1. Introduction:
With the rapid advancement of power electronics (PE) technology, people now demand higher levels of reliability, safety, and quality in power supply systems. However, the power grid faces numerous challenges due to the presence of non-linear loads and impact loads, including those from chemical industries, metallurgy, mining, and household appliances. High-power converter devices, thyristor rectifiers, and electric arc furnaces are particularly problematic, introducing transient shocks, reactive power issues, and increasingly severe problems like higher harmonics and three-phase imbalance. These issues pollute the power grid, increase energy loss, and degrade power quality, negatively impacting the safe and economical operation of power supply equipment. In particular, the interference caused by higher harmonics has become a significant public hazard affecting power quality in today's power grids. Thus, addressing harmonic suppression and reactive power compensation in power systems and ensuring power quality have become critical concerns for everyone.
2. Harm of Higher Harmonics and Modern Control System Requirements:
The voltage output from a three-phase alternator in the power system is essentially sinusoidal, meaning there is minimal DC or higher harmonic content in the waveform. Under normal conditions, the fundamental wave is a symmetrical component, and the sum of the three-phase vectors equals zero, thus not forming an external electromagnetic field. However, due to the sum of the three-phase vectors, the harmonic current component is not zero and can generate a strong magnetic field, leading to various harmful effects on the power grid.
2.1 Impact on Power Quality:
Nonlinear loads act as harmonic sources, injecting harmonic current components that are multiples of the fundamental frequency into the grid. These harmonic currents cause harmonic voltage drops across the grid, distorting the voltage and current waveforms and degrading power quality.
2.2 Effect on Distribution Networks:
In non-ferrous conductors, the distribution of fundamental current is approximately uniform across the cross-section. However, when harmonic currents flow through, the skin effect causes the current to concentrate on the outer surface of the conductor, increasing the resistance of the harmonic current loop and raising the effective resistance of the conductor. This results in increased power and energy losses in the power grid. Higher harmonics can also trigger voltage resonance in the power system, leading to high voltages on the lines that may damage the insulation of line equipment.
2.3 Effect on the Power Factor of the Power System:
Since the actual power factor of equipment is lower than its ideal power factor, higher harmonics increase the power consumed by the equipment and reduce the overall power factor of the system.
2.4 Requirements of Variable Frequency Speed Control Systems:
The frequency converter in variable frequency speed control transmission systems is a key component due to its high efficiency and energy-saving features. However, the rectifier bridge of the frequency converter is a nonlinear load to the power grid, and its inverter often uses PWM technology. Operating in switching mode and switching at high speeds generates a large amount of coupling noise, leading to serious EMI. This harsh electromagnetic environment can interfere with the inverter’s operation and result in higher harmonics on both the input and output sides. Therefore, the inverter must be designed to prevent external interference and avoid interfering with the outside world, i.e., achieving electromagnetic compatibility (EMC).
2.5 Requirements for Modern AC Motor Control Systems:
As new topologies for PE converters continue to emerge, the required computational and control functions have grown significantly. With the development of high-voltage and large-capacity PE devices, digital signal processor (DSP) control technology is becoming increasingly widespread. However, the electromagnetic environment of PE systems and motor control systems is becoming more complex. Due to their high operating frequencies, the anti-jamming capabilities of DSPs are generally weaker than those of microprocessors. Enhancing the anti-interference capabilities of DSPs and their peripheral circuits is crucial for ensuring reliable system operation. The "purification" of the power grid is an essential prerequisite for the development and application of modern PE 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 nonlinear loads to be compensated. These filters provide a low-resistance path for harmonics while supplying the necessary reactive power. This is the most common and practical method. Passive filters utilize inductors and capacitors as energy storage elements. By tuning the filter circuit according to the resonance principle, the harmonics to be eliminated are resonated, minimizing impedance at resonance. This effectively eliminates the specified harmonic currents, absorbing them locally near the harmonic source, preventing their injection into the power grid. Passive filters are reliable, easy to maintain, and cost-effective. They not only filter harmonics but also perform reactive power compensation. Despite these advantages, their performance is influenced by the grid's impedance, frequency, and operating conditions. They can only suppress fixed-frequency harmonics of a specific order, potentially amplifying other subharmonics and overloading the filter. Additionally, LC filter circuits can cause parallel resonance issues with the system due to changes in system impedance parameters, leading to serious consequences.
3.2 Application of Active Power Filters (APF):
Active power filters (APFs) are a new type of PE device designed to dynamically suppress harmonics. The filtering method involves detecting harmonic currents from the compensation object and then injecting a harmonic component (current or voltage) of the same amplitude but opposite phase into the harmonic source. This cancels out the total harmonic current of the power supply, achieving real-time compensation. Experience has shown that APFs are an ideal and flexible solution for suppressing harmonics and compensating reactive power, as detailed below.
4. Active Power Filter (APF):
APFs are the most effective PE devices 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 controllable power semiconductor switches. While a single pulse per half-cycle can be applied to the composite AC voltage, pulse-width modulation (PWM) is commonly used today for dynamic performance in most applications.
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