Transcript
Variable Frequency Drive Solutions for Low Harmonics
Harmonics – A Growing Concern For Customers Harmonics are deviations from the ideal sinusoidal AC line voltage and current waveforms. These distortions are typically low in magnitude and can have an impact on the performance of your connected devices. With the advent 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 the utility power lines.
Potentially Compromising Harmonic distortion can impact your electrical systems in a number of ways: Conductor insulation overheating, higher temperature rise in transformers and motors resulting in shorter life, and instrumentation signal interference. Depending on your facility and the system parameters, these issues may have a significant effect on various elements of your installation. Facility engineers need to determine if, how and where to mitigate harmonic distortion in the power distribution system to reduce the risk of any serious consequences.
Staightforward Solutions Fortunately, reducing harmonic distortion to acceptable limits is easily attainable. The Institute of Electrical and Electronics Engineers (IEEE) has created guidelines that set reasonable limits for harmonic current and voltage distortion for various systems and locations (IEEE 519-1992). Many software calculation programs are also available for you to help predict the level of harmonic content and determine the mitigation requirements for your installation.
Rockwell Automation® has the Answer By applying the appropriate technology, the effects of harmonics can be reduced, thereby minimizing the effects of potential damage to your assets. Rockwell Automation offers a variety of low and medium voltage VFD options in small footprint packages, engineered to minimize harmonic distortion at the source. Several proven harmonic mitigation techniques are available to meet your requirements for budget, space limitations and harmonic attenuation. Rockwell Automation has a proven track record and large installed base of low harmonic VFDs in various industrial and commercial applications. These technologies include 18-pulse and active front-end converters as well as both active and passive filters, employed to minimize harmonics and adhere to IEEE 519-1992 standards.
Harmonic Mitigation Techniques
Harmonic Solution Cost Comparison
Active Approximate Cost
Passive 18 Pulse AFE
10 HP
25 HP
50 HP
75 HP
100 HP
150 HP
200 HP
250 HP
300 HP
400 HP
500 HP
800 HP
Drive Horsepower Rating
DRIVE HARMONIC MITIGATION TECHNIQUE
COMMENTS
Active Power Filter
This is an external solution that actively monitors harmonic distortion levels and injects cancellation harmonic current onto the line to meet IEEE 519-1992 at the power line where the A active filter is connected. Cost effective on large common AC bus applications, such as low voltage motor control centers.
Passive Filters and 6-pulse AC Drives
B resonance condition in some installations. Use caution when applying this type of filter with
18-Pulse Converter with Auto Transformer
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 will C have a lower cost and smaller size (easier to mount in the enclosure line-up) than an isolation transformer. Comparable cost to 12-pulse. Cost effective over 250 horsepower.
Active Front End
This is a cost-effective solution at 200 horsepower and below. It may cause a power system generator power sources.
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 meet D IEEE 519-1992 at the input terminals of the drive. This method is most cost effective on large common 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.
Configurations Active Power Filter Drive Tra ns form e r xfm r %Z
DC Lin k Ch o ke Ifund
Ifund + Iharm
AC
DC DC
Iharm
AC
AC
M
DC Active Filter
Motor Loa d
The active power filter solution injects harmonic and reactive current to limit the harmonic distortion, and improves displacement power factor for electrical distribution systems. It measures the entire load current, removes the fundamental frequency component, and injects the inverse of the remaining waveform for nearly complete cancellation of harmonic current. The filter is not focused on specific frequencies; rather it creates the required waveform based upon the input of its sensing circuitry by monitoring the load through current transformers mounted on the AC line. This information is analyzed by 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 knowledge regarding the nature of the load, or the type of harmonics present. Due to the fact that active power filter is a parallel installed device, it can typically be sized for 30% of the current required by the load. It only has to be sized to provide the harmonic and out-of-phase current drawn by the non-linear load. The utility provides the resulting 60/50 Hz fundamental current.
Passive Filters and 6-Pulse AC Drives Drive
Tra n s fo rm e r
DC Lin k Ch o ke
xfm r %Z
AC
DC DC
AC
M Passive Filter
Mo to r Lo a d
The passive filter solution, when applied at the load, provides a low impedance path for the major harmonic currents demanded by the drive. This greatly reduces the amount of harmonic current flowing through the distribution system and results in improved power factor, lower RMS currents, lower harmonic current distortion, lower harmonic voltage distortion, and increased system capacity. Passive filters are a cost-effective and efficient solution up to 200 HP.
18-Pulse Converter with Auto Transformer Drive Tra n s fo rm e r xfm r %Z
3 3
DC Lin k Ch o ke
AC DC
3
DC
AC DC
3
AC
AC DC
18-Pulse Auto-transformer
M Motor Loa d
Rockwell Automation’s patented 18-pulse solution relies on a compact auto transformer feeding 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 harmonic distortion is significant. This solution’s simple topology results in a reliable method for harmonic mitigation and improving power factor. Another advantage of using a multi-pulse drive over the addition 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-pulse drive. 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.
Active Front End Drive Tra n s fo rm e r xfm r %Z
AC
DC DC
AC
M Notch Filter Mo to r Lo a d
Rockwell Automation’s active front end solution keeps the network current sinusoidal and a unity power factor by controlling the drive input to produce sinusoidal current without the harmonic components associated with conventional rectifiers. This solution uses a switching rate substantially higher than the network frequency, which effectively reduces the low frequency harmonics. Active front end technology is most cost effective over 300 HP and is available for both low and medium voltage drives with air or liquid-cooled power structures to meet the requirements of most applications.
Harmonic Mitigation and Compliance Application Guidelines 1 Take time to understand the benefits and drawbacks of each type of mitigation solution to assure you meet the requirements of the application and that you can live with any negative effects created by the chosen harmonic solution.
6 For passive filters on generator power, select a filter with a dropout contactor terminal block for the filter capacitors. This will limit the leading power factor at no-load operation.
2 Perform a preliminary harmonic analysis on your system and explore the effects of using various harmonic mitigation methods.
7 For an even number of equally sized drives, consider a pseudo 12-pulse solution by placing half of the load of a phase shifting delta-wye (delta-star) transformer.
3 Identify the required Point of Common Coupling (PCC) and apply most cost-effective mitigation techniques for that application.
8 Design the system to isolate linear and non-linear loads and create two systems with 5% and 10% voltage distortion limits.
4 Add a line reactor (or DC link choke if possible) to all unbuffered 6-pulse drives.
9 If the application requires regenerative operation and harmonic compliance, consider an active front-end.
5 Never use power factor correction capacitors at the input (or output) of a drive, or in parallel with passive filters.
10 Consider use of an active filter on a multiple drive system or MCC lineup to correct for harmonic distortion.
Harmonic Mitigation Solutions Check-List
Good
? Need to confirm application X May not meet IEEE 519-1992 Standards
6-Pulse Drive Typical Current Harmonic Distortion (ITHD)
20 - 45%
18-Pulse Drive 4.5 - 6%
X X
3 - 5%
Marginal
Yes
No
Meet IEEE General Applications
No
Meet IEEE Dedicated Applications
Yes
Yes
Effect of 1% Voltage Unbalance
Large
X
Moderate
Marginal
No
Yes
Potential System Resonance Typical Total Power Factor, no/full load
No
Yes
No
No
0.75 - 0.95
0.90 - 0.99
Efficiency
97%
Cost Effective
Good
Overall Size (Relative to 6-Pulse Drive)
1.0
Reliability
High
96.5% >250 HP 3.3 High
Active Filter
5 - 8%
Meet IEEE Special Applications
Potential Low DC Bus
Yes
Passive Filter
Yes Yes
Yes 0.3 - 1 lead 96.5% <200 HP 2-6 Medium
? ? ?
Yes Yes Minimal No No 0.90 - 0.99
Active Front- End 3 - 5%
Yes Yes Yes Minimal No
No 0.8 - 1 lead
96
96 - 97.5%
Lg. System
>300 HP
3.5 - 5
Regen, MV
Medium
Medium
Low Voltage AC Drives
Medium Voltage AC Drives
The PowerFlex® family of AC drives is a proven solution that offers a variety of options for harmonic mitigation to comply with even the most stringent harmonic requirements.
Lower your line harmonics simply and easily
Technology with a track record The PowerFlex family of AC drives represents the culmination of over 30 years of drive design and manufacturing experience. This compact, modular design offers advanced fan and pump control features designed to improve system performance and reduce energy consumption. In addition, built-in software algorithm adjusts output voltage diminishing standing waves from damaging motor insulation allowing up to 600 feet of cable between the VFD and motor.
Reducing line side harmonics helps protect your facility’s power quality. The PowerFlex 7000 Medium Voltage drive has several solutions to address harmonic concerns without increasing component count or complexity. The latest evolution of the PowerFlex 7000 Medium Voltage AC drive includes active front-end and “Direct-to-Drive” technology, allowing you to connect utility power directly to the drive without an isolation transformer. The Direct-to-Drive Active Front End rectifier uses semiconductor switching to reduce line current harmonic levels that comply with IEEE standards. This technology reduces the drive size, helps lower installation costs, increases energy savings, and provides a commonality in spare parts.
Decrease your footprint With the latest generation of IGBT power technology, some power rating sizes have been reduced, without sacrificing efficiency or performance. PowerFlex drives offer many enhancements. A sleep/wake feature is built-in to activate and deactivate the drive based on a reference signal threshold input.
Save time To reduce start-up time, a built-in SMART™ start menu allows you to program the basic drive operational settings without having to go through the entire menu of adjustments. This simplifies commissioning and reduces training.
Choose from a range of time-tested AC drive solutions
Packaging Configured Drives
Motor Control Centers
For your applications requiring low voltage AC drives mounted in wall or floor mount enclosures, the Configured Drives Program offers flexible packaging solutions for PowerFlex Drives. A wide range of power, control, communication and operator devices ensure specific application requirements can be addressed. Drive enclosures can be sized and designed to meet most IP/NEMA/UL Type requirements allowing for installation in any type of environment.
The CENTERLINE 2100 Motor Control Center (MCC) combines rugged durability and premium quality. CENTERLINE 2100 MCCs integrate control and power in one centralized package with a wide variety of motor control options.
Detailed product wiring documentation, test reports and consistent panel/component layouts allow for faster troubleshooting and start-up resulting in decreased installation costs. Specific industry solutions are also available to address unique requirements for oil & gas, water wastewater and heating/ventilation/air-conditioning markets.
The key feature of our MCCs is the proven CENTERLINE bus design providing an AC thru bus that supports fully isolated enclosures for maximum fault containment. This is a space saving design that maximizes section utilization reducing the overall footprint. This industry leading Motor Control Center has delivered safety, performance and reliability for over 35 years.
Local Drive Solutions With over 25 locations worldwide, Local Drive Solutions provides a local solution for your custom drives packaging requirements. From custom single drive enclosures to multi-drive panels with options ranging from special enclosures to active front end packaged drives, Local Drive Solutions can meet your application requirements with regional engineering and manufacturing support.
For further information: www.ab.com/drives/techpapers/page8.htm PowerFlex is a trademark of Rockwell Automation.
Publication DRIVES-BR011B-EN-P – August 2009
Copyright ©2009 Rockwell Automation, Inc. All Rights Reserved. Printed in Canada.
