P ower system anomalies that peri-odically occur in a facility’s elec-trical system can cause equipment

malfunction, data distortion, transformerand motor insulation failure, overheatingof neutral buses, nusiance tripping of cir-cuit breakers, and solid-state componentbreakdown. The cost of power qualityproblems can be enormous. Equipmentreplacement can cost tens of thousandsof dollars. Downtime, however, can runin the millions of dollars.

Power quality problems take manyforms. Voltage sags, transients, and spikesare probably the most understood of theseforms. Harmonic distortion, though, canoccur from adjustable-frequency drivesand other non-sinusoidal loads typicallywhere ac/dc conversion is present.

In any facility, the voltage supplied by apower system is generally not a pure sinewave. Rather, it usually possesses someamount of distortion, which has a funda-

mental frequency and harmonics at thatfrequency, (see box, “Harmonic basics”).

Distortion comes from various sources,particularly equipment with power-cir-cuitry devices that draw current in a non-linear fashion. Non-linear devices arethose that switch the current on and off,such as transistors, diode bridges, andSCRs. Two major classes of equipmentthat contain these devices and producecurrent harmonics are:

• Internal power supplies, such as com-puters, copiers, and electronic ballasts.

• Any type of static power converter,such as an uninterruptiblepower supply, dc drives, oradjustable-frequency con-trollers (AFC).

An AFC has a convertersection, which converts acline power to dc, and aninverter section, whichconverts dc to adjustablefrequency ac. Both con-tain non-linear devices intheir power circuitry, andtherefore produce har-monics on the input andoutput lines.

Input line harmonics are caused solelyby the converter section and are usually

referred to as line-side harmonics. Out-put line harmonics are caused solely bythe inverter section and are called load-side harmonics. They are completely iso-lated from each other. Thus, load-sideharmonics only affect the equipmentdriven by the AFC, while line-side har-monics affect the whole power system.

Standard three-phase PWM drives typi-cally use a six-diode rectifier bridge in theconverter section, Figure 1. These diodesdraw current non-linearly, in a 6-step con-verter waveform, Figure 2. The high lev-els (amplitude) of 5th and 7th harmonics

Eliminating harmonics from the facility power system

Adjustable-frequency drives and other devices can produce harmonics on a facility’s power system. These harmonics can corrupt data, damage equipment (usually by excess heat), and cause erraticequipment performance. But these effects can be controlled by monitoring and analyzing the whole

system, determining safe harmonic levels, and choosing the right attentuation solution.

KEVIN J. TORY and RICH POPE, Cutler-Hammer, Eaton Corp.

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Kevin J. Tory is application engineeringmanager, Solid State Motor Controls, Cutler-Hammer, and Rich Pope is product man-ager, Power Distribution Components Div.,Cutler-Hammer, Eaton Corp., Pittsburgh.

Figure 1 — Six-diode rectifier bridge inthe converter section of a standard three-phase PWM drive.

A B C

DC+

DC-

and the magnitude of each decreases asharmonic number increases. Harmonicsabove the 25th generally have little effectand are usually considered insignificant.

In this waveform, only certain har-monic orders are present: 5th, 7th, 11th,13th, 17th, 19th, 21st, and 25th. To deter-mine the number of harmonic orders in awave for a six-diode bridge, solve for:

h = 6k ± 1, where k is an integer.

Almost any three-phase system willpossess a degree of line unbalance, pro-ducing 3rd harmonics. In a balancedthree-phase system, however, these har-monics are in phase with each other andtheir effect is typically small.

Load-side harmonic effects

As mentioned earlier, load-side har-monics are generated by the inverter sec-tion of an AFC and may effect the motorand connecting cables. AFCs can de-crease motor life because of the addi-tional heating caused by the harmonics(see PTD, “Solutions to motor insulationfailures,” 8/95, p. 43). High-efficiencymotors with a service factor of 1.15 canhelp compensate for deleterious effects.

Line-sideharmonic effects

Unlike load-sideharmonics, line-sideharmonics effect thewhole power distri-bution system. Har-monic currentsdrawn from thesource give rise toharmonic voltagesthat affect otherequipment on thedistribution system.How much thesevoltages effect the

system depends on system load and im-pedance. Equipment considered sensi-tive to harmonics includes communica-

tion equipment, computers and compu-ter systems, diagnostic equipment,switchgear and relays, transformers, andstandby generators.

Communication equipment, comput-ers, and diagnostic equipment are de-signed for operation on smooth sinu-soidal input. Therefore, harmonics cancorrupt data or result in false commands.

Transformers may experience extraheating in the core and windings. Tocombat potential problems, many trans-former manufacturers rate their prod-ucts with a K-factor. This factor indicatesthe transformer’s ability to withstanddegradation from harmonic effects.Some manufacturers simply derate theirtransformers to compensate for potentialproblems. Others incorporate specialfeatures to better handle harmonic cur-

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I5 = 22.5%

I7 = 9.58%

I11 = 6.10%

I21 = 1.12%

I25 = 0.86%

I13 = 4.06%

I17 = 2.26%

I19 = 1.77%

Figure 2 — A 6-step converter waveform.

Harmonic basics

Harmonic currents and voltages are usuallymultiple sinusoidal waveforms that combine toform distorted waveforms. Figure A and FigureB waveforms are both pure sine waves, free ofdistortion, and differing only in frequency andamplitude. Figure B has five times thefrequency and one-fifth the amplitude of FigureA. The wave in Figure A is called thefundamental. The wave in Figure B is aharmonic of the wave in Figure A because itsfrequency is an integral multiple of it. Becausethe harmonic has a frequency five times that ofthe fundamental, it is called the 5th harmonicor 5th order harmonic.

If both waves are on the same power systemsimultaneously, they would add together,resulting in the waveform shown in Figure C.This resultant waveform is still periodic andhas the same frequency of Figure A, but it nowdeviates from the sine-wave shape of itsfundamental frequency.

Just as waveforms can be added to producedistorted waves, distorted waves may bedecomposed into fundamental and harmoniccomponents.

A

B

C

f x SinxSin x( ) = + 5

5

rents. These features include speciallydesigned cores and windings to reduceeddy currents and heating, and an over-sized neutral bus.

With standby generators and theirvoltage regulators, harmonics may causethem to put out a significantly high or lowvoltage. Or, because a generator has animpedance higher than typical distribu-tion transformers, harmonic currentsflowing in the generator can produce har-monic voltages three to four times thenormal levels. Sensitive equipment, suchas that used in hospitals and computercenters, may be severely affected. Thus,engineers should perform an extensivesystem study in any application of non-linear devices on systems with standbygenerators.

Take care when applying harmonic-generating equipment on systems withlarge amounts of capacitance in parallelwith inductance, such as systems withpower-factor correction capacitors or ca-pacitive welders. System resonance canoccur at one of the harmonic frequencies.Resonance can amplify the harmonics,which would exacerbate the effects. Cal-culating the harmonic resonance-fre-quency will help determine if a problemmay occur.

For example, with a 1,500 kVA trans-former with a 5.75% impedance con-nected to a capacitor load of 600 kVA:

where:hr = harmonic resonance frequencyTr = transformer rating, kVACr = capacitor load, kVAZ = impedance, %The harmonic point of this example is

the 6.59th harmonic, which can be prob-lematic because the system resonancemay be excited by the 5th and 7th har-monics. Attentuation of these two fre-quencies would likely avoid problems.

Detecting harmonics

The first step in eliminating thesepower quality problems is identifying andisolating their source through powermonitoring and data acquisition systems.Such systems provide real-time monitor-ing, sub-metering, electrical data trend-ing, and specific information on magni-tude, time, and direction of power qualityrelated events.

A power monitoring system can in-clude metering devices, protective relays,

circuit breaker trip units, and motorstarters. Data from these devices, passedto a control into available software pack-ages, can then be graphically displayedon a personal computer or other operatorinterface device.

A harmonic analyzer is another devicethat can determine if a harmonics prob-lem exists within a facility. Availablefrom several sources, it is essentially a

spectrum analyzer that decomposes a dis-torted wave into its component waveformand gives the relative amounts of each.

Third party companies also provideharmonic analysis for a fee. In some ar-eas, electric utilities will provide suchservices as well.

POWER TRANSMISSION DESIGN APRIL 1997 45

hT

Z C

h

rr

r

r

=

= ( ) =

1500

0 0575 6006 59

..

Figure 3— (Left side) Wave shape from a clean power rectifier for a 500-hp motor at460 V. (Right side) Waveform and harmonic contents of a 6-pulse system. With thistype of rectifier, no additional filters, inverters, or transformers are needed. It stopsharmonics at their source.

I5 = 22.5% I7 = 9.38%

I11 = 6.10% I13 = 4.06%

I17 = 2.26% I19 = 1.77%

I23 = 1.12% I25 = 0.86%

I5 = 0.16% I7 = 0.03%

I11 = 0.24% I13 = 0.1%

I17 = 0.86% I19 = 1.0%

I21 = 0.01% I25 = 0.01%

A power analyzer, such as the IQ Analyzer,can provide data on harmonic distortion,current and power demands, trending,and events and alarms. Information isaccessable in real time or can berecorded for later analysis.

Though harmonic analyzers provide anaccurate and cost-effective means for de-termining the harmonic content on a sys-tem, they do not predict the effects of fu-ture system modifications. Somecompanies will perform a system study todetermine if any harmonic attenuation isneeded prior to installation of AFCs, andmake recommendations.

Attentuation methods

To attentuate harmonics, users canuse passive filters, inductive reactors,phase-shifting transformers, active fil-ters, or multi-pulse converter sections.

Passive filters apply tuned series L-Ccircuits (circuits with inductance and ca-pacitance) that attentuate specific har-monic frequencies. Circuit elements,similar to notch filters, tune to a givenfrequency. The filter offers a low-imped-ance path-to-ground for the chosen fre-quencies. Though conceptually simple,this solution has potential complications.

For one, it is system dependent, suchthat any future system changes may re-quire re-tuning or resizing the filters.

Also, the filter cannot discriminate be-tween those harmonics created by anAFC and those created by other sources.Thus, a filter may be sized properly forone piece of equipment but undersizedfor the system.

Tuning filters is sometimes labor in-tensive, which may present high initialcosts and future costs if re-tuning is re-quired.

Inductive reactance, in the form ofline reactors or isolation transformers,can help attentuate higher order har-monics and reduce overall harmonic con-tent. This can be a simple, inexpensivemethod for attentuating harmonics, par-ticularly if just a small reduction isneeded.

An effective way of applying isolationtransformers is to supply balanced loadsfrom phase-shifting transformers. Oneload may be fed from a delta-delta trans-former and a similar load fed from adelta-wye transformer (see PTD, the“1997 Handbook,” p. A50-51). A 30-de-gree phase shift between the harmonicsources will cancel the 5th and 7th har-monics. Users can expect about a 50%

harmonic reduction from this method. Loads that are phase-shifted from one

another should be balanced for best ef-fect. Lesser cancellation can occur forloads not precisely balanced.

Active filters, or adaptive compen-sators, constantly monitor the current onthe line and inject equal and oppositeharmonics as necessary. These devicesare effective, but are still in the initialstages of development and implementa-tion, and somewhat costly. There is alsosome concern about reliability, sincemost designs have transistors that aresubject to the conditions on the line.

Multi-phase converters have separaterectifiers that are fed from phase shiftedsources. These devices can be an integralcomponent to some AFCs.

A 12-pulse multi-phase converter, forexample, uses two separate 6-diodebridges fed from a special transformer.The transformer supplies the bridgeswith two three-phase sources phase-shifted by a given amount. In theory, the12-pulse system generates harmonics ac-cording to the formula:

h = 12k ± 1It will eliminate harmonics to the 11th

and reduces the 11th and 13th harmon-ics. Rectifiers come in stages of 6 pulses,thus, the next level rectifier is an 18-pulse system, which eliminates all har-monics to the 17th, and offers small re-ductions in the 17th and 19th harmonics,Figure 3. To accomplish these reduc-tions, some rectifiers use a single 18-diode bridge with integral phase shifting.Thus, there are no additional bridges tobalance with one another.

In general, any bridge rectifier gener-ates harmonics according to h = nk ±1,where n is the number of pulses (ordiodes) and k is an integer. ■

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IEEE 519 standard

The IEEE originally recommended safe levels of harmonics in the standard IEEE 519-1981. At that time, voltage distortion limits were set as shown in Table 1.

The latestrevision of thisstandard sets morestringent limits oncurrent distortion inaddition to the

voltage distortion limits. As can be seen inTable 2, the amount of current distortionallowed depends on the available shortcircuit current and the maximum load currentat the point of common coupling (PCC).

As the system short circuit capacitybecomes large compared to the load at thePCC, the distortion limits become morelenient. This is because the harmonic effectsof small loads will be washed out on largersystems.

Application % Notch Notch V-class distortion depth µsec

Special system 3% 10% 16,400General system 5% 20% 22,800Dedicated system 10% 50% 36,500

Isc/IL TDD

< 20 5%20 to 50 8%50 to 100 12%

100 to 1,000 15%> 1,000 20%

Isc = Maximum short circuit current at the point of common coupling.

IL = Maximum load current (fundamental frequency) at point of common coupling.

TDD = Total Demand Distortion: Harmonic content distortion as a percent of maximum demand load current.