harmonic distortion - sept 21 2011

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  • Power Quality Management - Harmonic Distortion

    and Variable Frequency Drives

    Voltage and Current Harmonic Distortion Cause and Effect

    23/09/2011 Marek Farbis, Mirus International Inc. 1

  • We will focus on

    Introduction: the ideal vs. distorted waveform Definition of harmonics Effects of harmonic distortion What is a cause for harmonic voltage distortion? Definition and calculation of THD VFDs and harmonics Standards and recommendations Harmonic mitigation techniques Applications

    23/09/2011 Marek Farbis, Mirus International Inc. 2

  • Introduction Electricity generation is

    normally produced at constant frequencies of 50 Hz or 60 Hz and can be considered practically sinusoidal.

    Ideally, an electricity supply should invariably show a perfectly sinusoidal voltage signal at every customer location.

    In reality however these signals contain many types of disturbances.

    -800

    -600

    -400

    -200

    0

    200

    400

    600

    800

    0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018

    Vo

    lts

    Time [sec]

    3-Phase, 480V, 60Hz Power Supply

    V(A,B) V(B,C) V(C,A)

    Marek Farbis, Mirus International Inc.

  • Introduction

    The deviation of the voltage and current waveforms from sinusoidal is described in terms of the waveform distortion, often expressed as harmonic distortion.

    In nearly all cases harmonic distortion is produced by a customers equipment (non-linear loads) injecting electrical noise into the power system i.e. Variable Frequency Drives.

    Marek Farbis, Mirus International Inc.

  • Definition of Harmonics In a periodic signal the primary, desired frequency is

    the "Fundamental Frequency. The term harmonics was originated in the field of

    acoustics, where it was related to the vibration of a string or an air column at a frequency that is a multiple of the base frequency.

    A harmonic component in an AC power system is defined as a sinusoidal component of a periodic waveform that has a frequency equal to an integer multiple of the fundamental frequency of the system.

    French mathematician Jean Baptiste Joseph Fourier (1768-1830) found that any function of a variable can be expanded in a series of sines of multiples of the variable.

    23/09/2011 Marek Farbis, Mirus International Inc. 7

  • Distorted Waveform

    -2

    -1.5

    -1

    -0.5

    0

    0.5

    1

    1.5

    2

    Harmonics - Components of a Distorted Waveform

    Fourier Series f(t) = Ao+A1sin(wt+q1)+A2sin(2wt+q2)+A3sin(3wt+q3) ...

    Fundamental - 60 Hz

    -1.5

    -1

    -0.5

    0

    0.5

    1

    1.5

    5th Harmonic - 300 Hz

    -1.5

    -1

    -0.5

    0

    0.5

    1

    1.5

    7th Harmonic - 420 Hz

    -1.5

    -1

    -0.5

    0

    0.5

    1

    1.5

    Resultant Waveform

    -2

    -1.5

    -1

    -0.5

    0

    0.5

    1

    1.5

    2

    Resultant Waveform

    -2

    -1.5

    -1

    -0.5

    0

    0.5

    1

    1.5

    2

    Resultant Waveform

    -2

    -1.5

    -1

    -0.5

    0

    0.5

    1

    1.5

    2

    Harmonic Spectrum

    0

    20

    40

    60

    80

    100

    1 3 5 7 9 11 13

    Harmonic #

    % o

    f F

    un

    dam

    en

    tal

    Harmonic Spectrum

    0

    20

    40

    60

    80

    100

    1 3 5 7 9 11 13

    Harmonic #

    % o

    f F

    un

    dam

    en

    tal

    Harmonic Spectrum

    0

    20

    40

    60

    80

    100

    1 3 5 7 9 11 13

    Harmonic #

    % o

    f F

    un

    dam

    en

    tal

    Time domain

    Frequency domain

    300 Hz

    420 Hz

    60 Hz

    23/09/2011 Marek Farbis, Mirus International Inc. 8

  • Why is the harmonic distortion bad?

    Effect of penetration in the electrical system affecting adjacent installations.

    Thermal effect on electric rotating machines, transformers, capacitors, and cables (extra losses).

    Pulsating torques in rotating machines. Neutral conductor overloading. Increased risk of faults from overvoltage conditions

    developed on power factor correction capacitors and resonant conditions.

    Unexpected Fuse Operation. Abnormal operation of electronic relays. Abnormal operation of solid-state devices. Lower system power factor preventing effective utilization.

    23/09/2011 Marek Farbis, Mirus International Inc. 10

  • What causes a voltage distortion?

    Relationship between System Impedance and Voltage Distortion.

    23/09/2011 Marek Farbis, Mirus International Inc. 11

  • Relationship between System Impedance and Voltage Distortion.

    ~

    ZSh

    ZTh ZCh

    Ih

    Harmonic Current Source

    Sinusoidal Voltage Source

    VS VT VL

    Where:

    ZSh impedance of the source at harmonic h,

    ZTh impedance of the transformer at harmonic h,

    ZCh impedance of cables at harmonic h,

    VS = Ih x ZSh

  • Total Harmonic Distortion

    Fundamental Current refers to the current carried in the fundamental frequency, Ih1 (60 Hz).

    current Total Harmonic Distortion refers to the ratio of all harmonic currents to the fundamental current.

    %100

    1

    2

    2max

    h

    h

    h

    h

    I

    I

    iTHD

    Ratio of the root-sum-square (RSS) value of the harmonic content of the current to the RMS value of the fundamental current.

    23/09/2011 Marek Farbis, Mirus International Inc. 13

  • Standard Variable Frequency Drive (PWM)

    IGBT S = FAST KNIFE SWITCHES IGBT = Insulated-Gate Bipolar Transistor

    CONTROL VOLTAGE & FREQUENCY

    DIODE BRIDGE

    23/09/2011 Marek Farbis, Mirus International Inc. 14

  • VFD, 6-Pulse Rectifier Current Waveform

    -300.0

    -200.0

    -100.0

    0.0

    100.0

    200.0

    300.0

    0.017 0.022 0.027 0.032

    Cu

    rre

    nt

    [Am

    ps]

    time [msec]

    VFD input current

    23/09/2011 Marek Farbis, Mirus International Inc. 15

  • VFD, 6-Pulse Rectifier and Harmonics

    For simple diode bridge rectifiers:

    When, p = 6

    h = n 6 1 h = -- 5,7,--,11,13,--,17,19...

    0

    20

    40

    60

    80

    100

    1 3 5 7 9 11 13 15 17 19 21 23 25

    harmonic

    % F

    und.

    . ia

    Current Waveform and Spectrum

    h = n p 1 h = harmonic number p = # of pulses in rectification scheme n = any integer (1, 2, 3, etc.)

    23/09/2011 Marek Farbis, Mirus International Inc. 16

  • Harmonic distortion limits

    IEEE Standard 519 1992

    IEEE Recommended Practices and Requirements for Harmonic Control in Power Systems.

    IEEE Standard C57.110 1986

    IEEE Recommended Practice for Establishing Transformer Capability When Supplying Non-sinusoidal Load Currents.

    23/09/2011 Marek Farbis, Mirus International Inc. 17

  • IEEE Standard 519 General Overview

    Introduced in 1981 (Latest revision 1992)

    Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems o Sets limits for voltage and current distortion at Point

    of Common Coupling.

    o Recognizes responsibility of both User and Utility.

    Widely adopted in N. America Becoming more common globally

    23/09/2011 Marek Farbis, Mirus International Inc. 18

  • Definition of Terms

    Point of Common Coupling (PCC)

    A point of metering, or any point as long as both the utility and the consumer can either access the point for direct measurement of the harmonic indices meaningful to both or can estimate the harmonic indices at point of interference.

    Within an industrial plant the PCC is the point between the nonlinear load and the other loads.

    XFMR

    XFMR

    ZTh

    UTILITY

    MOTOR1

    ZCh3 ZCh4

    CUSTOMER/UTILITY

    VFD2

    ZCh2

    MOTOR2

    MOTOR3

    ZCh1

    ZSh

    VFD1

    23/09/2011 Marek Farbis, Mirus International Inc. 19

    IEEE Standard 519-1992, Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems

  • Definition of Terms

    Point of Common Coupling (PCC)

    A point of metering, or any point as long as both the utility and the consumer can either access the point for direct measurement of the harmonic indices meaningful to both or can estimate the harmonic indices at point of interference. Within an industrial plant the

    PCC is the point between the nonlinear load and the other loads.

    XFMR

    XFMR

    ZTh

    UTILITY

    MOTOR1

    ZCh3 ZCh4

    CUSTOMER/UTILITY

    VFD2

    ZCh2

    MOTOR2

    MOTOR3

    ZCh1

    ZSh

    VFD1

    Rarely

    convenient to

    measure on

    Utility Side.

    23/09/2011 Marek Farbis, Mirus International Inc. 20

    IEEE Standard 519-1992, Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems

  • Definition of Terms

    Short-Circuit Ratio (ISC/IL): Ratio of the short-circuit current (ISC) available at the PCC to the maximum fundamental load current (IL).

    Maximum Load Current (IL): Recommended to be the average current of the maximum demand for the preceding 12 months.

    XFMR

    XFMR

    ZTh

    UTILITY

    MOTOR1

    ZCh3 ZCh4

    CUSTOMER/UTILITY

    VFD2

    ZCh2

    MOTOR2

    MOTOR3

    ZCh1

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