Protective Relaying Research On Grid-connected And Islanded Operation Of Microgrid

Microgrid integrates micro-sources, energy storage devices, load, control and protection relays together to form a single controlled distribution network unit. Microgrid can improve power supply reliability of the user side, achieve high penetration of large-scale distributed and renewable generators, and make efficient cascade utilization of all kinds of energy.As the main body of Microgrid, the access location of micro-sources is flexibility. When Microgrid connected to power system, power flows tow-way within the network, resulting in that the traditional distribution network protection is no longer suitable for fault diagnosis and start in this double-ended power flow situation. Meanwhile, the interfaces of micro-sources are commonly electronic inverter. When failure occurs within the network, the micro-source output short-circuit current will be limited by the over-current saturation module in inverter, which further worsened the using environment of the traditional protection with amplitude criterion. Thus, the reliability, selectivity, sensitivity and speed continuity of traditional distributed network protection cannot be fully met. Therefore, Microgrid protection technology has become one of the bottlenecks to the applications of this technology.In order to solve the conflict between the bottleneck in the development and application of Microgrid technology and the limitations of the traditional protection, this paper analyzes and derivates the potential impact of the micro-source accession on the three-current protection and automatic reclosing of traditional network. Based on the conclusion obtained from the above analysis, look for protection methods from conventional protection technology that can meet the technical requirements and application environment of Microgrid protection. Longitudinal differential protection module is built accordingly. Based on the proposed relay module, Microgrid protection strategies are further proposed. As representation, the electrical and control models of a micro-gas turbine power generation systems and fuel cells are established to be used for steady and transient simulation. Based on the scientific and reliable Microgrid model, the effectiveness and feasibility of the relay module and protection strategy is verified through PSCAD/EMTDC.