Power Factor & Harmonics -...
Transcript of Power Factor & Harmonics -...
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Power Factor
& Harmonics
Andy Angrick
2014
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• Harmonic problems are becoming
more apparent because more
equipment that produce
harmonics are being applied
to power systems
Harmonic Distortion
Grounding
Surge Transient
Voltage Variations
Harmonics
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Electrical Loads of the Past
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Electrical Loads Today
Computers Fluorescent Lights
and Ballast's
Variable Frequency
Drives
All equipment that uses
an AC to DC power supply
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Which came first?…..
• In this case…the Egg! – Current distortion causes Voltage distortion
– Voltage distortion is created by pulling distorted current through an impedance
– Amount of voltage distortion depends on:
• System impedance
• Amount of distorted current pulled through the impedance
• If either increases, VTHD will increase
Voltage Distortion Current Distortion
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Filter
480 V
Xs
M
XT
+ -
M
Blocking
Filter
G
UPS
w/Filter
Welder
Low Distortion Electronic Ballast
Oversized
Generator
K-Rated
Active
Filt
er
12
Pulse M
Harmonic Solutions
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100%, 60 Hz
2%, 780 Hz
20%, 180 Hz
12%, 300 Hz
4%, 420 Hz
2%, 660 Hz
%100...
%1
2
4
2
3
2
2
I
IIITHD I
Total Harmonic Distortion (THD) –
(voltage or current) represents a ratio of the
root-mean-square of the harmonic content
to the fundamental quantity, expressed as a
percent of the fundamental.
Harmonics
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H = NP+/-1
i.e. 6 Pulse Drive - 5, 7, 11, 13, 17, 19,…
Source Typical Harmonics*
6 Pulse Drive/Rectifier 5, 7, 11, 13, 17, 19…
12 Pulse Drive /Rectifier 11, 13, 23, 25…
18 Pulse Drive 17, 19, 35, 37…
Switch-Mode Power Supply 3, 5, 7, 9, 11, 13…
Fluorescent Lights 3, 5, 7, 9, 11, 13…
Arcing Devices 2, 3, 4, 5, 7...
Transformer Energization 2, 3, 4
* Generally, magnitude decreases as harmonic order increases
Expected Harmonics
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“Harmonics are not a problem
unless they are a problem!”
Remember
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Harmonic Sources - Transformer
Inrush
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Harmonic Sources – Arc Furnace
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Harmonic Sources - Switch Mode
Power Supplies
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Harmonic Sources - Switch Mode
Power Supplies – Neutral of Panel
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1 Nph
Harmonic Sources - VFD’s
• The input side (AC) of all AC or DC drives are usually very similar.
For most drives today, the input is a 6-pulse rectifier circuit.
• The harmonic current produced is related to the drive pulse number
by the formula:
– where h = the harmonic number
– N = an integer multiplier beginning with 1
– p = the pulse number.
• Theoretically, for a 6-pulse drive, the harmonic current produced
would include
– 5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th, etc.
– Called the drive characteristic harmonics, and for the most part, their
magnitudes decrease as their harmonic number increases
– No even harmonics would be generated.
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• For a 12-pulse drive, the characteristic harmonic currents
produced would include
– 11th, 13th, 23rd, 25th, 35th, 37th, etc. No even harmonics would
be generated.
– Realistically, power electronic manufacturing tolerances, firing
circuit timing variations, and component degradation or failure
cause drives to produce harmonic currents in addition to the
characteristic harmonics.
– 12-pulse drives will produce some small magnitudes of 5th, 7th,
17th, 19th, etc. harmonics. These are called noncharacteristic
harmonics of the 12-pulse drive. Small magnitudes of the 2nd,
3rd, and 4th harmonics are also generated, usually 1 to 4
percent.
Harmonic Sources - VFD’s
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Harmonic Sources - VFD’s
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From
IEEE519A Draft
Active Filters
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Effect of Drive Line Reactors
(IEEE519A)
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Fundamental = 100%
Harm
onic
= 1
00%
Fundamental = 100%
Harm
onic
= 2
0%
Total Current
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Harmonic Sequence Harmonic Sequence
1 + 10 +
2 - 11 -
3 0 12 0
4 + 13 +
5 - 14 -
6 0 15 0
7 + 16 +
8 - 17 -
9 0 18 0
Motor Heating & Vibrations
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60 Hz Rotation
5th Harmonic
Rotation
“Negative Sequence Current”
– Tries to Rotate Motor in
Opposite Direction
– Causes Motor and Heating
and Vibrations
Motor Heating & Vibrations
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If a capacitor exists on
the power system
AND
Harmonic producing
loads are in use
You MUST check for
harmonic resonance
(series and parallel)
Harmonic Resonance
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• Capacitors not only supply reactive power to the loads
in an electrical distribution system they also change the
resonance frequency of the system.
• Capacitors are also a “sink” for harmonic currents
present in a system (series resonance).
• When the resonance frequency of a system with PF
correction capacitors is close to the frequency of a
harmonic current generating load parallel resonance
can occur.
Harmonic Resonance
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• AC circuits characteristically have inductive and capacitive components and have the
means to transfer energy between these components.
• Harmonic resonance occurs when the inductive reactance of a circuit is equal to the
capacitive reactance. Resonance can be either series or parallel.
• Recall that inductive reactance increases as the power system frequency increases and
the capacitive reactance decreases as the power system frequency increases by the
following equations:
and
where XL = inductive reactance in ohms
XC = capacitive reactance in ohms
f = power system frequency in Hz
L = component inductance in henries
C = component capacitance in farads.
• At 60 Hz, the capacitive components have a much higher impedance than the inductive
components.
LjLfjX L 2
Cj
CfjXC
1)(
2
1
Harmonic Resonance
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Harmonic Problems-Capacitors
Example:
Xc = 1 / 2fc
A capacitor bank has reactance, measured in Ohms.
Lets say we have a Power Correction capacitor that has a
reactance of 12 ohms at 60HZ.
Then,
12 ohms = 1 / 2 x 3.14 x 60Hz x .00022F
If this capacitor is in a 480 Volt circuit 40 Amps will flow
in the capacitor (480Volts / 12 ohms = 40 Amps).
If we have 3rd (180 Hz) harmonics present.
Then,
Reactance = 1 / 2 x 3.14 x 180Hz x .00022F
Reactance = 4 ohms
We will now see 120 Amps flow in the capacitor.
(480 Volts / 4 ohms = 120 Amps)
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MSB
480Y277
3000A
600A Branch
feeding a
600A Busway
Motor Motor Motor
160Amps
of
other
Equipment
480V
50 Hp
65 Amps
each
Power Correction Capacitors
- 480V, 12 Cap, 40 Amps each
Harmonic Problems-Capacitors
Linear Load Example:
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MSB
480Y/277
3000A
600A Branch
feeding a
600A Busway
Motor Motor Motor
160Amps
3rd
Harmonic
(180Hz)
Generating
Equipment
480V
50 Hp
65 Amps
each
Power Correction Capacitors - 480V, 12 Cap, 40 Amps each
120Amps each
4 ohms
with 180Hz
Harmonic Problems-Capacitors
Non-Linear Load Example:
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Parallel Resonance
Harmonic
Current
Source X
C
X
L
CL
CLEQUIVALENT
XjjX
XjjXX
)(
)(
Equivalent Parallel Resonant Circuit
• The parallel combination of impedance is:
• Since XL and XC have opposite signs, the denominator can equal
zero if XL = XC. In reality, the only limiting factor is the difference in
resistance between the capacitor and reactor.
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This graph shows a typical frequency scan for a power system with unfiltered capacitance. This
is a “borderline” case for which a detuned filter would typically be recommended. The scan
shows parallel resonance at the 7.9th harmonic order. The scan magnitude at the peak is
approximately 615 ohms. The scan magnitude at the 7th harmonic is about 19 ohms (which is
not particularly high). However, the parallel resonant point of an unfiltered capacitor is very
sensitive to both the upstream source impedance and capacitance on other substations. A
source impedance change (such as being switched to a different transformer) could easily
cause the parallel resonant point to shift to the 7th harmonic or lower.
Parallel Resonance
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This graph shows an updated frequency scan for the same power system with the addition of a
series tuning reactor immediately upstream of the capacitors. Note the series resonant point at the
4.2nd harmonic. The addition of the series resonant point “anchor” has forced the parallel resonant
point down to the 3.7th harmonic. Also note that the magnitude at the parallel resonant point has
been reduced to approximately 110 ohms. The chances of exciting this parallel resonant point
would be very low, for a typical industrial power system. If the electrical system has specialty loads
that generate large amounts of low-order harmonics, such as EAFs (Electric Arc Furnaces), custom
filter design and analysis would typically be necessary. The new parallel resonant point will exhibit
some shift for source impedance changes, but these shifts will generally be minimal. Typically, we
want the parallel resonant point to land at approximately the 3.5th harmonic with all steps of the PFC
filter online.
Parallel Resonance
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The series combination of impedance is:
Since XL and XC have opposite signs, the summation can
equal zero if XL = XC. In reality, the only limiting factor is the
difference in resistance between the capacitor and reactor.
Equivalent Series Resonant Circuit
Harmonic Current Source X
C
X
L
CLEQUIVALENT XjjXX )(
Frequency Scan
0.1
1
10
100
1000
60 180 300 420 540 660 780 900 1020 1140 1260 1380 1500
Frequency in Hz
Imp
ed
an
ce in
Oh
ms
Series Resonance
Frequency Scan for Series
Resonant Circuit
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Utility
Utility Bus
1000 kVA
480 V Bus
Capacitor Fdr
CAPACITORLOAD
If other loads on the
system are “Non-Linear”,
consider installing a
harmonic filter in lieu of an
“unfiltered” capacitor to
avoid RESONANCE
Harmonic Modeling - Example
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Parallel Resonant Frequencies for Various Capacitor Sizes
0
50
100
150
200
250
300
350
400
450
500
0
100
200
300
400
500
600
0 5 10 15 20 25
Harmonic Order
Ca
pa
cit
or
Siz
eHarmonic Modeling - Example
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Harmonic Distortion Standards
IEEE 519-1992 Maximum Harmonic Current Distortion
ISC / IL TDD
1-20 5%
20-50 8%
50-100 12%
100-1000 15%
1000+ 20%
ISC=Maximum short circuit current
IL= Maximum demand load
TDD= Total Demand Distortion
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IEEE 519-1992 -Current
Typical Office Building
1200A 208Y/120 service
30K AIC
The Maximum IEEE Harmonic
distortion is:
30,000 AIC / 960 = 31
31 on the IEEE chart is 8%
Example:
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IEEE 519-1992 -Current
IEEE 519-1992 Maximum Harmonic Voltage Distortion
Voltage TVD
69kV and below 5%
TVD=Total Voltage Distortion
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Troubleshooting Methods
IEEE1159
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Future of IEEE 519 (2006)
• More concise document
• PCC clarified
• New voltage range
– 1.0 kV and below
• 8% THDV
• 5% individual voltage harmonics
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Utility Source
2.3% THD
Generator
Source
5.7% THD
Harmonics & Generators
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Transformers & Harmonic Currents
• Many people incorrectly assume that ALL harmonics are
trapped by delta-wye transformers. The fact is:
– only the balanced third harmonics (and multiples of the third)
circulate in the delta winding and are therefore trapped.
– other harmonic currents (5th and 7th, for example) and the
unbalanced multiples of the third harmonics can pass through the
transformer
– harmonic currents are inductively coupled along with the 60 Hz
current by the ratio of the primary turns to the secondary turns.
• Higher order harmonics (> 25th) may or may not be
inductively coupled through the transformer.
• Sometimes, higher frequency harmonics are capacitively
coupled from secondary to primary (not by the turns ratio).
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Power Factor & Harmonics
END