The term "phase-in operation" of a generator refers to a condition where the excitation current is reduced, causing the generator to absorb reactive power from the grid instead of supplying it. In extreme cases, the generator may completely lose its excitation, leading to a situation where it draws reactive power from the system, which can lower the overall system voltage. When large-capacity units operate in this mode, it may trigger system oscillations, potentially affecting the stability of the entire power network.
When the excitation current decreases and the generator’s internal voltage drops, the power factor angle becomes leading. This means that while the generator continues to supply active power, it absorbs reactive power from the grid. This operating state is known as phase-in operation or under-excitation.
**Main consequences of phase-in operation:**
1. **Reduced static stability:** The generator’s ability to maintain stable operation decreases, making it more prone to instability.
2. **Increased stator end temperature:** The leakage current at the stator end causes excessive heating, which can damage insulation and reduce the lifespan of the generator.
3. **Lower plant power supply voltage:** As the generator absorbs reactive power, the voltage at the plant bus may drop, affecting the performance of critical equipment.
4. **Increased stator current:** With constant output power, the stator current rises due to the drop in terminal voltage, increasing the risk of overheating and overload.
**Damage caused by loss of magnetism (phase-in operation):**
Sudden loss of excitation in synchronous generators is a common fault in power systems. Common causes include open circuits in the excitation circuit, short circuits in the rotor winding, failure of the excitation regulator, incorrect tripping of the demagnetization switch, or human error.
After losing magnetism, the generator quickly transitions into phase-in operation, drawing reactive power from the grid to sustain its operation. This leads to a significant drop in stator voltage, increased current in electrical equipment, and several serious risks:
1. **Increased stator current:** The generator must draw a large amount of reactive power, causing the stator current to rise significantly, which can lead to overheating and damage to insulation.
2. **Rotor overheating and vibration:** A negative sequence magnetic field is created, inducing high-frequency currents in the rotor, causing severe overheating and mechanical stress.
3. **Impact on system voltage:** The generator's reactive power demand can cause a sharp voltage drop, especially if the unit has a large capacity and the system has limited strength, potentially leading to system instability or collapse.
4. **Effects on other equipment:** Reduced grid voltage can trigger low-voltage protection trips in nearby devices such as contactors and inverters, disrupting normal power consumption.
**Preventive measures to avoid phase-in operation:**
To prevent long-term demagnetization and phase-in operation, proper management of the excitation system is essential. Key steps include:
1. **Excitation channel switching:** Ensure the system is in “tracking†mode before switching channels. Follow a step-by-step procedure to avoid disruptions.
2. **Manual operation during faults:** Switch the excitation regulator to manual mode and adjust settings accordingly to stabilize the system.
3. **Magnetization/demagnetization operations:** Perform these carefully, avoiding rapid changes that could destabilize the system.
4. **Fault handling for excitation alarms:** Immediately check parameters and perform magnetization if necessary. If not possible, proceed with channel switching.
5. **Monitor reactive power output:** Avoid prolonged phase-in operation by keeping reactive power within safe limits.
6. **Limit load during loss of magnetism:** Allow only limited continuous operation at specific loads to prevent damage and ensure system stability.
**Phenomena and handling during phase-in operation:**
During phase-in operation, the generator’s reactive power becomes negative, and voltages across various buses decrease. Stator coil and core temperatures rise, requiring close monitoring. Key handling steps include maintaining system voltage, limiting voltage deviation, ensuring proper cooling, and closely monitoring temperature and current levels.
If out-of-step oscillation occurs, increase excitation or reduce active load to restore synchronization. Adjust phase depth smoothly and monitor all related parameters. Record temperatures regularly and take corrective actions if limits are exceeded.
**Conclusion:**
Phase-in operation due to loss of magnetism can severely impact generator performance and system stability, causing voltage fluctuations and triggering protective trips. By implementing strict monitoring, timely adjustments, and proper operational procedures, the risks can be minimized. Operators must remain vigilant, manage reactive power effectively, and avoid prolonged phase-in operation to ensure reliable and safe power generation.
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