On harmonics and harmonic mitigation

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1. What are harmonics? The fundamental cause of harmonics in power systems is nonlinear loads. When current flows through a load, it does not follow a linear relationship with the applied voltage; this results in a non-sinusoidal current, thereby generating harmonics. The harmonic frequency is an integer multiple of the fundamental frequency. According to the analysis principles established by the French mathematician M. Fourier, any repeating waveform can be decomposed into sine wave components that consist of the fundamental frequency and a series of harmonics that are multiples of the fundamental frequency. Harmonics are sine waves, and each harmonic has a different frequency, amplitude, and phase angle. Harmonics can be classified as even or odd. Those numbered 3, 5, 7 are odd harmonics, while 2, 4, 6, 8, etc., are even harmonics. For example, if the fundamental frequency is 50Hz, the 2nd harmonic is 100Hz, and the 3rd harmonic is 150Hz. Generally speaking, the hazards caused by odd harmonics are greater and more severe than those caused by even harmonics. In a balanced three-phase system, due to the symmetry, even harmonics have been eliminated, and only odd harmonics remain. For three-phase rectified loads, the harmonic currents that appear are the 6n±1 harmonics, such as 5, 7, 11, 13, 17, 19, etc.; the frequency converter primarily generates the 5th and 7th harmonics. “The term “harmonic” originates from acoustics. The mathematical analysis of harmonics laid a solid foundation in the 18th and 19th centuries. The harmonic analysis method proposed by Fourier and others is still widely used to this day. The issue of harmonics in power systems attracted attention as early as the 1920s and 1930s. At that time in Germany, voltage and current waveforms were distorted due to the use of stationary mercury-arc converters. The paper on converter harmonics published by J.C.Read in 1945 is a classic early work on harmonic research. In the 1950s and 1960s, due to the development of high-voltage direct current transmission technology, a large number of papers were published on the harmonic problems in power systems caused by converters. Since the 1970s, due to the rapid development of power electronics technology, various power electronic devices have been increasingly used in power systems, industry, transportation, and households, and the hazards caused by harmonics have become more severe. Countries around the world pay full attention to the issue of harmonics. Many academic conferences on harmonic issues have been held internationally, and numerous ** and international academic organizations have established standards and regulations to limit harmonics in power systems and electrical equipment. The significance of harmonic research lies in the fact that harmonics pose extremely serious hazards. Harmonics reduce the efficiency of electricity production, transmission, and utilization; they cause electrical equipment to overheat, generate vibration and noise, accelerate insulation aging, shorten its service life, and even lead to failures or burns. Harmonics can cause local parallel or series resonance in power systems, amplifying the harmonic content and leading to the destruction of devices such as capacitors. Harmonics can also cause misoperation of relay protection and automatic devices, leading to confusion in electricity metering. Outside the power system, harmonics can cause severe interference to communication devices and electronic equipment. 2. Harmonic suppression – To address the problem of harmonic pollution caused by power electronic devices and other harmonic sources, there are two basic approaches: one is to install harmonic compensation devices to offset these harmonics, and this approach is applicable to all types of harmonic sources ； Another approach is to modify the power electronic device itself so that it does not generate harmonics, and its power factor can be controlled to 1; this, of course, only applies to power electronic devices that serve as the main source of harmonics. The traditional method for installing harmonic compensation devices is to use LC tuned filters. This method can compensate both harmonics and reactive power, and due to its simple structure, it has been widely used. The main drawback of this method is that its compensation characteristics are affected by the grid impedance and operating conditions; it is prone to parallel resonance with the system, resulting in harmonic amplification and causing the LC filter to become overloaded or even burn out. Furthermore, it can only compensate for harmonics at fixed frequencies, and the compensation effect is not very satisfactory. 3. Reactive power compensation: It is quite easy for people to understand active power, but it is not so simple to gain a thorough understanding of reactive power. In sinusoidal circuits, the concept of reactive power is clear, but in the presence of harmonics, there is still no universally accepted definition for reactive power. However, there is consensus on the importance of the concept of reactive power, as well as on the significance of reactive power compensation. Reactive power compensation should include compensation for fundamental wave reactive power as well as compensation for harmonic reactive power. Reactive power is very important for the operation of both power supply systems and loads. The impedance of network elements in power systems is mainly inductive. Therefore, roughly speaking, in order to transmit active power, a phase difference between the voltage at the sending end and that at the receiving end is required, and this can be achieved over a fairly wide range ； To transmit reactive power, a voltage difference between the two ends is required, which can only be achieved within a very narrow range. Not only do most network components consume reactive power, but most loads also require reactive power. The reactive power required by network elements and loads must be obtained from somewhere in the network. Obviously, it is unreasonable and usually impossible to have all of this reactive power supplied by generators and transmitted over long distances. A reasonable approach is to generate reactive power where it is needed, and this is what reactive power compensation entails. The main functions of reactive power compensation are as follows: (1) Improving the power factor of the power supply and consumption system as well as the loads, reducing the required equipment capacity, and minimizing power losses. (2) Stabilize the voltage at the power receiving end and in the power grid, thereby improving the quality of power supply. Installing dynamic reactive power compensation devices at appropriate locations in long-distance transmission lines can also improve the stability of the transmission system and enhance its transmission capacity. (3) In situations such as electrified railways where the three-phase load is unbalanced, appropriate reactive power compensation can be used to balance the active and reactive loads across the three phases.