Synchronous Generators and Wind Turbines: Frequency Behavior on Grid Disconnection

When a synchronous generator is disconnected from the power grid, its frequency behavior varies based on its mechanical input and load conditions. Similarly, wind turbines exhibit different frequency behaviors depending on their power electronics and control systems. Understanding these behaviors is crucial for maintaining system stability and preventing damage.

Synchronous Generator

Disconnection from Grid: When a synchronous generator is disconnected from the grid, it no longer has a reference frequency from the grid. However, it continues to operate at its current speed, determined by the prime mover (e.g., steam turbine or water turbine).

Frequency Increase

When the generator receives mechanical input from a turbine and has no load, the generator's speed and frequency will increase. This occurs because the generator accelerates without any electrical load to absorb its power. The system may enter a state of over-frequency if the mechanical input continues to exceed the generator's capacity.

Frequency Decrease

Should the prime mover be unable to maintain the speed (e.g., due to a cut-off fuel supply), the frequency will decrease as the generator slows down due to power losses. This situation is referred to as under-frequency.

Wind Turbine

Disconnection from Grid: Modern wind turbines often use power electronics to control their output, allowing them to continue generating electricity independently of the grid. Their frequency behavior is influenced by the availability of wind and the control systems in place.

Stable Frequency

At the grid synchronization mode, a wind turbine maintains a stable frequency if it can continue generating electricity without being synchronized to the grid. This frequency remains constant or fluctuates based on wind speed and turbine control systems. However, if the turbine operates without a load or control system, it may experience an increase in speed and frequency until it reaches a limit. At this point, protective systems may engage to prevent damage and maintain stability.

Control Systems and Wind Conditions

The frequency behavior of a wind turbine in an isolated state can be highly variable. It depends on the wind conditions, the type of control systems, and the capacity of the generator. For instance, the turbine may exhibit a higher frequency if the wind is strong and the control systems are designed to maintain operation under such conditions. Conversely, if the wind speed decreases or the control systems are not functioning optimally, the frequency may drop, leading to potential under-frequency conditions.

Summary

Both synchronous generators and wind turbines can exhibit different frequency behaviors when disconnected from the power grid. Synchronous generators may experience frequency increases or decreases based on their prime mover's ability to maintain speed. Wind turbines, on the other hand, can maintain a stable frequency or show fluctuations depending on their power electronics and control systems. Protective measures are generally in place to prevent damage to the equipment in both scenarios.

Protective Measures and System Stability

For both synchronous generators and wind turbines, maintaining system stability is crucial. Protective measures such as frequency protection relays, trip systems, and over-speed governors are essential to prevent damage and ensure the equipment operates safely during grid disconnection events.