Common to any production operation is movement, usually driven by electric motors. Better control of this movement contributes to improved operational performance. Beyond performance, however, there's also the need to protect these motors to maximize return on investment.
For reliable control and improved protection, adjustable-speed drives are a common companion to motors. Although these drives offer inherent production benefits, they may also contribute to power quality issues, such as line current harmonics. As a result, you should take a closer look at harmonics when evaluating mitigation strategies.
Catch the wave. Compared to DC motor drives, AC motor drives cause very few problems. However, poorly designed applications can result in power line voltage distortions, as shown in the flat-topped waves of Fig. 1.
These voltage distortions can cause problems for other equipment connected to the same power lines, resulting in erratic operation of controls, dimming of lights, and overheating motors operating across the line. The distribution transformers and cables feeding these drives will also experience additional heating, which reduces the power use of those components.
Unlike the way in which a linear load draws current, such as an AC motor operating across the power line (Fig. 2), a typical AC drive draws current from a distribution transformer that's far from a sinusoidal waveform (Fig. 3). This occurs because the drive is taking current from the transformer only during certain times of the cycle to convert the 3-phase AC line voltage to a fixed DC voltage within the drive. The drive then pulse-width modulates this fixed DC voltage into variable frequency/variable voltage for the motor.
The AC-to-DC conversion is what causes the harmonics. Current flows only during part of the cycle, creating the odd-looking current waveform shown in Fig. 3. It's this distorted current that creates the voltage distortion. This is why a drive is considered a nonlinear load.
While the number of drives in an automation system may increase, it does not necessarily mean that those drives are the cause of all harmonic headaches. There are other pieces of equipment that rectify AC to DC and create harmonic distortion. This includes most of the equipment found on the plant floor, as well as phase-to-neutral nonlinear loads in the form of office machines like computer power supplies and copiers.
Even fluorescent lighting ballasts can create harmonic distortion. That's why you must analyze all of the electrical loads that could potentially cause problems for a system before you rush out and fit filters to every drive.
Where's the limit? How much current harmonic distortion will cause a problem? As mentioned earlier, the current harmonics drawn by nonlinear loads — both phase-to-phase and phase-to-neutral — cause voltage distortion, which can cause improper operation of other equipment. IEEE Standard 519-1992, “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems,” provides the answer. This standard recommends maximum current distortion levels caused by nonlinear loads to limit the voltage distortion they would create, thereby reducing the likelihood of equipment failure due to those harmonics. Table 1 and Table 2 below, reproduced from the IEEE Standard 519, list the limits applicable for most typical drive systems. These limits were determined for use at the metering point in the plant, also known as the point of common coupling (PCC).
Many engineers have taken these limits and used them at other locations throughout their facilities in order to reduce the likelihood of harmonic disturbances. There are opposing viewpoints on whether or not these other locations should be considered PCCs. (You can find past articles on this topic by going to www.ecmweb.com and keying in “PCC” or “point of common coupling” in the search field on the upper right of the Home page.)
Reducing harmonics. There are several methods and products you can use to reduce the line current harmonics created by drives. Similarly, there is usually more than one way to approach a specific problem.
It's not uncommon for electrical engineers and plant facility electrical maintenance personnel to oversize the solution in order to solve a perceived harmonics problem. This results in excess costs that could be avoided by better assessing the options. But cost is not the only factor that distinguishes one solution from another. For example, even though the addition of line reactors or passive filters can help reduce the current harmonics, they also will reduce the DC bus voltage within the drive at full-speed, full-load conditions. This prevents the drive from providing full power to the motor, limiting the motor's output to about 95% of its rating. This is why multi-pulse solutions could be a better fit for many situations, since no derating is necessary — and it may be less expensive than other mitigation methods for drives above 200 hp.
Table 3 lists various harmonic mitigation methods, along with comments on what you need to know, and the respective typical harmonic current associated with a 100-hp drive operation.
Not all drive applications have a harmonics problem.
If there is a problem, the solution doesn't always have to involve adding expensive hardware to the system.
While plants need to take precautions, remember that each application is different.
The bottom line is to make informed decisions. Measure the load, determine how many potential harmonic sources exist, assess future conditions, and carefully consider the various options available. Then, based on the cost and benefits provided, select the solution that best meets your particular application needs.
Hoadley is technical program manager for Rockwell Automation in Mequon, Wis.
Top 10 Rules For Mitigating Harmonics
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 reduction solution. Ask questions!
Identify the required point of common coupling (PCC), and apply techniques most cost effective for that location.
Perform a preliminary harmonic analysis on your system, and explore the effects of using various harmonic mitigation methods.
Add a line reactor (or DC link choke if possible) to all 6-pulse drives that do not have a DC link choke or an integral line reactor.
Design the system to separate linear and nonlinear loads to create a 5% voltage distortion system for the linear loads and a 10% voltage distortion system for the nonlinear loads.
For an even number of equally sized drives, consider a pseudo 12-pulse solution by placing half of the load on a phase shifting delta-wye (delta-star) transformer or using harmonic mitigating transformers.
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 and stand-by operation.
Never use power factor correction capacitors at the input (or output) of a drive or in parallel with passive filters.
Consider the use of an active filter on a multiple drive system or motor control center lineup to correct for harmonic distortion.
Consider an active front-end if the application requires regenerative operation and harmonic compliance.
Basics of Harmonics Harmonics are deviations from the sinusoidal fundamental AC line voltage and current. Most North American electrical power operates at a frequency of 60 Hz. A harmonic frequency is an integer multiple of this fundamental frequency. So in a 60-Hz system, the second harmonic would be 120 Hz, the fifth would be 300 Hz, and so on.
The addition of any harmonic to the sinusoidal fundamental current or voltage will create distortion. The greater the amplitudes of the harmonics present, the greater the distortion in the electrical waveform. Simply put, whenever a voltage or current does not look like a perfect sinusoidal waveform, it contains harmonics.