Research On Stability Improvement Of Power System With Wind Power Integration Using Energy Storage

The wind power generation, which differs from the operating characteristics of tradi-tional synchronous generators, is highly stochastic, low-controllable and possesses a weak anti-disturbance ability. Therefore, with the increasing penetration of wind power, the characteristic of power system operation and control will be substantial changed. Mean-while, the security and stability of power system with wind power integration will be seri-ously threatened. Hence how to guarantee the secure and stable operation of power system with large scale wind power integration has become an urgent problem. The energy storage technology could be an effective method to solve this problem due to its fast power re-sponse ability and strong stabilizing effect. Therefore, this dissertation will study on the ap-plication of energy storage technology to improve the security and stability of power sys-tem with wind power integration.This dissertation firstly introduces the current research status as well as development trend of wind power generation and energy storage technology, respectively. Afterwards, the main safety and stability problems exist in wind power system are summarized, and thereby the research method and main work of this dissertation are proposed. The research focuses include the application of energy storage technology to improve the fast frequency response characteristics of wind farms, aid wind farms to successfully ride through of low voltage, and improve the angle stability of wind power system. Main contents and contribu-tions are summarized as follows:(1) This dissertation defines the virtual inertia of a system with both wind power and energy storage device, analyses the energy storage capacity required to assist wind farms to have similar inertial response characteristics to that of a synchronous generator with the same capacity through theoretical deduction. Based on fuzzy logic inference method, a control strategy to provide inertia to wind farms and aid them to participate in fast frequen-cy regulation by using energy storage system is designed. This control strategy effectively improves the power supporting of wind farms to stable power system frequency. Mean-while, it could reduce the maximum active power output of the energy storage system dur- ing the power system frequency regulation. Additionally, the power capacity requirement of energy storage system could reach the minimum values. In this way, equipped with an en-ergy storage system with the capacity of only 5%of its rated capacity, a wind farm could possess similar inertia response characteristics to that of a synchronous generator with the same capacity.(2) During the low voltage ride through, wind farms will inject reactive current into power system according to the grid code, which will lead to frequency instability of the wind power system under the control of phase locked loop. The formation mechanism of this instable phenomenon is revealed through building a detailed mathematical model of the wind power generation system during the low voltage ride through process. Based on this, a system stabilizing strategy to improve the current-injection characteristics of wind farms under low voltage conditions by using energy storage devices is designed. This control strategy maintains frequency stability at the point of common coupling of the wind farm, which offers frequency-stabled signals to all phase locked loops of the wind generators within the wind farm. Thereby, the frequency instability of the wind power system could be avoided.(3) The small signal stability analysis model of (double fed induction generator, DFIG) based wind power transmission system, including the dynamic characteristics of phase locked loop, is established. Then, this dissertation analyzes the influences of wind power generation system's various operating conditions, strength of power grid and parameters of phase locked loop on the stability of wind power system. It is indicated that the oscillation in the phase locked loop will cause a 10 Hz power oscillation phenomenon when wind power integrated to the main power grid through long-distance transmission lines. Besides, the weaker the power grid the wind power integrated into is, and the larger the parameters of the phase locked loop are, the power oscillation of the wind power system is more likely to occur. For such oscillations, an energy storage damping controller is designed based on the phase compensation theory. It can successfully suppress the power oscillation that oc-curs in weak power grid to which large scale of wind power is connected through long-distance transmission lines.(4) Based on the detailed model of DFIG based wind power generator including the phase locked loop, the impact of wind power on small signal stability characteristics of tra- ditional generators is analyzed from two aspects. One is the change of system power flow and the other is the influence on electromagnetic torque of neighbor synchronous genera-tors. Reach results demonstrate that DFIG based wind power generator integration would deteriorate the damping ratio of oscillation mode between synchronous generators under most cirtuation. To enhance the adaptability of the damping controller under the versatile wind power system operation conditions, the paper establishes the transient energy function including the wind power and ESS integration. Then an energy storage damping controller based on energy function is proposed to decay the transient energy of the power system. The simulation results of 4 machines 2 areas power system shouw that:this controller could greatly improve the oscillation characteristics of traditional synchronous generators in the grid with wind power integration under various disturbance and operation conditions.