HVDC link is a connection between two ac grids in order to transmit very high power at different frequencies. It is also used to solve stability problems in ac grids. By using HVDC links, the power can be transmitted for longer distances. Basically, there are five types of HVDC links that are used for power transmission. They are,
- Monopolar HVDC Transmission System
- Bipolar HVDC Transmission System
- Homopolar HVDC Transmission System
- Back to Back Coupling System
- Multi-Terminal HVDC System.
Monopolar HVDC Transmission System :
An HVDC link that uses only a single conductor is known as a mono-polar link. Usually, in this type of link, only a single conductor with negative polarity is used, in order to reduce corona and interference. Earth or water is used as the return path.
However, a metallic conductor is used as a return path when earth resistivity is very high. The power and current flows only in one direction. For monopolar transmission systems, the rated current is from 200A to 1000A. The below figure represents the mono-polar HVDC link.
Advantages of Mono-polar Link :
- It uses only a single conductor. Hence, the design is very simple.
- It requires less maintenance.
- Because of high charging currents, these links are technically feasible than HVAC systems.
- It is economical.
Disadvantages of Mono-polar Link :
- When a fault occurs on the conductor the entire transmission system is shut down.
- These are used only for low-power rating links, like cable transmission.
- It affects the magnetic compasses of ships when it passes over underwater cables.
Bipolar HVDC Transmission System :
An HVDC link that uses two conductors for transmitting the power and current is known as bipolar links. Generally, these type of systems uses two conductors. One with positive polarity and the other with negative polarity. The below figure represents the bipolar HVDC link.
Under normal conditions, the current in the two poles is the same. Hence, the ground current is absent. Whenever a fault occurs on these systems then they automatically switch to the monopolar system by using earth as a return path conductor i.e., when one pole undergoes fault condition, the other will continue to supply the load.
A single bipolar high voltage direct current line is equal to two ac transmission lines. When compared to the monopolar link the voltage is twice between the poles in this system. The mid-point of the converters are grounded. The voltage rating is given as ± X KV where X represents voltage with its magnitude.
Advantages of Bipolar HVDC Link :
- The transmission of power between two stations or on the mainline is continuous.
- The fault on one link does not affect the operation of another link.
- During fault conditions, this link can also be used as the monopolar link.
- The direction of power flow can be changed by changing the polarities of the two poles.
- The voltage in the bipolar link is twice between the poles when compared to the voltage between the pole and the earth of a monopolar link.
Disadvantages of Bipolar HVDC Link :
- Corona and radio interference is more when compared with a homo-polar link.
- The connection of a converter to a pole is complicated.
- It is quite costly when compared to mono-polar links.
Homopolar HVDC Transmission System :
These links also use two conductors but of the same polarity. Usually of negative polarity. When a fault occurs on the conductor the converters of the healthy pole are quite enough to feed the remaining conductors. Which are able to supply more than 50% of the power. In this type of link, the earth is used as a return conductor. It also acts as a monopolar link during faulty conditions.
Advantages of Homopolar HVDC Link :
- It is comparatively cheaper than a three-phase ac line of the same ratings.
- Corona and radio interference are greatly reduced with the use of negative polarity conductors.
- These links can be operated independently under faulty conditions.
- The connection of the converter to the pole is not so complicated as the bipolar link.
Disadvantages of Homopolar HVDC Link :
- The presence of ground current may have an adverse effect on the pipelines passing through the nearby areas.
- It has limited applications due to the presence of ground currents.
- The cost of the line increases for higher voltages.
Back to Back HVDC Coupling System :
It has no dc transmission line. Rectification and Inversion are done in the same substation by a back-to-back converter. The figure below shows the back-to-back HVDC coupling.
The back-to-back HVDC coupling is mainly used to interconnect two ac networks operating at different frequencies. It also provides features like improving system stability, rapid variations in the power exchange, and control over the magnitude of voltage and frequency independently in two networks.
Multi-Terminal HVDC System :
A multi-terminal HVDC system consists of three or more converter substations in which some of the converter stations act as the rectifiers and some of them as the inverters. The substations are either connected in series or parallel according to the requirements. The below shows the bipolar multi-terminal HVDC system.
