Waving Good Bye to Harmonics


Consulting-Specifying Engineer. February 1996

Today’s electrical-distribution systems are increasingly at risk from both current-and voltage-wave distortion. A system wide approach can address the causes of these harmonics problems, not just the symptoms.

Increased dependence on electronics-in everything from fluorescent ballasts and desk-top computers to large-scale hospital radiation suites-has brought a growing awareness of harmonics and the havoc they can wreak on that same equipment. With instrumentation making it easier to determine the extent of harmonic problems, it is now possible to look at what problems harmonics are causing, which equipment is affected and how to handle the causes of harmonics-not just their systems.

Harmonic consequences

Harmonics-related problems can occur throughout commercial, industrial and institutional facilities and include:

  • Operation of overcurrent devices without a measurable 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 set-points.
  • Interactions between uninterruptible power supplies (UPS) 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.

Harmonics-related

Problems are possible

throughout any facility

  • 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.

Harmonics Order: A Definition

Harmonics are categorized by their order. The first-order harmonic or fundamental frequency for 60-Hertz power is 60 Hertz. As the order increases the frequency increases, thus a second-order harmonic would be 120 Hertz, a third-order is 180 Hertz 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. On the neutral conductor where the three phases connect, the triplen harmonics add together resulting in three times the current of a single-phase harmonic.

For most three-phase systems, even-order harmonics end up having very low magnitudes and are typically of no consequence. The odd harmonics at the third, fifth, seventh, ninth, and eleventh are of greatest interest for most power applications. For most three-phase systems, even-order harmonics end up having very low magnitudes and are typically of no consequence.

Current-wave distortion is the most common of these causes. It occurs when amperage demand by a particular device occurs out of phase with the electrical system’s normal sine wave. Such devices as silicon-controlled rectifiers turn on for short periods, drawing power inconsistent with a normal current wave’s cyclical nature. Other 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 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 through-out 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. In more severe cases, sensitive control systems cease 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 or understood than current wave distortion.

Equipment with the potential for generating voltage-wave distortion includes UPS systems, VFDs, solid-state elevator drives, are 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 circuits.

With increased recognition of triplen harmonics, oversized neutrals are being offered in all sorts of distribution equipment, such as alternating-current cable, 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, the engineer is left with the feeling that if the neutral is not doubled, 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 might be better off studying ways to reduce or eliminate them. For example, new electronic ballasts have total harmonic distortion (THD) of less than 20 percent, which is less than that of a standard magnetic ballast. And third-order harmonic filters now are available to reduce triplen harmonics to under 10 percent. Such systems limit the need for increased neutrals and k-factor transformers.

Voltage Distortion

While current-wave distortion can be handled with such solutions as oversized neutrals, k-factor transformers, power conditioners and isolation transformers, voltage-wave distortion has no such easy options. Where voltage-wave harmonics occur, there is little latitude left to the engineer to produce a workable, economical solution. Oddly enough, it appears from a brief review of published literature that voltage-wave harmonics are not a hot topic, perhaps because there seem to be two primary solutions available at this time: incorporating harmonic filters, or eliminating devices that produce voltage-wave distortion.

As guide, harmonics-producing equipment may 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:

  • Uninterruptable power supplies.
  • Variable-frequency drives.
  • Large battery chargers.
  • Elevators.
  • Synchronous clock systems.
  • Radiology equipment.
  • Large electronic dimming systems
  • Arc heating devices.

When Upgrading Efficiency Downgrades Power Quality

Installation of five kilovolt capacitors to improve power quality at a McKeesport, Pa., hospital backfired when one of the capacitors ruptured its case and two others suffered distorted cases due to elevated internal pressures.

Several years prior, the hospital had begun a program to lower electric utility costs. Included in this upgrade were variable-frequency drives 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 capacitor quantity relative to the actual interrupting duty of the incoming electrical hit exactly on the seventh harmonic based on the formula:

Harmonic Order= Ö KVAsc/KVA Capacitor

The significant levels of seventh harmonic found in the distribution system indicated the capacitor failure was caused by a resonant condition where harmonics were amplified to a case-rupturing level. Harmonic filters operating at the seventh and eleventh harmonics were recommended prior to reconnecting the capacitor bank. Areview of the capacitor's bank size prior to reconnection also was suggested, because harmonic filters have some inherent capacitance, and no electrical service should be corrected to a leading power factor due to the voltage-level increase.

Small systems include:

  • Personal computers
  • Desk-top printers
  • Small battery chargers.
  • Electric discharge lighting.
  • Electronic or electromagnetic ballasts for fluorescents.
  • Small electronic dimming systems

Harmonic filters

In the commercial market today there are various sizes, types and application for harmonic filters-each having its own best applications. 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 to prevent them from being passed back into the distribution systems. Attenuation levels are dependant on the available fault-duty of the distribution system but are generally designed to pass a maximum of

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 new problems, such as distortions in CRT readings, on harmonics generated by the new unit.

However, a review of voltage and current waveforms plots generated by a harmonics analyzer showed no voltage distortion, although significant current distortion was indicated on the new unit. Without voltage distortion, the two units could not affect each other, so harmonics were discounted as the immediate cause.

Voltage measurements on the original unit’s ground indicated a significant increase in voltage, which should not have existed. The ground conductor was found to be connected to the neutral bus in the distribution panel feeding both units. Lifting the ground conductor from the neutral bus and connecting it to the panel ground bus solved the operating problems.

Analysis showed no harmonics on the neutral bus in the distribution system during the 10 years the original unit was operating, even though the neutral and ground were incorrectly wired. 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 is installed, its harmonic filter dumped all generated harmonics to the neutral conductor, which traveled back to the distribution panel.

5 or 10 percent current total harmonic distortion 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 uses, matched to the device for which they are furnished. These models limit THD of the combination filter/utilization device to either 5 or 10 percent THD and are normally applied to large systems. When these filters are applied to 12-pulse rectifier input sections of UPS or VFD systems, they generally may be sized smaller than those used with similar systems having six pulse rectifiers.

For small-system harmonics, individual filters for each harmonics, frequency can be costly if, for example, a filter is required for each fluorescent ballast. In this case, shunt filters may be connected to a breaker or fused-switch on the distribution or branch-circuit panel feeding multiple harmonics producers.

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. If the number of devices increases, the harmonic filter may no longer be sized adequately.

Motor-generator sets can help eliminate the passage of harmonics from the generator side (load) to the motor side (line). However, designers should ensure that harmonics do not disrupt the generated wave form by interfering with the voltage regulator feedback circuit. The generator may be sized to at least twice the size of the load to address this concern. Two approaches that do not fit into the above categories are motor-generator sets and active filters.

Active filters are not now widely available, but they have appeared as prototypes developed by several manufacturers. These filters incorporate microprocessors to eliminate harmonics by rapidly compensating for sine-wave deviations from ideal wave forms. These models can correct all harmonic magnitudes up to their maximum capability. Such products could eliminate harmonics concerns in the future from electrical-distribution design.

Isolation transformers also have been proposed for selected installations as harmonic filters. However, because of their limited attenuation capabilities, they are not very effective for any but the lowest magnitudes of current-wave harmonics.

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 presence of harmonics and level of required filtering.

Doubled neutral conductors and k-factor transformers should be used only as a last resort when nothing else works, 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-reducing loads, can be the most cost-effective way to limit harmonics in the distribution system.

Filtering through the Evidence

Operators of a small blue-line print machine in a Pittsburgh office building began having problems with their equipment’s drive speed-it would not remain constant at slow speeds. As a result, final prints were unusable. After repair technicians tried replacing numerous parts of the power supply and variable-frequency drive, engineers were asked to look into why the unit would not operate. Power conditioners had no effect, but connecting the unit to another outlet in a remote part of the building solved the problems.

When another print machine, operating with no problems, was moved to the original outlet, it started having the same operating problems. Engineers concluded there was a harmonics problem at that outlet. After reviewing the building’s as-built drawings, engineers found the remote outlet was connected to a separate incoming electrical service from the suspect outlet. The problematic incoming service line had a UPS with no input filter.

Suspicions were confirmed after the UPS input power was turned off and the harmonics disappeared. An oscilloscope confirmed the voltage wave distortion was causing the harmonics.

Copyright © 2002 Cahners Business Information, a division of Reed Elsevier Inc. Reprinted with permission.

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