1 Introduction With the rapid advancement of power electronics (PE) technology, the demand for reliability, safety, and quality of power supplies in power systems has grown significantly. However, the power grid is burdened with numerous non-linear loads and impact loads, including industries like chemicals, metallurgy, mining, and home appliances, particularly high-power converter devices, thyristor rectifiers, and electric arc furnaces. These devices generate transient shocks and reactive power, leading to increasingly severe issues such as higher harmonics and three-phase imbalance. Such problems pollute the power grid, increase energy loss, and degrade the quality of power supply, which is detrimental to the safe and economical operation of power supply equipment. In particular, the interference caused by higher harmonics has become a significant "public nuisance," affecting the quality of the current power grid. Therefore, addressing harmonic suppression and reactive power compensation in power systems and ensuring the quality of power supply has become a critical issue of concern for everyone.
2 Harm of higher harmonics and requirements of modern control systems The voltage output from the three-phase alternator in the power system is essentially sinusoidal, meaning there is minimal DC or higher harmonic content in the waveform. Under normal circumstances, the fundamental wave is a symmetrical component, and the sum of the three-phase vectors equals zero, forming no external electromagnetic field. However, due to the non-zero sum of the three-phase vectors, the harmonic current component creates a strong magnetic field, which poses various harmful effects on the power grid.
2.1 Impact on power quality Nonlinear loads act as harmonic sources that inject harmonic current components, which are multiples of the fundamental frequency, into the grid. These harmonic currents result in harmonic voltage drops across the grid, causing waveform distortion of both grid voltage and current, ultimately degrading the overall power quality.
2.2 Impact on the distribution network In non-ferrous conductors, the distribution of the fundamental current is relatively uniform across the entire cross-section. When harmonic currents flow, the skin effect causes the current to concentrate near the surface of the conductor, increasing the resistance of the harmonic current loop and raising the effective resistance of the conductor. This leads to increased power losses and energy losses within the power grid. Higher harmonics can also trigger voltage resonance in the power system, potentially causing high voltages on the lines that could damage the insulation of line equipment.
2.3 Impact on the power factor of the power system Since the actual power factor of the equipment is lower than the ideal power factor, higher harmonics increase the power consumed by the power equipment and decrease the system's overall power factor. This reduction in power factor results in wasted energy and inefficiencies in the system.
2.4 Requirements of Variable Frequency Speed Control Systems The frequency converter in variable frequency speed control systems is an essential part of AC transmission due to its high efficiency and energy-saving features. However, the rectifier bridge of the frequency converter is a non-linear load to the power grid, and its inverter often employs PWM technology. When operating in switching mode at high speeds, the converter generates substantial coupling noise, leading to severe EMI. This harsh electromagnetic environment can interfere with the inverter's operation, generating higher harmonics on both the input and output sides. Thus, the inverter must be designed to prevent external interference while avoiding interference with the surrounding environment, achieving electromagnetic compatibility (EMC).
2.5 Requirements for Modern AC Motor Control Systems As new topologies for power electronic converters continue to emerge, the computational and control functions required have significantly increased. With advancements in high-voltage and large-capacity power electronic devices, the application of DSP (Digital Signal Processor) control technology is becoming increasingly widespread. However, the electromagnetic environment of power electronic systems and motor control systems is growing more complex. Due to the high operating frequencies involved, the anti-jamming capabilities of DSPs are often weaker than those of microprocessors. Enhancing the anti-interference abilities of DSPs and their peripheral circuits is crucial to ensuring the reliable operation of these systems. The "cleaning" of the power grid is a vital prerequisite for the development and application of modern power electronic systems and AC motor control systems.
3 Main Indicators for Suppressing Higher Harmonics
3.1 Installation of AC Filter Devices (Passive Filters)
In power distribution systems, the traditional approach to harmonic suppression and reactive power compensation involves connecting passive power filters in parallel with the non-linear loads to be compensated. These filters provide a low-resistance path for harmonics and supply the required reactive power. This is one of the most common and practical methods. The device uses inductors and capacitors as energy storage elements. By applying the resonance principle, the filter circuit tunes the specific harmonics to be eliminated, creating resonance. To achieve minimum impedance at resonance, the specified harmonic currents are effectively eliminated, and the harmonic current is absorbed locally near the harmonic source, preventing it from entering the power grid. Passive filters are advantageous due to their low investment costs, high efficiency, and simple structure. They are reliable, easy to maintain, and have low operating costs. Not only do they filter out harmonics, but they also perform reactive power compensation. Therefore, passive filters are an important tool for suppressing harmonics and reactive power compensation that are widely used today. However, the compensation characteristics of this method are influenced by the impedance, frequency, and operating conditions of the power grid. They can only suppress fixed-frequency harmonics of a specific order and may amplify other subharmonics, leading to overloading or even burning of the filter. Additionally, changes in the system's impedance parameters can cause parallel resonance with the LC filter circuit, resulting in serious consequences.
3.2 Application of Active Power Filters (APF)
Active Power Filters (APF) are a new type of power electronic device designed to dynamically suppress harmonics. The filtering method involves detecting the harmonic current from the compensation object and then injecting a harmonic component (current or voltage) of the same amplitude but opposite phase from the compensation device. This cancels out the harmonic component (current or voltage) of the harmonic source, making the total harmonic of the power supply zero and achieving real-time harmonic compensation. Experience has shown that APF is an ideal and flexible solution for suppressing harmonics and compensating for reactive power, which will be further discussed below.
4 Active Power Filter (APF)
APF is the most effective power electronic device for suppressing grid harmonics, compensating reactive power, and improving grid power quality. Most APF topologies utilize voltage source inverters, with capacitors serving as energy storage devices, as shown in Figure 1. The DC voltage is converted into an AC voltage by appropriately triggering a controllable power semiconductor switch. While a single pulse per half cycle can be applied to the composite AC voltage, pulse width modulation (PWM) is commonly used today to meet the dynamic performance requirements of most applications.
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