Power Transformer attenuates harmonics.pdf

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Power Electronics Technology October 2004 www.powerelectronics.com14

Power Transformer

Attenuates HarmonicsA novel line-frequency transformer design shapesthe transformers 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

The mains of the world were constructedas a giant copper causeway to transportelectricity from the generating stationto our homes and factories. Todaysproblems on the mains were unknown

a 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 through

the 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 doesnt rely on separate induc-tors, capacitors and resistors, but instead defines thefilter based on the transformers inherent internal charac-teristics of leakage inductance, internal capacitance andresistance.

Sources of HarmonicsHarmonics are currents and voltages at frequencies that

are 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 inampli tude 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 isnt 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 an d current wa veform for n onlinear load . The voltage waveform is sinusoidal, but current waveform is not.


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the problem has become more severe in the last few de-cades. Most of these products didnt 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-tor ted 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 repetit ive 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 PowerAlthough the root cause of the harmonic problem is

the same in different settings, the magnitude of the prob-lem is scalable and shows up at many levels. On an indus-trial commercial scale, its 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 find

their 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 transformer

secondary share the effects of the harmonicsso its acommunity issue.Most harmonics originate 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 todays solid-state equipment. With the advent of powersemiconductors and the use of switching power supplies,

Fig. 2. Waveforms show ing h arm onics typical in switchi ng d evices.

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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 industrial

sites are the always dreaded third, fifth, seventh, eleventh

and 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-

li es. Thus, it can be seen that on an industrial, commercialor residential scale the affects of harmonics are becom-ing a severe but hidden catalyst resulti ng in equipmentfailure, expensive downtime and low-efficiency power

utilization.On a smaller desktop scale, the problems

of 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 the

product 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 todays 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.

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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 from

1 to 100 at 50 Hz or 60 Hz). Therefore, for optimumattenuation, the impedance of the filter should be low aswell . In reali ty, 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 noi seis 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 fi ltering 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 prim ary o f the t ransformSeries ind ucto rs m ust carry full prim ary current, and C follo wi ng increases leakage current to g roun d.

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www.powerelectronics.com Power Electronics Technology October 200423

POWER TRANSFORMER

Fig. 6. Frequency respon se for a stand ard to roid p ow er transform er (a), bal anced po wer tran sformer design (b), a standa rd toro id w ith a com m ercial filter (c) an d an NBT design (d).

NBT can be adapted to most trans-former-based power applications.

An NBT transformer performs as alow-pass filter with a selected cornerfrequency. The system is based on twoprinciples that involve an increase of

the internal series inductance and thephase cancellation principle.We can use these two systems either

combined ( Fig. 4 ) or individually(Fig. 5 ). The value for leakage induc-tance L leak is calculated through the fol-lowing equation:

N Leak = Lleak i

where N is the number of turns,Leak is the leakage flux and i is the cur-

rent. Because leakage inductance isfunction of leakage flux, L leak can becontrolled by the design method. Aneffective fi lter can be achieved throughcontrolled leakage inductance. Thistypically offers good attenuation to10 MHz.

To extend the attenuation band-width into the gigahertz range, phasecancellation is obtained by connectinga bifilar-wound control winding incontraposition through a capacitor, C fp,as shown in Fig. 4 . The optimum valueof Cfp is computed through P-Spicemodeling, using Butterworth tuning:

Cfp = K (Lleak P2) / (Vp

2 Vs2)

where K is a constant, P is power, and V p and V s are pri-mary and secondary voltages. At low frequencies, the ca-pacitor acts as an open switch, allowing the power fre-quency (50 Hz or 60 Hz) to freely cross the transformer.

At high frequencies, the capacitor behaves as a closedswitch. Therefore, the magnetic flux of the two windingscancels one another and full deletion of high-frequencysignals occurs. By adjusting the series inductance and thecapacitor, the passing bandwidth of the transformer canbe controlled.

NBT resolves both differential and common-mode noisewith the help of increased series inductance, phase cancel-lation principal and a reduction in primary to secondarycapacitance. Typically, high-frequency fi lters are applied toremove the noise before the line voltage enters the powertransformer that supplies the electronic equipment. How-ever, with NBT the power transformer becomes an effec-tive noise rejection filter and the external components toremove the high-frequencies are no longer required.

The elimination of primary-side filter components not

only reduces parts count and cost, but also expedites safetyagency and approvals certifications. In addition, reducedleakage current results from eliminating line-to-groundcomponents.

How NBT WorksThe two techniques of NBTincreasing internal series

inductance and phase cancellationsatisfy different at-tenuation requirements. Simply adding leakage inductancehas the affect of inserting a series inductor and removingany current handling limitations. The increased leakageinductance L leak

is the prime factor in the performance of the NBT design. The combination of the leakage induc-tance with the transformed capacitance from secondary toprimary (C) and the primary dc-resistance (R) acts as asecond-order low-pass filter. The corner frequency of thefilter is determined by the combination of the L, C and Relements along with the load impedance Z L.

To increase the usable frequency range, a capacitor canbe added across a special control transformer winding. Asdepicted in Fig. 4 , the secondary winding is extended withan extra winding (control winding), which has an equal


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number of turns to the existing secondary winding but isconnected in reverse phase through a capacitor ( C fp).

At low frequencies, the impedance of the control ca-pacitor is high, the capacitor acts as an open switch, onlyone secondary winding functions, and the 50 Hz or 60 Hzis free to cross the transformer. At higher frequencies (above

1 MHz), the control capacitor begins to act as a closedswitch. Both secondary windings now generate magneticflux in the transformer core; however, with 180 degreesphase difference. Therefore, the magnetic flux of the twowindings cancels one another and full cancellation of high-frequency signals occurs. Then, there is no magnetic trans-fer of energy through the core to the secondary.

An advantage of this approach is the large impedanceof L at high frequencies. Noise from the line will not bereflected at the input terminals of the transformer, but ab-sorbed in L. At high frequencies, the NBT transformer willdeliver no load to the power lines.

When the control winding is used to deliver energy toan extra secondary load, more efficient use of copper ismade ( Fig. 4 ). When the loads are equal, the total effectivecurrent through connection (2) becomes zero (phase can-cellation) . When connecting (2) to ground, a clean groundreference is created without high-frequency noise.

To verify the performance of the NBT transformer, fre-quency response tests were carried out, with the results

shown in Fig. 6 . The primary of the transformers is fed byan oscillator and the voltages of the primaries, and theloaded secondaries are measured at frequencies rangingfrom 50 Hz to 1GHz. The ratio of the secondary to pri-mary voltage, in decibels, versus frequency is plotted foreach transformer. Four different toroidal transformer con-

structions were compared:Standard toroidal transformerBalanced power (biflar-center tapped secondary con-

nected to ground)Standard toroid with external fil terToroidal transformer incorporating NBT.

The standard toroid (a) has a high corner frequency of about 50 kHz and a low attenuation rate. Balanced power(b) has better performance with a reasonably low cornerfrequency of about 3 kHz and an attenuation of about -15dB around 100 kHz and -30 dB close to 1GHz. The stan-dard toroid with an external commercial fi lter (c) performs

much better in the range of 50 kHz to 40 MHz in compari-son with the balanced power, but has poor corner frequencyof about 60 kHz. NBT (d) outperforms the other designsin terms of both corner frequency and attenuation. It has alow corner frequency of approximately 1 kHz, which canbe design-adjusted to any reasonable value, with attenua-tion in excess of 60 dB around 1 MHz and 35 dB at 1GHz.

ApplicationsThe applications of NBT in power

supply transformers are various. Agood example is audio applications,where it is important that differentialhigh-frequency noise not enter thesensitive audio equipment. A secondapplication is found in general power-supply transformers in any electronicequipment where differential noisefi ltering is mandatory. The choice foran NBT-transformer is then based onthe balance of costs for an NBT trans-former compared to a standard powertransformer with an external differ-ential-mode filter.

In IT applications with uninter-rupted data transport over long dis-tances, the advantages of clean powerlines are obvious. In medical applica-tionsespecially safety-critical pa-tient connected devicesNBT trans-formers with electrostatic shields pro-vide clean power with low leakagecurrents. In large power applications,NBT can remove the higher harmon-ics with cutoff frequencies startingfrom 800 Hz. PETech

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Power Transformer attenuates harmonics.pdf