The Role of Reverse Power Protection in Large Generator Sets

 
Reverse power protection is a key technology for ensuring the safety and stability of large generator sets. Typically, these systems are equipped with two independent reverse power protection devices: programmed trip reverse power protection and simple electrical reverse power protection. The main purpose of these devices is to limit the amount of reverse power, usually not exceeding 10% of the rated active power. Effective control of reverse power significantly enhances the safety and reliability of generator sets.

Principles and Risks of Reverse Power Operation

 
Theoretically, diesel generators can operate in a reverse power state, but this should not be done in practice. When a generator runs in reverse power, it essentially acts as a motor, failing to deliver electrical energy while consuming power from the grid. Prolonged reverse power operation not only wastes energy but can also lead to severe equipment damage, including burned-out windings and generator failures. For this reason, the industry generally advises against extended reverse power operation to protect equipment and ensure system stability.
The reverse power protection function is designed to prevent the power system from sending power back to the generator after the diesel engine loses power. When the power direction changes, the generator not only absorbs power but also creates a reverse torque on the diesel engine, which can cause engine damage and unnecessary energy consumption. Therefore, proper configuration of reverse power protection is an essential part of generator set design.

Working Principles of Reverse Power Protection

 
Reverse power protection is a type of generator relay protection. Its basic principle involves comparing absolute values to create a power direction relay. The system's AC measurement circuit uses a differential connection method to obtain two quantities: the sum voltage vector (A1) and the difference voltage vector (A2).
Normal Operating State: When the generator operates normally, A2 can be used to trigger the programmed trip.
Programmed Reverse Power Protection: Despite the name including "protection," the primary function of programmed reverse power protection is to ensure safe shutdown under specific conditions. The critical point is that the protection triggers only when reverse power reaches a predetermined value.
During normal shutdown operations, when the load drops to zero, the diesel engine's main steam valve must first be closed, followed by the activation of the programmed reverse power protection to trip the generator. This step ensures that even if the main steam valve is not fully closed, the generator will not cause the diesel engine to run away after the circuit breaker opens, effectively protecting the equipment.

Reverse Power State of Steam Turbine Generators

 
When steam turbine generators operate in parallel with the grid, if the main steam valve closes for any reason, the generator will switch from outputting power to absorbing power from the grid to maintain synchronous speed and manage losses. In this state, the generator effectively becomes a motor, drawing active power to keep the turbine spinning.
 
Impact on the Generator: While this reverse power state does not harm the generator itself, extended operation without steam can raise the exhaust cylinder temperature and overheat the turbine blades, potentially causing mechanical failures. For example, overheating may lead to thermal fatigue in materials, shortening equipment lifespan.
 
Risks of Increased Temperature: With stagnant steam in the cylinder, friction can cause the turbine blades to overheat, and overall expansion in low-pressure cylinders can lead to unit vibrations. Such vibrations not only affect performance but can also result in long-term structural damage.

Monitoring and Response of Reverse Power Protection

 
The input for reverse power protection includes the generator's secondary three-phase voltage and current. When the generator absorbs active power, the reverse power protection system acts immediately, sending signals and automatically shutting down within a set timeframe.
When the steam turbine enters reverse power operation, the protection system first sends a signal and disconnects the generator after a preset delay, typically ranging from 1 to 3 minutes, depending on the permissible steam-free running time of the turbine. If programmed trip reverse power protection is implemented, the delay should be shortened to prevent turbine damage due to erroneous tripping.

Optimization and Strategies for Protective Measures

 
When the reverse power protection triggers and opens the generator output switch, the demagnetization switch should not engage simultaneously. If the generator operates in a non-full-phase state, the relevant power supply must be cut off immediately to prevent severe damage from negative sequence currents and vibrations. Non-full-phase operation can lead to rotor overheating, and in extreme cases, can burn out the rotor.
 
Importance of Timely Handling: If a generator is unloaded and enters non-full-phase operation, it may generate negative sequence currents, significantly impacting the system and the generator. Therefore, when reverse power protection is activated, the excitation current should be promptly reduced to maintain the generator in an unloaded state, followed by appropriate procedures. This not only helps protect equipment but also ensures overall system stability.

Conclusion

 
Reverse power protection plays a crucial role in large generator sets. It effectively prevents potential damage from reverse power operation to the generator and diesel engine, ensuring safety and stability in the system. Through well-planned protective strategies and timely response mechanisms, generator sets can maintain efficiency and reliability under various operating conditions, providing solid support for the safe operation of the power system.