accelerating power

Understanding Accelerating Power in Synchronous Machines: The Case of Short Circuits

In the realm of electrical engineering, synchronous machines play a crucial role in power generation and distribution. These machines, operating at a constant speed synchronized with the power grid, are subject to various phenomena, one of which is the concept of "accelerating power". This article delves into the concept of accelerating power and its significance, particularly in the context of short circuits near the terminals of a synchronous machine.

What is Accelerating Power?

Accelerating power refers to the excess electric power generated by a synchronous machine that cannot be transmitted to the load due to a fault, such as a short circuit. This excess power manifests as a torque acting on the rotor, causing it to accelerate. The accelerating power, denoted by P_acc, is given by the difference between the mechanical power input to the machine (P_m) and the electrical power output (P_e):

P_acc = P_m - P_e

Short Circuits and Accelerating Power:

When a short circuit occurs near the terminals of a synchronous machine, the electrical power output (P_e) drops drastically, often reaching zero. This sudden reduction in electrical load leads to a significant imbalance, with the mechanical power input (P_m) exceeding the electrical power output. This difference in power is manifested as accelerating power.

Consequences of Accelerating Power:

The accelerating power causes the rotor of the synchronous machine to accelerate, leading to an increase in the rotor angle (θ). This increase in rotor angle is a critical parameter as it can lead to several undesirable consequences:

• Loss of Synchronism: As the rotor angle increases, the synchronous machine may lose synchronism with the grid, resulting in instability and potential damage to the machine.
• Increased Stresses: The accelerating torque can impose significant mechanical stresses on the machine's rotor and bearings, potentially causing damage or failure.
• Voltage Instability: The sudden reduction in electrical output due to the short circuit can lead to voltage instability in the grid, impacting other equipment and potentially causing widespread blackouts.

Mitigating Accelerating Power:

To mitigate the effects of accelerating power, various techniques are employed in synchronous machine systems, including:

• Protective Relays: These devices detect short circuits and quickly isolate the faulty portion of the system, limiting the amount of accelerating power experienced by the machine.
• Automatic Voltage Regulators (AVRs): AVRs help maintain stable voltage levels during short circuits, reducing the impact on the grid and limiting the accelerating power.
• Under-excitation Limiters: These devices limit the excitation current to the synchronous machine, reducing the accelerating power during a short circuit.

Conclusion:

Understanding the concept of accelerating power is crucial for ensuring safe and reliable operation of synchronous machines. The potential consequences of a short circuit, specifically the excess power that cannot be transmitted to the load, must be addressed through appropriate protective measures and control strategies. By implementing these safeguards, engineers can effectively mitigate the risks associated with accelerating power and ensure the continued stability and integrity of electrical power systems.