Notes

Q 1(B) What is the power carrying capability of EHV AC transmission lines?Express the percent power loss in terms of power carrying capability?

The power carrying capability of Extra High Voltage (EHV) AC transmission lines is typically in the range of several thousand megawatts. The exact capacity depends on various factors such as the type of conductor used, the size of the towers, the height of the towers, the spacing between the towers, the weather conditions in the area, and the design of the transmission line.

The percent power loss in EHV AC transmission lines is usually expressed in terms of the power carrying capability of the line. The power loss is typically a small fraction of the total power transmitted, typically less than 2%. The power loss is due to resistance in the conductors and other components of the transmission line. The resistance results in heating of the conductors and power loss as heat. In addition, there may also be losses due to capacitance and inductance in the transmission line, which can cause additional power loss.

It's important to note that the power loss in transmission lines is an important factor to consider in the design and operation of power systems, as it affects the efficiency of the system and the amount of energy that can be transmitted over a given distance. Therefore, it's crucial to design EHV transmission lines to minimize power loss while ensuring reliable and safe operation.

Q1(c) What are the reactive power problems associated with EHV AC systems?
Reactive power problems in Extra High Voltage (EHV) AC systems can have significant impacts on the system performance and reliability. Some of the most common reactive power problems associated with EHV AC systems are:

Voltage Stability: Reactive power consumption by loads, especially non-linear loads, can result in voltage instability, leading to voltage sags or voltage collapse.

Power factor: Poor power factor due to reactive power consumption by inductive loads can cause increased losses in the transmission and distribution systems.

Harmonic distortion: Harmonic distortion caused by non-linear loads can result in increased losses, overheating of equipment, and reduced reliability of the system.

Overloading of transformers: Overloading of transformers due to reactive power consumption can cause thermal and mechanical stress, leading to reduced life of the transformers.

Contingency analysis: Reactive power imbalances can result in reduced reliability of the system during contingency events, such as faults or outages.

To mitigate these problems, it is important to monitor and control the reactive power flow in EHV AC systems and to ensure that the system is operated within its specified limits. This may involve the use of reactive power compensators, such as static VAR compensators (SVCs), and flexible AC transmission systems (FACTS) devices.

Q 1(d) What are the advantages and disadvantages of HVDC systems?

High-Voltage Direct Current (HVDC) systems offer a number of advantages and disadvantages compared to Alternating Current (AC) systems.

Advantages of HVDC systems:

Efficient power transmission: HVDC systems are more efficient than AC systems for long-distance power transmission, as they result in lower transmission losses over long distances.

Reduced transmission losses: HVDC systems can result in reduced transmission losses compared to AC systems, due to their lower resistance and lower reactance.

Improved system stability: HVDC systems can improve the stability of interconnected AC power systems by providing a means of controlling power flows between systems.

Ease of control: HVDC systems are easier to control and monitor than AC systems, as they do not experience the same issues with phase shift and harmonic distortion.

Improved reliability: HVDC systems are less affected by faults and are therefore more reliable than AC systems.

Disadvantages of HVDC systems:

High initial cost: The initial cost of HVDC systems is generally higher than that of AC systems, due to the need for rectifiers, converters, and DC transmission lines.

Limited availability of equipment: HVDC equipment and components may be less readily available than AC equipment, which can result in longer lead times and higher costs.

Maintenance requirements: HVDC systems require more maintenance than AC systems, due to the need to maintain the rectifiers, converters, and DC transmission lines.

Interference with communication systems: HVDC systems can interfere with communication systems, due to their DC nature and the presence of high-frequency harmonics.

Limitations on integration with AC systems: HVDC systems can be more difficult to integrate with AC systems, as they require specific transformers and AC/DC interfaces to be installed.

In conclusion, the choice of HVDC or AC systems depends on the specific requirements of each application, such as transmission distance, cost, reliability, and system stability.