1999 IEEE T&D Show New Orleans

1. 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.

2. Interharmonics in Power Systems • Definition • Standards • Sources • Impacts • Solutions

3. 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”

4. 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

5. 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).

6. Frequency Bin Grouping

7. Frequency Bin Grouping

8. Interharmonic Sources • Arc furnaces • Cycloconverters • Power line carrier communications • PWM power electronic systems • Interaction of controls and power system components

9. 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

10. Example System One Line Diagram for Harmonic and Flicker Evaluations

11. Frequency Response with Initial Filter Design 186 hertz

12. Voltage at the 26 kV Bus

13. Current at the 26 kV Bus

14. 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:

15. Impacts • Similar to impact of harmonic distortion • Heating • Altered/multiple zero crossing • Telecommunications interference • Unique impacts • Light flicker • Torsional oscillation excitation

16. Light Flicker due to Interharmonics

17. 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.

18. 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.