Q1. Why do we transpose transmission lines, and how does it prevent phase imbalance?

Answer: In an untransposed three-phase line, the conductors are arranged in a specific geometry (e.g., flat horizontal or asymmetrical triangular). Because the spacing between phase conductors is unequal, the magnetic flux linkages and electric field couplings differ for each phase. This results in unequal phase inductances ($L$) and phase capacitances ($C$), causing an unbalanced voltage drop even under balanced load currents.

By transposing the line (rotating the positions of the phase conductors at regular intervals like $1/3$ and $2/3$ of the line length), each phase conductor occupies all three physical positions for an equal distance. This averages out the inductances and capacitances across all three phases, restoring electrical balance.

Q2. What is the Ferranti Effect, and why does the receiving end voltage rise at no-load?

Answer: The Ferranti Effect refers to the phenomenon where the receiving end voltage ($V_r$) of a long transmission line exceeds the sending end voltage ($V_s$) under no-load or lightly-loaded conditions. It occurs because the distributed shunt capacitance of the line draws a significant leading charging current ($I_c$) from the source. When this charging current flows through the series line inductance ($L$), it causes a voltage rise that is in phase with the receiving end voltage.

The voltage rise ($\Delta V$) can be approximated using the following formula:

Q3. What is Surge Impedance Loading (SIL), and why is it called the "sweet spot" of transmission?

Answer: Surge Impedance Loading (SIL) is the electric power delivered by a transmission line when it is terminated by its characteristic impedance ($Z_c = \sqrt{L/C}$). Under this specific load, the reactive power generated by the shunt capacitance of the line ($Q_C = \omega C V^2$) is exactly equal to the reactive power consumed by the series inductance of the line ($Q_L = \omega L I^2$).

As a result, the voltage profile remains perfectly flat along the entire length of the line, the voltage and current are in phase everywhere, and the line behaves like a purely resistive circuit, meaning no external reactive compensation is needed to maintain voltage.

Q4. Why do we use bundle conductors instead of single thick conductors in EHV/UHV lines?

Answer: Bundle conductors consist of two or more parallel subconductors per phase, separated by spacer dampers. Spreading the conductor surface area increases the Geometric Mean Radius (GMR) of the phase without increasing the actual copper/aluminum cross-sectional area. The benefits are:

• Reduces Corona Loss: The self-capacitance increases, spreading the surface charge and lowering the localized electric field gradient, preventing air breakdown.
• Lowers Inductance: Since $L = 0.2 \ln(GMD / GMR_{L})$, a larger GMR directly reduces line inductance.
• Increases SIL: Higher capacitance and lower inductance decrease the surge impedance ($Z_c = \sqrt{L/C}$), raising the line's loadability limit.

Q5. How do we cascade ABCD parameters for series and parallel networks, and why is the matrix multiplication not commutative?

Answer: When two transmission components (e.g., a line and a transformer) are cascaded in series, their combined ABCD matrix is found by multiplying the individual matrices in order. Since matrix multiplication is non-commutative ($[T_1][T_2] \neq [T_2][T_1]$), the order of cascade matters. Power flow analysis relies on the exact physical sequence of nodes.

Q6. What is the physical meaning of the Propagation Constant ($\gamma$) and Characteristic Impedance ($Z_c$)?

Answer: The Propagation Constant ($\gamma = \alpha + j\beta$) describes how a voltage or current wave decays and shifts phase as it travels along a long line. The real part $\alpha$ (attenuation constant) accounts for real power losses ($I^2R$ heating and shunt leakage), while the imaginary part $\beta$ (phase constant) dictates the phase angle shift per unit length.

The Characteristic Impedance ($Z_c = \sqrt{z/y}$) is the ratio of the traveling voltage wave to the traveling current wave. For an infinite line, the input impedance equals $Z_c$. It does not depend on the line length and is typically $370 - 410\ \Omega$ for overhead EHV AC lines.

Q7. How does shunt reactor compensation mitigate the Ferranti effect, and how do you calculate the required Mvar rating?

Answer: A shunt reactor is an inductive coil connected from line to neutral (shunt). By consuming lagging reactive power ($Q_{absorbed}$), it compensates for the excess capacitive charging reactive power ($Q_{generated}$) produced by the line's shunt capacitance.

To completely eliminate the no-load voltage rise ($\Delta V = 0$), the required Mvar rating of the reactor is calculated by setting the net reactive power flow to zero:

Q8. Compare the Nominal-$\pi$ and Nominal-$T$ models: When do they diverge, and which one is preferred in load flow software?

Answer: Both models approximate medium length lines ($80-250\ \text{km}$) but differ in how the line's capacitance is lumped:

• Nominal-$\pi$: The line capacitance is split into two halves, each lumped at the sending and receiving ends. The series impedance ($Z$) remains in the middle.
• Nominal-$T$: The line capacitance is lumped in the center, while the series impedance is split in half ($Z/2$) on each side of the capacitor.

Preference: The Nominal-$\pi$ model is highly preferred in load flow and modal simulation software. Spreading the capacitance to the terminals allows cascaded pi sections to share terminal nodes, meaning it does not create additional internal midpoint nodes, minimizing matrix size in nodal analysis.

Q9. What is the Skin Effect, and how does it alter the AC resistance of transmission line conductors at standard frequencies?

Answer: The Skin Effect describes the concentration of alternating current (AC) near the outer surface (skin) of a conductor. In an AC conductor, internal magnetic flux linkages are higher at the center than at the perimeter. This creates a higher back-EMF (reactance) at the conductor's core, forcing current to take the path of least impedance near the surface.

This reduces the effective cross-sectional area of the conductor, raising the AC resistance ($R_{AC}$) compared to its DC resistance ($R_{DC}$). The skin depth (depth where current density drops to $1/e$ of surface density) is given by:

Q10. Explain Corona Loss: How does weather affect it, and how is the Critical Disruptive Voltage calculated?

Answer: Corona discharge is the ionization of air surrounding a high-voltage conductor when the localized electric field gradient exceeds the breakdown strength of air (normally $30\ \text{kV/cm}$ peak or $21.2\ \text{kV/cm}$ rms). It is accompanied by a purple glow, hissing sound, and chemical generation of ozone.

The Critical Disruptive Voltage ($V_d$), which is the threshold where corona starts, is calculated using Peek's Formula:

Where $m_0$ is the conductor surface irregularity factor, $g_0$ is the air breakdown strength, $\delta$ is the air density correction factor, $r$ is the radius, and $D$ is the phase spacing. Bad weather (rain/fog) reduces $\delta$ and $m_0$, lowering $V_d$ and significantly increasing corona power loss.