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.

In future issues

Watch for these topicsin upcoming is sues .

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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

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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.

Go to top

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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