Understanding of Harmonics in Power Distribution System

Dr. Adel. M. Sharaf Department of Electrical & Computer Engineering University of New Brunswick

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OutlinePower System Harmonics?Why Harmonics are Troublesome?Nonlinear Loads Producing Harmonic CurrentsHarmonic Distortion?Negative Effects of Sustained HarmonicsMitigation of the Effects of HarmonicsEvaluation of AC Power System Harmonics?Conclusions References

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What are Power System Harmonics?Harmonic: a mathematical definition, generally used when talking about Integral orders of Fundamental frequencies Power system harmonics: currents or voltages with frequencies that are integer multiples (h=0,1,2,N) 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 deviations from a perfect sinusoidal-waveform (voltage or current waveform).The distortion comes from a Nonlinearity caused by saturation, electronic-switching and nonlinear electric loads, Inrush/Temporal/Arc/Converter/Limiter/Threshold Type Loads.

Figure: 2 [1]

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Why Bother about Harmonics?50-60% of all electrical Ac Systems in North America operate with non-linear type loadsPower-Quality-PQ Issues & ProblemsDamage to Power Factor Correction capacitorsWaveform Distortion can create SAG/SWELL/NOTCHING/RINGING/All can cause damage effects to consumer loads and power systems due to Over-Current/Over-Voltage or Waveform DistortionAdditional Power/Energy Losses

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Loads Producing Harmonic Currents Electronic lighting ballasts/ControlsAdjustable speed Motor-DrivesElectric Arc Welding EquipmentSolid state Industrial RectifiersIndustrial Process Control SystemsUninterruptible Power Supplies ( UPS )systemsSaturated Inductors/TransformersLAN/Computer Networks

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Current vs. Voltage HarmonicsHarmonic current flowing through the AC Power System impedance result in harmonic voltage-drop at the load bus and along the Feeder!!

Figure: 3 [3]

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How to Quantify Harmonic Distortion?Total Harmonic Distortion-THD: the contribution of all harmonic frequency Currents/Voltages to the fundamental current. [3]The level of THD-for Current or Voltage is directly related to the frequencies and amplitudes of the Offending Quasi-Steady State persistent Harmonics.Individual Distortion Factor-(DF)-h quantify Distortion at h harmonic-order

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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-I (Eq-2)

Voltage THD-V (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 ground conductors [2]Trouble sustained type Harmonics: 3rd, 9th, 15th A 3-phase 4-wire system: single phase harmonic will add rather than cancel on the neutral conductor Malfunction/Mal-Operation of Sensitive Tele-control and Protection Relaying

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Negative Effects of Harmonics (cont d) False or spurious Relay operations and trips of circuit breakers [2]Failure of the Firing/Commutation circuits, found in DC motor-drives and AC drives with silicon controlled rectifiers (SCR-Thyristor) [1]Mal-Operation instability of voltage regulator [1]Power factor correction capacitor failure [1]Reactance (impedance)-Zc of a capacitor bank decreases as the frequency increases.Capacitor bank acts as a sink for higher harmonic currents. The System-Series and parallel Resonance can cause dielectric failure or rupture the power factor correction capacitor failure due to Over-Voltages & Over-Currents.

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Harmonics and Parallel Resonance CircuitHarmonic currents produced by variable speed motor-drives: can be amplified up to 10-15 times in parallel resonance circuit formed by the capacitance bank and network inductance [5]Amplified/intensified 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 CircuitThe voltage of upstream AC Network can be also distorted due to series/parallel resonance formed by capacitance of the capacitor bank and System/load inductance : Ca cause high harmonic current circulation through the capacitors [5]Parallel Resonance can also lead to high voltage distortion. 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 Multi-Meter: Giving correct readings for distortion-free sine waves and typically reading low when the current waveform is distorted

Use of Harmonic Meters-Single Phase or three Phase

Figure 6: True RMS Multi-Meter [3]

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Standards for Harmonics LimitationIEEE/IECIEEE 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 (contd)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 Of Harmonics [1]Ranging from variable frequency motor- drive to other nonlinear loads and equipments Power System Design:Limiting the non-linear load penetration to 30% of the maximum transformers capacity Limiting non-linear loads to 15% of the transformers capacity, when power factor correction capacitors are installed.Avoiding/Detuning resonant conditions on the AC System:

(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] (contd)Delta-Delta and Delta-Wye TransformersUsing two separate utility feed transformers with equal non-linear loadsShifting 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] (contd)Isolation-Interface 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 SCR/GTO/SSR.. as bridge rectifiersLine Isolation-Reactors More commonly used for their low costAdding a small reactor in series with capacitor bank forms a Blocking series Filter. Use diode bridge rectifier as a front end to avoid severe harmonic power quality problems

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Mitigation the Effects of Harmonics [1] (contd)Harmonic-Shunt or Trap Filters: Used in applications with a high non-linear ratio to system to eliminate harmonic currentsSized to withstand the RMS current as well as the value of current for the harmonicsProviding true distortion power factor correction

Figure 8: Typical Harmonic Trap Filter [1]

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Harmonic Trap Filters (contd)Tuned to a specific harmonic order such as the 5th, 7th, 11th, etc to meet requirements of IEEE 519-1992 StandardThe number of tuned filter-branches depends on the offending steady-state harmonics to be absorbed and on required reactive power level to be compensated Figure 9: Typical Filter Capacitor Bank [5]

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Harmonics Filter Types [6]Isolating harmonic current toprotect electrical equipment from damage due to harmonic voltage distortion Passive Filter-Low cost: Built-up by combinations of capacitors, inductors (reactors) and resistorsmost common and available for all voltage levels Active Power Filter APF:Inserting negative phase compensating harmonics into the AC-Network, thus eliminating the undesirable harmonics on the AC Power Network. APF-Used only for for low voltage networks

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Harmonic Filter Types (contd) [7]Unified Switched Capacitor Compensator USCS: 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 Loads or SMPS-Computer/LAN-Network loads.

Figure 10 [7]

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Harmonics Filter Types (contd) [7]The USCS is a switched/modulated capacitor bank using a pulse-width modulated (PWM/SPWM) Switching Strategy. The switching device uses either solid state switch SSR-(IGBT or GTO).

Figure 11 [7]

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Need 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 or damage to sensitive metering/relaying/control equipment.

During the Planning/Design stage of any facility comprising capacitor banks and nonlinear harmonic generating equipment.

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When to Evaluate System Harmonics? [1] (contd)In facilities where restrictive Electric Power Utility Company Standards/Guidelines limit the harmonic injection back into their system to very small magnitudes.

Industrial/Commercial Plant expansions that add significant harmonic generating nonlinear type equipment operating in conjunction with capacitor banks.

When coordinating and planning to add any emergency standby generator as an alternate/renewable power source

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ConclusionsThe harmonic distortion principally comes from Nonlinear-Type Loads.The application of power electronics is causing increased level of harmonics due to Switching!!Harmonic distortion can cause serious Failure/Damage problems.Harmonics are important aspect of power operation that requires Mitigation!!Over-Sizing and Power Filtering methods are commonly used to limit Overheating Effects of Sustained Harmonics.

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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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