AC transmission is typically more economical than DC for distances less than 500-800 km. DC transmission has technical advantages like full control of transmitted power, improved stability, and fast fault current limiting. While DC requires less land, simpler towers and conductors, it has higher costs for conversion equipment and lacks transformers for voltage changes. Both AC and DC systems can achieve 95% or more reliability when considering energy availability and transient performance.

1. Comparison of AC-DC
Transmission Systems
Prepared By:
K. H. Shedbalkar
Department of Electrical Engineering,
Sharad Institute of Technology, College of Engineering

2. Comparison of AC-DC Transmission
Systems
• We need to consider following parameters for
comparison of AC-DC transmission systems.
 Economics of transmission.
 Technical Performance.
 Reliability.

3. Economics of transmission
• Right of way.
• Transmission tower,
conductor, insulators.
• Terminal equipment.
Investment
Cost
• Cost of losses.
Operational
Cost

4. Insulator:
Characteristics of insulator changes with change in
voltage level. For simplicity we assume for AC & DC it is
same. It can be shown that DC line much power with two
conductors as an AC line with 3 conductors of same size.
This implies that for given power level, DC requires less
RoW, Simple & cheaper tower, Conductor & insulator cost
reduced.
 Power losses are reduced (67% that of AC of same
capacity).
 Absence of Skin Effect.
 The corona effect tends to be significantly low leads
to economics size of conductors.

5. • Above fig. shows AC tend to be more economical than DC for
distance less than d*, break even distance may varies 500 to
800 Km depending upon overhead line per unite cost.
Fig. 1 Variation of cost with line length.

6. Technical Performance
• DC transmission systems having following
advantages :
– Full control over power transmitted.
– The ability to enhance transient & steady state
stability.
– Fast control to limit fault current in DC line.

7. • Stability Limit
– The power transfer in AC lines depends upon angle
difference between voltage at both ends, angle
increases with increase in distance.
• Voltage Control
– Voltage control in AC is Complicated by line
charging & inductive voltage drops, voltage profile
varies with loading. The reactive power
requirements increases with length.
– The DC converter station requires reactive power
related to line loading but line doesn’t requires.
• Line Compensation
– AC lines requires shunt & series compensation is
required for long transmission lines , mainly to
overcome the problem of line charging & stability
limitations.

8. Disadvantages of DC Transmission
system
• The difficulty of breaking DC currents which
results in high cost of DC breakers.
• Inability to use transformers to change voltage
levels.
• High cost of conversion equipments.
• Generation of harmonics which requires AC &
DC filters.
• Complexity of control.

9. Reliability
Both energy availability & transient reliability of existing system is
95% or more.

10. Reference
• HVDC Power Transmission Systems, K R
Padiyar, New Age International.