Harmonic mitigating transformers (HMTs) are a relatively new tool being used by some power quality professionals to rid commercial and industrial facilities of the power quality problems associated with harmonic currents. Harmonics are imposed on distribution systems by adjustable speed drives; electronically ballasted lighting; and the power supplies of every computer, copier, and fax machine and much of the telecom equipment used in modern offices. The widespread and growing adoption of these technologies has greatly increased the flow of harmonic currents on facility distribution systems and has given rise to a host of related problems, including overheated transformers, motors, and neutral wires; nuisance breaker trips; voltage distortion, which can cause sensitive electronic equipment to malfunction or fail; and elevated neutral-to-ground voltage, which can cause local area networks to malfunction. HMTs have the ability to attenuate harmonic currents and thus alleviate these types of power quality problems. In addition, by canceling certain harmonic currents, HMTs can reduce the energy losses that harmonics would otherwise cause in conventional transformers. Given the right conditions, these energy savings can offset the premium price for HMTs (which can be as much as 200 percent) and yield an attractive payback period.

HMTs are available in all standard transformer nameplate capacities from 15 to 500 kilovolt-amperes. All HMTs employ one or both of two approaches to combat harmonics, each of which addresses different types of harmonics. The selection of the appropriate type of HMT depends on which harmonics are present in the electrical distribution system being treated.

Single-phase loads. Single-phase electronic loads generate harmonics at all odd multiples of the fundamental (50 or 60 hertz [Hz]), but the most vexing of these are usually the "triplen" harmonics, that is, those that oscillate at multiples of the third harmonic. Triplens add together in the neutral on the secondary side of a delta-wye transformer and can cause very high neutral currents. In conventional transformers, triplen harmonics are transferred to the primary (delta) winding, where they are trapped and circulate continuously. The distribution system upstream of this transformer is thus spared from having to supply triplen harmonics, but the harmonic currents cause excessive losses in the transformer. HMTs attenuate triplen harmonics by using a "zig-zag" winding on the transformer secondary. The zig-zag winding is a design that places half of the turns of each phase of the secondary around two of the legs of the transformer core (in the standard design, all turns for a given phase go around just one core leg). This technique causes cancellation of the magnetic flux established by triplen harmonic currents, so little or none is transferred to the primary windings.

Three-phase loads. Three-phase loads do not generate triplen harmonics. As a result, harmonic problems in industrial facilities dominated by three-phase loads will most often result from currents flowing at the 5th, 7th, 11th, or even higher order harmonics. For these non-triplen harmonics, HMTs use either dual secondary windings or pairs of transformers to achieve substantial attenuation of one or two of the most problematic frequencies. In either design, the two secondaries are electrically phase-shifted relative to each other. The degree of relative phase shift is selected such that the targeted harmonic currents from one secondary are close to or exactly 180 degrees out of phase with the targeted harmonic currents from the other secondary, and thus they cancel each other.

Any distribution circuit serving modern electronic devices will contain some degree of harmonic frequencies. The greater the power drawn by such devices, the greater the harmonic distortion of line power. In office settings, distribution transformers are often very lightly loaded and can therefore accommodate the additional losses and resultant heating caused by harmonics. Because of the light loading, power quality is infrequently degraded to the point where sensitive electronic equipment begins to malfunction. But where distribution transformers are already moderately to heavily loaded and/or harmonic content is unusually high, a variety of problems related to harmonics can arise, and, in some cases, HMTs are the right solution.

Even where harmonics are not causing noticeable problems, the energy savings HMTs offer in some applications can help to make them an attractive and cost-effective choice. Because distribution transformers are typically under load a very high proportion of the time and incur no-load losses continuously regardless of load level, small improvements in transformer efficiency often translate to rapid payback of the premium paid for the efficient transformer. HMTs can save energy by attenuating harmonics because transformer losses mount rapidly as harmonic currents increase (Figure 1). HMTs accrue additional savings upstream of the transformer, particularly where long, relatively high impedance risers would otherwise incur losses due to harmonic currents. Moreover, because they reduce harmonic-related losses in their windings, HMTs can be loaded to a much higher fraction of their rated capacity without overheating. This means that transformers that are overheating due to harmonic currents can be replaced by HMTs of the same (or sometimes even smaller) nameplate capacity.

Determine HMT cost-effectiveness. HMT cost-effectiveness will vary considerably from one application to the next, depending on factors such as the magnitudes and frequencies of load harmonic currents, how heavily loaded the transformer is, load duty cycle, differences in efficiency (at 60 Hz) between the HMT and the standard or K-rated transformer it is being compared to, utility rates, whether or not transformer harmonic losses contribute to the facility's cooling load, and, of course, transformer capital and installation costs. There are three categories of cost savings: energy, demand, and, in some cases, transformer capital cost (where harmonic mitigation permits installation of a smaller transformer or obviates the need for a larger one).

HMTs reduce electricity costs in two ways. They reduce losses directly by minimizing harmonic currents and their related losses in the transformer primary. If the HMT under consideration has a more-efficient core and/or windings than the transformer it is being compared to, then direct losses at 60 Hz will be reduced as well, with additional cost savings. (Keep in mind that the HMT could also be less efficient at 60 Hz than its competitor.) HMTs may also contribute indirect savings if the transformers are to be located in air-conditioned space. Approximately 1 kilowatt-hour (kWh) of cooling system energy is required for every 3 kWh of heat removed. For HMTs installed in air-conditioned space, energy savings are therefore increased by about 33 percent.

Use the online screening tool below to help determine whether an HMT makes sense for a particular application. To use the tool, first gather the following information:

• The nameplate rating, secondary voltage, average kW load, hours per day and days per year at that load level, load power factor, and full-load I2R and eddy-current losses in the primary windings of the existing distribution transformer(s).

• Utility energy ($/kWh) and demand ($/kW-month) rates.

• The premium you will have to pay to install an HMT rather than the conventional or K-rated transformer you are considering as an alternative replacement.

In addition, obtain a profile (current magnitude at each frequency) of the harmonic currents served by the transformer(s) you may replace. There are numerous devices available to measure and record this information, such as the Dranetz model 8000, BMI's 3030A Profiler, and Esterline Angus model PMT. These devices can often be rented, but it may be necessary or preferable to hire a power quality consultant to perform the required measurements.

With all of the above information in hand, the cost-effectiveness analysis tool will provide an estimate of simple payback based on energy savings. Note, however, that an HMT may also provide non-energy savings in some circumstances. If, for example, the alternative under consideration is a K-rated transformer, consider that the larger dimensions of the K-rated transformer may require extensive redesign of the electrical room (relocation of equipment, rerouting of conduit, and so on) that could entail additional labor and materials costs and could require prolonged downtime. Because HMTs don't have to dissipate as much heat, they are often the same size or smaller than the transformers they replace, and so can be swapped in with relative ease.

One caveat. Some HMT vendors grossly overstate the energy savings their products will provide. The calculator provided here should give you an estimate of the savings you can realistically expect from an HMT, provided you enter accurate information.

The technology behind the HMT is well understood and pretty stable at this point. Other than variations in the materials used to construct the transformers (to improve efficiency or reduce cost), don't look for much to change. An alternative technology to keep an eye on though is the neutral harmonic filter. This type of device is designed primarily to treat the triplen harmonics generated by single-phase loads, and it works by placing very high impedance to the flow of harmonic currents in the neutral. This has the effect of changing the way the power supplies of electronic devices draw current from the distribution system, substantially reducing harmonic components. Because harmonics are reduced throughout the distribution system, from the end use to the distribution transformer, the potential for energy savings will often be substantially greater than that available from an HMT.