ESL-IE-95-04-32
CURRENT GENERATED HARMONICS AND THEIR EFFECT UPON
ELECTRICAL INDUSTRIAL SYSTEMS
Harold R. Alexander, P.E.
Industrial Division
Electrical Section Head
Black & Veatch
Kansas City, MO.
ABSTRACT
This paper provides a general overview of
harmonics and addresses the causes of current
generated harmonics in electrical systems. [n
addition, problems caused by current generated
harmonics and their affects upon different types
of electrical equipment; such as cables, meters,
capacitors, motors, transformers, emergency
generators, etc. are prescnted. Recommendations
for solving harmonic problems are also provided.
The paper discusses and analyzes two actual cases
where harmonics caused problems in electrical
systems, one case a computer center and the
other an oil collection system using variable
frequency drives (VFDs) for oil well pumps. The
paper also discusses where the public utility
industry appears to be headed in addressing
harmonics.
Daniel S. Rogge, P.E.
Industrial Division
Senior Electrical Engineer
Black & Veatch
Kansas City, MO.
portions of the current or voltage wave form to
pass.
ODD NUMBERED HARMONICS
For 6O-Hz power systems with nonlinear
loads, the even numbered harmonics have bee
found to be considerably less likely to occur at
levels detrimental to electrical systems. This is
because most nonlinear loads generate odd­
numbered harmonics, which are associated wit a
current wave shape that is a distortion of the
normal 6O-Hz positive and negative half cycles.
This paper will concentrate on odd-numbered
harmonics.
Odd-numbered harmonics have positive,
negative, or zero sequences that, in a balanced ­
phase system, can be defmed as follows.
BACKGROUND
The 3-phase, 60 Hz power provided by the
electric utilities in this country produces current
and voltage, under normal conditions, having an
almost pure 6O-Hz sine wave. Any phenomenon
that modifies this wave form on a steady basis will
cause harmonic distortion.
• Positive sequence harmonics consist of thr
phasors, each equal in magnitude, separate fro
each other by a 120° phase displacement and
having the same phase sequence as phasors
representing the normal 6O-Hz current.
• Negative sequence harmonics also consist. f
three phasors, each equal in magnitude, separa e
from each other by a lWO phase displacement;
however, they have a phasc sequence opposite ~o
phasors representing the normal 6O-Hz current I
Electrical devices or equipment having
nonlinear impedance will require load currents
that are not proportional to voltage. Thus,
harmonics are generated thal, in turn, cause
distortion to the current of an electrical system.
Within a facility, the magnitude of this produced
harmonic distortion will vary, depending on the
size of the nonlinear loads relative to the facility's
electrical system.
• Zero sequence harmonics consist of three
phasors equal in magnitude and having a zero
phase displacement from each other. Therefor
the phasors are concurrent in direction, thus I
producing an amplitude that is triple of anyone
phasor when they combine on the neutral of an!
electrical system. These harmonics are called
triplen harmonics and are symptomatic of phas~­
to-neutral nonlinear loads, such as personal
computers, electronic ballasts, etc.
I
The most common sources of harmonics are
generators and nonlinear loads. Nonlinear loads
include solid state electric equipment or devices
that constantly switch ON and OFF to only allow
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ESL-IE-95-04-32
TABLE 1 shows the odd multiples of the
fundamental 60-Hz current and the associated
sequence (positive, negative, or zero). Note that
the first harmonic is actually the fundamental 60­
Hz current (1 X 60-Hz).
ELECTRIC GENERATOR AS A HARMONIC
SOURCE
Electric generators have always been a
source of harmonics because of the way these
machines arc designed and built. They do not
generate a perfect sine wave. Depending on the
pitch factor of windings and other design
parameters, the magnitude and frequency of the
harmonics generated will vary.
Positive sequence harmonics from three
phase nonlinear loads will cause a 3-phase motor
(either induction or synchronous) to turn in the
forward direction while negative sequence
harmonics will try to force motors to turn in the
reverse direction. Of course the rotation will
depend upon the magnitude of the harmonics
present compared to the normal 60- Hz current.
Depending upon the harmonics present (5th, 7th,
11th, etc.) and their magnitude, the effect on the
torque of the motor will vary and there will be
some torsional vibrations, which may cause
serious problems.
The 3rd, 5th, and 7th harmonics are
generated by what is considered a standard
machine in this country; that is, a generator
wound with 4/5 or 5/6 pitch. Generators can be
purchased with specially pitched windings (usually
2/3 pitch) that will not generate 3rd harmonic
currents. However, the 5th and 7th harmonics
generated by these machines are approximately
double those generated by a standard machine.
Harmonic
Sequence
Harmonic
Sequence
1
Positive
19
Positive
3
Zero
21
Zero
5
Negative
23
Negative
7
Positive
25
Positive
9
Zero
27
Zero
11
Negative
29
Negative
13
Positive
31
Positive
15
Zero
etc.
17
Negative
Table 1. Sequence of harmonics in a 3-phase power system.
At least one manufacturer of diesel-generator sets
offers as standard a 2/3 ritch machine, in ratings
up to 1500kW range, that eliminates the 3rd
harmonic.
The triplen harmonics (3rd, 9th, 15th, etc.)
will add together at the neutral or ground.
However, if no neutral or ground path is
available, they will not flow. If a transformer's
winding is a grounded wye/neutral configuration,
then the triplen harmonic current will pass
through this winding and combine in an additive
manner with unbalanced phase current at the
ground/neutral connection. The harmonics will
also be transformed to, and circulate in, a primary
delta winding. (1)
Historically, problems caused by generator
harmonics were associated with telephone
interference or large 3rd harmonic currents
circulating through wye connected equipment fed
from the generator bus. The telephone
interference was usually caused by telephone lines
lying parallel to distribution feeders that were
connected directly to the generator bus.
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NONLINEAR LOAD AS A HARMONIC
NATURE OF HARMONlC CURRENT FLOW
SOURCE
The amount of flow of harmonic currents in
an electric system is determined by the
impedances of the various components and
conncetcd equipment. The 60-Hz impedances a
available from manufacturers and industry
standards.
Nonlinear loads such as inverters, solid-state
rectificrs used in welders, DC power supplics,
variable frequency drives, and electronic ballasts
for lighting, are sources of harmonics in the
electrical system that feed these loads. In most
cases, there will be specific harmonics associated
with each itcm of equipment. Equipment
manufacturers can usually provide information on
the magnitude and order of harmonics generated
by their equipment. However, depending on the
design of the spceific item of equipment, thc
harmonics may vary in frequency and magnitude
as load changes on the equipment occur. Table 2
is a summary of the magnitude and order of
harmonics that have been encountered with
certain loads.
It's common knowledge that inductance
increases and capacitance dcereases as frequenc
increases. In theory, these arc linear
relationships. In actual practice, however,
equipment designed for 60-Hz operation often
havc characteristics that causc these relationshipS
to be nonlinear. Theoretically, resistance does
not change with frequency. In practice, however
subtle details of construction can cause the
resistance of a piece of equipment to vary
Harmonic Order
Load
Descri ption
I
I
I
5
3
1
<)
7
I
11
lJ
I
I
15
I
I
I
Six-pulse
rectifier
100
-
17
11
-
5
3
-
Twelve­
pulse
rectifier
100
-
3
2
-
5
3
-
Eighteen­
pulse
rectifier
100
-
3
2
-
1
0.5
-
Twenty­
four pulsc
rectifier
lO()
-
3
2
-
1
0.5
-
Electronic!
computer
100
56
33
II
5
4
2
1
Lighting!
electronic
100
lR
15
R
:1
2
1
0.5
Office with
PCs
100
51
2R
<)
6
4
2
2
YFD's
(range)
lOO
] to 9
40 to 65
4 to 8
3 to 8
17 to 41
1 to
<)
o to 2
Table 2. Harmonic currents with typical magnitudes produced by various types of equipment.
"YFDs" denotc variable frequency drives. The numbers under thc harmonic ordcr are expressed in
percent of the fundamental 6O-Hz current.
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considerably with frequency. The bottom line
is that without considcrable calculation elnd
testing of each piece of equipment, the actual
impedances of the equipment at each harmonic
frequency cannot be accurately determined. And
without accurate impedance valucs, it's impossible
to conduct an accurate Ilow analysis.
• The magnitude of current and voltage wave
form distortion in a facility's electric system will
be dependent upon the relative size of the
nonlinear loads with respect to that system. The
distortion increases as the percentage of nonlinear
loads increases. (2)
PROBLEMS ENCOUNTERED WITH
HARMONICS
However, from cxperience with harmonics
and from basic knowledge of electric system
characteristics, thc following guidelines generally
are true.
High Neutral Conductor Currcnts
Perhaps the dominant harmonic problem
encountered in commercial facilities and some
industrial plants has been the overheating of
neutral conductors of 3-phase, 4-wire branch and
feeder distribution systems.
• Bccause of the relatively high inductive
reactance of transformers, and since most
transformers arc furnishcd with delta primary
windings, harmonic problems are usually limited
to that equipment connected to the voltage at
which the harmonics arc generated.
In a balanced, 3-phase, 4-wire wye system
with phase-to-ncutral linear loads, the neutral
current will be zero. Even with a maximum
unbalance, the resulting neutral current will be no
grealcr than the maximum phasc current.
However, this samc system with certain nonlinear
loads will generate triplcn harmonic currents (3rd,
9th 15th, ctc.), which will add in the neutral
conductor. This can theoretically result in the
neutral conductor carrying up to 300% of the rms
current of a phase conductor, even in a balanccd
system. This has caused neutral cable and neutral
termination failurcs elt elcctric panels and
transformcrs.
• Capacitors, or equipment that have
capacitive reactanec characteristics will appear as
very low impedances to harmonics and will tend
to allract the now of harmonic currents.
• Resistive loads and resistive characteristics of
connected equipment (motors, transformers, etc.)
tend to dissipate harmonics. These dissipated
harmonics end up as heat losses in this
equipment. This can be a problem if thc
equipment was marginally sized or if harmonic
content is exccssive. Loads such as incandcscent
lighting and resistance heating normally arc not
affected by harmonic Ilow.
Thc harmonics that would cause this high
neutral current can be traccd back to the
switchcd-mode powcr supplies in computers and
other nonlinear loads which generate triplen
harmonics. These devices generate large 3rd
harmonic current, as well as other triplen
harmonic currents because they demand current
only at the peak of the voltage waveform.
• On most clectric systems where harmonic
currents arc present, they are usually created by
nonlinear loads. Usually, most of the othcr
equipment in a circuit is inductive equipment.
Such equipment has inductive impedance, which
provides a relatively high impedance to harmonic
currents. Kirchoff's Laws will not allow these
currents to disappear oncc gencrated. Therefore,
they must flow into the inductive equipmcnt.
Most of the energy in the positive and negative
sequence harmonics are dissipated as heat in this
equipment. In addition to the extra heat, the
highcr frequency currents tcnd to work corc
material harder and can cause prcmature corc
saturation. In the case of transformers, part of
the harmonics will be transmitted through the
transformer.
Unless filtered from the system at the
computcr equipment's power supply, thc triplen
harmonic currents will flow and seek the path of
the least impedance; that is, through thc neutral
conductors towards the transformer (source of
power).
With high neutral current and undersized
neutrals, a facility can cxperience excessive
neutral conductor heating, resulting in possible
fircs, short circuits, or bus failure.
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Static power converters, used in adjustable
speed drives and unintcrruptiblc power supplies
(UPS) change the AC current waveform and thus,
also contribute to the flow of harmonic currents.
harmonic currents. This has resulted in blown
fuses and disfigured capacitors. This is usually a
result of the circuit being tuned or resonating at
one of the harmonic frequencies.
Heat Losses
Increased losses, in the form of heat that is
dissipated in electric equipment, will occur in a
plant's electrical system because of harmonics.
These losses are real energy power losses (kW
losses). Therefore, a facility will see higher
electric energy charges because of harmonic flow.
Until fairly recently, these losses have typically
been ignored because they are hard to define.
Nevertheless, the user pays for the additional
energy losses in the electrical equipment.
Overeurrent Protective Device Operation
Thermal overcurrent protective devices, suc
as fuses and inverse-time circuit breakers, are
affected by increased skin-effcct heating at the
higher harmonic current levels. This excess
heating can cause shifts in thc devices time­
versus-current characteristics, resulting in
nuisance tripping. The magnetic trip function of
older circuit breakers, whose operation depends
on electro-magnetic force, is proportional to the
square of peak current, not rms current. A high
1rd harmonic current, resulting in an abnormally
high overall peak current, could cause these
breakers to trip prematurely. Most new circuit
breakers with electronic trip devices are now
designed to respond only to true rms current an
not peak current.
Skin Effect
Harmonic currents can cause overheating of
conductors and insulating materials as a resull of
a phenomenon callcd skin effect. This relates to
the increase in AC resistance of a conductor as
frequency increases. Normally, current density
within a conductor is greater near its surface or
skin. Skin effect, or depth of currcnt penetration,
is inversely proportional to the frequency of thc
current. Thus, the higher the frequency, the less
skin depth available in the conductor. When this
occurs in transformers, the result is cxtra losses in
the windings.
When protcctive relays are subjected to
system harmonics, relay misoperation is possible,
possibly resulting in undesircd pick-up values,
changes in voltage and current operating
characleristics, and falsc tripping.
Metering Errors
Harmonics can causc errors in induction
wall-hour meter readings. Since the induction
disks are designed to monitor non-distorted
fundamental current, harmonics will cause
measurement errors. This may result in the end
user paying more for electricity than if the same
rms current being drawn was sinusoidal.
Harmonic voltages and currents will increase
the rotor winding and stator winding losses in
motors. Since these losses are fR losses,
increased heating due to skin effect can be
expected at thc higher harmonic frequencies.
Transformer Derating
Waveform distortion will cause increased
heating in all types of transformers. This heating
is due to an increase in the frequcncy-depcndent
eddy current and hysteresis losses. Increased
heating can also be expected from skin effect
heating in thc windings. For transformers
experiencing large harmonic current flow,
derating of transformers may be required.
Elcctronic Equipment Malfunction
When the system voltage waveform become
distorted, electronic equipment also can
malfunction. For instance, electronic clocks that
count zero-crossings in the waveform may not
operate correctly because the distorted waveforn
provides more zcro-crossings t han a nondistorte
waveform. Thus, these clocks will run fast,
causing the equipment they control to incorrectl
operatc.
Capacitor Failure
Since the impedance of a capacitor is
frequency dcpendent (decreasing reactance with
increasing frequency), capacitors will be
negatively affected by harmonics. Thus, power
factor correction capacitors will appear as very
low impedances paths and tend to altract
Some generator voltage regulators measure
peak voltage in controlling the generator's outpU\
voltage. A distorted voltage waveform results in a
high peak-to-rms ratio. Thus, a generator wouldl
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"sec" higher peak voltages, causing the generator
voltage regulator to malfunction.
particularly effective when the 3rd harmonic is
presenl.
With communication systems, cross talk
caused by harmonic pickup on communication
links between sensitive electronic equipment can
occur, resulting in erroneous data transmission.
NEUTRAL AND PHASE CURRENT
COMPARISON
Another method to identify the presence of
triplen harmonics is to measure the phase and
neutral currents with a true rms meter. If the
neutral current is greater than what would
normally be the unbalance of the phase currents,
3rd or triplen harmonics are presenl.
MEASURING VOLTAGE OR CURRENT
HARMONICS
The presence of harmonics (i.e., waveform
distortion, particularly from the 3rd harmonic)
may be detected by taking rms and instantaneous
peak readings of either current or voltage. Since
all harmonics generated by nonlinear loads are
current generated, the current reading usually will
be more sensitive than voltage readings. Special
note: current readings must be taken at the
power source of the nonlinear loads while voltage
readings can be taken almost anywhere on the
bus.
In measuring current and voltage where
harmonics are present, true rms reading
instruments must be used. These meters are
usually calibrated in rms amps or volts, based on
a crest factor of 1.4] 4.
Conventional meters (analog and digital)
measure either the average value or the peak
value of a waveform, and then are calibrated to
read the equivalent rms value. The average-value
measuring meter is usually termed an average­
responding or average-calibrated meter. The
peak-value measuring meter is usually termed a
peak-sensing meter. These meters will give mis­
leading readings when harmonic distortion is
present. For example, on a square wave, the
average-calibrated meter will read rms values
about 11% high while the peak-calibrated unit will
read about 30% low. For pulses, the errors can
be tremendous, depending on the height of the
peak and the off-time between pulses. The
average-calibrated meler will read very low, (as
much as 50%) while the peak-calibrated unit will
read very high (sometimes more than 100%).
VIEW THE WAVEFORM
Another effective way to identify harmonics
is to look at the voltage and current waveforms
with an oscilloscope. Current transformers (CTs)
will be required to view the current waveforms.
These CTs must be of high quality (with a very
high band width) to sense the high frequencies
accurately, if the rms reading is to be accurate for
high-order harmonics and pulsed currents. (This
is not a problem for pure 60-Hz sine waves.
SPECTRUM ANALYZER MEASUREMENTS
A spectrum analyzer can record the current
and voltage waveforms, determine the magnitude
and types of harmonics present, and provide a
printout of these data. If equipment under
evaluation has its power supplied from on-site
emergency generators, readings of the current and
voltage waveforms at the equipment should be
taken with the generators on-line. This will
probably be the worst case because on-site
emergency generators typically represent a greater
source impedance than the utility system. As a
result, higher voltage distortion on the facility's
electrical system can be expected when the
emergency generators are running.
ACTUAL CASES WITH HARMONICS
Computer Center
Background
An existing computer center had been
operating without an emergency generator for
several years. However; because of an expansion
in the computer center and its increased criticality
to company operations, a decision was made to
install an emergency diesel-engine generator to
supply all of the computer center's operations
during loss of normal (utility company) AC
power. The computer center loads were served
PHASE CURRENT MEASUREMENTS
If the phase current readings of a true rms
sensing meter are significantly different than those
of an average responding meter, it's likely that
harmonics are present and arc distorting the
current waveform. The difference in readings is a
function of how the two types of meters measure,
as previously discussed. This method is
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unit did not experience any problems because of
barmonics.
power via a 12.5 kV to 4R0/277 volt step-down
transformer and 480/277 volt switchboard. The
emergency diesel-engine generator was installed
and connected at 12.5 kV to thc computer
center's step-down transformer using a ] 2.5 kV
automatic circuit breaker transfer scheme between
the emergency generator and utility power.
Oil Collection System Using VFDs for Oil Well
Pumps
Background
An electric power system was installed to
support an oil field installation. Because of the
location of the oil field, a remote location, the
power system was not connected to a utility
company grid. Power was produced by using
multiple 13.8 kV diesel-engine powered
generators and then transported to various areas
using two 69 kV transmission lines. For
reliability, the 69 kV system could be operated as
a closed loop system. Several distribution
substations were connected to the 69 kV
transmission system and in turn distributed l3.R
kV power to the oil wells. At each oil well, the
power was transformed down to 480 volts and
then connected to a variable frequency drive
(VFD) and the well pump motor. Approximatel
80 percent of the systcm load was the VFDs at
the oil wells.
Problem
During testing of the emergency generator, it
was discovered that the generating unit's controls
were not stable when serving the computer
center's loads. An oscilloscope showed that the
system voltage waveform at the emergency
generator was very distorted.
Investigation
An investigation revealed that most of the
computer center's loads were supplied via
uninterruptable power supply (UPS) modules that
utilizcd twelve-pulse rectifiers. The UPS modules
had been installed without input filters to limit
harmonic distortion on the electrical system.
Measurements were taken of each UPS module's
input voltage and input current waveforms.
Typical values measured for total harmonic
distortion (THO) were 5.5 percent of the 60-Hz
voltage and ]6.2 percent of the 60-Hz current
when operating on the utility company's source of
power. Though not measured, we would expect
the THD values to be higher when operating on
the emergency generator because of the generator
being a higher impedance source when compared
to thc utility company grid.
Problem
During starting and checkout of the system,
the manufacturer of the diesel-generators
measured the voltage waveform at their
equipment and expressed concern that an
overvoltage condition existed. Using an
oscilloscope, the manufacturer measured 11
percent peak-to-peak ovcrvoltage in the
waveform. Depending on system loading, 25
percent peak-to-peak overvoltage had been
detected. The manufacturer was concerned that
insulation material would deteriorate at a higher
rate than normal. RMS metering did not dcteet
the overvoltage peaks and plant operators were
unaware of the problem.
Solution
Input filters were installed on each of the
UPS modules and THO measurements retaken.
Typical THO valves measured were 2.7 percent of
the 60-Hz voltage and 6.3 percent of the 60-Hz
current when operating on the utility company's
source of power.
Investigation
In addition, it was discovered that the
generating unit's voltage regulator was sensitive to
harmonics because the regulator measured peak
voltage. The unit's controls and metering circuits
were also filtered to avoid harmonic problems.
A study was performed Lo evaluate and mak
recommendations addressing the diesel-engine
generator manufacturer's concerns. Field
measurements of current and voltage distortion
were taken at all operating oil wells using a
spectrum analyzer. Voltage readings were taken
at power plant and substation buses. The VFDs
were six pulse without any input filtering provideq
Aftcr the above modifications were made,
the generating unit was retested. The generating
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on the drives. Each VFO was fed by a delta-wye
pad-mounted transformer. THO of voltage
measured at the VFDs input varied between 19
percent and 28 percent, while the THO of current
varied between 24 percent and 59 percent,
depending on loading of VFO. In addition, the
largest harmonics measured, were the 5th and 7th
harmonics. The 5th harmonic current measured
varied between lR perccnt and 49 percent of
fundamental current, while the 7th harmonic
current varied between 3 percent and 37 percent
of fundamental current. The THO voltage
measured at the 13.8 kV power plant bus was
approximately 10 percent.
The HI-WAVE software used considers the
complete electric system model whcn analyzing
harmonics. U nfortunatcly, it will only provide
complete information on up to ten pre-selected
buses and branches within the model.
TheoretiealJy, a complete analysis of one
operating condition for a 300 buslbranch system
would require approximately ]5 computer runs.
Then, if you eonsidcr the number of combinations
of on-off operation of thirty wells, it becomcs
obvious that it is not practical to analyze a system
this large. rr we consider analyzing one substation
with eight wells, there would be 256 well on-off
operating combinations to be analyzed. If another
well is added, there would be 5]2 operating
combinations to analyze. With experience, it may
be possible to eliminate some of these cases.
However, the analysis required to provide proper
operation for all operating combinations of wells
with bus filters is substantial. Thc addition of one
well would require substantial additional analysis.
The cost of these studies and the inaccuracies
caused by not having known device impedances at
higher frequencies make the application of bus
filters in this application im practical.
The system was computer modeled using
field mcasurements taken for harmonic distortion
and system impedance data. Software used was
SKM Systems Analysis, Inc. Power * Tools HI­
WAVES. From the model developed, alternative
plans were investigated, including future load
growth.
The alternative plans considered VFO input
versus distribution bus filters. The plans using the
existing delta-wye transformers used filters tuned
to the fifth harmonic. Thc plans which
considered replacing half of the pad-mounted
transformers with delta-della transformers used
filters tuned to the 11th harmonic. The HI­
WA YES program includes a filter design feature.
This was used to size the filters considered. In
general, the vaJue of capacitance was selected and
the computer calculated the resistance and
reactance to tune the filter to the required
harmonic. When an effective filter with
reasonable values of capacitance, resistance, and
reactance was found, it was lIsed in this study.
Time did not permit optimization of filter design.
Solution
Jt was recommended to apply filters at the
line side of each variable frequency drive. These
filters essentially trap the harmonics at their
source. By doing this, you essentially eliminate
thc need for robust equipment designed to
tolerate the presence of harmonics. Also, the
losses due to harmonics heating electrical
equipment are minimized. These filters minimize
the harmonics on the distribution system,
therefore, minimize the exposure to resonance at
the various harmonic frequencies. Field data
taken with most of the wells in service, did not
indicate resonance at any of the wells.
Originally, it was thought that passive distributed
bus filters would provide thc best and least
expensive solution. However, the requirement to
have complete operating and expansion
capabilities proved to be a problem for the bus
filter. The computer analysis indicated that an
effective bus filter for one operating condition
could cause resonance at other operating
conditions. Also, the analysis indicated that it
was possible to have resonant conditions on the
distribution system, depending on the
configuration of lines, transformers, and loads.
ADDRESSING HARMONICS
Equipment Derating
A generally accepted method of coping with
harmonics is the derating of certain electrical
equipment to compensate for the resultant
additional heating. A 20 to 25% derating [actor
for transformers and generators is typically
employcd. However, this solution can still result
in equipment failures because the full extent of
losses and resultant heating may not he known.
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ESL-IE-95-04-32
extra charge for a "beefed-up" design to handle
the harmonics. Nevertheless, harmonics will still
he present.
With higher content of harmonic currents,
additional derating may he required.
Generator manufacturers are recommending
thc derating of their units hy as much as 50%,
depending upon the magnitude of harmonic
currents present in the load. This would rcsult in
generators sized as much as 200% of anticipated
load: a very expensive way of coping with
harmonics.
The key to eliminaling harmonic problems is
the use of filters at the respective nonlinear load.
Since a filler is designed to reduce the amplitude
of one or more fLXed frequency currents, a
harmonic filter is one means of reducing
waveform distortion. Since the harmonic
frequencies are very close to the fundamental
power frequency (60 Hz), a very selective filter is
required. This specially tuned filter will tune out
the dominant harmonic heing generated.
K-Factor Transformers
In the past few years, k-factor transformers
have been marketed for systems with triplen
harmonics. The design of these units eliminates
the harmful effects within the transformer caused
by harmonic problems. Such a transformer has a
special core, double sized neutral lug, and special
windings to handle the triplen currents and their
negative effects. These featurcs result in a more
expensive unit.
A frequency selective harmonic filter can be
used as either a series filler or a sh unt filter. A
series filter uses a high impedance to block the
power source from the now of harmonic current
being generated by the nonlinear load. A shunt
filter, on the other hand, provides a low
impedance path so that the harmonic current will
divert to ground. Although either method of
filtering will reduce harmonics, the shunt filter is
usually prefcrred, since the series filter must he
designed ror full line current and insulated for
line voltage.
Transformers with Multiple Secondary Outputs
Morc reccntly, the introduction of a
transformer with multiple secondary outputs has
been offered as a solution. [I lIses a wye
connected secondary winding with multiple
outputs phase shifted 15 electrical degrees from
each other. As a result, the positive and negative
sequence harmonics will tend to cancel in the
transformer. However, unless these harmonics
are equally balanced on the outputs, cancellation
will not be complete. Theoretical data shows that
for a 50% unbalanced load, the total harmonic
current distortion can he cut in half.
Our recommendation for most applications i.
to filter the harmonics at their sources (at the
nonlinear loads). By filtering at the harmonics'
source, the magnitude of harmonics nowing on
the electric system will be minimized. In today's
market, manufacturers can provide filters that
limit their equipment's current distortion.
Large solid state electric equipment should
he purchased with a filter which will filter its
input to a reasonable level.
SOLVlNG HARMONIC PROBLEMS
Derating equipment, using k-factor
transformers and/or transformers with multiple
secondary outputs does not resolve harmonic
problems.
WHAT WlLL BE PUBLIC UTILITY
COMPANIES RESPONSE TO INDUSTRY
GENERATED HARMONICS'?
The question today is, "How will public uti lit
companies respond to industry generated
harmonics?" To-date, most utility companies hav
not enforced penalties associated with industry
generated harmonics on their distribution system
unless the utility company is experiencing
problems with other customers. However, in the
future, we can expect this to change. IEEE
Standard 5] 9 has been rcvised to estahlish
recommended practices and requirements for
Harmonic currents still now in the system,
and possibly on higher voltage level systems
through transformers' windings. Additional real
power losses also are incurred in cahles and
motors. Additionally, more equipment failures
can he expected, especially for electric equipment
that is sized without any consideration for
additional heating from harmonic currents that
continue to now. And, specifying specialized
equipment means that this equipment will have an
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ESL-IE-95-04-32
harmonic control in electrical power systems for
both voltage and current distortion. We believe
utility companies will use this standard as the
basis for establishing guidelines concerning
allowable levels of harmonics generated by
industrial customers.
REFERENCES
1.
Alexander, H.R., and D.S. Rogge,
"Harmonics: Causes, Problems, Solutions ­
Parl 1," EC&M, January, ]994, pp. 35-38, 42.
2.
Alexander, H.R. and D.S. Rogge,
"Harmonics: Causes, Problems, Solutions ­
Part 2," EC&M, February, ]994, pp. 47-55.
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Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995