Variable Frequency Drive Solutionsfor Low Harmonics

Harmonics are deviations from the ideal sinusoidal AC line voltage and current waveforms. These distortions aretypically low in magnitude and can have an impact on the performance of your connected devices. With theadvent of power electronics, non-linear loads and power switching devices such as Variable Frequency Drives(VFDs), there is concern for a potential increase in the level of harmonic distortion conducted back onto theutility power lines.

Potentially Compromising

Harmonic distortion can impact your electrical systems in anumber of ways: Conductor insulation overheating, highertemperature rise in transformers and motors resulting in shorterlife, and instrumentation signal interference. Depending on yourfacility and the system parameters, these issues may have asignificant effect on various elements of your installation. Facilityengineers need to determine if, how and where to mitigateharmonic distortion in the power distribution system to reducethe risk of any serious consequences.

Staightforward Solutions

Fortunately, reducing harmonic distortion to acceptablelimits is easily attainable. The Institute of Electrical andElectronics Engineers (IEEE) has created guidelines that setreasonable limits for harmonic current and voltagedistortion for various systems and locations (IEEE 519-1992).Many software calculation programs are also available foryou to help predict the level of harmonic content anddetermine the mitigation requirements for your installation.

Rockwell Automation® has the Answer

By applying the appropriate technology, the effects ofharmonics can be reduced, thereby minimizing the effects ofpotential damage to your assets. Rockwell Automation offers avariety of low and medium voltage VFD options in small foot-print packages, engineered to minimize harmonic distortion atthe source. Several proven harmonic mitigation techniques areavailable to meet your requirements for budget, spacelimitations and harmonic attenuation. Rockwell Automation hasa proven track record and large installed base of low harmonicVFDs in various industrial and commercial applications. Thesetechnologies include 18-pulse and active front-end convertersas well as both active and passive filters, employed to minimizeharmonics and adhere to IEEE 519-1992 standards.

Harmonics – A Growing Concern For Customers

Harmonic Solution Cost Comparison

Drive Horsepower Rating

App

roxi

mat

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st

10 HP 25 HP 50 HP 75 HP 100 HP 150 HP 200 HP 250 HP 300 HP 400 HP 500 HP 800 HP

Active

Passive

18 Pulse

AFE

DRIVE HARMONIC MITIGATION TECHNIQUE COMMENTS

Active Power Filter A

This is an external solution that actively monitors harmonic distortion levels and injectscancellation harmonic current onto the line to meet IEEE 519-1992 at the power line where theactive filter is connected. Cost effective on large common AC bus applications, such as lowvoltage motor control centers.

Passive Filters and 6-pulse AC Drives

BThis is a cost-effective solution at 200 horsepower and below. It may cause a power systemresonance condition in some installations. Use caution when applying this type of filter withgenerator power sources.

18-Pulse Converterwith Auto

Transformer C

This meets IEEE 519-1992 general requirements at the drive input terminals without analysis,when power phases are balanced within 1/2% (1% when required). An auto transformer willhave a lower cost and smaller size (easier to mount in the enclosure line-up) than an isolationtransformer. Comparable cost to 12-pulse. Cost effective over 250 horsepower.

Active Front End D

This solution actively tracks and regulates input current to maintain sine wave current draw.This technique generates minimal voltage distortion allowing the input power converter to meetIEEE 519-1992 at the input terminals of the drive. This method is most cost effective on largecommon DC bus systems with many drives, or on single high horsepower ratings requiring braking.For medium voltage drive applications, this is generally the most cost-effective solution.

Harmonic Mitigation Techniques

The active power filter solution injects harmonic and reactive current to limit theharmonic distortion, and improves displacement power factor for electrical distributionsystems. It measures the entire load current, removes the fundamental frequencycomponent, and injects the inverse of the remaining waveform for nearly completecancellation of harmonic current. The filter is not focused on specific frequencies; rather itcreates the required waveform based upon the input of its sensing circuitry by monitoringthe load through current transformers mounted on the AC line. This information is analyzedby the logic to determine the amount of correction to be injected into the AC lines.

Implementation of this technology is simple as it does not require detailed knowledgeregarding the nature of the load, or the type of harmonics present. Due to the fact thatactive power filter is a parallel installed device, it can typically be sized for 30% of the currentrequired by the load. It only has to be sized to provide the harmonic and out-of-phasecurrent drawn by the non-linear load. The utility provides the resulting 60/50 Hzfundamental current.

Active Power Filter

M

Drive

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DC

AC

DC LinkChoke

xfmr% Z AC

DC

Ifund Ifund + Iharm

Iharm

Active Filter

AC

DC

Passive Filters and 6-Pulse AC Drives

M

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Tra ns forme r

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Drive

DC

AC

AC

DC

DC LinkChoke

Passive Filter

The passive filter solution, when applied at the load, provides a low impedance pathfor the major harmonic currents demanded by the drive. This greatly reduces the amountof harmonic current flowing through the distribution system and results in improvedpower factor, lower RMS currents, lower harmonic current distortion, lower harmonicvoltage distortion, and increased system capacity.

Passive filters are a cost-effective and efficient solution up to 200 HP.

Configurations

Rockwell Automation’s active front end solution keeps the network current sinusoidal anda unity power factor by controlling the drive input to produce sinusoidal current withoutthe harmonic components associated with conventional rectifiers. This solution uses aswitching rate substantially higher than the network frequency, which effectively reducesthe low frequency harmonics.

Active front end technology is most cost effective over 300 HP and is available for bothlow and medium voltage drives with air or liquid-cooled power structures to meet therequirements of most applications.

Active Front End

M

MotorLoa d

Tra ns forme r

xfmr

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Drive

DC

AC

AC

DC

Notch Filter

18-Pulse Converter with Auto Transformer

Tra ns forme r

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18-PulseAuto-transformer

Drive

AC

DC

AC

DC

AC

DC

DC LinkChoke

33

3

3

M

MotorLoa d

DC

AC

Rockwell Automation’s patented 18-pulse solution relies on a compact auto transformerfeeding a diode bridge to reduce the harmonic current created by the AC drive.Compared to 6-pulse converters, the reduction in harmonic currents and total harmonicdistortion is significant.

This solution’s simple topology results in a reliable method for harmonic mitigation andimproving power factor. Another advantage of using a multi-pulse drive over theaddition of line reactors is that there is no drop in DC bus voltage as the load is increased.In fact, the DC bus voltage is a little higher than the nominal bus voltage on a 6-pulsedrive. This provides a better voltage utilization to achieve full power output to the motor.

This solution is cost and size effective for applications over 250 HP.

Application Guidelines

1 Take time to understand the benefits anddrawbacks of each type of mitigation solution toassure you meet the requirements of the applicationand that you can live with any negative effects createdby the chosen harmonic solution.

2 Perform a preliminary harmonic analysis on yoursystem and explore the effects of using variousharmonic mitigation methods.

3 Identify the required Point of Common Coupling(PCC) and apply most cost-effective mitigationtechniques for that application.

4 Add a line reactor (or DC link choke if possible) toall unbuffered 6-pulse drives.

5 Never use power factor correction capacitors at theinput (or output) of a drive, or in parallel withpassive filters.

6 For passive filters on generator power, select afilter with a dropout contactor terminal block forthe filter capacitors. This will limit the leading powerfactor at no-load operation.

7 For an even number of equally sized drives,consider a pseudo 12-pulse solution by placinghalf of the load of a phase shifting delta-wye(delta-star) transformer.

8 Design the system to isolate linear and non-linearloads and create two systems with 5% and 10%voltage distortion limits.

9 If the application requires regenerative operationand harmonic compliance, consider an activefront-end.

10 Consider use of an active filter on a multiple drivesystem or MCC lineup to correct for harmonicdistortion.

Good ? Need to confirm application X May not meet IEEE 519-1992 Standards�

Harmonic Mitigation Solutions Check-List

Harmonic Mitigation and Compliance

6-Pulse Drive 18-Pulse Drive Passive Filter Active Filter Active Front- End

Typical Current Harmonic Distortion (ITHD) 20 - 45% 4.5 - 6% 5 - 8% 3 - 5% 3 - 5%

Meet IEEE Special Applications No X Yes Marginal Yes Yes

Meet IEEE General Applications No X Yes Yes Yes Yes

Meet IEEE Dedicated Applications Yes Yes Yes Yes Yes

Effect of 1% Voltage Unbalance Large X Moderate Marginal Minimal Minimal

Potential Low DC Bus No No Yes ? No No

Potential System Resonance No No Yes ? No No

Typical Total Power Factor, no/full load 0.75 - 0.95 0.90 - 0.99 0.3 - 1 lead ? 0.90 - 0.99 0.8 - 1 lead

Efficiency 97% 96.5% 96.5% 96 96 - 97.5%

Cost Effective Good >250 HP <200 HP Lg. System >300 HP

Overall Size (Relative to 6-Pulse Drive) 1.0 3.3 2 - 6 3.5 - 5 Regen, MV

Reliability High High Medium Medium Medium

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The PowerFlex® family of AC drives is a provensolution that offers a variety of options for harmonicmitigation to comply with even the most stringentharmonic requirements.

Technology with a track record

The PowerFlex family of AC drives represents theculmination of over 30 years of drive design andmanufacturing experience. This compact, modulardesign offers advanced fan and pump controlfeatures designed to improve system performanceand reduce energy consumption. In addition, built-insoftware algorithm adjusts output voltagediminishing standing waves from damaging motorinsulation allowing up to 600 feet of cable betweenthe VFD and motor.

Decrease your footprint

With the latest generation of IGBT power technology,some power rating sizes have been reduced, withoutsacrificing efficiency or performance. PowerFlexdrives offer many enhancements. A sleep/wakefeature is built-in to activate and deactivate the drivebased on a reference signal threshold input.

Save time

To reduce start-up time, abuilt-in SMART™ startmenu allows you toprogram the basic driveoperational settingswithout having to gothrough the entire menuof adjustments. Thissimplifies commissioningand reduces training.

Lower your line harmonics simply and easily

Reducing line side harmonics helps protect yourfacility’s power quality. The PowerFlex 7000 Medium Voltage drive has severalsolutions to address harmonic concerns withoutincreasing component count or complexity. The latestevolution of the PowerFlex 7000 Medium Voltage ACdrive includes active front-end and “Direct-to-Drive”technology, allowing you to connect utility powerdirectly to the drive without an isolation transformer.The Direct-to-Drive Active Front End rectifier usessemiconductor switching to reduce line currentharmonic levels that comply with IEEE standards. This technology reduces the drive size, helps lowerinstallation costs, increases energy savings, andprovides a commonality in spare parts.

Low Voltage AC Drives Medium Voltage AC Drives

Choose from a range of time-tested AC drive solutions

Publication DRIVES-BR011B-EN-P – August 2009 Copyright ©2009 Rockwell Automation, Inc. All Rights Reserved. Printed in Canada.

PackagingMotor Control Centers

Local Drive Solutions

Configured DrivesFor your applications requiring lowvoltage AC drives mounted in wall orfloor mount enclosures, the ConfiguredDrives Program offers flexiblepackaging solutions for PowerFlexDrives. A wide range of power, control,communication and operator devicesensure specific applicationrequirements can be addressed. Driveenclosures can be sized and designedto meet most IP/NEMA/UL Typerequirements allowing for installation inany type of environment.

Detailed product wiring documentation,test reports and consistentpanel/component layouts allow for fastertroubleshooting and start-up resulting indecreased installation costs. Specificindustry solutions are also available toaddress unique requirements for oil &gas, water wastewater andheating/ventilation/air-conditioning markets.

The CENTERLINE 2100 Motor Control Center (MCC) combinesrugged durability and premium quality. CENTERLINE 2100 MCCsintegrate control and power in one centralized package with awide variety of motor control options.

The key feature of our MCCs is the proven CENTERLINE bus designproviding an AC thru bus that supports fully isolated enclosuresfor maximum fault containment. This is a space saving designthat maximizes section utilization reducing the overall footprint.

This industry leading Motor Control Center has delivered safety, performance and reliability for over 35 years.

With over 25 locations worldwide, Local Drive Solutions provides alocal solution for your custom drives packaging requirements.

From custom single drive enclosures to multi-drive panels withoptions ranging from special enclosures to active front endpackaged drives, Local Drive Solutions can meet your applicationrequirements with regional engineering and manufacturing support.

PowerFlex is a trademark of Rockwell Automation.For further information: www.ab.com/drives/techpapers/page8.htm