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
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
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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
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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 customer’s 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.
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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
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0
0.5
1
1.5
2
Resultant Waveform
-2
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-1
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0
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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
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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.
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What causes a voltage distortion?
• Relationship between System Impedance and Voltage Distortion.
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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 <- The voltage will be the least distorted nearest to the
source.
VT = Ih x (ZSh+ZTh) <- Voltage Distortion at the Transformer at h
VL = Ih x (ZSh+ZTh+ZCh) <- more distorted nearer the load, as the
harmonic current flows through larger amounts of impedance.
cable xfmr
Non-linear Load
Ohm’s Law:
Vh = Ih x Zh Where: Zh – impedance at hth harmonic, Ih – current of hth harmonic, Vh – voltage of hth harmonic,
ZSh
ZCh
ZTh
CUSTOMER/UTILITY
UTILITY
VFD1
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While current travels only along the power path of the non-linear load, voltage distortion affects all loads connected to that particular bus or phase.
Harmonic voltage distortion is caused by the flow of harmonic currents through system impedance.
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.
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Standard Variable Frequency Drive (PWM)
IGBT ‘S = FAST KNIFE SWITCHES IGBT = Insulated-Gate Bipolar Transistor
CONTROL VOLTAGE & FREQUENCY
DIODE BRIDGE
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VFD, 6-Pulse Rectifier Current Waveform
-300.0
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100.0
200.0
300.0
0.017 0.022 0.027 0.032
Cu
rre
nt
[Am
ps]
time [msec]
VFD input current
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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
un
d.
. 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.)
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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.
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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
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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
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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.
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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
ZSh
VFD1
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The harmonic current limits are based on the size of the load with respect to the size of the power system to which the load is connected.
IEEE Standard 519-1992, “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems”
ISC
IL
Recommended Current Distortion Limits
Table 10.3, p72
Current Distortion Limits for General Distribution Systems (120 V Through 69,000 V)
Where:
ISC = maximum short-circuit current at PCC.
IL = maximum demand load current (fundamental frequency component) at PCC.
TDD = Total Demand Distortion (harmonic current distortion calculated in % of maximum demand load current)
THD = Total Harmonic Distortion (calculated based on actual load)
Maximum Harmonic Current Distortion in Percent of IL
Individual Harmonic Order (Odd Harmonics)
ISC/IL <11 11h<17 17h<23 23h<35 35h TDD
<20* 4.0 2.0 1.5 0.6 0.3 5.0
20<50 7.0 3.5 2.5 1.0 0.5 8.0
50<100 10.0 4.5 4.0 1.5 0.7 12.0
100<1000 12.0 5.5 5.0 2.0 1.0 15.0
>1000 15.0 7.0 6.0 2.5 1.4 20.0
IEEE Standard 519-1992, “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems”
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Recommended Voltage Distortion Limits
Table 10.2, p70
Low-Voltage System Classification and Distortion Limits
Special
Applications1
General
System
Dedicated
System2
Notch Depth 10% 20% 50%
THD (voltage) 3% 5% 10%
Notch Area (AN)3 16 400 22 800 36 500
NOTE: The Value AN for other than 480 V systems should be multiplied by V/480 1 Special applications include hospitals and airports. 2 A dedicated system is exclusively dedicated to the converter load. 3 In volt-microseconds at rated voltage and current.
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IEEE Standard 519-1992, “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems”
Current Distortion Criteria
• Intended to limit for the harmonic current injection from individual customers, so they will not cause unacceptable voltage distortion levels. – Current harmonics will distort voltage in proportion to
impedance of power system.
• Short circuit current, ISC is a measure of system impedance. – Higher ISC means lower impedance, therefore lower voltage
distortion.
• Short circuit ratio, ISC/IL allows for higher distortion levels at lighter loads. – As the size of the user load decreases with respect to the size of
the system, the % of harmonic current that the user is allowed to inject into the utility increases.
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IEEE Standard 519-1992, “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems”
Effect of a Stiff Source, ISC/IL > 100
vTHD = 2 % iTHD = 127 %
400 Hp VFD
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Effect of a Weak Source, ISC/IL = 8
vTHD = 16 % iTHD = 25 %
400 Hp VFD
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Load Level contribution to Harmonics
• A load’s maximum contribution to harmonic distortion is at rated load. – Harmonic current in Amps at full load is highest even
if iTHD might be higher at lighter loads. IEEE Std 519 uses TDD (Total Demand Distortion) for this purpose.
– If IEEE Std 519 limits can be met at full load, then both voltage distortion and harmonic overheating would be satisfied at all load levels.
• More practical to use a load’s rated current as the Demand Current.
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WHAT METHODS ARE USED TODAY TO MITIGATE HARMONIC CURRENTS GENERATED BY PWM VFD’S ?
1.) DO NOTHING.
2.) ADD AC LINE REACTORS OR DC LINK CHOKES
3.) TUNED TRAP FILTER
4.) LOW PASS FILTERS
5.) 18-PULSE VFD’S
6.) VFD/CW ACTIVE FRONT ENDS (AFE).
7.) VFD WITH PARALLEL ACTIVE HARMONIC FILTER
8.) MIRUS LINEATOR™ AUHF.
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LINEATOR™ Advanced Universal Harmonic Filter with 600V-480V AUTOXFMR
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23/09/2011 Marek Farbis, Mirus International Inc. 36
APPLICATIONS
Case Study, Current Distortion
McQuay 400Hp VFD Chiller with Mirus Lineator.
-250.0
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-150.0
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-50.00
0.000
50.00
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200.0
250.0
A
1:33:27.264 PM
7/20/2011
1:33:27.281 PM
7/20/2011
3 ms/Div
16.669 (ms)
Competitive 500Hp VFD chiller with 5% AC Line reactor.
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-50.00
0.000
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A
1:39:39.267 PM
7/20/2011
1:39:39.284 PM
7/20/2011
3 ms/Div
16.672 (ms)
A1 Waveform
163.27 Arms, 7.69 %THD
0
20
40
60
80
100
1 5 10 15 20 25 30 35 40 45 50
7/20/2011 - 1:33:27.264 PM
7.7 % THD at 65% Load = 5% TDD
A1 Waveform
216.05 Arms, 42.81 %THD
0
20
40
60
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100
1 5 10 15 20 25 30 35 40 45 50
7/20/2011 - 1:39:39.267 PM
42.8 % THD at 65% Load = 27.8% TDD
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U1 Waveform
466.20 Vrms, 3.55 %THD
0
20
40
60
80
100
1 5 10 15 20 25 30 35 40 45 50
8/23/2011 - 11:54:34.219 AM
U1 Waveform
587.71 Vrms, 0.86 %THD
0
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40
60
80
100
1 5 10 15 20 25 30 35 40 45 50
7/20/2011 - 1:33:27.264 PM
Case Study, Voltage Distortion result
McQuay 400Hp VFD chiller with Mirus Lineator.
-800.0
-600.0
-400.0
-200.0
0.000
200.0
400.0
600.0
800.0
V
1:33:27.264 PM
7/20/2011
1:33:27.281 PM
7/20/2011
3 ms/Div
16.669 (ms)
Competitive 500Hp VFD chiller with 5% AC Line reactor.
-600.0
-400.0
-200.0
0.000
200.0
400.0
600.0
V
11:54:34.219 AM
8/23/2011
11:54:34.236 AM
8/23/2011
3 ms/Div
16.669 (ms)
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Summary • By drawing non-sinusoidal current, VFD’s
generate harmonics. • The flow of harmonic currents through the power
system impedance creates voltage distortion. • Excessive voltage distortion will cause equipment
malfunction. • IEEE Std 519 harmonic limits can be met by
application of appropriately designed harmonic treatment.
• Harmonic treatment method must perform in ‘Real World’ conditions and when supplied by generator.
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How to Ensure your VFD Installation Meets IEEE 519 Limits
• Perform harmonic survey to determine existing conditions
• Obtain harmonic spectrum of load from manufacturer
• Use modeling software to analyze various treatment methods and system conditions
• Analyze impact on Generator or UPS
• Anticipate effect of future system or load changes
• Specify LINEATOR™ FOR
ALL LARGER VFD APPLICATION
Or
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Thank you
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