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1999 IEEE T&D ShowNew Orleans

Panel Session PN13

Non-periodic Currents:Causes, Effects and Identification

Interharmonics in Power Systems

Erich W. Gunther

Electrotek Concepts, Inc.


Interharmonics in Power Systems

Definition Standards Sources Impacts Solutions


Interharmonics - Definition

IEC-61000-2-1 defines Interharmonics as:

“Between the harmonics of the power frequency voltage and current, further frequencies can be

observed which are not an integer of the fundamental. They can appear as discrete frequencies or as a wide-band spectrum”


Standards and Working Groups

IEC 61000-4-7 - Measurements IEEE Task Force on Interharmonics IEEE 519 - under revision - adds interharmonics Cigre 36.05 - Voltage Quality Working Group


IEC Harmonic / InterharmonicMeasurement Standard: IEC 61000-4-7

Number of cycles to sample chosen to provide 5 Hz frequency bins 10 Cycles for 50 Hz Systems 12 Cycles for 60 Hz Systems

Grouping concept Harmonic factors calculated as the square root of the sum of

the squares of the harmonic bin and two adjacent bins. Interharmonic factors calculated as the square root of the

sum of the squares of the bins in between the harmonic bins (not including the bins directly adjacent to the harmonic bin).


Frequency Bin Grouping


Frequency Bin Grouping


Interharmonic Sources

Arc furnaces Cycloconverters Power line carrier communications PWM power electronic systems Interaction of controls and power system

components


Example Case Where dc Arc Furnace Caused Unacceptable

Flicker Levels

Converter control problems result in increased interharmonic generation

System resonance magnifies the interharmonic component (186 Hertz)

Result is 6 Hertz modulation that causes light flicker over a wide area


Example System One Line Diagram for Harmonic and Flicker

Evaluations

Customer

Customer


Frequency Response with Initial Filter Design

0 200 400 600 800 0

25

50

75

100

125

150

Frequency (Hz)

Dri

vin

g P

oin

t Im

ped

an

ce (

Oh

m) 186 hertz


Voltage at the 26 kV Bus


Current at the 26 kV Bus


Effect of the Rolling Mill

Modern rolling mills will typically use cycloconverters. They also generate significant interharmonic components.

Example waveform and spectrum used for design purposes:

CASE41>S35K3A-FEED3A(Type 9)

270 280 290 300-2000

-1000

0

1000

2000

Time (mS)

Cu

rren

t (A

)

Max:Min:Avg:Abs:

RMS:CF :FF :

1536.08 -1498.02 821.937 1536.08 940.961 1.63245 1.14481

DERIVED>S35K3A-FEED3A(Type 9)

0 6 12 18 24 0

10

20

30

40

Frequency (Hz pu)

Cu

rren

t (A

)

Freq :Fund :THD :TID :TD :

RMSh :RMSi :

RMSfh:RMS :

ASUM :TIF :IT :

1 966.448 6.40455 0 6.40455 61.8967 0 968.428 968.428 1349.88 146.679 142048


Impacts

Similar to impact of harmonic distortion Heating Altered/multiple zero crossing Telecommunications interference

Unique impacts Light flicker Torsional oscillation excitation


Light Flicker due to Interharmonics


Solutions

Fix control problems to reduce level of interharmonic generation.

Non-characteristic harmonics and interharmonics must be considered in the filter design for dc arc furnaces and other interharmonic producing loads.

Resonances created by the filters in parallel with the system inductance can magnify interharmonic components causing high distortion and flicker.

Damping should be included in filter designs to avoid interharmonic problems.


Conclusions

Interharmonics have always been around, they are just becoming more important and visible.

Power electronic advances are resulting in increasing levels of interharmonic distortion.

Traditional filter designs can result in resonances that make interharmonic problems worse.

Light flicker is the most common impact. Measurement is difficult, but standards make them

possible and the results comparable.

Documents

Http:// Interharmonics - 1 1999 IEEE T&D Show New Orleans Panel Session PN13 Non-periodic Currents: Causes,