Research On Identifications Of Important Node,Vulnerable Transmission Line And Backbone Grid

Several blackouts of power systems in recent years have caused enormous economic loss and threatened social security.They have aroused much attention to operation security of power system.The blackouts are mainly caused by internal and external factors.The internal factors include faults of control system,protection system or communication system,overload and so on.And the external factors mainly include natural disasters.The above mentioned factors may result in the outages of some vulnerable transmission lines or important nodes and then trigger cascading failures.The probability of power system blackout can be decreased by enhancing the protection of the important nodes and vulnerable transmission lines.And the consequence of power system blackout can also be mitigated by setting up the backbone grid before natural disasters happen.The backbone grid ensures power supply of the important loads during natural disaster and accelerates the reconstruction of power grid after natural disaster.The backbone grid is constructed by elevating the fortification level of some important nodes and vulnerable transmission lines for disaster resistant.This paper focuses on the study of node importance evaluation,vulnerable transmission line identification and backbone grid construction,which have the instructional significance to operation security of power grid.Considering both power grid and Internet are complex network,this paper summarizes the corresponding relations between power grid and Internet and proposes a new method to evaluate node importance of power grid based on Hypertext-Induced Topic Search(HITS)algorithm.HITS algorithm is a famous algorithm applied in web page ranking.Considering load,power source,and line flow,HITS algorithm is modified by adding nodes and introducing weights when evaluating node importance.This method defines authority score and hub score to evaluate node importance from the perspectives of inflow and outflow power.The sum of the two scores is adopted to evaluate node importance of power grid comprehensively for the differences between Internet and power grid,which makes the evaluation more reasonable.Based on the current studies on power system blackouts,a new method to identify vulnerable lines is proposed based on relative probability and consequence of line outage in this paper.Firstly,comprehensive load rate of unit entropy is defined to represent power grid vulnerability.This index also represents load rate and its distribution of transmission lines in power grid.The change of this index after a line outage is modified by node importance,which measures the consequence of line outage.Secondly,relative probability of line outage is calculated considering line failure rate and relative probability of line flow off-limit caused by random fluctuation of loads.The latter is calculated by DC power flow model considering topology,running state,distribution of power source and load,line flow limit.A new method to construct backbone grid is also proposed in this paper.This method can ensure the important nodes reserved in the backbone grid more directly and make the size of the backbone grid minimum at the same time.The minimum backbone grid includes as little and vulnerable transmission lines as possible to pursue economy.When using this method to construct backbone grid,the important nodes which must be reserved in the backbone grid are determined according to node importance evaluation firstly.And then the capacity match of load and power source are examined.Some reasonable nodes will be added into the backbone grid if the capacities of load and power source do not match.Secondly,the shortest paths and the corresponding transmission line sequences among the important nodes are calculated by the shortest path algorithm after setting the vulnerability value of transmission lines as their weights.And the minimum spanning tree of the graph constructed by the shortest paths among the reserved nodes is calculated by minimum spanning tree algorithm.The minimum spanning tree is regarded as the initial backbone grid after replacing the shortest path by the corresponding transmission line sequences.Finally,operation security constraints of the initial backbone grid are examined.The final backbone grid is determined after eliminating the overloaded transmission lines by adjusting running mode or adding reasonable transmission lines.