The Research Of Supplying Reliability On Urban Mid-voltage Distribution Grid

Distribution grid connects to both the main power grid on the upper level and thousands of households downward. The reliability of power supply is directly perceived by customers, being closely bound up with safe production and high-quality service. The management level of distribution grid determines the supply quality, supply capability and service level directly. For a long time, influenced by the idea of "higher transmission than distribution", there were many weak links existed in the construction and management of distribution grid relative to the main grid, which reveals a rather big gap between the international advanced level still.With the development of city construction, the users’need for supplying reliability is becoming more demanding, which calls for higher requirements of the quality of existing distribution equipment and the reliability of distribution grid. While the original low-voltage distribution networks mostly apply the radial supplying pattern which shows a number of defects, that a fault happened somewhere could cause the whole-line massive interruption toward the low-voltage users, leading to rather long interruption duration of low reliability. The loop-grid connection mode could be applied to change the situation, extending to gridding connection to link two or more power sources to supply for users, which is able to switch on different lines fast when a fault happens to reduce the duration, so as to enhance the distribution supplying reliability further.The paper analyzed the defects and insufficiencies of the existing distribution grid structure in the demonstration area, then proposed the optimization scheme: mid-voltage cables should apply mainly the single-loop or double-loop grid connection, gradually transit to the mode of double-loop grid plus interconnection; set node switching stations with breakers partly equipped on the basis of original ones to realize the cascade protection cooperating with the relay devices on the outgoing lines of substations; based on the above conditions, apply unified load flow controller to try out closed-cycle operation of distribution grid on the experimental lines.This article discussed two operation mode of urban low-voltage distribution loop grid, as the characteristics of open-loop operation and closed-cycle operation. Analyzing the differences on network structure, working mechanism and technical performance between both modes to obtain the conclusion that under the current conditions, the closed-cycle operation mode would reduce the interruption duration and line loss, meanwhile improve the power quality efficiently, being higher reliable on power supply. The article also introduced the controlling need of mid/low-voltage distribution grid in the closed-cycle operation mode, put forward the model system of distribution grid intelligent control, and analyzed the basic practicing method and performance to conclude finally that the integrated network intelligent control method of distribution grid possesses the advantages of high reliability, high security and low work load, which as a result is a control plan of current trend.