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

The interplay between wind power system (WPS) and grid is becoming increasinglysignificant with the increased penetration of wind power, as is shown in two aspects: Onone hand, since the wind energy resources are quite fluctuant and unstable, thecorresponding wind-generated electricity is also full of fluctuations, which has greatinfluence on the quality and stability of the grid power on condition that the penetration ofwind power is relatively high; on the other hand, as the proportion of wind power in thewhole electronic grid is gradually increasing, if the passively protective disconnectionmethod is still adopted when grid faults occur, the difficulty of recovering the whole systemwill be increased, and to make things worse, the faults may become more serious, whichwill have significantly negative effects on the safety of grid operation. Therefore,integrating the energy storage system (ESS) with certain capacity will help to smooth windpower fluctuations, enhance the low voltage ride-through (LVRT) capability, maintain thedynamic balance of power generation/load, and keep the voltage and frequency stable, andhence 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 storagebased direct drive WPS.1. According to the features of direct drive WPS operation, based on the two essentialissues (smoothing wind power fluctuation and low voltage riding-through) existing incurrent WPS, three comprehensive control strategies are proposed: Dual-mode control,direct power control and direct voltage control of energy storage system (ESS). Under thesethree controlling strategies, the direct drive WPS can output smoother active power undernormal conditions and maintain relatively high low voltage ride-through capability whengrid faults occur. More importantly, based on the direct voltage control of ESS andcombining the complementary advantages of supercapacitors and batteries to each other interms of technical performance, a comprehensive strategy for controlling hybrid ESS isfurther 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 forwind power, the WPS will be permitted to be cut off from the grid when the duration of gridfault goes beyond a certain range. In order to make the most of wind resources and improvethe reliability of the local loads power supply, a coordinated controlling strategy forsuperconducting magnetic energy storage-based direct drive WPS is introduced, so as torealize 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 ofthe state-of-charge for ESS, a double fuzzy logic control strategy optimizing themanagement of the ESS is proposed to effectively control the charging and dischargingpower of ESS. Therefore, the wind power fluctuations can be smoothed, overcharge anddeep discharge can be effectively avoided, and the transformation towards suitablestate-of-charge can be realized.4. Considering the relevant technical regulations of grid-connected WPS, this paperfurther 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 ESScharacteristic parameters–level-of-smoothing of wind output power is established. Thentaken the technical and economic demand into the comprehensive consideration, along-term mathematical model of ESS characteristic parameters-lever-of-smooth of windoutput power and ESS cost is then established so that the optimized characteristicparameters of the ESS can be obtained through genetic algorithm.