Power II
(A) Obtain the ABCE parameters for a medium transmission line using nominal –π network.
(B) List four (4) parameters that determine the performance of a transmission line.
Answers
Where A, B, C and D are the constants known as ‘generalized circuit constants’ of the transmission line. Their valves depend on the particular method adopted for solving the transmission network line.
Short line: in the case of lines up to 50 Km, the effect of capacitance on the line performance is negligible. Hence, here, Ls = LR and Vs – VR + LRZ
Comparing this with equation (i) and (ii) above, we get that A = 1, B = 2, C = 0 and D = 1
Incidentally it may be noted that AD – BC = 1
Medium line –nominal –T method
Vs = V1 + Is 2/2
Vi = VR + IR 2/2
Ic = Is – IR = ViY = Y[i/r + IR Z/2]
Is = V2Y + In[ Is + YZ/2]
Eliminating Vi from equation (iii) and (iv), we get:
Vs = VR + +
Substituting the value of Is, we get:
Vs = [ I + ] VR + [ 2 + [ 2 + Y22/4] IR
Comparing equation (iv) and (v) with equation (I) and (ii) respectively, it is found that:
A = D = I + ; B = [ I + ] and C = Y
It can again be proved that AD – BC = 1
Medium line _nominal π method.
Series impedance per phase = [ R + sx]
And admittance is Y = jwc
As seen Is = 1 + ICL = I + VS Y/2
I = IC1 + IR = IR + VRY/2
VS = VR + IZ = VR + Z [ IR + VRY/2
= [ I + Y2/2 ] VR + 2IR
Eliminating I from equation (vii) and (x), we get:
Is = IR + VRY/2 + VSY/2
Now substituting the value of Vs, we have:
Is = IR + VRY/2 + Y/2 { [I + Yz/2] VR + ZIR }
Is = Y[ I + Yz/4] VR + [ I + Yz/2] IR
Comparing equation (x) and (ii) with equation (i) and (xi) above, we get:
A = D = [ I + Yz/2] ; B = Z, C = Y [ I + Yz/4]
Again it can be shown that AD – BC = 1
(B)
State four (4) advantages of underground cables.
Answer:
Cables lay underground is very important because due to very small spacing of their conductors, they have a very slow inductance and hence, low inductive drops.
They are good for feeders which are not likely to be disturbed for tapping purposes.
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