Harmonics No More
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Increased dependence on electronics—in everything from fluorescent ballasts and desktop computers to large-scale hospital radiation suites—has created a much greater awareness of harmonic distortion and the havoc it wreaks on equipment. The good news is that tools are available to help determine exactly which pieces of equipment are being affected and to what extent. Looking at the system as a whole, we can pinpoint and address the causes of harmonics, rather than just the symptoms.
Harmonics-related problems can occur throughout commercial, industrial, and institutional facilities and include:
Operation of overcurrent devices without an overload or short-circuit condition
Random component failure in electronic devices, such as printers and personal computers
Operating problems with electronic devices untraceable to any identifiable component problems
Interaction between multiple variable frequency drives (VFDs) so that one or more drives do not follow their control setpoints
Interactions between uninterruptible power supplies (UPSs) and the emergency generator supplying power to them during extended utility power outages
System power-factor reduction, with associated distribution-system capacity loss and power-factor penalties applied by the serving electrical utility
Increased neutral currents, causing overheating of neutral conductors in panels, feeders, transformers, and other neutral locations
Problems with capacitor operation and life, such as resonant conditions, capacitor-case expansion, and capacitor rupture.
Most harmonics-related problems have one of two basic origins: current-wave distortion or voltage-wave distortion. A third factor, harmonic phase shift, results from a combination of the first two and is not as crucial. In all cases, harmonics have more impact in electrical distribution systems where there is a lower available fault duty and less impact in systems where there is a very high available fault duty.
Current-wave distortion is the most common indication of the presence of harmonics. It occurs when the amperage demand by a particular device occurs out of phase with the electrical system’s normal sine wave. Such devices as silicon-controlled rectifiers (SCRs) turn on for short periods, drawing power inconsistent with a normal current wave’s cyclical nature. Some examples of potentially current-distorting equipment include fluorescent ballasts, dimming systems, and personal computers.
While this distortion-producing equipment is more demanding on distribution networks than non-distorting loads, only systems directly feeding the harmonic-producing loads and the neutral conductors for these systems are affected. Current-wave distortion has no significant effect on other devices connected to the same distribution system.
Effects of voltage-wave distortion, however, are evident throughout the distribution system. This condition results when instantaneous current demand exceeds the distribution system’s ability to deliver power to the load.
In the best of cases, changes in the voltage sine wave are viewed by sensitive loads as very short-term low- or high-voltage conditions, such as spikes or dips, and react accordingly. In more severe cases, sensitive control systems stop functioning, resulting in total equipment failure. As a result, voltage-wave distortion has the greatest potential impact on other electrical devices, but it is less recognized and understood than current-wave distortion.
Equipment with the potential for generating voltage-wave distortion includes UPS systems, VFDs, solid-state elevator drives, arc heating units, and other devices with very large, short-term current demands.
SOLUTIONS IN THE MARKETPLACE
As awareness of harmonic-distortion problems has risen, so has the number of products for solving these dilemmas. One of the most popular approaches is the oversized neutral conductor. Electronic equipment carries with it significant potential for generating third-order harmonics—primary culprits in neutral overloads. However, third-order harmonics are not new problems for electrical-distribution systems. Branch circuits feeding high-intensity discharge and fluorescent ballasts always have required full-sized neutrals, along with dedicated neutrals for each dimmer branch circuit.
With increased recognition of triplen harmonics (see “Harmonics order: a definition,” page 8), oversized neutrals are being offered in all sorts of distribution equipment, such as armored cable (AC or MC cable per the National Electrical Code ), transformers, panel boards, and other components. Also, recommendations in current technical literature suggest doubling neutrals in feeders and branch circuits where harmonics could be present. In every case, engineers are left with the feeling that if they do not design their systems with double-sized neutrals, the neutral conductors will fail.
Other manufacturers offer k-factor transformers for applications where harmonics might occur, because standard transformers cannot handle harmonics without overheating. However, at least one manufacturer says its 150 C-rise transformer has a k-rating of one, its 115 C-rise has a k-rating of 4, and its 80 C transformer has a k-rating of 13.
Rather than looking at ways to accommodate third-order harmonics, designers and engineers would be better off employing ways to reduce or eliminate them.
For example, some electronic ballasts have total harmonic distortion (THD) of less than 20%, which is less than that of a standard magnetic ballast. Electronic ballasts that have THDs of less than 10% also are available at a slight increase in cost. It should be noted that there are electronic ballasts that have THDs approaching 150%, so merely because a ballast is electronic does not assure that it will have a low THD. Where higher order THD ballasts exist, third-order harmonic filters are available to reduce triplen harmonics to less than 10%. Such systems limit the need for increased neutrals and k-factor transformers.
While current-wave distortion can be handled with such solutions as oversized neutrals, k-factor transformers, and power conditioners, voltage-wave distortion lacks such simple remedies. Oddly enough, it appears from a brief review of published literature that voltage-wave harmonics are not a hot topic due to continued widespread lack of understanding of the causes and mitigation methods. There are two primary ways to eliminate voltage harmonics: by incorporating harmonic filters at selected locations, or by eliminating devices that produce voltage-wave distortion by purchasing devices that produce lower levels of harmonics.
In today’s commercial market, there are various sizes, types, and applications for harmonic filters, each suited for a specific situation. Harmonic filters may be single-frequency, multiple-frequency, component-specific, or general-use, each with various attenuation levels.
Filters can be applied at the device producing the harmonics, such as on the input of a VFD. They are designed to block frequencies the drive generates and prevent them from being reflected into the distribution systems. Attenuation levels depend on the available fault-duty of the distribution system but are generally designed to limit the maximum to 5% or 10% current THD into the distribution system. These designs can use either a resonant T or Pi shunt section for each frequency they are intended to attenuate.
Filters also are available for specific use and are matched to the device for which they are furnished. These models limit THD of the combination filter/utilization device to either 5% or 10% THD and are normally applied to large harmonics-producing equipment. When these filters are applied to 6-pulse rectifier input sections of UPS or VFD systems, they are significantly larger than those used with similar systems having 12-pulse rectifiers. When a VFD is applied to a large motor, the use of an 18-pulse input section in the VFD will virtually eliminate the need for a harmonic filter due to its inherit low-harmonic content.
For small-system harmonics, installing individual filters for each utilization device can be costly (e.g., providing a filter for each fluorescent ballast). For multiple small harmonics-producing units, shunt filters can be connected to a breaker or fused-switch on the distribution or branch-circuit panel feeding the equipment, and sized for the total harmonic load.
Shunt filters are typically single-frequency, band-pass filters that route all frequencies within a certain band to ground. At the inductor/capacitor combination’s resonant frequency, the filter appears to be a very low impedance to ground. However, at frequencies above or below the resonant frequency, the filter appears to be a high impedance, blocking the passage to ground of frequencies greater or less than the resonant frequency. These filters are sized to pass the anticipated harmonic current of the selected harmonic order for all harmonic producers connected to the panel.
Two additional types of passive, harmonic filters are widely available: series reactors and drive-isolation transformers. Both of these use some type of inductance insertion to reduce the level of harmonics and have been used this way for many years. While these filters provide very little reduction in the level of reflected-wave voltage harmonics and should not be used for this application, they are quite effective in mitigating the rise in voltage when the VFD is remote from its driven motor.
Other approaches that can prevent harmonics from entering the distribution system are motor-generator (MG) sets and active filters. MG sets will eliminate the passage of harmonics from the generator side (harmonics-producing load) to the motor side (distribution system) because the only connection between the line and loads sides of the MG set is the common shaft.
Active filters incorporate microprocessors to eliminate harmonics by rapidly compensating for sine-wave deviations from ideal wave forms by inverting the harmonic distortion and reinserting it into the feeder to cancel the harmonics. These models can correct all harmonic magnitudes up to their maximum capability and can eliminate harmonics concerns in electrical-distribution design.
One harmonics mitigation effort that does not work is the use of isolation transformers. These transformers have electrically isolated windings and an intervening electrostatic shield and are very effective for providing total DC isolation of sensitive circuits. However, because of their limited attenuation capabilities, they are not very effective at eliminating harmonics except for the lowest magnitudes of current-wave harmonics, which typically do not require any mitigation.
A SYSTEMS APPROACH
Without careful design considerations, harmonics can cause expensive, damaging problems. As a result, each potential harmonic producer should be investigated to determine the frequency and level of harmonics so the appropriate type of filtering may be specified.
Doubled neutral conductors and k-factor transformers should be used only as a last resort in existing installations to mitigate large triplen harmonic levels, not as a routine procedure for every facility. Judicious use of harmonic filters for either a dedicated, device-specific application or on a group basis, incorporating a single filter to handle a number of harmonic-producing loads, can be the most cost-effective way to limit harmonics in the distribution system.
|Lovorn is president of Lovorn Engineering Assocs. He has 39 years of design and engineering management experience with architect-engineers and consulting engineers designing electrical systems. His firm provides mechanical, electrical, and plumbing engineering for a wide range of projects, including health care, light industrial, historic, specialized lighting design, power distribution, harmonic analysis and mitigation, institutional, and LEED design and commissioning.|
Harmonics order: a definition
Harmonics are categorized by their order. The first-order harmonic or fundamental frequency for 60 Hz power is 60 Hz. As the order increases the frequency increases, thus a second-order harmonic would be 120 Hz, a third-order is 180 Hz, and so on.
Third-order and ninth-order harmonics also are called triplen harmonics. When these odd-numbered triplen harmonics are present on all three phases, the harmonics fall in phase. The triplen harmonics add together on the neutral conductor, resulting in three times the current of the single-phase harmonic.
For three-phase systems, the odd harmonics at the third, fifth, seventh, ninth, and 11th are of greatest interest for most power applications, while the even-order harmonics are typically of very low magnitude and of no consequence.
When upgrading efficiency downgrades power quality
Installation of five kilovolt capacitors to improve power quality at a Pennsylvania hospital backfired when elevated internal pressures ruptured one capacitor’s case and distorted the cases of two others.
Several years prior, the hospital had begun a program to lower electric utility costs. Included in this upgrade were VFDs for ventilation and air-handling units as well as electronic ballasts. A harmonic analyzer indicated significant voltage- and current-wave harmonics on feeders serving lighting and mechanical loads. Analysis of the distribution system showed the capacitance relative to the actual interrupting duty of the incoming electrical was exactly on the seventh harmonic based on the formula:
The significant levels of seventh- and eleventh-order harmonics were found in the distribution system, indicating that the capacitor failure was caused by a resonant condition. The high level seventh harmonic was of a magnitude that, with the capacitive resonance, was amplified to a case-rupturing level. Harmonic filters operating at the seventh and eleventh harmonics were recommended prior to reconnecting the capacitor bank. A review of the capacitor’s bank size prior to reconnection was performed to prevent a recurrence of the resonant condition because harmonic filters have some inherent capacitance. The total capacitance also was reviewed to ensure that the electrical service was not connected to a leading power factor, due to the problems that would result in the system voltage increase caused by a leading power factor.
Two categories of harmonics-producing equipment
As a guide, harmonics-producing equipment can be divided into two categories: large systems (voltage-distortion producers) and small systems (current-distortion producers). The line between the two can be imprecise because a load that causes only current distortion on a large, high-fault duty distribution system can cause significant voltage distortion if connected to a small, low-fault duty system.
Large systems include:
Uninterruptible power supplies
Variable frequency drives
Large battery chargers
Synchronous clock systems
Large electronic dimming systems
Arc heating devices.
A well-grounded solution
A special procedures radiology unit at a large hospital complex had operated for nearly 10 years with no electrical difficulties until a second special-procedures unit was added. Technicians first blamed the problems that occurred when operating the new unit, such as distortions in CRT readings, on unit-generated harmonics. However, a review of voltage and current waveforms plots generated by a harmonics analyzer showed no voltage distortion, although significant current distortion was measured at the new unit. Without voltage distortion, the two units could not affect each other, so harmonics were discounted as the cause.
Voltage measurements on the original unit’s ground indicated a significant rise in voltage on the ground conductor, which should not have existed. Discussions with the user found no operating problems were observed in the distribution system during the 10 years the original unit was operating, even though the neutral and ground were incorrectly wired. After extensive field investigation, the ground conductor was found to be connected to the neutral bus in the distribution panel feeding both units. The unit’s ground point served as the control signal reference for all controls in the unit, including the built-in automatic voltage-level adjustment. When the new unit was installed, its SCR-based power supply dumped all the reflected-wave harmonics onto the neutral conductor, which traveled back to the distribution panel. Lifting the ground conductor from the neutral bus and connecting it to the panel ground bus solved the operating problems.