VFDs, harmonics drive power quality debate

This month's panel of experts discusses how to mitigate harmonic effects, passive and active filters, and technological advances with these systems and VFDs.

03/01/2008


This month's panel of experts discusses how to mitigate harmonic effects, passive and active filters, and technological advances with these systems and VFDs.

CSE: In the design phase, what ensures that variable frequency drives (VFDs) don't cause harmonic distortion?

JIM NASH : The first step is to analyze what portion of the total connected load VFDs represent. Most commercial and industrial installations tolerate up to 20% drive loading before additional hardware needs to be considered. This assumes that the drives to be installed have at least 3% ac line reactance or dc link reactance as a part of their standard designs. If the drives don't include internal reactance, then the system tolerance level drops to about 10%. Likewise, special locations, such as airports and hospitals, have lower harmonic tolerance levels where 10% loading may represent the permissible threshold even when drive designs do include reactance as standard. If it is known that these levels will be exceeded, then a formal harmonic analysis should be made. The drive vendor may be able to assist in providing a first order analysis that includes harmonic mitigation recommendations. If considering this approach, be prepared to provide a network one-line diagram that includes both expected drive loads and other loads. Keep in mind that accurately establishing the ratio between nonlinear loads—such as VFDs—and linear loads (across-the-line motors, incandescent lighting, etc.) is vitally important to accurately predicting harmonic content. Establishing the kVA and impedance of the utility transformer (or generator) also is necessary. Finally, the short circuit capacity of the utility feed at the primary of the utility transformer should be obtained from the utility. The appropriate harmonic solution will vary based on various installation factors. If the level of excess harmonic content is relatively small, the addition of simple passive filters will probably be most cost-effective. If the level of excess harmonic content is high and many drives are involved, installation of an active filter will likely be most appropriate. Finally, if the level of harmonic content is high and only a small number of large drives are involved, then installing drives with active front-ends probably is the best solution.

SYED PEERAN : Much depends on the distribution system. Smaller drives, typically less than 50-hp rating, can be six-pulse with passive filters. Larger drives can be the harmonic-free type or the 18-pulse or higher pulse drives. Still larger drives can be the active front-end type drives. One must select the most cost-effective combination. Satisfactory harmonic performance must be verified by a comprehensive harmonic analysis.

KEN CARR : If the VFD is installed in an existing facility, there are several options to minimize harmonic distortion, in increasing order of benefit/cost: An ac line reactor: Specifying a 3% ac line reactor at the input to the drive usually will reduce the total harmonic current distortion (THID) to approximately half of what it would be otherwise. Harmonic filter: Consider a passive, resonant-based harmonic filter that effectively traps the lower current harmonics, such as the fifth, seventh, 11th, etc. Isolation transformer: Installing an isolation transformer to supply the VFD will reduce the primary-side current harmonics sufficiently to meet IEEE 519 at the point of common coupling. Active filter: Installing the right active filter at the input to the drive can reduce the THID to as low as 3%. Installing VFD in a new facility, there are several other VFD options available that will mitigate harmonics right out of the starting gate: 12- or 18-pulse rectifier front-end VFD. This VFD has either 12 or 18 rectifiers in its front end, which eliminates the lower-current harmonics, such as the fifth and seventh, allowing only the 11th for the 12-pulse, or 17th for the 18-pulse. Active switch rectifier VFD: An active front-end VFD uses insulated-gate bipolar transistors (IGBTs) or other semiconductor switches and pulse-width modulation techniques to shape the input current waveform sinusoidally with less than 5% THID, and control input power factor to any desired value between 0.8 leading to 0.8 lagging. An added benefit is that it can also convert mechanical energy into electrical, and return it to the ac input lines or power mains.

KEN LOVORN : The two key methods of harmonic mitigation in VFDs are input line filters and higher pulse count. However, it should be noted that in the instance where the distribution system is large in comparison to the size of the VFDs, only minor current harmonics will be generated. The pervasive and damaging portion of the harmonics are the voltage harmonics, which are mitigated when the distribution has a large available fault duty and the VFDs are small in relation to the system size.

CSE: For existing facilities that have a problem with harmonics caused by drives, what can consulting engineers do to help facilities assess electrical loads in a building to solve harmonic problems?

CARR : Generate a list of questions, such as: When did the problem occur? Did anything change in the facility recently that coincided with the problem showing up? Does the problem go away when a particular drive is shut down? Does the problem appear to be related more to the power input to the VFD or to its output? Is the problem more severe when the VFD/motor load is loaded versus unloaded?

PEERAN : First, the situation would be assessed by field measurements and by a harmonic analysis. If it is found necessary to mitigate the harmonics, several options such as replacing the older drives with 18-pulse drives or installing harmonic filters would be evaluated.

LOVORN : The expensive answer is to have a harmonic study performed at the main electrical service, each motor control center, and each distribution panel. However, a quick and dirty method is to measure the current with an averaging ammeter and then measure it with a true root mean square meter. If the two numbers are relatively close, there are no significant harmonics present. If the two numbers are more than 15% or 20% apart, then there is a potential for current harmonics. The same technique may be used to determine whether voltage harmonics are present in a system.

NASH : Sometimes the source of excessive harmonic content is obvious. For example, there may be only one large VFD present. Often, however, VFD loads are distributed throughout a facility and it is the combined effect of many non-linear loads that result in an unacceptable harmonic situation. In such cases it is difficult without a formal analysis to determine how to best address the problem. Passive and active filter vendors often will agree to provide a free harmonic analysis with recommended solutions. The only other approach that will achieve reliable results is to formally contract with an electrical consultant who specializes in harmonic analyses. Though more costly, this approach has the advantage that such consultants have broad exposure to many different solution techniques and are more likely than an individual vendor to provide a recommendation that optimizes both the technical and cost factors involved.

CSE: Are active and passive filters necessary in some cases? What are some of the recent technological developments in the area of harmonic filters?

PEERAN : Active filters have not had much success in the industry. Passive filters, however, find use in several situations, particularly in existing systems when new VFDs and nonlinear loads are added. The passive filter is simply a reactor-capacitor combination tuned to a particular harmonic frequency usually slightly lower than the expected harmonic frequency. The filter also offers power factor correction because it simply acts like a capacitor at power frequency. To prevent over-correction of power factor at low loads, it is better to use an individual passive filter for each VFD instead of a large centralized filter.

LOVORN : The requirement for filters has changed over the past few years. Previously, if you had a VFD, which was normally a six-pulse drive, you had to have some type of filter to prevent interactions between drives and to prevent disruption of the sensitive electronics in the rest of the facility. Today, 12- and 18-pulse drives, which generally do not require filtering, are relatively common and in many cases, are more economical than the six-pulse drives with the additional filters.

CARR : It may be helpful to differentiate between the two principal types of harmonics and the filters used to mitigate them. Drive designers and application engineers more and more are looking to power quality standards, such as IEEE 519-1992, for guidance with respect to the need for filters on the inputs to VFDs and adjustable speed drives. The allowable harmonic limits depend on the impedance of the power source, whether it be the utility or a local generator, on most VFD applications. Significant developments and advances have been made in the area of capacitors, magnetics, fast-switching power semiconductors (such as IGBTs), micro-controllers, digital signal processors, memory devices, and chips. All of these are used in active filters, and even passive filters such as harmonic (series L-C) filters have benefited greatly from new processes in polypropylene capacitor film, powdered-iron, ferrites, grain-oriented steels, etc.

NASH : The majority of drives sold today employ six-pulse pulse-width modulation technology. For practical cost reasons, this is likely to remain the case into the foreseeable future. Whenever such drives represent a significant portion of the total connected load (20% or more), some form of harmonic filtering needs to be considered. Besides insuring compliance with harmonic standards, filters improve system power factor and increase available transformer capacity. Passive filters have evolved from simple trap filters to include broadband filters and hybrid filters. These provide significantly better performance than older designs. Active filters are more sophisticated. Users now have the flexibility to determine filter set-up and functional emphasis through modification of digital parameters.

CSE: How does the location of filters affect how the generator reacts to VFDs in the system?

PEERAN : Filters are generally located either at the VFDs or at a switchboard or switchgear that feeds a number of VFDs. When the system is fed by the standby generator, harmonic distortion would be greater because of the larger impedance of the generator as compared to that of the utility transformer. Therefore, the harmonic filter is a little less effective with the generator operation. Irrespective of the utility operation or generator operation, it is best to locate the filter close to the harmonic source—i.e., at the VFD.

LOVORN : We believe that the filter should be located as close as possible to the VFD, even in the same enclosure. This prevents voltage harmonics from leaving the immediate vicinity of the VFD, thus not impacting the generator operation. Most generators are now permanent magnet generators that derive their voltage regulation from the magneto on the generator shaft, which has no electrical connection to the generator output, thereby lessening the impact of the VFD on the generator output.

CARR : Placing the filter as close to the VFD as possible also minimizes the wire length between the two. This wire should be shielded VFD-type cable to prevent electromagnetic interference (EMI). Failure to adequately contain high-frequency EMI at the source allows it to be radiated and picked up by power cables and control cables, and ultimately make its way to the generator or utility mains via alternate current paths.

NASH : Utility grids are so massive that individual VFD loads, even though they introduce harmonic content, do not upset the basic generation and regulation process. Local generators, on the other hand, are often of the same size, or only slightly larger in size, than the VFD loads connected to them. The stability of the generator's voltage regulator must be taken into consideration when operating with significant VFD loading (50% or more). If full generator capacity is to be used, it is normally necessary to provide some form of harmonic mitigation to insure that the effects of nonlinear current loading don't cause the generator's voltage regulator to malfunction. In all cases, filters should be positioned upstream of the drive loads so that the current loading as seen by the generator becomes as sinusoidal as possible.

CSE: What are ways you can mitigate the harmonic effects on the generator?

LOVORN : The best way is to use a permanent magnetic generator (PMG), so the voltage regulator does not have any electrical connection with the generator output. In addition, the generator manufacturers have available in-line reactors that they use in front of their more sensitive voltage regulators, but this has virtually no benefit when the generator is a PMG, since there is nothing from the generator output voltage to the regulator to filter.

PEERAN : Harmonics in the generator are of two types: internally generated harmonics due to the generator design and the pitch of the stator windings, and externally generated due to the nonlinear loads supplied by the generator. Both types cause additional heating in the generator. Predominant effect is due to the nonlinear loads. The generator must be sized to handle the expected nonlinear load. Means of mitigating such as the use of harmonic filters, 18-pulse drives etc. would also reduce the effect on the generator.



Participants

Ken Carr

Team LeaderBaldor ElectricFort Smith, Ark.

Ken Lovorn, PE

President, Chief Engineer

Lovorn EngineeringPittsburgh

Jim Nash

Principal EngineerABB Inc.New Berlin, Wis.

Syed Peeran, PE

Senior EngineerCamp Dresser & McKee Inc.Cambridge, Mass.

Ask the experts: motors, drives, and harmonics

CSE gives its readers and Web visitors the opportunity to pose questions directly to the panelists at

“We have 750 kVA generators in our press. Last week this generator malfunctioned. Meanwhile we arranged a generator of 1,200 kVA. Press operates perfectly on a 1,200 kVA generator and power utility company. We measured harmonics at load side. When load is connected to generator (750 kVA), 8.5% total harmonic distortion (THD) observed. Magnitude of third harmonic is 8.2%. When load is connected to main distribution system, THD became 2.3% with third harmonic 0.108% and fifth harmonic 2.077%. My questions: Why does the press operate perfectly at 1,200 kVA genset and main distribution system? Why is the third harmonic more when load is connected to 750 kVA generator? How should I mitigate the harmonics?” —Atif Haider

KEN LOVORN : The harmonics are higher when connected to generators because the utility has a higher available fault duty. Based on the information available, it is doubtful that the failure of the 750 kVA generator was caused by the harmonics. If the 750 kVA generator is more than 10 years old, then it is probably sensitive to harmonically rich loads. If the 1,200 kW generator is new it could be a permanent magnetic generator (PMG) as noted. But even if it's not a PMG, by nearly doubling the size of the generator the harmonics would have much less effect than on the 750 kVA. As far as mitigating the harmonics, the metered levels of harmonics are not of magnitude that should cause problems with either the generator or any other equipment. Most electronic equipment is designed to accommodate THDs up to 10%. If harmonic rich loads are added, you should install harmonic filters sized for each of the new loads.

SYED PEERAN : Because the 750 kVA generator has greater impedance, higher THD is expected. The harmonics may have caused malfunction of the press controls. Most of the voltage distortion (8.5% THD) is the due to the third harmonic (8.2%) when 750 kVA is online. With the utility power, most of the distortion (2.3%) is due to the fifth harmonic (2.03%). This suggests that the third harmonic is created by the generator. If the generator stator winding is not 2/3 pitch winding, the generator creates considerable third harmonic voltage. You can check this by measuring the harmonics of the generator voltage on no load. There is no easy way of mitigating the third harmonic. The generator can be rewound or a zigzag grounding transformer at the terminals of the generator can bypass the third harmonic currents from the load.



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