Harmonics
Harmonics in
in Low
Low Voltage
Voltage Three-Phase
Three-Phase
Four-Wire
Four-Wire Electric
Electric Distribution
Distribution Systems
Systems
and
and Filtering
Filtering Solutions
Solutions
Dr.
Dr. Prasad
Prasad Enjeti
Enjeti
Texas
Texas A&M
A&M University
University
Power
Power Electronics
Electronics and
and Power
Power Quality
Quality Laboratory
Laboratory
PS
PSERC
ERC Online
Online Seminar
Seminar
February
February 13,
13, 2001
2001
©©2001
2001Texas
TexasA&M
A&MUniversity.
University.All
Allrights
rightsreserved.
reserved.
Outline
Outline
1
Introduction
Introduction
2
Harmonic
Harmonic Current
Current Sources
Sources
3
Case
Case Studies
Studies
4
Solutions:
Solutions:
-- Derating
Derating
-- Passive
Passive Approach
Approach
-- Active
Active Approach
Approach
5
Conclusions
Conclusions
Introduction
Typical
Typical Building
Building Power
Power Distribution
Distribution Scheme
Scheme
Harmonic Current Sources
Harmonic
Harmonic Producing
Producing Equipment
Equipment in
in Office
Office Buildings
Buildings
Copy machines
Recorders
Electric typewriters
Televisions
Light fixture ballasts
Video tape players
Personal computers
SCR drives for motors
Computer systems
SCR drives for elevators
Computer terminals
UPS systems
Audio visual equipment
Laboratory testing equipment
Harmonic
Harmonic Related
Related Problems
Problems in
in Buildings
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 distribution
transformers
Case Studies
Typical
Typical Current
Current Waveforms
Waveforms of
of Appliances
Appliances
Typical
Typical Current
Current Waveforms
Waveforms of
of Appliances.
Appliances. Contd.
Contd.
Typical
Typical Current
Current Waveforms
Waveforms of
of Appliances.
Appliances. Contd.
Contd.
Typical
Typical Current
Current Waveforms
Waveforms of
of Appliances.
Appliances. Contd.
Contd.
Effect
Effect of
of Triplen
Triplen Harmonics
Harmonics in
in Neutral
Neutral Conductors
Conductors
Ia
Ib
I N ,rms = I a2,rms + I b2,rms + I c2,rms
I N ,rms = 3 * I phase ,rms
Ic
IN
Survey
Survey results,
results, Building
Building II
Survey
Survey results,
results, Building
Building II
II
Summary
Summary of
of Building
Building Measurement
Measurement
Phase
Building
A
B
C
N
Neutral to
Phase, %
Building I
66.8A
77.3A
72.5A
120.0A
170%
Building II
50.0A
65.6A
41.1A
53.7A
103%
Building III
12.8A
36.1A
24.5A
27.5A
80.9%
Building IV
80.6A
77.2A
90.0A
142.0A
171.3%
Building V
23.0A
17.4A
26.0A
12.7A
57.4%
Building VI
44.6A
41.2A
41.9A
60.7A
143%
Harmonic
Harmonic Profiles
Profiles for
for Typical
Typical Buildings
Buildings
OFFICE
OFFICE BUILDINGS
BUILDINGS
MEDICAL
MEDICAL FACILITIES
FACILITIES
• 9 Buildings surveyed.
• Typical loads include computers and
office equipment.
• Average neutral to phase ratio of 1.13
• Average crest factor of 2.12
• 22 facilities surveyed.
• Typical loads include computers, office
equipment, diagnostic and patient care
equipment.
• Average crest factor of 2.02
INDUSTRIAL
INDUSTRIAL FACILITIES
FACILITIES
• 14 sites surveyed.
• Typical loads include computers, laboratory
equipment, telecommunication equipment.
• Average neutral to phase of 1.04
• Average crest factor of 2.12
Harmonic
Harmonic Profiles
Profiles for
for Typical
Typical Buildings.
Buildings. Contd.
Contd.
GOVERNMENT
GOVERNMENT BUILDINGS
BUILDINGS
• 6 sites surveyed.
• Typical loads include computers, data
processing equipment, office
equipment.
• Average neutral to phase of 0.77
• Average crest of 1.78
AUDIO
AUDIO VISUAL
VISUAL STUDIOS
STUDIOS
•8 studios surveyed.
•Average neutral to phase of 0.65
•Average crest of 1.74
Common
Common Mode
Mode Noise,
Noise, Building
Building II
Line
Line Voltage
Voltage Distortion,
Distortion, Building
Building II
Harmonic
Harmonic Effects
Effects on
on Transformers
Transformers
TRANSFORMER
TRANSFORMER LOSSES
LOSSES DUE
DUE
TO
TO TRIPLEN
TRIPLEN HARMONICS
HARMONICS
EFFECTS
EFFECTS OF
OF
TRIPLEN
TRIPLEN HARMONICS
HARMONICS
• Losses in transformers are subdivided
into core and winding losses.
• Excessive stress due to heating
• Core loss is of minor concern since it
is due to flux generated in the core by
the bus voltage.
• Winding losses are increased due to
I2R and stray losses.
• Insulation breakdown
• Lower operating efficiency
• Short life span
• Acoustic noise
Solutions:
Solutions:
-Derating
-Derating
-Passive
-Passive Approach
Approach
-Active
-Active Approach
Approach
Recommendations
Recommendations
by
by CBEMA
CBEMA
1. Derate transformers or use K-Factor Transformers.
2. Oversize all neutral components for 1.73 times rated
full load amps.
3. Use separate neutral conductors for nonlinear loads
and avoid shared neutral conductors where practical.
4. Use neutral over current sensors to trip phase
conductors.
5. Use true rms. ammeters and instruments with
sufficient bandwidth for measurement.
Transformer
Transformer Derating
Derating Schemes
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 "crest
factor" method which provides a transformer harmonic derating
factor,THDF.
Example
Example of
of Transformer
Transformer Derating.
Derating. Building
Building II
Phase
True RMS
Phase Current
Peak Value of
Phase Current
A
70A
178A
B
76A
181A
C
73A
180A
HENCE, THE TRANSFORMER SHOULD BE DERATED
TO 57% OF ITS NAME PLATE RATING!!!
KK Factor
Factor Rated
Rated Transformers
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 triplen
harmonic currents.
•The magnetic core has a lower flux density as it is designed with
higher grades of iron.
•K factor transformers use smaller, insulated, secondary
conductors in parallel to reduce skin effect.
•K factor transformers are more expensive than conventional
transformers.
KK Factor
Factor Rated
Rated Transformers.
Transformers. Contd.
Contd.
K factor is a means of rating a transformer with respect to the
harmonic magnitude and frequency of the load.
The K factor numbers do not linearly indicate transformer
harmonic tolerance. For example, a K4 rated transformer has
four times the eddy current tolerance as a K1 transformer. A K13
rated transformer has approximately twice the tolerance of a K4
and K30 has twice of a K13.
Example
Example KK Factor
Factor Calculation.
Calculation. Building
Building II
K
K Factor
Factor == 8.84
8.84
Solutions.
Solutions. Neutral
Neutral Cable
Cable
Super
Super Neutral
Neutral Cable®
Cable®
Super
SuperNeutral
NeutralCable
Cableisisaametal-clad,
metal-clad,Type
TypeMC
MCCable,
Cable,
manufactured
manufacturedwith
withan
anoversized
oversizedneutral
neutralconductor
conductoror
orone
one
neutral
per
phase
for
three-phase/four-wire
power
supply
neutral per phase for three-phase/four-wire power supply
systems
systemsto
tocomputers
computers(with
(withdc
dcdrive
drivefan
fanmotors,
motors,tape
tapeand
and
disk
diskdrives)
drives)office
officemachines,
machines,programmable
programmablecontrollers,
controllers,and
and
similar
similarelectronic
electronicequipment
equipmentwhere
wherenon-linear
non-linearswitching
switching
loads
loadsproduce
produceadditive,
additive,odd
oddorder
orderharmonic
harmoniccurrents
currentswhich
which
may
create
overloaded
neutral
conductors.
may create overloaded neutral conductors.
The
Theoversized
oversizedneutral
neutralconductor(s)
conductor(s)are
aresized
sized150%
150%to
to200%
200%
of
ofthe
thephase
phaseconductor
conductorampacity
ampacityto
tominimize
minimizethe
theeffects
effectsof
of
harmonics
harmonicsgenerated
generatedby
bythe
thenon-linear
non-linearloads.
loads. The
Theneutral
neutral
per
phase
(striped
with
color
to
match
the
phase
conductor)
per phase (striped with color to match the phase conductor)
accomplishes
accomplishesthe
thesame
sameobjective.
objective.
Super
SuperNeutral
NeutralCable
Cablecan
canbe
beused
usedunder
undercomputer
computerroom
room
floors,
floors,raised
raisedfloors,
floors,or
oroverhead
overheadin
inthe
thespace
spaceabove
abovehung
hung
ceilings
ceilingsused
usedfor
forenvironmental
environmentalair
airhandling.
handling. ItIthandles
handles
branch
and
feeder,
plus
power,
lighting,
signal,
and
branch and feeder, plus power, lighting, signal, andcontrol
control
circuits
circuitsin
indry
drylocations.
locations.
Solutions.
Solutions. Transformers
Transformers
K-factor transformers:
Sola
Dongan
Disadvantages
Disadvantages and
and Risks
Risks
Derating a transformer is a temporary fix and often translates into
lower 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 typical
dry type transformer is not designed to supply harmonic loads. Hence,
it may be subject to a shorter life span and lower efficiency.
Oversizing of transformers, or selection of unnecessarily high K factor
ratings of transformers, can increase the harmonic currents due to
lower impedance.
Separate
Separateneutral
neutralconductors
conductorsfor
forcomputer
computerloads
loadsisisalmost
almostimpossible
impossible
to
toimplement,
implement,due
dueto
toaawide
widescattering
scatteringof
ofdata
dataprocessing
processingequipment
equipment
all
allover
overthe
thebuilding.
building.
Solutions:
Solutions:
-Derating
-Derating
-Passive
-Passive Approach
Approach
-Active
-Active Approach
Approach
Passive
Passive Approach
Approach for
for Neutral
Neutral Current
Current Reduction
Reduction
Circulates
Circulates excessive
excessive triplen
triplen harmonics
harmonics through
through the
the
filter
filter and
and back
back to
to the
the load.
load.
Protects
Protects the
the distribution
distribution transformer
transformer and
and neutral
neutral
conductor
conductor from
from excessive
excessive neutral
neutral currents.
currents.
Improves
Improves transformer
transformer power
power factor.
factor.
Zig-Zag
Zig-Zag Transformer
Transformer for
for Neutral
Neutral Current
Current Reduction
Reduction
VA
VA Rating
Rating Calculation
Calculation
T-Connected
T-Connected Transformer
Transformer for
for Neutral
Neutral
Current
Current Reduction
Reduction
VA
VA Rating
Rating Calculation
Calculation
kVA
kVA rating
rating is
is higher
higher
Star-Delta
Star-Delta Transformer
Transformer for
for
Neutral
Neutral Current
Current Reduction
Reduction
Transformer
Transformer kVA
kVA rating
rating is
is higher.
higher.
Harmonic
Harmonic filtering
filtering is
is impedance
impedance sensitive.
sensitive.
VA
VA Rating
Rating Calculation
Calculation
Zig-Zag
Zig-Zag Transformer
Transformer for
for Neutral
Neutral Current
Current Reduction
Reduction
Disadvantages
Disadvantages
Zero
Zero sequence
sequence impedance
impedance of
of the
the Zig-Zag
Zig-Zag transformer
transformer must
must be
be low.
low. This
This
requires
requires special
special design.
design.
rd
The
harmonic current
current
The effectiveness
effectiveness of
of the
the Zig-Zag
Zig-Zag transformer
transformer to
to divert
divert 33rd.. harmonic
is
is highly
highly dependent
dependent on
on the
the distribution
distribution system
system impedance.
impedance. In
In most
most cases
cases
only
only 50%
50% reduction
reduction can
can be
be guaranteed.
guaranteed.
Lower
Lower zero-sequence
zero-sequence impedance
impedance increases
increases the
the single-phase
single-phase fault
fault current
current
level.
level. Therefore,
Therefore, fusing
fusing and
and circuit
circuit breaker
breaker re-sizing
re-sizing may
may be
be necessary.
necessary.
The
The specially
specially designed
designed low
low impedance
impedance Zig-Zag
Zig-Zag transformer
transformer becomes
becomes aa low
low
impedance
impedance path
path for
for zero
zero sequence
sequence currents
currents from
from other
other parts
parts of
of the
the system
system
Due
Due to
to the
the reasons
reasons mentioned
mentioned above
above the
the passive
passive Zig-Zag
Zig-Zag transformer
transformer
approach
approach is
is larger
larger in
in size
size and
and weight.
weight.
Solutions.
Solutions. Passive
Passive Approach
Approach
Acme's
Acme's I-Trap™
I-Trap™ Reduces
Reduces Excessive
Excessive Neutral
Neutral
Currents
Currents Caused
Caused by
by Harmonic
Harmonic Distortion
Distortion
Acme
Acmehas
hasintroduced
introducedaaproduct
productthat
thatprovides
providesaacost
costeffective
effectivesolution
solutionto
to
high
neutral
currents
that
result
from
zero
sequence
harmonics
and
high neutral currents that result from zero sequence harmonics and
unbalanced
unbalancedloads.
loads.
The
TheI-TRAP™
I-TRAP™units
unitsare
aredesigned
designedfor
foruse
useon
on208Y/120V
208Y/120Vthree
threephase
phase60Hz
60Hz
systems.
They
provide
electrical
contractors,
system
designers
and
systems. They provide electrical contractors, system designers and
specifying
specifyingengineers
engineerswith
withan
aneconomical
economicalalternative
alternativeto
toreplacement
replacementof
of
overheated
system
neutral
conductors.
The
I-TRAP™
can
be
installed
overheated system neutral conductors. The I-TRAP™ can be installed
adjacent
adjacentto
todistribution
distributionpanels
panelsthat
thatwill
willfeed
feednon-linear
non-linearloads
loadsin
inplants,
plants,
offices,
offices,hi-rises
hi-risesand
andother
otherlocations.
locations.This
Thispermits
permitsthe
theuse
useof
ofsmaller,
smaller,less
less
expensive
neutral
cable
and
provides
protection
to
distribution
expensive neutral cable and provides protection to distribution
transformers
transformersthat
thatmay
mayoverheat
overheatdue
dueto
toharmonic
harmonicloads.
loads.
The
TheI-TRAP™
I-TRAP™isisfor
foruse
useon
onsystems
systemswith
withmaximum
maximumneutral
neutralcurrents
currentsof
of
100A,
150A,
300A
and
450A.
Each
unit
includes
a
built-in
ammeter
to
100A, 150A, 300A and 450A. Each unit includes a built-in ammeter to
provide
provideconstant
constantreading
readingof
ofthe
theneutral
neutralcurrent.
current.
Installation
Installationisisaccomplished
accomplishedby
byfour
foursimple
simpleterminal
terminalconnections
connectionsand
andthe
the
units
can
be
floor
or
wall
mounted
and
require
minimal
maintenance.
units can be floor or wall mounted and require minimal maintenance.
All
AllI-TRAP™
I-TRAP™units
unitsare
areUL
ULlisted
listedand
andCSA
CSAcertified,
certified,rated
ratedfor
for115
115deg.
deg.CC
temperature
temperaturerise
riseand
and220
220deg.
deg.CCinsulation
insulationclass,
class,and
andare
arecovered
coveredunder
under
Acme's
exclusive
10
year
warranty.
Acme's exclusive 10 year warranty.
The
TheI-TRAP™
I-TRAP™and
andAcme's
Acme'sfull
fullline
lineof
ofdry
drytype
typedistribution
distributiontransformers
transformers
are
available
through
a
network
of
reps
and
electrical
distributors
are available through a network of reps and electrical distributorsin
inthe
the
United
States
and
Canada.
United States and Canada.
Solutions:
Solutions:
-Derating
-Derating
-Passive
-Passive Approach
Approach
-Active
-Active Approach
Approach
Active
Active Neutral
Neutral Current
Current Filtering
Filtering Scheme
Scheme
- +
Single & ThreePhase Linear and
Non-linear
Distributed Loads
- +
- +
In
Harmonix TM:
Active Harmonic
Cancellation
System
I ref = 0
Circulates
Circulates excessive
excessive triplen
triplen harmonics
harmonics through
through the
the
filter
filter and
and back
back to
to the
the load.
load.
Protects
Protects the
the distribution
distribution transformer
transformer and
and neutral
neutral
conductor
conductor from
from excessive
excessive neutral
neutral currents.
currents.
Improves
Improves transformer
transformer power
power factor.
factor.
Active
Active Neutral
Neutral Current
Current Filtering
Filtering Scheme
Scheme
•Developed by Texas A&M University,
College Station & Current Technology,
Inc. Dallas, Texas.
•Design to Cancel Zero Sequence
Harmonic Current from a 3-Phase
4-Wire Distribution System.
•Patented Technology.
Active
Active Neutral
Neutral Current
Current Filtering
Filtering Scheme
Scheme
Active
Active Neutral
Neutral Current
Current Filtering
Filtering Scheme.
Scheme.
Functional
Functional Block
Block Diagram.
Diagram.
a
- +
- +
- +
I N /3
b
I N /3
c
n
S in g le & T h re e P h a s e L in e a r a n d
N o n -lin e a r
D is trib u te d L o a d s
I N /3
In
In - IN
IN
T1
I
se n sed
N
S e n s in g
C irc u itry
60H Z
N o tc h
F il t e r
E rro r
A m p li f i e r
I ref = 0
Lf
PW M
C o n tro lle r
3 -P h a se
B rid g e
R e c tifie r
P W M F u llB rid g e
In v e rte r
+
Active
Active Neutral
Neutral Current
Current Filter
Filter
Active
Active Neutral
Neutral Current
Current Filter.
Filter.
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
Active Neutral
Neutral Current
Current Filter.
Filter.
Connection.
Connection.
F ilte r
O n
F ilte r
O n / O ff
I-m a x
I-H a rm o n ic
I-R M S
A la rm
D is a b le d
A la rm
O n / O ff
H a rm o n ic F ilte r
C a n c e lla tio n C u rre n t
F a u lt
N o rm a l
T est
C u r r e n t T e c h n o lo g y
L ow
T h e rm a l
B a tte ry
O v e rlo a d
H a r m o n ic C o n tr o l P a n e l
4 .5
A m ps
F a c ility
N e u tra l C u rre n t
1 0 2 .8
A m ps
H A R M O N IX
A c tiv e H a r m o n ic C a n c e lla tio n
S y s te m / 1 0 0 a m p
A B C
N
3 Phase
Breaker
B re a k e r
Current
Sensor
120/208 - 225 Amps
Distribution Panel
F ilte r
O n
F ilte r
O n / O ff
I-m a x
I-H a rm o n ic
I-R M S
A la rm
D is a b le d
A la rm
O n / O ff
H a rm o n ic F ilte r
C a n c e lla tio n C u rre n t
F a u lt
N o rm a l
T est
C u r r e n t T e c h n o lo g y
L ow
T h e rm a l
B a tte ry
O v e rlo a d
H a r m o n ic C o n tr o l P a n e l
4 .5
A m ps
F a c ility
N e u tra l C u rre n t
1 0 2 .8
A m ps
H A R M O N IX
A c tiv e H a r m o n ic C a n c e lla tio n
S y s te m / 1 0 0 a m p
B re a k e r
Current
Resistor
Term.
Box
N
G
Active
Active Neutral
Neutral Current
Current Filter.
Filter.
Connection.
Connection.
Filter
Filter Specifications
Specifications
Voltage
120/208, 220/380, 277/480
Current
100 A rms per module
Frequency
50/60 Hz
Power Efficiency
90%
Cancellation Effectiveness
90-95%
Parallelability
Up to 4 filters can be
connected in parallel
Topology
Parallel connection
3-Phase, 4-Wire
Filter
Filter Specifications.
Specifications. Contd.
Contd.
Protection
Pulse-Pulse current protection
Thermal shutdown
Overcurrent protection
Monitoring
Facility current
Filter current
Harmonics or total rms current
Status Indication
Indicator lights, Audible alarm,
Max current indicator,
Dry contacts
Enclosure
21H x 24W x13D
Floor mount stackable
Weight
200 lbs (approx.)
Active
Active Neutral
Neutral Current
Current Filter.
Filter. Test
Test Results
Results
1 2 0 /2 0 8 D is tr ib u tio n
P a n e l B o a rd s
L o a d C e n te r
PC
PG 2
S in g le - p h a s e
L in e a r &
N o n - L in e a r
L oads
PE
H a r m o n ix
: A c tiv e
H a r m o n ic
C a n c e lla tio n
S y s te m
TM
O n e - L in e D ia g r a m - B e ta S ite
E le c tr ic R o o m 1 0 3
Active
Active Neutral
Neutral Current
Current Filter.
Filter. Test
Test Results
Results
Neutral Current without Active Harmonic Cancellation System
Neutral Current with Active Harmonic Cancellation System
Active
Active Neutral
Neutral Current
Current Filter.
Filter. Test
Test Results
Results
Variable
Without Filter
With Filter
Neutral Current
(A rms)
82.1
33.0
Zero-Sequence Harmonic
Current in Neutral (A rms)
71.5
4.14
60 Hz Zero-Sequence
Current in Neitral (A rms)
34.3
32.7
Voltage THD
(% Fundamental)
3.3
2.9
Current THD
(% Fundamental)
67.2
33.3
Cancellation Effectiveness
-
94.2
Neutral
Neutral Current
Current Filtering
Filtering Scheme
Scheme
Conclusions
Conclusions
The
The active
active filter
filter employs
employs aa conventional
conventional zig-zag
zig-zag transformer
transformer (not
(not
optimized
optimized for
for low
low zero
zero sequence
sequence impedance)
impedance) and
and cancels
cancels the
the neutral
neutral
current
current harmonics
harmonics by
by measurement
measurement and
and closed
closed loop
loop current
current control.
control.
The
The performance
performance of
of the
the active
active filter
filter is
is independent
independent of
of the
the system
system
impedance,
impedance, hence
hence is
is not
not location
location sensitive.
sensitive.
The
The active
active filter
filter compensates
compensates only
only for
for harmonic
harmonic currents
currents in
in the
the neutral
neutral
and
and does
does not
not become
become aa low
low impedance
impedance path
path to
to unbalanced
unbalanced 60Hz
60Hz currents.
currents.
The
The active
active filter
filter has
has an
an electronic
electronic built
built in
in current
current limit
limit and
and does
does not
not overload.
overload.
The
The proposed
proposed active
active filter
filter is
is rugged
rugged and
and stable
stable in
in operation.
operation. Notice
Notice the
the
power
power electronic
electronic components
components are
are not
not subjected
subjected to
to line
line disturbances
disturbances
such
such as
as over
over voltages,
voltages, voltage
voltage spikes
spikes etc.
etc.
The
The kVA
kVA of
of the
the power
power electronic
electronic active
active current
current source
source in
in low
low as
as the
the voltage
voltage
between
between the
the neutral
neutral of
of the
the zig-zag
zig-zag transformer
transformer and
and the
the system
system neutral
neutral is
is
near
near zero.
zero.