Two, the basic principle:
Active power filter, is the use of modern power electronics technology and high-speed DSP devices based on digital signal processing technology made of new power harmonics management equipment. It consists of command current operation circuit and compensation current generation circuit two main parts. The command current arithmetic circuit monitors the current in the line in real time and converts the analog current signal into a digital signal `, sends it to a high-speed digital signal processor (DSP) to process the signal, separates the harmonics from the fundamental wave and sends a driving pulse in the form of a pulse-width modulated (PWM) signal to the compensation current generating circuit to drive the IGBT or IPM power module, generating the harmonic currents of the same amplitude as those of the grid and injecting them into the compensation current with opposite polarity into the power grid, The compensation current with opposite polarity is injected into the power grid to compensate or offset the harmonic current and actively eliminate power harmonics.
Three, the basic application:
Harmonics of the main hazards:
6?1 Increase the load of power facilities, reduce the system power factor, reduce the effective capacity and efficiency of power generation, transmission and power equipment, resulting in waste of equipment, waste of lines and loss of power; 6?1 cause reactive power compensation capacitor resonance and amplification of harmonic currents, resulting in capacitor bank damage or can not be put into operation due to over-current or over-voltage. Over-voltage and damage to capacitor banks due to over-current or over-voltage or inability to put into operation; 6?1 generates pulsating torque resulting in motor vibration, affecting product quality and motor life; 6?1 due to eddy currents and skin effect, so that motors, transformers, transmission lines, etc., to produce additional power loss and overheating, wasting power and accelerating the aging of the insulation; 6?1 the harmonic voltage in the form of the peak voltage of the insulating media in the form of the peak voltage of the enhanced electric field strength, reducing the insulation 6?1 Harmonic currents in the form of peak voltages increase the electric field strength of the dielectric medium, reducing the service life of the equipment; 6?1 Zero-sequence (times 3) harmonic currents can lead to overloading of the center line of a three-phase four-wire system, and generate circulating currents inside the windings of a transformer with triangular connection, causing the winding current to exceed the rated value, and in serious cases, even leading to accidents. 6?1 Harmonics can change the action characteristics of protective relays, cause false action of relay protection facilities, and cause automatic devices such as relay protection to work in a disorderly manner; 6?1 Harmonics change the rate of change and peak value of voltage or current, delaying the arc extinguishing, and affecting the breaking capacity of the circuit breakers; 6?1 make the metering meters, especially inductive meters, produce measurement errors; 6?1 interfere with the neighboring power electronic equipment, and make the winding currents exceed the rated value. 1 interfere with neighboring power electronic equipment, industrial control equipment and communication equipment, affecting the normal operation of the equipment.Four, the advantages and disadvantages of active filtering:
Advantages: can dynamically filter out the harmonics, the harmonics of the system can be completely absorbed; will not produce resonance.
Disadvantages: the cost is too high; by the hardware limitations, in the large-capacity occasions can not be used: active filtering capacity of a single set of not more than 100KVA, the current maximum applicable grid voltage does not exceed 450V.
Fifth, the application
Active power filters can be widely used in industrial, commercial and institutional organizations of the distribution network, such as: electric power systems, electrolysis plating enterprises, water treatment equipment, petrochemical enterprises, the power system, the power system, the power system, the power system, the power system, the power system, the power system, the power system, the power system, the power system, the power system and the power system. Depending on the application object, the application of HTAPF-I active power filter will play a role in guaranteeing the reliability of power supply, reducing interference, improving product quality, increasing the life of the equipment and reducing equipment damage.
Six, the main state of development:
Because of the deficiencies and shortcomings of active filtering, the current domestic market is mainly passive filtering; international ABB, APS, Nokia, Schneider (Mairan Gerin), Siemens on behalf of the domestic Shanda Huatian, HIT, Xi'an Saibao on behalf of the other Tsinghua University, the Department of Electrical Engineering development of the CleanPower In addition, the CleanPower series active power filters developed by the Department of Electrical Engineering of Tsinghua University have made great progress in self-adaptive capability, stability and optimal compensation for various delays, and have become one of the most advanced products. With the advancement of power electronics technology, active power filters, with their great technical advantages, powerful functions, and gradually decreasing prices, will eventually replace the traditional capacitor-type reactive power compensation devices and occupy the mainstream of the market.
Basic principles and classification of active power filters Abstract: The author is a graduate of Shanghai Jiaotong University's on-the-job engineering masters, engaged in practical work for many years, engineering research direction for power active filters. Now directly on behalf of foreign companies to promote active filters, reactive power compensation products. This paper introduces the basic principles and classification of power active filters, basically covering the international trend of productization of large companies, hoping to provide the basis for the majority of domestic users to choose.
1. The basic principle of active power filters
The active power filter system is mainly composed of two major components, namely, the command current detection circuit and compensation current generation circuit. Figure 1 Schematic diagram of active filter The function of command current detection circuit is to separate harmonic current component and fundamental reactive current from the load current, and then generate the command signal of compensation current after reversing its polarity. The function of the current tracking control circuit is to calculate the trigger pulse of each switching device of the main circuit according to the compensation current generated by the main circuit, and this pulse is applied to the main circuit through the driving circuit. In this way, the power supply current contains only the active component of the fundamental wave, thus achieving the purpose of harmonic elimination and reactive power compensation. According to the same principle, active power filters can also compensate for the negative sequence current component of asymmetrical three-phase circuits. The main circuit of an active power filter generally consists of a PWM inverter. According to the different energy storage components on the DC side of the inverter, they can be divided into voltage-type active filters (the energy storage components are capacitors) and current-type active filters (the energy storage components are inductors). Voltage-type active filters need to control the capacitor voltage on the DC side to keep the DC side voltage unchanged, and thus the output of the AC side of the inverter is a PWM voltage wave. The current type active filter needs to control the DC side inductor current to keep the DC side current unchanged, so the output of the AC side of the inverter is PWM current wave. Voltage type active filter has the advantages of low loss and high efficiency, which is the main circuit structure adopted by most active filters at home and abroad. Current-type active filters are less frequently used due to the fact that there is always a current flowing through the inductor on the current side, and this current will produce a large loss in the inductor internal resistance. Figure 2 Voltage-type active filter Figure 3 Current-type active filter 2. Classification of active power filters
By circuit topology, power active filters can be divided into shunt-type, series-type, series-parallel-type and hybrid. Figure 4 Shunt type active filter The basic structure of shunt type active filter is shown in Figure 4. It is mainly suitable for harmonic current offsetting, reactive power compensation and balancing unbalanced currents in three-phase systems for current-source nonlinear loads. At present, the shunt type active filter is technically mature, and it is also the most widely used active filter topology. Figure 5 Series active filter Figure 5 shows the basic structure of series active filter. It connects the active filter in series between the power supply and the load through a matching transformer to eliminate voltage harmonics and balance or adjust the terminal voltage of the load. Compared with shunt type active filters, series type active filters have higher losses and more complex protection circuits, so series type active filters are rarely studied on their own, and most of them are studied as part of hybrid active filters. Figure 6 Hybrid active filter The basic structure of the hybrid active filter is shown in Figure 6. It is based on the series active filter using some large-capacity passive L-C filter networks to undertake the task of eliminating low harmonics and performing reactive power compensation. The series-type active filters only take on the task of eliminating high harmonics and damping the harmonic resonance generated by the passive LC network and the line impedance. Thus, the current and voltage ratings of series-type active filters are greatly reduced (power capacity can be reduced to less than 5% of the load capacity), reducing the cost and volume of active filters. From an economic point of view, this form of structure is a recommended program at present. However, as the performance of power electronic devices continues to improve and the cost continues to decline, the hybrid active filter may be replaced by the following type of active filter with a higher performance-to-price ratio. Fig. 7 Series-parallel active filter The basic structure of a series-parallel active filter is shown in Fig. 7. It combines the advantages of series active filters and shunt active filters to solve most of the power quality problems occurring in the electrical system, so it is also called the universal active filter or unified power quality conditioner (UPQC), the main problem of this type of active filter is the complexity of the control and high cost.