Control of EHV d.c. system

Introduction

The control of EHV (Extra High Voltage) d.c. system plays a crucial role in EHV A.C. and D.C. Transmission. It involves the management and regulation of various parameters to ensure the efficient and reliable operation of the system. This topic explores the desired features, control characteristics, and different control methods used in EHV d.c. systems.

Key Concepts and Principles

Desired Features of Control in EHV d.c. System

The control of EHV d.c. system aims to achieve the following desired features:

1. Stability: The system should be stable under normal and abnormal operating conditions.
2. Fast Response: The control system should respond quickly to changes in load and system conditions.
3. Accuracy: The control system should accurately regulate the desired parameters within acceptable limits.
4. Reliability: The control system should be reliable and capable of withstanding faults and disturbances.

Control Characteristics in EHV d.c. System

The control characteristics in EHV d.c. system can be broadly classified into three categories:

1. Constant Current Control: This control method maintains a constant current in the d.c. system by adjusting the firing angle of the converter.
2. Constant Extinction Angle Control: This control method maintains a constant extinction angle in the d.c. system by adjusting the firing angle of the converter.
3. Ignition Angle Control: This control method maintains a constant ignition angle in the d.c. system by adjusting the firing angle of the converter.

Constant Current Control

Constant current control is a widely used control method in EHV d.c. systems. It involves adjusting the firing angle of the converter to maintain a constant current in the system. The firing angle determines the conduction period of the converter, which directly affects the output current. By regulating the firing angle, the control system can adjust the output current to match the desired value.

Advantages of Constant Current Control:

• Provides a stable and constant current output
• Suitable for applications where a constant current is required

Disadvantages of Constant Current Control:

• Limited flexibility in adjusting other system parameters
• May result in higher losses and reduced efficiency

Constant Extinction Angle Control

Constant extinction angle control is another control method used in EHV d.c. systems. It involves adjusting the firing angle of the converter to maintain a constant extinction angle in the system. The extinction angle is the delay between the zero-crossing of the input voltage and the initiation of the converter conduction. By regulating the firing angle, the control system can adjust the extinction angle to match the desired value.

Advantages of Constant Extinction Angle Control:

• Provides a stable and constant extinction angle
• Suitable for applications where a constant extinction angle is required

Disadvantages of Constant Extinction Angle Control:

• Limited flexibility in adjusting other system parameters
• May result in higher losses and reduced efficiency

Ignition Angle Control

Ignition angle control is another control method used in EHV d.c. systems. It involves adjusting the firing angle of the converter to maintain a constant ignition angle in the system. The ignition angle is the delay between the zero-crossing of the input voltage and the initiation of the converter conduction. By regulating the firing angle, the control system can adjust the ignition angle to match the desired value.

Advantages of Ignition Angle Control:

• Provides a stable and constant ignition angle
• Suitable for applications where a constant ignition angle is required

Disadvantages of Ignition Angle Control:

• Limited flexibility in adjusting other system parameters
• May result in higher losses and reduced efficiency

Parallel Operation of HVAC & DC System

Parallel operation of HVAC (High Voltage Alternating Current) and DC (Direct Current) systems is a common practice in power transmission and distribution networks. It involves the simultaneous operation of both systems to meet the load demand and ensure system reliability. However, parallel operation can introduce various challenges and problems that need to be addressed.

Problems Associated with Parallel Operation

1. Voltage and Frequency Control Issues: When operating in parallel, the voltage and frequency of the HVAC and DC systems need to be synchronized. Any deviation in voltage or frequency can lead to power quality issues and affect the performance of connected loads.
2. Reactive Power Control Issues: The reactive power exchange between the HVAC and DC systems needs to be properly regulated to maintain system stability and voltage control.
3. Harmonic Distortion Issues: The operation of power electronic converters in the DC system can introduce harmonics, which can affect the overall power quality and cause interference with other connected equipment.

Solutions to the Problems

1. Voltage and Frequency Control Solutions: To address voltage and frequency control issues, advanced control algorithms and communication systems can be implemented to ensure synchronization between the HVAC and DC systems. This includes monitoring and adjusting the voltage and frequency at various points in the system.
2. Reactive Power Control Solutions: Reactive power control devices such as static VAR compensators (SVCs) and STATCOMs can be used to regulate the reactive power exchange between the HVAC and DC systems. These devices can provide fast and accurate control of reactive power to maintain system stability.
3. Harmonic Distortion Control Solutions: Harmonic filters and active power filters can be employed to mitigate the harmonic distortion caused by power electronic converters in the DC system. These filters can remove or reduce the harmonics, ensuring a clean and distortion-free power supply.

Real-world Applications and Examples

There are several real-world applications of EHV d.c. systems with different control mechanisms. Some examples include:

• High voltage direct current (HVDC) transmission systems used for long-distance power transmission
• Interconnection of asynchronous AC systems
• Integration of renewable energy sources into the grid

Case studies of successful implementation of control of EHV d.c. system can provide valuable insights into the practical challenges and solutions in real-world scenarios.

Advantages and Disadvantages of Control of EHV d.c. System

Advantages of Control of EHV d.c. System

• Improved power transfer capability: The control of EHV d.c. system allows for efficient and reliable power transmission over long distances.
• Enhanced system stability: The control mechanisms help maintain system stability under various operating conditions.
• Flexibility in power flow control: The control system enables the regulation of power flow and reactive power exchange between different systems.

Disadvantages of Control of EHV d.c. System

• Complex control algorithms: The control of EHV d.c. system requires sophisticated control algorithms and communication systems, which can be complex to design and implement.
• High initial cost: The installation and implementation of control systems in EHV d.c. systems can involve high initial costs.
• Maintenance and operational challenges: The control systems require regular maintenance and monitoring to ensure their proper functioning.

Conclusion

In conclusion, the control of EHV d.c. system is essential for the efficient and reliable operation of EHV A.C. and D.C. Transmission. It involves the management and regulation of various parameters to achieve desired features and control characteristics. Constant current control, constant extinction angle control, and ignition angle control are the main control methods used in EHV d.c. systems. Parallel operation of HVAC and DC systems can introduce challenges, but with proper control solutions, these challenges can be overcome. Real-world applications and case studies provide practical insights into the implementation of control of EHV d.c. system. While there are advantages to control, there are also disadvantages such as complex control algorithms and high initial costs. Overall, the control of EHV d.c. system plays a crucial role in ensuring the efficient and reliable operation of power transmission and distribution systems.

Summary

The control of EHV d.c. system is essential for the efficient and reliable operation of EHV A.C. and D.C. Transmission. It involves the management and regulation of various parameters to achieve desired features and control characteristics. Constant current control, constant extinction angle control, and ignition angle control are the main control methods used in EHV d.c. systems. Parallel operation of HVAC and DC systems can introduce challenges, but with proper control solutions, these challenges can be overcome. Real-world applications and case studies provide practical insights into the implementation of control of EHV d.c. system. While there are advantages to control, there are also disadvantages such as complex control algorithms and high initial costs. Overall, the control of EHV d.c. system plays a crucial role in ensuring the efficient and reliable operation of power transmission and distribution systems.

Analogy

Imagine a traffic control system that regulates the flow of vehicles at a busy intersection. The control system ensures that the traffic moves smoothly, without any congestion or accidents. Similarly, the control of EHV d.c. system regulates the flow of power in a power transmission network, ensuring efficient and reliable operation.