Harmonics in electrical systems are caused by nonlinear components, which distort the sine wave of the electrical current. As power electronics become more common, voltage distortion, or harmonics, also increases. Devices like variable frequency drives (VFDs), uninterruptible power supplies (UPS), and inverters introduce different types of harmonics into the system. These harmonics can lead to problems like device failures, overheating, and shorter equipment life. Using harmonic filters can help reduce or eliminate these harmful harmonics before they cause damage.
Understanding Harmonics in Electrical Power Systems
Harmonic currents result from the nonlinear behavior of electrical devices. These currents, and the harmonic voltages they cause, come from various sources that can vary greatly in size. Historically, devices like transformers or generators were significant sources of harmonics. Today, with the focus on energy efficiency, reducing harmonics is still important for many industries, such as water treatment and oil and gas.
Many facilities need to meet strict operational and environmental standards, often running 24/7. Most power problems within these facilities originate from the plant itself. Harmonics, which are common in these systems, can cause anything from minor annoyances to severe equipment failures. Filtering out these harmonics, either through active or passive solutions, is crucial for improving equipment performance and ensuring the reliability of the facility’s electrical system.
IEEE 519-2014 and Harmonic Filtering
The IEEE 519-2014 standard is a widely recognized guideline that specifies the maximum allowed levels of current and voltage distortion at the point of common coupling (PCC). These limits depend on the system's load and short-circuit current.
Using ECOsine® passive and active harmonic filters and reactors in the electrical system can bring harmonics down to levels that meet most international standards. These filters help reduce the load on lines and transformers upstream of nonlinear loads, which lowers system losses and operating temperatures. They also improve the power factor, keeping it close to unity even when the load is not at full capacity.
Impact of Harmonics on Equipment
Frequency inverters are commonly used for controlling AC motors in various applications like pumps, fans, and HVAC systems. These inverters often use high-speed semiconductor switching techniques that can create harmonic problems.
High-power applications like arc furnaces and welding equipment also generate significant harmonic distortion due to their nonlinear operation. This can lead to unusual harmonic patterns and challenges for the electrical system.
Sensitive electronic components, such as those in CNC machines, are particularly vulnerable to power system imperfections caused by harmonics. These issues can lead to equipment malfunctions, production delays, and quality problems, which can have financial consequences for the company.
Variable speed drives, often used in pumps and fans, contribute to harmonic distortion, which can overload the electrical infrastructure and cause malfunctions in sensitive equipment. In water treatment facilities, powerful VFD-driven air compressors can create high levels of harmonic distortion, leading to serious problems.
Harmonics can severely impact the electrical power network, leading to failed components, tripped breakers, blown fuses, overheating, insulation breakdown, and reduced equipment lifespan. These issues can cause production downtime and costly repairs, reducing company profits.
To fully understand and address harmonic issues, a site survey or engineering study may be needed. Many facilities follow the IEEE 519-2014 guidelines to ensure acceptable distortion levels. Reviewing the entire system, including any planned or new equipment, is essential for effective harmonic mitigation.
Passive Harmonic Filters
One way to reduce harmonics is by using passive harmonic filters. These filters are usually installed on a one-to-one basis, with one filter for each VFD. In some cases, a larger filter can handle multiple drives.
The best place to eliminate harmonics is directly at the source—the individual nonlinear load. A passive filter provides a low-resistance path for harmonic currents, significantly reducing the harmonics in the electrical system. This helps the system draw cleaner, sinusoidal current from the power source.
Installing passive harmonic filters immediately benefits the system by reducing harmonic amplitudes, lowering losses, and improving equipment efficiency and reliability. These filters also help maximize the capacity of the electrical system.
Passive filters can reduce total harmonic current distortion to acceptable levels, meeting IEEE 519 standards. They are designed to perform well under both full and partial load conditions.
Passive filters come in two types: those reducing total harmonic distortion to less than 5% (THDi) and those reducing it to 7-10% THDi. While IEEE 519 is often , sometimes a 7% or 8% improvement is sufficient for the end user. Generally, passive filters are more economical than active filters but may not address a wide range of harmonics.
Active Harmonic Filters
Another method to reduce harmonics is using active harmonic filters (AHF). These devices monitor the nonlinear load and dynamically inject a counter-current to cancel out harmonics. This keeps the current supplied by the power source sinusoidal and reduces harmonic distortion to below 5% THDi, meeting standards.
AHFs are designed to quickly adapt to load changes, handling harmonics up to the 50th order. They operate across a wide frequency range, adjusting as needed to the harmonic spectrum.
In addition to reducing harmonics, active harmonic filters can improve power factor by compensating for reactive current. They also balance phase loads. AHFs are compact, efficient, and can be installed at any point in a low-voltage AC network. They offer more functionality than passive filters, making them suitable for various applications.
AHFs are versatile and can be used for single or multiple nonlinear loads. They are especially useful in environments where traditional power factor correction methods are insufficient due to high harmonic content.
Example of Active Filter Application
In one application, power factor correction equipment suffered from losses due to harmonics. Installing an AHF with a 500 A compensation current provided the needed electrical and thermal relief, improved power quality, and made it possible to add a backup generator. The AHF’s ability to adapt to changing grid and load conditions ensured the continuous operation of the wastewater treatment plant.
Final Thoughts
After identifying harmonic problems, it's important to review and implement the right solution. Considering the many issues caused by harmonics—like equipment failure, maintenance costs, and production downtime—keeping records of these costs can help justify the investment in mitigation equipment and calculate the return on investment (ROI).
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