Harmonics are electrical disturbances in the input power waveform caused by non-linear loads such as lighting, switch-mode power supplies, and variable frequency drives (VFD). 

In the past few decades, due to the increased deployment of VFDs in the market, harmonics have become a bigger problem in the water and wastewater industry, which has had a positive impact on improving energy efficiency. However, the harmonics generated by these devices must also be managed. If not alleviated, harmonics can cause serious problems in the system, thereby increasing costs. There are several different mitigation methods-each has its own advantages and disadvantages. Multi-pulse drivers, passive filters, active front ends (AFE) and active filters are often the most popular. 

Every system may have harmonics to some degree, but the question is how and how to reduce them. The Institute of Electrical and Electronics Engineers (IEEE) has developed a guide (IEEE 519) that describes the limits of voltage and current distortion caused by harmonics at the point of common coupling (PCC) in electrical systems. The guide has been iterated many times, with 2014 being the most recent. In the most stringent case, IEEE 519-2014 recommends a system total demand distortion (current distortion/demand load current) of 5% and a total voltage distortion of 8%. Current distortion (THiD) affects the transformer in the system, and voltage distortion (THvD) may affect other loads in the system. When considering harmonics, both types of distortion should be considered. 

Harmonics are a system-wide issue, so when calculating harmonics, it is recommended to use the secondary side of the transformer as the PCC instead of calculating based on a single device. Before deciding whether to invest in mitigation measures, it is always recommended to conduct a harmonic analysis. There are a variety of software programs that can be used to calculate harmonics, including Danfoss' free MCT-31. Some signs that harmonics may be a problem include transformers with more than 90% of the load or more than 30% of the total VFD load. If not mitigated, current distortion may cause transformer overload or cable overheating. Voltage distortion may cause other electronic devices or overheated motors to malfunction. 

One of the early harmonic suppression methods was multi-pulse drive. In the United States, 18 pulses are usually 18 pulses, which is a well-known original harmonic suppression method and has been the default solution for decades. These drivers consist of three six-pulse rectifier sections that use transformers for phase shifting. With the increase of 18-pulse driver components, the panel size also increases, and the possibility of failure is also higher. The 18-pulse driver under ideal conditions reduces the current distortion to about 5%. However, when the load drops below about 80% or the voltage is unbalanced, the harmonic suppression will be greatly reduced. 

Recently, passive filters have become one of the most common harmonic suppression methods. The passive filter can be independent or inside a panel with a driver. If the system has a backup generator, please note that the capacitor disconnect should be used with the passive filter; otherwise, there may be a leading power factor, which may cause problems under low load/speed conditions. Passive filters are relatively small, simple to install, and cost-effective, especially at lower power ratings (below a few hundred amperes). As shown in Figure 1, high-quality passive filters reduce IEEE level (5% and 8%) harmonics to less than 60% of the load.

Active front end (AFE) drives originated in industrial applications where electric motors acted as generators. The user needs a way to handle the energy returned from the motor to the drive. Therefore, AFE was created. 

Both the back end and the front end of the AFE driver contain insulated gate bipolar transistors (IGBT). These dual groups of IGBTs allow energy to flow from the motor, through the drive, and then back to the grid. Another benefit of using IGBTs at the front end is that it can reduce harmonics. However, the IGBTs in the driver are one of the components most likely to fail because they are high-speed semiconductor switches that transmit electrical energy thousands of times per second. The energy level can be as high as hundreds of amperes, which generates a lot of heat. Effectively make it a moving part in the VFD, and the moving part will eventually fail first. A large amount of heat is most of the loss in a VFD, and for a standard VFD, it is usually about 3%. Therefore, doubling the number of IGBTs in the driver greatly increases the risk of failure and may increase the total cost of ownership. AFE drivers will meet the IEEE standards of 5% and 8%, but as the voltage imbalance increases or the load decreases, mitigation measures will be reduced.

Figure 1: Mitigation of multiple load points. Passive filter (PhD), AFE, and 18 pulses are compared with the IEEE 519 level as the load decreases for reference.

Figure 1: Mitigation of multiple load points. Passive filter (PhD), AFE, and 18 pulses are compared with the IEEE 519 level as the load decreases for reference.

Active filter is the latest and most advanced harmonic suppression technology. Not to be confused with AFE, the active filter is a separate device, it can be installed in the panel together with the driver, or used as a stand-alone device, thus providing greater implementation flexibility. The active filter will continuously detect the harmonics in the system and inject back signals to mitigate most of the harmonics (Figure 2). They can also be used to mitigate the harmonics of the entire system, not just a single drive, and the size of the active filter can be adjusted to handle harmonics from multiple parallel drives. This can save capital expenditure and physical space, which is increasingly becoming a commodity. 

Unlike most mitigation methods, the active filter is installed in parallel with the driver. Therefore, if there is a problem with the active filter, the drive can still operate. Depending on the system, the active filter can reduce the harmonics to less than 5% when the load is as low as 20%. Even in the case of voltage imbalance, it can maintain a strong mitigation effect, and the filter will complete the power factor compensation to ensure the efficient operation of the system. 

Figure 2: Waveform and active filter. If there is no active filter (left), the waveform that should be a sine wave will be severely distorted by harmonics. Conversely, when using an active filter (right), the sine wave is clearer and current consumption is reduced.

When trying to meet the IEEE 519-2014 voltage and current distortion standards, there are multiple harmonic suppression options available, and each option has its advantages. Higher mitigation brings higher costs, and cost-benefit analysis is essential to determine the best solution for your system. Figure 3 generally compares the cost and mitigation measures of each of the solutions described in this article. 

In addition, depending on the application, there may be different best methods. For installations using low-horsepower VFDs, passive filter solutions are often the best method. At higher horsepower, or when multiple VFDS are installed in one location, active filters can save space and cost, while also providing the best relief for the system. If the application supports the motor to return energy to the grid, an active front-end VFD may be a good solution. Please note that AC line reactors are not discussed as an option in this article, because using these reactors alone usually does not comply with the IEEE 519-2014 standard. In addition, many drives now have internal AC line reactors that are equivalent to DC reactors. The line reactor is equivalent to wearing a basic earphone to protect your ears while the pump is running. Passive filters, multi-pulse drivers, and AFE are equivalent to foam earplugs-the noise is no longer painful, but still noticeable. Active filters are high-end noise-canceling headphones. 

Finally, although harmonic suppression technologies have been around for more than 25 years, they have made tremendous progress in performance and capabilities. Although there is basically no universally perfect solution for all applications, the potential mitigation level, available installation space, and total cost of ownership should be considered when selecting mitigation technologies. 

Figure 3: Cost-benefit relationship. Various harmonic mitigation solutions have different costs and may be better or worse in terms of mitigation. Use Danfoss' free MCT-31 software to carefully analyze your system and calculate harmonics to help you make a decision. Please contact your local Danfoss contact for assistance. 

Edmund Post is a regional sales engineer-Danfoss Transmission Water. He manages municipal water/wastewater sales in North Carolina, South Carolina, Tennessee, Kentucky, Indiana and Southern Ohio. He holds a bachelor's degree in mechanical engineering from the University of Kansas. 

The Water and Waste Digest staff invited industry professionals to nominate the most outstanding and innovative water and wastewater projects they consider to be recognized in the annual reference guide question. All projects must be in the design or construction stage within the past 18 months.

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