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1、DESIGNING MODERN ELECTRICAL SYSTEMSWITH TRANSFORMERS THAT INHERENTLYREDUCE HARMONIC DISTORTIONIN A PC-RICH ENVIRONMENTbyPhilip J. A. Ling, P.Eng.Cyril J. Eldridge, B. Sc.POWERSMITHS INTERNATIONAL CORP416-439-1077ABSTRACTGenerally, the electrical community has come to accept the fact that todays offi
2、ce facilities havean abundance of electronic equipment that produce harmonics. Based on the results of hundredsof electrical system surveys we have determined that the predominant harmonics are triplens;however, a high degree of 5th and 7th harmonics are also present and need to be treated for amore
3、 comprehensive solution. These harmonic rich environments are known to cause seriousoperational problems for users as well as building maintenance personnel. Until recently, no cost-effective methods have been proposed that can universally deal with 3rd, 5th and 7th harmonicsduring the design and sp
4、ecification stage of three phase four wire electrical distribution systems.By integrating phase shifting into an extremely low zero phase sequence impedance transformerwith single or multiple outputs, substantial reduction of triplen, 5th and 7th harmonics can beachieved. The net result is that the
5、electrical distribution system predictably becomeselectromagnetically compatible with the electronic loads (e.g. personal computers) it has tosupply.1. INTRODUCTIONTraditional electrical system design had very little need to deal with harmonics because the loadstypically designed for were linear in
6、nature. Over the years, as more and more research andpractical experience was gathered with linear loads, the design process became more and morepredictable. With the proliferation of variable speed drives, electronic ballasts, personal computersand other electronic equipment, electrical system desi
7、gn strategies need to be adjusted. Because inmany cases a major portion of the loads today are nonlinear in nature, the loading due toharmonics created by these loads must also be taken into consideration. While this seems to be areasonable request , you might ask just how does one predict these new
8、 loading requirements andplan for them.Over the years, essentially two approaches evolved and became widely used to address harmonics. Phase-shifting transformers of different configurations , used for decades in industrial andcomputer facilities, typically treat harmonics produced by loads that are
9、 balanced and connectedphase to phase e.g. 5th, 7th, 11th, 13th harmonics. Zero sequence filters (zig-zag reactors, etc.)have been used in commercial and institutional settings to address triplen harmonics (3, 9, 15.) and associated problems (high neutral current, voltage distortion, etc.).Designing
10、 next generation electrical systemsThe evolution of electronic power supplies (switch-mode) has generated the need for a solutionencompassing the benefits of both previous approaches. While the 5th & 7th harmonics are presentand require treatment, the predominant harmonic is the 3rd, which not only
11、causes high neutralcurrent and neutral-to-ground voltage, but just as importantly causes a substantial increase involtage distortion which, as a whole, is more frequently problematic for electronic equipment.This paper describes how Symmetrical Components Theory has been used to design a completelyn
12、ew and innovative transformer that integrates the treatment of zero sequence harmonics (triplenetc.) as well as 5th & 7th harmonics. These transformers may be configured as single or multipleoutput units to accommodate various design strategies. Besides taking the guess work out of thedesign process
13、, these transformers can be included at the base building design stage in a similarway that traditional transformers would be allocated. This approach, in the majority of cases,eliminates the need for remedial equipment to be added later when problems are alreadyoccurring. Case studies of actual dis
14、tribution systems are presented that document theeffectiveness of this approach.2. BACKGROUNDUntil the mid-1980s, there was essentially no significant harmonic-generating equipment incommercial or institutional buildings. As a result, standard practices for electrical system designwere appropriate a
15、nd the biggest concern was maintaining the requisite Power Factor, whichcould be achieved by adding a capacitor bank of the necessary kVAR. From the point of view ofharmonics, these buildings were basically trouble-free: no unusually hot transformers or neutralconductors, few voltage distortion prob
16、lems, and infrequent cases of resonance or capacitor bankoverloading.The decade of the eighties brought digital electronics and the personal computer age. The arrivalof these radically new types of loads has meant problems of a magnitude that no one imagined.The consumption of these new (nonlinear) loads is far from the ideal sinusoidal waveform (linear)that power systems were designed to feed and the result is serious harmonic problems.This paper will focus on the impact of the Person
