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Harmonics and PFC The love story
By Pol Nisenblat
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THE FUTURE OF POWER QUALITY
DEFINITIONS
PROBLEMS
“DETUNED” SOLUTIONS
TUNED SOLUTIONS
HOW TO CHOOSE
OPEN DISCUSSION
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ELECTRICAL POWER SOURCE It is worldwide common practice to assume that utilities
generate a near perfect sine wave voltage Typical electrical energy source is introduced by “voltage”
source
Ideal Voltage Source
Z internal = 0
Z load = infinity
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DISTRIBUTION TRANSFORMER Typical Internal Impedance is only 2-7% of the fully loaded
Secondary
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IDEAL ENERGY SOURCE
Simplified power control by
periodical switching (chopping)Nonlinear load operation
generates current
waveform distortion
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HARMONICS - DEFINITIONS
Harmonics are integral multiples of some fundamental
frequency that, when added together, result in a distorted
waveform
f(x) = sin(x) f(x) = sin(5x)
5
+
f(x) = sin(x) + sin(5x)5
=
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HARMONICS - DEFINITIONS Harmonic order are MULTIPLES of the fundamental frequency.
Typical Harmonics are the 3rd, 5th, and 7th
Where H3 = 150 Hz, H5 = 250 Hz, H7 = 300 Hz (at 50Hz world)
In fact, any waveform may be constructed from a sine wave and some number of its harmonics like:
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HARMONICS – MOST IMPORTANT DEFINITIONS
Harmonics are originated at the load side! Harmonics are created in the current!
Harmonic sources are - current sources
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HARMONIC SOURCE
High internal impedance The harmonic current is being PUSHED towards lowest
external impedance path
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HARMONIC CURRENT FLOW
Typical harmonic current flow is towards distribution transformer
1 Ohm0.01 Ohm
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HARMONICS – PROBLEMS?
I2r losses + “skin” effect
Voltage distortions
Iron-core losses
Cables/transformers overheat
Upstream pollutions
But, the real problems are yet to come
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HARMONIC CURRENT WITH PFC
Where the H5 current should go now?
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PARALLEL RESONANCE
C
1
X C
LX l
Parallel resonance = Infinity impedanceXX CL
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PARALLEL RESONANCE High impedance at the resonance frequency The resonance frequency changes with changed number of
capacitor groups (N)
1 2 3 4 6 7
ResonancePoint (7th)
85
ResonancePoint (5th)
k
n
Decreased NIncreased N
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PARALLEL RESONANCE
0
0.5
1
1.5
2
2.550 100
150
200
250
300
350
400
450
500
550
Frequency
Imp
edan
ce (
Oh
m)
1 Step
2 Steps
3 Steps
4 Steps
5 Steps
6 Steps
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PARALLEL RESONANCE Harmonic current turns back to the loads Harmonic VOLTAGE raises dramatically Harmonic current circulates (ping pong) between capacitors
and distribution transformer
V
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PARALLEL RESONANCE – WHAT TO DO? Xl1 reactor in series to the capacitor moving parallel resonance
frequency downstream The new resonance frequency at:
Xc=Xl+Xl1 Locate parallel resonance frequency below lowest
dominant harmonic
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PARALLEL RESONANCE FREQUENCY –”RULE OF THUMB”
Where hr is the harmonic number of the parallel
resonance
Unfortunately, not accurate enough
hr = kVAsc
kVAC
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HOW TO CALCULATE PARALLEL RESONANCE FREQUENCY? For those who love formulas:
1
1
1
11
11
|| 1
; ; 1
22
2
22
2
NCLCL
jLCLZ
CjNCLCL
LLC
L
Z
LjLjCjN
LjLjCjN
ZZZZN
Z
LjZLjZCj
Z
TR
TR
TR
TRTR
TR
TR
LTRLC
LTRLTRC
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HOW TO CALCULATE PARALLEL RESONANCE FREQUENCY? Unfortunately, Xl (Xtr) value which is actually the total
distribution network inductance at the frequency of interest - is unknown
But, we do know that the parallel resonance frequency would always be located downstream to the serial resonance frequency between Xl1-Xc
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“DETUNED” SOLUTION
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WHAT FREQUENCIES ARE SAFE?
Even” harmonics are normally not present
“Triple” harmonics are canceled on DELTA connected loads
with balanced 3rd harmonic
On On balanced loads balanced loads and and 3 phase DELTA 3 phase DELTA capacitors cases, capacitors cases,
tuning in H3.5-H4.2 ranges are most popular tuning in H3.5-H4.2 ranges are most popular
)120*3**3sin(*)0*3**3sin(*
)120*3**3sin(*
)0*3**3sin(*
21)3(21
2)3(2
1)3(1
tAtAV
tAV
tAV
H
H
H
V1
V2V3
N120 Deg.
V 1-2
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DETUNED SOLUTION - DEFINITIONS
Tuned frequency is defined by serial resonance point
Serial connected reactors are defined by % rather then Henries
Reactor’s % defined as impedance at the fundamental
frequency with respect to the capacitor’s impedance at
the same frequencyReactor 50Hz 60Hz Harmonic
7% 189Hz 227Hz H3.78
6% 204Hz 245Hz H4.08
14% 134Hz 160Hz H2.67
5.67% 210Hz 252Hz H4.2
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7% “DETUNED” SOLUTION
0
0.05
0.1
0.15
0.2
0.2550 100
150
200
250
300
350
400
450
500
550
Frequency
Imp
edan
ce (
Oh
m)
1 Step
2 Steps
3 Steps
4 Steps
5 Steps
6 Steps
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TUNED SOLUTIONS - FILTERS
Passive filters are “tuned” to just below the harmonic
frequency Care should be taken – not to overload!
Low impedance path at 5th harmonic
Low impedance path at 5th harmonic
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TOO HIGH HARMONIC LEVELS?
Disconnect ALL PFC capacitors and check again
Significant difference? – Use detuned solution
No difference? – Use tuned solution 2nd and 4th harmonics too high? - Check voltage
converters Weak network? 14% detuned solutions are preferred Balanced 3rd harmonic in current? – Use 5.67-7%
solutions Unbalanced compensation? – Use 14% solutions only Voltage harmonics without current sources?
Don’t use filters! Speak to utility.