Understanding of Harmonics in Power Distribution System

1
Understanding of Harmonics in Power Distribution
System

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

2
Outline

• Power System Harmonics?
• Why Harmonics are Troublesome?
• Nonlinear Loads Producing Harmonic Currents
• Harmonic Distortion?
• Negative Effects of Sustained Harmonics
• Mitigation of the Effects of Harmonics
• Evaluation of AC Power System Harmonics?
• Conclusions
• References

3
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
  (h0,1,2,N) of the fundamental power frequency
  1
• 1st harmonic 60Hz
• 2nd harmonic 120Hz
• 3rd harmonic 180Hz
• 
  Figure 1 2
• 
  
  

4
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/Limi
  ter/Threshold Type Loads.
• 
  Figure 2 1

5
Why Bother about Harmonics?

• 50-60 of all electrical Ac Systems in North
  America operate with non-linear type loads
• Power-Quality-PQ Issues Problems
• Damage to Power Factor Correction capacitors
• Waveform 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 Distortion
• Additional Power/Energy Losses

6
Loads Producing Harmonic Currents

• Electronic lighting ballasts/Controls
• Adjustable speed Motor-Drives
• Electric Arc Welding Equipment
• Solid state Industrial Rectifiers
• Industrial Process Control Systems
• Uninterruptible Power Supplies ( UPS )systems
• Saturated Inductors/Transformers
• LAN/Computer Networks

7
Current vs. Voltage Harmonics

• Harmonic current flowing through the AC Power
  System impedance result in harmonic voltage-drop
  at the load bus and along the Feeder!!
• Figure 3 3

8
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

9
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)

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 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

11
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.

12
Harmonics and Parallel Resonance Circuit

• Harmonic 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

13
Harmonics and Series Resonance Circuit

• The 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
14
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

15
Standards for Harmonics LimitationIEEE/IEC

• 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
• 
  

Ratio Iscc / Iload Harmonic odd numbers (lt11) Harmonic odd numbers (gt35) THD-i
lt 20 4.0 0.3 5.0
20 - 50 7.0 0.5 8.0
50 - 100 10.0 0.7 12.0
gt1000 15.0 1.4 20.0
16
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

Bus Voltage Voltage Harmonic limit as () of Fundamental THD-v ()
lt 69Kv 3.0 5.0
69 - 161Kv 1.5 2.5
gt 161 Kv 1.0 1.5
17
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

18
Mitigation the Effects of Harmonics 1 (contd)

• 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

19
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 rectifiers
• Line Isolation-Reactors
• More commonly used for their low cost
• Adding 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

20
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 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

21
Harmonic Trap Filters (contd)

• Tuned to a specific harmonic order such as the
  5th, 7th, 11th, etc to meet requirements of IEEE
  519-1992 Standard
• The 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
22
Harmonics Filter Types 6

• Isolating harmonic current to protect electrical
  equipment from damage due to harmonic voltage
  distortion
• Passive Filter-Low cost
• Built-up by combinations of capacitors, inductors
  (reactors) and resistors
• most 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

23
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

24
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

25
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.

26
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

27
Conclusions

• The 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
• 4http//engr.calvin.edu/PRibeiro_WEBPAGE/IEEE/ie
  ee_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
• 6http//rfcomponents.globalspec.com/LearnMore/Co
  mmunications_Networking/RF_Microwave_Wireless_Comp
  onents/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
?