电力系统设计 外文文献

DESIGNING MODERN ELECTRICAL SYSTEMS WITH TRANSFORMERS THAT INHERENTLY REDUCE HARMONIC DISTORTION IN A PC-RICH ENVIRONMENT by Philip J. A. Ling, P.Eng. Cyril J. Eldridge, B. Sc. POWERSMITHS INTERNATIONAL CORP 416-439-1077ABSTRACTGenerally, the electrical community has come to accept the fact that todays office 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 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 specification 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 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 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 design 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 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 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 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 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 completelynew 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, 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 distribution 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 and 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 problems, 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 Personal Computer (PC) and similar electronicequipment on power distribution systems. The PC and PC-based workstations are moving rapidlyinto the workplace everywhere - from the office having traditionally little load, to computer roomsused to phase-phase and 3-phase loads. Another good example of where electronic loadsdominate the load profile is broadcast facilities.A few special notes should be made about this type of equipment. It is connected phase-neutral(120V), which means that the neutral conductors in the system are part of the current circuit.Whereas phase-phase (208V) load harmonic spectrums are rich in 5th, 7th, 11th and 13thharmonics, the 120V, phase-neutral PC draws a pulsed current with a rich harmonic spectrumdominated by the 3rd harmonic and with appreciable levels of 5th and in many cases 7th harmonics.While it has been associated for many years with causing high neutral current and hottransformers 1,2 , the same PC that is the source of the current harmonics, is itself sensitive tovoltage distortion that results. In our experience, the costs associated with downtime far exceedall other costs, yet they are the most difficult to measure. The relevant North American standardPOWERSMITHS INTERNATIONAL CORPpage 2Designing next generation electrical systems3 IEEE-519-1992 “Recommended Practice ” specifically singles out electronic equipment(section 6.6) and highlights its sensitivity to distortion of the supply voltage waveform listingconsequences of excessive distortion such as erratic equipment malfunction and premature failure.Recommended limits are set at 5% Total Harmonic Distortion (THD) with no individual harmonicexceeding 3% of the fundamental.It is clearly undesirable to be in a position of having to design a system knowing it feeds a PC-richenvironment, acknowledging that this means a harmonic-rich environment and thereforepotentially a problem-rich environment. However, you still resign yourself to waiting until theclient is moved-in, operating, and experiencing problems (complains) before attempting to treatthe harmonics because existing approaches require accurate information about load conditions anddetails of the specific harmonic spectrum at hand.What is required is an approach that can not only tackle the harmonics successfully, but that canbe applied at the design stage so that remedial action is very rarely required.3. WHICH HARMONICS TO DESIGN FORThe first step towards preemptively solving a harmonic problem is to develop a typical“fingerprint” of the expected loads. It is important to note that some environments have morestable loads than others and there can also be quite a diversity of equipment connected to thesame system. Also, in time the loads and their harmonic content could change. These reasons havebeen cited as evidence that it is not practical to quantify potential harmonic problems unless theload is well-defined and stable. While acknowledging that it is difficult to anticipate the precisemagnitude of the different harmonics beforehand especially where loading varies significantly, it ispossible to provide successful, off-the-shelf harmonic treatment by predicting the types ofharmonics rather than trying to quantify each.In the course of our research, we conducted and reviewed extensive field data with respect to thecurrent spectrum of loads in commercial, institutional and industrial settings. What emerged wereconsistent load profiles; these findings are perhaps surprisingly simple. There are two basic loadprofiles - one typical of phase-neutral loads and another typical of phase-phase and 3-phase loads(see figure 1).POWERSMITHS INTERNATIONAL CORPpage 3Designing next generation electrical systemsThe results can be summarized as follows: 1. Where there are phase-neutral loads, a rich harmonic current spectrum can be expected comprising mainly of 3rd, 5th & 7th harmonics with the 3rd being predominant yet the 5th and sometimes 7th being troublesome. 2. As expected, where phase-neutral loads were in the majority, neutral current exceeded the phase current by a wide margin, 1.5 times on average. The data we have compiled over the past several years shows a clear tendency for the level of neutral current to increase relative to the phases. Two years ago it was common to find neutral current about equal to phase current. We have recently measured several sites at 2.2 times! 3. Phase-phase and 3-phase loads show a predominance of 5th & 7th harmonics with a noted absence of 3rd harmonic. There is no neutral current since the neutral is not part of their circuit. 4. While there were instances of 11th & 13th harmonics dominating the spectrum (12-pulse UPS & drives) they were far less in overall proportion and were in very localized systems.These are the commonly found electrical loads in our facilities today.Personal Computer208V Power SupplyVFD or UPSWhat we can summarize from this information is that the harmonic spectrums were consistent,predictable, and dominated by only 3 harmonics: the 3rd, 5th & 7th. Neutral current is present andrequires attention where there are phase-neutral loads.The authors also reviewed historical data to see whether the continuing advances in switch-modepower supply technology were having an impact on the current spectrum. There is indeed a trendto higher harmonic levels. The harmonic spectrum is similar except that all harmonics tend to gethigher as a percent of the fundamental. The 9th harmonic in some cases is high enough to warranttreatment (the 9th adds in the neutral conductor similar to the 3rd).POWERSMITHS INTERNATIONAL CORPpage 4Designing next generation electrical systems4. THEORETICAL BASIS FOR TREATMENT OF PROBLEM HARMONICSWhen trying to resolve any problem, clear identification of the goal is of prime importance sinceboth the effectiveness and cost of the treatment are at stake. Our objectives can be listed asfollowed: 1. Reduce voltage harmonic levels to within limits set out in IEEE-519-1992 (5% THD voltage with no individual harmonic above 3%). 2. Ensure the transformer and cables are not overloaded. 3. Use passive approach for simplicity, highest reliability, lowest maintenance, lowest cost. 4. Avoid the use of capacitors (risk of resonance, etc.).When dealing with harmonics it is important to understand the relationship between currentharmonics and voltage harmonics. Current harmonics in themselves cause extra heating inelectrical components through which they flow, namely transformers and cables. However, theireffect can be more widespread. Because they flow through the system impedance, currentharmonics create voltage drops at their respective harmonic frequencies, distorting the voltagewaveform, which in turn affects other equipment connected to the system (see figure 3). Theterm used when discussing the interaction of equipment is “electromagnetic compatibility”.Figure 3: How Harmonic Currents Create Voltage DistortionVthd 1 Vt hd 2Z1Z2Vthd 3Z3Vt hd 4 Vthd 5Z4Z5Vt hd 6Z6VthdSourc eZs ourceIhV h= IhxZhE QUI P ME NT Vt hd LoadHa rmo nic cur re nt sou rceWhere:VhIhZh(Ohms Law)h th harmonic voltageh th harmonic currentsystem impedance for hth harmonicFigure 3 makes it very clear that in the treatment of voltage harmonic distortion, one can eitherreduce the magnitude of the harmonic current or the system impedance at the frequencies thatharmonic currents are present, or both.POWERSMITHS INTERNATIONAL CORPpage 5Designing next generation electrical systemsFrom a design point of view, there is a load requirement and provision must be made for thatcertain kVA of load. Although at the outset the exact profile of the load is generally not known,certain information is available. The local environment will be one of three configurations: allphase-neutral, all phase-phase or 3-phase, or a mixture of both types of loads. In terms ofapproach to treatment, the mixed case should be treated the same way as if the loads were allphase-neutral. The harmonic currents to be expected are as per our previous discussion, and theharmonics to be treated are either 3rd, 5th, 7th & neutral current for phase-neutral loads, or 5th & 7thfor phase-phase and 3-phase loads. If 12-pulse systems are used, 11th & 13th harmonics arepredominant.Symmetrical components & 2 families of harmonicsAn in-depth treatment of Symmetrical Components Theory is beyond the scope of this paper, butit needs to be touched upon in order to understand the basis upon which harmonics can betreated.In the early part of this century, an Engineer named Fortesque 4 developed a method by whichany system of 3 unbalanced vectors can be represented by a set of 3 balanced systems of vectorswhich he called positive, negative and zero sequence components (see figure 4).Figure 4: Example of Symmetrical ComponentsAA+rotationA0A-B-C-rotation=CBC+B+ P os it iv eS eq ue nc eU n ba la nc e d 3- Ph a se Sy s t e m+B0C0N e ga t iv eS eq ue nc e Z e roS eq ue nc eThe following chart shows the harmonic sequence based on Symmetrical Components Theory. 1234567etc.Harmonic (fund.)Sequence+-0+-0+Note: unbalanced portions of positive & negative sequence currents are also zero phase sequencein nature and appear in the neutral. For example the unbalanced portion of 60Hz that flows backin the neutral is zero sequence.The conversion involves complex mathematical manipulation, but what needs to be rememberedin our application is the behavior of the different symmetrical components with respect totransformers and cables. First of all, by their nature positive and negative sequence currents flowthrough transformers from the secondary into the primary system.POWERSMITHS INTERNATIONAL CORPpage 6Designing next generation electrical systemsZero Sequence ComponentsZero sequence currents are important for several reasons: they flow only in 3-phase, 4-wire systems, and because they are in phase in all three phases, they add together in the neutral conductor they are trapped circulating in the transformer primary delta windings causing additional heating they flow through the system impedance causing voltage distortionZero sequence impedance is of particular importance: zero sequence impedance of a delta-wye transformer is equal to its positive and negative sequence impedance (the nameplate value). This is important because the overwhelming majority of distribution transformers have this type of connection, which means the delta-wye transformer is a reasonably high impedance for 3rd harmonic current thus contributing significantly to high voltage distortion at 3rd harmonic (remember Ohms law). zero sequence impedance of a cable can be several times higher than its positive and negative sequence values or at least equal to them. This means that any significant cable run feeding phase-neutral loads will result in high 3rd harmonic voltage distortion at these loads.Neutral to ground voltage, sometimes referred to as common mode noise, is a direct product ofthe neutral current and zero phase sequence impedance of the cables.The notion of symmetrical components is important because all currents of the same type behavein the same way. A zig-zag reactor treats high neutral current on the basis that it is a zerosequence filter by its nature and since all currents flowing on the neutral are zero sequence, theapproach works. This approach is now widely used for removing neutral current.The zero sequence network and the behavior of zero sequence currents is key because thepredominant harmonic in phase-neutral loads - the 3rd harmonic, is zero sequence, therefore lowzero sequence impedance is vital to the overall success of any complete treatment involvingphase-neutral loads.Remember that because they are all in the same family, any modification of the zero sequencenetwork affects all zero sequence currents (3,9,15 & unbalanced portions of others).Positive & Negative Sequence ComponentsUnderstanding of positive & negative sequence currents is key to resolving the other harmonicswe have identified as needing to be treated - the 5th & 7th. As noted in the above table, 5th isnegative sequence and 7th is positive sequence in nature. The fact that they both flow throughtransformers and yet rotate in opposite direction allows us to use one phase-shift to remove pairsof positive and negative sequence harmonics from two separate sources. The case we are mostinterested in is the 30 phase-shift between two similar harmonic sources. As figure 5 illustrates,the phase sequence difference results in cancellation of both 5th & 7th harmonics (the result is thesame for 17th & 19th etc. as well). This method has been used for decades in aluminum andelectrochemical industries by using secondary phase shifted transformer windings to supply heavynon-linear rectifier loads.POWERSMITHS INTERNATIONAL CORPpage 7Designing next generation electrical systemsFigure 5: How a 30 phase-shift between 2 sources result in cancellation of both 5th & 7th harmonic currentsA: No ShiftB: 30 ShiftA7A530B 7 = 7 x 30 = 210counter-clockwise(7th is positive sequence)B7B5B 5 = 5 x 30 = 150clockwise(5th is negative sequence)RESULT: 5th harmonic in 7th harmonic inA opposes 5th inA opposes 7th inBB5. Inherently Treating Problem HarmonicsBecause of the direct relationship between harmonic current and system impedance in the creationof voltage distortion, it is clear that the location of highest voltage distortion will be at theharmonic-producing loads themselves - at the deepest point of your distribution system. Figure 6illustrates the impact of system components like transformers and cables as well as load onvoltage distortion. It is the combination of high zero sequence impedance of the components andhigh levels of 3rd harmonic current (which is zero sequence in nature) that causes a dramaticincrease in 3rd harmonic distortion at various points.The figure also illustrates that it is true that the transformer has blocked the zero sequencecurrents from flowing upstream, but at the expense of a substantial increase in voltage dist