While the turbine control system regulates frequency via speed, there is a parallel system whose job is to regulate something equally important: the generator's output voltage. This is the role of the Automatic Voltage Regulator (AVR).
Where Does a Generator's Voltage Originally Come From?
Recall from generator components that output voltage results from the rotating magnetic field (produced by the field/excitation current in the rotor) passing in front of the stator windings. The greater the excitation current, the stronger the magnetic field, and the higher the generated output voltage — for the same rotational speed.
Why Does Voltage Need Regulation?
When the load connected to the generator changes (loads drawn increase or decrease), the output voltage tends to change as well, even if the speed remains constant — due to the internal interaction of the generator with the drawn current. Since equipment and the grid require a stable voltage within tight limits, the voltage cannot simply be left to "do whatever happens."
How Does the AVR System Work?
| Step | Explanation |
|---|---|
| 1. Measurement | The system continuously measures the generator's output voltage |
| 2. Comparison | The measured voltage is compared to the required reference voltage |
| 3. Correction | If the voltage is below the required value, the excitation (field) current is increased — and if it is higher, it is decreased |
| 4. Stabilization | The output voltage is restored to the reference value within fractions of a second |
Similarity to Frequency Control
You can think of the AVR as the "sibling" of the turbine control system (governor) from the article on frequency control:
- Governor: regulates speed (and thus frequency) by controlling the mechanical driving energy (fuel/steam/water).
- AVR: regulates voltage by controlling the magnetic excitation (field) current.
Both are continuous automatic control loops, each operating on a different variable, and both are essential to the quality of electricity delivered to the grid.
When preparing a generator for synchronization with the grid (see generator synchronization), the operator uses the AVR to adjust the generator's voltage to precisely match the grid voltage — one of the four conditions for safe synchronization.
The excitation (field) current does not generate the generator's primary electrical power — it is a relatively small current whose only job is to create the magnetic field. The large primary energy comes from the mechanical rotation of the turbine. The AVR "adjusts" a generator that is already rotating; it does not "drive" it.
Sample answer: Each is an automatic control loop that regulates a different variable in the generator. The governor regulates the turbine's rotational speed, and therefore the generator's output frequency, by controlling the amount of incoming mechanical driving energy (fuel, steam, or water). The AVR, on the other hand, regulates the generator's output voltage by controlling the excitation (field) current that creates the magnetic field in the rotor: increasing the excitation current raises the voltage, and decreasing it lowers the voltage. Both work together continuously to keep frequency and voltage within required limits despite changes in load.
Thinking that increasing the excitation (field) current increases the generator's overall "power" output. The excitation current only creates the magnetic field needed to generate voltage, while the actual power generated is governed by the mechanical driving energy coming from the turbine — two separate matters controlled by two different systems (AVR and governor).
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