Abstract: the technology of transmission data by power line has been gradually mature. The paper analyzes some technical characteristics of transmission data by power line, and analyzes the characteristics of power line itself, including impedance of power line, relevant parameters of signal transmission on power line and some special analysis.
Key words: impedance; signal attenuation; interference
The pumping impedance and its alternating input impedance are important parameters to characterize the transmission characteristics of low voltage power lines. It is of great significance to study input impedance to improve the efficiency of transmitter and optimize the input power of the network. The relationship between input impedance and signal frequency: the research shows that the input impedance of low voltage power line is closely related to the transmitted signal frequency. In ideal case, when there is no load, the power line is equivalent to a uniformly distributed transmission line. Due to the influence of distributed inductance and distributed capacitance, the input impedance will decrease with the increase of frequency. When there is load on the power line, the input impedance of all frequencies will decrease. However, because of the different load types, the impedance changes of different frequencies are different, so the actual situation is very complex, even the change of input impedance is unpredictable.
The input impedance of power line changes dramatically with the frequency, which can be from 0.1. The change range of input impedance with frequency is not consistent with the law of frequency increasing and decreasing, even contrary to the general imagination. In order to explain this problem, we can consider the power line as a transmission line, which is connected with various complex loads. These loads and power lines themselves are combined into many resonant circuits, forming low impedance region in resonance frequency and its nearby frequency. These low impedance zones combine to violate the general law of the decrease of the impedance along the power line with the increase of the load. At the same time, it is precisely because the load will be connected or disconnected randomly on the power line, so the input impedance of the power line will change greatly at different times.
The attenuation of advanced signal and the attenuation of high frequency signal in low voltage power line is another practical difficulty in the carrier communication of low voltage power line. For high frequency signal, low voltage power line is a transmission line with non-uniform distribution. Various loads of different properties are randomly connected or disconnected at any position of the line. Therefore, the transmission of high frequency signal on low voltage power line must be attenuated. Obviously, this attenuation is closely related to communication distance, signal frequency and so on.
2.1 in general, the farther the distance of signal transmission, the more severe the attenuation of signal is. However, because the power line is a non-uniform and unbalanced transmission line, the impedance of the load connected to it does not match, so the signal will encounter complex phenomena such as reflection and standing wave. The combination of these complex phenomena makes the attenuation of signal with distance become very complex, and it is possible to appear that the attenuation of near distance point is larger than that of long distance point. For civil power grid, the load size and nature of the three-phase power supply are different, so the attenuation of the same intensity signal on three-phase is different. Sometimes, the position of receiver and transmitter is unchanged, and the BER of communication is different when connected to different phases. The signal frequency has a direct relationship with the attenuation of the signal. The influence of transmission distance on attenuation is very obvious. In some frequencies, the attenuation can change by more than 50dB. The attenuation of signals less than 60KHZ is about 25dB, and then the attenuation increases with the increase of frequency, and then it reaches zoomh:, and the attenuation is about 50dB. When the high frequency signal is transmitted across phases, the attenuation is generally larger than that of the same phase. Generally, this gap can reach more than iodb. However, in some cases, the attenuation of cross-phase propagation is not necessarily greater than that of the same phase propagation. The reason for this phenomenon is some of the coincidence capacitors between the three phase lines, and some three-phase power supply equipment, such as three-phase motor, high-power heater, etc. These devices use three-phase power supply symmetrically, which is equivalent to adding the disaster element between the three-phase power supply for high-frequency signal.