The reason why the active filter is called active, as the name implies, the device needs to provide power (to compensate the harmonics of the main circuit), and its application can overcome the shortcomings of traditional harmonic suppression and reactive power compensation methods such as LC filters (traditional It can only be fixed compensation), which realizes dynamic tracking compensation, and it can compensate both harmonics and reactive power.
Three-phase circuit instantaneous reactive power theory is the main basic theory for the development of APF. APF has two types: parallel type and series type. The former is mostly used; parallel active filters are mainly used to control current harmonics, and series active filters are mainly used Deal with the problems caused by voltage harmonics. Compared with passive filters, active filters have a better governance effect. They can mainly filter out multiple and higher harmonics at the same time without causing resonance, but the price is relatively high. The figure shows the active filter structure diagram.
Active power filter is a new type of special equipment for power harmonic control made with modern power electronic technology and digital signal processing technology based on high-speed DSP devices. It is composed of two main parts, the command current calculation circuit and the compensation current generation circuit. The command current arithmetic circuit monitors the current in the line in real time, converts the analog current signal into a digital signal, and sends it to a high-speed digital signal processor (DSP) to process the signal, separate the harmonic from the fundamental wave, and modulate it with pulse width ( PWM) signal form sends driving pulses to the compensation current generating circuit, drives IGBT or IPM power module, generates compensation current with the same amplitude and opposite polarity as the grid harmonic current, and injects it into the grid, compensates or cancels the harmonic current, and actively eliminates it Power harmonics.
The following figure is the most basic APF system structure principle diagram, the nonlinear load is the harmonic source, and the lower part is the APF.
It can be seen from the system structure diagram that the APF system contains two major parts, namely the command current calculation circuit and the compensation current generation circuit. The current topological structure of the main circuit adopts the PWM converter. The basic working principle of the APF shown in the figure is: detect the voltage and current of the compensation object, calculate the command signal of the compensation current through the command current calculation link, and drive and control the reflux to generate PWM pulses based on the command signal to act on the switching devices of the converter, so that It generates compensation current with the same magnitude and opposite direction as harmonics and reactive currents, so that the grid current waveform tends to a sine wave to achieve the purpose of compensation.
The role of active filtersThe main function of the active filter is to buffer the energy of the fundamental wave and harmonics, and input and output voltage or current according to a certain rule. Therefore, the main circuit of the active filter is composed of a four-quadrant converter and its energy storage link. According to the main circuit form of the active power filter, it can be divided into voltage type and current type according to the difference of its DC side energy storage components, as shown in the figure.
The energy storage link of the voltage-type converter uses capacitors, and the DC side voltage remains basically unchanged through closed-loop control, which can be equivalent to a controllable voltage source. Its control method is to control the output voltage according to the requirements, so that it is subtracted from the power supply voltage at the output end reactor to obtain the required compensation current. Voltage-type converters have the advantages of simple circuit structure, low requirements for semiconductor devices, low energy loss, and easy control. But it has a serious shortcoming: because the DC side is a capacitor, once a bridge arm short-circuit occurs, it will cause a DC through short circuit. Therefore, the reliability of the voltage-type converter is relatively low, and protective measures must be taken in hardware and software. That is, the upper and lower components of the same bridge arm must maintain an interlocking relationship and have a dead zone block time.
The energy storage link of the converter of the current-type active filter is an inductance element, which keeps the DC side current basically unchanged through closed-loop control, which can be equivalent to a controllable current source. It modulates the DC current into a pulse train, and then demodulates it into the required current through the AC filter on the AC output. The DC side current should match the maximum compensation current. Since the energy storage element on the DC side of the current-source converter is an inductance, it has the effect of limiting the rapid increase of the fault current when the device has a fault such as a through short circuit.
The IDA system is a new product that Sinco has put on the market in recent years. IDA is an abbreviation for Intelligent Data Collection. The main components of the system are: sensors, multi-channel modules, interface repeaters, terminal boxes, I/R modules, power supplies, personal computers, sensing cables, buses, various lightning protectors, bus power controllers, overvoltages Protector, manual reading adapter and portable reading display, etc. The multi-channel module converts the sensor signal into a digital form and transmits it to the personal computer on site via the IDA bus. The PC will convert the data obtained from the engineering unit and save it to a floppy disk. If necessary, the PC can also analyze and compare the input data to preset alarm limits and issue alarms. Data can be printed, displayed, and can be repeatedly returned from the front PC to the rear PC for many times. The front PC is used for data recording and the rear is used for data analysis, and the data is transmitted by modem or floppy disk. The rear PC is responsible for data archiving, data analysis, drawing and report generation of the database. The Tennessee Engineering Administration (TVA) in the United States adopted IDAS in the monitoring of the Fontena Dam. The system works well and can also be used for the monitoring of landslides and tailings dams.
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