Author: Noyafa–CCTV Tester
Wave Velocity and Wave Impedance in Cable 1. Wave Velocity It takes a certain amount of time for a traveling wave to travel from one end of the cable to the other. The ratio of the cable length to the propagation time is called the wave velocity V. It can be seen from the analysis that the wave velocity of the traveling wave in the cable can be expressed as: Among them: the wave velocity and the wave impedance in the S=310 cable The ratio is called the wave velocity V. It can be seen from the analysis that the wave velocity of the traveling wave in the cable can be expressed as: where: S=3×108 m/s, is the speed of light propagation;μis the relative permeability of the medium around the cable core; ε is the relative permittivity of the medium around the cable core.
It can be seen that the wave velocity in the cable is only related to the properties of the insulating medium of the cable, and has nothing to do with the material and cross-sectional area of the conductor core wire. For cables made of different conductor materials, as long as the insulating medium is the same, the wave velocity is constant. This must be noted, because many people take it for granted that the wave velocity of the cable is affected by the material and cross-sectional area of the core wire. The measurement shows that for oil-impregnated paper insulated cable V≈160 m/µs; plastic cable V≈170-200 m/µs for rubber cable V≈220 m/µs.
2. When the voltage wave in the wave impedance cable moves forward, the distributed capacitance is continuously charged to generate an accompanying current wave moving forward. The relationship between a pair of voltage and current waves is measured by wave impedance (also called characteristic impedance). Z0 to describe. After analysis, it can be seen that the wave impedance of the cable can be expressed as: L0 and C0 are not only related to the dielectric material, dielectric coefficient and permeability coefficient used in the cable, but also related to the cross-sectional area of the cable core wire and the distance between the core wire and the outer skin. . Therefore, different specifications and types of cables have different wave impedances.
The larger the cross-sectional area of the cable core, the smaller the wave impedance value. The wave impedance value of the general power cable is about 10-40 ohms. For the forward voltage wave U+ and the current wave i+, the relationship is satisfied: U+/i+=Z0 (2.1) and for the reverse voltage wave U- and the current wave i-, there is: U-/i-=- Z0 (2.2) can be seen from equations 2.1 and 2.2, the forward voltage and current waves have the same polarity, while the reverse voltage and current waves have opposite polarities.
ab Figure 2.3 Polarity of Current Traveling Wave Assuming that the polarity of the voltage traveling wave is positive, the direction of the current traveling wave on the line is the forward direction of the voltage traveling wave. The positive direction of the specified current is consistent with the positive direction of the distance coordinate X. Obviously, the forward current traveling wave flows in the same direction as the distance coordinate, which is positive, as shown in Figure 2.3.a; while the reverse current traveling wave flows in the opposite direction to the distance coordinate, which is negative, as shown in Figure 2.3.b.
The wave impedance of the cable is related to the structure of the cable itself, the insulating medium and the conductor material, and has nothing to do with the length of the cable. Even a small section of the cable has the same wave impedance everywhere. The wave impedance is the ratio of the amplitudes between a pair of forward or reverse voltage and current waves in the cable, not the ratio of the instantaneous amplitudes of the voltage and current at any point, because the instantaneous value of the voltage and current at any point in the cable is determined by It is formed by the superposition of many forward and reverse voltage and current traveling waves at this point.
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