Automatic Voltage Regulator (AVR) Definition: A synchronous generator automatic voltage regulator is used to maintain the synchronous generator voltage at a predetermined value or change the terminal voltage according to a plan. When the terminal voltage and reactive power of the synchronous generator change, the exciter's output current is automatically controlled according to the corresponding feedback signal, thereby achieving automatic adjustment of the synchronous generator's terminal voltage or reactive power. Automatic voltage regulators (AVRs) for synchronous generators can be divided into three categories: thyristor automatic voltage regulators, TD1 type carbon film resistor automatic voltage regulators, and multi-phase excitation automatic voltage regulators. The following are the working principles of these three types of synchronous generator automatic voltage regulators.

1. Thyristor Automatic Voltage Regulation: This voltage regulation method refers to connecting a thyristor in series or parallel in the excitation circuit to control the excitation current, thereby enabling the generator's output voltage to automatically adjust with changes in load. There are many methods for controlling a thyristor: one is to use an oscillating circuit composed of a unijunction transistor to generate a trigger pulse, changing the charging voltage of the capacitor, thereby controlling the generation time of the trigger pulse and thus changing the conduction angle of the thyristor; another is to use the switching characteristics of a transistor to change the charging voltage of the capacitor, controlling the conduction time of the transistor to generate a trigger pulse, which can also control the conduction angle of the thyristor.

2. TD1 Type Carbon Film Resistor Automatic Voltage Stabilizer
This voltage stabilization method is applied to the 6135ZD diesel generator set. Its working principle is as follows: when the generator load is at its rated value, the automatic voltage regulator remains stable. At this time, the generator excitation current, voltage, and main excitation current are all stable. As the generator load increases, the voltage decreases, and the automatic voltage regulator begins to adjust the carbon film resistor, decreasing its resistance, thereby increasing the generator excitation current and raising the generator output voltage; conversely, when the load decreases, the automatic voltage regulator adjusts the carbon film resistor, increasing its resistance, thereby decreasing the excitation current and lowering the voltage.

3. Phase-Compound Automatic Voltage Regulation

For special equipment with significant load variations during startup and operation, phase-compound automatic voltage regulation is preferable. Therefore, in the power supply systems of special equipment, the generator control section mostly adopts phase-compound automatic voltage regulation. The basic principle of phase-compound automatic voltage regulation is as follows: When the generator is unloaded, the residual magnetizing voltage of the armature tap winding is phase-shifted by 90° through a linear reactor. After rectification by a three-phase bridge rectifier, the output DC current flows to the magnetic field and the excitation winding. When the residual magnetizing voltage is too low, DC current can be used for charging.

When the generator is under load, its load current flows through the primary winding of the current transformer, generating a secondary current proportional to the primary winding current. When the load power factor changes, the required excitation current also changes accordingly, increasing or decreasing accordingly. Through appropriate parameter settings, the required excitation current can be provided to the generator, thereby automatically regulating the voltage to maintain stability within a certain range. Due to its characteristics, current transformers are widely used in engineering construction and special equipment fields.