Harmonics are voltage or current waveforms that are distorted from their fundamental sinusoidal shape. They are multiples of the fundamental frequency (50 or 60 Hz) and are generated by non-linear electrical loads. These are loads that draw current in abrupt pulses rather than in a smooth sinusoidal manner. Common sources include:
- Variable Frequency Drives (VFDs): Used in HVAC systems, pumps, and fans, VFDs can introduce significant harmonic distortion.
- Computers and Office Equipment: Devices with switch-mode power supplies, such as computers, printers, and copiers, are major contributors.
- LED Lighting: Modern lighting solutions, particularly those with electronic ballasts, can generate harmonics.
- Uninterruptible Power Supplies (UPS): While essential for power backup, UPS systems can also introduce harmonics into the electrical network.
- Industrial Equipment: Arc furnaces, welders, and other heavy industrial equipment can create high levels of harmonic distortion.
Effects of Harmonics on Commercial Buildings
Harmonics can have several detrimental effects on commercial buildings, impacting both electrical systems and the efficiency of the facility. They can cause additional heating in transformers, motors, and cables due to increased losses, which can lead to premature aging and failure of electrical equipment. Electrical systems operating with high harmonic distortion are also less efficient, leading to higher energy consumption and increased operational costs. Harmonics can interfere with the operation of sensitive electronic equipment, leading to malfunctions or unexpected shutdowns. In three-phase systems, the third harmonic and its multiples can potentially cause overheating and fire hazards. Lastly, harmonic currents can cause protective devices to trip unnecessarily, disrupting operations and increasing maintenance costs.
The term "mains borne harmonic currents" appears regularly in literature on the topic of harmonics. It refers to harmonic currents that are present and travel through the main electrical power supply system (or the "mains").
Harmonic currents occur at multiples of the fundamental frequency. For example, if the fundamental frequency is 50 Hz, a third harmonic current would have a frequency of 150 Hz.
Mains borne harmonics are being carried by the main power supply lines, meaning they are circulating through the building's electrical system or even spreading into the broader electrical grid.
In the context of harmonics, the term NP typically refers to the number of pulses in a rectifier or power converter system. The harmonic order (i.e., the frequency of the harmonic as a multiple of the fundamental frequency). Harmonics are often found just above and below multiples of the number of pulses. For a 6-pulse rectifier, harmonics would occur at the 5th and 7th harmonics, then at the 11th and 13th, and so on.
It's worth noting that not all systems will require filters to address harmonics. For example, if a system has only a low power pump, on a percentage basis, the harmonics produced will not be significant enough to require extra measures. With a larger number of pumps, and/or larger horsepower motors, the common 30 to 40% current harmonics represents a larger total harmonic distortion and a more significant challenge to the system.
Here are some common methods to reduce or mitigate harmonics:
Passive Filters
Passive filters are circuits made from inductors, capacitors, and sometimes resistors. They are designed to absorb or block specific harmonic frequencies from circulating in the electrical system. These filters are tuned to the harmonic frequencies present in the system and prevent those harmonic currents from entering or propagating through the mains. Passive filters are best used when harmonics are at known, specific frequencies, such as in systems with regular non-linear loads like motor drives or industrial equipment.
Active Filters
Active harmonic filters can dynamically monitor a system and inject opposing currents to cancel out harmonics. These filters use real-time electronics to identify and counteract harmonics, improving overall power quality. They are effective across a wide range of harmonic frequencies. Active filters are best used in environments with fluctuating loads or where there are multiple sources of harmonics across different frequencies, such as in commercial buildings or data centers.
Harmonic Mitigating Transformers (HMTs)
HMTs are transformers specifically designed to reduce harmonics by using phase-shifting techniques. These transformers reduce triplen harmonics (such as 3rd, 9th, and 15th) using phase cancellation. HMTs are best used in systems with significant harmonic generation from multiple non-linear loads, such as office buildings or manufacturing plants.
Line Reactors
Line reactors are inductors installed in series with equipment to limit the rate of current change and reduce the harmonics entering the system. By increasing the impedance in the circuit, line reactors slow the rate of current rise, effectively smoothing out harmonic components. Line reactors are best used in systems with variable frequency drives (VFDs) or other power electronics where harmonics are a byproduct of fast switching.
K-Rated Transformers
K-rated transformers are designed to handle the additional heat generated by harmonic currents. These transformers are built with more robust materials and higher thermal limits to safely operate in systems with higher harmonic distortion.
They are most effective in environments where harmonic distortion cannot be fully eliminated but the infrastructure needs to be protected - such as in data centers or hospitals with sensitive equipment.
Power Factor Correction (PFC) Equipment
PFC devices are often combined with harmonic filtering to improve power factor and reduce harmonic content in the system. By balancing reactive power and addressing harmonic distortion at the source, PFC equipment can both improve efficiency and lower the harmonic currents in the mains. This type of solution is best used in commercial or industrial facilities where a combination of reactive and non-linear loads are present.
In facilities where power quality is critical, such as hospitals, data centers, and large industrial plants, regular monitoring of the electrical system using power quality analyzers can help detect and mitigate harmonic issues before they cause significant damage. By identifying where harmonics are being generated and how they affect the system, targeted solutions can be applied.
Mitigating mains borne harmonic currents can improve power quality, reduce energy losses, and extend the life of electrical equipment. A combination of solutions may be necessary depending on the source and severity of the harmonics. Regular monitoring and targeted interventions, such as using filters or balancing loads, can effectively manage these currents.
As a valuable and important mitigating factor against harmonics, Design Envelope solutions include built-in AC line reactors (The most common line reactors have either 3% or 5% impedance).
While a harmonics filter does offer some protection for the drive, its main purpose is to minimize the drive’s emissions of electromagnetic interference, thereby protecting other devices and maintaining overall system performance.
The filter reduces the electromagnetic interference (EMI) emitted by the drive. By filtering out high-frequency noise that the drive might generate, the filter helps to meet electromagnetic compatibility (EMC) standards and prevent disturbances in the surrounding environment.
Many regulatory standards require electronic equipment, including drives, to limit the amount of EMI they produce. A filter helps the drive comply with these standards, thereby avoiding issues related to electromagnetic compatibility and ensuring that the drive does not disrupt other equipment.
If you have questions about harmonics in your mechanical system, and how you can assess the effect they may be having on components, please contact your local Armstrong representative or Regional Sales Manager.
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