Download Harmonic Distortion Explinations and Various Countermeasures

Power Quality Considerations
When Applying
Adjustable Frequency Drives
Explanations and Various Countermeasures
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Power Quality
Why the Renewed Interest in Power Quality?
• Copy Machines
• Fax machines
• Computers
• Elevator Controls
• Solid State Lighting Ballasts
• Devices that incorporate Static Power Converters SCRs, Diodes .. etc.
• Adjustable Frequency Drives
Common Issue among Common Devices
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Power
Quality Topics
•
•
•
•
•
•
What are Harmonics?
What is Harmonic Distortion?
Differences between current and voltage distortion
Possible effects of Harmonics
What Guidance is there in the Industry
What Solutions does Yaskawa Offer?
Harmonics are important to understand the
relationship between Power Quality and switch
mode power supplies!
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Definition
of Harmonics
• Harmonics are defined as currents or voltages with
frequencies that are integer multiples of the
fundamental power frequency
• SIMPLY PUT - Harmonics are used to
mathematically describe the shape of a curve that is
not sinusoidal.
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What is Harmonic
Distortion?
• Harmonic Distortion is a mathematical way of
describing how non-sinusoidal a wave shape appears
• Fourier Analysis - Sum of the Squares
TVD 

2
V
 h
h z
THD = 78.3%
THD = 1.2%
Every Wave shape has Harmonic Distortion!
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Fourier
Analysis
• Fourier Analysis of the waveforms
found in a three phase diode
rectifier shows low order harmonics
including the 5th, 7th, 11th , 13th,
etc.
Voltage
Normalized Harmonic Spectrum
100.00%
30.38%
7.16%
5.55%
1
2
3
4
5
6
7
8
9
10
11
4.83%
12
Harmonic Order
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Current
Figure 6.1
Magnitude (as % of Fundamental
• Calculation of true power factor
considers the energies contained
on these additional frequencies.
Figure 6-2 shows the resulting
harmonic spectrum based on
Fourier analysis of the current
waveform shown in figure 6-1.
700
600
500
400
300
200
100
0
-100
-200
-300
-400
-500
-600
-700
Three phase diode rectifier, line
voltage/current
13
4.32%
14
15
16
17
3.59%
18
19
Figure 6.2
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Types of
Harmonics
DC Drive - SCR Based
AC Drive - Diode Rectifier
SCR Rectification - Line Notching, Increases Voltage Distortion
Diode Rectification - Pulsed Current, Increases Current Distortion
New Technology May Solve Old Power Quality Problems
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Possible Effects of
Harmonics
• Increased Transformer Heating
recommended K-Factor of 4 to 13 on new installations
• Increased Conductor Heating
larger gauge wire
run two wires in parallel
• Electromagnetic Equipment
PLCs - more sensitive to Voltage Notching
• System resonance - Power Factor Correction
utilize input reactors to reduce likelihood of resonance
• Lower Power Factor for System
PFTrue  PFTotal  PowerRe al /  PowerRe al  PowerRe act .  PowerHarmonics 
Harmonic Distortion most likely will have no effect on
Power Distribution Performance
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How Harmonics
Lower Efficiency
Consider estimating power factor at the
terminals of an AC Drive in a system with
low source impedance (high available
short circuit current) with no input line
reactor or DC bus choke.
True factor is improved, when current
distortion is limited by system
impedance. (Including reactors, or bus
chokes.)
500
400
500
300
400
200
300
100
200
0
100
-100
0
-200
-100
-300
-200
-400
-300
-500
-400
Figure 9.1
-500
Figure 9.1
Power Factor Considering 92.8% I THD
Power Factor Considering 32.6% I THD
pf = kW/kVA
pf = kW/kVA
I THD = 92.8%
I THD = 32.6%
pf = 1/Sqrt(12+.9282)
pf = 1/Sqrt(12+.3262)
pf = 73.3%
pf = 95.08!
Figure 9.2
Figure 9.2
Impedance Improves Efficiency!
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Power Factor When
Harmonics Exist
•
From IEEE Std. 141-1993: Power is the
product of in-phase current times the
voltage or:
P60 = V60 * I60cos q
In the case of harmonics:
Ph = Vh * Ihcos q or S = (Sqrt(P2 + Q2 +D2)){R1]
•
Where P = Real Power, Q = Reactive
Power and D = Distortion Power.
•
System losses will be higher due to the
harmonic components, than with
equivalent 60 kVA.
Ph = I2h * Rh
True Power Factor Representation - Expanded
Q Reactive
Power
X (kVAr)
•
P Real Power (kW)
Figure 10.1
(Ohm’s law in harmonic-land)
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The Risk of Parallel
Resonance
•
•
•
Hp - the harmonic order (per-unit frequency) at parallel
resonant frequency
MVAsc - the system short-circuit capacity
MVArc - the power factor improvement capacitor
Power Factor Capacitors Relieve Load
X
XL
Hp - Sqrt(MVAsc / MVArc)
•
•
[R2]
c
ih
ih
Per IEEE Red Book (Std. 141-1993): “If the SCR (short
circuit ratio is less than 20), and there is a parallel
resonance condition near a characteristic harmonic of
the non-linear load, there will be a problem.”
Since all power systems have inductance and
capacitance, they will resonate at a given frequency.
When an exciting energy at that frequency, in a quantity
that is large enough to offset the natural dampening in
the system, is present, resonance will occur. In a typical
industrial or commercial building, the natural resonance
frequency is likely to be in the range of 1000 Hz. It is a
real world possibility that a system can resonate.
Figure 11.1
Resonance occurs when: Xc = XL
Parallel Resonance
Current measured at the capacitor,
showing 660Hz, (11th harmonic resonance)Figure 11.2
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What Guidance
is there in Industry?
Table Two: Voltage Distortion Limits
Bus Voltage at PCC
Individual
Total Voltage
Voltage
Distortion THD
Distortion (%)
(%)
69 kV and below
3.0
5.0
69.001 kV through 161 kV
1.5
2.5
161.01 kV and above
1.0
1.5
VTHD A
VTHD B
USER A
USER B
UTILITY
IEEE 519 - 81
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IEEE519-92
Table Three: Current Distortion Limits for General Distribution Systems
(120 V through 69 kV)
Maximum Harmonic Current Distortion
in Percent of Load Current




ISC/IL
<11
11 h<17
17 h<23 23 h<35 35 h
<20
4.0
2.0
1.5
0.6
0.3
20<50
7.0
3.5
2.5
1.0
0.5
50<100
10.0
4.5
4.0
1.5
0.7
100<1000
12.0
5.5
5.0
2.0
1.0
>1000
15.0
7.0
6.0
2.5
1.4
Even harmonics are limited to 25% of the odd harmonic limits above.
where
TDD
5.0
8.0
12.0
15.0
20.0
ISC = Maximum short-circuit current at PCC.
IL = Maximum demand load current (fundamental frequency
component) at PCC.
• Compare Short Circuit Capacity to Maximum Load Current
• Determine Point of Common Coupling
Specification has created a problem
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IEEE519-92
Issues:
•
•
•
•
•
Addressed Current Distortion
Does not clarify PCC
Limits do not increase sufficiently closer to non-linear device
Does not clarify injected harmonics
Is not clear that goal is reasonable Voltage Distortion
Results:
Pass / Fail Dependant on Point of
Measurement
Specification is not intended to comply
internally
All electrical products are lumped together
Double the price of a VFD package to make a
waveform look Pretty!
Don’t Make Your Customer Pay for Poor Guidance
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Countermeasures with
Yaskawa Drives
• Correctly Sized Input Transformer (Kfactor Rated)
• Standard AFD
• DC Link Choke (Standard from 30 Hp to 250 Hp)
• AC Input Reactor (Optional on All Drives)
• Custom Designed Low Pass or Broad Band Filters
(Optional on All Drives)
• 12-Pulse Transformer (Optional from 30 Hp thru 1000 Hp
The Solution to fit the Specification
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Standard
6-Pulse Front End
• Distortion Levels can vary from 60% to 130%
• Dependent on stiffness of power transformer
• Not a problem
• Input Current Waveform
200
Current
100
Amps
0 .
-100
-200
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DC Link Choke
• Reduce Distortion by 50% from standard 6 Pulse Drive
• Represent 3% input impedance to Line Power
• Current Waveform
100
Current
50
Amps
0 .
-50
-100
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AC Input Reactor
• Reduce Distortion by 50%, dependent on System
Impedance
• Reduce Nuisance Trips from surge voltages
• Current waveform
100
Current
50
Amps
0 .
-50
-100
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Low Pass /
Broad Band Filters
• Designed to take 5th and 7th Harmonic out of system
• Series Resonant Tuned
• Leading Power Factor
f  1
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Shunt Filters
The development of shunt filters to correct
harmonic distortion has lead to the acceptance of
two common technologies, under one heading, but
with dramatic differences.
Typical de-tuned p.f. bank
1.) Power factor capacitor providers often use
tuning reactors to de-tune power factor back, and
apply it on a bus. These filters require careful field
evaluation and harmonic analysis to assure
effectiveness and prevent against resonance. Some
are tuned to 240Hz, and are simply called de-tuned
p.f. banks, others are tuned to 300Hz, and act as a
shunt filters for harmonic energies.
Figure 20.1
Drive Applied Harmonic
Filter
2.) Drives applied filters which include a 5% series
inductor and are applied in front of a single drive
load, and correct harmonic distortion for that
particular drive. Drive applied filters generally do
not require rigorous system analysis, and there is
not a risk of resonance.
Figure 20.2
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12-Pulse Transformer
• Reduce Distortion by 92%
• Lowest Levels in the Industry
• Input Current Waveform
Current
100
50
Amps
0 .
-50
-100
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Multi-pulse Converters
Using
Phase Shifting Transformers
• Theoretical phase cancellation in
transformer primary eliminates low
harmonics. In practice, phase
cancellation is dependant on current
and voltage phase balance and
current sharing between bridges.
Twelve-pulse Drive using Zig-Zag Transformer
Transformer
To
DC
Bus
From
Source
• Can limit the level of current
harmonic distortion to 5-15%
depending on transformer
configuration, bridge symmetry, and
source impedance..
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From
DC
Bus
Figure 22.2
Rectifiers
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Example!
Information Needed:
- Simplified One Line Diagram
- Electrical Schedule (preferred)
- Ratio of linear load / Non Linear Load
System Properties:
Connected kVA = 769
Impedance: 5.75%
Non Linear kVA = 114
Xfmr Size = 1000 kVA
Quick Estimate based upon Electrical Schedule
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PCC Service at
Entrance of Bldg.
Power System Distribution Specification
1000 Kva @ 5.75 % impedance
655 Hp Linear Load
115 Hp MagneTek Drive Load
PCC = Service Entrance
ISC/IL = 22.7
Harmonic Reduction
15%
IEEE519-92, ITHD < 8.0%
13%
10%
5%
5%
4%
2%
5%
1%
1%
2%
1%1%
0%
6 Pulse
DC Link
AC Input
Reactor
Low Pass
Filter
12 Pulse
Xfmr
Power Distribution Rule of Thumb:
Keep Voltage Distortion Below 5% - Normal Conditions
7.5% Start Up, Unusual Conditions
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PCC at
VFD Terminals
Power System Distribution Specification
1000 Kva @ 5.75 % impedance
115 Hp MagneTek Drive Load
PCC = At VFD Terminals
ISC/IL = 153
Harmonic Reduction
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
87%
IEEE519-92, ITHD < 15.0%
34%
4%
6 Pulse
2%
DC Link
33%
1%
13%
1%
8%
1%
AC Input
Reactor
Low Pass
Filter
12 Pulse
Xfmr
Power Distribution Rule of Thumb:
Keep Voltage Distortion Below 5% - Normal Conditions
7.5% Start Up, Unusual Conditions
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Summary:
Effects of Harmonics
#1 Effect of Harmonic is increased heating of supply transformer
System Properties:
Xfrm Rating: 2500 / 3125 FC kVA
Impedance: 5.67%
Estimated ITHD = 23.9%
I Load = 3639 Amps
Connected HP: 2785
Demand Hp: 1950
Estimated Current Harmonics :
I Load * THD = 870 Amps
IRMS = (I Load^2 + I Harm ^2)^.5 = 3742 Amps
Supply Xfmr KVA = Voltage * Current * 1.73 = 480 * 3742 * 1.73 = 3111 kVA
Capacity Reduction due to Current Harmonics
KVA = Voltage * Current * 1.73 = 480 * 3639 * 1.73 = 3021 kVA
Difference = 3111 - 3021 = 89 kVA = 2.8 % Losses at Full Capacity
Simple Calculations to put End User at Ease!
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Comparative Cost of
Harmonic Mitigation Devices
•
•
•
Figure 27.2
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Assumptions:
10,000 installed base cost
of 6-pulse drive.
Values will vary for lower
HP drives.
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Benefits Outweigh
Challenges
• Educating on Power Quality
• Electrical Noise Issues
• Motor Design Compatibility
Speed
Windings
Long Lead Length
Retrofits
•
•
•
•
7/15/2002
Improve Building Owner’s Bottom Line Thru Energy Efficiency
Reduce Wear /Tear on Pumps, Belts, Seals, Bearings …etc
Eliminate Demand charge due to Inrush from Line starting Motor
Gain Precise Control on Varying Climate in Building thru Automation
PP.AFD.08
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