AC and DC Transmission Concepts – GATE Level Notes
This article explains AC and DC transmission concepts for GATE Electrical Engineering. It includes formulas, comparison, advantages, disadvantages, and key exam points.
1. AC Transmission
AC transmission uses alternating current to transmit power. Most power systems use 3-phase AC transmission.
Real Power: P = √3 × VL × IL × cosφ
Reactive Power: Q = √3 × VL × IL × sinφ
Apparent Power: S = √3 × VL × IL
AC transmission has resistance, inductance, capacitance, and conductance.
- Reactive power present
- Charging current present
- Skin effect present
- Ferranti effect present
2. DC Transmission
DC transmission uses direct current for power transmission.
Power: P = V × I
- No reactive power
- No skin effect
- No charging current
- No Ferranti effect
3. Power Flow Equation (AC Transmission)
P = (VS × VR / X) × sinδ
Maximum power occurs when:
δ = 90°
This is called steady state stability limit.
4. Charging Current
Ic = ωCV
Present only in AC transmission.
5. Skin Effect
Current flows mainly on conductor surface in AC.
Skin depth δ = √(2ρ / ωμ)
Absent in DC transmission.
6. Ferranti Effect
Receiving end voltage becomes greater than sending end voltage.
Occurs in:
- Long transmission lines
- Light load condition
7. HVDC Transmission
HVDC is used for long distance transmission.
Power: P = V² / R
Typical voltage levels:
- ±500 kV
- ±800 kV
8. Break-Even Distance
Distance where DC becomes economical compared to AC.
| Transmission Type | Break-Even Distance |
|---|---|
| Overhead Line | 500 – 800 km |
| Underground Cable | 50 – 100 km |
9. AC vs DC Transmission Comparison
| Parameter | AC Transmission | DC Transmission |
|---|---|---|
| Power factor | Present | Not present |
| Reactive power | Present | Absent |
| Skin effect | Present | Absent |
| Charging current | Present | Absent |
| Efficiency | Lower | Higher |
| Conductors | 3 | 2 |
10. GATE Exam Important Points
- Power flow equation is very important
- Ferranti effect occurs only in AC
- Skin effect occurs only in AC
- HVDC has no stability limit
- DC transmission has lower losses
These concepts are frequently asked in GATE Electrical Engineering exam.
