Research On Impedance Characteristics And Impedance Matching Of Low Voltage Narrowband Power Line Carrier

Low-voltage power lines is a common communication channel, and the impedance is one of the main factors affecting the reliability of power line carrier communication(PLC).Impedance matching circuit of is usually an unchanged passiv e circuit.A low-voltage power line impedance measuring method was designed according to improved ratio method, and a set of impedance measurement system was designed according to this. The impedance of the transformer district in Yunnan province combination area of city and country was measured by this system. The impedance measurement point, the impedance measurement time and device access were determined by analysis and investigate the topological structure, the impedance characteristics of transmission line and habits of local residents.Analyze the impedance measurement data in time domain and frequency domain after filtering the gross error with3σcriteria. The average impedance modulus showed significant periodicity in20ms, period of about10ms; Impedance modulus value decreases and waveform of impedance modulus distort seriously according to the branch lines increases; Impedance modulus in the morning was greater than it in the afternoon. Impedance modulus value within the range of80kHz～500kHz increased first and then decreased. There was a minimum in the range of95kHz-l00kHz, which was the inherent resonant frequency of grid, circuit trap phenomenon occurred. Impedance phase angle decreased with the frequency increased, greater than0°. The maximum value of the transformer impedance angle was the highest, which of topology impedance angle was the lowest. Impedance angle was less than0°in the high frequency at PM. It was suggested that low-voltage power line network was weakly inductive, the main factors affecting the impedance were grid branches and appliances, which were the important reason for the low success rate meter.Polynomial fitting of impedance variation with frequency at AM obtained by fitting the impedance modulus value versus frequency curve and the impedance phase angle versus frequency curve, and by averaging the four measurement points. Smoothened the impedance curve by low-pass filtering the impedance modulus data in20ms at carrier frequency of150kHz,260kHz and480kHz. Fitted function obtained through Gaussian and sum of sin functions.An automatic impedance matching system of PLC based on PSO (Particle Swarm Optimization) is designed, and the objective function is built. The changes of load impedance are matched through adjusting the two capacitors in the π-type passive network. PLC with the capacitive, resistive and inductive load is researched through simulation, and the selected carrier frequency points are150kHz,260kHz and480kHz. The results show that the objective function obtains the optimal value gradually after20generations. Experiments prove that the particle swarm optimization algorithm based on the π-type passive matching network cans automatic matching the changes of load impedance.