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, leading to a negative reactive power output. In extreme cases, the generator may lose all excitation, resulting in a complete loss of magnetism. When this happens, the generator draws reactive power from the system, which can cause a drop in overall system voltage. If a large generating unit experiences this condition, it could potentially lead to system instability or even oscillation.
When the excitation current is reduced, the generator’s internal voltage decreases, and the power factor angle becomes leading. As a result, the armature reaction becomes magnetically assisting, allowing the generator to deliver active power while absorbing reactive power. This operating state is known as phase-in operation.
**Main Consequences of Phase-In Operation:**
1. **Reduced Static Stability:** The generator’s ability to maintain stable operation under varying loads is significantly weakened.
2. **Increased Temperature at Stator End:** The leakage current at the stator end causes localized heating, which can damage insulation and reduce the lifespan of the machine.
3. **Lower Plant Power Supply Voltage:** The voltage at the plant’s auxiliary systems may decrease, affecting the performance of critical equipment.
4. **Higher Stator Current:** With constant active power output, the stator current increases due to the drop in terminal voltage, increasing the risk of overheating and overload.
**Damage Caused by Loss of Magnetism (Phase-In Operation):**
A sudden loss of excitation in a synchronous generator is a common fault in power systems. Common causes include open circuits in the excitation circuit, short circuits in the excitation winding, regulator failure, or human error such as incorrect switch operation.
Once a generator loses its excitation, the rotor’s magnetic field gradually diminishes, and the generator transitions into phase-in mode, drawing reactive power from the system. This leads to several serious consequences:
1. **Increased Stator Current:** The generator must draw more reactive power, causing a significant rise in stator current, which can overheat the windings and reduce insulation life.
2. **Rotor Overheating and Vibration:** A negative sequence magnetic field is induced in the stator, creating 100 Hz currents in the rotor, leading to excessive heat and potential mechanical damage.
3. **Voltage Drop in the System:** The generator’s loss of magnetism reduces the system voltage, which can cause other equipment to trip due to low-voltage protection.
4. **Impact on Other Equipment:** A drop in grid voltage can trigger tripping of nearby electrical devices, such as contactors and inverters, disrupting normal operations.
**Preventive Measures Against Phase-In Operation:**
To avoid prolonged demagnetization, operators must follow strict procedures for managing the excitation system. These include:
1. **Excitation Channel Switching:** Perform channel switching carefully, ensuring that the system remains stable during the process.
2. **Manual Operation During Faults:** Switch to manual mode if the automatic excitation system fails, allowing for controlled adjustments.
3. **Magnetization/De-magnetization Operations:** Perform these actions slowly and with caution to prevent sudden changes in system conditions.
4. **Fault Alarm Handling:** Immediately address any alarm signals by resetting or switching channels to restore normal operation.
5. **Monitor Reactive Power Output:** Operators should closely monitor the unit’s reactive power to prevent long-term phase-in operation.
6. **Load Limitations:** Define allowable load limits during loss-of-magnetism events to ensure safe operation within defined timeframes.
**Symptoms of Phase-In Operation:**
- Generator reactive power becomes negative.
- Voltage levels at the generator outlet, 220kV bus, and 6kV plant bus drop.
- Stator coil and core temperatures rise at the end of the machine.
**Handling Phase-In Operation:**
- Maintain system voltage and avoid entering phase-in mode when network voltage is low.
- Keep the generator outlet voltage within ±5% of rated value.
- Ensure proper cooling water parameters are maintained.
- Monitor stator temperature, especially at the end, to prevent overheating.
- Adjust reactive load manually if necessary.
- Avoid exceeding operational limits and keep active power changes minimal.
- Monitor and adjust phase depth carefully to prevent instability.
- If out-of-step oscillation occurs, increase excitation or reduce active load.
- Coordinate with dispatch to manage voltage curves and reactive power distribution.
- Regularly record generator temperatures during phase-in operation.
- If limits are reached, immediately return to late-phase operation to stabilize the system.
**Conclusion:**
Phase-in operation due to loss of magnetism can severely impact both the generator and the power system. It can lead to voltage fluctuations, equipment tripping, and even system collapse. By implementing proper monitoring, timely response, and control measures, the risks associated with phase-in operation can be minimized. Operators must remain vigilant, regularly check reactive power outputs, and take immediate action if a unit enters phase-in mode. This ensures the stability of both the generator and the broader power network.
Uv Curving Screen Protector,Uv Curl Screen Protector,Uv Screen Protector Tempered Glass,Anti Uv Computer Screen Protector
Shenzhen TUOLI Electronic Technology Co., Ltd. , https://www.tlhydrogelprotector.com