| The distributed generation technology based on renewable energy sources have attracted widespread attention and developed quickly in order to meet the various needs of energy development, environment protection and the improvement of power grid construction in recent years.Distributed generation generally connects to low-voltage distribution network, China traditional distribution network is generally a radial network with a single power source, and its relay protection is designed and set according to the radial network. Grid connected DG changes the structure of distribution network, which makes the distribution system change from single supply radial network to double source or multi-source network. The power flow is no longer one way flowing from substation to the load. And the traditional protection system, including the setting value, protection algorithm, protection between cooperation, as well as the cooperate relationship between the protection and the reclosing is likely to be affected. Considering the impact of DG on the distribution network, traditional relay protection can no longer meet the requirement of safe operation of the power grid. Therefore, it’s necessary to analyze the impact on traditional distribution network of DG and study on improving protection system to adapt new distribution network with grid-connected DG.This paper firstly introduces the main types and operating characteristics of DG, then the control methods of inverter interfaced DG are classified. It is necessary to analyze the control scheme and the characteristics of fault respondence of DG to complete the equivalent model of DG. And this model will be used for the analysis of the effects of DG on the protection system of the traditional distribution network.The effects of DG on the distribution network protection system are studied, including different locations and different capacity. Firstly the impact of DG connected to the same location while its capacity changes on distribution network protection system are analyzed, and followed by the analysis of impact of DG connected to different location while its capacity keeps the same on the traditional distribution network. Then simulation is carried out, the effects of DG discussed above is verified by simulation. Finally, the effects of DG connected to different locations on the distance protection are studied. The result shows that DG changes the traditional distribution network topology, when a fault occurs, the DG connected to distribution network could lead to the loss of protection’s reliability. A new protection scheme based on the coordination of zoned pilot protection and overcurrent protection is introduced. According to the location of DG, the feeder is divided into different zones, directional pilot protection is configured at the upstream side of DG, and the overcurrent protection is retained for the whole feeder. With this protection scheme, the fault can be cleared reliably and quickly. Finally, the advantages and problems of distance protection used in distribution network with DG are analyzed, and an example is carried out to demonstrate DG’s affect on the setting value of distance protection.A distributed decision-based wide-area protection scheme is applied to distribution network with DG. The network nodes are numbered and regions are divided according to multi-DG distribution network structure. Based on fault current characteristics, this paper proposes a new adaptive protection method which includes two steps:first and secondary fault location, and then remove fault to protect distribution network with DG. Through a case analysis, the validity and adaptivity of this protection scheme is verified. The validity of the criterions used in two fault location steps is verified by the simulation result. |
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