Capacity Optmization Of Energy Storages For Isolated Microgrids Based On Power Frequency Decomposition

With the energy crisis and environmental pollution, new clean energy like wind power are highly valued. However, large-scale wind power will affect the security and stability of the grid due to its intermittence and volatility. It would reduce the impact on power system if the wind power was connected to the power system as a microgrid, storage system in microgrid can effectively reduce the fluctuations caused by the wind power. Storage system can stabilize the fluctuations of active power, which plays an important role in the stable operation of microgrid, reasonable configuration of energy storage is important to improve reliability and economy of microgrid. This paper was focused on the power balance analysis in isolated microgrid, reliability assessment considering energy storage and capacity optimization of hybrid energy storage system. The following work was carried out:To characterize the fluctuation of wind power and load in micro-grid including wind farm, micro turbine and energy storage, the definition of discrepancy power in microgrid was given, which is the difference between load and wind power fluctuations; According to the fluctuations of discrepancy power, the discrepancy power decomposition model using Discrete Fourier Transformation was established in microgrid, which is to decompose the discrepancy power into low frequency and high frequency components and the high frequency component was defined as imbalance power. Meanwhile, micro turbine was used to provide the low frequency component and energy storge system to balance the high frequency component, which is proposed as MT-ES joint operating strategy. Taking an island microgrid as a numerical example, the results showed that: Under the premise of meetting the load demand and without increasing fuel consumption, the proposed joint operating strategy can effectively reduce the storage system capacity configuration requirement.Based on the MT-ES joint operating strategy, a six-state energy storage system reliability model was established, the energy storage system charge and discharge power adjustment strategy was proposed based on this model. Considering the abundance indices and wind power absorbility indicators of microgrid, the microgrid reliability evaluation model was established. System chronological state was simulated using sequential Monte Carlo method, the reliability model of each component and the operating reserve of MT was taken into account to calculate the system reliability indices, the results showed that: Compared to other operating strategies, MT-ES joint operating strategy can reduce fuel consumption while improving the reliability of micro grid. Reasonable configuration of energy storage capacity can improve system reliability while reducing investment costs.Two kinds of energy storage were implied to compensate the components of imbalance power which was deposed under a certern segmentation point, the energy storage configuration results would be different when the segmentation point was different. Based on the cycle characteristics of energy storage system, the imbalance power Fourier transform optimal segmentation principle was proposed; considering segmentation point being the decision variables, the optimization of hybrid energy storage system capacity configuration model was established. According to the cycle characteristics of energy storage system, the imbalance power was decoupled into day component and hour component which were balanced by compressed-air energy storage and sodium-sulfur cell respectively based on Discrete Fourier Transform. Considering the power balance, MT operation, energy storage operation, segmentation point and reliability as constraints, then considering energy storage investment operating cost, system loss of load cost and wind abandoning penalty cost comprehensively, the hybrid energy storage system capacity was optimally configured. The results showed that: compared to single energy storage system, hybrid energy storage system can effectively reduce the total cost, improve the reliability of the microgrid and wind power utilization, choosing an appropriate segmentation points can further improve the economy.