by Bill Hammel vice president/engineering

The industry standard for power quality, IEEE 519 (“Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems”), describes current distortion limits that apply to the point of common coupling (PCC) with the consumer-utility interface.

Figure 1—Linear and nonlinear loads and the PCC

The electrical stiffness of a system is expressed as the ratio (RSC) of the available short-circuit current (ISC) at the PCC to the maximum demand fundamental load current (IL), calculated as the average of the maximum demand over 15-minute intervals for the preceding 12 months.

       RSC = ISC/IL

The IEEE standard adjusts limits with respect to this ratio in a manner that recognizes that low kW loads, connected to systems with much higher kVA capacities, have a proportionally smaller effect on the system and are, therefore, allowed a higher distortion limit.

The amount of harmonic current produced by nonlinear loads, such as adjustable-speed drives with rectifier inputs, is also affected by the stiffness of the electrical system. Higher levels of harmonic current are produced by the drive as system stiffness increases, and lower levels are produced with softer, higher impedance systems. This effect can be quite dramatic for unfiltered drives—those having significant DC link capacitance directly connected to the input rectifier bridge. This effect is also apparent, though less dramatic, on harmonic current produced by filtered drives—those that utilize either DC link inductors or other techniques to achieve relatively low DC link current ripple.

Figure 2—Comparison of unfiltered (1 and 2) and filtered (3 and 4) drives

Limits for current harmonic distortion are described with respect to total demand distortion (TDD), which expresses total harmonic current distortion as a percentage of IL.

Figure 3 below graphically illustrates how the TDD limits recommended by IEEE 519 vary as a function of RSC. Also included are typical TDD levels produced by drives utilizing filtered 6-, 12-, 18-, or 24-pulse input rectifiers under the assumption that IL. is entirely comprised of that drive load.

Figure 3—IEEE 519 limits and harmonic distortion of various drive configurations as a function of short-circuit ratio

For the assumptions given, the 6-pulse configuration always exceeds the IEEE limits while the 12-pulse configuration sometimes exceeds it. These configurations require either additional effort or further assumptions in order to satisfy the limit.

Additional effort can come in the form of including a reactor between the drive and the PCC. This additional impedance reduces drive-injected TDD without affecting the system RSC and, therefore, without affecting the corresponding TDD limit. Figure 4 shows the effect of including 3%, 5%, and 10% reactors on 6- and 12-pulse configurations.

Figure 4—Harmonic distortion of 6- and 12-pulse drives with various reactors

While Figure 4 shows how including a reactor might help 12-pulse configurations satisfy the IEEE limits, it also illustrates that the 6-pulse configuration remains a challenge despite inclusion of a reactor. While 18- and 24-pulse configurations more directly comply with the IEEE limits, there is still hope in another form for both 6- and 12-pulse configurations when additional conditions apply.

Limits can more easily be met when the drive load represents only a fraction (RDD) of the total load, IL, and the remaining portion of the total load is comprised of either linear loads or nonlinear loads whose harmonic contribution is negligible."

       RDD = ID/IL

Figure 5 illustrates results for a 6-pulse configuration that includes a 5% reactor where the drive load is 10%, 20%, 50%, and 100% of IL, while the remaining portion of the load is assumed to contribute negligible harmonic distortion.

Figure 5—Affect of varying RDD on 6-pulse drive with a reactor

A 6-pulse configuration, with the inclusion of a 5% reactor, can satisfy the IEEE limits as long as the drive load is a small enough fraction of the total load. 

A cautionary note regarding the addition of reactors must be made. Since adding reactor impedance reduces TDD, one might wonder—why not merely add enough to achieve any desired TDD objective? An offsetting penalty is that as impedance is progressively increased, both power factor and available output voltage progressively decrease. While the penalty associated with adding a 5% or even 10% reactor impedance is often acceptable, the penalty of further increases might begin to outweigh the TDD benefit.

The discussion above has been intended to provide some insight into the influence of line impedance on meeting IEEE current distortion limits.  Impedance affects both the limit and the level of injected distortion. The figures above provide an overview of various drive configurations and the conditions under which they can meet the IEEE limit. If you need further assistance with understanding line impedance and harmonic issues, please contact us.