DC transmission possesses many technical and economic advantages over ac transmission.
Some of the advantages are given below:
Advantage # 1. Cheaper in Cost:
Bipolar HVDC transmission lines require two-pole conductors while ac system requires 3 conductors to carry power.
HVDC transmission can utilize earth return and, therefore, does not require a double circuit while EHV-AC transmission always requires a double circuit.
ADVERTISEMENTS:
The potential stress on the insulation in case of dc system is 1/√2 times of that in ac system for the same operating voltage. Hence for the same operating voltage less insulation is required.
The phase-to-phase clearances, phase-to-ground clearances and tower size are smaller in case of dc transmission as compared to those for ac transmission. Because of lesser load on the supporting structures tower designs are simpler and cheaper. Requirement of ‘right-of-way’ for a dc line is about 20-40 per cent lesser than that for an ac line of the same power transmission capability.
EHV-AC transmission needs intermediate substations at an interval of 300 km for compensation but HVDC transmission system does not require any such intermediate substation for compensation.
However, the dc line terminal equipment because of high cost of converters is considerably more expensive than ac line terminal equipment. HVDC transmission becomes economical over ac transmission above the break-even distance when the saving in overall costs of conductors, towers etc. compensates the additional cost of the terminal equipment (rectifiers and inverters).
ADVERTISEMENTS:
The break-even distance is generally different for different projects. For overhead lines it may be about 600-800 km while for submarine cables and underground cables it may lie in the range of 20 to 50 km and 50 to 100 km respectively. Sometimes it becomes necessary to adopt dc transmission, even for distances smaller than the break-even distances, because of technical advantages.
Advantage # 2. An HVDC Line can be Built in Stages:
The dc line can be built as a monopolar line with ground return in the initial stage and may be converted into a bipolar line on a later date when the load requirement increases.
Advantage # 3. No Skin Effect:
There is no skin effect in dc, so there is a uniform distribution of current over the section of the conductor. Thus there is full utilization of line conductor in case of dc transmission while it is not so in case of ac transmission.
Advantage # 4. Lower Transmission Losses:
HVDC transmission system needs only two conductors and, therefore, the power losses in a dc line are lesser than the losses in ac line of the same power transfer capability.
Advantage # 5. Voltage Regulation:
ADVERTISEMENTS:
There is no inductance; hence the voltage drop due to inductive reactance does not exist in dc transmission line. Thus voltage regulation is better in case of dc transmission.
Advantage # 6. Line Loading:
The permissible loading on an EHV-AC line is limited by transient stability limit and line reactance to almost one-third of thermal rating of conductors. No such limit exists in case of HVDC lines.
Advantage # 7. Surge Impedance Loading:
Long EHV-AC lines are loaded to less than 80 percent of natural load. No such condition is applicable to HVDC lines.
Advantage # 8. Greater Reliability:
A two-conductor bipolar dc line is more reliable than a 3-wire 3-phase ac line because the dc line may be operated in a mono-polar mode with ground return when the other line develops a fault.
Advantage # 9. Rapid Change of Energy Flow:
ADVERTISEMENTS:
The control of converter (thyristor) valves permits rapid changes in magnitude and direction of power flow when the two ac systems are interconnected by a dc line. Thus a more flexible coordination of system control at the two ends, a more economical use of cheap power generation in either of the two ac systems, and reduction of system reserve and standby capacity are available. Transient stability is also increased.
Advantage # 10. Independent Control:
The ac systems interconnected by a dc line can be controlled independently. They can be completely independent as regards frequency, system control, short-circuit rating, future extension etc.
Independence of frequencies of sending-end and receiving-end networks makes it possible to generate the power at one frequency (say 50 Hz) and to utilise it at some other frequency (say 60 Hz).
Advantage # 11. Lesser Dielectric Power Loss and Higher Current Carrying Capacity:
The cables have lesser dielectric power loss with dc in comparison with ac and, therefore, have higher current carrying capacity.
Advantage # 12. Negligible Sheath Losses:
ADVERTISEMENTS:
In case of dc, only leakage current flows in the cable sheath where as in case of ac charging, circulating and eddy currents flow through the sheath of cable. Thus in case of dc transmission with cables, the sheath losses in the cables is negligible.
Advantage # 13. Higher Natural Dielectric Strength and Longer Life of Cable Insulation:
The natural dielectric strength of cable insulation with dc is substantially greater than that with ac. Phenomenon of dielectric fatigue is also absent, therefore the cable insulation has longer life.
Advantage # 14. Absence of Charging Current and Limitations of Cable Lengths:
Because of large charging currents, the use of EHV-AC for underground transmission over long distances is prohibited but because of absence of charging current in dc systems there is no limit on the length of the cable.
In ac systems there is limit on length of cable depending upon rated voltage—about 60 km in case of 145 kV, 40 km in case of 245 kV and 25 km in case of 400 kV.
Advantage # 15. Low Short-Circuit Currents:
In ac transmission, addition of parallel lines results in larger short-circuit currents in the system. This is due to reduced equivalent reactance. When an existing ac system is interconnected with another ac system by ac transmission line, the fault level of both the systems increases.
Sometimes this necessitates the replacement of the existing circuit breakers by the ones of higher rating. However, when the two ac systems are interconnected by a dc line, the contribution of a dc line to the short-circuit current is only up to the rated current of the dc line.
During fault in dc line, grid control of converter can drastically reduce the fault current.
Advantage # 16. Lesser Corona Loss and Radio Interference:
The corona loss is directly proportional to (f + 25), where f is the supply frequency. So corona loss in dc system is lesser than those in ac systems for the same conductor diameter and operating voltage. This is because in dc system f is equal to zero. Corona loss and radio interference are directly related and, therefore, radio interference in case of dc is less as compared to dc. Also corona and radio interference slightly decrease by foul weather conditions (snow, rain or fog) in case of dc where as they increase appreciably in case of ac system.
Advantage # 17. Higher Operating Voltages:
Modern high voltage transmission lines are designed on the basis of expected switching surges rather than lightning surges because the former are more severe in comparison to the latter. The level of switching surges due to dc is lower than that due to ac and, therefore, the same size of conductors and string insulators can be employed for higher voltages in case of dc as compared to ac.
Advantage # 18. Reactive Power Compensation:
Long distance ac power transmission is feasible only with the use of series and shunt compensation, provided at intervals along the line. For such lines shunt compensation is required to absorb the line charging VARs during light load conditions and series compensation is provided for stability reasons. HDVC lines do not require any reactive power compensation like EHV-AC lines. It is because of absence of charging currents and unity power factor operation.
Advantage # 19. No Stability Limit:
There is no stability limit in dc systems and, therefore, transmission line may be of any length whereas in ac-systems the theoretical length of a transmission line which can be operated without loss of stability is only 1.43 times its natural impedance (mostly it is about 500 km). However, with series compensation the length of ac line can be somewhat longer than this value.
Advantage # 20. Use as an Asynchronous Link between Two AC Systems:
Parallel operation of ac with dc which increases the stability limit of the system or interconnection of two large ac systems by a dc transmission line. Here the dc line is an asynchronous link between two rigid (frequency constant) systems where otherwise slight difference in frequency of the two large systems would produce serious problems of power transfer control in the small capacity link.