Contemporary industrial automation control systems are employing variable frequency drives (VFDs) in ever-increasing numbers. VFDs give control engineers flexibility to precisely regulate the speed and torque of motors in a wide array of applications. The proliferation of these VFDs has brought increased attention to harmonic distortion created by these drives and their effects on the power system. A standard-pulse drive with no built-in harmonic mitigation controls may interfere with neighboring equipment, reduce equipment life, and create a serious negative impact on the quality of utility power. Looking at the theory of operation for the following harmonic mitigation techniques and their typical performance levels may help take the guesswork out of harmonic reduction for these power systems.
Reactors to Reduce Harmonics
Standard 6-pulse drives (nonlinear loads) draw current only at the ± peaks of the AC line voltage. Since the current waveform is not sinusoidal, it may contain harmonic distortion. For a 3-phase phase input converter (used to convert AC to DC in the drive) using six diodes and a capacitor bank as shown in Figure 1, input current may contain as much as 85% or more total harmonic distortion (THID). Notice the high peaks and distortion created. This can be a problem for the power system that may need correcting.
Though harmonic mitigation is an important reason to use a line reactor, most drives at the 10-horsepower rating and above include a DC link choke. The link choke is a reactor put in the DC bus between the rectifier bridge and the capacitor bank, providing harmonic mitigation. Since it is in the DC bus, it can be made smaller and cheaper than a 3-phase input reactor.
With the addition of the DC choke, the peak current is reduced and somewhat broadened out, making the current more sinusoidal and lowering the harmonic level immensely. This effect is also beneficial to the DC filter capacitors since the ripple current is reduced. The drive capacitors can be smaller, run cooler, and last longer.
Generally speaking, a DC link choke becomes the first and best method for improving the harmonic performance of a 6-pulse drive, taking the distortion levels down from perhaps 85% THID to about 30% THID (See Figure 2). If a DC choke is not present, a properly sized line reactor might be installed that would provide roughly the same effect. If the drive already has a DC link choke, adding a line reactor will not provide much in the way of further harmonic reduction, perhaps only 3 to 5%.
Filters for Improved Harmonic Reduction
While the link choke or reactor provides the best “bang for the buck,” the 30% THID levels are still quite high and well outside the limits set by the IEEE-519 guidelines. In order to bring a system into compliance with the aforementioned guidelines, more must be done to reduce the harmonic content of the current. A passive filter then might be the next step in harmonic reduction. The traditional passive filter consists of a tuned LC (Inductive Capacitive) network. It is worth stating here that in a balanced 3-phase system, 6-pulse loads do not generate even or tripling harmonics. So the first harmonic element that becomes a problem is the 5th harmonic.
The 5th harmonic is generally the largest, so filters get tuned just below that point. For a 60-Hz filter, this would be just below the 300 Hz point or about 290 Hz. A reactor added to the input prevents the filter from trying to clean up any voltage distortion on the power feed and “de-tunes” it from the line. A reactor at the output reduces peak currents in the rectifier and cap bank. With a filter, it is the line current that gets cleaned up with typically less than 5% THID, as seen in Figure 3. However, the filter output or load side current (and voltage) remains polluted. For this reason, it is never a good idea to run any type of control power from the filter output.
While more expensive than a reactor alone, a filter can provide increased performance needed to meet IEEE-519. In order to get to this level of performance, selecting the right filter is important. Filter topology can vary widely, as can the harmonic reduction levels. Traditional filter designs work well above 80% load, but can fall off sharply below that 80% level. With established filter designs, if the load never reaches 80% or runs below 80% most of the time, the system will not see all the benefits of using a filter. Furthermore, filters utilize capacitors in the tuned network that can create concerns in certain instances.
Capacitors in a filter circuit create a phenomenon called “voltage magnification,” where the no-load filter output voltage might increase 10% or more as compared to the input voltage. There can also be a significant amount of leading power factor at light or no-load that may interfere with operation from generator power. Lastly, the filter might become problematic by resonating with the power line and any power factor correction capacitors that are in place on the power grid.
A new technology in harmonic filter design has been brought forth that addresses many of these nagging problems found in outmoded filter design schemes. This new topology utilizes patented magnetic material in the core gap of the main I/O filter reactor. It allows the impedance to decrease as load increases. The benefits here are multiple and substantial. First, lowering the impedance at heavy loads reduces the volt drop from input to output under full load, preventing loss of voltage to the motor. Second, reduced volt drop means improved efficiency and lower heat loss. Third, as seen in Figure 4, the filter will have a very flat performance curve, allowing excellent operational performance at lighter loads or where the filter may have been over-sized for one reason or another. It also allows for multiple drives to be run from one filter where some drive loads may be turned down or off. Fourth, the filter will require less total capacitance, resulting in lower voltage boost and less leading power factor at no-load or lightly loaded conditions. Finally, since the impedance of the main reactor varies, it is nearly impossible to create a stable resonance condition with any power line components.
Using a Combination of Methods
For larger systems where compliance to IEEE-519 is required at the utility PCC1, or Point of Common Coupling, a combination of drives with filters and reactors may be the most cost-effective. For systems with a large amount of linear load, little or no harmonic mitigation may be needed for system compliance. Where many 6-pulse drive loads are present, filters should be added, starting with the larger drives and reactors placed on smaller drives that may not have link chokes. Since a drive without a link choke is a gross harmonic polluter with distortion that can reach between 80% and 200% THID, it is imperative that these drives get reactors or link chokes added. Some drives have provisions for externally mounted DC chokes, but if not, the AC input reactor will provide the same harmonic reduction benefit as the DC choke, though at slightly higher cost. Several drive manufacturers provide free harmonic estimation tools that may prove helpful in determining the best mix of filters and/or reactors to bring a system in compliance.
This article was written by John T. Streicher, Sales and Applications Support Manager, and Chad Burks, Commercial Programs Manager, at MTE Corp., Menomonee Falls, WI. For more information, Click Here .