Question

is dc current making comeback over ac current in transmission ? if that is so explain how ? in future will dc current be used for transmission

Answer 1

Here are some specific applications where HVDC transmission provides some technical and economical benefits compared to AC transmission schemes :

• The potential for long distance transmission, in particular for long undersea/underground cables transmission schemes (typically when there is a long submarine cable link >40 km). Long undersea/underground high-voltage cables have a high electrical capacitance compared with overhead transmission lines because the conductors within the cable are surrounded by a thin layer of insulation. The geometry is that of a long co-axial capacitor. Thus, the total capacitance increases with the lenght of the cable. This implies that the maximum feasible length and power transmission capacity of HVAC cable is limited. As AC cable lenght and voltage increases, the capacitance and hence the AC charging current increases in proportion (the charging current is the current flowing just to charge the line capacitance and is equal to the line to ground voltage divided by the cable’s capacitive reactance that is 1/jCw). Where alternating current is used for cable transmission,charging current is so significant that it reduces the capability of the line to carry useful current to the load at the remote end. There is only reactive power and no real active power can be transmitted. Thus, the reactive power that is inherently generated in HV cables under AC voltage increases with both the voltage level and the cable length. For increasing transmission distances and voltage levels, reactive power compensation will be required at both cable ends to product active power. The absence of compensators on DC linesmakes it possible to realize several hundred of kilometers of undersea links without technical constraint.
• Lower energy losses : HVDC comprises active power only and causes 20% lower losses than HVAC which comprises reactive power. In the case of HVAC cables, the additional current flowing in the cable to charge its capacitance causes added energy loss via dissipation of heat in the conductors of the cable, raising its temperature (dielectric losses or heating effectin the cable insulation). Under AC these losses are mainly related to the skin effect that is absent under DC voltage. Let’s remind that the skin effect is the tendency of an alternating current to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor. The electric current flows mainly at the "skin" of the conductor, between the outer surface and the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor (R=pl/S). Therefore, for the same conductor losses, a given conductor can carry more currentto the load when operating with HVDC than with HVAC.
• Asynchronous connection : HVDC allows power transmission between unsynchronized AC distribution systems and is able to transfer power between countries that use AC at different frequencies! For example it allows the connection of 50Hz and 60Hz networks. Because the transfer can occur in both directon, it increases the stability of both networks by allowing them to draw on each other in emergencies and failures.
• Integration of renewable resources such as wind into the transmission grid is an other area for HVDC applications that is growing around the world.

Many countries are considering building HVDC systems at voltage level of +800kV and above, creating a need for additionnal research to develop components to operate at these Ultra High Voltage (UHV) level. In countries such as India, China, Brazil and South Africa, the demand for power is growing and there is a need to transmit power with UHV DC lines over 1000–2000 km at 800kV. However it may take a while for North America and Europe to consider UHV DC tranmission as they have less need for large amounts of power transfer over long distances.

Conclusion : The use of HVDC for future transmission is growing faster than ever for economic and technical reasons. HVDC transmission has low environmental impact and is the only technical solution for long links. These features are turning HVDC links to be the preferred choice for grid interconnection. Given the increasing importance of HVDC networks, diagnostic techniques able to infer degradation processes and prevent failures are becoming more and more important. These techniques focuses especially on Partial Discharges detection in insulating cables under HVDC conditions because Partial Discharge phenomenon is considered as one of the main cause of degradation of insulators in High Voltage cables.