Advantages HVDC Transmission

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Advantages of HVDC transmission system 

1. HVDC system is Cheaper in cost 
  • The bipolar HVDC transmission line requires only two conductors for transmission of a bulk amount of power as compared to the ac system which requires three conductors for transmission of power. So there are saving in conductor material in the case of the HVDC transmission system as compared to the HVAC system. so the cost of the HVDC system is reduced as compared to the HVAC system. 
  • In an HVDC system, double-circuit lines are not required but in an HVAC system, double-circuit lines are required. So HVDC transmission lines are cheaper than HVAC lines. 
  • For the same operating voltage more insulation is required in the HVAC transmission line as compared to the HVAC line. So HVDV lines are less costly as compared to HVAC transmission lines. 
  • The phase-to-phase clearance, phase-to-ground clearance and tower size are smaller in the case of HVDC lines as compared to HVAC lines. 
2. An HVDC line can be built in multiple stages 
  • The DC lines were first built as monopolar lines with ground return in the initial stage. 
  • When load requirement increases these monopolar lines are converted into bipolar lines with two conductors. 
3. No skin Effect in the HVDC system 
  • The DC voltage has zero frequency. so the skin effect in the HVDC system is zero. 
  • So there is a uniform distribution of current over the full cross-section of the conductor in the case of the HVDC system. 
  • So there is a full utilization of line conductor in HVDC transmission lines as compared to the HVAC lines. 
4. Lower Transmission losses due to less number of conductors
  • In HVDC transmission lines requires only two conductors and power losses in DC lines lesser than the AC lines for the same power transfer capability. 
  • In AC lines, Voltage waves and current waves change their direction for regular intervals of time. So heat losses are takes place in the case of the AC lines. This is not possible in DC lines because DC lines have Zero frequency. 
5. Better Voltage Regulations in HVDC system
  • In the case of DC transmission lines, there is no inductance, so voltage drop due to the inductive reactance does not exist in dc transmission lines.
  • So voltage regulation of the power system is better in the case of the DC transmission lines. 
6. No limit on Line loading 
  • The permissible line loading on AC lines is limited by transient stability limit and line reactance to one-third of the thermal rating of conductors. 
  • There are no such line loading limits exist in the case of HVDC transmission lines. 
7. Surge Impedance loading is not applicable in HVDC systems
  • Long High voltage AC transmission lines are loaded to less than 80 per cent of natural load. 
  • There is no such condition applicable to HVDC transmission lines. 
8. HVDC systems have greater reliability Greater Reliability 
  • The two conductors bipolar Direct current lines are more reliable than three wire three phase Ac lines. 
  • The DC lines are operated in a monopolar mode with ground return and other lines develop a fault. 
9. The Rapid change of energy flow possible in HVDC systems
  • The control of thyristor valves permits rapid changes in magnitude and direction of power flow when two alternating current lines are interconnected by the DC lines. 
  • So there is more flexible coordination of system control at the sending and receiving end stations and more economical use of cheap power generation either of the two ac systems available at sending end and receiving end. 
  • Due to this, a reduction in system reserve and standby capacity is available and also transient stability is also increased in the case of HVDC transmission lines. 
10. Independent frequency control possible for two ac stations 
  • In HVDC transmission lines, two ac systems are interconnected by the DC lines and can be controlled independently. 
  • Both End Ac systems are completely independent as regards frequency, system control, short circuit rating, and future extension. 
  • The Frequency at sending end and receiving end networks remain independent making it possible to generate the power at one frequency like 50 Hz and utilize it at some other frequency like 60 Hz.
11. The cable line has less dielectric power loss and higher current carrying capacity
  • The electrical cable has lesser dielectric power loss in DC lines as compared to ac lines. 
  • So the cables have a higher current carrying capacity in DC lines as compared to the AC lines. 
  • 12. Negligible sheath losses 
  • In the case of dc, only leakage current flows in the sheath of the cable. 
  • In the case of ac, charging current and eddy current flows into the sheath of the cable and causes the sheath losses. 
  • So in the case of dc lines, sheath losses are negligible. 
13. In an HVDC system cable has higher natural dielectric strength and longer life
  • The natural dielectric strength of the electrical cable is higher in the case of the dc as compared to the ac. 
  • The phenomenon of dielectric fatigue is absent in the case of the dc lines and therefore cable insulation has longer life. 
14. Absent of charging current and limitation of cable length 
  • In the case of dc lines, the charging current is absent so there is no limit on the length of the cable. 
  • In the case of the ac lines, the charging current is flowing through the sheath of the cable, so sheath losses have occurred and the length of the cable is limited.
  • In AC system, for 145 Kv - 60km, 245Kv - 40km - 400Kv - 25km length are limited. 
15. In an HVDC system, the short circuit current is low
  • In an ac transmission system, the addition of parallel lines results in larger short circuit currents in the system because of the reduced equivalent reactance. 
  • When the one ac system is interconnected with another ac system with ac transmission lines, the fault level of both systems increases. 
  • When the two ac systems are interconnected with dc transmission lines, the short circuit current is only up to the rated current of the dc line. So short circuit current in the HVDC system is low. 
16. In an HVDC system corona loss and radio interference are lesser 
  • The corona loss is directly proportional to the frequency of supply voltage. In the dc system frequency of the supply voltage is equal to zero. So corona loss in the dc system is lesser than in the ac system. 
  • The corona loss and radio interference are directly related to each other. So radio interference in Dc lines is lesser than in the ac lines. 
  • The corona loss and radio interference are decreased in bad weather conditions like snow, and rain in the case of the dc system. The corona loss and radio interference are increased in bad weather conditions in the case of the ac system. 
17. In HVDC systems, operating voltages are higher
  • The high voltage transmission lines are designed on the basis of switching surges. 
  • The switching surges are lesser in the dc system as compared to the ac system. 
  • So same size of conductor and insulator string can be employed for higher voltages in the case of dc as compared to ac. 
18. The reactive power compensation equipment is not required in the HVDC system
  • For long distances, ac transmission lines require series and shunt compensation at regular intervals along the line. 
  • The shunt compensation is used to absorb line charging VARs during the light load condition in the ac system.
  • The series compensation is used for stability reasons in the ac system. 
  • A High voltage dc system, does not require any reactive power compensation because of the absence of charging current and unity power factor operation. 
19. There are no stability limits in HVDC systems
  • There is no stability limit in the dc transmission line as compared to dc. so the length of the dc line is not limited but the length of the ac line is limited. 
  • In the Ac system, the length of the line can be increased by using series compensation. 
20. The HVDC system is used as an asynchronous tie between two AC systems by Dc lines
  • Interconnecting the two ac systems through the dc lines increases the stability limit of the system. 
  • Here the Dc line is an asynchronous link between the two systems whose frequency remains constant where a slight difference in frequency between the two large systems would produce the serious problem of power transfer control in the small capacity link. 
21. Decrease the right of way 
  • The right of way is less in the HVDC system as compared to the HVAC system. 
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