Interview questions & answers
Q1. What are the primary constants of a transmission line and what phenomena does each cause?
The four primary constants are: resistance (R, Ω/km) causing I²R loss and voltage drop; inductance (L, mH/km) causing series voltage drop and phase shift; capacitance (C, µF/km) causing charging current and Ferranti effect on lightly loaded lines; and conductance (G, S/km) representing dielectric leakage through insulation, negligible at normal operating conditions. A 400 kV, 400 km Moose-ACSR double circuit line has R ≈ 0.02 Ω/km, L ≈ 1 mH/km, and C ≈ 10 nF/km — the charging reactive power at 400 kV is (400)² × ω × C × length ≈ 96 MVAR total for both circuits.
Follow-up: What is the skin effect and how does it affect the resistance of a transmission line at 50 Hz?
Q2. What are ABCD parameters of a transmission line and what do each represent?
ABCD parameters (transmission parameters) relate the sending-end and receiving-end voltages and currents: Vs = A×Vr + B×Ir and Is = C×Vr + D×Ir. A and D are dimensionless voltage and current gain ratios (A = D for a reciprocal network), B (Ω) is the transfer impedance, and C (S) is the transfer admittance. For a 200 km, 132 kV medium-length line modelled as a nominal-π circuit, A ≈ 0.98 pu (sending voltage is about 2% higher than receiving for equal current), and B ≈ 35 Ω at 50 Hz — these values go directly into power circle diagrams and stability studies.
Follow-up: How are ABCD parameters of two networks in cascade combined?
Q3. What is the nominal-π model of a transmission line and when is it valid?
The nominal-π model represents a medium-length transmission line (80–250 km) by lumping the total series impedance (Z = R + jωL per unit length × length) in the centre and splitting the total shunt capacitance (Y = jωC per unit length × length) equally at each end. It is valid when line length is 80–250 km — for the 230 km, 220 kV Kota–Agra line, the nominal-π gives voltage and current errors below 0.5% compared to the exact distributed parameter model. For longer lines (250+ km, especially at 400 kV and above), the equivalent-π with hyperbolic correction factors must be used to account for distributed parameter effects.
Follow-up: What is the short-line model and for what line lengths is it applicable?
Q4. What is surge impedance loading (SIL) and what is its significance for line loading?
SIL is the resistive load at which the reactive power generated by line capacitance exactly equals the reactive power absorbed by line inductance: SIL = V²/Zs where Zs = √(L/C) is the surge impedance (typically 250–300 Ω for 400 kV overhead lines). At SIL, the voltage profile is flat along the line. Loading above SIL means inductive reactive demand exceeds capacitive supply — voltage drops along the line requiring reactive compensation. A 400 kV line with SIL ≈ 500 MW can economically transfer 600–700 MW with mid-line reactive compensation (switched capacitor banks), but beyond 150% SIL requires series compensation.
Follow-up: How does adding mid-line series capacitor compensation affect the effective SIL of a long transmission line?
Q5. Explain the Ferranti effect and under what conditions it is most severe.
The Ferranti effect is the rise of the receiving-end (load-end) voltage above the sending-end voltage on a lightly loaded or open-circuited transmission line — caused by the leading current (charging current) from the line''s shunt capacitance flowing through the series inductance and adding to the sending-end voltage phasorially. The effect worsens with line length, higher voltage (more total charging current), and lighter load — a 765 kV, 600 km line energised at no-load can have receiving-end voltage 10–15% above the sending-end. The 2012 India blackout post-event analysis showed 400 kV lines in western India running near no-load had high receiving-end voltages that made the post-fault voltage profile initially appear healthy, masking the true system stress.
Follow-up: How is the Ferranti effect controlled in practice during night-time low-load conditions?
Q6. What is the characteristic impedance (surge impedance) of a transmission line and what is its importance for power systems?
Characteristic impedance Zc = √(Z/Y) = √((R+jωL)/(G+jωC)) and for a lossless line Zs = √(L/C) — typically 50–300 Ω for overhead lines and 30–50 Ω for underground cables. It represents the impedance that a travelling wave ''sees'' at any point on an infinite (or perfectly terminated) line — a line terminated in its characteristic impedance has no reflections. For a 400 kV overhead line, Zs ≈ 275 Ω gives SIL = 400²/275 ≈ 581 MW — the natural loading concept is used directly in grid planning guidelines for 400 kV corridor capacity.
Follow-up: Why is the characteristic impedance of an underground cable much lower than an overhead line of the same voltage?
Q7. What is the voltage regulation of a transmission line and how is it calculated for a long line?
Voltage regulation of a transmission line is the percentage rise in receiving-end voltage when load is removed: VR = (Vr_NL - Vr_FL) / Vr_FL × 100%. For a 400 km, 132 kV line supplying 50 MW at 0.85 pf lagging, voltage regulation using nominal-π analysis is typically 15–25% — much worse than a transformer''s 3–5% because the line has far more series reactance relative to its capacity. Voltage regulation is improved by reactive compensation (shunt capacitors at the receiving end) and by operating the line closer to unity power factor, which reduces the reactive component of the series voltage drop.
Follow-up: At what power factor does a transmission line have zero voltage regulation, and is this achievable in practice?
Q8. What is the propagation constant of a transmission line and what are its real and imaginary parts?
The propagation constant γ = √(ZY) = α + jβ, where the real part α (Np/km) is the attenuation constant describing amplitude decay per unit length, and the imaginary part β (rad/km) is the phase constant describing phase shift per unit length. For a 50 Hz, 400 kV ACSR line, β ≈ 0.00106 rad/km, so a 500 km line has a phase shift of 0.53 radians (30°) between sending and receiving end at no load. The wavelength λ = 2π/β ≈ 6000 km at 50 Hz — this is why transmission line distributed effects are only significant for lines longer than λ/10 ≈ 600 km.
Follow-up: What is the velocity of propagation of an electromagnetic wave on a transmission line and how does it compare to the speed of light?
Q9. What is a travelling wave on a transmission line and what causes voltage doubling at an open end?
A travelling wave is a transient voltage or current disturbance that propagates along a transmission line at near-speed-of-light velocity when a switching operation, lightning strike, or fault creates a sudden voltage step. At an open-circuited line end (infinite impedance termination), the reflection coefficient is +1 — the incident wave reflects completely and adds to itself, doubling the voltage momentarily. A 400 kV line struck by lightning injects a 1000 kV travelling wave — at the transformer terminal end (which appears nearly open-circuit at the steep wave front), the voltage momentarily reaches 2000 kV before surge arresters clamp it, justifying the 2500 kV Basic Insulation Level (BIL) of 400 kV transformer bushings.
Follow-up: What is the reflection coefficient at a line junction with a different characteristic impedance, and how is it calculated?
Q10. How does thermal rating of a transmission line conductor differ from its stability rating?
Thermal rating is the maximum current the conductor can carry without exceeding its temperature limit (typically 75°C for ACSR — above which sag increases beyond clearance limits and tensile strength degrades). The stability rating is the maximum power the line can transfer while remaining transiently stable — it is an electrical limit set by the power-angle characteristic. For a short, heavily loaded 132 kV line, the thermal limit (say 300 A = 68 MW) may be reached before the stability limit; for a long 400 kV line, the stability limit (say 900 MW) may be reached well before the thermal limit. PGCIL classifies its 400 kV network circuits as either thermally limited or stability limited when allocating transmission capacity.
Follow-up: What is the effect of ambient temperature and wind speed on the thermal rating of an overhead conductor?
Common misconceptions
Misconception: The Ferranti effect causes the sending-end voltage to be lower than the receiving-end voltage on loaded lines.
Correct: The Ferranti effect occurs specifically on lightly loaded or open-circuited long lines — on heavily loaded lines, the receiving-end voltage is typically lower than the sending-end due to load voltage drops.
Misconception: A longer transmission line always has higher resistance and thus higher losses.
Correct: Longer lines have higher series resistance per unit, but losses depend on I²R — a longer line at lower current (same power at higher voltage) may have lower losses than a shorter line at higher current.
Misconception: The nominal-π and exact-π models give identical results for all line lengths.
Correct: The nominal-π uses lumped parameters directly; the exact-π uses hyperbolic correction factors that account for the distributed nature of line parameters — they diverge for lines longer than 250 km.
Misconception: Reactive power can be freely transmitted over long transmission lines without affecting voltage.
Correct: Reactive power transmission over inductive lines causes progressive voltage drop — each km of transmission consumes reactive power in the line inductance, making remote reactive compensation far less effective than local compensation.