Power-line pollution — voltage and current harmonics — has been with us a long time, but, until recently, caused few problems and was often ignored. However, the increasing use of nonlinear-load devices — the cause of power-line pollution — makes reducing harmonics now a necessity. Examples of nonlinear-load devices include ac and dc adjustable-speed drives, static uninterruptable power supplies, programmable controllers, rectifiers, switching-type power supplies, static lighting ballasts, computers, and xray devices.
Power-line pollution can cause such problems as power factor reduction, power system overloading, system resonance, and failure of motors, transformers, and electronic equipment.
Acceptable limits
In 1992, in response to the concerns of electric utilities, IEEE-509 “IEEE Recommended Practices and Requirements for Harmonic Control in Power Systems” was revised to redefine the acceptable limits of individual and total current and voltage harmonics allowable at the point of common coupling (PCC) with power lines. These guidelines protect other users on the same power feeder as well as the utility, which is required to furnish a certain quality of voltage to customers. Utilities also want to reduce the high-frequency currents that produce additional heating in motors and transformers that can cause premature failure. Moreover, utilities are attempting to reduce the use of power-factor correcting capacitors. Switching these capacitors on and off creates harmonics thereby distorting the power sign wave.
Of the three tables in IEEE-Std. 519-1992 specifying current distortion limits, table 10.3 (reproduced in this article as Table 1) applies to most industrial installations.
Solutions
There are three approaches to meet the limits of current and voltage distortion as specified in IEEE Std. 519-1992:
Drive filters. A specific filter for each drive, Figure 1, although not cost effective, does trap and eliminate unwanted harmonics as close to the source as possible so they do not affect other equipment.
An alternate, less-costly approach uses one filter to reduce the harmonics of a group. Such a filter is connected at the PCC or inboard of the plant main transformer. Although less costly than individual filters, the group filter enables drives within that group to affect the operation of other drives within the same group. For example, harmonics produced by one drive many cause erratic operation of another drive or other electronic devices. (See PTD, 11/90, for details on designing harmonic filters.)
If a filter is required and if electric costs require adding power-factor correcting capacitors, plan both at the same time, because part of the filter design may require capacitors. Also, be sure that the filter design does not produce a system resonance.
Simulating 12-pulse systems. By using individual drive transformers connected delta-wye for half the load and the other transformers connected in deltadelta, Figure 2, you can produce a combined harmonic content that approaches the content of a 12-pulse drive.
This method works well in reducing harmonics at the PCC, Table 2, if the sum of delta-wye loads equals the delta-delta loads. To use this solution, analyze your power system and balance the delta-wye and delta-delta loading. Once this is accomplished, you should monitor your plant’s power system at the PCC to determine individual and total current and voltage distortions and power factor. This monitoring should consist of numerous “snap shots” over time because harmonics and power factor are a function of the specific equipment operating at any one time.
This transformer configuration can be cost-effective if your present drive-isolation transformers have the same primary and secondary voltages, and the secondary is reconnectable to a delta configuration and maintains the proper output voltage.
Dedicated 12-pulse system. For larger drives (>500 hp) consider creating a dedicated 12-pulse system from two sixpulse drives that have load-sharing capability or buying a 12-pulse drive. The resultant harmonics remain fairly constant. This approach will reduce the amount of filtering required and its associated cost.
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There are two basic configurations:
• Two separate isolation transformers (one connected delta-wye and the second delta-delta), Figure 3.
• A single transformer with two secondaries. In this case, the primary is in delta and one secondary is in wye and the second is in delta, Figure 4.
In the first method, should either of the two transformers fail, replacement is relative easy, because both are standard, model number items. However, the installed cost of this system is higher, because two sets of transformer primary protection are required. Also, harmonic reduction takes place at the PCC, so the original harmonics caused by each sixpulse drive will be seen by any other loads connected to the plant main transformer.
With the alternate arrangement, Figure 4, harmonic reduction takes place in the transformer primary. Also, installed cost is lower, because the single transformer requires only one set of transformer primary protection. But, because the transformer has two secondary windings — one delta, the other wye — if something should happen to the transformer, repair or replacement would take time.
For more information from Ted Bryda, on reducing harmonics, please circle 460 on the reader service card.
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Theodore J. Bryda, B.S.E.E., Rolling Meadows, Ill., (708-397-2959) is a consultant specializing in electrical adjustable-speed drives and instrumentation.
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