2022/07/19

Author: Noyafa–**CCTV Tester**

Basic Concepts and Equivalent Circuits of Long Lines Power cables are a type of transmission line. When the length of the transmission line itself is compared with the wavelength of the signal it propagates, it is called a long line. For pulsed voltage and current waves in cables, the pulse width is less than one microsecond.

The basic concept and equivalent circuit of long line power cable is a kind of transmission line. When the length of the transmission line itself is compared with the wavelength of the signal it propagates, it is called a long line. For pulsed voltage and current waves in cables, the pulse width is less than one microsecond.

The propagation distance of a wave in one microsecond is only about 200 meters, so it is necessary to treat the cable line as a long line to study the propagation process of voltage and current waves. The cable line (hereinafter referred to as the cable) can be regarded as being composed of many resistance R, conductance G, capacitance C and inductance L elements (equivalent elements) connected, these elements are called the distribution parameters of the cable. The equivalent circuit of a short length of cable is shown in Figure 2.1.

Figure 2.1 Equivalent circuit of a small section of cable When the signal current flows through the series resistance R and the inductance L on each section of the circuit, a voltage drop will be generated, and the signal current will also pass through the capacitor C and the conductance G on each section of the line. return. If the propagation loss of the line is ignored, that is, R=G=0, the line is called a lossless line, and the capacitance and inductance values per unit length are represented by C0 and L0 respectively. Unless otherwise specified, the lines discussed in this chapter refer to such lossless lines.

The voltage and current values at any point on the distributed parameter line are actually the algebraic sum of many voltage and current wave values propagating in two different directions. These voltage and current waves move at a certain speed, so they are called traveling waves. We call a traveling wave whose motion direction is consistent with the specified direction as a forward traveling wave, and a traveling wave whose motion direction is opposite to the specified direction is called a reverse traveling wave.

Assuming that there is a cable line MN as shown in Figure 2.2, the direction of the specified distance coordinate X is from the M end to the N end, then the wave moving toward the N end on the line is called forward traveling wave, and the wave moving toward the M end is called reverse traveling. Wave. 1111MicrosoftInternetExplorer402DocumentNotSpecified7.8 ptNormal0.

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