Which of the following options is/are correct for the Automatic Generation Control (AGC) and Automatic Voltage Regulator (AVR) installed with synchronous generators ?

The correct options are:
1. AGC response has a global effect on frequency while AVR response has a local effect on voltage.
2. AGC regulates the generator’s mechanical power input while AVR regulates the field current of the synchronous generator.

1. Understanding the Functions and Control Actions (AGC vs. AVR):
Let us analyze how each control loop operates and what it regulates:
• Automatic Generation Control (AGC): The primary objective of AGC (also known as load-frequency control) is to maintain the system frequency and establish net power interchanges between control areas. Frequency is directly tied to the rotation speed of the synchronous generators, which in turn depends on the balance between total generation and total load. If the load increases, the generators slow down, causing the frequency to drop. To restore the frequency, AGC adjusts the governor speed-changer position, which regulates the mechanical power input (steam, water, or gas flow) to the generator's turbine.
• Automatic Voltage Regulator (AVR): The primary objective of AVR is to regulate the terminal voltage of the synchronous generator. It accomplishes this by adjusting the exciter voltage, which in turn regulates the field current flowing through the generator's rotor winding. Adjusting the field current directly controls the internal generated electromotive force (EMF) and the reactive power output, thereby regulating the terminal voltage.

2. Analyzing the Spatial Scope of Response (Global vs. Local Effects):
• System Frequency (Global Effect): In an interconnected AC power system, all synchronous generators operate in synchronism at the same electrical frequency. A change in load or generation anywhere in the network affects the rotational speed of all connected machines, meaning the system frequency is uniform across the entire grid. Therefore, the frequency control action of AGC has a global effect on the network.
• System Voltage (Local Effect): Voltage profiles are determined by the local balance of reactive power. Unlike active power and frequency, reactive power cannot travel long distances across transmission lines without causing significant voltage drops and line losses. Consequently, adjusting the field excitation at one generator location primarily controls the voltage in its immediate vicinity. Therefore, the voltage control action of AVR has a local effect on the network.