Optimal Control Strategy Of Direct-Drive Wind Power System Based On Energy Storage

The interplay between wind power system (WPS) and grid is becoming increasingly significant with the increased penetration of wind power, as is shown in two aspects:On one hand, since the wind energy resources are quite fluctuant and unstable, the corresponding wind-generated electricity is also full of fluctuations, which has great influence on the quality and stability of the grid power on condition that the penetration of wind power is relatively high; on the other hand, as the proportion of wind power in the whole electronic grid is gradually increasing, if the passively protective disconnection method is still adopted when grid faults occur, the difficulty of recovering the whole system will be increased, and to make things worse, the faults may become more serious, which will have significantly negative effects on the safety of grid operation. Therefore, integrating the energy storage system (ESS) with certain capacity will help to smooth wind power fluctuations, enhance the low voltage ride-through (LVRT) capability, maintain the dynamic balance of power generation/load, and keep the voltage and frequency stable, and hence realize the safe, economic, effective and high-quality operation of WPS. In this paper, studies are carried out on the optimizing and controlling strategies toward energy storage based direct drive WPS.1. According to the features of direct drive WPS operation, based on the two essential issues (smoothing wind power fluctuation and low voltage riding-through) existing in current WPS, three comprehensive control strategies are proposed:Dual-mode control, direct power control and direct voltage control of energy storage system (ESS). Under these three controlling strategies, the direct drive WPS can output smoother active power under normal conditions and maintain relatively high low voltage ride-through capability when grid faults occur. More importantly, based on the direct voltage control of ESS and combining the complementary advantages of supercapacitors and batteries to each other in terms of technical performance, a comprehensive strategy for controlling hybrid ESS is further brought up, which can hence greatly enhance the economic and technical performance of the system.2. According to the related technical requirements of low voltage ride-through for wind power, the WPS will be permitted to be cut off from the grid when the duration of grid fault goes beyond a certain range. In order to make the most of wind resources and improve the reliability of the local loads power supply, a coordinated controlling strategy for superconducting magnetic energy storage-based direct drive WPS is introduced, so as to realize the fast and smooth switch between grid-connected and off-grid modes.3. According to the short-period prediction of wind power and real-time detection of the state-of-charge for ESS, a double fuzzy logic control strategy optimizing the management of the ESS is proposed to effectively control the charging and discharging power of ESS. Therefore, the wind power fluctuations can be smoothed, overcharge and deep discharge can be effectively avoided, and the transformation towards suitable state-of-charge can be realized.4. Considering the relevant technical regulations of grid-connected WPS, this paper further comes up with a judgmental standard to quantitatively reflect the level-of-smoothing of power curves, based on which the short-period neural network model of ESS characteristic parameters-level-of-smoothing of wind output power is established. Then taken the technical and economic demand into the comprehensive consideration, a long-term mathematical model of ESS characteristic parameters-lever-of-smooth of wind output power and ESS cost is then established so that the optimized characteristic parameters of the ESS can be obtained through genetic algorithm.