n article appearing in PLANT ENGI- NEERING nine years ago covered all A of the power system aberrations like-

ly to cause malfunction or damage to sensi- tive electronic equipment in industrial plants.* Notably omitted from the six-page discussion of power system plagues was any mention of harmonics. The omission was not an oversight. For practical purposes, the effect of harmonics on electronic equip- ment in manufacturing plants was essen- tially a nonproblem as recently as nine years ago.

An Old Malady with a Host of New Victims In years past, harmonics were essentially

a problem experienced‘ by electric utility companies, with a relatively few types of industries impressing a serious degree of harmonics onto the utility system. Typical among these industries were primary met- als, where a disproportionate share of the load comprised such notorious harmonic

equipment creates a problem that feeds on itself: (a) solid-state electronic equipment is a flagrant generator of harmonics, and (b) solid-state equipment is, itself, intoler- ant of harmonics and is susceptible to mal- function and failure when served from a harmonic-laden power source.

How Harmonics Are Created Simply stated, harmonic currents are the

result of nonlinear loads, in which the re- sultant current waveform does not conform to the shape of the applied voltage wave- form. Common types of equipment in which this phenomenon occurs are given in the section, “Causes of Harmonic Distor- tion.” It is significant that many of the har- monic polluters identified in this section were just beginning to come into their own a decade or two ago.

How electronic equipment develops har- monic currents is described in the section “How Harmonics are Generated.” The ex-

LIVINGWITH Power System Harmodcs

Here is informatzo n on the causes and effects of harmonics, and bow to cope with the problems they create

ED PALKO, Senior Editor generators as arc furnaces and mercury-arc rectifiers. Utilities typically solved their harmonic problems by imposing limits on the amount of harmonics that a customer was permitted to reflect onto the utility sys- tem, and assessing heavy financial penal- ties for noncompliance.

Plants with significant harmonic content simply had to endure or compensate for all of the harmful effects - save one. The ef- fect of harmonics on electronic equipment was a relatively minor consideration, be- cause solid-state electronic equipment had not yet become a major factor in industrial plants.

However, the proliferation of solid-state electronic equipment over the past decade has brought harmonic problems to bear in plants spanning the full spectrum of m a p - facturing industries. Today’s electronic

ample given is for a full-wave rectifier served from a symmetrical sine-wave volt- age source. Resultant current flow, howev- er, does not conform to a sine wave, with current flowing only when the rectifier conducts.

The result is a series of harmonic fre- quencies superimposed on the fundamen- tal (60 Hz) frequency. These harmonic fre- quencies are multiples of the fundamental and generally decrease in magnitude as their harmonic order increases. Harmonic order is defined as a multiple of the funda- mental. For example, a harmonic frequen- cy of 300 Hz developed on a 60-Hz system is called the 5th harmonic.

The distorting effect of a 5th harmonic superimposed on a 60-Hz fundamental-fre- quency waveform is described in the sec- tion, “How Harmonics Distort.” The con- tribution of all harmonic-fresuencv

‘ProvidingClean. Stable Power to Sensitive Electronic ment, PE, 03/17/83, p 32, File 0501.

currents to the fundamental Curient i’S known as total harmonic distortion (THD)

and is expressed as a percentage value of the fundamental current. It is significant that the heating effect of current is a function of the square of the rms (root-mean-square) value of current. Rms current value is determined by the area under the current wave, regardless of waveform. Conventional ammeters, how- ever, are narrow-band instruments that will yield a true rms reading only on a symmet- rical 60 Hz waveform. Such ammeters, therefore, do not yield a true reading on a circuit containing harmonics (see section

Problems That Harmonics Create Typical major problems caused by har-

monics are summarized in the section, “Harmonic Distortion Consequences.” Here are how the harmful effects are created.. .

Conductor overheating. Conductor heat- ing is a function of the square of the rms

“Don’t Kid Yourself”). ‘\

value of current per unit volume of conduc- tor. Obviously, an increase in rms current brought on by harmonics results in in- creased conductor temperature. Less un- derstood, however, is the increased con- ductor heating brought on by “skin effect.” Skin effect increases with frequency - and harmonic frequencies are always higher than the fundamental (60 Hz) frequency.

Skin effect is a phenomenon likened to centrifugal force, whereby electron flow is thrown toward the periphery of the con- ductor at higher frequencies. The net effect is to reduce the in-use cross-section of the conductor and increase current density at the conductor periphery.

On circuits bearing triplen harmonics, neutral conductors are subjected to still more heating than their companion phase conductors. Triplen harmonics are those integer harmonic orders evenly divisible by three - such as the 3rd; 6th, 9th; 12th, and 15th harmonic. On a balanced three-phase

Display and printout of harmonic data varies with the particular instrument. Shown are two of several printout conjigurations that can be derived from one in- strument. Accompanying waveform is displayed on instrument CRT, and re- tained in memory for subse- quent display or printout on external equipment. (Courtesy Angus Electronics eo.)

FILE 0501 JUNE 18,1992 PLANT ENGINEERING 49

~-

At the onse4 preventfon is an anachronistic approach to harmonicprobGems caused by ekch-onic equi@”t

system containing no harmonics, line cur- rents cancel each other in the neutral. Tri- plen harmonics, however, are additive in the neutral, and can result in severe neutral conductor overheating. Neutral overheat- ing might require replacing the neutral with a larger conductor, or supplementing it with a paralleled conductor.

Electromagnetic equipment. Equipment that operates on the principle of electro- magnetic induction (transformers, motors, electromagnetic lighting ballasts, solenoids, etc.) is also “double whammied” by har- monics. In addition to increased heating caused by higher rms currents, it is subject- ed to increased heating caused by eddy-cur- rent core losses; eddy current losses in- crease as the square of the frequency. Harmonics can also cause increased audi- ble noise in electromagnetic equipment.

Power factor improvement capacitors. Harmonics can create special problems on circuits containing power factor improve- ment capacitors. Inductive reactance varies directly with frequency, while capacitive reactance varies inversely with frequency.

On any circuit containing both types of reactance, there is one or more frequencies at which inductive and capacitive reac- tance are equal, creating a resonant condi- tion. The result is amplification of system parameters, analogous to the manner in which resonance induces or exacerbates vi- brations in a mechanical system. Extremely high currents can flow, blowing fuses or de- stroying capacitors, and dangerously high voltages can appear across system components.

The condition can be alleviated or elimi-

nated by filters or wave-traps containing proper amounts of capacitance and induc- tance (see section “Solving Harmonic Problems on Circuits Containing Capaci- tors”). Solutions, however, cannot be ap- plied unless the harmonic order and magni- tude of the offending frequencies are known.

Electronic equipment. Electronic equip- ment is especially susceptible to both mal- function and damage from harmonics, It is self-evident that electronic component damage can be caused by the additional heating induced by harmonic currents. Less obvious, however, are insidious mal- functions that can occur simply by virtue of waveform distortion. Much electronic equipment employs electronic clocks, with timing of the current-wave zero-crossing critical to proper operation. Harmonics can shift the zero-crossing point, with the offset creating havoc with the operating sequence.

Why Can’t Harmonics be Prevented? The harmonics prevalent in the typical

manufacturing plant today are primarily generated by electronic equipment, and electronic equipment designed and built for minimum harmonic output is readily available. One of the many design ap- proaches that can be applied is to build the equipment with integral harmonic filters.

All approaches, however, contribute sig- nificantly to the cost of the equipment. At the onset, prevention is an anachronistic approach to the inplant harmonic prob- lems that are brought on by electronic equipment, because prevention by the

How Harmonics

7

I I I I I 0 90 180 270 360

Electrical Degrees

Are Generated

Typical Harmonic Signature (THD=20-40%)

1 3 5 7 9 1 1 1 3

Wave-cbopping eflect of a full-wave recttjier operating at apring angle of 90 deg pro- duces currentpow that is not consonant with the applied sine waveform. Magnitude of individual harmonics varies with the particular unit, but total harmonic distortion for a full-wave recttjier typically ranges from 20 to 40%.

50 PLANT ENGINEERING JUNE 18,1992 FILE 0501

ounce can cost tons of money. Ironically, the additional cost of such equipment can seldom be justified unless harmonics are already causing problems in the plant.

The harmonic-producing equipment causing inplant problems today did not ap- pear overnight; it was added a piece at a time. And for the most part, each addition was well-diluted by existing linear loads - and, of itself, made a relatively insignifi- cant contribution to THD.

At some point, however, THD can achieve problem proportions. Unfortu- nately, the problem seldom announces it- self with a blare of trumpets. The disease creeps in insidiously in the form of symp- toms such as blown capacitors, unex- plained equipment overheating and failure, unaccounted-for blown fuses and breaker trips, chronic failure of electronic compo- nents, and “gremlin” misoperation of elec- tronic equipment.

Detection and Analysis While preventive medicine is not neces-

sarily a cost-effective approach to potential harmonic ills, diagnosis is highly advisable to detect or refute the existence of harmon- ic maladies, and to prescribe proper treat- ment if necessary. Harmonic analysis pro- vides answers to these questions:

Are harmonics of insignificant magni- tude, presenting no immediate cause for concern?

Can harmonics be dismissed as the cause of unexplained problems?

Is a uroblem condition beinn

ble for the problem? Of the equipment contributing to the

problem, which are the most serious offenders?

What is the frequency and magnitude of the offending harmonic currents?

Identifying frequency and magnitude fa- cilitates pinpointing the source of prob- lems, because much harmonic-polluting equipment can be identified by its harmon- ic signature (see section “Frequency Spec- tra of Some Typical Harmonic Genera- tors”). Knowing the amount of THD permits the extent of the problem to be quantified, and identifying the magnitude

- approached?

If a problem exists, is it a new or a long- standing problem?

Which items of equipment are responsi-

Three options for developing tuned circuits to solve harmonic resonance problems on circuits containingpower factor improvement capacitors are shown. Any of the solutions requires that the harmonic frequencies be identified and quantified. (Courtesy Dranetz Technologies, Znc.)

Pure, undistorted sine wave (left) has a unit magnitude of 1.0. Superimposing a 5th harmonic of 0.2 magnitude on this wave (center) develops the resultant waveform shown at right. The resultant 1.2 unit-magnitude waveform peak can trigger older solid-state overcurrent relays, which interpret average current as a function of the peak value of a presumed sine waveform.

FILE 0501 JUNE 18,1992 PLANT ENGINEERING - 5

Harmonic analysis instruments specz@uz@y for phnt en.’neering use appeared only about six years ago

Something is screwy. You have problems that bear all of the symptoms of current overload - fuses are blowing and break- ers are tripping under no-fault conditions, overload relays are operating, and trans- former temperatures are abnormally high. Yet, ammeter checks indicate no current overload. So what goes on here?

The problem could be that you are reading currents on circuits containing harmonics, and taking the readings with a conventional ammeter, rather than a true- rms-reading instrument. Conventional ammeters will yield a true rms reading

Don’t Kid Yourself only on undistorted 60 Hz waveforms, and do not provide a true reading on cir- cuits containing harmonics. On rare oc- casions they will over-read, but in almost all cases, read less than the true rms (root-mean-square) value of current. And it is the rms value of current that deter- mines the heating effect of current.

When harmonics are present, some older solid-state overload relays can also false-trip when current is actually within prescribed limits. Early solid-state relays interpreted current as a function of the peak value of a presumed undistorted

sine wave. Cognizant of the emergence of har-

monics as a widespread problem, instru- ment manufacturers developed product lines of ammeters that read true rms cur- rent, regardless of the waveshape. Such ammeters are commonly available today.

The existence of harmonics can be de- termined by taking comparative readings with a conventional vs an rms-reading ammeter. A disparity in the readings indi- cates harmonics are present. Determina- tion of specific frequencies, magnitudes, and source requires harmonic analysis.

and harmonic order permits solutions to be developed.

Instruments for Monitoring and Analysis Instruments for conducting harmonic

analysis have been available for many years, but were of the laboratory type in- tended for use primarily by physicists. Har- monic analysis instruments intended spe- cifically for plant engineering use appeared on the scene only about six years ago. Plant engineers today can choose from instru- ments offered by a number of suppliers (see section “Harmonic Monitoring and Analy- sis Instrument Sourcing Guide”).

Instrument cost ranges from less than $1500 to more than $17,000. As might be expected, instrument capabilities encom- pass a similarly broad range. At the low end are instruments that are essentially monti- tors only. Such instruments generally pro- vide only realtime capture and logging ca- pability for downloading and subsequent analysis on a personal computer or other external device.

Top-line instruments have integral anal- ysis capability, and contain such features as user-selectable scan rates, a variety of methods by which data can be displayed and retrieved, and multifunction monitor- ing/logging capability. Multifunction mon- itoring and logging capability includes power quality aberrations such as frequen- cy deviation, voltage instability, voltage transients, electrical noise, and power in- terruptions, and power system parameters such as volts, amperes, kW, LVA, kVAr, power factor, and kWh.

Each has its place, from the loftiest to the lowliest. A practical approach to the con- trol of harmonic problems is to purchase a number of low-cost instruments, so that problems can be monitored and detected at multiple locations throughout the plant. One or more “top gun” instruments can

then be brought into play for refined analy- sis when problems are detected by the low- cost units.

Surviving in the Harmonic Jungle When harmonics have been determined

to be a serious problem, consideration should be given to selecting and specifying new electronic equipment on the basis of low harmonic content. New electronic equipment should also be specified on the basis of its ability to operate on a power source of high THD. Consideration should be given to isolating critical loads that are especially sensitive to harmonics - either by serving them from a dedicated trans- former or a UPS system.

Transformers might have to be supple- mented with additional units, replaced with units of larger capacity, or replaced by “High K-Factor” units. High K-Factor transformers are designed for high harmon- ic withstand, and are described in ANSI/ IEEE (37 .1 I O Recommended Practice .for Establishing Transformer Compatibility When Supplying Non-Sinusoidal Load Currents.

The size of neutral conductors on three- phase systems might have to be increased, or the neutrals supplemented with paral- leled conductors. Overheating of neutral conductors can also be alleviated by bal- ancing the phase conductor loads not only on the basis of ampere load, but also on the basis of harmonic content.

Circuits containing capacitors might have to be modified to eliminate resonance problems, and dedicated harmonic filters/ wave traps might have to be installed at strategic locations. Existing capacitors can also be applied as the capacitive element of such harmonic filters, provided that they create no resonance problem.

Surveying should be conducted on an on- going basis, not only at known problem lo-

- PLANT ENGINEERING JUNE 1 8 , 1 9 9 2 FILE 0501

Frequency Spectra of Some Typical Harmonic Generators

Typical Unfiltered I *

Load Phases Typical Frequency Spectrum Current THD r I I I .#l I 1

I Variable-voltage Like six or twelve-pulse rectifier I adjustable-speed drive

Variable-frequency adjustable-speed drive 3 m h 1530%

1 6 12

I Fluorescent lighting 10-30%

I

Power conditioners and

supplies

Rectifier/inverter type UPSs and PCDXs appear as rectifiers to the ac supply bus uninterruptible power 1-3

cations on the power system, but also at locations bearing no history of problems.

Surveying should be conducted under varying conditions of load utilization, and whenever equipment is added or removed. It should be noted that harmonic problems can be created by the removal, as well as the addition, of a load. Large motors provide a case in point. Although motor loads gener- ate some harmonics, their relatively mini- mal harmonic contribution can result in a diluting effect that actually reduces THD when the motor is in service.

The author extends special appreciation to Dr. G.T. Heydt of Purdue University, Jerry Spindler of Angus Electronics CO., and Jim Toy of Dranetz Technologies, Inc., for their assis- tance in developing this article.

For more information. . . answer questions regarding this article.

Ed Palko is available at 708-390-2689 to

For information on how to order copies of this article circle 10 on post card

Harmonic Distortion Consequences

Electrical conductor overheating

Transformer overheat- ing and premature failure

Motor overheating and failure

Solenoid coil overheat- ing and failure

Fluorescent and HID lighting ballast failure

Capacitor bank over- heating and failure

High voltages devel- oped by capacitive/in- ductive resonance

Nuisance breaker trips and blown fuses

Unstable operation of adjustable-speed motors

UPS system failure Electronic component

failure Unreliable operation of

electronic equipment Communication sys-

tems interference

Harmonic Monitoring and Analysis Instrument Sourcing Guide

PLANT ENGINEERING acknowledges with appreciation these manufacturers who contributed information for this article. For more information on the harmonic monitors and analyzers offered by these manufacturers, circle the applicable reader service numbers on the post card in this issue.

Circle Company

t 280 Angus Electronics Co.

I 281 Basic Measurement Instruments

I ~. ~ -- -

282 Dranetz Technologies, Inc.

I 283 Eastern Time Designs

I .. -

284 John Fluke Mfg. Co.. Inc.

I 285 Power Measurement Ltd.

t 280 ACC Electronics

I 287 Tektronix. Inc.

I 288 Voltech, Inc.

I 289 Myron Zucker. Inc.

JUNE 18,1992 PLANT ENGINEERING 53 FILE 0501