Transcript Document

Variable Frequency Drives
Harmonics Overview
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Harmonics Overview
What causes harmonics ?
Non-linear load devices create harmonics when they convert
ac to dc, dc to dc, dc to ac, and ac to ac.
This can produce high levels of harmonic voltage and current
distortion.
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Harmonics Overview
• Harmonics operate at a frequency that is a multiple of the
fundamental
• Harmonics operate at a magnitude that is inversely
proportional to the magnitude of the fundamental
• In other words... the higher the harmonic order, the less effect
it has on the waveform.
• In most cases, harmonic analysis is limited to the 13th
harmonic.
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Harmonics Overview
• Frequency is defined as the number of times a given
waveform repeats itself every second.
• Example: the typical household line current impresses 115
volts AC, 60 times per second.
Repeated
60 times
every second
0
166 msec
1 cycle
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Harmonics Overview
• Harmonic currents operate at the same time as the
fundamental, but at faster rates
• Harmonic currents are additive, producing a “distorted” sine
wave.
Fundamental = 60Hz
5th Harmonic = 300Hz
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Harmonics Overview
• Harmonic Currents are currents that flow at the same time as
a fundamental current, 60Hz frequency, but at multiples of
the fundamental.
• For a typical 6 pulse inverter, these multiples are:
– Fundamental = 60 Hz
– 5th Harmonic = 60 x 5 = 300 Hz
– 7th Harmonic = 60 x 7 = 420 Hz
– 11th Harmonic = 60 x 11 = 660 Hz
– 13th Harmonic = 60 x 13 = 780 Hz
– 17th Harmonic = 60 x 17 = 1,020 Hz
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Harmonics Overview
• A harmonic current operates in “harmony” with the
fundamental but faster; i.e. the 5th harmonic of the 60 Hz
fundamental repeats itself 300 (60 x 5) times per second.
Repeated
300 times
every second
0
30 msec
1 cycle
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Harmonics Overview
Harmonics have several harmful side effects:
• Can interfere with electronic equipment and people
– through radiated (RFI) interference through the air
– emitted (EMI) interference over power and control wires.
• Can cause an increase in supplied power (KVA) over what is
really needed (kW or HP)
– Entire AC supply line must be sized accordingly
– End user transformers, switchgear, wires, contactors, and fuses
have to be bigger.
– The power company generator
• Can cause Poor Power
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Harmonics Overview
Poor Power. . .
Reduces the Load Carrying Capacity of Your Electrical
System!
•
Transformers & wiring are sized by current capacity. If additional
current is required due to low power factor, capacity for work
producing current is displaced. Everything in your building is
oversized to make up for poor power factor.
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Harmonics – DC Capacitance Link
•
A DC capacitance link provides voltage to the inverter between the
peaks of the mains voltage waveform. Current only flows from the
mains into the DC link when the mains voltage exceeds that of the
capacitor. Current only flows when the mains voltage is near its peak,
as shown.
•
The larger the DC link capacitor, the shorter the period of current
flow, and the higher the peak current.
Single or
V
+
Three
Phase Power
C
C
Current
-
Rectifier
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D C Link
Inverter
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Harmonic - Current Wave Form
Ideal current wave but not attainable with
a 6-pulse drive
What the current wave form looks like
with a 6-pulse drive
Approaching a perfect current wave
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Typical Drive Input Current Waveform
MM4 45kW Line current waveform (low impedance mains)
MM4 45kW
Line Current Waveform (low impedance mains)
200.00
150.00
100.00
Amps
50.00
0.00
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
-50.00
-100.00
-150.00
-200.00
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I RMS = 75A
Power = 12kW (x3)
If current is sinusoidal
Only 52A is required
for the Same power.
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SED2 Typical Waveform
11kW
Line
current Waveform
waveform
SED2SED2
11kW
Line
Current
30.00
20.00
Amps
10.00
0.00
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
-10.00
-20.00
I RMS = 53A
Power = 12kW (x3)
-30.00
If current is sinusoidal
Only 52A is required
for the same power.
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DC Link Design Results
0.18
0.16
0.14
15HP Typical VFD
0.12
0.1
Series1
0.08
Line Current
Harmonics Spectrum
0.06
0.04
0.02
0
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0.2
0.18
0.16
15 HP SED2
0.14
0.12
0.1
Series1
0.08
0.06
Line Current
Harmonics
Spectrum
0.04
Meter deflection
0.02
0
0
500
1000
1500
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2000
2500
3000
3500
4000
4500
5000
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Harmonics Competitive Analysis
How do other VFD manufacturers address Harmonics?
• Some build in DC link chokes
– This adds cost, weight and current losses
• Some offer line reactors
– Rarely built-in to VFD - often offered as a separate option
– Commonly mounted in bypass cabinet, or supplied in its
own enclosure
– Adds cost, weight and can cause nuisance tripping of drive
on undervoltage
• Some build special 12 and 18 pulse drives
– Extremely expense and not practical for most application
under 100HP
• Some just ignore the problem
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Harmonics - The SED2 Advantage
• SED2 is specifically designed for HVAC applications
– Fans and pumps with variable torque load characteristics
• Sophisticated Siemens control technology enables the SED2
to significantly reduce current harmonics by dramatically
reducing the size (capacitance) of the DC link.
• In lieu of large (high uF) electrolytic capacitors used in most
VFDs, SED2 uses much lower value (low uF) plastic
capacitors.
•Typical 15Hp VFD: DC link capacitance of 1000uF
•SED2 15HP VFD: DC link capacitance of 20uF
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Siemens SED2 VFD Advantages
• Low harmonics design
• Small enclosure size
• Many standard
features
• Reduces operating
cost
• Low power losses/high
efficiency
• Minimal effect on
customer power
• Wide power range –
from ½ to 125 HP
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