High Voltage DC Transmission Interview Questions and Answers:

1. Why is voltage regulation better in case of dc transmission ?

Ans. Because of absence of inductance in dc systems.

2. Why there is no limit on the length of the cable for dc system while it is in case of ac system ?

Ans. Because of absence of charging currents in dc system.

3. Why HVDC line do not require any reactive power compensation ?

Ans. Because of absence of charging currents and unity power factor operation.

4. Why dc systems invariably use ground or sea return ?

Ans. Because of economy and reliability.

5. Why is dc used for high voltage transmission of electric power ?

Ans. There is a limit to the distance that the ac can be transmitted by overhead lines because of the surge impedance limitation producing high voltages at the receiving end. That is why dc is used for high voltage transmission of electric power as an alternative.

6. Why iron and graphite cannot be employed as material for earth electrode ?

Ans. Because iron has high rate of corrosion and graphite when buried in earth causes significant loss of material due to corrosion.

7. Why dc transmission is economical and preferable over ac transmission for large distances only ?

Ans. Because with larger distances, the saving in cost of dc overhead lines becomes greater than the additional expenditure on terminal equipment.

8. What is the most modern method of triggering of thyristors ?

Ans. Optronics employing light guide system.

9. What is normal value of inductance of the dc reactor ?

Ans. The value of inductance of the dc reactor should be such that a resonance of the dc circuit does not occur at power frequency and is in the range of 0.4-1.0 H.

10. What are the equipment’s that supply reactive power in HVDC converter stations ?

Ans. AC filters, static shunt capacitors, synchronous condensers, static VAR compensators etc.