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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 l.jpg

Understanding of Harmonics in Power Distribution System

By

Wei Wu

Instructor: Dr. Adel. M. Sharaf

Department of Electrical & Computer Engineering

University of New Brunswick


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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


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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]

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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]

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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

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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

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Current vs. Voltage Harmonics

  • Harmonic currents flowing through the system impedance result in harmonic voltages at the load

    Figure: 3 [3]

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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.

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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)

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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

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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.

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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]

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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]

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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]

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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]

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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]

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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

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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]

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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

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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]

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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]

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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

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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]

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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]

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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.

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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.

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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


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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

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Question

?

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