Umair Gaines
Harmonics in electrical systems are a result of distortion in the waveform caused by the presence of multiple frequencies in systems that utilize non-linear loads. Non-linear loads, such as rectifiers, computers, printers, and variable-speed drives, draw currents in abrupt pulses, creating harmonic currents that distort the waveform and lead to power problems. This can cause issues such as flickering lights, increased heating in equipment, and unexpected resonances. The state of harmonics in a system can be expressed as Total Harmonic Distortion (THD), which is the sum of all harmonic effects. To mitigate harmonics, filters can be added to block or redirect harmonic currents, and transformer connections can be utilized to reduce harmonic currents in three-phase systems.
| Characteristics | Values |
|---|---|
| Definition | Harmonics in electrical systems are distortions in a waveform that result from the presence of multiple frequencies in systems that utilize non-linear loads. |
| Causes | Non-linear loads, such as rectifiers, power converters, electronic devices, and high-frequency fluorescent lights. |
| Effects | Increased current in the system, particularly the third harmonic, which can cause a sharp increase in the zero-sequence current and increase heating in equipment. Other effects include malfunctions of motors and generators, added efficiency losses, unexpected resonances, and disturbances in electronic equipment. |
| Mitigation | Adding filters to block or siphon off harmonic currents, modifying the frequency response with filters/inductors/capacitors, providing transformer connections, and changing capacitor size. |
| Measurement | K-factor, Total Harmonic Distortion (THD), and IEEE 519 Standards are used to measure and address harmonic issues. |
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What You'll Learn
Causes of harmonics in electrical systems
Harmonics in electrical systems are caused by non-linear loads. In a normal alternating current power system, the current varies sinusoidally at a specific frequency, usually 50 or 60 hertz. When a linear load is connected to the system, it draws a sinusoidal current at the same frequency as the voltage, although not always in phase with the voltage. However, when a non-linear load, such as a rectifier, is connected to the system, it draws a current that is not sinusoidal, causing waveform distortion. This distortion can be quite complex, depending on the type of load and its interaction with other components of the system.
Non-linear loads can include semiconductor devices like transistors, IGBTs, MOSFETs, and diodes. Other examples include common office equipment such as computers and printers, fluorescent lighting, battery chargers, and variable-speed drives. Electric motors do not normally contribute significantly to harmonic generation, but they can create harmonics when they are over-fluxed or saturated.
In an industrial environment, the causes of harmonic distortion are most often the electrical equipment in operation. Modern industrial plants contain many pieces of equipment that may contribute to overall distortion, including variable frequency drives and electrical motors driven by inverters. Additionally, machinery with rapid fluctuations in load current or voltage, such as large motors during startup, machinery with cyclo-converters, and machines using static frequency converters, can cause flickering lights, indicating a potential power quality issue.
Harmonics can also be caused by transformers under no load or light loads, saturated reactors, thyristor-controlled motor drives, arc furnaces, arc welders, conduction furnaces, high-pressure mercury vapour lamps, energy conservation devices, rectifiers, UPS systems, static VAR compensators, and HVDC transmission systems. These sources of harmonics can degrade power system performance, reliability, and safety, and may require corrective measures to be taken.
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Effects of harmonics on equipment
Harmonics in electrical systems can have detrimental effects on equipment. Harmonics are caused by non-linear loads, such as rectifiers, semiconductor devices, and common office equipment, which create distortions in the pure sinusoidal voltage waveform. These distortions can lead to several issues in electrical equipment:
Firstly, harmonics can increase the current in the system, particularly the third harmonic, resulting in a sharp increase in the zero-sequence current and the current in the neutral conductor. This requires special consideration in the design of electric systems serving non-linear loads.
Secondly, electrical equipment such as electric motors can experience losses due to hysteresis and eddy currents in their iron cores. As harmonics are at higher frequencies, they produce higher core losses, resulting in increased heating of the motor core. Excessive heating can reduce the lifespan of the motor.
Harmonics can also cause problems in transformers, leading to overheating and energy wastage. In some cases, harmonic distortions may even result in catastrophic damage to transformers. Additionally, capacitors in power systems with large voltage or current harmonics may experience failures due to exceeding their limitations.
Furthermore, harmonics can cause issues in variable-speed drives, such as misfiring, and torque pulsations in motors and generators. In a harmonic-rich environment, both series and parallel resonance occur, and during resonant conditions, there is a high probability of damage to capacitor banks and other electrical equipment.
To mitigate these issues, it is essential to perform harmonic surveys and measurements to identify the sources of harmonic distortion and implement suitable solutions, such as filters or system upgrades.
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Troubleshooting power quality issues
Harmonics are a major cause of power quality issues. They are currents or voltages with frequencies that are integer multiples of the fundamental power frequency, which is usually 50 or 60 Hertz. Harmonics are caused by non-linear loads, such as rectifiers, power converters, and electronic devices like computers and printers. These non-linear loads draw current in abrupt pulses, creating distorted current waveforms and causing harmonic currents to flow back into other parts of the power system. This can result in resonance and increased heating of the motor core, potentially shortening its life.
To determine if you have a harmonics problem, you need to measure the true-rms value and the instantaneous peak value of the wave shape using specialised equipment such as clamp meters, multimeters, or power quality analysers. Before taking any measures to address harmonics issues, it is recommended to consult a power quality expert for tailored advice.
Some methods to reduce harmonic voltage and current distortion from non-linear loads include the use of line side reactors, broadband filters, and voltage regulators. Line side reactors help prevent noise and interference in sensitive equipment and reduce flicker in fluorescent lighting. Broadband filters are useful in water and pumping systems with adjustable frequency drives (AFDs). Voltage regulators can be used to maintain a constant voltage level, reducing the impact of voltage fluctuations on the system.
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Harmonic distortion and waveform analysis
Harmonics are a form of voltage or current waveform distortion. Harmonics occur when the current or voltage waveforms deviate from a purely sinusoidal waveform. In a normal alternating current power system, the current varies sinusoidally at a specific frequency, usually 50 or 60 hertz. When a linear load is connected to the system, it draws a sinusoidal current at the same frequency as the voltage. However, when a non-linear load, such as a rectifier, is connected to the system, it draws a current that is not sinusoidal, causing waveform distortion.
Harmonics are sinusoidal waveform components with frequencies that are integer multiples of the fundamental frequency. These harmonics can combine with the fundamental frequency to create a distorted waveform. The degree of distortion caused by harmonics can be measured using Total Harmonic Distortion (THD), which represents the sum impact of all harmonics within an electrical system. A high THD indicates excessive distortion, which can reduce the efficiency and lifespan of electrical equipment, leading to increased costs and reliability issues.
Harmonics are typically generated by non-linear loads, such as semiconductor devices (transistors, IGBTs, MOSFETs, diodes), office equipment (computers, printers), fluorescent lighting, battery chargers, and variable-speed drives. Electric motors and transformers can also create harmonics when they are over-fluxed or saturated. These non-linear loads introduce harmonics that distort the pure sinusoidal voltage waveform supplied by the utility, resulting in resonance and increased heating in the system.
To address harmonic distortion, power quality surveys and measurements are essential. By regularly monitoring and evaluating power quality, potential problems can be identified early, and solutions can be implemented to maintain equipment health and optimize energy usage. One method to analyse waveform distortion is by using mathematical techniques, such as the Fourier series transform, which can deconstruct a complex waveform into a series of simple sinusoids, allowing for a detailed understanding of the distortion and its sources.
Additionally, certain techniques and components can be employed to mitigate the effects of harmonics. For example, delta connections are used to reduce the impact of third-order harmonics by redirecting the current away from the neutral conductor. K-factor rated transformers can also be used to address the heating effects caused by harmonics, although they come with higher costs and installation downtime. Overall, a comprehensive approach to harmonic distortion analysis and management is crucial for maintaining power quality and the longevity of electrical equipment.
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Reducing harmonics with filters
Harmonics in power systems are caused by non-linear loads. These include semiconductor devices like transistors, IGBTs, MOSFETs, diodes, and common office equipment such as computers and printers, as well as fluorescent lighting, battery chargers, and variable-speed drives. Electric motors do not usually contribute significantly to harmonic generation, but they can create harmonics when they are over-fluxed or saturated. Non-linear load currents distort the pure sinusoidal voltage waveform supplied by the utility, and this can result in resonance.
Harmonics can also cause an increase in current in the system, particularly the third harmonic, which increases the current in the neutral conductor. This can lead to higher core losses in a motor than the power frequency, resulting in increased heating of the motor core and potentially shortening the life of the motor.
To reduce harmonics with filters, harmonic filters are series or parallel resonant circuits designed to block harmonic currents and reduce harmonic voltage distortion in the system. Passive filters are often used to reduce lower-order harmonics, while active filters deal with higher-order harmonics. Passive filters are also used when the supply voltage is disturbed. An LC circuit, tuned to each harmonic order to be filtered, is installed in parallel with the non-linear load. This bypass circuit absorbs the harmonics, preventing them from flowing in the distribution network.
Hybrid filters combine passive and active filters to offer a balance between cost-effectiveness and efficiency. They are made up of capacitors and reactors with parallel resistances to provide relatively low Q-tuned circuits. Damped filters are used to control higher-order harmonics as a group and are typically tuned between corresponding pairs of harmonics to provide maximum harmonic reduction at those frequencies.
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Frequently asked questions
Harmonics in electrical systems refer to the distortion of a waveform that results from the presence of multiple frequencies in systems that utilize non-linear loads.
Harmonics are caused by non-linear loads. When a non-linear load, such as a rectifier, is connected to the system, it draws a current that is not sinusoidal. Semiconductor devices like transistors, IGBTs, MOSFETs, diodes, etc. are all non-linear loads.
Harmonics can have several negative effects on an electrical system, including malfunctions of motors and generators, added efficiency losses, unexpected resonances, and disturbances in electronic equipment. Harmonics can also cause increased heating in equipment and conductors, misfiring in variable-speed drives, and torque pulsations in motors.
Harmonics in an electrical system can be reduced by adding filters to either siphon the harmonic currents off the system or supply the harmonic currents locally. Another method is to modify the frequency response of the system with filters, inductors, or capacitors.
