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GATE 2024 EE Q.47: solution and answer

GATE 2024 · EEQ.47 · 2 marks · MSQ

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

  1. (A)

    AGC response has a local effect on frequency while AVR response has a global effect on voltage.

  2. (B)

    AGC response has a global effect on frequency while AVR response has a local effect on voltage.

  3. (C)

    AGC regulates the field current of the synchronous generator while AVR regulates the generator's mechanical power input.

  4. (D)

    AGC regulates the generator's mechanical power input while AVR regulates the field current of the synchronous generator.

See the question in the paper
GATE 2024 EE question 47Tap to open full size

Answer and solution

ANSWER

Options (B) and (D) are correct.

Official answer: B, DSource: Official answer key, GATE 2024 EE (IISc Bengaluru)

  1. Identify what each controller acts on. AGC (load-frequency control) changes the load reference setpoint of the turbine governor, which changes the prime-mover (turbine) output. So AGC regulates the generator's mechanical power input .

  2. AVR measures the terminal voltage and adjusts the exciter output, which changes the field current of the synchronous generator. So (D) is correct. (C) swaps the two actuators, so it is wrong.

  3. Frequency is common to the whole interconnected synchronous system. A real-power imbalance moves the speed of all the synchronous machines together, and the steady-state change is So the effect of AGC on frequency is global.

  4. Voltage depends on reactive power flow. Reactive power is supplied mainly by nearby sources, because carrying it over long distances causes large voltage drops across the line reactances. So the effect of AVR on voltage is local, near the generator bus. Hence (B) is correct. (A) swaps global and local, so it is wrong.

  5. Correct options: B and D.

KEY IDEA

AGC acts on the turbine's mechanical power (real power, frequency), and its effect on frequency is system-wide. AVR acts on the field current (reactive power, voltage), and its effect on voltage is local.

WATCH OUT FOR

Swapping the two loops: thinking AGC controls the field current and AVR controls the mechanical input. Or thinking frequency is a local quantity and voltage is a global one. It is the opposite: frequency is common to the whole synchronous grid, while voltage is set by local reactive flow.

From the GATE 2024 EE paper. Every question of this paper · Download the full paper · All 9 Dispatch & control questions, with notes

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