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A short length of cable is connected between dead-end tower and sub-station at the end of a transmission line. Which of the following will decrease, when voltage wave is entering from overhead line to cable?
Velocity of propagation of voltage wave
Steepness of voltage wave
Magnitude of voltage wave
Select the correct answer using the codes given below
1 and 2
2 and 3
3 and 1
1, 2 and 3
1, 2 and 3
When a voltage surge travels from an overhead line with high surge impedance to a cable with low surge impedance, both the transmitted voltage magnitude and the propagation velocity decrease. The discontinuity at the junction also causes the wavefront to become less steep (longer rise time) due to the higher shunt capacitance of the cable.
When a voltage surge travels from an overhead line with high surge impedance to a cable with low surge impedance, both the transmitted voltage magnitude and the propagation velocity decrease. The discontinuity at the junction also causes the wavefront to become less steep (longer rise time) due to the higher shunt capacitance of the cable.
v=LCтАЛ1тАЛ тАФ Velocity of propagation
╬▓=Z1тАЛ+Z2тАЛ2Z2тАЛтАЛ тАФ Refraction coefficient
The surge impedance of an overhead line is typically 400тИТ500╬й, whereas that of a cable is 20тИТ50╬й. Because the cable has a higher permittivity (epsilonrтАЛ>1), the velocity v=LCтАЛ1тАЛ is lower. Furthermore, the refraction coefficient ╬▓=Z1тАЛ+Z2тАЛ2Z2тАЛтАЛ results in a transmitted voltage magnitude less than the incident wave. The cable's high capacitance filters high-frequency components, reducing wave steepness.
Surge impedance of overhead lines is much higher than that of power cables.
The transition from high to low surge impedance acts as a shunt capacitive load.
Reduced velocity in cables is due to higher relative permittivity of insulation materials.
Refraction coefficient determines the voltage magnitude on the cable side.
Cable sections help attenuate high-frequency surges protecting substation equipment.
Underground entry minimizes lightning strike probability.
Increased capacitive current requirement for the line.
Reflections at the junction can stress the overhead line insulation.
Line-to-cable transition at power substations.
Distribution network entry points.
The phenomenon is governed by the boundary conditions of Maxwell's equations at the discontinuity.
The decrease in steepness is often described as 'surge modification' or 'surge suppression' due to the cable's distributed capacitance.
D is correct тАФ The transition from an overhead line to a cable reduces velocity due to higher permittivity, reduces magnitude due to the refraction coefficient, and reduces steepness due to the cable's increased shunt capacitance.
Remember that velocity in a medium is inversely proportional to the square root of permittivity; since cables have higher permittivity than air, the wave always slows down when entering a cable.