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Understanding of Harmonics in Power Distribution System

Understanding of Harmonics in Power Distribution System . By Wei Wu Instructor: Dr. Adel. M. Sharaf Department of Electrical & Computer Engineering University of New Brunswick. Outline. What are Power System Harmonics?

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Understanding of Harmonics in Power Distribution System

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  1. Understanding of Harmonics in Power Distribution System By Wei Wu Instructor: Dr. Adel. M. Sharaf Department of Electrical & Computer Engineering University of New Brunswick

  2. Outline • What are Power System Harmonics? • Why Bother about Harmonics? • Loads Producing Harmonic Currents • How to QuantifyHarmonic Distortion? • Negative Effects of Harmonics • Mitigation the Effects of Harmonics • When to Evaluate System Harmonics? • Conclusion • Reference EE 6633 Seminar 1

  3. What are Power System Harmonics? • Harmonic: a mathematical definition, generally used when talking about frequencies • Power system harmonics: currents or voltages with frequencies that are integer multiples of the fundamental power frequency [1] • 1st harmonic: 60Hz • 2nd harmonic: 120Hz • 3rd harmonic: 180Hz Figure: 1 [2] EE 6633 Seminar 1

  4. How are Harmonics Produced ? • Power system harmonics: presenting deviation from a perfect sine waveform (voltage or current waveform). • The distortion comes from electronic and nonlinear devices, principally loads. Figure: 2 [1] EE 6633 Seminar 1

  5. Why Bother about Harmonics? • 75% of all electrical devices in North American operate with non-linear current draw • Important aspect of power quality • Affecting power factor correction capacitors • Combining with the fundamental frequency to create distortion • Causing damage effects to consumer loads and power system EE 6633 Seminar 1

  6. Loads Producing Harmonic Currents • Electronic lighting ballasts • Adjustable speed drives • Electric welding equipment • Solid state rectifiers • Industrial process controls • Uninterruptible Power Supplies ( UPS )systems • Saturated transformers • Computer system EE 6633 Seminar 1

  7. Current vs. Voltage Harmonics • Harmonic currents flowing through the system impedance result in harmonic voltages at the load Figure: 3 [3] EE 6633 Seminar 1

  8. How to QuantifyHarmonic Distortion? • Total Harmonic Distortion: the contribution of all harmonic frequency currents to the fundamental current. [3] • The level of distortion: directly related to the frequencies and amplitudes of the harmonic current. EE 6633 Seminar 1

  9. Calculation of THD • THD: Ratio of the RMS of the harmonic content to the RMS of the Fundamental [3] (Eq-1) • Current THD (Eq-2) • Voltage THD (Eq-3) EE 6633 Seminar 1

  10. Negative Effects of Harmonics • Overheating and premature failure of distribution transformers [1] • Increasing iron and copper losses or eddy currents due to stray flux losses • Overheating and mechanical oscillations in the motor-load system [1] • Producing rotating magnitude field, which is opposite to the fundamental magnitude field. • Overheating and damage of neutral conductors [2] • Trouble Harmonics: 3rd, 9th, 15th … • A 3-phase 4-wire system: single phase harmonic will add rather than cancel on the neutral conductor EE 6633 Seminar 1

  11. Negative Effects of Harmonics (cont’ d) • False or spurious operations and trips of circuit breakers [2] • Failure of the commutation circuits, found in DC drives and AC drives with silicon controlled rectifiers (SCRs) [1] • Interference and operation instability of voltage regulator [1] • Power factor correction capacitor failure [1] • Reactance (impedance) of a capacitor bank decreases as the frequency increases. • Capacitors bank acts as a sink for higher harmonic currents. • The overvoltage and resonance cause dielectric failure or rupture the power factor correction capacitor failure. EE 6633 Seminar 1

  12. Harmonics and Parallel Resonance Circuit • Harmonic currents produced by variable speed drives: amplified up to 10-15 times in parallel resonance circuit formed by the capacitance bank and network inductance[5] • Amplified harmonic currents: leading to internal overheating of the capacitor unit. • Higher frequency currents: causing more losses than 60hz currents having same amplitude Figure 4: Parallel resonance circuit and its equivalent circuit[5] EE 6633 Seminar 1

  13. Harmonics and Series Resonance Circuit • The voltage of upstream network is distorted the series resonance circuit, formed by capacitance of the capacitor bank and short circuit inductance of the supplying transformer: drawing high harmonic currents through the capacitors[5] • leading to high voltage distortion level at low voltage side of the transformer Figure 5: Series resonance circuit and its equivalent circuit [5] EE 6633 Seminar 1

  14. Measure Equipments of Harmonics • Digital Oscilloscope: Wave shape, THD and Amplitude of each harmonic • “True RMS” Multimeter: Giving correct readings for distortion-free sine waves and typically reading low when the current waveform is distorted Figure 6: “True RMS” Multimeter[3] EE 6633 Seminar 1

  15. Standard of Harmonics Limitation • IEEE 519-1992 Standard: Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems(Current Distortion Limits for 120v-69kv DS) Table 1: Current Harmonic Limits [4] EE 6633 Seminar 1

  16. Standard of Harmonics Limitation (cont’d) • IEEE 519-1992 Standard: Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems(Voltage Distortion Limits) Table 2: Voltage Harmonic Limits[4] EE 6633 Seminar 1

  17. Mitigation the Effects of Harmonics [1] • Ranging from variable frequency drive designs to the addition of auxiliary equipments • Power System Design: • Limiting the non-linear load to 30% of the maximum transformer’s capacity • Limiting non-linear loads to 15% of the transformer’s capacity, when power factor correction capacitors are installed. • Determining if resonant condition on the distribution could occur: (Eq-4) hr = resonant frequency as a multiple of the fundamental frequency kVAsc = short circuit current as the point of study kVARc = capacitor rating at the system voltage EE 6633 Seminar 1

  18. Mitigation the Effects of Harmonics [1] (cont’d) • Delta-Delta and Delta-Wye Transformers • Using two separate utility feed transformers with equal non-linear loads • Shifting the phase relationship to various six-pulse converters through cancellation techniques Figure 7: Delta-Delta and Delta-Wye Transformers [1] EE 6633 Seminar 1

  19. Mitigation the Effects of Harmonics [1] (cont’d) • Isolation Transformers • The potential to “voltage match” by stepping up or stepping down the system voltage, and by providing a neutral ground reference for nuisance ground faults • The best solution when utilizing AC or DC drives that use SCRs as bridge rectifiers • Line Reactors • More commonly used for size and cost • Adding reactor in series with capacitor bank • The best suitable to AC drives that use diode bridge rectifier front ends EE 6633 Seminar 1

  20. Mitigation the Effects of Harmonics [1] (cont’d) • Harmonic Trap Filters: • Used in applications with a high non-linear ratio to system to eliminate harmonic currents • Sized to withstand the RMS current as well as the value of current for the harmonics • Providing true distortion power factor correction Figure 8: Typical Harmonic Trap Filter [1] EE 6633 Seminar 1

  21. Harmonic Trap Filters (cont’d) • Tuned to a specific harmonic such as the 5th, 7th, 11th, etc to meet requirements of IEEE 519-1992 Standard • The number of turned branches depends on the harmonics to be absorbed and on required reactive power to be compensated Figure 9: Typical Filter Capacitor Bank [5] EE 6633 Seminar 1

  22. Harmonics Filter Types [6] • Isolating harmonic current to protect electrical equipment from damage due to harmonic voltage distortion • Passtive Filter: • are built-up by combinations of capacitors, inductors (reactors) and resistors • most common and available for all voltage levels • Active Filter: • Inserting negative harmonics into the network, thus eliminating the undesirable harmonics on the network. • mainly for low voltage networks EE 6633 Seminar 1

  23. Harmonics Filter Types (cont’d) [7] • Unified Switched Capacitor Compensator: The single line diagram (SLD) of the utilization (single-phase) or (three-phase- 4-wire) feeder and the connection of the Unified Switched Capacitor Compensator (USCS) to the nonlinear temporal inrush /Arc type or SMPS-computer network loads. Figure 10 [7] EE 6633 Seminar 1

  24. Harmonics Filter Types (cont’d) [7] • The USCS is a switched/modulated capacitor bank using a pulse-width modulated (F'WM) strategy. The switching device uses either solid state switch (IGBT or GTO). Figure 11[7] EE 6633 Seminar 1

  25. When to Evaluate System Harmonics? [1] • The application of capacitor banks in systems where 20% or more of the load includes other harmonic generating equipment. • The facility has a history of harmonic related problems, including excessive capacitor fuse operation. • During the design stage of a facility composed of capacitor banks and harmonic generating equipment. EE 6633 Seminar 1

  26. When to Evaluate System Harmonics? [1] (cont’d) • In facilities where restrictive power company requirements limit the harmonic injection back into their system to very small magnitudes. • Plant expansions that add significant harmonic generating equipment operating in conjunction with capacitor banks. • When coordinating and planning to add an emergency standby generator as an alternate power source in an industrial facility. EE 6633 Seminar 1

  27. Conclusion • The harmonics distortion principally comes from nonlinear loads. • The application of power electronics is causing increased level of harmonics. • Harmonics distortion can cause serious problem for uses of electric power. • Harmonics are important aspect of power quality. • Oversizing and Filtering methods are commonly used to limit effects of harmonics. EE 6633 Seminar 1

  28. References [1] www-ppd.fnal.gov/EEDOffice-w/Projects/CMS/LVPS/mg/8803PD9402.pdf [2] www.pge.com/docs/pdfs/biz/power_quality/power_quality_notes/harmonics.pdf [3] www.metersandinstruments.com/images/power_meas.pdf [4]http://engr.calvin.edu/PRibeiro_WEBPAGE/IEEE/ieee_cd/chapters/CHAP_9/c9toc/c9_frame.htm [5] www.nokiancapacitors.com.es/.../EN-TH04-11_ 2004- Harmonics_and_Reactive_Power_Compensation_in_Practice.pdf [6]http://rfcomponents.globalspec.com/LearnMore/Communications_Networking/RF_Microwave_Wireless_Components/Harmonic_Filters [7]A.M. Sharaf & Pierre Kreidi, POWERQ UALITYE NHANCEMEUNSTI NGA UNIFIEDSW ITCHED CAPACITOCRO MPENSATOR, CCECE 2003 - CCGEI 2003, Montreal, Mayimai 2003 0-7803-7781-8/03/$17.00 0 2003 IEEE EE 6633 Seminar 1

  29. Question ? EE 6633 Seminar 1

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