The usual value of the surge impedance of a telephone line is
Answers
Surge impedance of a transmission line or an underground cable is defined as √(L/C).The lines are assumed to be lossless, i.e., R=0 and G=0..Here L, R are series inductance and resistance per unit length, C,G are shunt Capacitance and Conductance per unit length of the line. Its value is about 400 ohms for overhead lines and about 40 ohms for underground cables. In power transmission at 50/60 Hz it is used to find the Surge Impedance Loading (SIL).and is defined as (V^2/Z), where V= tr line voltage (line to line) in kV and Z=Surge Impedance in ohms and SIL is in MW (mega watts). When a lossless line is loaded to its SIL, the voltage profile along the line is flat. Power Engineers represent line loadings in terms of SIL. The maximum load a short line (less than 50 miles) can transmit without any compensation is about 1.2 to 1.5 times SIL. Thus a 110 kV uncompensated short line can be loaded to about 1.5 x [(110)^2]/400 = 45 MW (approx), and a 400 kV line can carry 1.5 x [(400)^2]/400 = 600 MW (approx). Thus given the line voltage you can quickly estimate its loadability. Alternately you can calculate the number of lines required to carry, say 2500 MW of power at 400, 765, or 1000 kV. Thus you can find the appropriate choice of voltage of transmission to transmit a given power. It may be noted that Z reduces from about 400 to about 250 ohms at say 765 and 1000 kV.
Another application of Z is in finding the lightning Surge Voltage ( V ) as it is related to the Lightning Surge Current ( I ) as : I = V/Z. This helps is selecting the Lightning Arrestor ratings for the protection of power equipments.
Explanation:
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