--- title: "Electronic automatic voltage regulator" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:electronic-avr" source: https://www.bctengineer.com/q/semester_8/power_plant_design/electronic-avr site: BCT Engineering --- # Electronic automatic voltage regulator Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2079 Chaitra, 8 marks - Video: [Automatic Voltage Regulator(AVR) For Alternator - How Does It Work | AVR for Generator | Hindi](https://www.youtube.com/watch?v=3FCBovUH0-M) — ElectricalTalks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/electronic-avr - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Describe the working principle of electronic based automatic voltage regulator with suitable diagrams. ## Short answer An electronic AVR is a closed-loop PID controller that rectifies terminal voltage, compares it with a stable reference, and adjusts thyristor firing angle to control exciter field. Limiters and stabilising feedback prevent hunting and overheating. ## Answer An **electronic automatic voltage regulator (AVR)** keeps the generator terminal voltage at the set value by measuring it continuously and adjusting the field current through power-electronic switches (thyristors or transistors), with no moving parts. #### Principle It is a **closed-loop (feedback) control system**. The error between the reference voltage $V_{ref}$ and the measured voltage $V_t$ is amplified and used to change the exciter field: $$ E_{fd} = K_A\,(V_{ref} - V_t + V_s) $$ where $K_A$ is the amplifier gain and $V_s$ is the signal from the stabiliser (PSS). If $V_t$ falls (for example, when load increases), the error rises, the field current increases and the voltage comes back. #### Block diagram ```text Vref -->( + )--> Amplifier -> Firing -> Thyristor ^ - (PID) circuit bridge | | | v | Exciter / field | | Rectifier v & filter <------- PT <------- Generator Vt ^ Stabilising feedback / limiters ``` #### Main parts and their work 1. **Sensing (PT + rectifier + filter):** steps down the terminal voltage, rectifies and smooths it into a DC signal proportional to $V_t$. Often the reactive current from a CT is added (**load compensation/droop**) so parallel units share vars. 2. **Comparator (error detector):** compares the sensed signal with a stable **reference** (zener or digital set-point). 3. **Amplifier / controller:** op-amp or digital PID amplifies the error. 4. **Firing circuit:** converts the control voltage into a firing angle $\alpha$ for the thyristors. Smaller $\alpha$ gives more field voltage, since $V_{dc} = 1.35\,V_{ac}\cos\alpha$ for a full bridge. 5. **Power stage:** thyristor bridge feeding the exciter field (brushless) or main field (static). 6. **Stabilising feedback:** rate feedback from the field voltage, or a PSS, damps hunting and oscillations. 7. **Limiters and protection:** over-excitation limiter, under-excitation limiter, V/Hz limiter, stator current limiter. #### Operation - Steady state: firing angle fixed, $V_t = V_{ref}$. - Load increases or lagging power factor: $V_t$ drops, error positive, $\alpha$ decreases, field current rises, $V_t$ restored. - Load rejection: $V_t$ rises, $\alpha$ increases (or inverts), field reduced. #### Modes - **Auto mode:** voltage control as above. - **Manual mode:** field current control, used for testing or when the PT fails. #### Advantages over electromechanical regulators - Fast response with no dead band. - No moving contacts, so little maintenance. - Accurate regulation (about ±0.5 %). - Easy to add limiters, PSS and remote control. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What type of control system is an electronic AVR? A. Manual only B. Closed-loop feedback comparing Vref with Vt C. Mechanical governor D. Open-loop Answer: B. The error between reference and measured voltage is amplified to adjust the field. (Key point: Principle) 2. If load increases and Vt falls, what does the AVR do? A. Increases firing angle so field falls B. Does nothing C. Decreases firing angle so field current rises D. Trips the unit Answer: C. A smaller alpha gives more field voltage (Vdc = 1.35 Vac cos alpha), restoring Vt. (Key point: Response to load) 3. Why is reactive current from a CT added in the sensing circuit? A. To flash the field B. To filter the PT C. To raise the speed D. Load compensation/droop so parallel units share vars Answer: D. This droop prevents parallel generators from fighting each other over reactive power. (Key point: Droop) 4. What is the role of stabilising feedback or PSS? A. Measures speed B. Damps hunting and oscillations C. Increases the gain D. Replaces the PT Answer: B. It damps oscillations that a high-gain AVR would otherwise cause. (Key point: Stabiliser) ## Related in other subjects - [Zener voltage regulator working](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Azener-voltage-regulator) — Basic Electronics Engineering, chapter 2 - [Boost regulator working principle](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aboost-regulator-principle) — Advanced Electronics, chapter 8, asked 2× - [Brayton cycle working principle with P-v and T-s](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Abrayton-cycle-working-principle) — Fundamental of Thermodynamics & Heat Transfer, chapter 6 - [Ideal diesel cycle working principle](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Adiesel-cycle-working-principle) — Fundamental of Thermodynamics & Heat Transfer, chapter 6