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Generator excitation method and working principle
**Generator Excitation**
The generator excitation system refers to the power supply and its associated equipment that provide 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. The excitation system plays a critical role in maintaining stable operation of the power system by regulating the generator’s voltage, reactive power, and overall stability.
Over the past decade, advancements in technology have led to continuous improvements in excitation systems. New methods and devices have emerged, making excitation systems more efficient, reliable, and responsive. In particular, automatic excitation regulators have evolved significantly, offering better control over the generator’s performance during both steady-state and transient conditions.
**System Composition**
The excitation power unit is responsible for supplying the DC current needed to energize the rotor windings. Meanwhile, the excitation regulator controls the output of the power unit based on input signals and predefined control criteria. This system ensures that the generator operates efficiently and maintains the desired terminal voltage under varying load conditions.
The excitation system also includes various components such as transformers, rectifiers, and control units. Together, these elements form a control system that manages the excitation process and enhances the stability of the power grid. The importance of the excitation system cannot be overstated, as it directly affects the safe and stable operation of both the generator and the broader power network.
**Main Functions of the Excitation System**
1. Adjust the excitation current to maintain the generator's terminal voltage at a constant level.
2. Control the distribution of reactive power among parallel-operating generators.
3. Improve the static stability of the power system.
4. Enhance the transient stability of the system during disturbances.
5. Initiate demagnetization in case of internal faults to minimize damage.
6. Apply maximum and minimum excitation limits based on operational requirements.
**Methods of Obtaining Excitation Current**
1. **DC Exciter System**: A dedicated DC generator, known as a DC exciter, is used to supply the excitation current. It is usually mounted on the same shaft as the generator. While this method offers reliable operation, it has slower response times and higher maintenance requirements, making it less common in modern large-scale units.
2. **AC Exciter System**: An AC exciter is used to generate alternating current, which is then rectified and supplied to the generator’s rotor. This system is often referred to as a static excitation system. It provides faster response times and improved reliability compared to traditional DC exciters.
3. **Self-Excited Static Excitation**: In this method, the generator itself acts as the source of excitation power. The excitation current is derived from the generator’s output through a rectifier transformer. This system is simpler and requires fewer components, but it may introduce harmonic distortions into the system.
**Characteristics of Generator and Excitation Current**
1. **Voltage Regulation**: The excitation system adjusts the generator’s voltage by controlling the excitation current. As the reactive load increases, the terminal voltage tends to drop. To maintain stability, the excitation current must be adjusted accordingly.
2. **Reactive Power Adjustment**: When operating in parallel with the grid, changes in the excitation current affect the generator’s reactive power output. Proper adjustment ensures that the generator contributes the correct amount of reactive power to the system.
3. **Reactive Load Distribution**: Parallel-operating generators distribute reactive load based on their capacity. The excitation system helps maintain a balanced distribution by adjusting the excitation current of each generator.
**Automatic Adjustment of Excitation Current**
Rather than directly adjusting the rotor circuit, which involves high currents, the excitation system typically modifies the excitation current of the exciter. This can be done by changing the resistance in the excitation circuit, adjusting the conduction angle of thyristors, or using other electronic control methods.
Modern automatic excitation systems use solid-state devices such as transistors and thyristors. These systems offer fast response times, high reliability, and the ability to suppress overvoltage during faults. They are composed of several key units, including a measurement unit, synchronization unit, amplification unit, and limiting unit, all working together to ensure precise control.
**Components and Auxiliary Equipment**
The excitation system includes various components such as voltage and current transformers, excitation transformers, and control relays. These devices provide essential signals and power to the excitation regulator. Additionally, protective features like demagnetization switches and overcurrent protection are integrated to ensure safe operation.
In the event of an internal fault, the system must quickly demagnetize the rotor to prevent damage. This is achieved through either linear or nonlinear resistance-based demagnetization methods, depending on the system design.
With ongoing technological advancements, digital excitation systems are becoming more prevalent. These systems use microprocessors to provide more accurate and adaptive control, further enhancing the performance and stability of power generation systems.