Wiktor Wise
Harmonics in electrical systems are a result of the distortion of a waveform caused by the presence of multiple frequencies in systems that utilize non-linear loads. Non-linear loads are electrical equipment with non-linear impedance characteristics, which cause the power supply frequency and waveform to be distorted. The third harmonic is a type of harmonic that is generated in non-linear loads and has a frequency three times that of the fundamental harmonic. This type of harmonic is particularly significant as it causes a sharp increase in the zero-sequence current, increasing the current in the neutral conductor, which can lead to problems in the power system.
| Characteristics | Values |
|---|---|
| Definition | A harmonic of a voltage or current waveform is a sinusoidal wave whose frequency is an integer multiple of the fundamental frequency. |
| Cause | Non-linear loads such as rectifiers, discharge lighting, saturated electric machines, transistors, electrical motors, and non-ideal transformers. |
| Effects | Increase in current in the system, increased heating of the motor core, increased power consumption, and problems in the power system. |
| Frequency | The third harmonic has a frequency that is three times that of the fundamental frequency, i.e., if the fundamental frequency is 50Hz, the third harmonic is 150Hz. |
| IEEE Standards | IEEE 519-2022 provides standards and goals for the design of electrical systems with linear and nonlinear loads, including waveform distortion limits. |
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What You'll Learn
Third harmonic current and neutral current
In a normal alternating current power system, the current varies sinusoidally at a specific frequency, typically 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, but usually not in phase with it. Current harmonics, on the other hand, are caused by non-linear loads. Non-linear loads, such as rectifiers, discharge lighting, or saturated electric machines, draw a current that is not sinusoidal. This non-sinusoidal current waveform can become quite complex, depending on the load and its interaction with the system.
Harmonics in power systems are generated by these non-linear loads. Semiconductor devices like transistors, electric motors, and transformers are all examples of non-linear loads. Other examples include common office equipment such as computers and printers, fluorescent lighting, and battery chargers. Electric motors, however, do not usually contribute significantly to harmonic generation.
In power systems, harmonics are defined as positive integer multiples of the fundamental frequency. The third harmonic, therefore, is the third multiple of the fundamental frequency. One of the major effects of power system harmonics is to increase the current in the system, particularly for the third harmonic, which causes a sharp increase in the zero sequence current. This, in turn, increases the current in the neutral conductor.
In a three-phase system, if the three phases are balanced, they sum to zero, and the size of neutral conductors can be reduced or even omitted. However, a balanced third harmonic current will not add up to zero in the neutral. Instead, the third harmonic will add constructively across the three phases, leading to a current in the neutral conductor at three times the fundamental frequency. This can cause issues if the system is not designed to handle it. For example, in North America, building wiring designs sometimes take advantage of the cancellation of currents in the phases, allowing for a smaller neutral wire. However, harmonic currents created by computers can cause this system to fail, as the third harmonic currents add up instead of cancelling each other out on the neutral wire.
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Non-linear loads and waveform distortion
Harmonics in power systems are generated by non-linear loads. Semiconductor devices like transistors, IGBTs, MOSFETs, diodes, etc. are all non-linear loads. Other examples of non-linear loads 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. However, both motors and transformers will create harmonics when they are over-fluxed or saturated.
Non-linear load currents create distortion in the pure sinusoidal voltage waveform supplied by the utility, and this may result in resonance. The even harmonics do not normally exist in power systems due to the symmetry between the positive and negative halves of a cycle. If the waveforms of the three phases are symmetrical, the harmonic multiples of three are suppressed by delta (Δ) connection of transformers and motors.
When a linear electrical 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. Current 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 necessarily sinusoidal. The current waveform can become quite complex, depending on the type of load and its interaction with other components of the system.
One of the major effects of power system harmonics is to increase the current in the system. This is particularly the case for the third harmonic, which causes a sharp increase in the zero-sequence current, and therefore increases the current in the neutral conductor. This effect can require special consideration in the design of an electric system to serve non-linear loads.
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Triplen harmonics and their effects
Harmonics in electrical power systems are generated by non-linear loads. Examples of non-linear loads include semiconductor devices like transistors, electrical motors, and transformers. Harmonics are classified by the type of signal (voltage or current) and the order of the harmonic (even, odd, triplen, or non-triplen odd).
Triplen harmonics are odd multiples of the third harmonic (h = 3, 9, 15, etc.). They are also known as zero-sequence harmonics, as they are in phase in time for a given frequency or order. Triplen harmonics are of particular concern because the system response is often considerably different from other harmonics. For example, in a three-phase system, the third harmonic will add constructively across the three phases, resulting in a current in the neutral conductor at three times the fundamental frequency. This can cause problems if the system is not designed to handle it, such as overloaded neutrals and telephone interference.
Transformers, especially neutral connections, are susceptible to overheating when serving single-phase loads with high third harmonic content. This can lead to increased heating of the motor core, potentially shortening the life of the motor. Additionally, measuring the current on the delta side of a transformer may not accurately represent the heating effects of triplen harmonics.
To mitigate the impact of triplen harmonics, Delta connections are used as attenuators, or third harmonic shorts, to circulate the current within the Delta connection instead of flowing through the neutral conductor of a Y-Δ transformer (Wye connection). This helps to reduce the risk of dangerous oscillating currents in the neutral wire, which is designed to carry minimal current.
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Power quality and equipment issues
Power harmonics in electrical systems refer to the distortion of a waveform that results from the presence of multiple frequencies in systems that use non-linear loads. Non-linear loads are those in which the current does not have the same waveform as the supply voltage. When non-linear loads draw current from the power supply, they generate harmonic currents that create a distorted waveform, which can cause problems in the power system.
The third harmonic is particularly notable as it causes a sharp increase in the zero-sequence current, leading to an increase in the current in the neutral conductor. This can cause issues if the system is not designed to accommodate this, such as using conductors sized only for normal operation. The third harmonic will add constructively across the three phases, resulting in a current in the neutral conductor at three times the fundamental frequency. This can be mitigated through the use of Delta connections, which act as attenuators, or third harmonic shorts, as the current circulates in the Delta connection instead of flowing in the neutral conductor.
The impact of power harmonics on equipment and power quality can be significant. Electric motors, for example, experience losses due to hysteresis and eddy currents in the iron core, which are proportional to the frequency of the current. As harmonics are at higher frequencies, they produce higher core losses, resulting in increased heating of the motor core. This can lead to reduced lifespan for the motor if not addressed.
Harmonics are also a frequent cause of equipment heating, misfiring in variable-speed drives, and torque pulsations in motors and generators. They can further result in increased energy costs and potential damage to equipment. To address these issues, it is important to conduct routine power quality surveys, including semi-regular measurements, to identify and rectify any problems early on.
To prevent the generation of harmonics, equipment manufacturers often include electronic filters in their devices, ensuring they meet the required EMC standard. Additionally, IEEE 519 standards provide guidelines for voltage and current distortion limits in installations with harmonics, with the most recent update being IEEE 519-2022.
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Design considerations for electric systems
Harmonics in power systems are generated by non-linear loads, such as rectifiers, discharge lighting, electric motors, transformers, and other semiconductor devices. These harmonics can cause a range of issues, including increased equipment heating, misfiring in variable speed drives, and torque pulsations in motors and generators. Third harmonics are particularly problematic as they cause a sharp increase in the zero-sequence current, leading to increased current in the neutral conductor, which can be dangerous. Therefore, when designing electric systems, there are several considerations to keep in mind to mitigate the impact of harmonics, especially third harmonics.
Firstly, it is important to understand the characteristics of third harmonics and how they interact within a three-phase system. In a three-phase system, each phase is 120 degrees apart, and the three phases can theoretically sum to zero, reducing the size of neutral conductors or even omitting them. However, when third harmonics are present, they add constructively across the three phases, resulting in an oscillating current in the neutral conductor, which can be dangerous if not designed to carry higher currents.
To address this issue, Delta connections can be used to cycle the current around the connection instead of combining it into the neutral of a Wye connection. This prevents the third harmonics from adding together and causing excessive current in the neutral conductor. Additionally, it is important to consider the use of harmonic mitigating transformers, such as non-phase-shift transformers, which can effectively treat triplen harmonic currents, including third harmonics. These transformers work by treating the harmonic currents in the secondary windings due to their low zero-sequence impedance.
Another design consideration is the integration of harmonic correction into the equipment. Many modern power electronic devices, such as 12- and 18-pulse VFDs, and active front-end VFDs, already have harmonic correction capabilities. By utilizing these devices, the impact of harmonics can be reduced within the system. Furthermore, active harmonic filters (AHFs) can be employed to monitor the load current in real time and react to changes, including the cancellation of harmonic content. AHF equipment can be implemented at various points within a facility's electrical system, including at the PCC and within distribution and control equipment.
Lastly, a thorough cost-benefit analysis should be conducted to evaluate and select the optimal solution for a specific application. The greatest benefit is typically achieved when harmonic mitigation solutions are placed close to the loads generating excessive harmonic currents. By following these design considerations, the impact of third harmonics in electric systems can be effectively mitigated, ensuring the safe and efficient operation of the system.
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Frequently asked questions
Harmonics in an electrical system refer to the distortion of a waveform that results from the presence of multiple frequencies in systems that utilize non-linear loads.
3rd harmonics, or triplen harmonics, are odd multiples of the fundamental frequency. In other words, the frequency of the 3rd harmonic is three times that of the fundamental harmonic.
3rd harmonics are dangerous because they cause an abnormal increase in the neutral current, which can lead to overheating, power waste, and equipment failure.
