Harmonic mitigating transformers use phase-shifting, electromagnetic flux cancellation, and source impedance to reduce harmonic currents in electrical distribution systems. By reducing the amount of heat that harmonics generate, these devices enable transformers to operate at lower temperatures. The result is lower cooling costs as well as longer device life and reduced maintenance. Harmonic mitigating transformers also enable the use of a transformer’s full capacity and ensure compliance with harmonic limits defined by the Institute of Electric and Electronic Engineers and other organizations.
There are several types of harmonic mitigating transformers. Single phase devices are designed to operate with single phase voltage. With three phase transformers, the three primary windings are connected together. The three secondary windings are connected together, too. With step-up transformers, the secondary voltage is larger than the primary voltage. With step-down transformers, the secondary voltage is smaller than the primary voltage. Variable harmonic mitigating transformers have a setting for changing the turn ratio as needed. One-to-one devices have a transformer or turn ratio of 1:1, or near 1:1.
Performance specifications for harmonic mitigating transformers include operating frequency range, maximum primary voltage rating, maximum secondary voltage rating, maximum secondary current rating, power rating, and operating temperature. Because transformers with several nominal voltages need more than one primary winding, the maximum voltage should be specified for the application. Power rating, the rated power of the device, is expressed in VA (volts x amps) for all of the secondary windings. The range of operating temperatures is also important to consider when selecting harmonic mitigating transformers.
Harmonic mitigating transformers differ in terms of mounting style or form factor. Chassis-mount devices have tabs so that the device can be screwed into place. Chip transformers have an integrated circuit (IC) form factor. Dish or disk mounted devices are attached with a rubber washer and metal disk. H-frame mounting is used in applications where high vibration is present. The modular jack form factor (RJ-45) incorporates magnetics to insure high common noise immunity while maintaining signal integrity. Harmonic mitigating transformers may also be mounted on concrete pads, telephone poles, or printed circuit boards (PCBs).
Some harmonic mitigating transformers are seismic-qualified or rated for outdoor use. Others are waterproof or feature current limiting protection. Harmonic mitigating transformers that meet enclosure or acoustic requirements from the National Electrical Manufacturers Association (NEMA) are also available. Specialty products include medical-grade transformers.