Harmonics: Sources, Effects and ControlTechniques

Niharika Singh I, N K Sharma' and P Tiwari:'

EN Dept., KIET, Muradnagar, Ghaziabad, UP, INDIAdrnikhlesh@gmail.com

Abstract-Power systems are designed to operate atfrequencies of 50 or 60 hertz. However certain type of loadproduce voltage and currents having frequency that are aninteger multiple of fundamental frequency. These are caUedharmonics. The pure sinusoidal wave shape generated byelectrical utilities is distorted by the harmonics produced by theincreased use of non linear loads. The harmonics lead to adverseeffects like excessive heating and transmission losses. This paperdiscusses various sources of power harmonics, its impacts andcontrol techniques to minimize harmonics.

Keywords- THD, Crest factor.Power factor,non linear load,PCC

1. INTRODUCTION

The rapid development in the field of electronics has led tothe development of highly sophisticated, small size, lightweight and efficient power system to meet the demand of theusers. These demands can be met by using the high frequencyswitching transients which widely use non linear electronicdevices for switching. non linear loads occur when impedanceis not constant i.e. current is not proportional or same asvoltage. The current drawn by non-linear load is not sinusoidalhence customer pays more for unused energy due to bothvoltage and current distortion [1). This increased use of nonlinear electronic device has given rise to phenomena calledpower harmonics. The power system are designed to operateat frequencies of 50 or 60 hertz.howewer certain type of loadproduce currents or voltages with frequencies that are a integermultiple of fundamental frequency. These higher frequenciesare a form of electrical pollution known as power harmonics.harmonics are steady state periodic phenomena that producescontinuous distortion in output voltage and current. Harmonicscurrent generated by single phase and three phase non-linearelectric loads will cause significant loss of performance. Theeffect of harmonics using active or passive harmonic filters atpower supply input point.

II. GENERAL DEFINITIO OF HARMONICS

The general definition of harmonics is any periodic signalcan be described by series of sine and cosine function, alsoknown as Fourier series. There are several ways of describingthe degree of distortion of current or voltage. The two termsthat are frequently used are crest factor and total harmonicdistortion. The crest factor of a voltage is equal to the peak

value divided by effective value. In case of sinusoidal voltagecrest factor is 1.4l4.The total harmonic distortion of a currentor voltage is equal to the effective value of all the harmonicsdivided by effective value of the fundamental.

CUllen! MaximumTHD true pf20% 0.9850~o 0.89100% 0.71

Table I Maximum true power factor of a non-linear load.THD=IH/IF

where, IH is rms value of combined harmonic components andIF is the rms value of fundamental component of line current. Ithas been observed that the sinusoidal voltages and currentshave the THD of zero. Table I shows the maximum true powerfactor of a non linear load

III. SOURCES OF HARMONICS

Harmonics are created by non linear loads that draw currentin abrupt pulses rather than smooth sinosodial manner.Thereare two general categories of harmonic sources-saturabledevices and power electronic devices. Saturable devicesproduces harmonics mainly due to iron saturation, as in case oftransformers,machines and flurescent lamps.Flurescent lampwith magnetic ballasts are usually rather benign sources ofharmonics. Their current distortion is due to arc and to theballast.fig 1 shows harmonic current of flurescent lamp with astandard magnetic ballast.

0.2 I \ / \\ J \ J\ / \ II

\ J \/

U.J

,g 0.1

~ 0.0'-8 -0.1

-0.2

-0.30 30 4020(a) Time (ms)

Fig. 1 Harmonic current of flurescent lamp

10

NIET Journal of Engineering & Technology, Vol. 1, Issue 1, 2012 34

4OO0~---LS----~~~15~--~W~--~----~L---~35

(a) Time (ms)

100

80

~ 80e••5 40o

W

00

I I I I I I I I_L __ L __ L __ L __ L __ L __ L __ L __I I I I I I I__ L __ L __ L __ L __ L __ L __ L __I I I I I I I__ L __ L __ L __ L __ L __ L __ L __I I I I I I I__ L __ L __ L __ L __ L __

I I I I

200 400 600 600 1000 1200 1400 1600 1600 WOO(b) Frequency (Hz)

.Fig. 2 Current and harmonic spectrum of a typical voltagesource PWM converter for ASD application.

Harmonic(a) Source

(b)HarmonicSource

Fig. 3 Two cases of resonance (a) parallel resonance (b)series resonance

The input to the the PWM is typically a 3 phase version ofsingle phase mode power supply[2][3]. The current THD isusually 40-60%.All circuits containing both capacitance andinductance have one or more natural resonant frequencies.When one of these frequencies corresponds to an excitingfrequency being produced by non linear loads, harmonicresonance can occur. Voltage and current will be dominated byresonant frequency and can be highly distorted. Thus responseof the power system at each harmonic frequency determinesthe true impact of non linear load on harmonic voltagedistortion [4]. To better understand resonance, consider thesimple parallel and series cases shown in figure 3.

Parallel resonance occurs when power system presents aparallel combination of power system inductance and powerfactor correction capacitors at non linear load. The product ofthe harmonic impedances and injection current produces highharmonic voltages. Series resonance occurs when the systemimpedance and capacitance are in series with respect to non

linear load point[8]. For parallel resonance, the highest voltagedistortion is at non linear load, however for series resonance,the highest voltage distortion is at remote point. Hence the totalcurrent given by non linear load is given by

IIFIF+IH

IFis fundamental current

IHis total harmonic current

The schematic of a diode rectifier bridge used in converteris shown in fig 4

The circuit draws current during peaks of voltagewaveform as shown in fig 5 to charge the capacitor to peak ofline voltage. This type of power supplies drawing current inshort pulses during the voltage waveform peak has improvedefficiency of the equipment but at cost of generating highfrequency harmonics [6].

Typical current waveform of a PC measured at power inputis in Fig. 6.

itI-r

+

Line * ..• ::input ;;; lidC.

f

Fig. 4 Schemetic of a diode rectifier bridge

Fig. 5 Current waveform with respect to supply voltage incase ofVSI.

· -.· ..· ...P':., •..... ; .... :__.

· . . ..~..... - .· . . .- .-.,,!nT;1'·~".. .

" ,-

0000)00 __ ), \i--Jjlt.~r--;'. . . .

... :. ~.: ~.. '..:..:... .; :. . ~.-

_:~t).__~.'~T~_. ~- •• " ~ •• I .: • • !_..

•• 0 r'"f" ..~...~-..

Fig. 6 Current waveform of PC measured at power input

NIET Journal of Engineering & Technology, Vol. 1, Issue 1, 2012 35

IV. EFFECTS OF HARMONICS

Harmonics have a number of undesirable effects on powersystem components and loads. They fall in two majorcategories-short term and long term. Short term effects areusually most noticeble and related to excessive voltagedistortion[7]. On the other hand, long term effects often gounnoticeble and are related to increased resistance losses orvoltage stresses, some of the cornman problems caused byharmonic currents are as follows.

a) Neutral conductor over heatingIn a 3 phase system, the voltage waveform from each phase

to neutral star point is displaced by 120 degree so that eachphase is equally loaded; in the combined current in neutral iszero. When the loads are not balanced, only the balance currentflows in neutral, howewer the fundamental currents cancel out,harmonic currents do not, get added in the neutral which isshown in the Fig. 7.

Fig. 7 Triplen currents added in neutral

Suqlly Impedonce

Non-linear load

lrvrvrvf\;

LoodCurremSupply Vottoge_arm

VoIoge WmefQf""rlapplOed 10 load

Currenl.,Uneorlood

Fig. 8 Voltage distortion caused by non-linear loads

In this diagram, the phase current, shown at the top areintroduced at 120 degree intervals.The third harmonic of eachphase is identical being 3 times the frequency and one third offundamental offset. The effective third harmonic neutralcurrent is shown at bottam,In this case 70% third harmoniccurrent in each phase results in 210% current in neutral[5].

b) Effect on transformersTransformers are affected in two ways by harmonics.

Firstly the eddy current losses, normally about 10% of loss atfull load increases with square of harmonic number.The secondeffect concern the triple harmonics. When reflected back todelta winding they are all in phase, so the triple N harmoniccurrent circulate in the winding.

c) Heating effect on conductorsThe increased use of harmonic producing equipment like

power electronics converters in industrial management has ledto increase in harmonic levels in power distribution system.Heating effect on conductors is one of the main problemscaused by harmonic currents. Higher frequency harmoniccurrent generally flow on outer sides of the conductor due toskin effect, thus effectively reducing the area of cross section.Overheating of neutral wires are caused by harmonicsproduced in anyone of a balanced 3 phase system.

d) Effect on motorsHarmonic voltage distortion causes increased eddy current

losses in motor in same way as in transformers, howeweradditional losses arise due to generation of harmonic fields instator, each of which is trying to rotate the motor at differentspeed either forward or backwards. This high frequency currentinduced in rotor furnace increase further losses.

e) Harmonic problem effecting the supplyWhen a harmonic current is drawn from supply, it gives

rise to harmonic voltage drop proportional to source impedanceat point of conman coupling(PCC) and current. Since thesupply network is generally inductive, source impedance ishigher at higher frequencies. The voltage at PCC is alreadydistorted by harmonic currents drawn by other consumers andby distortion inherent in transformers, and each consumermakes an additional contribution.

V. CONTROL TECHNIQUES FOR HARMONICS

The mitigation methods fall broadly into three groupsnamely: passive filters, isolation and harmonic reductiontransformers and active solutions.[7] Each approach hasadvantages and disadvantages of its own

a) Passive filtersPassive filters are used to provide a low impedance path

for harmonic currents so that they flow in filter and not insupply. The filter may be designed for a single harmonic or fora broadband depending on requirements. Sometimes it isnecessary to design a more complex filter to increase the seriesimpedance at harmonic frequencies and so reduce the portionof current that flows back onto the supply.

NIET Journal of Engineering & Technology, Vol. 1, Issue 1, 2012 36

Source lmpedcnce

t t17ft<Shun! filter

(band pass)

Vv-

Supply Installation

Fig.9 Passiveharmonicshuntfilter.

Suppfy lood

Fig.transformer

10 Delta star isolation

Fundamental Current teed CUfrent: I "'Jno....... I ooc •....•

'E

j t.~ t!"l '1-1:

EoI

t tlS.h

Supply fnstellcficn

Fig.II Activeharmonicconditioner.

b) Isolation transformersTriplen currents circulate in delta windings of transformers.

Although this is a problem for transformers manufacturers, theextra load has to be taken into account, it is beneficial tosystem designers because it isolates triplen harmonics from thesupply.

c) Active filtersActive filter is a shunt device. A current transformer

measures the harmonic content of load current. Since theharmonic current is sourced from the active conditioner, onlyfundamental current is drawn from the supply. In practice,harmonic current magnitude is reduced by 90% and becausethe source impedance at harmonic frequency is reduced,voltage distortion is reduced.

VI. IEEE STANDARD519To minimize the impact of facility harmonic distortion on

utility power system and on neighboring facilities, IEEEstandard 519 was developed and published in 1982. IEEE

standard 519 provides recommended limits for total harmonicvoltage and current distortion.

VII. CONCLUSIONHarmonic distortion is not a new phenomena, however, the

widespread application of digital systems and power electronicbased loads continue to increase concern over harmonicdistortion. The current drawn by the electronic loads can bemade virtually distortion free and is the subject of ongoingdebate between equipment manufacturers and electric utilitycompanies in standard making activities. Two main questionsarise during these discussions

What are the acceptable levels of current distortionsproduced by the non linear loads.

Who should be responsible for controlling harmonics,theend user or utility companies.

IEEE 519 provides an excellent groundwork to addressboth concerns and practicing engineers must adhere to theseguidelines.

REFERENCES

[1] D.J Pileggi, N.H Chandra, A.E Emanuel: Production ofharmonic voltages in distribution system", IEEE Transactionspower Apparatus and systems, Vol. PAS-lOO, No.3 pp 1307-1315, March 1981.

[2] M. Sakui, H.Fujita: Calculation of harmonic currents in a 3phase converter with unbalanced power supply conditions, IEEETransactions on Power Electronics, Vo1.l46, No.1, Jan 1999,pp. 478- 494.

[3] S.G Jakli, R.H Lasseter: A study of non linear harmonicinteraction between a single phase line comrnutated converterand a power system, IEEE trans.power delivery, vol 9"No-3"july 1994, pp. 1616-1624

[4] L.Hu, K.Arnei and R.E morrison": AC side equivalent circuitbased method for harmonic analysis of a converter system"IEEE PROC-Electric power applications,vol 146,no-l,pp 103-110,jan 1999

[5] W.Xu, K. Arnei, J.R Marti and H.W Damrnel,Harmonic analysisof systems with static compensators, IEEE Transactions powerdelivery, Vol. 6, No.1, Feb 1991, pp. 183-190.

[6] B.C Smith,J.Arrillaga A.R. Wood: A review of iterativeharmonic analysis for AC-DC power systems", IEEETransactions ,power delivery, Vol.l3, No. I, Jan 1998, pp. 180-185

[7] G.Mahesh, R. Ganesan, Sisir.K.Das: Effects ofpower harmonicsand its control techniques, Centre of Electro magnetics, 2nd crossRoad,CIT campus,Taramani,Chennai-113.

[8] Ned Mohan, Tore. M.Undeland, William P.Robbins, Powerelectronics converters,applications and design, John utility andsons

NIET Journal of Engineering & Technology, Vol. 1, Issue 1, 2012 37