The increasing use of nonlinear electronic components in power systems has led to a growing problem of harmonic pollution. When the level of harmonics exceeds acceptable limits, it can cause serious damage to the grid and users, increase line losses, reduce transmission capacity, and interfere with communication signals. Therefore, it is essential to find an effective method to control this type of pollution.
One of the most effective solutions for controlling harmonic pollution is the Active Power Filter (APF). The principle of APF is to generate a compensation current that is equal in magnitude but opposite in direction to the reactive and harmonic currents in real time. This helps cancel out the unwanted components, ensuring that only active current flows into the grid. Thus, real-time detection of reactive and harmonic currents becomes critical for effective compensation. Detecting these currents essentially involves designing a low-pass filter. In this paper, we use MATLAB’s FDAtool to design a digital low-pass filter model quickly and implement it on an FPGA using VHDL.
Digital filters are generally divided into analog and digital types. Digital filters are preferred here because they are more stable over time, temperature, and voltage, and they can achieve near-ideal responses with linear phase characteristics. These advantages make them suitable for real-time and accurate harmonic detection applications.
There are two main types of digital filters: Infinite Impulse Response (IIR) and Finite Impulse Response (FIR). While FIR filters offer strict linear phase, IIR filters can achieve similar performance with much lower order due to their feedback structure. This makes IIR filters more efficient in terms of computational resources and real-time performance. Given the low phase requirements and high-speed processing needs in harmonic detection, IIR filters were chosen as the focus of this study.
MATLAB provides powerful tools for digital filter design, especially through its Signal Processing Toolbox. However, many existing design methods are time-consuming and lack visual feedback. In this paper, the FDAtool was used to simplify the process of designing a digital low-pass filter. For a three-phase system, the main harmonics on the AC side are the 5th and 7th. Therefore, the designed low-pass filter has a sampling frequency of 2000 Hz, a passband cutoff frequency of 100 Hz, a stopband cutoff frequency of 200 Hz, a maximum passband ripple of 0.1 dB, and a minimum stopband attenuation of 32 dB. Using the Elliptic IIR filter type, the filter was successfully designed with an order of 4 and 2 sections.
To verify the performance of the filter, Simulink was used. A new model was created, and the filter was imported from FDAtool. Three sine waves at 50 Hz, 250 Hz, and 350 Hz were added, and the output was observed using a Scope. The results showed that the 50 Hz signal was preserved with a slight delay, while the higher-frequency components were effectively filtered out, confirming that the filter meets the required specifications.
The implementation of digital filters can be done in several ways: using basic building blocks like adders, multipliers, and delay elements; using general-purpose processors with specialized acceleration units; or using programmable DSP chips. In this study, the filter was implemented on an FPGA using VHDL, which allows for high-speed, real-time processing and flexible reconfiguration. This approach ensures efficient and reliable harmonic filtering in power systems.
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