Active Power Filter Technology

Technical Research Power Active Filtering Technology Wu Zhengguo Naval University of Engineering 430030 Wuhan source filter is widely used. This article describes the basic principles and classification of the active power filter technology, several major control strategies that have been proposed, and the development status of the APF technology at home and abroad and some issues that should be considered in the application.

The basic diagram of the active power filter system is composed of two major parts. The power electronics technology is an important pillar for the future development of science and technology. Some people predict that the power electronics technology together with the motion control and computer technology will become the two most important technologies in the 21st century.

However, while the power electronics technology brings convenience, high efficiency, and great benefits, its non-linear, impactful, and unbalanced power consumption characteristics also cause serious pollution to the power supply of the utility grid, causing significant harmonics to the utility grid. Reactive power. On the other hand, with the widespread use of a large number of sensitive devices represented by computers, people are demanding higher and higher quality of power supply for the public power grid, and more stringent harmonic content in the power grid and the power factor of the power equipment are more stringent. Claim.

This article first briefly introduces the basic principle and classification of the active power filter technology; then it focuses on the proposed several main control strategies in the active power filter; Finally, it should consider the development status and application of the APF technology at home and abroad. Some questions are briefly introduced in order to arouse everyone's interest in the popularization and application of active power filters.

1 The basic principle of an active power filter The schematic diagram of the most basic configuration of an active power filter system is shown. In the figure, /s stands for AC power and the load is a non-linear load, which generates harmonics and consumes reactive power.

That is, a command current operation circuit and a compensation current generation circuit (consisting of a current tracking control circuit, a drive circuit, and a main circuit).

The function of the command current operation circuit is mainly to separate the harmonic current component and the fundamental wave reactive current component iLv from the load current and then apply the reverse polarity effect to generate the compensation current command signal=-(iU+h). 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 principle of the compensation current generated by the main circuit.

This pulse is applied to the main circuit after the drive circuit, generating a compensation power of 2mlk. That is, the power supply current contains only the active component p of the fundamental wave, thereby achieving the purpose of eliminating harmonics and performing reactive power compensation.

According to the same principle, an active power filter can also compensate for the negative-sequence current component of an asymmetric three-phase circuit.

The main circuit of an active power filter generally consists of a PWM inverter. According to the different DC-side energy storage components of the inverter, it can be divided into voltage type APF (energy storage element is capacitance) and current type APF (energy storage element is inductance). The voltage type APF needs to control the DC side capacitor voltage during operation so that the DC side voltage remains unchanged, and thus the AC side output of the inverter is a PWM voltage wave. The current-type APF needs to control the DC-side inductor current during operation so that the DC-side current remains unchanged, and thus the AC-side output of the inverter is a PWM current wave. The voltage-type APF has the advantages of less loss and high efficiency, and is the main circuit structure adopted by most APFs at home and abroad. The current-type APF is used less frequently because the current-side inductance always has a current flowing therethrough, and this current will cause a large loss in the internal resistance of the inductor. However, the current-type APF will not cause a short-circuit phenomenon due to switching devices. With the development of research on superconducting energy storage magnets, it will also promote more current-type APFs to be practical.

From the above principle, it can be seen that the active power filter uses transient filter forming technology to “correct” non-sinusoidal waveforms containing harmonic and reactive components. This and the steady-state spectrum-based instrument have a fast response speed, and can perform dynamic compensation for varying harmonics and reactive power, and its compensation characteristics are less affected by the grid impedance parameters.

2 Classification of the active power filter According to the classification of the circuit topology, the active power filter can be divided into parallel type, series type, series-parallel type and hybrid type.

The hybrid APF is shown as a shunt APF. It is mainly applied to harmonic current cancellation, reactive power compensation and unbalanced current in balanced three-phase systems for current source nonlinear loads. At present, the parallel APF is relatively mature in technology, and it is also the most widely used APF topology at present.

In addition to using the inverter as the compensation circuit, the shunt APF has recently proposed a new structure with a switched capacitor filter, a *T* filter, or a voltage regulator as a compensation circuit. The outstanding advantage of these new circuit structures is that there are fewer high-frequency harmonics in the compensated current and electromagnetic compatibility is better. However, at present, they are only suitable for single-phase circuit compensation.

The basic structure of the tandem APF is shown. It uses a matching transformer to connect the APF in series between the power supply and the load to eliminate voltage harmonics and balance or adjust the terminal voltage of the load.

Compared with parallel APFs, series APFs have large losses and various protection circuits are also complicated. Therefore, tandem APFs used seldom are studied, but most of them are studied as part of hybrid APFs.

The basic structure of the hybrid APF is shown. It is based on the series APF using some large-capacity passive LC filter network to undertake the task of eliminating low-order harmonics and performing reactive power compensation, while the series APF is only responsible for eliminating high-order resonance and damping passive LC networks. The task of resonance with the harmonics generated by the line impedance. As a result, the current and voltage ratings of the series APF are greatly reduced (the power capacity can be reduced to less than 5% of the load capacity), reducing the cost and size of the APF. From an economic point of view, this structural form is currently a worthy proposal.

However, as the performance of power electronic devices continues to increase, the cost of continuous APF parallel APF drops, and the hybrid APF may be replaced by a higher-cost APF.

The string-parallel APF shows the basic structure of a string-and-parallel APF. It combines the advantages of series APF and parallel APF, and can solve most of the power quality problems that occur in the electrical system, so it is also called universal APF or Unified Power Quality Regulator (UPQC). This type of APF is still in the experimental stage. Its main problems are its complicated control and high cost.

According to the type of power supply, APF can be divided into single-phase APF, three-phase three-wire APF, three-phase four-wire APF, and active line conditioner (APLC).

The three-phase four-wire APF is mainly used to compensate for current harmonics, reactive power, and unbalance between three phases on the power line. When the power rating is small, the main circuit can directly use a three-phase inverter, and the midpoint of the DC side capacitor is connected to the midpoint of the power supply. When the load power is large, a four-leg inverter can be used, and the fourth bridge arm alone is used to compensate the neutral line. In order to realize three-phase independent regulation, more complicated three-phase bridge inverters can also be used for compensation.

An active line conditioner injects a harmonic elimination compensation current into one (or several) preferred nodes of the power grid to achieve comprehensive power quality control of the power grid within a certain range. At present, higher-level active power filter technology is still in the research stage abroad.

3. Control of active power filter As mentioned above, the control of the active power filter is mainly the operation of command current and the generation of compensation current.

(1) Calculation of command current. The instruction current L is mainly calculated in the following ways: Based on the frequency domain operation method: This is the earliest method applied to the instruction current operation. The basic idea is to use frequency-domain filtering (using a bandpass filter) to first separate the fundamental and harmonic components of the load current, and then use the calculation method in the circuit theory to decompose the fundamental current into fundamentals. Active components and fundamental reactive components. Due to the use of a sharp cut-off bandpass filter, the additional phase shift is large. In addition, its filter characteristics are also sensitive to grid frequency fluctuations and circuit component parameters. Therefore, this method has been used less frequently, and it has turned to an all-digital frequency domain filtering method based on a fast Fourier transform, and can automatically track fluctuations of the grid frequency and adaptively extract the fundamental wave components. However, this method still has problems such as large time delay, poor real-time performance, and poor compensation effect.

Instantaneous space vector method: The instantaneous space vector method based on instantaneous reactive power theory is the most widely used command current calculation method in three-phase active power filters. It was first proposed by the Japanese scholar H*Akagi in 1984. It is only applicable to symmetrical three-phase circuits. After continuous improvement, it now includes Pq method, ip-iq method, and dp method. The pq method is the earliest application, only applicable to symmetrical three-phase and undistorted power grid; ip-iq method is not only effective for power supply voltage distortion, but also applicable to asymmetric three-phase power grid; dp method based on synchronous rotation park transform not only simplifies symmetry Distortion-free command current operation, but also for asymmetrical, distorted grids.

The method based on modern control theory: The earliest applied method based on PI controller, due to the characteristics of the PI controller can not adapt to changes in the load and power grid, and later proposed a modern control method based on sliding mode control and fuzzy control. They are based on the direct current side voltage (voltage type APF) or current (current type APF) of the inverter, to obtain the required fundamental current amplitude of the grid current, so as to obtain the required compensation current command value. i... This method is suitable for single-phase and three-phase APFs, as well as for grid voltage distortion.

Adaptive detection method. This method is based on the principle of adaptive interference cancellation in adaptive filtering. The fundamental active component is eliminated from the load current, and the required compensation current command value is obtained. The outstanding advantage of this method is that it has better self-adjusting ability to the grid voltage distortion, frequency offset and grid parameter change, but its dynamic response speed is still relatively slow. Later, an adaptive detection method using neural networks was proposed.

The source active power filter of the source inverter, there are mainly four kinds of compensating current generation methods at present: Triangle carrier linear control. This is the simplest linear control method. It uses the difference between the command current and the actual compensation current L as the modulation signal and is compared with the high-frequency triangular carrier to obtain the control signal required by the inverter switching device. Its advantages are good dynamic response, fixed switching frequency, and simple circuit.

The disadvantage is that the switching loss is large, and the output waveform contains high-frequency distortion components of the carrier frequency and its harmonic frequency.

Hysteresis comparison control. This method inputs the difference between the command current value and the actual compensation current into a comparator with a hysteresis characteristic, and then uses the output of the comparator to control the switching device of the inverter. Compared with the triangular carrier control method, this method has a small switching loss and fast dynamic response. However, this method causes a large change in the switching frequency and easily causes pulse current and switching noise. Later, in order to limit the maximum value of the switching frequency, an improved algorithm for changing the hysteresis loop bandwidth was proposed, which will inevitably affect the response speed and the compensation current tracking accuracy.

No beat control. This method is a fully digital control technology. It uses the instruction current value and the actual compensation current value at the previous moment to calculate the switching pattern to be satisfied at the next moment of the inverter according to the space vector theory. The advantage is that the dynamic response is fast and easy for the computer to execute. However, its disadvantage is that it has a large amount of calculations and is dependent on the system parameters. Later, there are some improved ways to simplify its calculations. With the increasing popularity of digital signal processing microcontroller (DSP) applications, this is a promising control method.

Nonlinear control. In order to adapt to the nonlinear characteristics of the AC grid with nonlinear loads and achieve the best results of harmonic cancellation and reactive compensation, in recent years, nonlinear control such as internal model control, sliding mode 1 control, and neural network control have been proposed. The method is applied to a compensation current generation scheme. These nonlinear control schemes have good application prospects, but the compensation characteristics under various load conditions still need further study.

The above control strategies are just some of the main methods I have seen so far. The control strategy of APF is emerging with the development of DSP technology and intelligent control theory. With the improvement of control strategies, the characteristics of APF will continue to increase, and the corresponding prices will also decline.

4 Application of Active Power Filters As a key technology for improving the quality of power supply, active power filters have become increasingly mature abroad. In Japan alone, more than 500 APFs have been put into operation, and their capacity has reached 160 MVA. In the application of APFs, generally the following issues should be considered: the selection of component parameters. The first is the choice of switching devices. For APFs with a capacity of less than 2 MVA, IG-BT is generally used, and GTO and multiplexing techniques are used for capacities greater than 5 MVA. Second, the switching ripple to reduce power to the grid is also reduced. Without reducing the compensation characteristic of APF, the output inductance and passive ripple filter of the voltage-type inverter should be carefully designed; finally, in order to ensure the stability of the DC-side voltage of the inverter, the DC-side capacitance value should be properly selected.

It was proposed in the 1970s, but it was not until the 1990s that APF technology was put into practical use. One of the important reasons was that the actual cost of APF was too high. Therefore, when choosing to apply APF, you must consider its cost price. At least in the current state of the art, a hybrid active power filter with a small rated APF combined with passive filters is a viable solution. Of course, as the price of switching devices decreases and the cost of DSP chips decreases, the series: parallel active power filters are also promising.

APF type selection. The application of APF should choose different active power filters according to different application goals. According to the current technical level recommended by foreign countries in 2000, a selection scheme is shown in Table 1, which is available for application. (The “I” in the table indicates the selection of the type of active power filter of the first table. \\Application \\Series Parallel Hybrid Type Series-Parallel APF Current i Waves Reactive Power Load Balancing Neutral Current Voltage Harmonic Voltage Balanced voltage flicker voltage drop voltage regulation for the study and application of active power filter, the country lags far behind foreign.Apart of a few APF has been put into industrial trial operation, most of the others are still in the development stage.But with China's The increasing attention paid to the control of harmonic pollution in power grids, and the growing demand for “green power electronics”, active power filters are bound to be widely promoted.

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