A filter is a frequency-selective device that is selective to the frequency of the signal. Its function is to allow specific frequency components in the signal to pass, while greatly attenuating other frequency components. The principle is that when signals of different frequencies pass through the circuit, they have different amplitude attenuations, the signal attenuation in the passband is small, and the signal attenuation in the stopband is large.
passive filter
A passive filter is a filter circuit composed of resistors, reactors and capacitors. At the resonant frequency, the circuit impedance value is the smallest. At the non-resonant frequency, the circuit impedance ratio is very large. Adjusting the value of the circuit components to a certain characteristic harmonic frequency can filter out the harmonic current; when several harmonic frequencies When the tuned circuits are combined together, the corresponding characteristic harmonic frequencies can be filtered out, and the main order harmonics (3, 5, 7) can be filtered through a low-impedance bypass. The main principle is to design the impedance of the harmonic frequency to be very small for different orders of harmonics to realize the shunt effect of the harmonic current, that is, to provide a bypass channel for the pre-filtered high-order harmonics to realize the purification of the waveform.
active filter
The main function of the active filter is not only to dynamically track and suppress harmonics, but also to compensate for the lower reactive components in the power grid. It can compensate the high-order harmonic components with fluctuating amplitude and frequency, and dynamically compensate the reactive components of the system, which overcomes the shortcomings of traditional harmonic control schemes and reactive power compensation, and achieves dynamic tracking compensation. Effect. The basic principle of APF is to detect the voltage and current signal of the current system, generate a compensation current signal through the operation of the instruction current operation circuit, and amplify the instruction by the compensation current generation circuit according to the harmonic signal, so as to obtain the compensation current, and then communicate with the system The high-order harmonic components and reactive power in the system are offset to realize the sinusoidalization of the system waveform, filter out the harmonics of the power grid, and improve the power quality. The difference between active filters and passive filters is that they require power supply and can compensate for both harmonics and reactive power. The command current calculation circuit and the compensation current generation circuit are two important components of the active filter. The function of the command current calculation circuit is to detect the harmonic components and reactive components that the system needs to compensate for. The function of the compensation current generating circuit is to issue a compensation current command according to the detected harmonic components and reactive components to generate a compensation current that can be offset. It is mainly composed of three parts: current tracking control circuit, drive circuit and main circuit.
How to distinguish active filter from passive filter
If the filter circuit is only composed of passive components (resistors, capacitors, inductors), it is called a passive filter; if the filter circuit is not only composed of passive components, but also active components (bipolar tubes, unipolar tubes, Integrated op amp) is called an active filter. The difference is mainly reflected in the following aspects:
(1) Active filters are electronic and passive filters are mechanical.
(2) The active filter detects a set harmonic order and cancels it, and the passive filter forms a harmonic channel to absorb harmonics through the cooperation of reactors and capacitors.
(3) The use of passive filters can improve the power factor because of the presence of capacitors. The use of active filters only eliminates harmonics and has nothing to do with power factors.
(4) The cost of active filters is more than three times that of passive filters, the technology is relatively immature, and the maintenance cost is high; the cost of passive filters is relatively low, the technology is relatively mature, and basically maintenance-free after installation.
(5) Active filters are used for small currents, and passive filters can be used for large currents.
