In power distribution management, "harmonics" is a frequently mentioned term, but many people do not have a deep understanding of them. This article will provide readers with a comprehensive and detailed introduction from three aspects: the definition of harmonics, the hazards of harmonics, and methods for harmonic mitigation.
1. Basic Concepts
In AC power grids, due to the operation of many nonlinear electrical devices, the voltage and current waveforms are not actually perfect sinusoidal waveforms, but rather non-sinusoidal waves with varying degrees of distortion. Fourier series decomposition of periodic AC quantities yields components with the same frequency as the power frequency, called the fundamental frequency; components with frequencies greater than integer multiples of the fundamental frequency, called harmonics (HR); and components with frequencies not equal to integer multiples of the fundamental frequency, called interharmonics (IHR). Any repeating waveform can be decomposed into sinusoidal components containing the fundamental frequency and a series of harmonics that are multiples of the fundamental frequency.
Harmonics, as a special component of electric current, typically have frequencies that are integer multiples of the fundamental frequency. After decomposing a periodic non-sinusoidal electrical charge into a Fourier series, we find that in addition to the charge at the fundamental frequency, there are other current components with frequencies higher than the fundamental frequency; these components are called harmonics. Through Fourier transform, we can observe that a periodic non-sinusoidal wave is formed by the superposition of the fundamental frequency and sinusoidal waves with frequencies that are integer multiples of the fundamental frequency. Harmonics are high-frequency components in electric current, with frequencies that are integer multiples of the fundamental frequency; these high-frequency components can be decomposed using Fourier transform.
In industrial production, we expect to obtain pure power at the power grid frequency. However, in reality, harmonics are generated at every stage of the power system—generation, transformation, transmission, distribution, and consumption—wherever current passes through a nonlinear load. Nonlinear loads in the power system lead to the generation of harmonics, affecting the power quality of the grid. These harmonics have a profound impact on the power system, and their harm cannot be ignored.
The fundamental frequency is the power grid frequency (50Hz). The harmonic order (h) is the integer ratio of the harmonic frequency to the fundamental frequency, and the intermittent frequency (ih) is the ratio of the intermittent frequency to the fundamental frequency. For example, if the fundamental frequency is 50Hz, the second harmonic is 100Hz, and the third harmonic is 150Hz.
Harmonics are classified according to phase sequence into positive sequence harmonics (4th, 7th, 10th, ..., 3h+1th), negative sequence harmonics (2nd, 5th, 8th, ..., 3h-1st), and zero sequence harmonics (3rd, 6th, 9th, ..., 3hth). They are also classified according to their order into even harmonics, odd harmonics, and intermittent harmonics (non-integer harmonics).
Generally speaking, odd harmonics cause more and greater damage than even harmonics. In a balanced three-phase system, due to the symmetry, even harmonics are eliminated, and only odd harmonics exist. For a three-phase rectified load, the harmonic currents are 6n±1 harmonics, such as 5th, 7th, 11th, 13th, etc. Frequency converters mainly generate 5th and 7th harmonics.
2. Harmonic sources and harmonic current values generated by commonly used equipment
Over the past 30 years, the application of power electronic devices has become increasingly widespread, making them the largest source of harmonics.
Electrical equipment that injects harmonic current into the public power grid or generates harmonic voltage in the public power grid is collectively referred to as a harmonic source. Common harmonic sources mainly include converter equipment, electric arc furnaces, iron core equipment, lighting equipment, and certain household appliances, which are nonlinear electrical equipment.
3. The hazards of harmonics
An ideal public power grid should provide a single, stable frequency and a specified voltage amplitude. However, the harm caused by harmonics to the public power grid and other systems cannot be ignored, mainly in the following aspects:
When harmonic current flows through a transformer, it significantly increases core losses, causing the transformer to overheat and thus shortening its service life.
When harmonic currents pass through an AC motor, they not only increase the loss of the motor core, but also cause rotor vibration, which seriously affects the quality of machined products.
Capacitors have very low impedance to high-order harmonics, so when a voltage containing high-order harmonics is applied across the capacitor, the capacitor can easily be overloaded and damaged.
Harmonic currents can lead to increased power loss in power lines.
Harmonics can cause voltage resonance in power systems, generating overvoltages and potentially damaging the insulation of line equipment. When a large amount of third harmonics flows through the neutral conductor, the line may overheat or even start a fire.
Harmonics can cause malfunctions in the system's relay protection and automatic devices, and also lead to inaccurate measurements by electrical measuring instruments.
The additional magnetic field interference generated by harmonics can affect the normal operation of electronic instruments and communication systems, thereby reducing communication quality.
01 Overview of Harmonic and Interference Issues
◆ Introduction and Harmonic Generation
With the widespread application of frequency converters in motor systems, the interference problems they bring have gradually become prominent. Due to their nonlinear rectifier and switching devices, frequency converters generate harmonics, affecting the power grid and surrounding equipment. The operating characteristics of frequency converters, such as the use of nonlinear rectifiers, lead to harmonic generation, causing conducted interference to the power grid, inducing voltage distortion, and thus affecting power supply quality. Simultaneously, the switching devices used in their output section also generate electromagnetic radiation interference while outputting energy, affecting surrounding electrical appliances.
◆ Hazards of Harmonics and Electromagnetic Radiation
The presence of harmonics poses numerous hazards to power grids and other systems. First, it increases additional harmonic losses in electrical components, leading to decreased equipment efficiency, potential interference or even damage to equipment, and impacting the accuracy of metering and control systems. Second, harmonics are conducted through the power grid to other electrical appliances, interfering with their normal operation and potentially damaging critical equipment such as transformers. Furthermore, harmonics can cause series or parallel resonances in the power grid, further amplifying the harmonics. More seriously, harmonic or electromagnetic interference can cause relay protection devices to malfunction, affecting the accuracy of electrical instrument readings and even leading to control system malfunctions.
◆ Harmonic standards and domestic and international standards
Current relevant standards include IEC, EN, IEEE standards, and the Chinese national standard GB/T14549-93, providing guidance for harmonic mitigation. These mainly include: international standards IEC61000-2-2 and IEC61000-2-4, European standards EN61000-3-2 and EN61000-3-12, and the IEEE recommendation standard IEEE519-1992. The domestic standard GB/T14549-93 also makes clear provisions for harmonic issues. These standards not only provide a basis for harmonic mitigation but also offer important references for understanding harmonic interference problems from frequency converters.
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◆ Harmonic Interference Countermeasures
To reduce the impact of inverter harmonics on other equipment, several measures can be taken. One effective method is to add AC/DC reactors. By adding reactors at the inverter's input line, the THDv of the input current can be significantly reduced, thereby reducing the impact of harmonics on the power grid and other equipment. Reactors, passive filters, and isolation transformers can be used to reduce harmonics, while simultaneously strengthening anti-interference measures for signal lines. Passive filters are also an effective solution; after use, the THDv in the input line can be reduced to 5%~10% under full load. Furthermore, adding an AC reactor between the inverter and the motor can also reduce electromagnetic radiation.
To address the interference issues caused by inverter harmonics and electromagnetic radiation, in addition to the measures mentioned above, isolation transformers can be used to combat conducted interference. Isolation transformers block conducted interference and also transform the power supply voltage. They are commonly used for anti-interference protection of low-voltage, low-power equipment such as instruments and PLCs in control systems. Furthermore, filter modules or components are also important means of improving the anti-interference capability of equipment.
To further reduce interference on signal lines, especially electromagnetic radiation from space, RC filters or dual-T filters can be added to the input circuit to suppress normal interference. At the same time, the length of the input line should be minimized, and close contact between signal lines and power lines should be avoided to prevent signal distortion.
In conclusion, appropriate measures should be selected based on the actual situation during the anti-interference process. The effectiveness of the anti-interference measures must be considered, along with cost and on-site conditions. Excessive anti-interference measures may introduce new interference problems; therefore, a flexible approach based on specific circumstances is necessary.
The definition of power supply system harmonics is to perform Fourier series decomposition on periodic non-sinusoidal electrical quantities. In addition to obtaining components with the same frequency as the fundamental frequency of the power grid, a series of components with frequencies higher than the fundamental frequency of the power grid are also obtained. This part of the electrical quantity is called harmonics.
The ratio of the harmonic frequency to the fundamental frequency (n=fn/f1) is called the harmonic order. Sometimes, non-integer multiples of harmonics also exist in power grids; these are called non-harmonics or fractional harmonics.
For the 50Hz power supply used in my country, the fundamental frequency is 50Hz, the third harmonic is 150Hz, the fifth harmonic is 250Hz, and so on.
To address the harmonic pollution problem from power electronic devices and other harmonic sources, there are two basic approaches: one is to install harmonic compensation devices to compensate for harmonics, which is applicable to all types of harmonic sources; the other is to modify the power electronic devices themselves so that they do not generate harmonics and the power factor can be controlled to 1, which of course only applies to power electronic devices that are the main harmonic source.
The traditional method for installing harmonic compensation devices is to use LC tuned filters. This method can compensate for both harmonics and reactive power, and its simple structure has led to its widespread use. The main drawbacks of this method are that its compensation characteristics are affected by grid impedance and operating conditions, and it is prone to parallel resonance with the system, leading to harmonic amplification, overload of the LC filter, or even burnout. Furthermore, it can only compensate for harmonics of a fixed frequency, and the compensation effect is not ideal.
Generation of harmonics
1. Harmonics generated by the power generation system
Because it is difficult to achieve perfect symmetry in the manufacturing process of the three-phase windings of a generator, and it is also difficult to achieve absolute uniformity in the iron core, some harmonics will be generated during power generation, although the overall amount is relatively small.
2. Harmonics generated by the substation system
Due to the saturation of the power transformer core and the nonlinearity of the magnetization curve, coupled with economic considerations in the design, the operating magnetic flux density is selected close to the saturation segment of the magnetization curve. This results in the magnetizing current exhibiting a peaked waveform, thus containing odd harmonics. The magnitude of these harmonics is related to the structure of the magnetic circuit and the degree of core saturation. The higher the saturation, the further the transformer's operating point deviates from linearity, resulting in a larger harmonic current.
3. Harmonics generated by the power supply system
Because of nonlinear loads in the power supply system, when the current flowing through the system and the applied voltage are not linearly related, nonsinusoidal currents will occur; these are harmonic currents. Nonlinear load devices include switching power supplies, uninterruptible power supplies, variable frequency drives, electronic fluorescent lamp ballasts, devices containing magnetic cores, and some household appliances such as televisions and computers.
1) Semiconductor rectifier equipment
Because semiconductors are widely used in many applications such as switching power supplies and uninterruptible power supplies, the harmonics they generate cause significant electrical pollution to the power grid. Semiconductor rectifiers use phase-shifting control to absorb a portion of the sine wave missing from the grid, leaving the grid with another portion of the sine wave missing from the grid. This remaining portion contains a large number of harmonics. Statistics show that harmonics generated by rectifiers account for approximately 40% of all harmonics in the power grid, making them the largest source of harmonics.
2) Variable frequency speed control device
Due to the switching characteristics of the inverter circuit in a variable frequency speed control device, it forms a typical nonlinear load on the power supply. Furthermore, because it employs phase control, the harmonic composition is complex, containing not only numerous integer harmonics but also fractional harmonics. These devices generally have high power ratings, and with the widespread application of variable frequency speed control, the harmonics they generate on the power grid will increase. The harmonics generated by frequency converters are mainly the 5th and 7th harmonics.
3) Nonlinear lighting devices
Nonlinear lighting devices mainly include fluorescent lamps, high-pressure gas discharge lamps, and metal halide lamps. By measuring and analyzing the volt-ampere characteristics of these devices, it can be seen that their nonlinearity is very serious, and some even have negative volt-ampere characteristics. They all cause odd-order harmonic currents to the power grid.
4) Household appliances
Appliances such as induction cookers, televisions, VCRs, and computers generate numerous odd harmonics due to their internal voltage regulating and rectifying devices. In these winding devices, variations in unbalanced current can also alter the waveform. Although these household appliances have relatively low power consumption, their large number makes them a major source of harmonics.
The dangers of harmonics
In non-sinusoidal alternating current, besides the fundamental wave with angular frequency ω, all other harmonic components with different angular frequencies are higher harmonics. If these higher harmonics are not suppressed, they will intrude into the system and other users, causing many adverse consequences.
1. It distorts the voltage and current waveforms of the power grid, resulting in poor power quality and even rendering some precision instruments unusable.
2. It increases the iron and copper losses of electrical equipment such as transformers and motors, causing the equipment to overheat during operation, reducing normal output, and increasing noise.
3. It affects the working accuracy and reliability of control, protection and detection devices. Electromagnetic relays, transistor relays and induction relays are very susceptible to harmonics, which can cause malfunctions or failures to operate.
4. Harmonics can cause overheating and burnout of the insulation layers of certain capacitive electrical equipment, such as capacitors, and electrical materials, such as cables. Harmonics can increase the voltage at the capacitor's output, increase the capacitor current, and increase the power loss of the capacitor, thereby causing abnormal heating and accelerating the aging of the insulation medium. In severe cases, the capacitor may break down or even explode.
5. It causes severe interference to low-voltage systems such as communication lines and control circuits, distorting signals and even preventing the low-voltage systems from functioning properly. Nonlinear loads between phase and neutral lines generate third harmonic currents, which are superimposed on the neutral line. Because the third harmonic current and its multiples exhibit zero-sequence characteristics, the third harmonic current on the neutral line is the sum of all third harmonic currents in the three phases, causing current and voltage distortion and generating a 150Hz electromagnetic field, interfering with surrounding communication equipment and systems, electronic control, and protection equipment.
6. More seriously, under certain high-order harmonic conditions, it may cause parallel or series resonance in the network, resulting in resonant overvoltage or overcurrent, which is even more destructive and has extremely serious consequences.
