1. Power System Harmonics
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Power System Harmonics
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2. INDEX What are Harmonics? Where do they come from?
Problems caused by harmonic currents
Common Sources of Harmonics
Some important definitions
Input Current Of Different Non-linear Loads
Recommended Limits IEEE-519
Resonance
Harmonics Mitigating Techniques
Power factor in presence of Harmonics
Conclusion
References
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3. What are Harmonics?
Definition: Harmonics are integral multiples of some fundamental frequency that, when added together, result in a distorted waveform.
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4. Where do they come from?
The power company typically supplies a smooth sinusoidal waveform as shown:
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5. Where do they come from?
...but nonlinear devices will draw distorted waveforms, which are comprised of harmonics of the source:
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6. Where do they come from?
When a non-linear load draws current, that current passes through all of the impedance that is between the load and the system source. As a result of the current flow, harmonic voltages are produced by impedance in the system.
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7. Where do they come from?
These voltages sum and when added to the nominal voltage produce voltage distortion. The magnitude of the voltage distortion depends on the source impedance and the harmonic voltages produced.
If the source impedance is low then the voltage distortion will be low. If a significant portion of the load becomes non-linear (harmonic currents increase) and/or (system impedance increases), the voltage can increase dramatically.
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8. Problems caused by Harmonic Currents
Equipment heating
Equipment malfunction
Equipment failure
Communications interference
Fuse and breaker mis-operation
Process problems
Conductor heating
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9. Common Sources of Harmonics
Fluorescent lighting
Computer switch mode power supplies
Static VAR compensators
Variable frequency motor drives (VFD)
DC-DC converters
Inverters
Television power supplies
AC or DC motor drives
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10. Some Important Definitions
Harmonic: A sinusoidal waveform with a frequency that is an integral multiple of the fundamental 50/60 Hz frequency.
50/60 Hz fundamental
100/120 Hz 2nd harmonic
150/180 Hz 3rd harmonic
200/240 Hz 4th harmonic, etc 
Triplen Harmonics: Odd multiple of the 3rd harmonic (3rd, 9th, 15th, 21st, etc.)
Harmonic Distortion: Non-linear distortion of a system characterized by the appearance in the output of harmonic currents (voltages) when the input is sinusoidal.
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11. Some Important Definitions
Voltage Harmonic Distortion (VHD): Voltage harmonic distortion is distortion caused by harmonic currents flowing through the system impedance. The utility power system has relatively low system impedance, and the VHD is very low. VHD on the distribution power system can be significant due to its relatively high system impedance.
Total Harmonic Distortion (THD): The square root of the sum of the square of all harmonic currents present in the load excluding the 60 Hz fundamental. It is usually expressed as a percent of the fundamental.
Where IDn is the magnitude of the nth harmonic as a percentage of the fundamental (individual distortion).   
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12. Input Current Of Different Non-linear Loads
TYPE OF NONLINEAR LOAD
TYPICAL WAREFORM
THD%
1-φ
Uncontrolled Rectifier
80% (high 3rd component)
Semicontrolled Rectifier Bridge
2nd, 3rd, 4th ,......
harmonic components
6 –Pulse Rectifier
with output voltage filtering and without input reactor filter
80%
5, 7, 11, ……….
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13. Input Current Of Different Non-linear Loads
6 - Pulse Rectifier
with large output inductor
28%
5, 7, 11, ………..
with output voltage filtering and with 3% reactor filter or with continues output current
40%
12 - Pulse Rectifier
15%
11, 13, ………..
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14. Recommended limits - IEEE 519
The Institute of Electrical and Electronics Engineers (IEEE) has set recommended limits on both current and voltage distortion in IEEE
Voltage distortion limits low-voltage bus):
Application Class
THD (Voltage)
Special system 3%
General system 5%
Dedicated system
10%
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15. Recommended limits - IEEE 519
Maximum Harmonic Current Distortion
In percent of IL
Individual harmonic number (odd harmonics)
Isc: Maximum short-circuit current at the Point of Common Coupling (PCC).
IL: Maximum demand load current (fundamental) at the PCC.
Isc/IL
<11
11<h<17
17<h<23
23<h<35
TDD
<20
4.0
2.0
1.5
0.6
5.0
20-50
7.0
3.5
2.5
1.0
8.0
50-100
10.0
4.5
12.0
5.5
15.0
>1000
6.0
20.0
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16. Recommended limits - IEEE 519
The PCC is generally defined as the utility/customer connection point. It is this point at which the current distortion limits apply. The other concept that is important is that of Total Demand Distortion (TDD)
The idea behind the standard is that harmonic limits are placed on a customer on the basis of current distortion relative to the total plant load.
The limits do not apply to a specific non-linear load in the plant.
The harmonic current limits change depending on the ratio of short circuit current to maximum demand load current at the PCC.
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17. Resonance
Resonance occurs when a harmonic frequency produced by a non-linear load closely coincides with a power system natural frequency. There are 2 forms of resonance which can occur: parallel resonance and series resonance.
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18. Mitigation Techniques
Inductive Reactance
Method: Add a line reactor or isolation transformer to attenuate harmonics.
Benefits: Low cost and Technically simple.
Concerns: Tends to offer reductions in only higher order harmonics. Has little effect on the 5th and 7th harmonics. Because of the associated voltage drop, there are limits to the amount of reactance that may be added.
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19. Mitigation Techniques
Passive Filters
Method: Provide a low-impedance path to ground for the harmonic frequencies.
Benefits: May be tuned to a frequency between two prevalent harmonics so as to help attenuate both.
Concerns: Tuning the filters may be a labor-intensive process. Filters are difficult to size, because they offer a path for harmonics from any source. Quite sensitive to any future system changes.
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20. Mitigation Techniques
Active Filters
Method: Inject equal and opposite harmonics onto the power system to cancel those generated by other equipment.
Benefits: Have proven very effective in reducing harmonics well below required levels.
Concerns: The high performance inverter required for the harmonic injection is costly. Power transistors are exposed to conditions of the line, so reliability may be a problem.
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21. Mitigation Techniques
12-pulse Rectifiers
Method: Two separate rectifier bridges supply a single DC bus. The two bridges are fed from phase- shifted supplies.
Benefits: Very effective in the elimination of 5th and 7th harmonics. Stops harmonics at the source. Insensitive to future system changes.
Concerns:May not meet the IEEE standards in every Case. Does little to attenuate the 11th and 13th harmonics.
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22. Mitigation Techniques
18-pulse Rectifiers
Method: An integral phase-shift transformer and rectifier. Input which draws an almost purely sinusoidal waveform from the source.
Benefits: Meets the IEEE standards in every case! Attenuates all harmonics up to the 35th.Stops harmonics at the source. Insensitive to future system changes.
Concerns: Can be expensive at smaller HP’s
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23. Comparison of waveforms
6-pulse converter
note the level of distortion
and steep current rise.
12-pulse converter
the waveform appears more
sinusoidal, but still not very
smooth.
18-pulse converter
virtually indistinguishable
from the source current
waveform.
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24. Power factor in the presence of Harmonics
There are two different types of power factor that must be considered when voltage and current waveforms are not perfectly sinusoidal.
Input Displacement Factor (IDF) which refers to the cosine of the angle between the 50/60 Hz voltage and current waveforms.
Distortion Factor (DF) is defined as follows:
Total Power Factor (PF) is the product of the Input Displacement Factor and the Distortion Factor as follows:
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25. Conclusion
Although the likelihood of harmonic problems is very low, the cases in which they do occur can result in decreasing power system reliability.
An understanding of the causes, potential effects and mitigation means for harmonics can help to prevent harmonic related problems at the design stage and reduce the probability of undesired effects occurring on start-up.
It should be kept in mind that if the harmonic producing loads are small in relation to the total plant load, then harmonics are not an issue. When the non-linear loads become a substantial portion of the total load, it becomes worthwhile to give some consideration to harmonics.
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26. References
4. P519A Task Force, Harmonics Working Group (IEEE PES T&D Committee) and SCC22 – Power Quality, “Guide for Applying Harmonic Limits on Power Systems” (Un-approved draft)
IEEE Standard , IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.
R. G. Ellis, “Harmonic analysis of industrial power systems,” IEEE Trans. Industry Applications, vol. 32, Mar. 1996, pp
P. E. Sutherland, ‘Harmonic measurements in industrial power systems’, IEEE Trans. Ind. Appl., 31(1) (1995), 175.
IEEE Working Group on Power System Harmonics, ‘Power system harmonics : an overview’, IEEE Trans. Power Apparatus and Systems, PAS 102(8) (1983), 2455.
D. Sutanto and M. Bou-Rabee, ‘Harmonic filters for industrial power system’, AUPCE’91, 3–4 October 1991, pp. 308–388.
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27. THANKS
VISIT for more…….
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