The Failure Analysis And Protection Study Of The Grid With Micro-grid

Due to the influence of the energy crisis and the problem of power supply in some areas, the distributed generation has been developing faster and faster. Distributed generation connected to the electricity grid scattered, however, it may cause varies aspects of problems on the grid, such as electric energy quality, the coordinated control, protection and management. Therefore, for the problem of multiple distributed generation accessing the power grid, a concept called micro-grid was puts forward. Micro-grid concentrates multiple distributed generation for power supply and convenient management. Now the micro-grids are getting a greater appreciation because of its superior to the characteristics of distributed generation.Although micro-grid has many advantages, however, it brings some problems to the grid due to the access of distributed generation. First the topology structure of the network has been changed from the traditional electricity supply mode with a single supply voltage to multiple distributed generation supply electric power. So it caused some problems such as bi-directional current existed. Second a large number of power electronic devices are contained in the distributed generation, what made a large difference in the short circuit between the traditional grid and micro-grid. So we should consider both of the bi-directional current and the change of short circuit when we solve the relay protection problem of the micro-grid. The traditional protection strategy can’t suitable for the grid which contains micro-grid. So the efficient protection scheme should be put forward.The characteristics of the distributed generation are analyzed in this paper. First, the distributed generation model and a micro grid are established. After profoundly analysis for relay protection problem of micro grid interconnection of grid and the influence of the containing micro grid fault characteristic, a sequence component protection scheme to improve differential current method is proposed. The faults in the grid model are simulated and analyzed, the feasibility of the protection scheme is verified. Concrete study content as follows:(1)Some frequently-used distributed generation are introduced and analyzed, electric generation principle and the merits and demerits about solar power, wind power generation unit and the mathematical model of the micro gas turbine are analyzed, the energy storage unit of super capacitor are different from traditional energy storage device are compared, that the merits and demerits and the use of occasions are analyzed; Then analyze several kinds of micro grid model, mainly for CERTS model, the difference distributed generation type and application of bus are analyzed.(2)According to different kinds of distributed generation supply for mathematical modeling based on PSCAD software, analyzes grid topology modeling, mainly analysis on grid inverter control. In order to guarantee the stability of the grid voltage and output, the control of inverter using different control methods through respectively on the PQ control and V/f control according to the grid mode or island mode. Finally, micro grid model was build based on the model of distributed generation.(3)The difference between the grid contains micro grid failure and traditional electrical protection are analyzed, it analyzed the fault occurred in the influence of different position of micro grid relay protection from the distributed generation access position, micro power supply capacity of these two aspects; It analyzed the impact on the automatic reclosing because of micro-grid and those protection coordination problems because of multiple distributed generation at the same time into a feeder. The faults in the grid model are built is simulated and analyzed, the failure problems in relay protection are verified.(4)The main protection strategies of micro-grid are analyzed, and against the problem that upstream region of single-phase ground fault can’t be detected by differential current method, this paper put forward a protection method based on sequence component, which can cover the shortage of differential current method. In consequence, this paper proposed improvement program that combining the two methods, which can reach a good protection effect. Finally using the model of microgrid to make simulation, determine the protection setting value from the value of fault current, make sure of the feasibility of the protection scheme that based on sequence components.