All questions of Voltage & Frequency Control for Electrical Engineering (EE) Exam

Shunt Capacitance in EHV Line

Shunt capacitance is an important parameter of an EHV transmission line. It is the capacitance between the line conductors and the ground.

Importance of Shunt Capacitance

- Shunt capacitance helps in maintaining the voltage level of the transmission line.
- It also helps in reducing the corona losses.
- Shunt capacitance is responsible for the charging current of the transmission line.
- It affects the overall transmission line impedance.

Restoring Shunt Capacitance to Improve Voltage

The shunt capacitance in an EHV line is restored to improve the voltage level of the transmission line. When the shunt capacitance is low, the voltage level of the transmission line decreases. This can lead to power losses and also affect the stability of the system.

By restoring the shunt capacitance, the voltage level can be improved. This is particularly important in long transmission lines where the voltage drop is significant. The shunt capacitance helps in maintaining the voltage level within acceptable limits.

Conclusion

Shunt capacitance is an important parameter of an EHV transmission line. It helps in maintaining the voltage level and reducing corona losses. Restoring shunt capacitance can improve the voltage level of the transmission line and ensure stable operation.

Understanding Series Capacitors in Transmission Lines
Series capacitors are used in transmission lines primarily to improve power factor and increase the transmission capacity by compensating for inductive reactance. However, their effectiveness varies based on the load conditions.
Load VAR Requirements
- Large Load VAR Requirement:
- When there is a large load VAR requirement, series capacitors can be beneficial as they provide sufficient reactive power support, improving voltage stability and reducing line losses.
- Small Load VAR Requirement:
- In cases where the load VAR requirement is small, series capacitors become less effective. This is because:
- Overcompensation: A small VAR requirement may lead to overcompensation, causing the system to operate at an undesirable leading power factor.
- Voltage Regulation Issues: If the capacitors are too large for the small load, they may cause voltage regulation problems, leading to voltage rise and potential equipment damage.
- Fluctuating Load VAR Requirement:
- Series capacitors can also be beneficial in situations with fluctuating VAR demands. They can adjust to varying loads, thus providing necessary reactive power dynamically.
Conclusion
In summary, series capacitors are less useful when the load VAR requirement is small due to the risk of overcompensation and subsequent voltage regulation issues. Understanding the specific load conditions is crucial for effective application of series capacitors in transmission line systems.

Between voltage and current, b) improves power factor, and c) reduces reactive power demand.

Synchronous phase modifier is a device that is used to control the reactive power flow in a power system. It is connected in parallel with the transmission line and can either supply or absorb reactive power depending on the requirement. Let's discuss the answer option by option.

Option A - Both active and reactive power: This option is incorrect because a synchronous phase modifier can only control the reactive power flow and cannot supply active power.

Option B - Both lagging and leading reactive power: This option is correct. A synchronous phase modifier can supply both lagging and leading reactive power. When the load is inductive, it absorbs lagging reactive power. The synchronous phase modifier can supply this reactive power to the load. On the other hand, when the load is capacitive, it requires leading reactive power. The synchronous phase modifier can absorb this reactive power from the load.

Option C - Inductive reactive power only: This option is incorrect because a synchronous phase modifier can supply both lagging and leading reactive power.

Option D - None of the above: This option is incorrect because option B is the correct answer.

Therefore, option B is the correct answer.

Passive Element for Stable Operation of Interconnected System

Passive elements play a crucial role in the stable operation of interconnected systems. These elements do not introduce energy into the system but rather modify the existing energy in the system. The passive element that can be used as an interconnecting element is a reactor.

Reactor as an Interconnecting Element

A reactor is a passive element that is used to introduce inductance into the circuit. It is used to limit the flow of current in the circuit and to stabilize the voltage. Reactors are commonly used in power systems to limit current surges and to reduce the impedance of the system.

Advantages of Using Reactors as Interconnecting Elements

- Limiting Current Surges: Reactors can limit the flow of current in the circuit, preventing current surges that can damage the system.

- Stabilizing Voltage: Reactors can stabilize the voltage in the system by reducing the impedance of the system.

- Preventing Harmonic Distortion: Reactors can prevent harmonic distortion in the system by limiting the flow of harmonic currents.

Conclusion

In conclusion, a reactor is a passive element that can be used as an interconnecting element in interconnected systems. It can limit current surges, stabilize voltage, and prevent harmonic distortion. Therefore, the use of reactors is crucial for the stable operation of interconnected systems.

Introduction:
Phase modifiers are installed in transmission lines to improve the performance and stability of the system. They are used to compensate for the reactive power in long length transmission lines.

Explanation:
1. Short transmission lines:
Short transmission lines are typically less than 80 km in length. These lines have a low voltage drop and low reactance. Since the length is short, the power factor correction can be achieved using other devices such as capacitors and reactors. Therefore, phase modifiers are not normally installed in the case of short transmission lines.

2. Medium length lines:
Medium length transmission lines are typically between 80 km and 250 km in length. These lines have a moderate voltage drop and moderate reactance. In some cases, phase modifiers may be required to compensate for the reactive power and improve power factor correction. However, it is not a common practice to install phase modifiers in medium length lines.

3. Long length lines:
Long length transmission lines are typically greater than 250 km in length. These lines have a significant voltage drop and high reactance. The high reactance causes a large phase angle difference between the voltage and current, resulting in a poor power factor. Phase modifiers are installed in long length lines to compensate for this reactive power and improve power factor correction. They help in maintaining voltage stability, reducing losses, and improving the overall performance of the transmission system.

4. For all length lines:
Although phase modifiers are not normally installed in short and medium length transmission lines, there may be exceptional cases where phase modifiers are required based on specific system requirements. However, as a general rule, phase modifiers are commonly installed in long length transmission lines to address the reactive power issues and improve power factor correction.

Conclusion:
Phase modifiers are normally installed in the case of long length transmission lines to compensate for the reactive power and improve power factor correction. While short and medium length lines may not require phase modifiers, there may be exceptional cases where they are needed based on specific system requirements.