Harmonic filters isolate harmonic current to protect electrical equipment from damage due to harmonic voltage distortion. Harmonic filters mitigate the detrimental effects of harmonic distortion, which can be manifested in many different ways. Examples include increased heating effect on electrical distribution equipment and cables, electronics miss-timings (computers, fax machines, etc.), capacitor overloads, fluorescent light flickering, and others.

Filter type and signal type are important specifications to consider when searching for harmonic filters. There are two choices for filter type: passive and active. Passive harmonic filters are built with a series of passive components such as resistors, inductors and capacitors. These devices are the most common type of harmonic filters, and are available for all voltage levels. Active harmonic filters are used mainly in low-voltage networks. These are very fast electronic devices that insert negative harmonics in order to eliminate undesirable harmonics from a network. Active harmonic filters are built with active components such as insulated gate bipolar transistors (IGBT) and can eliminate many harmonic frequencies. Choices for signal type are single-phase AC, three-phase AC, and DC.

Performance specifications for harmonic filters include THID, rated power, nominal input voltage, and frequency. A signal’s total harmonic current distortion (THID) is defined as the ratio of the sum of the powers of all harmonic frequencies above the fundamental frequency to the power of the fundamental frequency. Rated power or reactive power is the mathematical product of voltage and current consumed by reactive loads. Examples of reactive loads include capacitors and inductors. These types of loads, when connected to an AC voltage source, will draw current; however, since the current is 90 degrees out-of- phase with the applied voltage, they actually consume no real power in the ideal sense. Nominal input voltages for harmonic filters include 110 V, 208 V, 240 V, 380 V, 480 V, and 600 V. Frequency choices are 50 Hz and 60 Hz.

Applicable standards and optional features are also important factors to consider when selecting harmonic filters. National and international standards include IEEE 519, EN 61000, AS 2279, and BS G5/3. Features for harmonic filters include UL Marks, performance monitors, operating temperature, and form factor. The UL Mark denotes that Underwriters Laboratories Inc. (UL) has found that the harmonic filters met UL's published standards for safety. Harmonic filters with performance monitors have indicators or meters that monitor variables such as current THD, voltage THD, and power. Choices for form factor include NEMA 1 enclosure, NEMA 2 enclosure, NEMA 3 enclosure, NEMA 4 enclosure, NEMA 3R enclosure, NEMA 12 enclosure, rack-mounted, and panel-mounted.

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