Study On Novel Sufficient Criterion Protection Strategy For Distribution Networks With Distributed Generations

In recent years, large-scale applications of renewable energy based distributed generations (DG) have improved the reliability of power distribution systems, as well as changed the topological structures of power supply. Traditional power distribution protection methods for radial network no longer handle the complicated network integrated with distributed generations. Currently proposed DG integrated distribution system protection studies are based majorly on large amount of data, which is infeasible to use with the existing network. To face the challenge of the DG integrated distribution network protection, the thesis comes up with the new sufficient criterion protection strategy, which is outstanding in selectivity and sensitivity of protection.To protect the DG integrated distribution system, sufficient criterion strategy makes decision under the actual information-lacking and imprecisely communicational situation. DG auto-disconnecting strategy is also considered to make fault locating decision selectively. The new sufficient criterion protection comes up with five basic characteristics, which are sufficiency, adaptability, optimization, discrete inverse-time characteristic and reliability.Under the theory of sufficient criterion, the current phase variation based differential protection is proposed. The specific relation between fault currents’ intersection angle and fault current phase variation of each end in distribution feeder is analyzed, and the indirectly calculating method of intersection angle between fault currents is proposed thereunder. The sufficient criterion method overcomes certain degree of synchronization error in DG integrated distribution system.The current amplitude based pilot protection method is also proposed, which reflects the current amplitude sufficient fault feature directly. As current amplitudes of each end of the feeder are almost the same under external fault and normal condition, internal fault is determined once difference is found between current amplitudes. The relative protective criterion makes it selectively and sensitively in fault locating calculation.To make a supplementary of the sufficient criterion protective strategy, the voltage difference searching protection method is proposed as well. Its searching direction is based on the voltage difference of specific nodes in the system. Still, fault locating determination is made selectively.With the application of sufficient criterion protection, DG is no longer immediately isolated after fault. However, because of the informational and communicational impreciseness in distribution network, sufficient criterion protection might perform with malfunction under few fault condition. DG should be disconnected from grid thereunder, and the multi-sources system transforms into traditional single-source radial system. Sufficient criterion protection and over-current are used to locate fault afterwards. According to its advantage in waveform singular points distinguishing, the db4 wavelet is used to decomposing voltage signal of the PCC, and the DG disconnecting criterion is proposed thereunder.The sufficient criterion protection is able to locate fault before or after DG is disconnected, and the protection configures with hierarchy in different system topology. The thesis builds a model of 10kV DG interconnected distribution system using MATLAB/Simulink to test the correctness of sufficient criterion protection method. The result shows the sufficient criterion protection performs efficiently and correctly under different fault condition.