Filters can be divided into active filters and passive filters, and we've discussed the differences between these two types in previous articles. To enhance your understanding of active filters, this article will introduce the selection methods for active filters and the choice between active and passive filters for loop filters. If you're interested in filters, please continue reading.
I. How to select an active filter
Active power filters are a reasonable method for controlling harmonics, making the selection of the right active power filter a challenge. Factors to consider when choosing an active power filter include: frequency range, gain, suppression of electromagnetic interference from the power grid, and amplifier noise. The parameters of the active power filter should be determined based on the application requirements to ensure they meet those requirements.
1. Selection of wiring method
① Active power filters are available in three-phase three-wire and three-phase four-wire series, tailored to the characteristics of the power grid and load. The three-phase three-wire series is particularly suitable for industrial loads with large, discrete loads, such as electronic rectifiers, frequency converters, large UPS systems, and medium-frequency induction furnaces. It offers effective filtering capabilities, simultaneously filtering out harmonics from the 2nd to the 51st order, with a response time of less than 20ms and a one-to-four expansion function. Applicable to various operating conditions, it is an ideal solution for harmonic control in industrial loads with large, discrete loads.
② The three-phase four-wire series APF is suitable for electrical control systems in office buildings. These systems often utilize fluorescent tubes, computers, UPS systems, elevator cars, and variable frequency central air conditioning units, which not only pollute the power grid but also cause third harmonics to accumulate at the neutral point, leading to neutral line heating and seriously threatening power system safety. This series of APFs can completely eliminate neutral line harmonic currents caused by the third harmonic and eliminate all or selected harmonics from the 2nd to the 51st orders. This series of APFs is small in size, has full communication functions, and is an important electrical device for ensuring power system safety.
2. Capacity Selection
The capacity of an active power filter is calculated based on the amount of harmonic current. Generally, the larger the capacity of an active power filter, the higher its cost. A 150A active power filter from the same brand is significantly more expensive than a 100A one. By measuring the power supply to the grid, the harmonic current can be determined. For example, if the total harmonic current distortion rate under full load is measured to be 30.4%, and the peak apparent current is 249A, then the total harmonic distortion of the system is 249 * 30.4% = 75.69A. Therefore, the capacity of the active power filter can be determined to be 75A.
3. Selection of Product Installation Type
There are three types of active power filters: modular, wall-mounted, and cabinet-type. They are all composed of APF power units. The only difference is their external structural dimensions. Project owners can choose the appropriate active power filter with the appropriate external structure and dimensions based on the reserved dimensions of the system's power distribution room.
2. Should the loop filter be an active or passive filter?
First, we need to understand what a loop filter is. A loop filter is a filter based on a resonant circuit, mainly used for signal denoising and frequency selection. Its working principle is to use an inductor and a capacitor to form a resonant circuit, achieving a high voltage gain at the resonant frequency, thus achieving the filtering purpose.
When designing a loop filter, we need to select appropriate inductors and capacitors, and match them with a suitable amplifier. Depending on the amplifier type, loop filters can be divided into active loop filters and passive loop filters.
An active loop filter is a filter that incorporates an amplifier into the resonant circuit for signal amplification and frequency selection. Common active loop filters include dual-T networks, Sallen-Key networks, and Butterworth networks. These filters' amplifiers can gain the signal, altering the system's impedance characteristics to make it more flexible and efficient. Simultaneously, active loop filters can induce a positive phase shift in high-frequency signals, increasing phase margin and improving system stability and performance.
A passive loop filter is a resonant circuit that contains only passive components such as inductors, capacitors, and resistors, without the involvement of an amplifier. Common passive loop filters include LC resonant circuits, LRC resonant circuits, and tetrode resonant circuits. These passive filters are generally less expensive and simpler in structure than active filters, and produce less distortion and spurious signals. However, their frequency response and gain parameters are limited by the quality factor and voltage rating of the inductors and capacitors, making them difficult to adjust and optimize.
Therefore, when selecting a loop filter, it is necessary to comprehensively consider factors such as filtering performance, cost, and reliability to choose the appropriate filter type. In practical applications, some situations require high input impedance, while others require high output impedance. Active filters, due to their gain and voltage following characteristics, are suitable for amplifying and distributing precise signals and are among the most commonly used filters; while passive filters, due to their simplicity, stability, and reliability, are suitable for low-frequency, small-signal applications.
In summary, regardless of whether an active or passive filter is used, it is necessary to select the appropriate filter type and parameters based on the specific application and requirements to fully utilize its filtering and signal processing capabilities. At the same time, attention must be paid to the filter's anti-interference capability, stability, temperature drift, and other performance indicators to ensure its effective use and reliability.
