Abstract:The rapid development of renewable energy calls for feasible and reliable technologies to transmit and integrate power into grids. Voltage Source Converter (VSC)-Direct Current (DC) technology is considered as a promising solution for its independent control of active and reactive power. Modeling and coordinated control of a large-scale concentrating photovoltaic integration system with VSC-MVDC (Voltage Source Converter-Medium Voltage Direct Current) technology have been investigated in this paper. The average controlled-source model of PhotoVoltaic (PV) integration system is firstly established. Then, a novel control strategy without fast communication is proposed to improve the reliability of the coordinated control system. An extra voltage loop is added to the basic control block, which is able to assure stable operation of the PV system in various conditions. Finally, the proposed control strategy is verified with simulation results.
Compared with the AC micro-grid, the DC micro-grid has low energy loss and no issues of frequency stability, which makes it more accessible for distributed energy. Thus, the DC micro-grid has good potential for development. A variety of renewable energy is included in the DC micro-grid, which is easily affected by the environment, causing fluctuation of the DC voltage. For grid-connected DC micro-grid with droop control strategy, the tie-line power is affected by fluctuations in the DC voltage, which sets higher requirements for coordinated control of the DC micro-grid. This paper presents a simplified control method to maintain a constant tie-line power that is suitable for the DC micro-grid with the droop control strategy. By coordinating the designs of the droop control characteristics of generators, energy storage units and grid-connected inverter, a dead band is introduced to the droop control to improve the system performance. The tie-line power in the steady state is constant. When a large disturbance occurs, the AC power grid can provide power support to the micro-grid in time. The simulation example verifies the effectiveness of the proposed control strategy.
Minimal cut sets are the basis of reliability analysis using analytical techniques. At the present stage, minimal cut sets are mainly obtained by dealing with minimal path sets, which involves cumbersome steps and slower operational speed. The speed of reliability analysis is limited by that of calculating minimal cut sets. In consideration of the characteristics of microgrid, a hierarchical approach for fast calculating minimal cut sets is proposed in this paper. Firstly, an equivalent principle is proposed to convert topology structure into network node diagram; then grades of nodes are designated based on their original connection and the breadth-first search approach; afterwards, root-leaf matrices and selected matrix are created to specify the direction and order of the search. Next, all possible combinations of minimal cut sets are vertically traversed out by replacing root nodes with leaf nodes to achieve a more rapid access to minimal cut sets. Finally, taking the electrical structure of type A380 more electric aircraft as an example, mainstream methods are compared to show the correctness and advantage of our proposed method.
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