Attenuation Constant and Propagation Velocity

Modes 1-3 in the solidly bonded cable shown in Table 3.3a-4 are coaxial modes and are the same as mode 3 in the homogeneous cross-bonded cable model. Although the attenuations of the inter-core modes (modes 1 and 2) shown in Table 3.3b-4 are almost identical to that of the coaxial mode of the solidly bonded cable, the velocities are lower than that of the coaxial mode. The velocity of the coaxial mode is determined by the permittivity of the main insulator Sj = 2.3 shown in Table 3.2. 

The velocity is converged to the aforesaid value as increases the frequency. The attenuation and velocity of the earth-return mode (mode 4) in both cable models are identical to each other. 

Because the cable is installed in a tunnel, that is, the cable is in the air, the attenuation constant and the propagation velocity of mode 4 (earth-return) and modes 5 and 6 (first and second intersheath) in the solidly bonded cable show a similar characteristic to those of an overhead line [1]. The attenuation constants of the modes 5 and 6 are far smaller, and the propagation velocity is far greater in the solidly bonded cable than those in the other modes. 

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Table 3.3a shows calculated results of the impedance, the admittance, modal attenuation constant, and propagation velocity on the solidly bonded case and Table 3.3b on the cross-bonded case with the aforesaid homogeneous model at frequency/= 100 kHz. 

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