Abnormal Operating Conditions in Large Generator Sets

As the power industry continues to grow rapidly, especially with the increasing capacity of large generator sets, the role and importance of generator sets in the power system have become increasingly significant. Large generators are not only core equipment for power supply but also play a crucial role in the stable operation of the power system, the quality of electricity supply, and the long-term operation of equipment. However, with the continuous increase in generator capacity, the operating environment of generators has become more complex, which raises the risks of operational failures and abnormal conditions. In addition to common faults, some abnormal operating conditions, such as stator winding overload, demagnetization, loss of synchronism, reverse power operation, and single-phase operation, while relatively common, are often overlooked. Therefore, studying these abnormal operating conditions and their protective measures is essential not only for improving the reliability of generator sets but also for providing strong support to ensure the stable operation of the power system.

Low Excitation and Demagnetization Protection

Low excitation and demagnetization are common abnormal operational issues in generators. Demagnetization occurs when the excitation current is insufficient, causing the generator to lose its ability to generate stable power and maintain synchronism. Low excitation refers to the failure of the excitation system to supply enough excitation current, which may cause the generator to lose synchronism with the grid and even result in loss of synchronism or reverse power operation.

Protection Mechanism

For small generators with a capacity below 100kW, demagnetization is generally not allowed. When a DC excitation machine is used, it is essential to ensure that when the demagnetization switch is opened, the generator circuit breaker also trips to prevent asynchronous operation. For generators larger than 100kW, demagnetization protection is critical. The function of demagnetization protection is to prevent the generator from losing synchronism and to minimize the adverse effects caused by demagnetization.
In hydroelectric generators, demagnetization protection devices act when the generator is disconnected, ensuring the generator operates off-grid to avoid negative effects. For diesel generators, demagnetization protection reduces output power to shorten the duration of asynchronous operation and quickly restore synchronism. If demagnetization causes the bus voltage to fall below an allowable threshold, the protection device will quickly disconnect the generator from the grid to prevent equipment damage.

Stator Overcurrent and Overload Protection

The stator winding carries the main current load of the generator. If the stator current exceeds the rated range or remains in an overload state for too long, it may lead to overheating of the generator, damage to the winding insulation, or even serious electrical faults. Overcurrent and overload are common causes of generator damage, so it is crucial to continuously monitor the stator current during operation and take timely protective measures.

Protection Mechanism

To prevent the potential risks of overcurrent and overload, the stator and excitation windings should be equipped with time-limit and inverse-time overload protection. Time-limit overload protection relies on the load signal from the generator set. When the load exceeds a set threshold, the protection device activates to reduce the load or lower the excitation current to relieve the overload. Inverse-time overload protection monitors the operating status of the generator. When an overload is detected, the protection device will quickly disconnect the generator to prevent excessive damage, ensuring a safe shutdown.

Reverse Power Protection

Reverse power typically occurs when a generator experiences a grid connection fault, causing power to flow back from the grid to the generator. This situation not only risks loss of synchronism but also can damage the electrical and mechanical structures of the generator, leading to equipment burnouts, increased vibrations, and reduced lifespan.

Protection Mechanism

Reverse power protection is especially critical for diesel generators with a capacity greater than 200kW. The primary function is to monitor the power flow direction and protect the generator from reverse power operation. When reverse power occurs, the protection device activates according to the set time parameters, first issuing a warning through a short time limit. If reverse power persists, the device will disconnect the generator using a long time limit to prevent it from staying in a dangerous state.
For diesel generators, reverse power protection typically follows these steps: First, close the main steam valve, then, after the reverse power relay operates, trip the generator breaker and demagnetize the generator. For hydroelectric generators, the procedure involves first closing the guide vanes to the no-load position, then tripping the generator breaker and demagnetizing. These measures can quickly eliminate the negative effects of reverse power on the generator set, ensuring safe operation.

Loss of Synchronism Protection

Loss of synchronism refers to the situation where the generator loses synchrony with the grid, resulting in a mismatch between its speed and the grid frequency. Loss of synchronism can prevent the generator from operating in parallel with the grid and may lead to large-scale system failures. It not only reduces the generator's efficiency but also subjects the equipment to mechanical stress and thermal damage, affecting long-term reliability.

Protection Mechanism

For generators with a capacity of 300kW or more, loss of synchronism protection is essential. When a generator loses synchronism, the protection device will activate and take appropriate actions to disconnect it from the grid. There are two main methods to restore synchronism: one is to increase the excitation current of the generator to enhance its synchronizing ability, and the other is to reduce the active power of the generator that has lost synchronism, allowing it to regain synchrony. These measures help restore stable operation of the generator and prevent any negative impact on the power system.

Single-Phase Operation Protection

Single-phase operation occurs when one of the phases in the generator's three-phase power supply is disconnected or unbalanced, often during the disconnection or paralleling process. Single-phase operation leads to excessive negative-sequence current, causing additional mechanical and thermal stress. In severe cases, it can cause rotor burnout, insulation damage, or even equipment failure.

Protection and Preventive Measures

To prevent the risks of single-phase operation, it is necessary to strictly follow the correct sequence of operations during generator startup or shutdown. First, ensure that the generator's excitation current is stable, the speed is balanced, and the output power is within the specified range. By controlling the stator current and reducing the negative-sequence current caused by single-phase operation, the damage to the generator can be effectively minimized.

Analysis of Single-Phase Operation Faults

The characteristics of single-phase operation faults are closely related to factors such as the generator's wiring method, the main transformer grounding method, and the form of phase loss. To effectively analyze single-phase operation faults, the symmetrical component method is often used. This method can decompose asymmetric three-phase currents (or voltages) into positive, negative, and zero sequence components. These components provide critical fault information, helping to promptly identify single-phase operation and take corresponding protective actions.

Conclusion

With the continuous development of the power industry, generator capacities are increasing, and the operating environment and fault modes are becoming more complex. Abnormal conditions such as low excitation, demagnetization, stator overload, reverse power, loss of synchronism, and single-phase operation, though relatively common, are often neglected. However, these abnormal operating conditions can have more severe impacts on generator sets than faults themselves. Therefore, it is crucial to effectively study and protect against these conditions. By conducting in-depth analysis and implementing protective measures—such as demagnetization protection, overload protection, reverse power protection, loss of synchronism protection, and single-phase operation protection—we can significantly improve the operational safety and reliability of generator sets, ensuring the stability of the power system.