A Variable Frequency Drive (VFD) controls the speed of an AC motor. The VFD enhances efficiency by limiting energy consumption and reducing equipment wear. Its use, however, may create power quality issues in the form of harmonics.

Adding Variable Frequency Drive (VFD) to control motor speed enhances efficiency by limiting energy consumption and reducing equipment wear. The use of VFD, however, may create power quality issues in the form of harmonics.

Harmonics are deviations from the desired model sinusoidal AC line voltage and current waveforms. These deviations, or harmonic distortions, generally have low magnitude. However, the magnitude can increase if there is greater use of power electronics, non-linear commercial and industrial loads, and VFDs. Harmonics can cause problems in the power system. Higher-order harmonics may interfere with sensitive electronics such as programmable logic controllers (PLCs), distributed control systems (DCS), and communications systems. In contrast, lower-order harmonics can cause overheating in motors, transformers, and conductors.

Harmonics have a different acceptable level of distortion, defined by standards such as IEEE 519-1992.

Figure 1: 5th–harmonic notching of current from a variable-frequency drive

VFD options for mitigating harmonics

Some power utilities now impose penalties for introducing harmonics onto their grid, incentivizing owners to reduce harmonics. You can mitigate harmonic distortions using several methods. A few involve an additional cost in the overall power system, while some techniques provide other benefits outside harmonic mitigation. The commonly used methods and their benefits for harmonic mitigation are as follows:

  1. Line reactors:  AC line reactors will make the current drawn from the power line more sinusoidal. It reduces the input current distortion to 30-40 percent compared to 70-100 percent in case there is no reactor in the drive. The reactors are standard inclusions in drives of 5 hp and above.

    The inclusion of a line reactor or an isolation transformer to attenuate harmonics gives you a low-cost, technically simple solution. However, this solution alleviates only higher-order harmonics and has minimal effect on the 5th and 7th harmonics.

    The typical reactance is either AC or DC, delivering similar harmonic mitigation. You can use AC reactors to provide additional protection to the drive rectifier bridge. Reactors are typically rated from 1.5 percent to 5 percent, with 3 percent being the industry standard due to the diminishing returns delivered by higher levels and associated voltage drop issues.

    Figure 2: Line reactors
  2. Multi-pulse drives (12 and 18-pulse):  You can also use 12-pulse drives and 18-pulse drives to diminish the harmonics. These drives contain multiple rectifiers and an expensive transformer with one primary and multiple secondary. Such configurations cancel some of the lower level, higher amplitude harmonic currents.

    Current distortion at the input terminals is approximately 10 percent for 12-pulse drives and 5 percent for 18-pulse drives. The 18-pulse converter is the most cost-effective solution at 50 hp or higher.

    Figure 3: Multi-pulse drives (12 and 18-pulse)
  3. Active harmonic filters:  Active harmonic filters use smart electronics and IGBTs to inject harmonics of equal amplitude and opposite phases into the power system to correct the harmonics generated by non-linear loads. These systems are installed parallel to the utility lines. Sensors determine and monitor the quantity of corrective current that must be injected into the power system to provide a sinusoidal waveform.

    Active harmonic filters can compensate for harmonic distortion and power factor, and one unit can compensate for multiple non-linear loads. The unit can operate at its maximum current rating without overloading, even if new loads are installed. You can install multiple active harmonic filters in parallel to supply corrective current for more extensive applications. They can function appropriately if voltage imbalances exist. Active harmonic correction filters also can make an excellent retrofit for existing systems because they are shunt-connected devices. However, this technology has the drawbacks of high cost per amp and low energy efficiency.

    Figure 4: Active filter
  4. Passive filters: Passive filters use a capacitive and inductive filter to block harmonics from being transferred to the electrical distribution system. A primary inductor uses relatively high impedance blocks to check the higher-order harmonics. The 5th and 7th order harmonics are reduced using a shunt-connected tuned reactor connected with a capacitor.

    These filters are cheaper than 12- 18-pulse converters and are slightly more tolerant of line imbalances but also have losses associated with using them. The use of passive filters in the installed drives increases the total cost by 200 to 500 percent, depending on the level of mitigation.

    Figure 5: Passive filter
  5. Active front end:  The active front-end drive is a bi-directional power converter for the front end of a typical DC bus drive lineup. It is immune to voltage imbalance and is available in 10–2000 hp. You can use the converter on multiple drives with a single front end. It provides voltage sag ride-through capabilities, a unity power factor, and a regenerative power flow. Although the converter does not affect other harmonics and cannot retrofit into existing drives, it does reduce total harmonics at any load to 2–3 percent THD.

    The active front-end drive is a newer technology for regenerative loads, such as test stands and centrifuges. It requires an inductive-capacitive filter to filter the high-frequency IGBT switching from the line. The added technology introduced by the additional IGBTs increases the cost of the active front-end drive in many applications. Also, the capacitive filter and the IGBTs are not as robust as the simple and reliable magnetics and diode technology of the 18-pulse drives.

    Figure 6: Active front end

    Farnell has a wide array of products to reduce harmonics caused by non-linear loads. You can apply several solutions to meet your harmonic requirements, facility limitations, and budget needs. VFDs enabled with low harmonic solutions such as active front-end technology, 18-pulse arrangements, and active and passive harmonic filters have proven effective and can be configured to meet IEEE 519 standards.


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