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Harmonics in Low Voltage Three-PhaseFour-Wire Electric Distribution Systems
and Filtering Solutions
Harmonics in Low Voltage Three-PhaseFour-Wire Electric Distribution Systems
and Filtering Solutions
Dr. Prasad EnjetiTexas A&M University
Power Electronics and Power Quality Laboratory
Dr. Prasad EnjetiTexas A&M University
Power Electronics and Power Quality Laboratory
PSERC Online Seminar
February 13, 2001 © 2001 Texas A&M University. All rights reserved.
PSERC Online Seminar
February 13, 2001 © 2001 Texas A&M University. All rights reserved.
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Introduction
Introduction
Outline
Outline
1
11
Harmonic Current Sources
Harmonic Current Sources2
22
Case Studies
Case Studies3
33
Conclusions
Conclusions5
55
Solutions:- Derating- Passive Approach
- Active Approach
Solutions:- Derating- Passive Approach
- Active Approach
4
44
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Introduction
Introduction
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Typical Building Power Distribution SchemeTypical Building Power Distribution Scheme
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Harmonic Current SourcesHarmonic Current Sources
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Copy machines
Electric typewriters
Light fixture ballasts
Personal computers
Computer systems
Audio visual equipment
Recorders
Televisions
Video tape players
SCR drives for motors
SCR drives for elevators
UPS systemsComputer terminals
Harmonic Producing Equipment in Office BuildingsHarmonic Producing Equipment in Office Buildings
Laboratory testing equipment
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Harmonic Related Problems in BuildingsHarmonic Related Problems in Buildings
•Overheating and damage to neutral conductors
•Overheating and damage to panel board feeders
•Line voltage distortion
•Higher Common mode voltage
•Nuisance tripping of circuit breakers
•Overheating and premature failure of distributiontransformers
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Case StudiesCase Studies
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Typical Current Waveforms of AppliancesTypical Current Waveforms of Appliances
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Typical Current Waveforms of Appliances. Contd.Typical Current Waveforms of Appliances. Contd.
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Typical Current Waveforms of Appliances. Contd.Typical Current Waveforms of Appliances. Contd.
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Typical Current Waveforms of Appliances. Contd.Typical Current Waveforms of Appliances. Contd.
Eff f l l d
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Effect of Triplen Harmonics in Neutral ConductorsEffect of Triplen Harmonics in Neutral Conductors
Ia
Ib
Ic
IN
2
,
2
,
2
,, rmscrmsbrmsarms N I I I I ++=
rms phaserms N I I ,, *3=
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Survey results, Building ISurvey results, Building I
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Survey results, Building IISurvey results, Building II
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Building I
Building II
Building III
Building IV
Building V
66.8A
50.0A
12.8A
80.6A
23.0A
77.3A
65.6A
36.1A
77.2A
17.4A
72.5A
41.1A
24.5A
90.0A
26.0A
PhaseBuilding
A B C
Summary of Building MeasurementSummary of Building Measurement
120.0A
53.7A
27.5A
142.0A
12.7A
170%
103%
80.9%
171.3%
57.4%
NNeutral toPhase, %
Building VI 44.6A 41.2A 41.9A 60.7A 143%
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OFFICE BUILDINGSOFFICE BUILDINGSOFFICE BUILDINGS
• 9 Buildings surveyed.• Typical loads include computers andoffice equipment.
• Average neutral to phase ratio of 1.13• Average crest factor of 2.12
Harmonic Profiles for Typical BuildingsHarmonic Profiles for Typical Buildings
MEDICAL FACILITIESMEDICAL FACILITIESMEDICAL FACILITIES
• 22 facilities surveyed.• Typical loads include computers, officeequipment, diagnostic and patient careequipment.
• Average crest factor of 2.02
INDUSTRIAL FACILITIESINDUSTRIAL FACILITIESINDUSTRIAL FACILITIES
• 14 sites surveyed.• Typical loads include computers, laboratoryequipment, telecommunication equipment.
• Average neutral to phase of 1.04• Average crest factor of 2.12
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GOVERNMENT BUILDINGSGOVERNMENT BUILDINGSGOVERNMENT BUILDINGS
Harmonic Profiles for Typical Buildings. Contd.Harmonic Profiles for Typical Buildings. Contd.
AUDIO VISUAL STUDIOSAUDIO VISUAL STUDIOSAUDIO VISUAL STUDIOS
• 6 sites surveyed.
• Typical loads include computers, dataprocessing equipment, officeequipment.
• Average neutral to phase of 0.77
• Average crest of 1.78
•8 studios surveyed.•Average neutral to phase of 0.65•Average crest of 1.74
C M d N i B ildi I
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Common Mode Noise, Building ICommon Mode Noise, Building I
Li V lt Di t ti B ildi I
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Line Voltage Distortion, Building ILine Voltage Distortion, Building I
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Harmonic Effects on TransformersHarmonic Effects on Transformers
• Excessive stress due to heating
• Insulation breakdown
• Lower operating efficiency
• Short life span
• Acoustic noise
• Losses in transformers are subdividedinto core and winding losses.
• Core loss is of minor concern since it
is due to flux generated in the core bythe bus voltage.
• Winding losses are increased due toI2R and stray losses.
TRANSFORMER LOSSES DUETO TRIPLEN HARMONICS
TRANSFORMER LOSSES DUETRANSFORMER LOSSES DUE
TO TRIPLEN HARMONICSTO TRIPLEN HARMONICSEFFECTS OF
TRIPLEN HARMONICS
EFFECTS OFEFFECTS OF
TRIPLEN HARMONICSTRIPLEN HARMONICS
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Solutions:-Derating
-Passive Approach-Active Approach
Solutions:-Derating
-Passive Approach-Active Approach
d
R mm d ti s
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Recommendationsby CBEMA
Recommendationsby CBEMA
1. Derate transformers or use K-Factor Transformers.
2. Oversize all neutral components for 1.73 times ratedfull load amps.
3. Use separate neutral conductors for nonlinear loadsand avoid shared neutral conductors where practical.
4. Use neutral over current sensors to trip phase
conductors.
5. Use true rms. ammeters and instruments withsufficient bandwidth for measurement.
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Transformer Derating SchemesTransformer Derating Schemes
Derating is a means of determining the maximum load that may be
safely placed on a transformer that supplies harmonic loads.
The most common derating method is the CBEMA approved "crestfactor" method which provides a transformer harmonic derating
factor,THDF.
E l f T f D ti B ildi I
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A
B
C
70A
76A
73A
178A
181A
180A
Phase True RMSPhase Current Peak Value ofPhase Current
HENCE, THE TRANSFORMER SHOULD BE DERATED
TO 57% OF ITS NAME PLATE RATING!!!
Example of Transformer Derating. Building IExample of Transformer Derating. Building I
K F t R t d T f
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K Factor Rated TransformersK Factor Rated Transformers
•K factor was developed by Underwriters Laboratory in UL 1561.
•K factor transformers are designed to supply nonsinusoial loads.
•They contain enlarged primary windings to carry circulating triplenharmonic currents.
•The magnetic core has a lower flux density as it is designed withhigher grades of iron.
•K factor transformers use smaller, insulated, secondary
conductors in parallel to reduce skin effect.
•K factor transformers are more expensive than conventionaltransformers.
K F R d T f C d
K F t R t d T nsf m s C ntd
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K Factor Rated Transformers. Contd.K Factor Rated Transformers. Contd.
K factor is a means of rating a transformer with respect to theharmonic magnitude and frequency of the load.
The K factor numbers do not linearly indicate transformerharmonic tolerance. For example, a K4 rated transformer hasfour times the eddy current tolerance as a K1 transformer. A K13rated transformer has approximately twice the tolerance of a K4and K30 has twice of a K13.
E l K F C l l i B ildi I
Example K Factor Calculation Building I
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Example K Factor Calculation. Building IExample K Factor Calculation. Building I
K Factor = 8.84K Factor = 8.84
Solutions Neutral Cable
Solutions Neutral Cable
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Solutions. Neutral CableSolutions. Neutral Cable
Super Neutral Cable®
Super Neutral Cable is a metal-clad, Type MC Cable,
manufactured with an oversized neutral conductor or one
neutral per phase for three-phase/four-wire power supply
systems to computers (with dc drive fan motors, tape anddisk drives) office machines, programmable controllers, and
similar electronic equipment where non-linear switching
loads produce additive, odd order harmonic currents which
may create overloaded neutral conductors.
The oversized neutral conductor(s) are sized 150% to 200%
of the phase conductor ampacity to minimize the effects of
harmonics generated by the non-linear loads. The neutral
per phase (striped with color to match the phase conductor)
accomplishes the same objective.
Super Neutral Cable can be used under computer room
floors, raised floors, or overhead in the space above hung
ceilings used for environmental air handling. It handles
branch and feeder, plus power, lighting, signal, and control
circuits in dry locations.
Super Neutral Cable®
Super Neutral Cable is a metal-clad, Type MC Cable,
manufactured with an oversized neutral conductor or one
neutral per phase for three-phase/four-wire power supply
systems to computers (with dc drive fan motors, tape anddisk drives) office machines, programmable controllers, and
similar electronic equipment where non-linear switching
loads produce additive, odd order harmonic currents which
may create overloaded neutral conductors.
The oversized neutral conductor(s) are sized 150% to 200%
of the phase conductor ampacity to minimize the effects of
harmonics generated by the non-linear loads. The neutral
per phase (striped with color to match the phase conductor)
accomplishes the same objective.
Super Neutral Cable can be used under computer room
floors, raised floors, or overhead in the space above hung
ceilings used for environmental air handling. It handles
branch and feeder, plus power, lighting, signal, and controlcircuits in dry locations.
S l ti T f
Solutions Transformers
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Solutions. TransformersSolutions. Transformers
K-factor transformers:
Sola
Dongan
Disadvantages and Risks
Disadvantages and Risks
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Disadvantages and RisksDisadvantages and Risks
Derating a transformer is a temporary fix and often translates intolower efficiency operation and increased heat for losses.
Derated transformers also run the risk of being perceived to be
partially loaded and future load additions are possible.
While derating removes some stresses from the transformer, a typicaldry type transformer is not designed to supply harmonic loads. Hence,
it may be subject to a shorter life span and lower efficiency.
Separate neutral conductors for computer loads is almost impossibleto implement, due to a wide scattering of data processing equipment
all over the building.
Separate neutral conductors for computer loads is almost impossible
to implement, due to a wide scattering of data processing equipment
all over the building.
Oversizing of transformers, or selection of unnecessarily high K factor
ratings of transformers, can increase the harmonic currents due to
lower impedance.
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Solutions:-Derating
-Passive Approach-Active Approach
Solutions:-Derating
-Passive Approach-Active Approach
Passive Approach for Neutral Current Reduction
Passive Approach for Neutral Current Reduction
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Circulates excessive triplen harmonics through thefilter and back to the load.
Protects the distribution transformer and neutral
conductor from excessive neutral currents.
Improves transformer power factor.
Circulates excessive triplen harmonics through thefilter and back to the load.
Protects the distribution transformer and neutralconductor from excessive neutral currents.
Improves transformer power factor.
Passive Approach for Neutral Current ReductionPassive Approach for Neutral Current Reduction
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T-Connected Transformer for Neutral
T-Connected Transformer for Neutral
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T Connected Transformer for NeutralCurrent Reduction
f m fCurrent Reduction
VA Rating CalculationVA Rating CalculationVA Rating Calculation
kVA rating is higherkVA rating is higher
Star Delta Transformer for
Star-Delta Transformer for
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Star-Delta Transformer forNeutral Current Reduction
Star-Delta Transformer forNeutral Current Reduction
VA Rating CalculationVA Rating CalculationVA Rating Calculation
Transformer kVA rating is higher.
Harmonic filtering is impedance sensitive.
Transformer kVA rating is higher.
Harmonic filtering is impedance sensitive.
Zi Z T f f N t l C t R d ti
Zig Zag Transformer for Neutral Current Reduction
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Zero sequence impedance of the Zig-Zag transformer must be low. Thisrequires special design.
The effectiveness of the Zig-Zag transformer to divert 3rd. harmonic currentis highly dependent on the distribution system impedance. In most cases
only 50% reduction can be guaranteed.
Lower zero-sequence impedance increases the single-phase fault currentlevel. Therefore, fusing and circuit breaker re-sizing may be necessary.
The specially designed low impedance Zig-Zag transformer becomes a lowimpedance path for zero sequence currents from other parts of the system
Due to the reasons mentioned above the passive Zig-Zag transformerapproach is larger in size and weight.
Zero sequence impedance of the Zig-Zag transformer must be low. This
requires special design.
The effectiveness of the Zig-Zag transformer to divert 3rd. harmonic currentis highly dependent on the distribution system impedance. In most cases
only 50% reduction can be guaranteed.
Lower zero-sequence impedance increases the single-phase fault currentlevel. Therefore, fusing and circuit breaker re-sizing may be necessary.
The specially designed low impedance Zig-Zag transformer becomes a lowimpedance path for zero sequence currents from other parts of the system
Due to the reasons mentioned above the passive Zig-Zag transformer
approach is larger in size and weight.
Zig-Zag Transformer for Neutral Current ReductionZig-Zag Transformer for Neutral Current Reduction
DisadvantagesDisadvantagesDisadvantages
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Solutions:-Derating
-Passive Approach-Active Approach
Solutions:-Derating
-Passive Approach-Active Approach
Active Neutral Current Filtering Scheme
Active Neutral Current Filtering Scheme
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Active Neutral Current Filtering Schemeg
Circulates excessive triplen harmonics through thefilter and back to the load.
Protects the distribution transformer and neutralconductor from excessive neutral currents.
Improves transformer power factor.
Circulates excessive triplen harmonics through the
filter and back to the load.
Protects the distribution transformer and neutralconductor from excessive neutral currents.
Improves transformer power factor.
Single & Three-
Phase Linear and
Non-linear
Distributed Loads
HarmonixTM
:
Active HarmonicCancellation
System
- +
- +
- +
I ref = 0I n
A i N l C Fil i S h
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Active Neutral Current Filtering SchemeActive Neutral Current Filtering Scheme
•Developed by Texas A&M University,
College Station & Current Technology,Inc. Dallas, Texas.
•Design to Cancel Zero SequenceHarmonic Current from a 3-Phase4-Wire Distribution System.
•Patented Technology.
A ti N t l C t Filt i S h
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Active Neutral Current Filtering SchemeActive Neutral Current Filtering Scheme
Active Neutral Current Filtering Scheme.
Active Neutral Current Filtering Scheme.
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Act ve Neutral Current F lter ng Scheme.Functional Block Diagram.
gFunctional Block Diagram.
S i n g le & T h r e e -
P h a s e L in e a r a n d
N o n - l i n e a rD i s t r i b u t e d L o a d s
- +
- +
- +
I s e n s e d
L f P W M F u ll -
B r i d g e
I n v e r t e r
3 - P h a s e
B r i d g e
R e c t i f ie r
+N
S e n s i n g
C i r c u i t r y
P W M
C o n t r o l l e r
E r r o r
A m p l if ie r6 0 H Z
N o t c h
F i l t e r
I r e f = 0
T 1I N
I N /3
I N /3
I N /3I nI n - I N
a
b
c
n
l F l
A ti N t l C t Filt
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Active Neutral Current FilterActive Neutral Current Filter
Active Neutral Current Filter.
Active Neutral Current Filter.
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Features.Features.
•High Cancellation Effectiveness : 90 - 95%.
•Performance independent of System Impedance.
•Cancels neutral current harmonics by measurement
and close-loop control.
•No low-impedance path for Zero-Sequence 60 Hz.
•Built-in pulse-pulse current limit (No Overloading).
•Significant improvement in Voltage and Current THD’s.
•Fast-response characteristics.
Active Neutral Current Filter.
Active Neutral Current Filter.
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Connection.Connection.
L o w
B a t t e r y
A l a rm
O n / O f f
T h e r m a l
O v e r lo a d
A l a r m
D i s a b l e d
I -m a x
F a u l t
I -H a r m o n i c
I -R M S
N o r m a lT e s t
F i l t e r
O n / O f f F i l t e r
O n4 . 5
1 0 2 . 8
H A R M O N I X
C u r r e n t T e c h n o l o g y H a r m o n i c C o n t r o l P a n e l
A c t i v e H a r m o n i c C a n c e l l a t i o n
S y s t e m / 1 0 0 a m p A m p s
A m p s
H a r m o n i c F i l te r
C a n c e l l a ti o n C u r r e n t
F a c i l i t y
N e u t ra l C u r r e n t
B r e a k e r
L o w
B a t t e r y
A l a rm
O n / O f f
T h e r m a l
O v e r lo a d
A l a r m
D i s a b l e d
I -m a x
F a u l t
I -H a r m o n i c
I -R M S
N o r m a l
T e s t
F i l t e r
O n / O f f
F i l t e r
O n4 . 5
1 0 2 . 8
H A R M O N I X
C u r r e n t T e c h n o l o g y H a r m o n i c C o n t r o l P a n e l
A c t i v e H a r m o n i c C a n c e l l a t i o n
S y s t e m / 1 0 0 a m p A m p s
A m p s
H a r m o n i c F i l te r
C a n c e l l a ti o n C u r r e n t
F a c i l i t y
N e u t ra l C u r r e n t
B r e a k e r3 Phase
Breaker
G
A B C N
Current
Resistor
Term.
Box
120/208 - 225 Amps
Distribution Panel
CurrentSensor
N
Active Neutral Current Filter.Connection
Active Neutral Current Filter.Connection
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Connection.Connection.
Filter Specifications
Filter Specifications
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Filter Specificationsp
Voltage
Current
Frequency
Power Efficiency
Cancellation Effectiveness
Topology
120/208, 220/380, 277/480
100 A rms per module
50/60 Hz
90%
90-95%
Up to 4 filters can be
connected in parallelParallelability
Parallel connection3-Phase, 4-Wire
Filter Specifications. Contd.
Filter Specifications. Contd.
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Pulse-Pulse current protectionThermal shutdown
Overcurrent protectionProtection
Facility currentFilter current
Harmonics or total rms currentMonitoring
Indicator lights, Audible alarm,Max current indicator,
Dry contactsStatus Indication
21H x 24W x13DFloor mount stackable
Enclosure
200 lbs (approx.)Weight
Active Neutral Current Filter. Test Results
Active Neutral Current Filter. Test Results
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Active Neutral Current Filter. Test Results
P C
P G 2
1 2 0 /2 0 8 D i st ri b u t io n
P a n e l B o a r d s
S i n g l e - p h a s e
L in e a r &N o n - L i n e a r
L o a d s
O n e - L i n e D ia g r a m - B e t a S ite
E l e c t r ic R o o m 1 0 3
P E
H a r m o n i xT M
: A c t iv e
H a r m o n i c
C a n c e l l a t i o n
S s te m
L o a d C e n te r
Active Neutral Current Filter. Test Results
Active Neutral Current Filter. Test Results
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Neutral Current without Active Harmonic Cancellation System
Neutral Current with Active Harmonic Cancellation System
Active Neutral Current Filter. Test Results
Active Neutral Current Filter. Test Results
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Neutral Current(A rms)
Zero-Sequence HarmonicCurrent in Neutral (A rms)
60 Hz Zero-SequenceCurrent in Neitral (A rms)
Voltage THD
(% Fundamental)
82.1
71.5
34.3
3.3
Cancellation Effectiveness -
Current THD(% Fundamental)
67.2
33.0
4.14
32.7
2.9
94.2
33.3
Variable Without Filter With Filter
Neutral Current Filtering Scheme
Neutral Current Filtering Scheme
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Conclusions
Conclusions
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The active filter employs a conventional zig-zag transformer (notoptimized for low zero sequence impedance) and cancels the neutralcurrent harmonics by measurement and closed loop current control.
The performance of the active filter is independent of the systemimpedance, hence is not location sensitive.
The active filter compensates only for harmonic currents in the neutraland does not become a low impedance path to unbalanced 60Hz currents.
The active filter has an electronic built in current limit and does not overload.
The proposed active filter is rugged and stable in operation. Notice the
power electronic components are not subjected to line disturbancessuch as over voltages, voltage spikes etc.
The kVA of the power electronic active current source in low as the voltage
between the neutral of the zig-zag transformer and the system neutral isnear zero.
The active filter employs a conventional zig-zag transformer (notoptimized for low zero sequence impedance) and cancels the neutralcurrent harmonics by measurement and closed loop current control.
The performance of the active filter is independent of the systemimpedance, hence is not location sensitive.
The active filter compensates only for harmonic currents in the neutraland does not become a low impedance path to unbalanced 60Hz currents.
The active filter has an electronic built in current limit and does not overload.
The proposed active filter is rugged and stable in operation. Notice the
power electronic components are not subjected to line disturbancessuch as over voltages, voltage spikes etc.
The kVA of the power electronic active current source in low as the voltagebetween the neutral of the zig-zag transformer and the system neutral isnear zero.