harmonic currents.doc

8/10/2019 Harmonic Currents.doc

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Harmonic Currents

Power quality used to be to the ability of the electric utilities to provide electricpower without interruption. Today, power quality encompasses any deviation froma perfect sinusoidal waveform. This includes EMI and !I noise, transients,

sur"es, sa"s, brown outs, blac# outs, and any other distortions to the sinusoidalwaveform. $ne distortion to the sinusoidal waveform which has dan"erous

consequences is harmonics.

Harmonics can be present in volta"e, current or both. It is forecast that before theend of the century, half of all electrical devices will operate with nonlinear currentdraw. These nonlinear loads are the cause of current harmonics.

The term % harmonic currents& is a popular bu'' word in the power quality industryand the layman needs some information to understand the term and reali'e they can

be a serious problem if not treated properly. The treatment and the details ofharmonics is better left to the e(pert with the proper tools and trainin".

Harmonic currents cause overheatin" of electrical distribution system wirin",transformer overheatin" and shortened transformer service life. Electrical firesresultin" from distribution system wirin" and transformer overheatin" were rareoccurrences until harmonic currents became a problem. The frequency of such firesis now becomin" more common. The life safety issues of electrical fires are not partof this discussion, but should be considered.

Ima"ine the cost to a company such as a stoc# bro#era"e firm if a transformer andthe distribution wirin" failed durin" a busy tradin" day. Power is lost to their

tradin" computer equipment and they cannot carry out their normal business of

buyin" and sellin" financial instruments. )n electrical system fire could result anddestroy part of the electrical distribution system.

These events could ta#e days to correct to put this firm bac# in business. The entiredistribution system would have to be surveyed for dama"e. )ll dama"ed or

wea#ened distribution system equipment and wirin" would have to be replaced.This list of dama"es could include dama"ed conduit, transformers, switch "ear,

electrical panels, wire, etc. This list of dama"ed equipment probably will includeequipment connected to the electrical distribution system at the time of systembrea#down. *urin" such a brea#down, other power quality problems will beencountered includin" shortin" of transformers and wirin". Ima"ine the effect on

sensitive computer equipment not isolated from the electrical distribution system.Transient activity can have a catastrophic effect on computer and

telecommunications networ#s.

In effect, the stoc# bro#era"e firm in our e(ample has been put out of business byharmonics until repairs can be completed or operations moved to another location.+hat could be the bottom line cost Millions of dollars would be a conservativeestimate if they were a small firm. Estimate the cost to your own company if you

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were out of business for days or a wee# or lon"er. +ho would service yourcustomer +ould it be your competition Could your company financially survivesuch a catastrophic event

Harmonic currents cause false circuit brea#er trippin". Pea# sensin" circuit

brea#ers often will trip even thou"h the ampera"e value has not been e(ceeded.Harmonic current Pea# values can be many times hi"her than sinusoidal waveforms.

In electronic equipment that relies on the 'ero crossin" of the sinusoidal waveform,such as cloc# timin" devices, heavy harmonic content can cause a 'ero crossin" pointoffset.

In many cases, power factor correction capacitor failure can be directly attributed toharmonic content. Capacitors appear as e(tremely low impedance values and are

more susceptible to harmonics. Hi"h Harmonic currents have been #nown tooverheat correction capacitors, causin" premature failure and sometimes resultin" in

e(plosion.

-tilities invest millions of dollars annually to ensure the power supplies theircustomers receive are as close as possible to a sinusoidal waveform. esistive linearloads such as the incandescent li"ht bulb result in sinusoidal waveforms, butswitchin" loads do not. The power supply in a modern personal computer is a "ood

e(ample of a nonlinear load. The switchin" action of the computer power supplyresults in distortion of the current waveform. There are many other switchin" loads

and a few are listed in the chart below.

/$)*0 P$*-CI12 H)M$1IC C-E1T0

Electronic /i"htin" 3allast )d4ustable 0peed *rivesElectric )rc !urnaces Personal Computers

Electric +eldin" Equipment 0olid 0tate ectifiersIndustrial Process Controls -P0 0ystems

0aturated Transformers 0olid 0tate Elevator ControlsMedical Equipment

The above list is not complete and is bein" e(panded every day. The commoncharacteristic shared by most of the equipment on this list is seen more today thanbefore the microprocessor. Most of this equipment relies on an internal *C power

supply for operation.

/ar"e commercial buildin"s have many different si'es and types of loads. The mostcommon power distribution to office systems and equipment is 5678956 transformers

in a +ye confi"uration. This confi"uration does not include what are #nown ashi"h volta"e loads such as air handlers or hi"h volta"e li"htin" that are powered by:76 *elta or +ye confi"uration transformers. *elta and +ye distribution systems

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are also used in some facilities for speciali'ed equipment. !or the purpose of ourdiscussion we will consider the distribution system to be :76 5678956 +ye.

The followin" are definitions that will assist your understandin" of this section.

Harmonic; ) sinusoidal waveform with a frequency that is an inte"ral multiple ofthe fundamental volta"esA when the input is

sinusoidal.

Bolta"e Harmonic *istortion >BH*A; Bolta"e harmonic distortion is distortioncaused by harmonic currents flowin" throu"h the system impedance. The utilitypower system has relatively low system impedance, and the BH* is very low. BH*on the distribution power system can be si"nificant due to its relatively hi"h systemimpedance. E ID $hms /aw

Total Harmonic *istortion >TH*A; The square root of the sum of the square of allharmonic currents present in the load e(cludin" the


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The amount of harmonics produced by a "iven load is represented by the term %F&factor. The lar"er the %F& factor, the more harmonics are present. The chartbelow provides a "uide of typical %F& factors of different loads. This chart is only a"uide and the %F& factor of equipment will vary from one manufacturer to another.The best way to determine %F& factor is by spectrum analysis and should be left to a

professional with the proper power quality analysis equipment.

F!actor 3y Type of /oad

F9 esistance Heatin" F9= Telecommunications EquipmentIncandescent /i"htin" 3ranch Circuits in Classrooms

Electric Motors Health Care !acilitiesControl Transformers*istribution Transformers

F: +elders F56 Main !rame ComputersInduction Heaters )C Bariable 0peed *rivesHI* /i"htin" Circuits +ith *P Equipment!luorescent /i"htin" Personal Computers

0olid 0tate Controls Computer Terminals

Harmonics are produced by the diodecapacitor input section of power supplies.The diodecapacitor section rectifies the )C input power into the *C volta"e used bythe internal circuits. The personal computer uses *C volta"e internally to powerthe various circuits and boards that ma#e up the computer. The circuit of the

power supply only draws current from the )C line durin" the pea#s of the volta"ewaveform, thereby char"in" a capacitor to the Pea# of the line volta"e. The *C

equipment requirements are fed from this capacitor and, as a result, the currentwaveform becomes distorted.

The harmonics in the electric power distribution system combine with thefundamental >


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most ad4ustable speed drives create. +hen this situation arises, capacitor ban#sshould be resi'ed to shift the resonant point to another frequency.

Modern electronics "rade distribution panels have lu"s for neutrals 5J the phaseconductor si'e in anticipation of neutral currents. F rated transformers are also

built to accommodate hi"h neutral currents.

)nother useful parameter is the *istortion !actor, or G*!. G*! is the TotalHarmonic *istortion referenced to the total M0 si"nal. The G*! is e(pressed as apercenta"e and may not be "rater than 966G. International standard IEC@@@ hasrequirements for equipment that must be met.

The !ollowin" are steps to ta#e in alleviatin" the many problems encountered when

harmonics are present in an electric distribution system. 0tep L9 can be started bythe layman, after that, a professional should be called in.

9. Inventory all equipment that may "enerate harmonic currents.

5. /ist the nonlinear loads which are on each branch circuit.=. ecord true M0 current in each phase at the service entrance.:. ecord the neutral current of the transformer secondary.@. Compare the measured neutral current to the anticipated current

due to phase imbalance. If the phase currents are equal, the vectorsum of the neutral currents will add up to 'ero. If e(cessive

amounts of triplen harmonics are present in the neutral, neutralcurrent may e(ceed phase current. Consult the 1EC for thema(imum capacity for each of the conductors that have beenmeasured.


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Bolta"e Harmonics

Bolta"e harmonics may cause havoc within the electrical distribution system.Motors are typically considered to be linear loadsN however, when the source volta"e

supply is rich in harmonics, the motor will draw harmonic current. The result istypically a hi"her than normal operation temperature and shortened service life.

*ifferent frequency harmonic currents can cause additional rotatin" fields in themotor. *ependin" on the frequency, the motor will rotate in the opposite direction>countertorqueA. The fifth harmonic, which is very prevalent, is a ne"ativesequence harmonic causin" the motor to have a bac#ward rotation, shortenin" theservice life.

1oise can be pic#ed up in computer networ#s, communications equipment and

telephone systems when harmonics are at audio or radio frequencies. +ith the

increase in speed of computer networ#s, the future will brin" these systems into thefrequencies where they will be more affected by harmonic "enerated noise. Thenoise is inductively or capacitively coupled into the communications and data lines.

+hen inductiondisc watthour meters are monitorin" non linear loads, dependin"

on the content of the harmonics, the dis# may run slower or faster, resultin" inerroneous readin"s.

The ma4ority of "enerators and transformers base their operatin" characteristics onnon disturbed


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Crest !actor >C!A K Pea# Balue divided by M0 Balue

*eratin" certain types of electrical equipment is the easiest way to limit the effects

increased heatin" has on equipment. ) 5@G deratin" for transformers and"enerators is commonly employed in industry.

!ilterin" is currently the most common method used to limit the effects thatharmonics present to the rest of the system. !ilters typically consist of tuned series

/C circuits. !ilter impedance is ne"li"ible with respect to the rest of thedistribution system. These filter products are commercially available under

different trade names. Most filter products are no more than @6G effective. Thebest solution is to install transformers, with the appropriate F ratin" and wirin"that is si'ed to meet the equipment and systems needs.

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