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Variable Frequency Drives (VFDs) are commonly used to control motor speed and optimize energy consumption in industrial systems.
However, VFDs work as nonlinear loads. This means they draw currents that are not smooth, even if the supply voltage is a perfect sine wave.
This happens because VFDs change AC power into DC. Then, they convert it back into a modified AC signal. This process helps control motor speed.
Despite advancements in technology, such as multipulse converters (12-, 18-, or 24-pulse systems), active front-end converters, passive harmonic filters, or line reactors, harmonic currents can still flow through the system, causing voltage distortion. These harmonic currents are problematic because they affect the overall quality of the power supplied to other equipment, potentially leading to operational disruptions.
Overheating of transformers and motors can happen because of extra currents. This heat can damage insulation and shorten the life of electrical parts.
Malfunctioning of electronic instruments, sensors, or meters: Harmonic distortion can interfere with the accurate operation of sensitive devices.
Overloading of power factor correction capacitors: The presence of harmonic currents can cause capacitors to operate outside their rated capacity, leading to potential failures.
Premature failure of circuit breakers can happen when there are too many harmonic currents. These currents can make breakers trip or wear out faster. This is because the breakers cannot handle these uneven loads.
Harmonics disrupt the normal operation of power delivery systems. Nonlinear loads generate both voltage and current harmonics that adversely affect equipment like transformers, cables, and circuit breakers. These distortions increase heating losses, making power systems less efficient and more prone to failure. The effects can be immediate or delayed, depending on the type of equipment affected.
Voltage distortion caused by harmonics can significantly affect power delivery, especially in facilities with high automation. Even small levels of distortion (as low as 2%) can disrupt operations in critical systems. Voltage notching and waveform distortion from harmonic currents can harm sensitive devices. This includes solid-state equipment, timing systems, and controllers.
The issue is even more pronounced in systems powered by standby generators. These generators have higher reactive impedance, between 15% and 20%. In contrast, utility transformers have lower impedance, from 2% to 5%. This difference can increase voltage drop and harmonic distortion effects.
Generators with higher internal impedance may have trouble handling harmonic loads. This can affect their ability to keep stable voltage levels.
To mitigate these effects, larger generators with reduced impedance can help manage harmonic loads more effectively. Proper harmonic analysis is important. It helps make sure that both utility and generator systems can manage harmonic distortion. This way, power quality is not harmed.
Harmonic-related problems often become noticeable when VFDs are first energized. At first, the issues may seem minor and go undetected. However, as people add more VFDs, motors, and power factor correction systems, these issues can escalate.
Over time, symptoms like equipment overheating, malfunctions, and distorted voltage waveforms become clearer. This shows the need for active harmonic management. Without proper intervention, these problems can result in expensive repairs, downtime, and even equipment failure.
Modern Facilities with Sensitive Electronics: Locations such as pharmaceutical manufacturing plants, research laboratories, and chemical processing facilities are highly susceptible to harmonic distortion. These sites depend on precise control systems.
These include temperature regulators and electronic monitors. Even small power quality issues can cause operational disruptions. Clean and stable power is critical for ensuring the accuracy of processes and measurements.
Facilities with Mixed Loads: Hospitals, airports, telecom centers, and banks often have a mix of loads. These include diagnostic devices, HVAC systems, lighting, and computers.
When nonlinear loads predominate, they introduce harmonic distortion into the AC power supply, affecting the performance of connected equipment. These facilities are especially at risk if they use standby generators. The higher impedance of these systems can make harmonic issues worse.
VFD-Driven Systems: VFDs inherently draw nonlinear currents, which create harmonic distortion across the system. These currents can lead to voltage spikes, particularly at motor terminals, when combined with long feeder cables.
Voltage peaks can exceed 1,500 V on a 480 V system, which stresses motor insulation and accelerates wear. To prevent this, engineers often add impedance load reactors. These are usually 3% or 5%. They install them on the VFD’s load side when motor leads are longer than 110 feet.
Generator-Connected Systems: Systems powered by standby generators require careful planning to handle the nonlinear loads produced by VFDs. Oversized alternators, similar to those used in soft-start systems, can improve generator performance under such loads.
Using technologies like 12-pulse insulated-gate bipolar transistors (IGBTs) can improve how variable frequency drives (VFDs) work with generators. Harmonic filters can also help with this. This is done by reducing harmonic effects.
Critical Alarm Systems: Alarm systems, like fire and burglar alarms, are sensitive to voltage distortion. This is especially true in weak AC systems. These harmonics especially affect hospitals and critical infrastructure facilities that rely on alarms for safety and security. Managing harmonics is critical to ensuring the reliability of these systems.
To manage harmonic problems effectively, a harmonic analysis should be performed early on. You can conduct this assessment in collaboration with the utility or through an independent power quality consultant. It is important to follow the IEEE 519 guidelines for harmonic limits at your facility. This helps ensure compliance and avoid problems.
You can use power quality meters to monitor harmonic levels and identify distortion in current waveforms. Monitoring the health of secondary transformers, standby generators, and sensitive electronics is essential to detect potential problems before they escalate.
To address existing harmonic issues, solutions include active harmonic filters, line reactors, and upgrading to 12- or 18-pulse VFDs. YT Electric designs its active filters to cancel harmonic currents. This helps improve power factor and boosts overall system efficiency. Additionally, impedance load reactors can reduce voltage peaks and protect motors from long-feeder cable issues.
For new installations, engineers should design systems that account for harmonic distortion. Using 12-pulse or 18-pulse rectifiers is a reliable method to reduce harmonic distortion significantly, ensuring long-term operational stability.
YT Electric’s advanced solutions offer a reliable approach to addressing harmonics, improving power system efficiency, and safeguarding sensitive equipment.
YT Electric is the biggest OEM manufacturer of low-voltage AHF and SVG with more than 15 years' experience. All products hold certifications for ISO9001, CE, and CQC standards, and type test reports support them.
For more information on how our SVGs can improve power quality: contact us at sales@ytelect.com.
keywords: harmonic mitigation, electrical system, distribution system, point of common coupling
loads draw, linear loads, line voltage, total harmonic distortions thd, dc bus, homogenous mixture, liquid solutions, fundamental frequency
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