Abstract: The technology of transmission data by power line has been gradually mature. This paper briefly analyzes some technical characteristics of transmission data by power line, and analyzes the characteristics of the power line itself, including the impedance of power line, the parameters related to signal transmission on the power line and some characteristic 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. The study of input impedance is of great significance for improving the efficiency of transmitter and optimizing the input power of network. Relationship between input impedance and signal frequency: Studies have shown that the input impedance of a low-voltage power line is closely related to the frequency of the transmitted signal. In an ideal world, when there is no load, the power line is equivalent to a uniformly distributed transmission line. Due to the influence of distributed inductance and capacitance, the input impedance will decrease with the increase of frequency. When there is a load on a power line, the input impedance at all frequencies decreases. However, due to the different types of loads, the impedance changes at different frequencies are also different, so the actual situation is very complex, even making the input impedance changes unpredictable.
The input impedance on a power line varies dramatically with frequency and can range from 0.1. It's greater than 100Q, over a factor of 1,000! Moreover, in the frequency range measured in the experiment, the variation of input impedance with frequency does not accord with the variation law of decreasing with the increase of frequency in general imagination, or even contrary to it. To explain this, think of a power line as a transmission line connected with a variety of complex loads. These loads and the power lines themselves combine into a number of resonant circuits that form low impedance regions at and near the resonant frequency. The combination of these low impedance areas locally violates the general rule that impedance decreases with increasing load on a power line. At the same time, it is precisely because the load is randomly connected or disconnected on the power lines that the input impedance of the power lines changes greatly at different times.
The attenuation of advanced signal at the low voltage power level and the attenuation of alternating high frequency signal on the low voltage power line is another practical difficulty encountered in low voltage power line carrier communication. For high-frequency signals, a low-voltage power line is a non-uniformly distributed transmission line in which loads of different properties are randomly connected or disconnected at any location. Therefore, the transmission of high frequency signals on low voltage power lines must attenuate. Obviously, the attenuation is closely related to communication distance, signal frequency and so on.
2.1 Relationship between signal attenuation and distance and frequency In general, the farther the signal is transmitted, the more severe the signal attenuation is. However, because the power line is not uniform unbalanced transmission line, the impedance of the load connected to it is not matched, so the signal will encounter reflection, standing wave and other complex phenomena. The combination of these complex phenomena makes the relationship between signal attenuation and distance change very complicated, and it is possible that the attenuation of the near point is greater than that of the distant point. For the civil power grid, the load size and nature of the three-phase power supply are different, so the attenuation of the signal with the same strength on the three phase is also different. This phenomenon is sometimes shown as the receiver and transmitter position is unchanged, connected in different phases, communication bit error rate is different. There is a direct relationship between signal frequency and signal attenuation. The effect of transmission distance on attenuation is very obvious. At some frequencies, the variation of attenuation can exceed 50dB. For signals less than 60KHz, the attenuation is about 25dB, and then the attenuation increases with the increase of frequency. In ZooKH:, the attenuation is about 50dB. The attenuation of high frequency signal is generally greater than that of the same phase signal when it is propagated across phases. Normally this gap would work above the IOdB. However, sometimes the attenuation of cross-phase propagation is not necessarily greater than that of in-phase propagation. This phenomenon is caused by the existence of some junction capacitors between three phase lines, as well as some three-phase power supply electrical equipment, such as three-phase motors, high-power heaters, etc.. These devices use a three-phase power supply symmetrically, which is equivalent to adding a junction element between the three-phase power supply for the high-frequency signal.