EHV AC stands for Extra High Voltage Alternating current and HVDC stands for High Voltage Direct current.
As compared to EHV AC, HVDC has following advantages and disadvantages.
Advantages
1. HVDC systems are economical for long distance bulk power transmission by overhead line (reduced tower costs).
2. There is greater power per conductor and simpler line construction.
3. Ground return is possible.
4. There is no charging current and skin effect.
5. The voltage regulation problem is much less serious for DC since only the IR drop is involved (IX = 0). For the same reason, steady-state stability is no longer a major problem.
6. There are easy reversibility and controllability of power flow through a DC link.
7. The DC line is an asynchronous link and it can interconnect two rigid systems operating at different frequencies.
8. Each conductor can be operated as an independent circuit.
9. A smaller amount of right of way and narrower tower are required.
10. AC line requires more insulation between the tower and conductors as well as greater clearance above the earth as compared to DC line.
11. Line losses are smaller.
12. No reactive compensation of DC line is required.
13. Corona loss, radio interference and audible emissions are less as compared to AC.
Disadvantages
1. The systems are costly since the expensive installation of complicated converters and DC switchgear is expensive.
2. Converters require considerable reactive power.
3. Harmonics are generated which require filters.
4. Lack of HVDC circuit breakers hampers network operation.
5. There is nothing like DC transformers which can change the voltage level. Voltage transformation has to be provided on the AC sides of the system.
6. Reactive power required by the load is to be supplied locally as no reactive power can be transmitted over a DC link.
7. Pollution affects DC more than AC. More frequency clearing of insulators is required.
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