What is HVDC Transmission System?
High voltage direct current (HVDC) power systems use direct current (DC) for the transmission of bulk power over long distances. HVDC transmission lines are less expensive and have lower losses than alternating current (AC) transmission lines, making them a more efficient and cost-effective option for transmitting power over long distances. HVDC lines can also be used to interconnect networks that have different frequencies and characteristics.
In a combined AC and DC transmission system, the generated AC voltage is converted into DC at the sending end using a rectifier. The DC voltage is then inverted back into AC at the receiving end using an inverter, for distribution purposes. This requires the use of conversion and inversion equipment at both ends of the transmission line.
HVDC transmission is typically only economical for long distance transmission lines with a length of more than 600 km and for underground cables with a length of more than 50 km. For shorter distances, AC transmission is typically more cost-effective.
Types of HVDC System:
- Monopolar HVDC Transmission System
- Bipolar HVDC Transmission System
- Homopolar HVDC Transmission System
- Back-to-Back Coupling System
- Multi-Terminal HVDC System.
1. Monopolar HVDC Transmission System:
A monopolar transmission link is a type of electrical transmission system that uses a single conductor to transmit electricity over a long distance. The single conductor carries a direct current (DC) of negative polarity, while the return path for the current is provided by the earth or sea. Alternatively, a metallic return conductor may also be used.In a monopolar transmission link, two converters are located at the end of each pole, which are used to convert the DC electricity into alternating current (AC) for consumers. The poles of the transmission link are grounded using earth electrodes placed a significant distance away from the terminal stations, typically 15 to 55 km.
One of the main disadvantages of a monopolar transmission link is that it uses earth or sea as the return path for the current, which can result in large currents flowing through the earth or sea. This can cause problems with other underground utilities and structures and may also have environmental impacts. Additionally, the use of earth or sea as the return path can result in voltage drop over long distances, reducing the transmission link's efficiency.
Due to these disadvantages, monopolar transmission links are not widely used in modern electrical transmission systems. Instead, other types of transmission systems, such as bipolar or multiterminal DC systems, are typically used. These systems use multiple conductors and return paths, providing higher transmission efficiency and reducing the potential for interference with other underground utilities and structures.
Advantages:
- Simple design: Because a monopolar transmission link uses only a single conductor, the design is relatively simple compared to other types of transmission systems.
- Low maintenance: Monopolar transmission links may require less maintenance than other types of transmission systems because they have fewer components.
- High charging currents: Monopolar transmission links can handle high charging currents, making them technically feasible for certain applications.
- Economical: Monopolar transmission links may be more economical to install and operate than other types of transmission systems, particularly for low-power rating links such as cable transmission.
Disadvantages:
- Shut down on fault: If a fault occurs on the conductor in a monopolar transmission link, the entire transmission system must be shut down until the fault is repaired. This can be a significant disadvantage if the fault occurs in a remote location and it takes a long time to repair.
- Limited power rating: Monopolar transmission links are typically only used for low-power rating links such as cable transmission. They may not be suitable for transmitting large amounts of power over long distances.
- Affects compasses: Monopolar transmission links may affect the accuracy of magnetic compasses on ships when they pass over underwater cables. This can be a problem for navigation.
2. Bipolar HVDC Transmission System:
A bipolar high voltage direct current (HVDC) transmission link is a type of electrical transmission system that uses two conductors to transmit electricity over a long distance. One conductor carries a direct current (DC) of positive polarity, while the other conductor carries a DC of negative polarity. Under normal conditions, the current in the two conductors is the same, and there is no ground current.
One of the main advantages of a bipolar HVDC transmission link is that it can transmit large amounts of power over long distances more efficiently than an alternating current (AC) transmission link. This is because DC electricity does not suffer from the same losses as AC electricity when it is transmitted over long distances.
A single bipolar HVDC transmission link is equivalent to two AC transmission lines, as it can transmit the same amount of power as two AC lines. The voltage rating of a bipolar HVDC transmission link is given as ±X kV, where X represents the voltage magnitude. The midpoint of the converters in a bipolar HVDC transmission link is grounded, which helps to stabilize the voltage.
In the event of a fault on a bipolar HVDC transmission link, the system can automatically switch to a monopolar mode by using earth as the return path conductor. This allows the other conductor to continue supplying power to the load while the fault is being repaired.
Advantages:
- Continuous transmission: A bipolar HVDC transmission link allows for continuous transmission of power between two stations or on the mainline, as it uses two conductors to transmit the power.
- Fault isolation: If a fault occurs on one link in a bipolar HVDC transmission system, it does not affect the operation of the other links, as each link is independent of the others.
- Monopolar mode: In the event of a fault, a bipolar HVDC transmission link can automatically switch to a monopolar mode, using earth as the return path conductor. This allows the other conductor to continue supplying power to the load while the fault is being repaired.
- Reversible power flow: The direction of power flow in a bipolar HVDC transmission link can be changed by reversing the polarities of the two conductors.
- Higher voltage: The voltage in a bipolar HVDC transmission link is twice that of a monopolar link between the poles, which allows for more efficient transmission of power over long distances.
Disadvantages:
- Corona and radio interference: Bipolar HVDC transmission links may produce more corona and radio interference than other types of transmission systems, such as homopolar links.
- Complicated connection: The connection of a converter to a pole in a bipolar HVDC transmission link can be complicated, as it requires two conductors to be connected.
- High cost: Bipolar HVDC transmission links may be more costly to install and maintain than other types of transmission systems, such as monopolar links.
3. Homopolar HVDC Transmission System:
A homopolar high voltage direct current (HVDC) transmission link is a type of electrical transmission system that uses two conductors of the same polarity, usually negative, to transmit electricity over a long distance. In the event of a fault on one of the conductors, the converters of the healthy pole are able to supply more than 50% of the power to the remaining conductors.
One of the main advantages of a homopolar HVDC transmission link is that it can continue to operate even if a fault occurs on one of the conductors. This makes it a reliable option for transmitting power over long distances.
In a homopolar HVDC transmission link, earth is used as the return conductor. This means that the current flows through earth as it returns to the converter station. During faulty conditions, the homopolar HVDC transmission link can also operate as a monopolar link, using earth as the return path conductor.
Advantages:
- Lower cost: Homopolar HVDC transmission links are generally cheaper to install and operate than three-phase AC transmission lines of the same ratings.
- Reduced corona and radio interference: The use of negative polarity conductors in a homopolar HVDC transmission link can greatly reduce corona and radio interference.
- Independent operation: Homopolar HVDC transmission links can continue to operate independently even if a fault occurs on one of the conductors.
- Simplified connection: The connection of the converter to the pole in a homopolar HVDC transmission link is generally less complicated than in a bipolar HVDC transmission link, as it uses only one polarity of conductor.
Disadvantages:
- Ground current effects: The presence of ground currents in a homopolar HVDC transmission link may have adverse effects on pipelines passing through nearby areas.
- Limited applications: Homopolar HVDC transmission links may have limited applications due to the presence of ground currents.
- Higher cost at higher voltages: The cost of a homopolar HVDC transmission link may increase at higher voltages due to the need for additional equipment and infrastructure.
4. Back-to-Back HVDC Coupling System:
A back-to-back HVDC coupling system is a type of electrical transmission system that connects two alternating current (AC) networks operating at different frequencies. It uses a back-to-back converter to rectify and invert the AC electricity in the same substation, eliminating the need for a direct current (DC) transmission line.
Back-to-back HVDC coupling systems are mainly used to improve system stability and control the magnitude of voltage and frequency independently in two networks. They can also be used to rapidly exchange power between the two networks.
Overall, a back-to-back HVDC coupling system is a useful tool for connecting two AC networks operating at different frequencies and can provide a range of benefits such as improved system stability and the ability to control voltage and frequency independently.
Advantages:
- Allows for the independent control of voltage and frequency in two networks
- Power flow is fast, accurate, and fully controllable
- Allows for the determination of power flow in the link
- Short circuit levels can be limited
- Can couple two networks at different frequencies
- Allows for the determination of daily and seasonal costs
Disadvantages:
- Generates harmonics
- Expensive due to complicated converters and DC switchgear
- Can lead to the contamination of water with insulators when located near the seacoast.
5. Multi-Terminal HVDC System:
A multi-terminal high voltage direct current (HVDC) system is a type of electrical transmission system that consists of three or more converter substations. Some of the converter stations in a multi-terminal HVDC system act as rectifiers, converting AC electricity into DC electricity, while others act as inverters, converting DC electricity back into AC electricity.
Multi-terminal HVDC systems can be connected in either series or parallel, depending on the specific requirements of the transmission system. In a series connection, the converter stations are connected in a chain, with the DC current flowing from one converter station to the next. In a parallel connection, the converter stations are connected alongside each other, with the DC current flowing through multiple paths.
Overall, a multi-terminal HVDC system is a flexible and efficient way to transmit large amounts of power over long distances. It is commonly used in modern electrical transmission systems to connect multiple AC networks or to transmit power between distant locations.
Advantages:
- Allows for the transmission of power between multiple points, rather than just two
- Can be used to connect isolated AC systems or to transmit power over long distances
- Can control the power flow between different points, allowing for efficient transmission of power
Disadvantages:
- Can be more complex and costly to install and maintain compared to traditional HVDC systems
- May require more advanced control and protection systems
- May require more complex converter stations due to the increased number of terminals
Difference Between HVDC links
- A monopolar HVDC transmission system uses a single conductor with negative polarity to transmit power, with earth or water being used as the return path. This type of system is simple and requires less maintenance, but it is only suitable for low-power rating links and can be affected by faults on the conductor.
- A bipolar HVDC transmission system uses two conductors, one with positive polarity and one with negative polarity, to transmit power. The voltage between the poles in a bipolar link is twice that of a monopolar link, and the direction of power flow can be changed by changing the polarities of the two poles. Bipolar links are continuous and can continue operating in monopolar mode in the event of a fault on one link. However, they are more expensive and may cause more corona and radio interference than other types of links.
- A homopolar HVDC transmission system also uses two conductors, but they are both of the same polarity, usually negative. This type of system is cheaper than a three-phase AC line of the same ratings and has reduced corona and radio interference. However, it may have an adverse effect on pipelines passing through nearby areas due to the presence of ground currents and has limited applications because of these ground currents.
- A back-to-back HVDC coupling system does not have a DC transmission line and instead uses a back-to-back converter to rectify and invert AC power in the same substation. This type of system is used to connect two AC systems that cannot be directly connected or to stabilize an AC system. It is generally used for small power transfer and has a low capital cost, but it is limited in terms of the power it can transmit.
- A multi-terminal HVDC system consists of three or more converter substations that are connected in series or parallel according to the requirements of the power transmission system. Some of the converter stations act as rectifiers, converting AC power to DC power, while others act as inverters, converting DC power back to AC power. This type of system allows for the transmission of power between multiple points, rather than just two, as is the case with traditional HVDC systems.