What are the main types of frequency control?
Frequency control in power systems integrating wind energy is fundamentally different from conventional thermal generation due to the variable nature of wind resources. The 2011 literature on this topic identifies several distinct types of frequency control mechanisms that are critical for maintaining grid stability when wind power constitutes a significant share of total generation. These mechanisms operate on different time scales and rely on both the inherent characteristics of wind turbines and external grid support systems.
Primary Frequency Control
Primary frequency control is the fastest response mechanism, typically activating within seconds of a frequency deviation. In conventional synchronous generators, this is achieved through governor action that adjusts mechanical power input. For wind turbines, primary control is not inherent unless specific technologies are employed. Modern wind turbines equipped with power electronics, such as doubly-fed induction generators (DFIG) and full-converter systems, can provide primary frequency response by adjusting their active power output. This is often achieved through rotor kinetic energy extraction, where the turbine temporarily increases power output by slowing down the rotor, thus releasing stored kinetic energy into the grid.
Secondary Frequency Control
Secondary frequency control, also known as automatic generation control (AGC), operates on a time scale of minutes. It aims to restore the system frequency to its nominal value and relieve the primary control reserves. In wind-dominated systems, secondary control is crucial because wind power fluctuations can be more pronounced than those of thermal units. This control type involves adjusting the setpoints of controllable generators, including wind farms equipped with energy storage systems or flexible operation capabilities. The integration of wind power requires enhanced communication and control strategies to coordinate the response of multiple wind farms and other generating units.
Tertiary Frequency Control
Tertiary frequency control, or economic dispatch, operates on a longer time scale, ranging from 15 minutes to several hours. It involves the rescheduling of generating units to optimize the economic operation of the power system while maintaining adequate frequency reserves. For wind power integration, tertiary control is essential for managing the uncertainty and variability of wind generation. This may involve adjusting the output of thermal plants, hydroelectric units, and demand response programs to compensate for wind power fluctuations. The 2011 discussions on frequency control emphasize the need for flexible tertiary control strategies that can accommodate the stochastic nature of wind energy.
Role of Energy Storage
Energy storage systems play an increasingly important role in frequency control for wind power integration. Batteries, pumped hydro storage, and other storage technologies can provide fast-acting frequency response, complementing the natural inertia of synchronous generators. In the context of the 2011 literature, energy storage was recognized as a key enabler for enhancing the grid-friendly characteristics of wind power. Storage systems can absorb excess wind generation during periods of high wind speeds and release energy during low wind periods, thus smoothing the power output and reducing the burden on conventional frequency control mechanisms.
Grid Code Requirements
To ensure effective frequency control, grid codes have been updated to specify the performance requirements for wind turbines. These requirements often include provisions for primary frequency response, reactive power support, and fault ride-through capabilities. The 2011 discussions highlight the importance of harmonizing grid code requirements across different regions to facilitate the integration of wind power. Compliance with these requirements ensures that wind turbines contribute to system stability and frequency control, rather than merely acting as passive loads on the grid.
See also
- Combined heat and power
- Nuclear Power Plant Security and Vulnerabilities: Congressional Research Service Report
- Nuclear power in Russia
- Nuclear power in Germany
- International Energy Agency: Structure, Mandate, and Global Energy Policy