A study on capacity of distributed generation and its effect on short circuit current at micro-grid operation mode

Abstract

Now more and more distributed generations will be connected to distributed power system. It's found that different distributed generation has its own operation characteristic such as fluctuation and uncertainty. When micro-grid is to be island the voltage and frequency will change, so different control modes have been used on different distributed generation. Different kinds of distributed generation have different behavior when system operates normally or fault occurs. Classification and equivalence were adopted to deal with all kinds of the distributed generations in system. The distributed generations with same control mode can be combined to an equivalent generation. When system operates on grid-connection mode the short circuit current relies on the capacity of grid so it is rather large for the capacity of distribution grid is much more than that of distributed generations. But when micro-grid operates on island mode the short circuit current may decrease to a relative small value. Over current protection is widely used in distribution system, and the performance of it is dependent on operation mode, so the value estimate of short circuit current is necessary for verification of credibility and selectivity. Two summations of capacity were proposed to estimate the minimum and maximum short-circuit current in island.