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Abstract:With the rapid development of wind power generation, the inertial response of wind turbines (WTs) has become a topic of wide concern recently, due to its influence on grid frequency dynamics and stability. This paper proposes and defines the inner potential to summarize and understand the inertia control methods and inertial response of type-3 and type-4 WTs, which is analogous to typical synchronous generators (SGs), to make it more easy to understand by system operators and engineers with a traditional power system background. The dynamics of the defined inner potential of the wind turbine without any inertia control is different from SGs, thus the electromechanical inertia is completely hidden. The rapid power control loop and synchronization control loop are the major reasons that the WT's inertial response is disenabled. On the basis of the defined inner potential's dynamic, the existing inertia control method for WTs are reviewed and summarized as three approaches, i.e., optimizing the power control or synchronization control or both. At last, the main challenges and issues of these inertia controls are attempted to explain and address.

Due to decoupling between mechanical part and electrical part of doubly fed induction generator (DFIG), the DFIG has no natural frequency response capability, which results in decreasing of total rotary inertia of power grid, and the frequency stability of power grid will face larger challenges. This study proposes an assessment method of equivalent inertia time constant, further gives the assessment value of equivalent inertia time constant of DFIG. Then, a virtual inertia optimisation control of DFIG using rotor current direct control based on status assessment value is proposed, which includes conventional function, status assessment and additional virtual inertia control. The assessment values of equivalent inertia time constant of wind farms, synchronous generator and power grid connected with multi-type generators are calculated, which are related to penetration of wind power and control strategies. The simulation results verify the efficacious of the proposed virtual inertia optimisation control and the accuracy of assessment method of equivalent inertia time constant, and the frequency stability of power grid is improved in the condition of different active power of wind farms and penetration of wind power.

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