Studies On Several Key Technologies Of Fault Risk Assessment And Fault Restoration In Urban Power Grid

As the most significant part of modern power system,the urban power grid burdens the tasks of maintaining city economic development and residents' daily life.The urban power grid in China has been developed to have the following features:diversity of power-supply services,complexity of demand-side responds,high fusion of cyber and physical power equipments.As a result,the urban power grid is expanding rapidly and the structure is becoming more and more complex and the operating mode is becoming more and more flexible.Besides,the related technologies of urban power grids have been highly upgraded and modernized.Once a fault occurs in an urban power system,there may be wide-area outage happening,which brings massive damage not only to power consumers but also to national security.Under this circumstance,it brings in severe challenges such as the fault identification,control and fault restoration,which needs the efficient,reliable and safe communication technology.The efficient information exchanging helps to achieve the goal of the power grid monitoring,assessment,analysis and aid-decision making.Aiming at meeting the demand of smart urban power grids' safeguard by the wide-area information accessing technology,this dissertation studies on the several key technologies of the fault risk assessment and fault restoration in urban power grid.In the respect of fault risk assessment,the dissertation focuses on three key technologies including the fault assessment index system,the basic information accessing and processing and the high speed risk calculation methods.To quantify the safety and economic efficiency of urban power system,risk index system including equipment assets loss,safety margin,structure integrity and power loss is proposed in allusion to urban power grid operating characteristic and dispatching operators'concern.Based on the information demand of two aspects,probability and consequence,of risk assessment,system architecture for information accessing is proposed to acquire basic data for fault risk assessment.Common information models are resolved and analyzed to tackle the issue of acquiring real-time data such as power grid topology,operating mode and etc.,which helps to meet the data demand of real time risk assessment.Transmission and distribution lines are one of the most important components in urban power grids.The acquirement of transmission line online monitoring data is the key technology for fault risk assessment in practice.Some communication technologies such as wireless public network,Wi-Fi and EPON fiber optical have been applied in transmission line online monitoring system so far.However,they have shortcomings such as high operating cost,poor coverage in remote area and difficult installation.To conquer problems of bandwidth bottleneck and high latency in chain-type wireless communication network,a hybrid hierarchical communication network architecture is presented based on dedicated WSN and OPGW communications.An optimal model and multi-object-based decision for hybrid hierarchical communication network planning are presented to get the fiber-optic separation placements.This model is formulated with object of cost and end-to-end latency optimization,while satisfying constraints of graph-based path connectivity and bandwidth.To meet the time demand of fault risk assessment,fast calculation technologies including fault scenarios probability calculation and scenario screening have been studied.Conventional power equipment's outage models don't consider the same consequences of faults because of different disconnector-failure modes,which leads to repetitive and inefficient computation of fault scenarios.To tackle this problem,a fault scenario probability calculation method is proposed considering disconnector-failure modes.Besides,a fault scenario screening method is proposed to reduce the complexity of multiple combinations,which lays the foundation of online fault risk assessment.In the respect of fault restoration,the key technologies include fast restoration of communication outage and distribution faults.The fast restoration of communication guarantees the services of protection and stability control.Fault location in distribution network is the foundation of distribution power outage restoration.Once the fault is found,network reconfiguration approach can be determined and maintenance of fault lines can be quickly executed.Natural disasters may have significant impacts on overhead transmission lines and cause communication outage of pilot protection.With the development of smart grids,wireless sensor network has been a potential approach to realizing reconstructed communication channel without further installations.Fault restoration of pilot differential protection communication based on wireless sensor network is proposed in this dissertation.For a reliable design,system arrangement and communication structure are presented.A theoretical planning of sensor nodes is formulated,which enjoys the advantages such as high reliability,cost optimization,and capacity of satisfying the connectivity of the communication network.To meet the need of time delay,a novel routing protocol for WSN is proposed with three stages including route establishment,route discovery and route maintenance,which assures the directional propagation of data packets.Practical experiments and simulation results indicate that the proposed RCC scheme can satisfy time delay of protection relaying in emergency communication channel,as well as guarantee the connectivity of network when some WSN nodes are damaged.Fault location is the foundation of fault restoration in urban distribution network.Key technologies including travelling wave propagation characteristic,signal detection,measurement position and fault location algorithm are discussed to locate single-line-to-ground fault in urban low-voltage distribution network.Since the high frequency components of the ground mode travelling wave decay seriously with the fault distance and frequency,the singularity detection of ground mode travelling wave is not accurate.Therefore a selection method of wavelet basis for singularity detection is proposed.Based on the data gathered from all measuring devices,a fault section algorithm is proposed.Besides,least square estimation is used to identify erroneous measurements and get error-tolerance effect.At the end of this dissertation,research work and results are summarized and the directions of further research are presented.