Self-healing Technology In Distribution Grid With Distributed Photovoltaic Generator

Although the distributed photovoltaic generators tied to Nanj ing’s distribution grid have increased not only in number but also in their size during recent years, power lines with PV still depend on traditional self-healing devices, such as protective relay and automatic circuit reclosers, due to the relatively slow construction speed of infrastructure for automation system. In another aspect, the feasibility of self-healing strategies which now being applied to Nanj ing’s grid has not been fully analyzed under such circumstance, because the characteristic of multi-source network had not been considered seriously while the system being built by technicians. Therefore, a thorough study to self-healing technology in multi-source distribution network is needed, not just for the urgent need to self-healing devices from PV owners, but also for the effectiveness of automatic self-healing technology on power lines with grid-connected PV system.Based on an introduction to the internal structure of PV system and its control scheme, the characteristic of grid-tied PV as an inverter-based current source is discussed not only under a normal circumstance or but also when fault happens. Then a line model with one single on-grid PV is simulated on RSCAD software, for evaluating the potential impact on fault current caused by PV’s output. The result shows that both a current drop from the substation as well as a current rise into the fault point could be detected under simulation environment, which are respectively named as the decrease effect and the accumulation effect. Besides, another line model with multi PV sources is also examined, and shows a certain pattern of the way PV sources influencing the fault current in different areas, which relates to the position of PV’s interconnection point (POI).From this point, potential challenges are analyzed, ranging from the relay’s unselective or delayed response to short-circuit fault, to the reclosers’ failure in closing breaker because of the unsuccessful synchronization between utility grid and photovoltaic DG, and the solution to each device’s problem is proposed respectively for ensuring their effectiveness in a grid tied with PV source.After that, the algorithm now used by Nanj ing’s automation system is evaluated under different situations with photovoltaic generators’connected to the distribution network, especially the potential failure in the three key steps during the automatic self-healing process:fault locating, isolation and recovery of health branches. And as it turns out that the fault locating step may be mistaken by PV’s reverse energy flow, two different locating rules are proposed, based on a grid model with multi-direction energy flows which is described with a directed graph. Then four possible algorithms are evaluated, by comparing their simplicity, feasibility, as well as their tolerance to wrong signals from terminal unities. And one algorithm, in which gird is described with point-line array and fault district is located according to hot-zone rule, is finally picked for its advantages in all of the three aspects. Along with fault location, the algorithms for fault isolation and power recovery in healthy branches are also modified, so that accidental island can be prevented while penetration rate can be kept under certain limit in Nanjing’s distribution grid.