Transmission Lines, Fault Analysis & Protection
Line models, symmetrical components, relaying and circuit breakers — the fault-clearing story every power interviewer wants complete.
Key formulas
Fault MVA = √3 · V_LL(kV) · I_f(kA); per-unit Z = Z_actual / Z_base
Fault MVA / base conversion: Per-unitisation removes transformer ratios — the first step of any fault calculation.
SIL = V_LL² / Z_c (Z_c ≈ 400 Ω for overhead lines)
Surge impedance loading: At SIL the line's reactive power generation equals consumption — a natural loading benchmark.
η = V_string / (n · V_disc,1) × 100%
String efficiency: Guard rings / grading capacitors equalise disc voltages and raise string efficiency.
Cheat sheet
- ▸Line models: short lines (< 80 km, lumped series impedance), medium (80–250 km, nominal π/T), long (> 250 km, distributed parameters with ABCD constants).
- ▸Unsymmetrical faults (L-G most common, then L-L, L-L-G) are analysed with symmetrical components: positive, negative and zero-sequence networks connected appropriately.
- ▸Sequence networks: for L-G fault, all three sequence networks in series; for L-L fault, positive and negative in parallel; zero-sequence only flows with a ground path.
- ▸Relay scheme: CT/VT sense → relay compares with settings → trip coil of circuit breaker. Zones overlap to guarantee no blind spots; zone-1 covers ~80% of line (instantaneous), zone-2 extends past the remote bus with time delay.
- ▸Circuit breakers must interrupt the current at a current zero with adequate dielectric recovery; arc-quenching media rank air < oil < SF6 < vacuum.
- ▸Insulators: pin (≤ 33 kV), suspension (higher voltages, string of discs); string efficiency = voltage across string / (n × voltage across nearest disc).
Sequence-network connection per fault type
| Fault | Positive | Negative | Zero | Connection |
|---|---|---|---|---|
| 3-phase (symmetrical) | Yes | No | No | Positive network only |
| Single line-to-ground | Yes | Yes | Yes | All three in series |
| Line-to-line | Yes | Yes | No | Positive ∥ negative |
| Double line-to-ground | Yes | Yes | Yes | All three in parallel |
Visual
Distance protection zones (line A→B): |----Zone 1 (80%, inst.)----|---Zone 2 (+20% + next line, 0.3-0.5s) A B---Zone 3 (backup, remote, 1s+) Zones overlap so every point is covered by at least two relays.
Top 4 campus interview questions
MCQ mastery quiz
1. The most frequent fault type on overhead transmission lines is…
2. For a line-to-line fault, which sequence networks carry current?
3. A transmission line longer than 250 km is represented by…
4. Zone-1 of a distance relay is typically set to reach…