Abstract
Hybrid energy storage system (HESS) is widely used in microgrids, and its research mainly focuses on energy management, power allocation, topology design and so on. For the power allocation issue, most research only involves the fundamental power allocation within a single HESS. While the allocation of reactive power, negative sequence power and harmonic power, and the coordinated operation of multiple HESS units are rarely be studied. In response to the above situation, this paper proposes a coordination control strategy for the HESS group under the unbalanced load and nonlinear load conditions. The proposed strategy is based on droop control method and can realize interconnection and power sharing for multiple HESS units under weak communication condition. Inside the HESS, the battery (BAT) power conversion system (PCS) works in droop mode, providing energy and only outputting fundamental active power; and the ultracapacitor (UC) PCS works in the compensation mode, analyzing the output power of HESS and providing the reactive, negative sequence and harmonic powers. This strategy can provide better system performance in unbalanced and nonlinear load conditions. It utilizes UC-PCS to deal with the inherent power sharing issues of droop control method, and enhance the transient process. Meanwhile, the stability of microgrid and the service life of the BAT-PCS are increased with the improvement of power quality. Moreover, the HESS adopts a dual inverter structure, which is conducive for the upgrading of existing equipment. Finally, the validity of the proposed strategy is verified by simulation and experimental results.
Highlights
With the increased concerns on environment and cost of energy, more renewable energy sources (RES) such as photovoltaic cells, small wind turbines, and microturbines are integrated into the power grid in the form of distributed generation (DG) units which are normally interfaced to the grid through power electronics converters
Since this paper studies the AC power allocation issue under unbalanced and nonlinear load conditions, the topology shown in Figure 3(d) is selected
SIMULATION RESULTS A microgrid model is established in Matlab/Simulink to validate the proposed coordination control strategy
Summary
With the increased concerns on environment and cost of energy, more renewable energy sources (RES) such as photovoltaic cells, small wind turbines, and microturbines are integrated into the power grid in the form of distributed generation (DG) units which are normally interfaced to the grid through power electronics converters. The BAT-PCS, as the main unit in the HESS, adopts the droop control method to ensure active power sharing among HESSs; the UC-PCS, as the auxiliary unit, works in compensation mode, outputting reactive, negative sequence and harmonic powers. To solve the power sharing issue in islanded microgrid, the conventional droop control method is usually applied to the BAT-PCS, and its control expressions are as follows:. That when all the DG units operate under the same frequency and voltage magnitude, the larger capacity DG units can output more real and reactive powers according to (1) and (2) This is the power sharing principle of droop control. D. CONTROL STRATEGY OF THE UC-PCS The current of the HESS may contain fundamental positive, negative sequence and harmonic components according to the load, while the BAT-PCS only outputs the fundamental active current i+1d due to the compensation effect of the UC-PCS. Due to the assist role of the UC-PCS, the BAT-PCS can operate in a favorable state and improve the voltage quality of the microgrid
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