Jiangmen Hongli Energy Co.ltd , https://www.honglienergy.com
Danger of generator phase-in operation
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 rather than supply it. This typically occurs when the generator's internal voltage drops below the system voltage, resulting in a negative reactive power output. In extreme cases, the generator may lose all excitation, which can lead to severe system instability and even oscillation, especially when large units are involved.
During phase-in operation, the generator’s power factor becomes leading, meaning it delivers active power but consumes reactive power. This happens due to the armature reaction caused by the load current, which has a magnetizing effect on the stator. The operating state is known as "in-phase operation" or "under-excitation."
**Main consequences of phase-in operation:**
1. **Reduced static stability**: The generator's ability to maintain stable operation decreases, increasing the risk of loss of synchronism.
2. **Increased temperature at the stator end**: The leakage current causes higher temperatures, potentially damaging insulation and reducing the generator's lifespan.
3. **Lower plant power supply voltage**: As the generator absorbs reactive power, the voltage at the plant bus may drop, affecting auxiliary equipment.
4. **Higher stator current**: When the generator maintains constant active power output, the stator current increases due to the lower terminal voltage, risking overload and overheating.
**Damage caused by loss of magnetization during phase-in operation:**
Sudden loss of excitation is a common fault in power systems, often caused by open circuits, short circuits, regulator failures, or human error. Once a synchronous generator loses magnetization, it quickly transitions into phase-in mode, drawing reactive power from the system to maintain its magnetic field. This leads to several serious issues:
1. **Increased stator current**: The generator draws a large amount of reactive power, causing the stator current to rise significantly, leading to overheating and potential damage to the winding insulation.
2. **Rotor overheating and vibration**: The negative sequence magnetic field induced by asynchronous operation creates high-frequency currents in the rotor, causing excessive heating and mechanical stress.
3. **Voltage drop in the system**: As the generator absorbs reactive power, system voltage levels decrease, which can cause other equipment to trip or malfunction.
4. **Impact on other electrical devices**: Low voltage can trigger protective trips in nearby equipment, such as contactors and inverters, disrupting normal operations.
**Measures to prevent phase-in operation:**
To avoid long-term demagnetization and ensure safe operation, several preventive actions are taken:
1. **Excitation channel switching**: Ensure proper switching between channels without disrupting the system. Always check the regulator status before performing any switch.
2. **Manual operation during faults**: Switch to manual control if the excitation regulator fails, allowing for more precise adjustments.
3. **Magnetization/demagnetization operations**: Perform these carefully and gradually to avoid sudden changes that could destabilize the system.
4. **Fault handling procedures**: Address alarms promptly, and if necessary, perform magnetization or switch channels to restore normal operation.
5. **Reactive power monitoring**: Keep an eye on the unit’s reactive power output to prevent prolonged phase-in operation.
6. **Load limits during loss of magnetization**: Define allowable load and duration for asynchronous operation to avoid exceeding safe limits.
**Symptoms of phase-in operation:**
- Negative reactive power output
- Drop in generator outlet, 220kV bus, and 6kV plant bus voltages
- Increased temperature in the stator windings and core
**Handling phase-in operation:**
- Maintain system voltage within ±5% of rated value.
- Monitor stator temperature and ensure it does not exceed 120°C.
- Adjust reactive load manually if necessary.
- Avoid overloading the generator and keep active power changes minimal.
- If out-of-step oscillations occur, increase excitation or reduce active load.
- Record generator temperatures hourly and report any abnormal conditions immediately.
**Conclusion:**
Phase-in operation due to loss of magnetization can severely impact both the generator and the overall power system. It can lead to voltage instability, equipment tripping, and potential system collapse. By implementing strict monitoring, timely intervention, and proper operational procedures, the risks associated with phase-in operation can be minimized. Operators must remain vigilant, monitor reactive power closely, and take immediate action if signs of phase-in operation appear.