TUTORIAL ON HARMONICS Theory and Computation Techniques C
- Slides: 96
TUTORIAL ON HARMONICS Theory and Computation Techniques C. J. Hatziadoniu: hatz@siu. edu
AC Drive Harmonics Harmonic Sources: l Power converter switching action l Motor own generated harmonics (spatial distribution of windings, stator saturation) l Transformer/inductor iron core saturation l Harmonics flowing between generator and motor sides
Potential Problems due to Harmonics l Power losses and heating: reduced efficiency, equipment de-rating l Over-voltage and voltage spiking, due to resonance: insulation stressing, limiting the forward and reverse blocking voltage of power semiconductor devices, heating, de-rating l EMI: noise, control inaccuracy or instability l Torque pulsation: mechanical fatigue, start-up limitation
Power Loss and Heating Losses into the resistive and magnetic components • Resistive losses: skin effect • Magnetic losses: Eddy currents and hysterisis losses increase with frequency
Over-Voltage, Over-Current (due to resonance) Capacitor loss due to harmonics (insulation loss)
Interference with Control, EMI
What Are Harmonics? l. Technical Description A high frequency sinusoidal current or voltage produced by certain nonlinear and switching processes in the system during normal periodic operation (steady state); l l The harmonic frequency is an integer multiple of the system operating frequency (fundamental). The non-sinusoidal part in a periodic voltage or current is the harmonic ripple or harmonic distortion—comprised of harmonic frequencies. l. Mathematical Definition l Sine and cosine functions of time with frequencies that are integer multiples of a fundamental frequency l Harmonic sine and cosine functions sum up to a periodic (nonsinusoidal) function l Terms of the Fourier series expansion of a periodic function;
Harmonic Analysis l What is it? l Principles, properties and methods for expressing periodic functions as sum of (harmonic) sine and cosine terms: l l Fourier Series Fourier Transform Discrete Fourier Transform Where is it used? l Obtain the response of a system to arbitrary periodic inputs; quantify/assess harmonic effects at each frequency l Framework for describing the quality of the system input and output signals (spectrum)
Superposition l A LTI system responds linearly to its inputs l ui 1 uo 1, ui 2 uo 2 l l aui 1+bui 2 auo 1+buo 2 For sinusoidal inputs:
Application preview: DC Drive Find the armature current io(t) below
DC Voltage Approximation
Source Superposition
Output Response
Procedure to obtain response Step 1: Obtain the harmonic composition of the input (Fourier Analysis) Step 2: Obtain the system output at each input frequency (equivalent circuit, T. F. frequency response) Step 3: Sum the outputs from Step 2.
Fundamental Theory Outline l Harmonic Fundamental Theory—Part a: Periodic Signals—sinusoidal function approximation l Fourier Series—definition, computation l Forms of the Fourier Series l Signal Spectrum l Applications of the FS in LTI l Wave Form Quality of Periodic Signals l
Measures Describing the Magnitude of a Signal l Amplitude l Average l Root and Peak Value or dc Offset Mean Square Value (RMS) or Power
Amplitude and Peak Value l Peak of a Symmetric Oscillation
Non-Symmetric Signals l Peak-to-peak variation
Average Value l Signal=(constant part) + (oscillating terms)
Examples
AC Signals l Zero Average Value
DC and Unidirectional Signals
Root Mean Square Value (RMS) For periodic signals, time window equals one period
Remarks on RMS l RMS is a measure of the overall magnitude of the signal (also referred to as norm or power of the signal). l The rms of current and voltage is directly related to power. l Electric equipment rating and size is given in voltage and current rms values.
Examples of Signal RMS
Effect of DC Offset New RMS= SQRT [ (RMS of Unshifted)2+(DC offset)2]
Examples of Signals with equal RMS
RMS and Amplitude l Amplitude: Local effects in time; Device insulation, voltage withstand break down, hot spots l RMS: Sustained effects in time; Heat dissipation, power output
Harmonic Analysis: Problem Statement l Approximate the square pulse function by a sinusoidal function in the interval [–T/2 , T/2]
General Problem l Find a cosine function of period T that best fits a given function f(t) in the interval [0, T] l Assumptions: f(t) is periodic of period T
Approximation Error l Error: Objective: Minimize the error e(t) Method: Find value of A that gives the Least Mean Square Error
Procedure Define the average square error as : E is a quadratic function of A. The optimum choice of A is the one minimizing E.
Optimum Value of A Find d. E/d. A: Set d. E/d. A equal to zero
EXAMPLE: SQUARE PULSE
A geometrical interpretation Norm of a function, error, etc is defined as:
Shifted Pulse
Approximation with many harmonic terms Average Square Error :
Harmonic Basis The terms From an orthogonal basis Orthogonality property:
Optimum coefficients l The property of orthogonality eliminates the cross harmonic product terms from the Sq. error l For each n, set
Optimum coefficients l Obtain the optimum expansion coefficients:
Example—Square Wave Pulse
n A B 1 4/p 0 2 0 0 3 -4/3 p 0 4 0 0 5 4/5 p 0 6 0 0 n ± 4/np 0
Waveform Recovery n=1 -3 n=1 -9 n=1 -7 n=1 -5 n=1
Example: Sawtooth
Odd Symmetry
n An Bn 0 0 0 1 0 2/p 2 0 -1/p 3 0 2/3 p 4 0 -1/2 p 5 0 2/5 p 6 0 -1/3 p 7 0 2/7 p n 0
Periodic Approximation
Approximation of the Rectified sine A periodic signal= (constant part)+ (oscillating part)
Average Value
Harmonic Terms
Summary
Numerical Problem: DC Drive
Input Harmonic Approximation Average or dc component Harmonic Expansion Truncated Approximation (n=2, 4, and 6)
Equivalent Circuit
Superimpose Sources: DC Source
Superposition: n=2, f=120 Hz
Superposition: n=4, f=240 Hz
Superposition: n=6, f=360 Hz
Summary Freq. , Hz Vo ampl, V Io ampl, A Za magn, W Power loss, W 0 (dc) 216. 1 66. 1 1 120 144. 1 36. 9 3. 9 680. 8 240 28. 8 3. 78 7. 61 7. 14 360 12. 3 1. 08 11. 35 . 583 71. 1 Total Power Loss RMS 240 Output Power (66. 1 A)(150 V) 4, 369. 2 5, 057. 7 9, 915
Output Time and Frequency Response
Generalization: Fourier Series The Fourier theorem states that a bounded periodic function f(t) with limited finite number of discontinuities can be described by an infinite series of sine and cosine terms of frequency that is the integer multiple of the fundamental frequency of f(t): Where is the zero frequency or average value of f(t).
Waveform Symmetry l Half Wave Symmetry l Quarter Wave Symmetry Odd l Even l
Half Wave Symmetry • Half-wave symmetry is independent of the function shift w. r. t the time axis • Even harmonics have zero coefficient
Square Wave
Triangular
Saw Tooth—Counter Example
Quarter Wave Symmetry l Half wave and odd symmetry l Half wave and even symmetry
Half-wave: odd and even
Quarter Wave Symmetry Simplification
Forms of the Fourier Transform l Trigonometric l Combined Trigonometric l Exponential
• Trigonometric form • Combined Trigonometric
• Exponential
Relations between the different forms of the FS
Summary of FS Formulas
Time Shift
Example: SQP -90° Shift original shifted n 2|Cn| qn -np/2 1 4/p 0 -p/2 2 0 0 - 3 4/3 p p -p/2 4 0 0 - 5 4/5 p 0 -p/2 6 0 0 - 7 5/7 p p -p/2 n 4/np (n-1)p/2 -p/2
Example: SQP -60° Shift original shifted n 2|Cn| qn -np/3 1 4/p 0 -p/3 2 0 0 - 3 4/3 p p 0 4 0 0 - 5 4/5 p 0 p/3 6 0 0 - 7 5/7 p p 2 p/3 n 4/np (n-1)p/2 (n-3)p/6
SPECTRUM: SQ. Pulse (amplitude=1)
SPECTRUM: Sawtooth (amplitude=1)
SPECTRUM: Triangular wave (amplitude=1)
SPECTRUM: Rectified SINE (peak=1)
Using FS to Find the Steady State Response of an LTI System Input periodic, fundamental freq. =f 1=60 Hz Voltage Division
Square Pulse Excitation Harm. Order Inp. |Uin(n)|, <Uin(n) Circ. TF |H(n)|, <H(n) Out. |Uout(n)|, <Uout(n) 0 1 0 0 0 1 0. 4686 -1. 083 1. 2732 0 0. 5967 -1. 083 2 0. 2564 -1. 3115 0 0 3 0. 1741 -1. 3958 0. 4244 -3. 1416 0. 0739 1. 7458 4 0. 1315 -1. 4389 0 0 5 0. 1055 -1. 4651 0. 2546 0 0. 0269 -1. 4651 6 0. 0881 -1. 4826 0 0 7 0. 0756 -1. 4952 0. 1819 -3. 1416 0. 0137 1. 6464 8 0. 0662 -1. 5046 0 0 9 0. 0588 -1. 5119 0. 1415 0 0. 0083 -1. 5119 10 0. 053 -1. 5178 0 0 11 0. 0482 -1. 5226 0. 1157 -3. 1416 0. 0056 1. 619 12 0. 0442 -1. 5266 0 0 13 0. 0408 -1. 53 0. 0979 0 0. 004 -1. 53 14 0. 0379 -1. 5329 0 0 15 0. 0353 -1. 5354 0. 0849 -3. 1416 0. 003 1. 6061
SQUARE PULSE Excitation
Rectified SINE Wave Harm. Order Inp. |Uin(n)|, <Uin(n) Circ. TF |H(n)|, <H(n) Out. |Uout(n)|, <Uout(n) 0 1 0 0. 6366 0 1 0. 4686 -1. 083 0 0 2 0. 2564 -1. 3115 0. 0849 3. 1416 0. 0218 1. 8301 3 0. 1741 -1. 3958 0 0 4 0. 1315 -1. 4389 0. 0202 3. 1416 0. 0027 1. 7027 5 0. 1055 -1. 4651 0 0 6 0. 0881 -1. 4826 0. 0089 3. 1416 0. 0008 1. 659 7 0. 0756 -1. 4952 0 0 8 0. 0662 -1. 5046 0. 005 3. 1416 0. 0003 1. 637 9 0. 0588 -1. 5119 0 0 10 0. 053 -1. 5178 0. 0032 3. 1416 0. 0002 1. 6238 11 0. 0482 -1. 5226 0 0 12 0. 0442 -1. 5266 0. 0022 3. 1416 0. 0001 1. 615 13 0. 0408 -1. 53 0 0 14 0. 0379 -1. 5329 0. 0016 3. 1416 0. 0001 1. 6087 15 0. 0353 -1. 5354 0 0
Rect. SINE wave
Total RMS of A Signal Rewrite the FS as: Nth harmonic rms (except for n=0) Total rms of the wave form:
Total RMS and the FS Terms Using the orthogonality between the terms: For ac wave forms (A 0=0) it is convenient to write:
Waveform Quality-AC Signals Total Harmonic Distortion Index
Waveform Quality-DC SIgnals (A 0≠ 0) Ripple Factor
Example: W. F. Q. of the circuit driven by a Sq. P. Inp. Rms: |Uin(n)|/√ 2 Harm. Order Out. Rms: |Uout(n)|/√ 2 0 0 0 1 0. 900288 0. 421931 2 0 0 3 0. 300096 0. 052255 4 0 0 5 0. 180029 0. 019021 6 2 – 0. 42192)=0. 0575{ 0 √(0. 4258 0 7 0. 128623 0. 009687 8 0 0 9 0. 100056 0. 005869 10 0 0 11 0. 081812 0. 00396 12 0 0 13 0. 069226 0. 002828 14 0 0 15 0. 060033 0. 002121 RMS 1. 00 (excact) 0. 4258 %THD 48. 43 (exact) 13. 6
Example: WFQ of the circuit driven by a rect. sine Inp. Rms: |Uin(n)|/√ 2 Harm. Order Out. Rms: |Uout(n)|/√ 2 0 0. 6366=Uin(0) 0. 6366=Uout(0) 1 0 0 2 0. 060033 0. 015415 3 0 0 4 0. 014284 0. 001909 5 0 0 6 0. 006293 0. 000566 7 0 0 8 0. 003536 0. 000212 9 0 0 10 0. 002263 0. 000141 11 0 0 12 0. 001556 7. 07 E-05 13 0 0 14 0. 001131 7. 07 E-05 15 0 0 RMS 0. 707 (exact) 0. 6368 %RF 48. 35 (exact) 2. 5
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