The excitation system of a generator is one of the most crucial systems in a generator and the entire power system, and is often referred to as the heart of the generator. Today, we will provide a comprehensive and detailed explanation from three aspects: principle, working mode, and function.
I. Basic Principle
Core concept: Electricity generates magnetism, and magnetism generates electricity.
2. The basis of power generation: The generator operates based on Faraday's law of electromagnetic induction - a closed conductor moving through a magnetic field and cutting the magnetic lines of force will generate an electric current.
3. Source of the magnetic field: A strong magnetic field is required. This magnetic field is generated by an excitation winding (i.e., the rotor winding) that carries direct current. This direct current is referred to as the excitation current.
4. Key to control: By adjusting the magnitude of the excitation current, the strength of the magnetic field can be controlled, thereby determining the voltage output by the generator and the amount of reactive power it generates.
Increase the excitation current → Strengthen the magnetic field → Increase the electric potential within the generator → Tend to raise the voltage and output more inductive reactive power.
Reducing the excitation current → Weakening the magnetic field → Decrease in the electrical potential within the generator → Tendency to lower the voltage, output less inductive reactive power (even absorbing reactive power).
Therefore, the fundamental task of the excitation system is to provide a controllable direct current excitation current for the rotor of the generator.
II. Working Method (System Composition and Process)
1. The modern excitation system mainly adopts the static self-excited system. The working principle flowchart is shown as follows:
2. The core components involved in the above process and their functions are as follows:
1) Excitation Transformer: It receives energy from the machine end, serving the functions of voltage reduction and electrical isolation, thereby protecting low-voltage equipment such as the rectifier bridge.
2) Thyristor rectifier bridge: Converts alternating current into controllable direct current. By changing the trigger angle, the magnitude of the direct current output voltage can be smoothly adjusted, thereby precisely controlling the excitation current.
3) Automatic Voltage Regulator (AVR): The brain of the system. It continuously monitors the terminal voltage of the generator and compares it with a set value. Once a deviation is detected (such as a decrease in voltage due to an increase in load), it immediately issues an instruction to change the trigger angle of the thyristors, increase the excitation current, and bring the voltage back to the set value. This is a high-speed, continuous closed-loop control process.
4) Magnetization suppression and rotor overvoltage protection device: The guardian of the system.
5) Magnetization Elimination: In the event of internal faults or shutdown of the generator, it is necessary to quickly and safely eliminate the rotor magnetic field. This is achieved by transferring the magnetic energy to the magnetization elimination resistor for dissipation, and it is the most effective measure for protecting the generator.
6) Overvoltage Protection: Prevent damage to insulation in the rotor circuit caused by system surges, asynchronous operation, etc., which may generate high voltages.
Other methods: Besides the mainstream self-excited system, there are also the traditional but still in use excitation from separately excited system (which is powered by an independent excitation machine) and the brushless excitation system (which uses rotating diodes for rectification and does not require carbon brushes or slip rings).
III. Main Functions
The excitation system is far more than just providing direct current. It also performs other important functions:
1. Voltage control and regulation
This is the most fundamental function. Through the automatic regulation of AVR, the voltage at the generator terminal is maintained at the rated level, ensuring the quality of electrical energy and not being affected by changes in load.
2. Control and distribution of reactive power
When operating in grid connection mode, adjusting the excitation current can control the reactive power sent or absorbed by the generator to the grid.
Boosting excitation: Outputting inductive reactive power to the grid to support grid voltage.
Demagnetizing: Reducing the reactive power output, or even absorbing inductive reactive power from the grid (asymmetric operation) to reduce excessively high system voltage.
This is crucial for maintaining the voltage stability and reactive power balance of the grid.
3. Improving the parallel operation stability of synchronous generators
Static stability: A powerful and responsive excitation system (such as using PSS power system stabilizer) can effectively suppress small oscillations in the system and increase the power limit of transmission lines.
Transient stability: When a serious fault such as a short circuit occurs in the grid, the voltage drops sharply.
The excitation system will perform forced excitation, instantly raising the excitation voltage to its maximum value, injecting a large amount of reactive power into the system, helping to restore the grid voltage and maintaining the generator from losing synchronization.
4. Protection Function
Through magnetization interruption and overvoltage protection, the safety of the generator rotor body is ensured.
IV. Summary
1. Principle: Control the excitation current to regulate the magnetic field strength, and ultimately adjust the output voltage and reactive power of the generator.
2. Working mode: Usually adopts a self-excited system: powered by the excitation transformer, rectified by thyristors, and automatically controlled by AVR to achieve precise and rapid regulation.
3. Functions
1) Voltage regulation: Maintain voltage stability.
2) Reactive power regulation: Control the reactive power flow in the power grid.
3) Stability maintenance: Enhance the static and transient stability of the system.
4) Safety protection: Magnetization extinction and overvoltage protection.
It can be said that without a high-performance excitation system, modern large synchronous generators would not be able to be reliably and stably connected to the power grid, and we would not be able to enjoy high-quality and highly reliable electricity.