An active filter (activefilter, known as af) started in the early 1970s when scholars proposed the basic principles of a power filter, but it could only be compensated for currents by means of linear amplifiers because of the lack of large power switches and the corresponding control techniques at the time, with fatal weaknesses such as inefficiency, high cost and difficulty of large-capacity. With the increased performance of the semiconductor switch element of electricity and the corresponding development of the pwm technology, it has been possible to develop a high-capacity low-depletion harmonization current generator, thereby making the active filtering technology operational, and when a coordinated current occurs in the system, to compensate for the current in a way that is equal in size and at a different level, and to connect the circuits that are the source of the condensation wave, so that the current on the direct side is only a base wave fraction and does not contain a condening component. When the high-intensity current generated by the source of a condensation cannot be predicted and changes at any time, the coordinated current ih signal must be detected from the load current il, the detected coordinated current ih signal must be modulated by the modem and converted to the working mode of the current reverser by switch, to compensate for current and inject it into the circuit, in order to offset the dc/ac-wide retrovert main circuit, where the switch element can be used as a large-power-controlled semiconductor component such as gto, gtr, sit or igbt, to control the output current by the breakout of the switch element, and to generate the required compensation。
