Abstract
The large-scale application of wind power eases the shortage of conventional energy, but it also brings great hidden danger to the stability and security of the power grid because wind power has no ability for frequency regulation. When doubly-fed induction generator (DFIG) based wind turbines use rotor kinetic energy to participate in frequency regulation, it can effectively respond to frequency fluctuation, but has the problems of secondary frequency drop and output power loss. Furthermore, it cannot provide long-term power support. To solve these problems, a coordinated frequency control strategy based on rotor kinetic energy and supercapacitor was proposed in this paper. In order to ensure the DFIG provides fast and long-term power support, a supercapacitor was used to realize the droop characteristic, and rotor kinetic energy was used to realize the inertia characteristic like synchronous generator (SG). Additionally, the supercapacitor is also controlled to compensate for the power dip of the DFIG when rotor kinetic energy exits inertia support to avoid secondary frequency drop. Additionally, a new tracking curve of DFIG rotor speed and output power was adopted to reduce the power loss during rotor speed recovery.
Highlights
In recent years, the penetration of renewable energy connected to the power grid continues to increase
Doubly-fed induction generator (DFIG) is one of the main types in the current wind power market. It usually operates at a maximum power point tracking (MPPT) state [2], in which the rotor speed is completely decoupled from system frequency
In a conventional frequency control strategy based on rotor kinetic energy, the control block diagram is shown in Figure 2 [7], and the expression of the DFIG output active power is as follows
Summary
The penetration of renewable energy connected to the power grid continues to increase. In the control strategy of keeping reserve power, the DFIG maintains a certain reserve power through the means of over-speed control or pitch control [10,11,12], so the DFIG can provide primary frequency regulation and long-term power support In this method, the DFIG operates at the load shedding state in normal condition, which causes a great waste of wind energy resources. A torque limitthe strategy to inertial response capability of a battery-embedded DFIG and used it to provide power support in control rotor kinetic energy and participate in frequency regulation, and utilized a battery to solve the synchronous, supersynchronous, and subsynchronous operations; [22] utilized an adaptive fuzzy secondary control frequency dropthecaused by the of torque limit control. (3) By controlling the output power mode of supercapacitor, power dip of the DFIG happened at the rotor kinetic energy exiting, power support is solved, and secondary frequency drop is avoided
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