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									Understanding of Harmonics
in Power Distribution System

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

2/23/2006            EE 6633 Seminar 1            2
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 loads
   Power-Quality-PQ Issues & Problems
   Damage to Power Factor Correction capacitors
   Waveform Distortion can create
   All can cause damage effects to consumer loads
    and power systems due to Over-Current/Over-
    Voltage or Waveform Distortion
   Additional Power/Energy Losses
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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
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       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]

2/23/2006            EE 6633 Seminar 1              7
    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
   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]


   Current THD-I

    Voltage THD-V

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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.

    2/23/2006                        EE 6633 Seminar 1                            11
    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]

2/23/2006                             EE 6633 Seminar 1                             12
     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]

2/23/2006                              EE 6633 Seminar 1                           13
    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 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]

               Ratio        Harmonic odd        Harmonic odd      THD-i
            Iscc / Iload    numbers (<11)       numbers (>35)
               < 20              4.0 %                0.3 %       5.0 %

             20 - 50             7.0 %                0.5 %       8.0 %
            50 - 100            10.0 %                0.7 %       12.0 %
             >1000              15.0 %                1.4 %       20.0 %

2/23/2006                                EE 6633 Seminar 1                 15
       Standard of Harmonics Limitation
   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             THD-v (%)
                               as (%) of Fundamental
             <= 69Kv                     3.0                        5.0

            69 - 161Kv                   1.5                        2.5

            >= 161 Kv                    1.0                        1.5

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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
            transformer’s capacity
           Limiting non-linear loads to 15% of the transformer’s capacity,
            when power factor correction capacitors are installed.
           Avoiding/Detuning resonant conditions on the AC System:

            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

2/23/2006                                  EE 6633 Seminar 1                           17
    Mitigation the Effects of Harmonics                                [1]

   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]
   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

2/23/2006                       EE 6633 Seminar 1                   19
    Mitigation the Effects of Harmonics                            [1]

   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]
2/23/2006                         EE 6633 Seminar 1                      20
            Harmonic Trap Filters (cont’d)

   Tuned to a specific harmonic order such as the 5th, 7th,
    11th,… etc to meet requirements of IEEE 519-1992
   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]

2/23/2006                   EE 6633 Seminar 1                    21
                  Harmonics Filter Types              [6]

   Isolating harmonic current to protect electrical
    equipment from damage due to harmonic voltage
   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
2/23/2006                      EE 6633 Seminar 1                   22
            Harmonic Filter Types (cont’d)             [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 (cont’d)      [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]

2/23/2006             EE 6633 Seminar 1            24
 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.
2/23/2006               EE 6633 Seminar 1                25
      When to Evaluate System Harmonics?                  [1]

   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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   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.

2/23/2006              EE 6633 Seminar 1              27




[5] 2004-


    ITCHED CAPACITOCRO MPENSATOR, CCECE 2003 - CCGEI 2003, Montreal, Mayimai 2003
    0-7803-7781-8/03/$17.00 0 2003 IEEE

    2/23/2006                           EE 6633 Seminar 1                            28

2/23/2006    EE 6633 Seminar 1   29

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