Electronic automatic voltage regulator

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

 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.