Power System Harmonics 8051.in Power System Harmonics 8051.in

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 8051.in

What are Harmonics? Definition: Harmonics are integral multiples of some fundamental frequency that, when added together, result in a distorted waveform. 8051.in

Where do they come from? The power company typically supplies a smooth sinusoidal waveform as shown: 8051.in

Where do they come from? ...but nonlinear devices will draw distorted waveforms, which are comprised of harmonics of the source: 8051.in

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. 8051.in

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. 8051.in

Problems caused by Harmonic Currents Equipment heating Equipment malfunction Equipment failure Communications interference Fuse and breaker mis-operation Process problems Conductor heating 8051.in

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 8051.in

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. 8051.in

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).    8051.in

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, ………. 8051.in

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, ……….. 8051.in

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% 8051.in

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 12.0 100-1000 5.5 15.0 >1000 6.0 20.0 8051.in

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. 8051.in

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. 8051.in

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. 8051.in

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. 8051.in

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. 8051.in

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. 8051.in

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 8051.in

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. 8051.in

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: 8051.in

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. 8051.in

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 519-1992, 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. 417-421. 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. 8051.in

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