Introduction:
The reactive power delivered by a synchronous generator can be controlled by changing the excitation. This control mechanism allows for the adjustment of the generator's reactive power output, which is important for maintaining the stability and reliability of power systems.

Explanation:
The synchronous generator is an important component of a power system as it provides both active power (real power) and reactive power. The active power is responsible for driving electrical loads, while the reactive power is required to maintain voltage levels and support the operation of inductive loads.

Importance of Reactive Power Control:
Reactive power control is essential for maintaining voltage stability, reducing transmission losses, and ensuring the reliable operation of power systems. By controlling the reactive power output of a synchronous generator, it is possible to regulate the voltage levels and improve the power factor of the system.

Controlling Reactive Power:
There are various methods to control the reactive power output of a synchronous generator, including the use of automatic voltage regulators (AVRs) and excitation control systems. Among these methods, changing the excitation of the generator is the most common and effective way to control reactive power.

Role of Excitation:
The excitation system of a synchronous generator is responsible for controlling the magnetic field strength in the generator's rotor. By adjusting the excitation level, the generator's reactive power output can be controlled. The excitation system typically consists of an exciter, which supplies the necessary DC current to the generator's field winding, and an automatic voltage regulator (AVR), which controls the excitation level based on the system requirements.

Effect of Excitation on Reactive Power:
When the generator's excitation is increased, the magnetic field strength in the rotor is strengthened. This results in an increase in the reactive power output of the generator. Conversely, when the excitation is decreased, the reactive power output decreases.

Benefits of Reactive Power Control:
By controlling the reactive power output of a synchronous generator, power system operators can regulate the voltage levels and improve the power factor. This helps in reducing transmission losses, improving voltage stability, and ensuring the reliable operation of the power system.

Conclusion:
In summary, the reactive power delivered by a synchronous generator can be controlled by changing the excitation. By adjusting the excitation level, the generator's reactive power output can be regulated, which is essential for maintaining voltage stability and supporting the operation of inductive loads.