The generator excitation system is the core system that provides controllable direct current (excitation current) to the rotor windings of the generator. It directly determines the voltage stability, power factor regulation, and grid-connected operation capability of the generator. The following elaborates in five key dimensions:
I. Core Functions of the System
1. Provide excitation current: Inject direct current into the rotor windings of the generator to create a rotating magnetic field, which cuts the magnetic lines of the stator windings to achieve the energy conversion from "mechanical energy to electrical energy".
2. Stabilize terminal voltage: When the grid load or rotational speed changes, adjust the magnitude of the excitation current in real time to maintain the output terminal voltage of the generator at the rated value (such as 10.5 kV), avoiding damage to equipment due to excessive or insufficient voltage.
3. Adjust power factor: By changing the excitation current ("over-excitation" or "under-excitation"), adjust the reactive power generated by the generator to optimize the power factor of the grid and reduce line losses.
4. Ensure grid connection synchronization: During grid connection, through excitation regulation, make the generator voltage and frequency match those of the grid, achieving smooth grid connection; after grid connection, maintain the synchronous operation of the unit and the grid.
5. Fault protection: When a short circuit or loss of synchronization occurs in the system, quickly cut off or reduce the excitation current (magnetization extinction) to prevent the rotor winding from overheating or the generator from being damaged.
II. Main Equipment and Functions
Excitation power supply: The "source" that provides excitation current. It is divided into two types: - DC exciter: Commonly used in small units, directly outputting DC; - Static excitation system (mainstream): Converts AC (taken from the generator stator or the grid) into DC through a rectifier device.
Excitation regulator (AVR): The "brain" of the system, controlling the excitation current. It collects real-time voltage and current signals from the generator, compares them with the rated values, and outputs control instructions to regulate the excitation power supply to ensure voltage stability.
Generator rotor winding: Generates a rotating magnetic field. After applying excitation current, it forms an electromagnet, which rotates and generates an alternating magnetic field, cutting the stator winding to produce an induced electromotive force.
Destruction device: Quickly cuts off the excitation current in case of a fault. The core consists of "destruction switch + destruction resistor": In case of a fault, it disconnects the rotor circuit and simultaneously connects the resistor, absorbing the remaining magnetic field energy of the rotor winding to avoid overvoltage.
Excitation transformer: Provides power for the static excitation system. It takes power from the generator stator side or the grid, reduces the voltage, and supplies it to the rectifier device to ensure the stability of the excitation power supply voltage.
Rectifier device: Converts AC to DC excitation current. Commonly used is the thyristor rectifier bridge. According to the excitation regulator's instructions, it adjusts the conduction angle to change the output DC current size.
III. Core Working Principle (Taking the mainstream "static excitation system" as an example)
1. Excitation current generation:
The excitation transformer takes alternating current from the generator stator or the grid and sends it to the rectifier device. The rectifier device converts the alternating current into direct current and inputs it into the rotor windings of the generator.
2. Voltage regulation logic:
- The excitation regulator (AVR) continuously collects signals such as generator output voltage and stator current;
- Compares the collected actual voltage with the "rated voltage" and calculates the deviation;
- If the actual voltage is low: The AVR instructs the rectifier device to increase the conduction angle → the output direct current (excitation current) increases → the rotor magnetic field strengthens → the stator induced voltage rises, returning to the rated value;
- If the actual voltage is high: The regulation logic is reversed, reducing the excitation current and lowering the stator voltage.
3. Fault excitation de-magnetization process:
When the generator experiences faults such as short circuit or loss of synchronization, the protection system triggers the excitation de-magnetization switch to disconnect the rotor circuit, and simultaneously connects the excitation resistor; The magnetic energy stored in the rotor windings is consumed through the excitation resistor (converted into heat energy), rapidly reducing the rotor current to zero, avoiding overheating of the windings or insulation damage.
IV. Key Points for Operation and Maintenance
1. Daily inspection (must be conducted every day)
- Check the excitation regulator (AVR) panel: The indicator lights are normal (no fault alarms), the voltage and current display is consistent with the rated values (deviation ≤ ±5%).
- Check the rectifier device: The thyristor modules have no overheating or discharge sparks, and the cooling fan (or heat sink) is operating normally.
- Check the rotor circuit: The carbon brushes (if any) have a wear of ≤ 1/3, are in good contact with the slip ring (no sparks, no carbon deposits), and the slip ring surface is smooth without scratches.
- Check the de-magnetization device: The de-magnetization switch status is correct (closed during operation), the connection terminals are not loose or overheated.
2. Regular maintenance (monthly/quarterly)
- Cleaning: Blow away the dust inside the excitation regulator and rectifier device to prevent short circuits or poor heat dissipation caused by dust.
- Calibration: Calibrate the voltage sampling circuit and current sensor of the AVR to ensure accurate signal collection.
- Insulation check: Use a multimeter to test the insulation resistance of the rotor winding (at room temperature, ≥ 0.5 MΩ) to avoid aging insulation and leakage.
- Component inspection: Tighten the connection terminals of the excitation transformer and the extinction resistor to prevent loosening and overheating; check that the cooling system pipelines have no water leakage or oil leakage.
3. Condition Monitoring (Long-term)
- Online Monitoring: Using PLC or DCS systems, real-time data such as excitation current, voltage, and rectifier device temperature are recorded. Over-limit alarms (such as an alarm when the excitation current exceeds the rated value by 10%) are set.
- Trend Analysis: Regularly analyze the changing trends of excitation current and voltage. If a gradual increase in deviation is detected, the regulator or rectifier device faults can be proactively investigated.
V. Common Faults and Their Solutions
Fault type:
Excitation current disappearance (loss of excitation): The generator voltage drops sharply, the speed increases, and the unit emits a "buzzing" abnormal sound. Causes:
1. The de-magnetization switch malfunctions;
2. The rectification device fails (thyristors are damaged);
3. The rotor winding breaks.
Handling method:
1. Immediately disconnect the generator (open the grid connection switch) to prevent loss of synchronization and damage to the unit;
2. Check the status of the de-magnetization switch. If it malfunctions, reset it;
3. Test the rectification device and rotor winding. After replacing the damaged components, conduct a re-excitation test.
Fault type:
Abnormal generator terminal voltage (high/low): The voltage display deviates from the rated value, and the reactive power of the grid fluctuates.
Causes:
1. Parameter drift of the excitation regulator (AVR);
2. Fault in the voltage sampling signal (sensor damage);
3. Abnormal conduction angle of the rectifier device.
Handling methods:
1. Switch to the "manual excitation" mode, manually adjust the excitation current to temporarily stabilize the voltage;
2. Calibrate the AVR parameters, check the voltage sensor;
3. Detect the conduction status of the thyristors in the rectifier device, and replace the faulty components.
Fault type:
Excitation regulator (AVR) fault: The fault indicator on the AVR panel lights up, and the excitation current fluctuates frequently.
Causes:
1. AVR power supply failure;
2. Internal logic circuit damage;
3. Feedback signal interference.
Handling method:
1. Immediately switch to the backup excitation regulator (if there is a "primary-secondary switching" function);
2. Check the AVR power supply circuit, eliminate short circuits/short connections;
3. If repair is impossible, contact the manufacturer to replace the AVR module, and conduct a load test after replacement.
Fault type:
Rotor slip ring and carbon brush fault: There are a large number of sparks at the slip ring, and the carbon brush is overheating and wearing out too quickly.
Causes:
1. Insufficient contact pressure between the carbon brush and the slip ring;
2. Oil stains or carbon deposits on the slip ring surface;
3. Incompatible carbon brush model.
Treatment method:
1. Reduce the generator load (or shut down), clean the surface of the slip ring (wipe with alcohol);
2. Adjust the pressure of the carbon brush spring (ensure tight contact);
3. Replace with a matching model carbon brush, and run it for 1-2 hours after replacement before operating at full load.
Key Considerations
- Before handling any faults in the excitation system, the excitation power supply must be disconnected to prevent electric shock. When conducting maintenance on rotor windings or excitation de-magnetization devices, it is necessary to discharge (release the remaining magnetic energy) first.
- During routine maintenance, it is strictly prohibited to remove carbon brushes or plug/unplug AVR components while the excitation system is running, as this may trigger protection actions.
- If a backup excitation system is available, it should undergo a "primary-backup switching test" every month to ensure it can be quickly activated in case of a failure.