Research On Transmission Section Security Protection And Preventing Cascading Overload Control Strategy

When power system overloads in heavy load operation, the power flow transferring caused by voerload removal may cause other lines in system cascading overload. Traditional overload protection mostly takes the mode of delay tripping to remove overload with no consideration of the influence of voerload removal to system security, which may cause cascading overload trip under certain operating conditions because of a series of overload protection devices actting successively. Therefore, in the emergency time of overload, in order to avoid cascading overload trip caused by overload removal, it is needed to fast estimate the effect of voerload removal to system security. When taking control measures to eliminate overload, in order to avoid new lines overload, it is required to fast find out the normal lines prone to overload when adjustment and to study the control strategies which can ensure these normal lines won’t overload when adjustment.The main work and achievements are as follows:With the reasons of some typical blackouts are analyzed, it is pointed out that the power flow transferring caused by voerload removal and the unreasonable action of protection devices are the important reasons to cause the accidents worse.By analyzing the features of flow transferring caused by overload removal, the concept of flow transferring factor (FTF) is proposed and the new definition of transmission section is given. On this basis, a new algorithm for fast searching transmission section is proposed. Based on dynamic programming theory, by converting the real-time power network into a topological graph, it fast finds out the K shortest paths between the two nodes of the overload line in a subgraph with overload line as its center, then finds out the transmission section suffered greatly from power flow transferring. The security analysis of oveload removal to the whole system is thus reduced to the analysis of a transmission section. In such a condition, the workload for further analysis is decreased.An algorithm for fast searching transmission section based on deviation path is given. It firstly searches the first shortest path between the two nodes of overload line by the algorithm of Shortest Path Search based on graph theory, then takes the intermediate nodes as deviated nodes to derive deviation paths and finds out the shortest path in these deviation paths as the second shortest path between the two nodes of overload line, by the same way finds out the K shortest paths in a region which the electrical distance is shorter to the overload line. The use of multi-path searching mode helps avoiding the case of miss-searching some lines due to too small searching region through the mode of single-path searching. The related transmission section of overlaod line can also be completely gained by searching only a small number of paths. Therefore, this algorithm is fast and effective.The concept of generalized transmission section (GTS) is proposed and a method to determine GTS is given. Besides, a strategy to estimate the security of GTS is studied. The power grid is firstly divided into several regions based on the improved Dijkstra algorithm to form the generalized grid topology. The generalized nodes in the nearby region of the generalized source node are searched by using the algorithm of improved Breadth First Search (BFS). The GTS is composed with the lines in the generalized nodes searched and the lines with smaller power flow redundancy, so the scope of transmission section is extended. When estimating the influence of voerload removal to transmission section security, the mode of distributed parallel computing is used. Taking each generalized node in GTS as a computing unit, when overload occurs, these units parallel judge if the lines in its own unit will overload afeter the overload line removal. In such a condition, the time of online analysis is saved, which is adapted to the rapidity requirement of real-time estimation.The concept of minimum limiting factor (MLF) is given and a strategy to estimate the security of transmission section based on MLF is studied. It is analyzed that when overload occurs, the effect of overload removal to system security can be estimated by judging whether the overload removal can cause the line with MLF cascading overload. In order to avoid searching that line through whole network computing, two related theorems are introduced, which determines the transmission section by reasonably setting distribution factor limit and current redundancy limit. When overload occurs, the analysis of whether the line with MLF will cascading overload after the overload line removal can be replaced by that of whether the lines in the transmission section will, so the calculation amount is reduced. When estimating the effect of overload removal to transmission section security, in order to meet the rapidity requirement of real-time estimation, the generalized transmission section subset and the generalized sub-region of each generalized node in the generalized grid topology are respectively determined during normal operation of power system. Each computing unit calculates the minimum current redundancy of the lines in its own unit and the minimum current redundancy of the lines with distribution factors greater than the distribution factor limit. When overload occurs, these related computing units only need to parallel judge whether the lines in its own unit will overload after the overload line removal. The online computing time is saved and the efficiency of transmission section security estimation is improved.When taking emergency control measures to eliminate overload, in order to avoid new lines overload, the constraint of normal line’s power flow to the control quantity are considered, the concept of security constraint set (SCS) is given and a control strategy to prevent cascading overload based on SCS is studied. The generalized nodes in the nearby region of generalized control node in the generalized grid topology are fast searched firstly, then the SCS is composed with the lines in the generalized nodes searched and the lines close to thermal limit, so the scope of normal line’s constraint set is extended. New methods to select the control nodes and to determine the control quantity combined with SCS are given. A control strategy based on the mode of decision center and regional terminal to prevent cascading overload is given. Taking each generalized node as a regional terminal, the related regional terminals parallel calculate the maximum security control quantity when overload occurs, so the computing time is saved and the control speed is improved.