Side-by-side comparison
| Parameter | Overhead Conductor | Underground Cable |
|---|---|---|
| Capital Cost (33 kV, 10 km) | ₹1–1.5 Cr approximate | ₹8–12 Cr approximate |
| Conductor/Insulation | ACSR, ACCC — bare, air insulated | XLPE, PILC — solid dielectric insulation |
| Charging Current | Very low; negligible for lines < 100 km | High; XLPE 33 kV cable ~1–2 A/km charging current |
| Fault Rate | 3–5 faults/100 km/year (storms, birds) | < 0.5 faults/100 km/year |
| Fault Location | Visual inspection; easy | TDR (Time Domain Reflectometry) required |
| Thermal Rating (typical 33 kV) | ACSR 150 mm²: ~170 A continuous | XLPE 150 mm²: ~290 A (better heat dissipation in duct) |
| Maintenance | Regular ROW clearing, hardware inspection | Low routine maintenance; joints are weak points |
| Reactive Power | Inductive — absorbs reactive power | Capacitive — generates reactive power; needs reactors |
| Typical Voltage Limit | Up to 765 kV (India's 765 kV PGCIL network) | Practical limit ~220 kV; 400 kV feasible but costly |
| Environmental Impact | Visual obstruction, EMF in corridor | No visual impact; land corridor freed |
Key differences
The biggest technical difference is reactive power behaviour: overhead lines are inductive and consume reactive power, while XLPE cables generate it (capacitive charging current 1–2 A/km at 33 kV), requiring shunt reactors on long cable runs to avoid ferranti effect. Fault location on underground cable demands TDR or bridge methods; overhead faults are visible. Above 220 kV, underground cables become economically impractical for bulk transmission. At distribution voltages (11–33 kV), cables win on reliability but lose on installed cost and reactive compensation requirements.
When to use Overhead Conductor
Use overhead conductors for rural transmission and sub-transmission above 33 kV where right-of-way is available and capital cost dominates. Example: PGCIL's 400 kV ACSR "Moose" line from Vindhyachal to Satna is overhead because underground HVAC cable at that voltage is not commercially viable over long distances.
When to use Underground Cable
Use underground XLPE cable in dense urban areas, river crossings, or airport vicinities where overhead lines are prohibited. Example: Mumbai's 33 kV urban network uses 300 mm² XLPE cables in RCC ducts because overhead lines cannot navigate the city's dense building footprint.
Recommendation
For any 11 kV or 33 kV urban distribution project, choose XLPE underground cable — reliability and aesthetics justify the premium. For rural transmission above 66 kV, choose overhead ACSR; the economics and voltage levels make it the only practical option. Never mix them on the same feeder without recalculating protection reach.
Exam tip: Examiners frequently ask students to explain the Ferranti effect on an unloaded underground cable and calculate the receiving-end voltage rise — know that cable charging capacitance raises voltage and a shunt reactor is the fix.
Interview tip: A core power interviewer will ask why you cannot simply replace all 33 kV overhead lines with cables — expect to explain charging current magnitude, shunt reactor need, TDR fault location, and the 8–10× cost difference clearly.