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Harmonic reduction
The document presents information on harmonic reduction in inverter output voltage. It defines harmonics as integral multiples of a fundamental frequency that result in a distorted waveform when added together. Common sources of harmonics are identified as lighting ballasts, UPS systems, AC drives, and DC drives. Methods for attenuating harmonics discussed include inductive reactance, passive filters, active filters, 12-pulse and 18-pulse rectifiers, PWM, transformer connections, stepped wave inverters, and multilevel inverters. The document recommends limits on voltage and current distortion set by IEEE 519 and compares harmonic reduction performance of different converter and inverter configurations.
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Harmonic reduction
- 1. A PRESENTATION ON Harmonic reduction ininverter output voltage By RAJ KUMAR TIWARI M.E
- 2. What are Harmonics? Definition: Harmonicsare integral multiples of some fundamental frequency that, when added together, result in a distorted waveform.
- 3. What are Harmonics? f(x)= sin(x) f(x) = sin(5x) 5 + The resulting wave shows a strong departure from the smooth waves comprising it: f(x) = sin(x) + sin(5x) 5=
- 4. What are Harmonics? Infact, any function may be constructed from a sine wave and some number of its harmonics:
- 5. Where do theycome from? The power company typically supplies a reasonably smooth sinusoidal waveform:
- 6. Harmonic Distortion • DCDrives, UPSs, DC power supplies (computers, duplicators, fax’s) will cause current (and voltage) harmonics • Single phase – 3rd , 6th , etc (triplens) can cause transformer neutral conductor overheating • Three phase – 5th , 7th , 11th , 13th , etc can cause equipment malfunctions • Big questions – “How much?” and “How much is too much?”
- 7. Common sources ofHarmonics Lighting ballasts UPS systems M AC and DC drives
- 8. AC drives andHarmonics Converter DC bus & smoothing Inverter Determine the line-side harmonics Determines load-side harmonics
- 9. AC drives andHarmonics Inverter Determines load-side harmonics EFFECTS OF LOAD-SIDE HARMONICS: Have implications for the motor insulation and windings. Essentially have zero effect on other equipment on the power system.
- 10. AC drives andHarmonics Converter DC bus & smoothing Determine the line-side harmonics LINE-SIDE HARMONICS CAN HAVE FAR-REACHING EFFECTS ON THE POWER SYSTEM: Distribution transformers Standby generators Communications equipment Switchgear and relays Computers, computer systems Diagnostic equipment
- 11. AC drives andHarmonics Typical 6-step converter waveform: Harmonic Content I5 = 22.5% I7 = 9.38% I11 = 6.10% I13 = 4.06% I17 = 2.26% I19 = 1.77% I23 = 1.12% I25 = 0.86%
- 12. Recommended limits -IEEE 519 The Institute of Electrical and Electronics Engineers (IEEE) has set recommended limits on both current and voltage distortion in IEEE 519-1992. Voltage distortion limits (@ low-voltage bus): Application class THD (voltage) Special system 3 % General system 5 % Dedicated system 10 %
- 13. Recommended limits -IEEE 519 MAXIMUM HARMONIC CURRENT DISTORTION in percent of IL Individual harmonic number (odd harmonics) 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 4.0 1.5 12.0 100-1000 12.0 5.5 5.0 2.0 15.0 >1000 15.0 7.0 6.0 2.5 20.0 Isc: Maximum short-circuit current at the Point of Common Coupling (PCC). IL: Maximum demand load current (fundamental) at the PCC.
- 14. Attenuation of Harmonics InductiveReactance Method: Add a line reactor or isolation transformer to attenuate harmonics. Benefits: Low cost. 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.
- 15. Attenuation of Harmonics PassiveFilters 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.
- 16. Attenuation of Harmonics ActiveFilters 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.
- 17. Attenuation of Harmonics 12-pulseRectifiers 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.
- 18. Attenuation of Harmonics 18-pulseRectifier 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
- 19. Comparison of waveforms 6-pulseconverter 12-pulse converter 18-pulse converter note the level of distortion and steep current rise. the waveform appears more sinusoidal, but still not very smooth. virtually indistinguishable from the source current waveform.
- 20. HARMONIC REDUCTION BYPWM (pulse width modulation)
- 22. • Amplitude offundamental component for values of α1 and α2 b1= 4Vs/π{ 1-2cos 23.62+ 2cos 33.304} =1.0684 Vs • Amplitude of the fundamental component of unmodulated output voltage is: b1=4Vs/ π=1.27324 Vs • Un modulated voltage wave ,the amplitude of the 7th ,9th ,11th harmonics are 24.78%,40.86%,30.39 % • 3rd and 5th harmonics are been eliminated • Amp of fundamental output voltage =83.91% i.e. .8391 amp of fundamental unmodulated voltage wave. Harmonics reduction =16.09%
- 23. Harmonics reduction byTransformer connections • Two or more inverters can be combined by means of transformers to get a net output voltage with reduced harmonic content . • Output voltage waveforms from the inverters must be similar but phase-shifted from each other.
- 24. By stepped waveinverters • Pulses of different widths and heights are superimposed to produce a resultant stepped wave with reduced harmonic content . • two transformers are used have different turn ratio from primary to secondary . • Turns ratio from primary to secondary is assumed 3 for transformer 1and 1 for transformer 2. • Inverter 1 is gated that its output voltage is vo1 • During first half cycle output voltage woud be zero or negative.
- 25. • This outputvoltage waveform is named as TWO –LEVEL MODULATION . • When inverter2 is triggered output voltage is v02 • level of this voltage is positive, negative or zero for first half cycle. • For this the inverter has THREE LEVEL MODULATION • By superimposing both the waveforms the output voltage is 4Vs and wave for four steps. • 3rd ,5th ,and 7th harmonics are eliminated .
- 26. • A NovelApproach of Harmonic Reduction with TransformerConnected 3-Phase Multilevel Inverter • By:Vipul Rastogi,Sudhanshu Tripathi (Iss.7| July. 2014 ) • This paper proposes a multilevel inverter arrangement employing a series connectedtransformer to suppress 5th,7th,11th &13th order harmonics. • eliminates the need of output filter
- 28. Harmonics reduction inthree phase multilevel inverters using space vector modulation Published in: 19-20 March 2015 • Harmonic reduction in three phase multilevel inverter is an imperative factor to obtain the quality of output voltage and current. • This paper proposes the switching strategy of space vector pulse width modulation uniquely to minimize THD. Publisher:IEEE INSPEC Accession Number:15380959 • To reduce the total harmonic distortion and study the harmonic analysis of five-level inverter in diode clamped inverter, flying capacitor inverter and cascaded H-bridge multilevel inverters are simulated in MATLAB
- 29. International Journal ofEngineering Technology, Management and Applied Sciences www.ijetmas.com June 2015, Volume 3, Issue 6, ISSN 2349-4476 • New Approach for Harmonics Reduction in Inverters
- 30. • This paperprimarily compares and analyzes the two DC- AC power conversion approaches. These two approaches are • 1. Cascaded H Bridge inverter • 2. Multilevel scheme with H-Bridge and level modules. • The primary advantage of cascaded topology is that it uses multi-level DC voltage to accurately synthesize an expected AC voltage .The level of output voltage shape can be calculated by using appropriate level module. MATLAB Simulink is used to design the above mentioned two approaches. Comparatively, the THD produced in the second approach is more efficient and productive than the first approach.
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Editor's Notes
- #3 The “first harmonic” is really just the fundamental frequency. The second is twice that frequency, the third is three times, and so on. Thus for a power system, the fifth harmonic would be 300 hz. Harmonics may appear in any system that oscillates at a given frequency (e.g. light waves, sound pressure waves, mechanical vibrations, etc.
- #4 Though the resultant waveform has the same frequency as the fundamental and is still symmetrical, it is easily seen that its sinusoidal character is lost.
- #5 Since any function whatsoever may be constructed from a fundamental wave and a sum of its harmonics, any type of distortion can be represented in this fashion. Thus a device that draws a non-sinusoidal waveform is, by definition, drawing harmonics as well.
- #6 The term “reasonably” is used because the supplied waveform is never purely sinusoidal due to distortions on the power grid itself (oftentimes due to harmonics generated by other power customers).
- #8 Both AC and DC drives have similar front ends in that they convert the 60 hz AC to DC. This conversion employs switching devices which draw nonlinear currents.
- #10 The DC bus is responsible of this isolation of output harmonics from the rest of the power system. In reality, there is some passing of harmonics, but the effect is negligible.
- #11 Complex diagnostic equipment such as that used in hospitals or laboratories is particularly sensitive to power line distortion.
- #12 The waveform pictured here is the actual input current of an AFC with a six-diode rectifier bridge. Harmonics above the 25th become increasingly smaller and are usually ignored.
- #13 Special applications include hospitals, airports, computer complexes, and laboratories. A dedicated system is one that is exclusively dedicated to converter loads.
- #15 Line reactors are a good solution when a reduction of only a few percent is necessary.
- #16 A man-week of labor is not uncommon to properly install and tune passive filters. If a filter is tuned to the 5th harmonic, for example, and some other facility on the power grid begins to put 5th harmonics on the system, then this filter will be a path for those currents as well! Filters will likely have to be retuned at some later date if system changes are made.
- #17 Transistors in the output stage of a standard drive are isolated from the conditions of the line by the DC bus. This is not true for the transistors used in an active filter.
- #18 Remember the “nk+/-1” rule. From this, it is seen that 12-pulse drives still generate 11th, 13th, 23rd and 25th harmonics. In some cases this may be enough to exceed IEEE limits.
- #19 The Clean Power Option is available only on Accutrol 700’s, from 100-600 hp.