EHV a.c. and d.c. links

Introduction

EHV (Extra High Voltage) a.c. and d.c. links play a crucial role in power transmission. These links are used to transmit electricity over long distances and connect different power systems. Understanding the fundamentals of EHV a.c. and d.c. transmission is essential to comprehend the constitution, limitations, advantages, and applications of these links.

Limitations and Advantages of a.c. and d.c. transmission

Limitations of a.c. transmission

1. Power Loss: A.c. transmission systems suffer from higher power losses due to the resistance and reactance of the transmission lines.
2. Voltage Drop: Voltage drop occurs in a.c. transmission lines, leading to a decrease in the delivered voltage at the load end.
3. Limited Distance: The distance over which a.c. transmission is feasible is limited due to the reactive power requirements and voltage drop.

Advantages of a.c. transmission

1. Easy Conversion: A.c. voltage can be easily converted to different voltage levels using transformers.
2. Existing Infrastructure: A.c. transmission can utilize the existing infrastructure of power systems.
3. Lower Cost: A.c. transmission systems are generally more cost-effective compared to d.c. transmission systems.

Limitations of d.c. transmission

1. Higher Cost: D.c. transmission systems are generally more expensive to install and maintain compared to a.c. transmission systems.
2. Limited Voltage Conversion: D.c. voltage conversion is more complex and requires additional equipment compared to a.c. transmission.
3. Limited Applications: D.c. transmission is suitable for specific applications such as long-distance transmission and interconnecting different power systems.

Advantages of d.c. transmission

1. Lower Power Loss: D.c. transmission systems have lower power losses compared to a.c. transmission systems.
2. Higher Power Handling Capacity: D.c. transmission can handle higher power levels compared to a.c. transmission.
3. Improved Voltage Control: D.c. transmission allows for better voltage control and stability.

Principal application of a.c. and d.c. transmission

Applications of a.c. transmission

A.c. transmission is widely used in the following applications:

1. Distribution Networks: A.c. transmission is used for distributing electricity to residential, commercial, and industrial consumers.
2. Grid Interconnections: A.c. transmission links different power systems and enables the exchange of power between them.
3. Short-Distance Transmission: A.c. transmission is suitable for short-distance transmission within a power system.

Applications of d.c. transmission

D.c. transmission finds its principal applications in the following areas:

1. Long-Distance Transmission: D.c. transmission is preferred for long-distance transmission due to its lower power losses and higher power handling capacity.
2. Undersea Cables: D.c. transmission is used for transmitting power through undersea cables.
3. HVDC Systems: High Voltage Direct Current (HVDC) systems utilize d.c. transmission for interconnecting power systems and transmitting power over long distances.

Trends in EHV a.c. and d.c. transmission

Advancements in EHV a.c. and d.c. transmission have led to improved efficiency, reliability, and power handling capacity. Some of the trends in EHV transmission include:

1. Higher Voltage Levels: EHV transmission systems are operating at higher voltage levels to minimize power losses and increase power transfer capability.
2. Advanced Converter Technology: The development of advanced converter technology has improved the performance of EHV d.c. transmission systems.
3. Integration of Renewable Energy: EHV transmission systems are being adapted to integrate renewable energy sources into the grid.

Power handling capacity

The power handling capacity of EHV a.c. and d.c. links depends on various factors such as the voltage level, current rating, and system design. The power handling capacity can be increased by:

1. Increasing Voltage Level: Higher voltage levels allow for the transmission of more power.
2. Optimizing System Design: Efficient system design and component selection can enhance the power handling capacity.
3. Improving Converter Technology: Advancements in converter technology can improve the power handling capability of EHV d.c. links.

Constitution of EHV a.c. and d.c. links

Components of EHV a.c. links

EHV a.c. links consist of the following components:

1. Generators: Generators produce the electrical power that is to be transmitted.
2. Step-up Transformers: Step-up transformers increase the voltage level of the generated power for efficient transmission.
3. Transmission Lines: Transmission lines carry the high-voltage a.c. power over long distances.
4. Step-down Transformers: Step-down transformers decrease the voltage level of the transmitted power for distribution.
5. Substations: Substations are used for switching, controlling, and monitoring the flow of power in the transmission system.

Components of EHV d.c. links

EHV d.c. links consist of the following components:

1. Converter Stations: Converter stations convert a.c. power to d.c. power at the sending end and convert d.c. power back to a.c. power at the receiving end.
2. Converter Transformers: Converter transformers are used to step-up or step-down the voltage level of the d.c. power.
3. D.C. Transmission Lines: D.C. transmission lines carry the d.c. power over long distances.
4. Filtering Equipment: Filtering equipment is used to reduce harmonics and ensure smooth transmission of d.c. power.
5. Substations: Substations are used for switching, controlling, and monitoring the flow of power in the transmission system.

Types of d.c. links

There are two main types of d.c. links used in EHV transmission:

Line-commutated converters

Line-commutated converters (LCC) are widely used in EHV d.c. transmission systems. These converters utilize thyristors for the conversion of a.c. power to d.c. power and vice versa. LCCs offer good controllability and are suitable for long-distance transmission.

Voltage-source converters

Voltage-source converters (VSC) are another type of d.c. link used in EHV transmission. These converters use insulated-gate bipolar transistors (IGBTs) or gate turn-off thyristors (GTOs) for the conversion of a.c. power to d.c. power and vice versa. VSCs provide better controllability and are suitable for applications requiring fast response and grid integration.

Advantages and disadvantages of EHV a.c. and d.c. links

Advantages of EHV a.c. links

1. Existing Infrastructure: EHV a.c. links can utilize the existing infrastructure of power systems, reducing the need for additional investments.
2. Easy Voltage Conversion: A.c. voltage can be easily converted to different voltage levels using transformers.
3. Lower Cost: EHV a.c. transmission systems are generally more cost-effective compared to d.c. transmission systems.

Disadvantages of EHV a.c. links

1. Higher Power Loss: A.c. transmission systems suffer from higher power losses due to the resistance and reactance of the transmission lines.
2. Limited Power Handling Capacity: EHV a.c. links have a limited power handling capacity compared to d.c. links.
3. Voltage Drop: Voltage drop occurs in a.c. transmission lines, leading to a decrease in the delivered voltage at the load end.

Advantages of EHV d.c. links

1. Lower Power Loss: EHV d.c. transmission systems have lower power losses compared to a.c. transmission systems.
2. Higher Power Handling Capacity: EHV d.c. links can handle higher power levels compared to a.c. links.
3. Improved Voltage Control: D.c. transmission allows for better voltage control and stability.

Disadvantages of EHV d.c. links

1. Higher Cost: EHV d.c. transmission systems are generally more expensive to install and maintain compared to a.c. transmission systems.
2. Limited Voltage Conversion: D.c. voltage conversion is more complex and requires additional equipment compared to a.c. transmission.
3. Limited Applications: EHV d.c. transmission is suitable for specific applications such as long-distance transmission and interconnecting different power systems.

Summary

EHV a.c. and d.c. links are essential for power transmission over long distances and interconnecting different power systems. A.c. transmission has advantages such as easy voltage conversion and utilization of existing infrastructure, but it suffers from higher power losses and limited power handling capacity. D.c. transmission, on the other hand, offers lower power losses, higher power handling capacity, and improved voltage control, but it is more expensive and has limited applications. The constitution of EHV a.c. and d.c. links involves various components such as generators, transformers, transmission lines, and substations. There are two main types of d.c. links: line-commutated converters (LCC) and voltage-source converters (VSC). Advancements in EHV transmission include higher voltage levels, advanced converter technology, and integration of renewable energy. Understanding the advantages and disadvantages of EHV a.c. and d.c. links is crucial for selecting the appropriate transmission system for specific applications.

Analogy

Imagine EHV transmission as a highway system. A.c. transmission can be compared to a multi-lane highway with existing infrastructure, allowing for easy access and conversion between different routes. However, due to traffic congestion and limited lane capacity, the multi-lane highway has limitations in terms of power handling capacity and efficiency. On the other hand, d.c. transmission can be compared to a dedicated expressway with fewer entry points but higher power handling capacity and smoother traffic flow. The expressway requires additional investment in terms of construction and maintenance, but it offers advantages in terms of lower power losses and improved control.