Influ ec oL impda M t g E519 Harmonic Limits - … · Figure 1—Linear and nonlinear loads and the...
Transcript of Influ ec oL impda M t g E519 Harmonic Limits - … · Figure 1—Linear and nonlinear loads and the...
Figure 1—Linear and nonlinear loadsand the PCC
Line impedance andIEEE 519How line impedanceaffects efforts to meetthe IEEE 519 harmoniclimits .
Harmonics calculator
New online calculatorprovides s impleharmonic analys is fordifferent drive types .
AHD™ Technology
Active HarmonicDamping technologyintegral to the drivehelps sys tems meetIEEE 519.
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Oil & Gas Automation
Solutions is a publication
of Unico, Inc .
Since 1967
Unico, Inc .
3725 N icholson Rd.
P . O . Box 0505
Franksville, WI
53126-0505
262.886.5678
262.504.7396 fax
oilgas@unicous .com
unicous .com
Influence of Line Impedance on Meeting IEEE 519
Harmonic Limits
by Bill Hammel vice pres ident/engineering
The industry standard for power quality, IEEE 519 (“Recommended
Practices and Requirements for Harmonic Control in Electrical Power
Systems”), describes current distortion limits that apply to the point of
common coupling (PCC) with the consumer-utility interface.
The intent of the IEEE current
distortion limits is to ultimately
limit voltage distortion to levels
that will generally avoid
interference with neighboring
electrical equipment. Harmonic
voltage distortion will be a
function of total injected
harmonic current and the system
impedance at the PCC.
Therefore, stiffer, lower-
impedance systems can
accommodate relatively higher current distortion limits.
The electrical stiffness of a system is expressed as the ratio (RSC) of the
available short-circuit current (ISC) at the PCC to the maximum demand
fundamental load current (IL), calculated as the average of the maximum
demand over 15-minute intervals for the preceding 12 months.
RSC = ISC/IL
The IEEE standard adjusts limits with respect to this ratio in a manner
that recognizes that low kW loads, connected to systems with much
C opyright © 2010Unico, Inc .A ll rights reserved.
All trade des ignationsare provided withoutreference to the rights oftheir respective owners .
higher kVA capacities, have a proportionally smaller effect on the system
and are, therefore, allowed a higher distortion limit.
The amount of harmonic current produced by nonlinear loads, such as
adjustable-speed drives with rectifier inputs, is also affected by the
stiffness of the electrical system. Higher levels of harmonic current are
produced by the drive as system stiffness increases, and lower levels are
produced with softer, higher impedance systems. This effect can be quite
dramatic for unfiltered drives—those having significant DC link capacitance
directly connected to the input rectifier bridge. This effect is also apparent,
though less dramatic, on harmonic current produced by filtered drives—
those that utilize either DC link inductors or other techniques to achieve
relatively low DC link current ripple.
Figure 2—Comparison of unfiltered (1 and 2) and filtered (3 and 4) drives
Limits for current harmonic distortion are described with respect to total
demand distortion (TDD), which expresses total harmonic current
distortion as a percentage of IL.
Figure 3 below graphically illustrates how the TDD limits recommended by
IEEE 519 vary as a function of RSC. Also included are typical TDD levels
produced by drives utilizing filtered 6-, 12-, 18-, or 24-pulse input rectifiers
under the assumption that IL. is entirely comprised of that drive load.
Figure 3—IEEE 519 limits and harmonic distortion of various drive
configurations as a function of short-circuit ratio
For the assumptions given, the 6-pulse configuration always exceeds the
IEEE limits while the 12-pulse configuration sometimes exceeds it. These
configurations require either additional effort or further assumptions in
order to satisfy the limit.
Additional effort can come in the form of including a reactor between the
drive and the PCC. This additional impedance reduces drive-injected TDD
without affecting the system RSC and, therefore, without affecting the
corresponding TDD limit. Figure 4 shows the effect of including 3%, 5%,
and 10% reactors on 6- and 12-pulse configurations.
Figure 4—Harmonic distortion of 6- and 12-pulse drives with various reactors
While Figure 4 shows how including a reactor might help 12-pulse
configurations satisfy the IEEE limits, it also illustrates that the 6-pulse
configuration remains a challenge despite inclusion of a reactor. While 18-
and 24-pulse configurations more directly comply with the IEEE limits,
there is still hope in another form for both 6- and 12-pulse configurations
when additional conditions apply.
Limits can more easily be met when the drive load represents only a
fraction (RDD) of the total load, IL, and the remaining portion of the total
load is comprised of either linear loads or nonlinear loads whose harmonic
contribution is negligible."
RDD = ID/IL
Figure 5 illustrates results for a 6-pulse configuration that includes a 5%
reactor where the drive load is 10%, 20%, 50%, and 100% of IL, while the
remaining portion of the load is assumed to contribute negligible harmonic
distortion.
Figure 5—Affect of varying RDD on 6-pulse drive with a reactor
A 6-pulse configuration, with the inclusion of a 5% reactor, can satisfy the
IEEE limits as long as the drive load is a small enough fraction of the total
load.
A cautionary note regarding the addition of reactors must be made. Since
adding reactor impedance reduces TDD, one might wonder—why not
merely add enough to achieve any desired TDD objective? An offsetting
penalty is that as impedance is progressively increased, both power factor
and available output voltage progressively decrease. While the penalty
associated with adding a 5% or even 10% reactor impedance is often
acceptable, the penalty of further increases might begin to outweigh the
TDD benefit.
The discussion above has been intended to provide some insight into the
influence of line impedance on meeting IEEE current distortion limits.
Impedance affects both the limit and the level of injected distortion. The
figures above provide an overview of various drive configurations and the
conditions under which they can meet the IEEE limit. If you need further
assistance with understanding line impedance and harmonic issues,
please contact us.
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Harmonics Calculator Now Online
A new online Harmonics Calculator provides simple harmonic analysis for
various variable-
speed drive
configurations. The
calculator
determines the
harmonic distortion
of the drive and
allows users to
quickly see if the
drive meets the
IEEE 519 harmonic
distortion recommendations. The calculator can be found at
http://www.unicous.com/oilgas/harmonicscalc.php.
To use the calculator, the user must input the supply short-circuit ratio,
drive demand ratio, and drive reactor impedance. The short-circuit ratio is
a measure of the stiffness of the line and is the ratio of the short-circuit
current to the rated capacity of the line. The larger the load or the weaker
the system, the greater the impact of harmonics on the utility associated
with a lower short-circuit ratio. The drive demand ratio is the percentage of
the total load on the supply that the drive load represents. The larger a
drive is with respect to the capacity of the line, the greater the impact its
harmonics will have. The last input is the impedance of the line reactor, if
one is used. The calculator shows the resulting effective supply
impedance and total effective impedance.
The results table shows the calculated current distortion for 6-, 12-, 18-,
and 24-pulse drive configurations. Distortion is calculated for individual
harmonics as well as total harmonic distortion. Individual odd-numbered
harmonics are shown through the 49th harmonic, although the total
distortion calculations incorporate contributions through the 97th
harmonic.
The IEEE 519 Standard
The Institute of Electrical and Electronic Engineers (IEEE) has
created a standard to minimize problems associated with
nonlinear equipment like drive systems that generate harmonic
currents. The IEEE 519 recommendations specify the maximum
acceptable levels of harmonic components and total harmonic
distortion (THD) as a function of the stiffness of the power source,
which is given by the short-circuit ratio (RSC). The guideline
expresses limits for current harmonics and distortion as
percentages of load current, which is defined as the average
current of the maximum demand, measured over 15-minute
intervals, for the preceding 12 months. The THD of current
calculated using that definition is referred to as total demand
distortion (TDD). TDD addresses the fact that a small current may
have a high THD but be of little concern, such as an adjustable-
speed drive operating at very light loads. All harmonics are
assessed at the point of common coupling (PCC).
IEEE 519 Maximum Current Distortion Limits (% of IL)
Individual Harmonic Order (Odd Harmonics)
(Even harmonics are limited to 25% of values shown)
RSC (ISC/IL) h<11 11≤h<17 17≤h<23 23≤h<35 35≤h TDD
< 20 4.0 2.0 1.5 0.6 0.3 5.0
20 < 50 7.0 3.5 2.5 1.0 0.5 8.0
50 < 100 10.0 4.5 4.0 1.5 0.7 12.0
100 < 1 ,000 12.0 5.5 5.0 2.0 1.0 15.0
> 1 ,000 15.0 7.0 6.0 2.5 1.4 20.0
Acceptable levels of harmonics as a function of stiffness of the powersource (RSC), where ISC is the maximum short-circuit current at thepoint of common coupling (PCC) and IL is the maximum demand-loadcurrent (fundamental frequency component) at the PCC. From IEEE519-1992, “Recommended Practices for Harmonic Control in ElectricalPower Systems.”
When it comes to satisfying IEEE 519, it is the impact of harmonic current
distortion on the power line voltage distortion that is important. The
supply column reflects the portion of the drive's harmonic distortion that is
injected back onto the line, which is calculated using the drive demand
ratio. The IEEE 519 recommendations specify not only the maximum
acceptable level of demand distortion as a function of the short-circuit
ratio, but also individual distortion limits based upon the harmonic order.
Individual and total supply harmonics that exceed the IEEE 519 limits are
highlighted in red by the calculator along with the corresponding limit.
The Harmonics Calculator is another in a series of online tools provided for
your convenience. We hope you find it useful. As always, your questions
and comments are welcome and appreciated.
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Self-Mitigating Variable-Speed Drives Feature
AHD™ Active Harmonic Damping Technology
It's an unavoidable fact that
all electronic drives create
harmonic distortion.
Harmonics are an
undesirable side effect of the
frequency conversion
process. However, instead of
merely contributing to power
quality problems like most
drives, Unico's 1200 series
drives are part of the
solution, thanks to AHD™
Active Harmonic Damping
technology.
AHD™ technology incorporated within the drive actively mitigates
harmonics at the source. The unique, patent-pending software technique
works hand-in-hand with the 1200 series hardware, which uses metal-film
rather than conventional electrolytic capacitors. AHD™ technology takes
advantage of the relatively low bus capacitance of this topology to
precisely control the bus voltage and minimize harmonic currents. Input
harmonic currents appear as fluctuations in the bus voltage that are
multiples of six times the power line frequency. The AHD™ control
automatically detects and damps those fluctuations to minimize harmonic
distortion.
AHD™ technology help to increase energy utlization, extend equipment
life, and improve system reliablity and productivity. When used in concert
with other harmonic solutions, the technology provides an economical
path to satisfying the IEEE 519 harmonic distortion guidelines. The 1200
series drives with AHD™ control are an essential part of a portfolio of
harmonic solutions that includes line reactors, multiphase techniques (12-,
18-, and 24-pulse drives), harmonic filters, autotransfomers, and hybrid
configurations. By making the drive self-mitigating, integral AHD™ control
not only enhances the performance of any other technique with which it is
paired, but it also lowers the cost, complexity, and footprint of the
harmonic solution.
Typical AHD™ Harmonic Mitigation Results
Method THD
C onventional drive More than 100%
A HD™ control with 3% A C line reac tor Less than 30%
A HD™ control with pass ive harmonic filter Less than 8%
A HD™ control with 12-pulse drive and isolation trans former Less than 12%
A HD™ control with 18-pulse drive and isolation trans former Less than 8%
A HD™ control with 24-pulse drive and isolation trans former Less than 5%
A HD™ control with 12-pulse drive and autotrans former Less than 12%
A HD™ control with 18-pulse drive and autotrans former Less than 8%
A HD™ control with 24-pulse drive and autotrans former Less than 5%
A brochure explaining AHD™ technology and comparing various harmonic
mitigation approaches is available online. If you have questions about
AHD™ technology, please contact us.
In Future Issues...
Look for the following articles in upcoming issues of Oil & Gas Automation
Solutions:
Field tests of methods to eliminate rod pump gas locking andinterference
Reducing power consumption and improving power factor of beampumps
Using a torque economizer mode to improve efficiency and reducegearbox stress
Control options to ride through power disturbances
Loss of methane gas production due to overpumping CBM wells
Use of low-profile CRP® and LRP® pumping units with travelingirrigation systems
Air counterbalance increases LRP® linear rod pump lift capacity
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