Harmonic Mitigation in AC Drives
Adjustable speed drives inherently create harmonic currents as they take three-phase ac sinusoidal power and convert it to dc power as part of ac induction motor control. With ac drive-related harmonics and mitigation, remember: 1) Whether the power converter is made up of diodes, silicon control rectifiers (SCRs), or insulated gate bipolar transistors (IGBTs) with free-wheeling diodes, they al...
Adjustable speed drives inherently create harmonic currents as they take three-phase ac sinusoidal power and convert it to dc power as part of ac induction motor control. With ac drive-related harmonics and mitigation, remember:
1) Whether the power converter is made up of diodes, silicon control rectifiers (SCRs), or insulated gate bipolar transistors (IGBTs) with free-wheeling diodes, they all generate harmonics because they turn on and off, creating a current waveform that is not sinusoidal.
2) Harmonic currents create voltage distortions on the power lines that feed the power converters.
3) Voltage distortion can also be created by line notching caused by some types of power converters.
Compared to dc motor drives, ac motor drives cause very few problems due to their harmonics. Poorly designed applications can result in problems for power distribution equipment and other equipment attached to the same power distribution system.
As the number of drives in automation systems increases, so does the misconception that drives are the only cause of harmonic problems. Every piece of equipment that rectifies ac to dc creates harmonic distortion, including most plant-floor equipment and office machines (such as computer power supplies, phone chargers, and copiers). Even ballasts for fluorescent lights create harmonic distortion. That's why it's important to analyze all electrical loads that could potentially cause problems for a system before rushing out and fitting filters to every drive in the facility.
In reality, there have been very few harmonic-related problems caused by ac drives. Harmonic problems typically appear as overheating of transformers and wiring feeding the drives because the designer did not account for additional harmonic currents being drawn. Fuses can blow prematurely and circuit breakers can trip for the same reason.
Harmonic currents create voltage distortion on power lines, which can then cause problems for other equipment connected to the same power lines: dimming of lights and overheating motors operating across the line.
Some devices, such as dc drives or unfiltered regenerative converters, also create line notches in addition to the harmonic currents. These voltage notches easily can cause erratic behavior in other equipment in the facility. Such effects perversely manifest themselves as misbehavior of perfectly good equipment while the problematical unit(s) appear to work properly.
When designing a new system, or when expanding the number of adjustable speed drives already in place on a power system, the designer needs to keep in mind the harmonics each piece of equipment contributes to the system and how those harmonics may interact.
Meet industry standards
IEEE Standard 519-1992, “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems,” provides some guidelines to reduce likelihood of equipment problems due to harmonics. Section 10 covers “Recommended Practices for Individual Consumers,” with two key tables that also apply to most industrial applications.
The first table, “Low voltage system classification and distortion limits,” provides guidelines for voltage distortion limits and line notches measurable at any location within a plant where linear and non-linear loads are tied together, at transformer secondaries, and at the power meter feeding the plant.
If a transformer supplies power to only ac drives, for example, the voltage distortion could be allowed to go as high as 10% without affecting their operation. If there is a mix of across-the-line motors and ac drives, however, or linear and non-linear loads, then the voltage distortion should be kept below 5%. In hospitals and airports, lives of people are at stake, so maximum voltage distortion is limited to 3%.
The standard also provides limits for depth and area of line notches to reduce the probability of erratic behavior in other loads connected to the power distribution system.
In addition to voltage distortion limits, IEEE-519 recommends current distortion limits, as specified in the second table, “Current distortion limits for general distribution systems.” These are the recommended current distortion limits to be measured at the interface between the utility and the customer, rather than at equipment terminals. This is where the point of common coupling (PCC) for this table is defined by IEEE 519.
Again, this standard was intended to serve as a guideline for utilities and their customers. Limitations were provided as a fair method to allow every customer that may be supplied power by the utility access to relatively distortion-free voltage for their plant. No one customer should be allowed to draw so much harmonic current that they badly distort the voltage provided to other customers. Even though data shown from IEEE 519 was not meant as an equipment standard, many consultants have applied it to current distortion drawn by each non-linear load or ASD. This usually means that the customer may be paying more for more filtering that what is recommended according to IEEE 519.
Several methods of harmonic distortion mitigation are available if line current harmonics drawn by an ac drive need to be reduced. The methods reduce harmonics and voltage distortion, but also affect other aspects of the power and drive systems in different ways. Here's what the drive system designer needs to know:
Basic drive current profile shows distortion of about 110%.
• Basic ac drive—For comparison, consider a basic ac drive as a three-phase diode bridge converter, a dc bus capacitor bank filter, and a three-phase IGBT bridge inverter. It does not contain a dc link choke or ac line reactor. Its line current looks like the “Basic ac drive current” graphic, with a current distortion of about 110%.
• Line Reactors—The easiest and least expensive method for reducing line current harmonics is to add a 3% line reactor ahead of each drive, or ahead of a group of three drives. This will reduce the current distortion to about 50% and create a waveform like that shown in the “Line current with 3% ac line reactor” graphic. One drawback to line reactors is that they also cause the dc bus voltage within the ac drive to sag with increases in motor speed and load.
Line reactor reduces current distortion to about 50% but can cause dc bus voltage sag.
A 3% line reactor will cause the dc bus voltage to sag about 3% at full speed, full load. This means that when operating at full speed and full load, the motor will not receive rated voltage, causing increased rotor slip and current draw., and causing the motor to run hotter. If you don't need to go above 97% speed at full load, or if you have high line voltage to begin with, then this would not be a problem. Similarly, a 5% line reactor would cause about a 5% sag in the dc bus voltage.
• DC Link Chokes—A dc link choke is located within the drive, between the output of the converter and the dc bus capacitor bank. It will reduce the distortion current to about 40%, but without the voltage sag on the dc bus at full speed, full load. Some drive vendors supply dc link chokes as an option for their drives or as a standard feature. Most drives larger than 5 hp have a built-in dc link choke.
• Passive Harmonic Filters—Passive harmonic filters are a popular choice for reducing line current harmonics, and are cost effective below about 150 hp. The passive filter becomes the reservoir for harmonic currents produced in the power conversion process. Since the filter supplies the currents, the transformer does not need to, so overheating and voltage distortion are reduced. These filters reduce distortion currents to about 8%, as the “Line current for a basic ac drive with passive harmonic filter” graphic shows.
Passive filter reduces current distortion to about 8% for motors below 150 hp.
Passive harmonic filters' biggest drawback arises when the process requires the ac drive to operate at low speeds or low loads for extended time periods. In that case, the passive filter can draw leading power factor current from the transformer, leading to overheating. This could also be a problem if the ac drive operates from a back-up generator.
To help compensate for these effects, some filter vendors supply a contactor to disconnect the capacitor bank within the filter whenever the drive is stopped or operates at a low speed. At full load on the drive, however, the power factor into the filter is close to unity. Before adding 10 or more drives with passive filters to a transformer, check with the filter vendor to ensure there will not be any voltage or current resonance issues among the filters
• Active Harmonic Filters—Active harmonic filters are a great choice for a single drive, but especially when several drives reside in a motor control center. The filter supplies the harmonic currents required by all the drives, and will actively modify its operation based on drive requirements, typically reducing the distortion current to 5%. Active harmonic filters do not cause leading power factor under no-load conditions. When the drives are stopped, and no harmonic current is needed, it will automatically reduce its output to zero. There are no technical or system drawbacks seen with active harmonic filters.
Active front end filter achieves highest distortion reduction to about 5%.
• Multi-pulse Transformers—Transformers have been used for phase shifting for several decades. Eighteen-pulse transformers and converters have become popular since they can easily achieve distortion currents of 5%.
Multi-phase transformers can be wound as auto-transformers or isolation types. Auto-transformers are less expensive and physically smaller than isolation transformers because they need fewer; 12-pulse transformers have been dropping in popularity recently, since they can only achieve 9-15% current distortion. Twenty-four-pulse systems and higher are also available, but the typical distortion improvement (to 4.5%) seldom pays for their additional cost.
On the positive side, there is no leading power factor at no load, and the dc bus voltage remains at a reasonable level throughout the drive's speed and load range. This option is often very cost effective above 150 hp.
• Harmonic Mitigating Transformers—HMTs come in specific phase shifts of 0, 15, 30, and 45 degrees, and are useful for configurations where multiple drives can be placed into two or four groups where the total drive power within each group is about the same. By using two groups of drives, one group on a 0-degree phase shift, and the other on a 30-degree phase shift, it's possible to cancel most of the 5th and 7th harmonics, mimicking a 12-pulse system. The same is true with 15-degree and 45-degree phase shift transformers. With four groups of drives, placing a group onto each of the four phase shifts will cancel most of the 5th, 7th, 11th and 13th harmonics, mimicking an 18-pulse system. If some of the drives are stopped, there would be less cancellation. The worst case condition for voltage distortion is when all of the drives are at full load.
• Active Front-End—AFE users usually purchase them as an integral part of a drive, and not as a retrofit, although they can be used as a separate module for several drives on a common dc bus.
An AFE consists of a three-phase IGBT bridge converter, similar to the inverter section of an ac drive, along with line reactors of about 10%. They act like a boost converter and can control the dc bus voltage during motoring and regenerating operation. If the IGBTs were to modulate at a carrier frequency of 3kHz, the harmonics would be equivalent to those from a 50-pulse system. AFEs can achieve current distortion of 5% as seen in the “Line current with active front end” graphic. However, a small notch filter is required to minimize the line notches created during modulation. Without this notch filter, the line notching could cause operational problems for other equipment connected to the same transformer as the AFE.
Drive-system design engineers need to be aware of how each harmonic mitigation method influences power and drive systems. More than just Ithd (input current total harmonic distortion) is affected. Within the plant, focus should be on keeping the voltage distortion and line notches within limits to prevent problems with other equipment. At the utility interface, the focus should be on keeping the current distortion within recommended limits to prevent problems with the utility and other customers. That is the goal of IEEE 519, and the system designer's goal.
Low-voltage system classification and distortion limits
Data from IEEE 519, showing voltage distortion limits, notch depth limits, and notch area limits for various applications
Special applications– hospitals and airports
Dedicated system– exclusively converter load
Notch area in V-us at rated voltage and current
Current distortion limits for general distribution systems
(120V through 69,000V)
Macimum harmonic current distortion in % of IL
Even harmonics are limited to 25% of the odd harmonic limits above
Rick Hoadley is technical program manager, Rockwell Automation,