Power active filtering

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

Received diagram Basic power active filter system diagram The power active filter system is mainly composed of two parts. Power electronics technology is an important pillar of future science and technology development. It has been predicted that power electronics technology along with motion control and computer technology will become the two most important technologies in the 21st century.

However, while power electronics technology brings convenience, efficiency and great benefits, its non-linear, impact and unbalanced power characteristics also cause serious pollution to the power supply of the utility grid, and a large amount of harmonics are injected into the utility grid. Reactive power. On the other hand, with the widespread application of a large number of sensitive devices represented by computers, people have higher and higher requirements for the power supply quality of the public power grid, and put more stringent requirements on the harmonic content of the power grid and the power factor of the power equipment. Claim.

This paper first briefly introduces the basic principles and classification of power active filtering technology; then it focuses on several main control strategies that have been proposed in power active filters; finally, the development status and application of APF technology should be considered at home and abroad. Some questions are briefly introduced in order to arouse everyone's interest in the promotion and application of power active filters.

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

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

The function of the command current operation circuit is mainly to separate the harmonic current component and the fundamental reactive current component iLv from the load current and then apply the reverse polarity to generate a 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 compensating current generated by the main circuit and tracking.

The pulse acts on the main circuit after the driving circuit, and generates 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, the active power filter can also compensate for the negative sequence current component of the asymmetric three-phase circuit.

The main circuit of the active power filter is generally composed of a PWM inverter. According to the different energy storage components of the DC side of the inverter, it can be divided into a voltage type APF (the energy storage element is a capacitor) and a current type APF (the energy storage element is an inductor). 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 inverter AC side output 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 inverter AC side output 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. Since the current type APF always has a current flowing through the current side inductance, the current will cause a large loss in the internal resistance of the inductor, so it is rarely used at present. However, the current-type APF does not cause a through-short phenomenon due to the switching device. With the progress of research on superconducting energy storage magnets, more current-type APFs will be promoted and put into practical use.

It can be seen from the above principle that the power active filter uses a transient filtering forming technique to "correct" a non-sinusoidal waveform containing harmonic and reactive components. This has a fast response speed with the steady-state spectrum based device, dynamic compensation for both varying harmonics and reactive power, and its compensation characteristics are less affected by the grid impedance parameters.

2 The classification of power active filters is classified according to circuit topology. Power active filters can be divided into parallel type, series type, series-parallel type and hybrid type.

The hybrid APF is shown as a parallel APF. It is mainly suitable for harmonic current cancellation of current source type non-linear loads, reactive power compensation, and unbalanced current in balanced three-phase systems. At present, the parallel APF is mature in technology, and it is also the most widely used APF topology.

In addition to the structure of the inverter as the compensation circuit, the parallel APF has proposed a new structure with a switched capacitor filter, a *T* filter or a voltage regulator as the compensation circuit. The outstanding advantages of these new circuit structures are that the generated compensation current has less high frequency harmonics and better electromagnetic compatibility. But currently they are only suitable for single-phase circuit compensation.

Shown is the basic structure of a tandem APF. It connects the APF in series between the power supply and the load through a matching transformer to eliminate voltage harmonics, balance or adjust the terminal voltage of the load.

Compared with the parallel APF, the series APF loss is large, and various protection circuits are also complicated. Therefore, the tandem APF used alone is rarely studied, and most of them are studied as part of a hybrid APF.

The basic structure of the hybrid APF is shown. It uses some large-capacity passive LC filter networks to perform the task of eliminating low-order harmonics and performing reactive power compensation based on the series APF. The series APF only undertakes to eliminate high-order resonance and damped passive LC networks. The task of resonating with harmonics generated by line impedance. Therefore, the current and voltage rating of the series APF is greatly reduced (the power capacity can be reduced to less than 5% of the load capacity), which reduces the cost and volume of the APF. From an economic perspective, this form of structure is currently a recommended solution.

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 the following APF with higher performance and price ratio.

The string-parallel APF is shown as the basic structure of the string-and-parallel APF. It combines the advantages of series APF and parallel APF to solve most power quality problems in electrical systems, so it is also called universal APF or unified power quality regulator (UPQC). This type of APF is still in the experimental stage, and its main problems at present are 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 regulator (APLC).

The three-phase four-wire APF is mainly used to compensate for current harmonics, reactive power, and unbalanced problems between the three phases of the power supply. 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, the inverter of the four-bridge arm can be used, and the fourth bridge arm is used alone to compensate the center line. In order to achieve three-phase independent regulation, more complex three single-phase bridge inverters can also be used for separate compensation.

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

3 Control of Power Active Filter As described above, the control of the power active filter is mainly the calculation of the command current and the generation of the compensation current.

(1) Calculation of command current. The operation method of the command current L mainly has the following methods: based on the frequency domain operation method: This is the first method used in the command current calculation. The basic idea is to use the frequency domain filtering method (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 to decompose the fundamental current into the fundamental. Active component and fundamental reactive component. Since the sharp-cut 篼-order bandpass filter is required, the additional phase shift is large. In addition, its filter characteristics are also sensitive to grid frequency fluctuations and circuit component parameters. Therefore, the method has been adopted less and turned to the all-digital frequency domain filtering method based on the fast Fourier transform, and the fundamental wave component can be adaptively extracted by automatically tracking the fluctuation of the grid frequency. However, this method still has problems such as large 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 one of the most widely used command current calculation methods in current three-phase power active filters. It was first proposed by Japanese scholar H*Akagi in 1984. It is only applicable to symmetrical three-phase circuits. After continuous improvement, it has included Pq method, ip-iq method and dp method. The pq method was first applied only to symmetric three-phase and undistorted power grids; the ip-iq method is not only effective for power supply voltage distortion, but also for asymmetric three-phase power grids; dp method based on synchronous rotating park transform not only simplifies symmetry Command current calculation without distortion, but also for asymmetric, distorted power grids.

The method based on modern control theory: The earliest application method based on PI controller, because the characteristics of PI controller can not adapt to the load and grid changes, and later proposed modern control methods based on sliding mode control and fuzzy control. They are based on the voltage on the DC side of the inverter (voltage type APF) or current (current type APF) to find the amplitude of the fundamental active component of the required grid current, so as to obtain the command value of the required compensation current. i... This method is suitable for single-phase and three-phase APF, and is also suitable for grid voltage distortion.

Adaptive detection method. The method is based on the principle of adaptive interference cancellation in adaptive filtering, and the active component of the fundamental wave is cancelled from the load current, thereby obtaining the required compensation current command value. The outstanding advantage of this method is that it has better adaptive adjustment ability for grid voltage distortion, frequency offset and grid parameter change, but its dynamic response speed is still slow. Later, an adaptive detection method implemented by neural network was proposed.

The power active filter of the source inverter has four main methods for generating the compensation current: triangular carrier linear control. This is the simplest method of linear control. It uses the difference between the command current and the actual compensation current L as a modulation signal, and compares it with the high frequency triangular carrier to obtain the control signal required by the inverter switching device. The advantage is that the dynamic response is good, the switching frequency is fixed, and the circuit is simple.

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

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

No beat control. This method is an all-digital control technology. It uses the command current value and the actual compensation current value at the previous moment to calculate the switching mode that the inverter should satisfy at the next moment according to the space vector theory. The advantage is that the dynamic response is fast and easy to perform on the computer. However, its disadvantage is that it has a large amount of calculation and is highly dependent on system parameters. Later, there were some improved methods to simplify their 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 AC power grid with nonlinear load to achieve the best effect of harmonic cancellation and reactive power compensation, in recent years, nonlinear control such as internal model control, sliding mode 1 control and neural network control are proposed. The method is applied to a scheme that compensates for current generation. These nonlinear control schemes have good application prospects, but the compensation characteristics under various load conditions need further study.

The above control strategy is just a few of the main methods I have seen so far. The control strategy for APF is emerging with the development of DSP technology and intelligent control theory. As the control strategy improves, the characteristics of APF will continue to increase, and the corresponding price will also decline.

4 Application of Power Active Filters Power active filters, as a key technology to improve power quality, have become increasingly mature abroad. In Japan alone, there are more than 500 APF investors operating, and its capacity has reached 160MVA. In the application of APF, the following aspects should generally be considered: the selection of component parameters. The first is the choice of switching devices. For APFs with a capacity less than 2MVA, IG-BT is used. When the capacity is greater than 5MVA, GTO and multiplexing technology can be used. Secondly, to reduce the switching ripple of the inverter to the grid, Without reducing the compensation characteristics of the APF, the output inductor of the voltage type inverter and the passive ripple filter 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 appropriately selected.

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

APF type selection. The application of APF should select different power active filters according to different application goals. A selection scheme recommended by foreign countries in 2000 according to the current state of the art is shown in Table 1, when it is available for application. ("I" in the table indicates the selection of the first active power filter type of the first table \\ type application \\ series parallel type hybrid type string-parallel type APF current i all wave reactive power load balancing center line current voltage harmonic voltage Balanced voltage flicker voltage drop voltage regulation For the research and application of power active filters, China is far behind in foreign countries. Except for a few APFs that have been put into industrial trial operation, most of them are still in the development stage. With the increasing emphasis on power grid harmonic pollution control, the voice of “green power electronics” is getting higher and higher, and power active filters will inevitably be widely promoted.

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