| As the global energy crisis becomes more serious, wind energy, solar energy and other new energy have become the focus of attention. But the new energy source like wind and solar have the characteristics of volatility, intermittent and uncertainty. Therefore, large-scale new energy connect to power systems will bring a great challenge to the safe operation of the power grid. In large wind farm, typically configured reactive compensation devices and energy storage systems, which will solve the problem well. If the two device constructed separately, it will cause the high cost and big volume and other defects, if the two devices can be combined together, the efficiency of large wind reservoir engineering can improve greatly. Based on this, this paper studies cascade multilevel STATCOM combined with hybrid energy storage system, and the following several aspects are studied.1.Hybrid energy storage system (HESS) based on the foundation of lithium-ion battery and super capacitor is studied, and the characteristic and model of each energy storage technology is analyzed. In order to enhance the accuracy and rapidity of the management of charge and discharge process, different hybrid methods of hybrid energy storage system are compared and the method that two energy storage devices are both connected to DC side of the converter through DC/DC link is adopted. A control strategy of DC/DC converter in which the SOC of battery and terminal voltage of the super capacitor are considered simultaneously is put forward in order to prevent the energy storage system overcharging and over discharging. Finally, a control strategy of smoothing wind power fluctuations based on complex filtering algorithm according to properties of battery and super capacitor. Using this control strategy will reduce the design capacity of the super capacitor and lower the charging and discharging frequency of lithium-ion battery and then prolong the lifetime of the energy storage system.2. The topology and operating principle of cascaded multilevel STATCOM is researched and the advantage of cascaded multilevel topology in combination with energy storage system is explained. Then the mathematical model of cascaded multilevel STATCOM is built. The p-q analysis method based on instantaneous reactive power theory, the decoupling control strategy of voltage and current double closed-loop and the phase shift PWM modulation strategies are studied. The reason of DC voltage unbalance is analyzed and the stable control of DC capacitor voltage is achieved using the outer voltage loop control method.3. According to the aforementioned hybrid energy storage system and cascade multilevel STATCOM, this paper proposes a topology of STATCOM/HESS. Based on the simplified equivalent of topological structure, and then analyze its mathematical model. Park transformation are used to achieve the independence of P-Q decoupling control. Based on this, the dynamic coordinated control strategy of STATCOM/HESS considering the compensation capacity limits is put forward. According to the state of the bus voltage adjustment control target, smooth wind power fluctuations under steady state, in the event of an emergency sacrifice part of active ability to provide dynamic reactive power to improve the level of power grid voltage. In PSCAD environment established the simulation model of STATCOM/HESS, validate the correctness of the proposed coordinated control strategy. |
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