Line and Load
Considerations
for AC Drives:
Harmonics
1
Outline
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What are harmonics?
What problems do they cause?
What are the standards?
How can I reduce the drive harmonics?
Summary
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What are Harmonics?
• A sinusoidal waveform
does not contain any harmonics
• This is an example of a linear load
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Harmonics 101
• Harmonics are simply integer multiples of the
fundamental frequency
– for example, if 50Hz is the fundamental
(sometimes referred to as the 1st harmonic),
then the 2nd harmonic is 100Hz, the 3rd
harmonic is 150Hz, etc.
• Any non-sinusoidal waveform can be created
by the addition of harmonics at various
amplitudes and phase angles
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Fundamental (1st harmonic) Only
• 70.7A of fundamental current
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Fundamental, Harmonics, Total
• Sum the harmonics with the
fundamental
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What problems do they cause?
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Increased Utility current requirement inability to expand or utilize
equipment
Component overheating
distribution transformers & wires
Nuisance tripping causing lost productivity sensitive equipment
Equipment malfunction
due to multiple or loss of zero crossing
Noise transfer to other loads
possible other utility customers
Incorrect meter readings, relays malfunction
maintenance time
Communication or Telephone Interference problems
Excitation of Power System Resonance's creating over-voltage’s
Voltage Flat Topping Problem
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IEEE 519 I(TDD)
25
IEEE 519 Maximum Acceptable I(TDD), %
20
15
10
5
0
1 Isc / Iload 10
100
1000
10000
Rule of Thumb: If non-linear load is >30% of total load, may
have harmonic issues.
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How can we reduce the harmonic current?
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DC link choke within the drive
line reactor
passive filter
active filter
multi-pulse
– 12 pulse
– 18 pulse
• active converter
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Passive Harmonic Filter
• 3rd party hardware solutions
packaged with drives
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Summary of Mitigation Costs
Relative Installed Cost, %
0
50
100
150
200
250
300
Basic Drive without dc link choke
Addition of dc link choke
Addition of 3% line reactor
Addition of passive filter
12 pulse auto transformer
18 pulse auto transformer
Active filter
Synchronous converter
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Conclusions
• We have the knowledge base for understanding
harmonics
• We have tools to estimate harmonics
• We have methods to reduce I(THD)
• We have cost effective solutions to reduce drive line
current harmonics
• We have tools to verify on site I(THD)
• For further information, contact your local distributor
or your local Rockwell Automation office
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