There are two types of transmission lines; HVDC transmission line and HVAC transmission line.
Both systems have their own advantages and disadvantages. The HVAC transmission line is a convention line that uses in the present power system network.
But the HVDC transmission line is a new technology in the electrical power transmission system. For a long distance of the line, the HVDC line is better than the HVAC line . There are many other applications which explained in detail.
1) Interconnection of two AC systems
DC link is an economical option than the AC link to interconnect two AC systems. This system is more effective, efficient and technically superior compared to the AC link.
The biggest advantage is the there is no effect of frequency in the DC link. And the frequency disturbance of one system does not transfer to other systems.
It does not affect the transient stability and there is no change in the short circuit levels of both the systems.
The direction of power flow maintains properly through the DC link.
2) Long-distance power transmission line
This is the main purpose to use the HVDC system. Because in the HVAC system, the length of a line is the biggest constraint. The length of the line cannot more than a certain length to keep control of the thermal effect of the conductor. And it needs an intermediate substation every 300 km of line.
But this problem solved by the use of the HVDC line. In the HVDC line, the generated AC power is stepped up by the transformer. The high voltage AC converted in High voltage DC with the help of a rectifier at sending end of the line.
Power transmitted to long-distance with the help of the HVDC line. To transmit more power bipolar HVDC system used.
At the receiving end, high voltage DC power converted into high voltage AC with the help of an inverter.
The HVDC line is economical only for long-distance. The breakeven point is at 800 km. hence, at 800 km of line, the HVDC line is more economical than the HVAC line.
And there is no need to build an intermediate substation in between of lines irrespective the length of a line.
The cost of the tower and the conductor is less in the HVDC line.
3) Multi-terminal HVDC interconnection
The frequency does not affect in DC system. Therefore, if the frequency is not the same, then also these systems can connect with the HVDC link.
Three or more AC systems can be interconnected asynchronous using a multi-terminal HVDC system. Due to this, bulk power can be transferred.
4) Parallel AC and DC link
DC link operates with the parallel to the existing AC line. In this way, more amount of power can transmit.
Due to this, there is a decrease in the fault level and an increase in the stability of the system.
5) Underground or submarine cable transmission
In the AC system, it is difficult to transmit power through underground cable or submarine cable because of the temperature rise due to the charging current.
This will limit the length of the line. But this problem solved in the HVDC line as an absence of the charging current.
Therefore, it is easy to implement the underground and submarine cable with the HVDC transmission line.
6) Back to back asynchronous tie station
If two tie lines have different frequencies than it tends to not possible to interconnect. Therefore, back to back asynchronous tie station becomes very useful for the interconnection of two AC systems which has different frequencies.
For example. One tie line has 50 Hz and the other has 60 Hz frequency. (Generally, is much of frequency difference will not create in the same country. But tie lines which connect the different countries which use different frequencies.) The interconnection of these tie lines can be done by the HVDC system. This cannot do by the AC system.
The converter substation used to connect two asynchronous AC systems. There is no DC transmission line used.
The two AC lines connected through back to back converters. Power flow can easily control from one system to other systems.
Smoothing reactor, filters and converter transformer used in this station.
Scope of HVDC system
1) High voltage transmission line
According to the availability of fuel, the power plants placed far from the load. Therefore, a bulk amount of power can be transmitted for long-distance.
The HVDC transmission line is a more economical option than the HVAC line for long-distance and bulk power transmission.
2) HVDC scheme for submarine line
It is possible to use seaway as a return path. So, it is easy to implement on the island which is far from the generating stations.
3) Asynchronous interconnection
In a conventional power system interconnected network, the grids connected with the AC link. Because of the fluctuation in load and other reason frequency fluctuate continuously.
So, it is difficult to interconnect.
But if the interconnection is done through the HVDC link, the effect of frequency disturbance of one system cannot transfer in other systems.
There are many other scopes of the HVDC system and if it is feasible economically and technically, there are many chances of development.