Handling Diesel Generator Sets Running in a Demagnetized State

Diesel generator sets play a crucial role in power systems, especially in emergency power supply and maintaining system stability. The phenomenon of demagnetization in diesel generator sets can not only affect their normal operation but also pose a serious threat to the stability of the power system. This article will provide a detailed analysis of the impact of demagnetized operation in diesel generator sets, the conditions under which demagnetized operation is allowed, and the measures and precautions for handling it.

Impact of Demagnetized Operation in Diesel Generator Sets

When a generator operates in a demagnetized state, the stator current increases, resulting in higher electrical losses, rising temperatures, and even mechanical vibrations. For diesel generator sets, the effects of demagnetization are more specific and include the following.

1. Loss of Reactive Power and Voltage Drop

After demagnetization, a diesel generator can no longer supply reactive power to the grid, and instead, it may draw reactive power from the grid. The lack of reactive power directly leads to a drop in system voltage, which can affect the stability of the power system. If the system cannot provide enough reactive power to compensate for the shortfall, a voltage collapse may occur, leading to the breakdown of the power system.

2. Temperature Rise and Increased Losses in the Generator

During demagnetization, the increase in stator current raises the temperature of the stator windings, potentially causing the insulation material to age or even be damaged. In extreme cases of leading power factor operation, the magnetic field distribution becomes uneven, and the core and components at the ends may heat up due to additional losses. The differential currents in the rotor may also generate extra losses, leading to local overheating.

3. Vibration Issues

The electromagnetic imbalance caused by demagnetized operation generates pulsating torque in the rotor, which can lead to vibrations in the generator and its foundation. Prolonged vibrations may damage the generator's structure, reducing its lifespan.

Conditions for Allowing Demagnetized Operation of Diesel Generator Sets

While demagnetized operation can have several adverse effects, in certain situations, a diesel generator set may be allowed to operate in this state, provided the power system has sufficient reactive power reserves. To ensure the stability of the system and safe operation of the generator, the following conditions must be met.

1. Sufficient Reactive Power Reserve in the System

In the event of demagnetization, the system must have enough reactive power reserves to maintain the voltage at no less than 90% of the rated value. If the system's reactive power source cannot provide adequate support, demagnetized operation will severely affect system stability, potentially leading to a voltage collapse.

2. Stator Current Limitation

During demagnetized operation, the stator current must not exceed the value specified in the generator's operating guidelines, usually up to 1.1 times the rated value. Exceeding this limit may cause overheating of the equipment, leading to faults or damage.

3. Temperature Control

When demagnetized operation is allowed, the temperature of the stator ends and other critical components must not exceed the set safety temperature to prevent generator damage due to overheating.

4. Cooling Capacity

For air-cooled diesel generators, the additional losses in the rotor may cause difficulty in dissipating heat. Therefore, when operating in a demagnetized state, the excitation loss of an air-cooled diesel generator must not exceed 0.5 times the rated excitation loss.

Measures for Handling Demagnetized Operation of Diesel Generator Sets

Taking timely and effective measures to address the demagnetization issue is key to ensuring the safe operation of the generator set and maintaining system stability. The handling methods vary depending on whether demagnetized operation is allowed. Below are the steps for dealing with both scenarios.

1. Steps for Handling Demagnetized Operation When Not Allowed

In cases where the power system's reactive power reserves are insufficient or for other reasons, demagnetized operation of the diesel generator is not allowed. The following steps should be taken immediately.

Quick Diagnosis of the Cause: Use meters and signal displays to identify the root cause of demagnetization. Prompt diagnosis helps take targeted actions.
Disconnect the Generator from the Grid: If the demagnetization protection device fails to act, manually disconnect the generator from the system to prevent further damage to the power system.
Switch to Backup Excitation: If the demagnetized generator's excitation can be switched to a backup excitation source, and other components are still functioning normally, quickly disconnect and switch to the backup excitation, then reconnect to the grid.
Monitor System Voltage: After disconnecting the demagnetized generator, increase the excitation current of other non-demagnetized generators to help maintain system voltage stability.
Ensure Normal Supply of Auxiliary Power: After disconnecting the demagnetized generator, closely monitor the voltage of the auxiliary power system. Switch to backup power if necessary to ensure uninterrupted auxiliary power supply.

2. Steps for Handling Demagnetized Operation When Allowed

If the power system has sufficient reactive power reserves, the diesel generator set can continue operating in a demagnetized state for a certain period. The following steps should be taken.

Reduce Active Power: For heavily loaded diesel generators, reduce the active power to an acceptable level within the specified time to minimize the impact on the system and auxiliary power. For lightly loaded generators, active power reduction may not be necessary.
Monitor Current and Voltage: During demagnetized operation, continuously monitor the stator current (which should not exceed 1.1 times the rated current) and the stator voltage (which should not drop below 90% of the rated voltage). Also, monitor the temperature at the stator ends to prevent overheating.
Restore Excitation Current: Within the allowable running time, restore the excitation current as soon as possible. If the automatic voltage regulator (AVR) is malfunctioning, switch to the backup excitation system.
Fault Handling: If excitation cannot be restored within the allowed running time, transfer the active power load to other generators and promptly disconnect the demagnetized generator to prevent further damage to the power system.

Conclusion

Demagnetized operation of diesel generator sets is a complex and potentially risky situation. In power systems, it is essential to carefully evaluate the feasibility of running a generator in a demagnetized state based on reactive power reserves and load conditions. With proper management and timely intervention, the duration of demagnetized operation can be extended while ensuring system safety. For generators where demagnetized operation is not permitted, quickly disconnecting from the grid and restoring excitation is critical to preventing system collapse. For those allowed to operate in a demagnetized state, strict monitoring of operating parameters is necessary to ensure both system stability and equipment safety.