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Power Transformer attenuates harmonics.pdf
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  Power Electronics Technology October 2004www.powerelectronics.com 14 Power TransformerAttenuates Harmonics A novel line-frequency transformer design shapesthe transformer’s leakage inductance, internalcapacitance and resistance to create a low-passfilter that suppresses noise generated by non-linear loads and RF interference. By Brian Gladstone, Director of Engineering, and HenryPajooman,  Technical Editor, Plitron Manufacturing Inc., Toronto, Canada  T he mains of the world were constructedas a giant copper causeway to transportelectricity from the generating stationto our homes and factories. Today’sproblems on the mains were unknowna few years ago, when power grids were expanding all overthe continent. No provision existed to carry frequenciesthat are thousands of times higher than the fundamentalpower bandwidth.A new form of pollution envelops us from within ourwalls and is interfering with the operation of our appli-ances and our lives. Uncontrolled harmonics can diminishthe life span of equipment and accelerate failures; can causeexcessive heat in many appliances, leading to shock andfire hazard; and can increase power consumption and re-duce system efficiency. Most insidiously, these harmonicscan propagate through the power grid and infect everyonein the neighborhood.A common approach to cleaning the mains is throughthe use of an LCR filter network. However, an innovativealternative addresses the causes of line distortion andoffers a simple transformer-based solution to clean upthe mains. This approach doesn’t rely on separate induc-tors, capacitors and resistors, but instead defines thefilter based on the transformer’s inherent internal charac-teristics of leakage inductance, internal capacitance andresistance. Sources of Harmonics Harmonics are currents and voltages at frequencies thatare integer multiples of the fundamental power frequency.As a result, power lines contain pure undistorted 50-Hz or60-Hz sine wave voltages as well as other signals. The sinewave is distorted and consequently harmonics of the50-Hz or 60-Hz fundamental are found. At higher frequen-cies, switching transients appear from rectifiers, motordrives and other sources. In addition, at frequencies above50 kHz, strong HF signals from radio, TV and computersare superimposed on the line and appear across the pri-mary winding of a transformer.These extra signals, called noise or distortion, appear intwo ways on the power lines. At frequencies above 1 MHz,noise is mostly common mode, which refers to bothline and neutral containing an equal amount of amplitudeand phase distortion. For frequencies below 1 MHz,the major component of the noise is typically differentialmode, where the noise on line and neutral sides is equal inamplitude and opposite in phase. Differential-mode noisegenerates a real noise voltage difference between line andneutral.If all these harmonics and noise on the line are detri-mental and dangerous, why isn’t there some sort of con-trol over the power quality leaving the generation station?As a matter of fact, there is. The power leaving the plantand fed into the power grids is clean, green and sinusoidal Fig. 1.  Voltage and current waveform for nonlinear load. The voltage waveform is sinusoidal, but current waveform is not.  Power Electronics Technology October 2004www.powerelectronics.com 16 the problem has become more severe in the last few de-cades. Most of these products didn’t exist 30 years ago, thusthe trouble is recent and a direct result of technologicalinnovation.A nonlinear load draws current in a non-sinusoidalmanner, despite the fact the voltage may be perfectly sinu-soidal ( Fig. 1 ). Nonlinear loads draw current during a por-tion of the incoming voltage waveform, not continuouslyas with a light bulb. Current is drawn in bursts or plannedabrupt pulses, as required by the product. The result is dis-torted current wave shapes, the harmonic content of whichcan flow back and contaminate other parts of the powersupply ( Fig. 2 ).Harmonics and the resulting harmonic distortion are aconstant repetitive occurrence within a product. Sometimestransients on the line are confused with harmonics, butthey are not the same. Transients typically are not relatedto normal operating conditions and are a random occur-rence with no repeatable time signature or frequency. Electronics Demand Clean Power Although the root cause of the harmonic problem isthe same in different settings, the magnitude of the prob-lem is scalable and shows up at many levels. On an indus-trial commercial scale, it’s not uncommon for a buildingor plant engineer to face nonlinear loads in excess of 25%.in nature. It is rarethat the lowly stateof power founddownstream isrelated to thesource generator.We must lookelsewhere for thesource, not to thegeneration of power.The harmonicsgenerated down-stream can findtheir way back onto the utility lines and affect all powerusers on the system, and ultimately adversely affect theoperation of utility and distribution power transformersall down the line. All loads in common with the transformersecondary share the effects of the harmonics—so it’s acommunity issue.Most harmonics srcinate from the generation of harmonic currents caused by nonlinear load signatures. Anonlinear load is characteristic in products such as com-puters, printers, lighting and motor controllers, and muchof today’s solid-state equipment. With the advent of powersemiconductors and the use of switching power supplies, Fig. 2.  Waveforms showing harmonics typical in switching devices.  POWER TRANSFORMER CIRCLE214on Reader ServiceCard or freeproductinfo.net/pet Kooler Inductors P.O. Box 11422 ã Pittsburgh, PA 15238-0422Phone 412.696.1333 ã Fax 412.696.0333 1-800-245-3984 email: magnetics@spang.comã www.mag-inc.com Inductors made fromMAGNETICS’ ® Kool Mµ ® E coresrun cooler than those made with gapped ferrite cores.Eddy currents, caused by the fringing flux across thediscrete air gaps of a gapped ferrite, can lead toexcessive heat due to heavy copper losses. Thedistributed air gaps inherent in Kool Mµcan provide amuch cooler inductor.Kool MµE cores are available in many industrystandard sizes. MAGNETICS nowoffers cores in 13sizes (from12 mmto 80 mm) and four permeabilities(26µ, 40µ, 60µ, and 90µ). Newsizes are beingadded. Standard bobbins are also available.If you are using gapped ferrite E cores for inductorapplications, see what Kool MµE cores can do for you. You may even be able to reduce core size in additionto having a cooler unit. Production quantities are nowin stock. For more information, contact MAGNETICS. New Sizes Available!New Sizes Available! Visit us at Power Systems World, Booth #1619  Power Electronics Technology October 2004www.powerelectronics.com 18  POWER TRANSFORMER Most installations can manage 10% to 15%. However,seemingly unaccountable though symptomatically predict-able things begin to occur when the total harmonic distor-tion (THD) levels rise above this range. Local distributiontransformers can become inductively overheated for noapparent reason and suffer minute levels of daily deterio-ration. Their usable life is shortened and early failure willresult.Problematic harmonics for commercial and industrialsites are the always dreaded third, fifth, seventh, eleventhand a few other assorted odd numbers. Within the affectedsite, other harmonic-induced problems will be experienced,such as electronic equipment shutting down as a resultof voltage distortion, nuisance fuse interruptions, motorfailures due to overcurrent caused by undervoltage, andvarious other destructive, mysterious equipment anoma-lies. Thus, it can be seen that on an industrial, commercialor residential scale the affects of harmonics are becom-ing a severe but hidden catalyst resulting in equipmentfailure, expensive downtime and low-efficiency powerutilization.On a smaller desktop scale, the problemsof an uncontrolled harmonic-rich environ-ment manifest themselves in other ways.Our focus will be on desktop and house-hold scale. Electronic equipment is sensi-tive to noise entering through the powerline. This unwanted noise may affect theproduct in many ways, including perfor-mance degradation and malfunctions. Theproblematic harmonics for computers andequipment are higher in frequency thanthose that plague power systems. Whendealing above 100 kHz or so, we would re-fer to them in terms of frequency ratherthan harmonic number, so the terminol-ogy would speak of a 5-MHz componentand not the Nth harmonic.Processing speeds are increasing at a fastrate. The clock frequencies and ultrahigh-speed operation of today’s electronicswould seem like science fiction to engineersa few decades ago. But because of the highclock rates associated with modern micro-processors and the high switching frequen-cies associated with switching power sup-plies, PCs and other equipment are guiltyof generating and kicking back massiveamounts of distortion into the line.Ironically, the same equipment thatgenerates this distortion demands cleanpower to operate. Modern electronic equip-ment depends on a low-distortion voltagesupply to operate to spec, and there ishigh sensitivity to fluctuations and tran-sients. In addition, large pulsating currentscan cause flat topping of the voltage wave-form. Noise can be introduced into suscep-tible cables or other components fromhigh-frequency circulating currents, caus-ing havoc with microprocessors and othersensitive components. Common Solutions toAttenuate Noise Effective noise rejection filters should www.resistor.com  2470 Fox Hill Road ã State College, PA 16803-1797 Call Toll Free: 1-800-458-3401 Fax: 814-355-2714 ã e-mail: sales@resistor.com Stateof theArt,Inc. MADE I N USA  R    E    G   I  S  T   E    R      E       D    S  O  T  A   I S O  9 0 0 1 Reliable Resistors   Full range of thick and thin filmresistive products ideal for ã Medical electronics ã Defense systems ã Microwave communications ã Aerospace electronics ã Satellite systems  Unsurpassed established reliability failure levels forMIL-PRF-55342 chip resistors,including S level (0.001% per1,000 hours) and T (space) level  Exclusive MIL-PRF-914 surfacemount network qualification  Your systemis only as reliableas the components you use. Why risk a great design onanything less than the world'shighest reliability resistors?State of the Art resistors. Why risk it?Why risk it? 50 million par ts in s tock! CIRCLE216on Reader ServiceCard or freeproductinfo.net/pet  Power Electronics Technology October 2004www.powerelectronics.com 20    pass the 50-Hz or 60-Hz fundamental and remove all higherfrequencies. However, the line source impedance, combinedwith the impedance of the actual load, is low (ranging from1   to 100   at 50 Hz or 60 Hz). Therefore, for optimumattenuation, the impedance of the filter should be low aswell. In reality, however, this would require impracticablylarge and expensive capacitors and inductors.A more practical approach is to start filtering noise atfrequencies above 1 kHz, where most of the unwanted noiseis found and where such interference causes malfunctionof electronic equipment. The filter should be of the low-pass type with second- or higher-order slopes. The inter-nal capacitance and inductance inside the transformer arethe tools to create the desired filter. A common solutionis the installation of an off-the-shelf line filter, which isavailable in a variety of configurations from variouscompanies ( Fig. 3 ).Sometimes, line filters are packaged in the same box withother primary circuit modules, such as an input selectorswitch (dials 100 Vac, 120 Vac, 220 Vac or 240 Vac posi-tions), or an IEC connector for power cord or fuse hous-ing assembly. These devices offer good filtering and attenu-ation and have proven successful in many products.Upfront line filters usually are specified for reasons suchas compliance to CE or other legislated standards for radi-ated emissions, or in products where noise enteringthrough the primary circuit is detrimental to the opera-tion of the product.External line filters are installed in series with the pri-mary circuit, as shown in Fig. 3 , and thus must carry thefull primary current (load current passing through the in-ductors L). Therefore, there may be some power or perfor-mance limitation imposed by current handling capabilityof the series inductor as it must grow in size, weight andthermal dissipation to accommodate higher power devices.In addition, the high-pass shunt capacitors from line andneutral to ground increase system leakage current toground. This becomes a significant factor where low levelsof leakage current are demanded, such as in medical appli-cations in patient care devices.Another common solution is the use of K transformers. Fig. 3.  An external LC filter connected to the primary of the transformer.Series inductors must carry full primary current, and C following increases leakage current to ground. POWER TRANSFORMER CIRCLE218on Reader ServiceCard or freeproductinfo.net/pet Visit us at PowerSystems World, Booth #1714
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