There are five types of HVDC links
1. Monopolar link
2. Bipolar link
3. Homopolar link
4. Back-to-back HVDC system
5. Multi-terminal HVDC system
1. Monopolar link
The only single conductor is used in monopolar line and having negative polarity and uses ground or sea as a return path for current. In monopolar link, sometimes metallic or water return is used when the earth resistivity is very higher.
In a monopolar link, two converter stations are placed at sending and receiving ends. Each converter station is earthed through the earth electrode placed about 15 to 55km away from the converter station. Each converter station and earth electrode are connected through an electrode line. The monopolar lines operate with negative polarity with respect to earth. So corona loss and radio interference are reduced in the monopolar link.
The monopolar links are more economical than a bipolar line because only one conductor is used in monopolar links so cost and losses in it are reduced. The monopolar links are only used for lower power ratting and cable transmission lines. The monopolar links are not more used because monopolar links use the ground as the return path.
The rated current for the monopolar link is 200A to 1000A and the current flows in one direction only.
Advantages
1. This system has a simple design.
2. This system requires less maintenance because only one conductor is used in this system.
3. This system is very economical.
Disadvantages
1. If the fault occurred in the conductor then the entire system failed to operate.
2. This system is only used for low power rating cable transmission lines.
2. Bipolar link
Bipolar link is mostly used in HVDC systems compared to other HVDC systems. The bipolar link consists of two conductors. One is a positive conductor and the second is a negative conductor with respect to earth. In normal operating conditions, the current in both the conductor is the same and the current in-ground is absent.
In a bipolar HVDC system voltage difference between two conductors is twice and the voltage of the earth electrode is half of the phase conductor of the HVDC system. The bipolar link is mostly used to transmit a bulk amount of power over long distances.
The major advantage of the bipolar link is that if any one of the links is failed to operate in bipolar links, then the remaining links operated as a monopolar link with the ground return. In monopolar links ground current increased and cause excessive heat on the earth electrode if the earth electrode resistance is very high. So half of the HVDC system continues to work as a monopolar link and supply the power. The bipolar link is mainly used link in the HVDC system.
Advantages
1. In an HVDC line, the direction of power flow can be changed by changing the polarities of the two poles.
2. This link works as a monopolar link during the fault.
3. This system is mostly used HVDC system for bulk amounts of power transfer.
Disadvantages
1. The corona loss and radio interference are more in the bipolar link.
2. The connection of converter poles is very complicated.
3. This system uses two conductors so this system is costly as compared to monopolar link.
3. Homopolar link
The homopolar link consists of a two-conductor with negative polarity with respect to the ground return or metallic return. If the poles of the homopolar links work in parallel then the insulation cost of the equipment is reduced.
In homopolar links, corona loss and radio interference are reduced. If one conductor is failed to transmit the power then the second conductor supplies the power with ground return with a capacity of 50%. The main disadvantage of this system is the larger return current in-ground which causes the losses.
The homopolar link provides the following features
1. Improve the system stability
2. Rapid variation in power exchange
3. Control over the magnitude of voltage and frequency.
Advantages
1. This system is cheaper than ac transmission line.
2. The conductor polarity in the homopolar link is negative so radio interference and corona loss are reduced in the homopolar link.
3. The homopolar link is work as monopolar links under fault conditions.
4. The connection of the converter is not complicated as compared to the bipolar link.
Disadvantages
1. This system is used for limited application due to the presence of the earth's current.
2. For higher voltages cost of the system increased.
4. Back-to-back HVDC system
The back-to-back HVDC system does not have any dc line between two conversion stations. The inversion and rectification are carried out in the same substation by back to back converter. This system is also known as the High Voltage DC coupling system.
This system is used to connect two ac systems at the same locations. This system is used to make the asynchronous interconnection between two independently(having different frequencies) connected ac substations.
Advantages
1. In Back to the back system, Frequency and voltage are controlled independently.
2. In Back to the back system, power flow is fast and accurate.
3. In back to the back system, power flow is determined in link and the short circuit level is limited.
4. By using back to a back system, coupling of two networks with different frequencies is possible.
Disadvantages
1. In back to the back system, harmonics are generated and this system is very complicated and expensive.
5. Multi-terminal HVDC links
These HVDC systems have three or more number of substations. Some of them are connected as inverter stations and some of them are acts as rectifier stations. These substations are connected in series or parallel as per requirement.