How to design an active filter? How to select active filter?

An active filter is a type of analog circuit that implements an electronic filter using the active component, usually an amplifier. The amplifier included in the filter design can be used to improve the cost, performance and predictability of the filter.

First, how to design active filter

The design method of active filter can be roughly divided into two categories: cascade method and simulation method.

1. Cascade method

According to the requirements of the technical indicators, the transfer function that can be physically realized (usually can be found in the existing active filter data and manuals) is obtained, and it is decomposed into the product of low-order functions (mainly second-order functions), and these low-order functions are respectively realized by the active circuit and then cascaded together to achieve the original transfer function. The circuit that realizes the low-order function is called the basic section, and there are many typical second-order basic power saving circuits for designers to choose. According to the number of amplifiers used in the basic section, it can be divided into single amplifier circuit, double amplifier circuit, three amplifier circuit and four amplifier circuit. Figure 1 shows several examples of basic sections. Cascade method design process is relatively simple, the circuit characteristic adjustment is easy, the circuit realized is relatively economical, is commonly used method.

2. Simulation method

Firstly, the LC filter which can meet the requirements of technical specifications is designed as the design prototype, and then the active circuit is used to simulate the realization. This method can be divided into two categories: component simulation method and functional simulation method, and most of the LC ladder filter with double terminal resistance is used as the prototype. Generally, the analog method requires more components than the cascade method.

(1) Component simulation method

The inductor element in the LC filter is replaced by an analog inductor (an active circuit without inductor element that can achieve the inductance characteristics). The performance of the existing floating analog inductor circuit is not good enough, so it is used less. When the LC filter contains floating inductance, it is often eliminated by means of transformation. The RLC – CRD transformation is a commonly used one. It uses the factor K/s(s is a complex frequency, K is a real constant) to increase the impedance of each component in the circuit by K/s times. This transformation does not change the transmission characteristics of the original circuit, but makes the R, L and C components in the original circuit become C, R and D(frequency variable negative resistance) components respectively. Figure 2 is an example.

(2) The signal flow diagram of each voltage and current in the LC prototype filter circuit is first made by the functional simulation method, and then the active circuit such as integrator, adder and multiplier is used to achieve it.

Second, how to select active filter

Active power filter is a reasonable method to regulate harmonics, so how to choose active power filter becomes difficult. When choosing an active filter, factors that should be considered include: frequency range, gain, suppression of electromagnetic interference in the power grid, amplifier noise, etc. The parameters of the active filter should be determined according to the requirements of the application to ensure that it can meet the requirements of the application.

1. Selection of wiring method

① Active filter according to the power grid and load characteristics, each has a three-phase three-wire series products and three-phase four-wire series products digital power amplifier power engineering filtration equipment, three-phase three-wire series products for electronic rectifier, frequency conversion governor, large and medium-sized UPS, medium frequency induction furnace, medium frequency furnace and other industrial production large space discrete system load, has a reasonable filtering capacity. It can filter out 2 ~ 51 harmonics at the same time, and the response speed is lower than 20ms, with a drag and four expansion effect. Suitable for all kinds of working conditions, it is an ideal solution for load harmonics rectification of large space discrete systems for industrial production.

② Three-phase four-wire series products APF for office building electrical control system, because office building electrical control system many applications of fluorescent tubes, computers, UPS, elevator car, frequency conversion central air conditioning and other machinery and equipment, not only environmental pollution grid, and the three harmonics caused by the accumulation of the neutral point, so that the neutral line heating, a serious threat to the safety of the power system. This series of APF can completely eliminate the neutral harmonic current caused by the third harmonic, and eliminate all or selected sub-harmonics from 2 to 51. This series of APF is small in size and has complete communication function, which is an important electrical equipment to ensure the safety of power system.

2. Capacity selection

The capacity of the active filter device is calculated according to the harmonic current, in general, the larger the capacity of the active filter, the higher the cost. The same brand of active filter 150A is much more expensive than 100A. Through the measured power supply of the grid, the harmonic current is obtained, for example, the total harmonic current distortion rate under full load operation is 30.4%, and the peak apparent current is 249A, then the total harmonic of the system is 249*30.4%=75.69, and the capacity of the active filter can be determined as 75A.

3. Selection of product installation type

There are three types of active filters: modular, wall-mounted and full-cabinet, they are composed of APF power units, the only difference is: the shape structure size is different, the project can choose the corresponding shape structure and size of the active filter according to the reserved size of the system distribution room.

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