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Understanding of Harmonics in Power Distribution System

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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

- 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

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- 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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- 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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- 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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- 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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- Harmonic currents flowing through the system impedance result in harmonic voltages at the load
Figure: 3 [3]

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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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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- 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.

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[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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