Chapter 1

HarmonicRequirements

1.1 INTRODUCTION

Placing limits upon the effects that nonlinear loads may produce on users ofelectric power requires definition of system and equipment parameters. TheIEEE Std 519-1992 document provides many of those definitions that are repro-duced at the end of this chapter. They offer a standardized terminology that facil-itates discussion of system harmonic issues. The basic requirements of voltagedistortion and current distortion are guides for many users. When followed theyeliminate most of the power system concerns relating to application of solidstate equipment.

Telephone interference factor (TIF) is still under review, but the harmonicsof voltage and current are critical parameters. By addressing these and conform-ing to IEEE Std 519-1992, some control of telephone interference is automati-cally provided.

The original IEEE Std 519 specification, issued in 1981, focused almost en-tirely on the matter of system voltage distortion, which is heavily dependentupon system characteristics. To determine voltage distortion, potential equip-ment suppliers often had to perform detailed system studies. Unwanted effectscould be remedied by system as well as equipment changes; however, it wasoften unclear who should change what.

In the revised IEEE Std 519-1992 document the harmonic currents drawnby a users' equipment are also defined. This is something that manufacturerscan address in equipment design. They are doing so, and it is hoped this bookwill provide additional help for designers and users. There is still a system fac-tor involved because tolerable harmonic currents are defined relative to thetotal system load. This is as it should be; however, the system definition can beless detailed for this and performance expectations are more readily deter-mined.

1

Chapter 1 Harmonic Requirements

1.2 VOLTAGE DISTORTION

Voltage distortion defines the relationship between the total harmonic voltageand the total fundamental voltage. Thus, if the fundamental ac line to neutralvoltage is VLN and the total line to neutral harmonic voltage is Vw then

total harmonic voltage distortion =

whereYL-N

^=VZ:h=25

<h=2 vh2

(1.1)

(1.2)

An upper summation limit of h = 25 is chosen for calculation purposes. Itgives good practical results. Recommended voltage distortion limits are summa-rized in Table 1-1.

TABLE 1-1.

CLASSIFICATION AND VOLTAGE DISTORTION LIMITSFOR INDIVIDUAL USERS (LOW-VOLTAGE SYSTEMS)

Class of System

tSpecial applications

General system

Dedicated system

Total HarmonicDistortion

3%

5%10%

Notch AreaVolt—jxsec*

16,400

22,800

36,500

Notch Depth

10%

20%

50%

*Multiply this value by V7480 for other than 480 V systems.fSpecial applications include hospitals and airports.

1.2.1 Line Notching Calculations and Limits

Notching refers to the effects that commutation has on the ac line voltage. Itis most easily demonstrated with respect to a 6-pulse three-phase converterbridge with dc filter inductor as shown in Figure 1-1.

Line notching results when two semiconductors of the same polarity are si-multaneously contributing to the load current Id. This occurs, for example, whendevice S2 starts to conduct current and supply the current previously suppliedthrough device Sv Changeover of current conduction from one device to theother takes time (commutation time), and during that interval the voltage differ-ence between line A and line B is zero because the devices Sl and S2 ideally haveno voltage drop. Figure 9-1 shows currents during commutation in detail.

With diode operation (gating at 0° in Figure 1-1), the current changes natu-rally from one device to the other, and the notch has one fast rising side. With

2

VH

Section 1.2 Voltage Distortion

Gated at 0° Gated at -30°

* d c

Figure 1-1 Illustrating line voltage notching effects.

SCR phase back, such as —30° in Figure 1-1, the notch will have two fast-changing sides. This gives a greater likelihood of high-frequency interference.

When the converter is operated from the line without phase shift, as inFigure 1-1, the line voltage shows one large notch and two smaller notches. TheIEEE Std 519-1992 specification defines the volt-seconds area relative to thelarger notch.

During commutation the inductance LTcomm in one line has a change in cur-rent from Id to zero. In the other phase, a similar change of current Id occurs ex-cept that this current goes from zero to 1(V To change current in a linear inductorrequires an expenditure of volt seconds equal to the product of inductance andcurrent. These volt-seconds are subtracted from the source line voltage. At theconverter terminals in Figure 1-1 the notch area is ideally given by

notch volt-seconds = 2 LTcommId (1-3)

At a point closer to the source, where for example the inductance back tothe source is only LT{comm, the corresponding notch volt-seconds are reduced bythe ratio Lncomjn/Lrcomm.

3

u

VA-B VA-B

*A

wicomm

*-Tcomm

uA-B A *Y S1 1 * 2 X 3 3

k'

cB

C

L-6c

Chapter 1 Harmonic Requirements

1.2.2 Notching with Delta/WyeTransformers

If the converter is operated from a phase-shifting transformer, the pattern ofnotching changes. In the case of a delta/wye transformer with 30° phase shift,there will now be two large notches. Waveforms for this example are shown inFigure 1-2.

The apparently simple notch calculation in equation (1.3) has to be modi-fied for delta/wye systems. Specifically, the 2 multiplier becomes V3, as shownin Figure 1-3, page 6.

Gated at 0° Gated at -30°

* d c

Figure 1-2 Showing the different notch pattern when converter is fed fromdelta/wye transformer

1.2.3 Effects of dc Filter Inductanceon Notching

Practical 6-pulse converters often have a significant amount of ripple in the dccurrent, and at the instant of commutation, the current is usually less than the av-erage value of ld assumed in equation (1.3). When the dc inductance is negligi-ble, the voltage waveshapes and notch patterns are greatly changed. For exam-ple, in diode bridge simulations for this case the major notch area was 28% less

4

K M

U

VA-B

u ,

>AA_

A-B

B

C

*-T1comm

L7comm

$1 52 ' S3

Id

c

Section 1.2 Voltage Distortion

than that predicted by equation (1.3) when the ac line reactance was 6% and38% less for 3% reactance. For 6-pulse phase-controlled converters, dc induc-tance is essential and notch patterns are more predictable. The notching limitsspecified in IEEE Std 519-1992 are reproduced in Table 1-1.

1.2.4 System Notches Caused by MultipleConverter Loads

In general the system load will include a mix of converter types and ratings. Inthis event the commutation of one unit affects commutation of another, and it isnot possible to determine the total notch area by summing the effects of individ-ual units. For analysis, a full-scale computer simulation is recommended to cal-culate the line voltage waveshapes. The author has his own favorite methods;however, various software packages are available. Practical oscillograms of sys-tem ac line voltages can be similarly analyzed to determine notch effects.

1.2.5 Notching in Multipulse Circuits

Multipulse circuits are introduced in Chapter 3 as a means for filtering powerconverters to produce more nearly sinusoidal currents. With smoother currents,the concept of notch volt-seconds becomes less viable. In the limit, when thereare so many pulses that the line current is completely sinusoidal, there is still avoltage drop in the source reactance. This volt-seconds loss in each half cycle canbe much greater than the IEEE Std 519-1992 limits without causing operatingproblems.

Multipulse arrangements cause less individual notch area than do 6-pulsecircuits. For example, in a 12-pulse circuit formed from two 6-pulse circuits, theload current being commutated is reduced to one-half. Intuitively we would ex-pect the notch area to be reduced by a factor of about 2; Figure 1-3 shows oneexample. In multipulse systems with reduced device conduction, the commutat-ing reactance in the phase-shifting transformer secondary is greatly magnifiedwhen it is referred to the source voltage. The result is a nearly sinusoidal sourcecurrent and negligible notch effects. Exact analysis of notches is of limited use.In specific cases, a simulation provides accurate results.

Another method for estimating notching considers the net effective changein current when steps of current occur. For example, consider the 6-pulse and12-pulse currents *', and i2 in Figure 1-3. At commutation the volt-sees absorbedwill be the appropriate Lcomm times the current change in each line. Specifically,for balanced ac line inductance:

total notch volt-sees = Lcomm (step in /x — step in i2)

This technique is illustrated in Figure 1-3. The concept can be applied toany converter circuit.

5

total notch volt-sees = Lcomm (step in /x — step in i2)

Chapter 1 Harmonic Requirements

1.115 /̂

-0.56 ld

12-pulse

notch volt-seconds = Lcomm (A^ - Ai2)

Figure 1-3 Calculating notch volt-sees from current steps.

1.3 CURRENT DISTORTION

In general, current distortion defines the relationship between the total har-monic current and the fundamental current in much the same way as voltagedistortion. However, there are some application differences which need to berecognized. These include

• Current harmonic limits depend upon the system short-circuit current ca-pability at the point of interest.

• Current harmonic percentages apply to individual harmonic currents.They are expressed relative to the total system fundamental load current

6

^Tcomm'1

'2^-ncomm

-1/V3/W2/V3 ld

' i I

k6-pulse DIY

0.97 L

0-(A/i-AM I

V3V---

0

(A/i-A/2)|

Section 1.3 Current Distortion

for worst case normal operating conditions lasting more than one hour.(They are not expressed relative to the fundamental current load of thenonlinear equipment.) The worst case operating conditions are expressedrelative to the average current of maximum demand, preferably for thepreceding 12 months.

Total demand distortion TDD is the total harmonic current distortiongiven by

TDD = ^- (1.4)

where IL is the maximum demand load current (fundamental frequencycomponent) at the PCC derived from a 15-minute or 30-rninute billingdemand kVA. And IH is given by

'if^XfcM (1.5)The upper summation limit of h = 25 is chosen for calculation purposes. It

gives good practical results.The system harmonic current limits recommended in IEEE Std 519-1992 are

shown in Table 1-2 for 6-pulse systems. For higher-pulse numbers, larger charac-teristic harmonics are allowed in the ratio (pulse number/6),0<5 provided that non-characteristic harmonics are less than 25% of the limits specified in the table.

TABLE 1-2. Table 10.3 in IEEE Std 519-1992. Reprinted with permission.

Maximum Harmonic Current Distortionin Percent of/̂

Individual Harmonic Order (Odd Harmonics)

IJIU <11 ll</i<17 17<//<23 23</i<35 35</i TDD

<20*20<50

50<100

100<1000

>1000

4.07.0

10.0

12.0

15.0

2.03.5

4.5

5.5

7.0

1.52.5

4.0

5.0

6.0

0.61.0

1.5

2.0

2.5

0.30.5

0.7

1.01.4

5.08.0

12.0

15.0

20.0

Even harmonics are limited to 25'/? of the odd harmonic limits above.

Current distortions that result in a dc offset, e.g., half-wave converters, are notallowed.

*A11 power generation equipment is limited to these values of current distortion,regardless of actual /SC//L-

where

/sc = maximum short-circuit current at PCC./L = maximum demand load current < fundamental frequency component) at

PCC.

7

Chapter 1 Harmonic Requirements

1.3.1 Current Distortion and Transformers

The previous discussion regarding total demand distortion (TDD) and calcula-tion of harmonic current distortion relates to the IEEE Std 519-1992 specifica-tion. Another specification, ANSI/IEEE Std C57.110-1986, relates to the effectthat harmonic currents have on power transformers covered by ANSI/IEEEC57.12.01-1979 and to power transformers up to 50-MVA maximum nameplaterating covered by ANSI/IEEE Std C57.12.00-1987. This specification does notapply to rectifier or special transformers. In these specifications, a definition of"harmonic factor" is used for current distortion. It is used to determine the trans-former rating when the harmonic factor exceeds 0.05 per unit. This harmonicfactor relates to the ratio of the effective harmonic current to the fundamentalcurrent. Thus, for transformer rating, it may not be sufficient to determine thesystem total demand distortion. The issue of transformer derating is dealt with inChapter 7, Section 7.7.

1.4 TELEPHONE INTERFERENCE

There is as yet no formal specification for the allowable telephone influence fac-tor; however, two formulas for calculation are given in IEEE Std 519-1992.Each uses the 1960 curves for telephone interference weighting factor. This fac-tor takes into account the response of telephone sets and the human ear. Also,each formula directly incorporates line harmonic currents up to 5000 Hz.

One useful form of the formulas, for gauging the possibility of telephoneinterference, is given by the root-sum-square (RSS) of the product of individ-ual harmonic current Ih and telephone interference factor Th, namely, / • T. It isgiven by

^T = ̂ UlhTh)2 (1-6)

where H corresponds to 5000 Hz.Specific frequency values for Th up to the 49th harmonic of a 60-Hz con-

verter are given in Table 1-3. Practical results from standard waveform analyzerscover this range. A larger range for TIF, up to 5000 Hz, is given in IEEE Std519-1992. Table 1-4 gives calculated results for / • T using the idealized currentamplitude harmonics of l/(kq ± 1), where q is the pulse number and k is anypositive integer. Higher pulse number converters are seen to reduce the possibil-ity of telephone interference.

In practice, results are much reduced because of the filtering affects ofequipment reactance. For 50 Hz converters, the idealized I*T factors are reducedby approximately 8 percent.

8

I*T = ^ti(hTh)2 (1.6)

Section 1.5 Definitions of Terms

TABLE 1-3.

SINGLE-FREQUENCY TIF (Tf) VALUES FOR HARMONICS OF 60 Hz

/ i #

1

2357

9111213

15

TIF

0.5

15

30

225

650

1,320

2,260

2,760

3,360

4,350

h#

17

18

19

21

23

24

25

27

29

30

TIF

5,100

5,400

5,630

6,050

6,370

6,560

6,680

6,970

7,320

7,570

//#

31

33

35

36

37

39

41

43

47

49

TIF

7,820

8,330

8,830

9,080

9,330

9,840

10,340

10,600

10,210

9,820

TABLE 1-4.

IDEALIZED / • T FACTORS FOR 60-HZ CONVERTERS(PER FUNDAMENTAL AMPERE) (UP TO 49TH HARMONIC)

6-PuIse

997

954

12-Pulse

705

686

18-Pulse

594

552*

*From measured data, practical 18-pulse equipments produce an / • 7* that is only 18% to31% of this value. For example, a 480-V, 125-A, 18-pulse converter causes an / • T of12,000. Referred to a 12-kV bus, this reduces to 7 • T = 480.

1.5 DEFINITIONS OF TERMS

Selected definitions in this section are from IEEE Std 519-1992. They are repro-duced with permission of the Institute of Electrical and Electronic Engineers.

Definitions given here are tailored specifically to the harmonics generatedby static power converters at utility system frequencies. Additional useful guide-lines will be found in IEEE Std 100-1992, IEEE Std 223-1966, IEEE Std59-1962, ANSI Std C34.2 1968, and IEEE Std 444-1973.

9

10 Chapter 1 Harmonic Requirements

commutation. The transfer of unidirectional current between thyristor(or diode) converter circuit elements that conduct in succession.

converter. A device that changes electrical energy from one form to an-other. A semiconductor converter is a converter that uses semiconductors as theactive elements in the conversion process.

distortion factor (harmonic factor). The ratio of the root mean squareof the harmonic content to the root mean square of the fundamental quantity, ex-pressed as a percentage of the fundamental.

^,_ I sum of squares of amplitudes of all harmonics , _ _ _,DF= •100%

\ square of amplitude of fundamentalfilter. A generic term used to describe those types of equipment whose

purpose is to reduce the harmonic current or voltage flowing in or being im-pressed upon specific parts of an electrical power system or both.

filter, damped. A filter generally consisting of combinations of capaci-tors, inductors, and resistors that have been selected in such a way as to presenta low impedance over a broad range of frequencies. The filter usually has a rela-tively low Q (X/R).

filter, high-pass. A filter having a single transmission frequency extend-ing from some cutoff frequency, not zero, up to infinite frequency.

filter, series. A type of filter that reduces harmonics by putting a high se-ries impedance between the harmonic source and the system to be protected.

filter, shunt. A type of filter that reduces harmonics by providing a lowimpedance path to shunt the harmonics away from the system to be protected.

filter, tuned. A filter generally consisting of combinations of capacitors,inductors, and resistors that have been selected in such a way as to present a rel-atively minimum (maximum) impedance to one or more specific frequencies.For a shunt (series) filter the impedance is a minimum (maximum). Tuned filtersgenerally have a high Q (X/R).

harmonic. A sinusoidal component of a periodic wave or quantity havinga frequency that is an integral multiple of the fundamental frequency. Note, forexample, a component, the frequency of which is twice the fundamental fre-quency, is called a second harmonic.

harmonic, characteristic. Those harmonics produced by semiconductorconverter equipment in the course of normal operation. In a six-pulse converterthe characteristic harmonics are the nontriple odd harmonics, for example, the5th, 7th, 11th, and 13th.

Section 1.5 Definition of Terms 11

harmonic, characteristic, (continued for a 6-pulse converter)

h = kq±\

k = any integer

q = pulse number of converter

harmonic, noncharacteristic. Harmonics that are not produced by semi-conductor converter equipment in the course of normal operation. These may bea result of beat frequencies, a demodulation of characteristic harmonics and thefundamental, or an imbalance in the ac power system, asymmetrical delay angle,or cycloconverter operation.

harmonic factor. The ratio of the RSS value of all the harmonics to therms of the fundamental.

A/^3+£"52 + £? . . .

harmonic factor (for voltage) = —3

J/32+/5

2+/72 . . .

harmonic factor (for current) = —

h/ • T product. The inductive influence expressed in terms of the product

of its rms magnitude (I), in amperes, times its telephone influence factor.

kV • T product. Inductive influence expressed in terms of the product ofits rms magnitude, in kilovolts, times its telephone influence factor.

line voltage notch. The dip in the supply voltage to a converter due tothe momentary short circuit of the ac lines during a commutation interval.Alternatively, the momentary dip in supply voltage caused by the reactive dropsin the supply circuit during the high rates of change in currents occurring in theac lines during commutation.

nonlinear load. A load that draws a nonsinusoidal current wave whensupplied by a sinusoidal voltage source.

notch depth. The average depth of the line voltage notch from the sinewave of voltage.

notch area. The area of the line voltage notch. It is the product of thenotch depth, in volts, times the width of the notch in microseconds.

power factor, displacement. The displacement component of powerfactor; the ratio of the active power of the fundamental wave, in watts, to the ap-

h = kq±\

k = any integer

q = pulse number of converter

12 Chapter 1 Harmonic Requirements

parent power of the fundamental wave, in volt-amperes (including the excitingcurrent of the converter transformer).1'2

pulse number. The total number of successive nonsimultaneous commu-tations occurring within the converter circuit during each cycle when operatedwithout phase control. It is also equal to the order of the principal harmonic inthe direct voltage, that is, the number of pulses present in the dc output voltagein one cycle of the supply voltage.

short-circuit ratio. For a semiconductor converter, the ratio of the short-circuit capacity of the bus, in MVA, at the point of converter connection, to therating of the converter, in megawatts.

telephone influence factor (TIF). For a voltage or current wave in anelectric supply circuit, the ratio of the square root of the sum of the squares ofthe weighted rms values of all the sine wave components (including alternatingcurrent waves both fundamental and harmonic) to the rms value (unweighted) ofthe entire wave.

total demand distortion (TDD), The total RSS harmonic current distor-tion, as a percentage of the maximum demand load current (15- or 30-minute de-mand).

total harmonic distortion (THD). This term has come into commonusage to define either voltage or current "distortion factor." See distortion factor.

1.6 OTHER HARMONIC SPECIFICATIONS

The work in this book focuses on power systems used in the United States. Forthis reason the primary specification addressed is IEEE Std 519-1992.

In the United Kingdom, specification G.5/3 is a harmonics specification inthe form of an engineering recommendation from The Electricity Council ChiefEngineers' Conference. It is titled "Limits for Harmonics in the United KingdomElectricity Supply System." In addition to specifying various harmonic limits, itprecludes the use of certain power levels of converter equipment in differentpower systems. Some of the power equipment designs in this book, which con-form to IEEE Std 519-1992, will be very effective at addressing the concerns ofthe G.5/3 specification.

'This definition includes the effect of harmonic components of current and voltage (distortion powerfactor), the effect of phase displacement between current and voltage, and the exciting current of thetransformer. Volt-amperes are the product of rms voltage and rms current.2The power factor is determined at the line terminals of the converter.

Section 1.6 Other Harmonic Specifications 13

The International Electrotechnical Commission (IEC) published a first edi-tion of publication IEC 555-2 in 1982 to address the impact of electrical equip-ment and appliances used in the home. The specification is entitled "Dis-turbances in Supply Systems Caused by Household Appliances and SimilarElectrical Equipment, Part 2, Harmonics." CENELEC approved IEC-555-2 as aEuropean standard (EN 60555-2) in December 1991. It includes individual sin-gle-phase equipment up to 16 A and is important to those involved in Europeanmarkets. At the time of this writing, the standard is under revision [24].

It is desirable to address the generation of power line harmonics by con-sumer and professional electronic equipments; however, most of the equipmentis single phase. In this case, individual 120-V ratings are limited to 15 A. Indi-vidually, these types of equipment are not considered as "power" electronics inthis book. However, when large quantities of single-phase equipment are con-nected to a power system, significant harmonic currents result. The 3rd har-monic of current is especially of concern and is important in determining theampacity of the neutral conductor in branch circuits. Discussion on this is pro-vided in Chapter 7, Section 7.8.

Military specifications such as MIL-STD-1399 (NAVY) SECTION 300Aalso define acceptable levels of harmonic current generation.