Long Transmission Lines

Demonstrative Video


Revision of Important Formulas

\[\begin{aligned} V_{x}&=\left(\dfrac{V_{R}+I_{R} Z_{C}}{2}\right) e^{\gamma x}+\left(\dfrac{V_{R}-I_{R} Z_{c}}{2}\right) e^{-\gamma x} \\ I_{x}&=\left(\dfrac{V_{R} / Z_{c}+I_{R}}{2}\right) e^{\gamma x}+\left(\dfrac{V_{R} / Z_{c}-I_{R}}{2}\right) e^{-\gamma x} \end{aligned}\]

\[\begin{aligned} V_{x}&=V_{R}\left(\frac{e^{\gamma x}+e^{-\gamma x}}{2}\right)+I_{R} Z_{c}\left(\frac{e^{\gamma x}-e^{-\gamma x}}{2}\right)\\ &=V_{R} \cosh (\gamma x)+I_{R} Z_{c} \sinh (\gamma x) \\ I_{x}&=V_{R} \frac{1}{Z_{c}}\left(\frac{e^{\gamma x}-e^{-\gamma x}}{2}\right) e^{\gamma x}+I_{R}\left(\frac{e^{\gamma x}+e^{-\gamma x}}{2}\right)\\ &=I_{R} \cosh (\gamma x)+V_{R} \frac{1}{Z_{c}} \sinh (\gamma x) \end{aligned}\]

when \(x = l, \quad V_{x} = V_{s}, \quad I_{x} = I_{s}\)

\[ \boxed{ \left[\begin{array}{c} V_S \\ I_S \end{array}\right]=\left[\begin{array}{cc} \cosh (\gamma l) & Z_c \sinh (\gamma l) \\ \frac{1}{Z_c} \sinh (\gamma l) & \cosh (\gamma l) \end{array}\right]\left[\begin{array}{c} V_R \\ I_R \end{array}\right] } \]
\[\begin{aligned} V_{x}&=\left|\frac{V_{R}+I_{R} Z_{c}}{2}\right| e^{\alpha x} e^{j\left(\beta x+\phi_{1}\right)}+\left|\frac{V_{R}-I_{R} Z_{c}}{2}\right| e^{-\alpha x} e^{-j\left(\beta x-\phi_{2}\right)} \end{aligned}\]
where
\[\begin{aligned} &\boxed{Z_{c}=\sqrt{\left(\dfrac{z}{y}\right)}} \\ &\boxed{\gamma=\sqrt{y z}} \\ \gamma&=\sqrt{y z}=\boxed{\alpha+j \beta} \end{aligned}\]
\[\begin{aligned} \boxed{\cosh (\alpha l+j \beta l)} & =\cosh (\alpha l) \cos (\beta l)+j \sinh (\alpha l) \sin (\beta l) \\ \boxed{\sinh (\alpha l+j \beta l)}& =\sinh (\alpha l) \cos (\beta l)+j \cosh (\alpha l) \sin (\beta l) \end{aligned}\]

Problem

\[R = 0.2~ \Omega, ~ L = 1.3~\text{mH}, ~ \text{and}~ C = 0.01~\mu F\]
TL has the following parameters per km: A single circuit 50 Hz, 3-
  1. The incident voltage to neutral at the receiving end (reference)

  2. The reflected voltage to neutral at the receiving end

  3. The incident and reflected voltage to neutral at 120 km from the receiving end

Also,

Solution

\[\begin{aligned} I_{r} &=218.7 \angle-36.8^{\circ} \\ V_{s}^{+} &=\frac{V_{r}+I_{r} Z_{c}}{2} e^{\alpha x} e^{j \beta x} \\ &=\frac{76200+380 \times 218.7 \angle-13.06 \angle-36.8}{2} \times 1.033 \angle 8.02^{\circ} \\ &=74.63 \angle-18^{\circ} \\ V_{s}^{-} &=\frac{V_{r}-I_{r} Z_{c}}{2} e^{-\alpha x} e^{-j \beta x} \\ &=\frac{76200-380 \times 218.7 \angle-49.86}{2} \times 0.968 \angle-8.02^{\circ} \\ &=32.619 \angle 62.37 \mathrm{kV} \\ V_{s} &=V_{s}^{+}+V_{s}^{-}=74.63 \angle-18^{\circ}+32.619 \angle 62.37^{\circ} \\ &=86077+j 5751=86.26 \angle 3.82^{\circ} \end{aligned}\]
as the reference, Taking
\[\begin{aligned} I_{s} &=\frac{V_{r} / Z_{c}+I_{r}}{2} e^{\alpha x} e^{j \beta x}-\frac{V_{r} / Z_{c}-I_{r}}{2} e^{-\alpha x} e^{-j \beta x} \\ &=\frac{V_{s}^{+}}{Z_{c}}-\frac{V_{s}^{-}}{Z_{c}}\\ &=\left(\frac{74.63 \angle-18^{\circ}}{380 \angle-13.06^{\circ}}-\frac{32.619 \angle 62.37^{\circ}}{380 \angle-13.06^{\circ}}\right) \mathrm{kA} \\ &=200.39 \angle-29.9^{\circ} \end{aligned}\]

Efficiency

ABCD parameters

\[\begin{aligned} & \gamma l=(0.2714+j 1.169) 120 \times 10^{-3}\\ &=0.03254+j 0.1402 \end{aligned}\]
\[\begin{aligned} A = D &=\cosh \gamma l\\ &=\cosh (0.03254+j 0.1402) \\ &=\cosh 0.03254 \cos 0.1402+j \sinh 0.03254 \sin 0.1402 \\ &=0.99+j 0.004435=0.99 \angle 0.26^{\circ} \end{aligned}\]
\[\begin{aligned} \sinh \gamma l &=\sinh \alpha l \cos \beta l+j \cosh \alpha l \sin \beta l \\ &=\sinh 0.03254 \cos 0.1402+j \cosh 0.03254 \sin 0.1402 \\ &=0.031958+j 0.1386 \\ &=0.1422 \angle 77^{\circ} \end{aligned}\]
\[\begin{aligned} B &=Z_{c} \cdot \sinh \gamma l=380 \angle-13.06^{\circ} \times 0.1422 \angle 77^{\circ} \\ &=54.03 \angle 64^{\circ} \end{aligned}\]
\[\begin{aligned} C &=\dfrac{1}{Z_{c}} \cdot \sinh \gamma l\\ & =\dfrac{1}{380 \angle-13.06^{\circ}} \times 0.1422 \angle 77^{\circ} \\ &=3.74\times 10^{-4} \angle 90.06^{\circ} \end{aligned}\]
\[\begin{aligned} V_{s} &=A V_{r}+B I_{r} \\ &=0.99 \angle 0.26 \times 76200+54.03 \angle 64^{\circ} \times 218.7 \angle-36.8^{\circ} \\ &=75438+11772 \angle 27.2^{\circ} \\ &=85908+j 5380 \\ &=86.07 \angle 3.588^{\circ} \end{aligned}\]

Nominal-\(\pi\)

The nominal- \(\pi\) circuit will be

image
\[\begin{aligned} I_{r} &=218.7(0.8-j 0.6)=174.96-j 131.22 \\ I_{c_{1}} &=j 314 \times 0.6 \times 10^{-6} \times 76200=j 14.356 \mathrm{amp} \\ I_{l} &=I_{c_{1}}+I_{r}=174.96-j 116.86=210.39 \angle 33.73^{\circ}\\ V_{s} &=76200+(174.96-j 116.86)(24+j 48.38) \\ &=76200+4199+j 8596-j 2804+5723 \\ &=86122+j 5765 \\ &=86314 \angle 3.82^{\circ} \mathrm{volts} \\ \text { The loss } &=3 \times 210.39^{2} \times 24=3.187 \mathrm{MW} \\ \% \eta &=\frac{40 \times 100}{43.187}=92.69 \% \end{aligned}\]
Taking receiving end voltage as reference,

Nominal-T

\[\begin{aligned} V_{c} &=76200(0.8+j 0.6)+218.7(12+j 24.49) \\ &=60960+j 45720+2624+j 5356 \\ &=63584+j 51076 \\ I_{c} &=j 314 \times 1.2 \times 10^{-6}(63584+j 51076) \\ &=j 23.95-19.24 \\ I_{s} &=218.7+j 23.95-19.24=199.46+j 23.95 \\ &=200.89 \angle 6.8^{\circ} \\ V_{s} &=63584+j 51076+(199.46+j 23.95)(12+j 24.49) \\ &=63584+j 51076+2393+j 4884.7+j 287.4-586.5 \\ &=65390+j 56248 \\ &=86.25 \angle 40.70^{\circ} \\ \text { The loss }&=3 \times 12\left(200.89^{2}+218.7^{2}\right)\\ &=3.174~ \mathrm{MW} \\ \% \eta & =\frac{40}{43.174} \times 100\\ &=92.64 \% \end{aligned}\]
Taking receiving end current as reference