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Generator excitation method and working principle
**Generator Excitation**
The generator excitation system is a comprehensive set of power supply and associated equipment that provides the direct current (DC) required to energize the rotor of a synchronous generator. It typically consists of two main components: the excitation power unit and the excitation regulator. These systems play a critical role in maintaining stable and efficient operation of the generator within the power grid.
Over the past decade, advancements in technology have significantly influenced the development of excitation systems. New methods, such as static excitation and brushless excitation, have emerged, offering improved performance and reliability. In particular, automatic excitation regulators have evolved to enhance the stability of parallel-operating units within power systems. As modern power grids become more complex, the need for advanced excitation control has increased, leading to continuous improvements in excitation technology.
**System Composition**
The excitation power unit supplies the DC current needed for the generator's rotor, while the excitation regulator controls this output based on input signals and predefined adjustment criteria. Together, they form the excitation control system, which is essential for the safe and stable operation of the generator and the overall power system.
The excitation system can be divided into two primary parts: the power source that provides the excitation current and the regulator that manages it. The image below illustrates the basic layout of a typical excitation system:
**Main Functions of the Excitation System**
1. Adjust the excitation current according to load changes to maintain a constant terminal voltage.
2. Control the distribution of reactive power between parallel generators.
3. Enhance the static stability of the power system during normal operations.
4. Improve the transient stability during disturbances or faults.
5. Perform demagnetization quickly in case of internal generator faults to minimize damage.
6. Apply maximum and minimum excitation limits based on operational requirements.
**Excitation Methods**
1. **DC Exciter Excitation**: This traditional method uses a dedicated DC generator, known as a DC exciter, which is usually mounted on the same shaft as the main generator. While reliable and independent, it suffers from slow response and high maintenance costs, making it less common in large-scale applications.
2. **AC Exciter Excitation**: In this method, an AC exciter generates alternating current, which is then rectified and supplied to the generator rotor. This approach offers faster response times and reduced mechanical wear compared to DC excitation. It is widely used in modern power plants.
3. **Self-Excited Static Excitation**: This method does not require a separate exciter. Instead, the generator itself supplies the excitation current after being rectified. It is cost-effective and simple but may introduce harmonic distortions and noise.
**Adjusting the Excitation Current**
Direct adjustment of the excitation current in the rotor circuit is impractical due to high current levels. Instead, the excitation current of the exciter is modified to indirectly control the generator’s field current. Common techniques include varying the resistance in the excitation circuit, adjusting thyristor conduction angles, or using additional excitation sources.
Modern excitation systems often use thyristor-based controllers, which offer fast response, high precision, and better fault handling. These devices are composed of transistors and thyristors, providing reliable and efficient control.
**Components of Automatic Excitation Systems**
An automatic excitation system typically includes the following components:
- Measurement unit: Monitors voltage, current, and other parameters.
- Synchronization unit: Ensures proper triggering of thyristors.
- Amplification unit: Boosts the signal to control the excitation.
- Adjustment unit: Regulates the excitation current to maintain stability.
- Stabilization unit: Improves system stability.
- Limitation unit: Prevents over-excitation or under-excitation.
Additional components such as transformers, relays, and protection circuits are also included to ensure safe and reliable operation.
**Brushless Excitation Systems**
In large synchronous generators, brushless excitation systems are commonly used. These systems eliminate the need for slip rings and brushes, reducing maintenance and improving reliability. The excitation current is generated by a rotating armature and rectified directly on the rotor, making the system more efficient and durable.
As power systems continue to evolve, so too will excitation technologies. The integration of digital control systems and microprocessors is paving the way for smarter, more responsive excitation solutions, ensuring the continued stability and efficiency of power generation.