Study On Sufficient Directional Protection For Distribution Network With DGS

As the crisis of fossil energy and the increasing problems of serious environment, distributed generation (DG) technology has been paid more attention. With the advantage of flexibility, economy and reliability, the application of DG in distribution network becomes inevitable trend and direction of development. However, the application of DG in distribution network changes the topology of traditional distribution network from the radiation structure of single power supply network to complex structure of multiple power supply network. It changes the short-circuit current and the directions of power flow. The phenomenon of two-way power flow even appears in some lines.According to the regulation of IEEE 1547, in order to restore the radial structure of distribution network with DGs and remove fault with three-sectional current protection, DGs must cut off immediately when fault occurs in distribution network with DGs. However, this protection strategies not only damage the the benefit of DG generators and DG users, but also the development of new energy power generation industry has been limited in a certain extent. On the other hand, it also highlights the current urgent need for a more rational and effective protection strategy for distribution network with DGs.This paper analyses research status of directional protection in distribution network with DGs after analysis of impact of DG on protection of distribution network, and then point out its shortage and areas for improvement. Based on it, starting from the strategic level of protection, a new type of "sufficient" protection strategy has been put forward, which basic idea is making full use of the incomplete information of distribution network to find out a "sufficient" fault location and isolation scheme for distribution network with DGs. The strategy is not required to solve all problems of distribution network with DGs, but must be able to solve most problems. Compared with traditional protection strategy, this strategy is more suitable for current distribution networkBased on the idea of "sufficient" protection strategy, from the point of view of the whole distribution network, this paper analyses characteristics of fault current in distribution network in-depth. The criterion of the amplitude and phase of negative sequence current is proposed to determine the direction of asymmetric fault and the criterion of amplitude and phase of positive sequence fault current is proposed to determine the direction of symmetrical fault. Based on it, scheme of fault region determination has been put forward. Then fault line can be determined in fault region by compare fault current phase of two terminals based on load current phase. The scheme can not only realize the accurate location of fault line, but also can reduce the amount of communication in distribution network. Finally, a protection scheme for distribution network based on "sufficient" directional protection has been summed up, and then the structure and workflow of it has been analyzed in detail.The model of distribution network with DGs is built based on Matlab/Simulink platform. The conclusions show that this scheme can remove fault quickly and accurately with any kind of fault.