility, typical daily transients created outside the plant are amplified by the tuning and can create power-quality related problems. As a result, the power-quality community is moving toward tuning all power factor correction capacitors.
HOADLEY : From a manufacturer’s standpoint, we are seeing more inquiries and specifications from consultants requesting that new equipment incorporating adjustable-speed drives for AC motors meet IEEE Standard 519-1992. The biggest change is that in the specifications, the point of common coupling has been moved from the location of the power meter to the terminals of the drive. This requires some form of harmonic mitigation because most AC drives use a simple three-phase diode bridge for the converter. The solutions include the addition of DC link chokes within the drive, AC line reactors, various harmonic filters, multi-pulse—12-, 18- and 24-pulse—phase-shifting transformers and converters, and synchronous converters.
CSE: It is often said that the first step in harmonics mitigation is a thorough assessment of electrical loads in a building. Briefly outline the essential requirements of an effective program for identifying and solving harmonics problems.
VANDYNE : The first step is to not just assess electrical loads in the plant, but to assess the electric system itself. In a rush to provide for an expansion, engineers sometimes overlook basic analyses, such as voltage drop calculations, motor start analyses and short-circuit studies.
Often, power-quality problems are the result of ignoring the basics of good electrical design. Although power-quality problems often come to light shortly after the application of new technology, most of the time it is not caused by that new technology. It simply acts like a canary on the power system: the first to be affected by the problem, and bring it to light.
FLICKINGER : For existing facilities, a site survey is essential to identify any significant harmonic distortion and its sources. Field measurements at each panel board are still the primary method for identifying harmonic sources. In recent years, manufacturers have developed multitesters, specifically designed for the measurement of harmonics. Not only do they graphically represent harmonics, they have the capacity to capture waveforms and generate printouts of the information.
Several products on the market can be used to treat harmonics problems, but they are not all the same. Most methods used in the past have only treated the symptoms. For example, oversized neutral conductors and k-rated transformers are built to withstand the heating effects of harmonics, rather than reduce or eliminate the actual harmonics. Harmonics cancellation transformers and filters, on the other hand, actually mitigate the effects of harmonics by eliminating them close to the source.
CSE: Are harmonics a challenge for the consulting engineer, or is it more an issue for plant engineers and other technical personnel?
VANDYNE : Since good basic design is the first step in dealing with harmonics, the consulting engineer does play a role in the design phase. Concentration on fundamentals, such as voltage drop and motor starting, can go a long way toward mitigating problems down the road. Further basic steps, such as 200% rated neutrals to, and in low-voltage panels, and filtering for any power-factor correction capacitors, should be part of the design.
The M/E engineer also needs to have a better awareness of the production equipment that is to be installed by the client and ask questions like: During voltage drop studies, what minimum voltage must be maintained for the production equipment?
HOUDEK : It is typically more cost effective to consider power quality in the system design phase than to add this equipment at a later date, especially if the system power quality causes interference or damage in the meantime. Typically, the design engineer analyzes the loads associated with the new construction and considers the power-quality requirements and mitigation techniques to be employed. Once completed, the plant engineer typically maintains clean power as non-linear loads are added, as linear loads are replaced with new technology or as linear loads are simply removed from the system. This practice typically involves adding mitigation equipment at the source of harmonics.
CSE: How does one take a preventative approach in the design phase, both for new and retrofit projects?
HOADLEY : With the increase in the awareness of harmonics, the emphasis has changed from solving harmonic problems to preventing them. The first step is to “check your pulse.” With the easy availability of power-quality monitoring test equipment, plant engineers should:
Determine the point(s) for common coupling.
Obtain a harmonic picture of present operating conditions.
Understand how the harmonics change over several months.
Understand what equipment is creating the harmonics and their spectrum.
Document the notches and distortion of current and voltage waveforms.
Determine that power-factor correction capacitors and/or harmonic filters are not being overstressed.
HOUDEK : It is quite simple to take preventive measures against harmonics. In general, each VFD installation should include a 5% impedance line reactor. This will achieve 28% to 35% total harmonic distortion (THD) under full load conditions.
FLICKINGER : Obviously, it is more desirable to have the inherent design of the electrical distribution system incorporate the necessary components for the mitigation of harmonic sources. This is accomplished by proper specification of filters for VFDs, low-THD electronic ballasts, k-rated transformers and oversized neutrals.
CSE: What about active harmonic filtering? Has this proved an effective method?
HOUDEK : Active harmonic filtering techniques have become more readily available and economical, and the same can be said for passive filtering technology, which has advanced significantly in recent years with the introduction of cost-effective low-pass filters. However, due to the number of electronic components involved in the construction of active filters, it remains doubtful that their reliability can approach that of passive filters.
FLICKINGER : Active harmonic filters are more expensive, but they control the waveform distortion more efficiently by dynamically injecting current back into the electrical distribution system. The active system responds quickly to current waveform excursions and generally is able to keep THD to under 5%.
VANDYNE : Active filtering has proven effective where there is a high level of harmonic distortion, but there needs to be a relatively good power factor. Additionally, a high degree of filtering is needed due to a relatively weak power system. Traditional capacitor-based filters will not suffice because the capacitor controller only responds to a kVAR load on the system, and operating in the lead can create a high-voltage condition.
In these cases, the engineer can utilize 12- or 18-pulse technology, or 6-pulse drives with active filtering. Cost and space considerations drive the decision from this point, as 12- and 18-pulse drives are significantly more expensive, and require substantially more floor space.
HOADLEY : In my opinion, the key advantage of active harmonic filtering over passive is the realization that future changes in the power system will not require another harmonic study to determine whether harmonic-resonant frequencies may be present.
CSE: How does the strategy for transformers change with different types of facilities? For example, how would an engineer weigh the advantages of k-rated vs. de-rated transformers?
HOUDEK : K-rated transformers are probably better suited for applications where non-linear loads are expected, because not only can they withstand the additional heating effects of harmonics, they also include oversize neutral conductors—important when supplying single-phase loads from four-wire transformers.
VANDYNE : From a strictly technical perspective, a de-rated transformer will perform the same as a k-rated transformer, faraday shielding aside. Two problems can drive the decision toward using k-rated transformers. Often, the inspecting authority may demand k-rated transformers as a strategy for accommodating harmonics over simple de-rating. Although the de-rated transformer will perform adequately, the plans examiner and inspector have authority to demand k-rating. The feeling is that down the road, facility personnel may inadvertently overload the transformer by loading to nameplate, rather than the de-rated capacity.
When the k-rated transformer is used, there is no misunderstanding about the capacity of the equipment because the nameplate reflects the actual conditions. Further supporting the use of k-rated units is the presence of shielding that can further improve power quality on the load side of the transformer.
HOADLEY : If the harmonic current for an application exceeds 5% of the rating of the transformer, the heating effect in the transformer should be evaluated by the engineer, and he or she should consider the use of a k-rated transformer for that application.
FLICKINGER : If the facility is to be used exclusively for computer-type applications—offices, schools, laboratories, hospitals, courthouses, detention facilities—one will utilize k-rated transformers for all of the panel boards serving computer loads.
For manufacturing applications, more of the electrical load is purely three-phase, so the need is more toward power delivery than harmonic mitigation.
CSE: It would seem that in the vast majority of cases, harmonics issue from within the affected facility. Describe any cases where a harmonics problem came from the utility, or in which the utility helped solve the problem.
VANDYNE : Often, these cases are similar to the case of the chicken and the egg. The conditions for harmonic problems are created by one entity and set off by another.
In one case, the utility had in place power-factor correction capacitors with no consideration for the harmonic resonance conditions that were created. Under no load, the voltage distortion was nonexistent, but when the customer’s welders operated, substantial harmonic transients were created throughout the system.
In another case, a utility installed a capacitor at a point on the system where the system impedance, coupled with the impedance of the capacitor, resonated directly at the fifth harmonic. When the down-stream customers drew a relatively minor amount of 300-Hz current, the voltage distortion increased to unacceptable levels. The solution was simply to relocate the capacitor bank.
FLICKINGER : The solution for a large manufacturing facility—with 30-megawatt load of induction furnaces and arc welders, located adjacent to a business park—was to serve all mission-critical loads in the business operation with uninterruptible power supplies. This did not, however, completely eliminate the voltage flicker in the lighting system. Eventually, with the involvement of the electrical utility, the manufacturer corrected the voltage waveform clipping at his location.
CSE: Any predictions for the future, such as harmonic mitigation technologies that haven’t yet been proven, but are in the research and planning stages?
FLICKINGER : As new solid-state power supplies are developed for computers, printers and laser copiers, one would expect a decline for need of aggressive harmonic mitigation since there will be fewer harmonic sources on the electrical system. I am not certain that improved power supplies alone will preclude the need for some harmonic filtering; however, as more and more VFDs and electronic ballasts are specified with filtering, one certainly could surmise that there would be less of a need for harmonic filtering of electrical distribution systems.
VANDYNE : Refinement of emerging active filtering technology into standard 6-pulse drives may help control harmonics at low-voltage levels. The question is whether the market will support the premium demanded by the enhanced drive.
HOADLEY : As power semiconductor costs continue to decrease, I see a growth in the use of synchronous converters. Advantages include low harmonics when utilizing a high carrier frequency and small filter; the ability to maintain a constant DC bus voltage under voltage-sag or low-line conditions, or to maintain rated DC-bus voltage for full pulse-width modulation voltage utilization at full speed; the ability to regenerate power from the rotating load to the power lines; and the ability to operate at unity power factor, at a slightly leading or lagging power factor.
Thomas Flickinger, P.E., CSI , Managing Principal, Durrant, Madison, Wis.
Rick Hoadley , Technical Program Manager, Custom & Configured Drives, Rockwell Automation, Mequon, Wis.
John A. Houdek , Vice President, Marketing & Sales, MTE Corporation, Menomonee Falls, Wis.
David A. VanDyne, P.E. , President, IDC Engineering, Inc., Lima, Ohio
Keeping Harmonics out of Harm’s Way
One harmonics case involved the application of filters on a facility’s power-factor correction capacitors. The client had a medium-voltage capacitor bank that switched with load, and transients were present in the system.
The initial thought was that the switching of the capacitor was creating the transient, but after metering the system, it was discovered that the transients were occurring without capacitor switching. An analysis of the circuit revealed that the three-step bank resonated at the 11
The answer was to remove the medium-voltage capacitors—there was little room in the substation for the medium voltage reactors that would have been required to retain the capacitors at this level—and install tuned low-voltage capacitors at the 480-volt level.
College Cures Harmonics
In 1997, Madison Area Technical College’s Truax Campus in Madison, Wis., commissioned a complete analysis of the entire electrical system. In the study, a serious harmonics problem, which is quite common when numerous personal computers are powered on one system, was discovered.
The 850,000 sq.-ft. campus had originally used a single mainframe computer connected to several hundred terminals. This operation was served by a power distribution system of two medium-voltage unit substations supplying six power centers. The power centers were used to serve multiple stepdown transformers, which in turn distributed 120/208-volt power to receptacle loads.
In addition to the current load, the college had purchased $4 million worth of computers, which were being stored in a warehouse until the campus had additional power circuits to operate them, so this drove the need for an expedient solution.
The engineers’ analysis of the conditions indicated that the 120/208-V electrical distribution system had significant harmonic current in all three-phase conductors and the neutrals of the circuits, which serve computer loads. The solution for new panel installations was a harmonic cancellation transformer, which would not adversely affect the system as the k-rated transformer does.
For the existing panels supplied by standard transformers, a harmonic cancellation filter was installed near the panel. These harmonic cancellation filters were designed to provide the ultra-low zero-sequence impedance parallel path to remove the harmonic currents. The resultant harmonics are then contained within the reactor at the specific panel board and do not migrate at the power distribution system, thus eliminating panel board neutral heating, feeder conductor heating and stepdown transformer heating. The engineering team also installed four new harmonic cancellation transformers and added 22 harmonic cancellation filters at existing panel locations.