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That is not to say there are no applications for which a 12- or 18-pulse drive is appropriate. Take, for instance, a cinderblock pump station in a residential neighborhood. This author observed one in which there were three 300-hp VFDs, overhead fluorescent lighting, and a wall-mounted programmable logic controller (PLC). The pump station was fed by a dedicated 480-v transformer. Virtually the entire load on the transformer was non-linear. The VFD non-linear load represented approximately 1,100 amps. The PLC and fluorescent-light loads totaled a couple of amps. That was an ideal application for 18-pulse or other ultralow-harmonic VFD technology.

In a commercial office building, if VFDs are installed on every fan and pump, they typically will use less than 20 percent of the electrical demand load. In almost all such cases, standard six-pulse drives are a good choice.

Contrary to popular belief, ANSI/IEEE Standard 519 is not a law or government/utility regulation; it is a “recommended practice.” It states that strict adherence to its recommended harmonic limits “will not always prevent problems from arising.” The contrary also is true: A facility may have harmonics in excess of the standard's maximum recommended limits and not experience difficulties.

TECHNOLOGIES

The simplest and least-expensive method of mitigating VFD-generated harmonics is adding impedance at a VFD. This can be accomplished with an input line reactor (Figure 3) or a DC link reactor (bus choke) (Figure 4). In a 1-percent-source-impedance system, a 3-percent line reactor can reduce harmonic-current content at the input to a VFD to about 40 percent at full-load output.

The next-most-common type of harmonic-mitigation technology is the 12-pulse VFD (Figure 5). A 12-pulse VFD reduces harmonic-current content to about 10 percent.

Also common are broad-band and passive filters (Figure 6). These hybrid filters reduce harmonic-current content to approximately 7 percent.

The next-most-effective technology is the 18-pulse drive (Figure 7), which typically presents approximately 5-percent current distortion at VFD inputs. Compared with a VFD with no impedance, total harmonic reduction is in the range of 93 percent.

Relatively new technologies are the active harmonic filter (Figure 8) and the active-front-end VFD (Figure 9). A single active filter can filter the harmonics of several VFDs or an entire facility. Meanwhile, the THDI content of a VFD with an active front end — measured at the VFD input — typically is less than 4 percent, while the total-harmonic-current-content reduction is 95 percent.

Table 2 lists the expected current distortion, percent current-distortion reduction, and relative cost of the various harmonic-reduction technologies. The estimates are based on a 1-percent-source-impedance system and a perfectly balanced voltage supply.

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