Due to the large-scale use of nonlinear electronic components in the power system, the problem of harmonic pollution is becoming more and more serious. When the harmonic content exceeds a certain limit, it may cause great harm to the power grid and users, and increase the line loss and reduce the transmission capacity of the line. Interference with communication signals, etc. Therefore, we should actively seek an effective means to control harmonic pollution.
At present, active power filter (APF) is an effective means to control harmonic pollution of power grids. The compensation principle of APF is to generate a compensation current with the same magnitude and opposite direction in the system as the reactive power and harmonic current in real time. The reactive power and harmonic current generated by the nonlinear load are cancelled, so that the current flowing into the grid is all active component, so real-time detection of reactive power and harmonic current becomes the key to real-time compensation. The detection of harmonics and reactive current is essentially the design of a low-pass filter. In this paper, the FDAtool in MATLAB is used to realize the rapid design of the digital low-pass filter model, and it is implemented on the FPGA using VHDL language.
1 Low pass filter selectionThe filter has two types of analog filters and digital filters. The digital filter is chosen here because it is easier to perform filter algebraic operations than the analog filter, and it does not drift over time, temperature, and voltage like analog filters, and it can achieve near-ideal response and linear phase. Therefore, it is better to meet the real-time and accuracy requirements of harmonic detection.
Digital filters are available in Infinite Impulse Response (IIR) systems and Finite Impulse Response (FIR) systems. Although the FIR filter can obtain a strict linear phase with respect to the IIR filter, under the same technical specifications, the IIR filter can use the order of about 1/10~1/5 FIR filter due to the feedback of the output to the input. Meet the requirements of the indicator, so that the space used is small, the number of operations is small, and the real-time performance is strong. Since the harmonic detection link has low phase requirements and the calculation speed is extremely high, the IIR filter is selected as the research object.
2 Designing IIR Filters with MATLABAt present, there are many ready-made high-level language design programs for digital filter design, but they all have shortcomings such as low design efficiency, no visual graphics, and inconvenient to modify parameters. Matlab provides an intuitive way for digital filtering research and application. Efficient and convenient tools. Based on matrix operations, it combines computation, visualization, and programming into an interactive work environment. In particular, the Matlab toolbox enables researchers in all fields to conduct scientific and engineering applications intuitively and conveniently. Among them, the signal processing toolbox, image processing toolbox, wavelet toolbox, etc., provide the possibility for the development of digital filtering research.
2.1 Simulation design in Matlab
In many literatures, when designing IIR digital filters with Matlab, an analog filter is designed first, and then the transfer function of the analog filter is converted into a system function of the digital filter by using the impulse response invariant method or the bilinear transform method. The design process is more complicated, the programming is longer, and no professional knowledge is not designed. In this paper, the digital filter can be designed very conveniently using the FDAFool tool.
FDATool (Filter Design & Analysis Tool) is a graphical user tool in Matlab that provides a comprehensive and simple graphical user interface for filter design. In this paper, we design a low-pass filter in the harmonic detection circuit. For the three-phase circuit, the AC side is mainly the 5th and 7th harmonics. Therefore, the low-pass filter designed in this paper is: analog signal sampling. Frequency Fs=2000Hz, passband cutoff frequency Fpass=100Hz, stopband cutoff frequency Fstop=200Hz, passband ripple is not more than 0.1dB, and stopband attenuation is not less than 32dB. Set the parameters in the interface of Figure 1, and select the EllipTIc filter in the IIR, then click Design Filter to get the model of the designed filter: Order = 4, SecTIons = 2, and observe Its amplitude response, phase response, and observation of the impulse response, step response, the pole-zero plot of the filter, and the various coefficients of the system function.
2.2 Using Simulink to verify the filtering effect
Open Simulink to create a new file unTItledl as shown in Figure 2. Click on Realize Model in the FDATool interface, import the designed filter model into Simulink, and then add three sine waves with frequencies of 50Hz, 250Hz, and 350Hz, and use Scope to see through The filtering effect of the digital low pass filter Filter.
Scopel displays a 50Hz sine wave, Scope2 displays a 50Hz superimposed 250Hz, 350Hz sine wave, and Scope shows the waveform after passing through our low-pass filter. Compare Figure 3(a), (b) It can be seen that the filtered waveform retains the 50 Hz signal with a slight delay and effectively filters out the high frequency portion, indicating that the designed filter meets the requirements.
The implementation methods of digital filters generally have the following types:
(1) A filter circuit designed by an adder, a multiplier, and a delayer.
(2) A special acceleration processor design is implemented in the general computer system.
(3) Realized with a general-purpose programmable DSP chip.
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