Automatic Voltage Regulators (AVR) for Generator

It is important to keep voltage constant (within limits) at various levels in a power system. There are various methods of voltage control at various levels. An Automatic Voltage Regulator (AVR) is used for controlling the output voltage of a generator (alternator) in a power station.
The terminal voltage of a generator fluctuates due to change in supplied load or several other reasons. Therefore, an automatic voltage regulator (AVR) is provided with every generator in a power station. There are many types of automatic voltage regulators. Earlier AVR for generators were electromechanical systems, but a modern AVR uses solid-state devices such as thyristors.

Working principle of an Automatic Voltage Regulator

Basically, an Automatic Voltage Regulator is a feedback control system. The output voltage of the Generator is measured using a Potential Transformer then rectified, filtered and fed to a comparator. This comparator copares the obtained output voltage with a set reference voltage. The difference between generator output voltage and the reference voltage is called as the error voltage. This error voltage is amplified and then used to control the excitation of the Generator.

An AVR controls the terminal voltage using excitation control method, i.e. the output voltage of the terminal can be kept by varying the field current of the alternator.

If the excitation current of an alternator is increased, the output voltage of the alternator will also increase. The relationship between the excitation current (field current) and the output voltage of an alternator can be described by the following equation:

  • V is the output voltage of the alternator.
  • K is a constant that depends on the characteristics of the alternator.
  • Φ is the magnetic flux produced by the rotor (field current).
  • N is the number of turns in the stator windings.
  • f is the frequency of the output voltage.

From this equation, you can see that the output voltage (V) is directly proportional to the excitation current (Φ). Therefore, if you increase the excitation current by adjusting the excitation system (typically controlled by an Automatic Voltage Regulator or AVR), the magnetic flux (Φ) increases, resulting in a higher output voltage (V).

Conversely, if you decrease the excitation current, the magnetic flux decreases, leading to a lower output voltage. This relationship allows for precise control of the alternator's output voltage, which is essential for maintaining a stable and regulated voltage supply in power generation and distribution systems.

Voltage Sensing: The AVR constantly monitors the terminal voltage of the generator. This voltage sensing is typically done through a potential transformer (PT) or a voltage transformer (VT), which steps down the generator's voltage to a manageable level for the AVR to process.

Comparison and Error Detection: The AVR compares the sensed voltage to a reference voltage or setpoint. This reference voltage is the desired output voltage that the generator should maintain. Any deviation from this setpoint voltage is considered an error.

Error Amplification: To correct the error, the AVR uses an error amplifier. This amplifier magnifies the error signal to a level suitable for controlling the generator's excitation system.

Excitation System Control: The excitation system is responsible for providing the necessary field current to the generator's rotor windings. This field current determines the strength of the magnetic field and, consequently, the generator's output voltage. The AVR adjusts the excitation system to either increase or decrease the field current based on the error signal.

Adjustment of Field Current: If the sensed voltage is below the setpoint voltage, the AVR increases the field current, strengthening the magnetic field. This, in turn, boosts the generator's terminal voltage. Conversely, if the sensed voltage is above the setpoint, the AVR reduces the field current, weakening the magnetic field and lowering the generator's voltage.

Feedback Loop: The entire process is iterative and continuous, creating a feedback loop. As the AVR makes adjustments, it keeps monitoring the voltage, ensuring it stays within the desired range. This continuous correction maintains a stable and regulated output voltage.

Thyristor Control (Modern AVRs): In modern AVRs, solid-state devices like thyristors are used for precise control of the excitation system. Thyristors are semiconductor devices that allow for rapid and accurate adjustments to the field current. They offer a faster response time compared to older electromechanical systems, enabling tighter voltage control.

Protection Features: AVRs often come with built-in protection features. These features can include over-voltage protection, under-voltage protection, over-excitation protection, and more. They safeguard the generator and connected equipment from voltage-related issues that could cause damage.

Communication and Monitoring: In many setups, AVRs are integrated into a larger control and monitoring system. This allows operators to remotely monitor the generator's voltage and receive alerts or alarms if any voltage-related issues arise.

In conclusion, an Automatic Voltage Regulator plays a vital role in maintaining a stable voltage output from a generator. It continuously monitors the voltage, compares it to a setpoint, and adjusts the excitation system to ensure that the generator produces the desired voltage. With the advent of solid-state devices like thyristors, modern AVRs offer precise and efficient voltage control, contributing to the reliability and performance of power systems in various applications.

Quick Voltage Control Using Overshooting the Mark Principle

Many quick-acting automatic voltage regulators (AVRs) employ "overshooting the mark principle" to maintain voltage stability. For example, Tirril Regulator and Brown-Boveri Regulator.

When the system encounters increased load, the AVR will try to increase the excitation current by increasing the exciter voltage. But due to large inductance of the alternator, the field current (excitation current) will take some time to reach the desired steady value. This delay can be too slow to react to sudden load changes.

To address this, when the load on the alternator rises, the regulator reacts by initially boosting the excitation more than required. This quick increase speeds up the voltage rise. But, before the voltage gets too high, the regulator steps in to lower the excitation to the correct level. This approach ensures a faster response to load changes without allowing voltage to overshoot its target.