Study On Wide Area Backup Protection Control Strategy Against Cascading Outages

With the construction of new pattern about "one EHV and three LARGE" for power system, the scale of power networks in China becomes larger and larger. The large-scale transmission networks improve the economy of power system; but at the same time the probability of great cascading failure is increased too. In recent years, many widely spread outages have been happened in many countries which reveal the defects in protection and stability and emergency control. By analyzing these outages, it could be known that the unwanted operation of backup protection which based on local information worsened the situation in the expanding of outages and accelerate the collapse of power system no matter what the reason and course of the outages. Therefore, the wide area information is introduced into the backup protection system in this dissertation in order not to protect the transmission line but also to realize the cooperation between the backup protection of other lines and the safety automatic device to prevent the power network from cascading failures. Major work can be summarized as follows.The definition and searching method of RCTL (relative cluster of transmission line) which can reflect the transferring direction of power flow is presented based on the fact that when the faulted line is cut, the transferring of original power flow will seriously affect some transmission lines. The cut of the branch line will lead to the redistribution of the flow of total power network. However, only several power flow of transmission lines are affected severely. In a certain operation mode, each transmission line possesses its fixed and close related transmission lines. So in the dissertation the mathematic description and definition of RCTL is processed, and two searching methods-decision tree theory based and shortest paths based-are also presented. For wide area backup protection, the collection of wide area information is limited. The definition of RCTL not only decides the range in choosing wide area information, but also establishes the base of realizing the load-shedding controlling strategy of backup protection.Based on the RCTL, line relative factor (RF) is defined and two calculating methods-parallel route method and matrix method-are also presented. Quantitative analysis of power flow transferring caused by tripped branch circuit can be processed by using line relative factor. By calculating the line relative factor, the power flow transferring distribution in each branch circuit within RCTL can be accurately forecast, which can provide the reliable data for load-shedding controlling strategy of backup protection.The searching method and calculating method of RCTL and line relative factor when multi-lines are cut off are presented to deal with the power flow transferring problem caused by cascading cut of multi-lines. The calculation formula of RCTL and line relative factor of cascading cut of multi-lines is obtained by mathematical derivations, which extends the RCTL from single line cutoff to multi-lines cutoff and establishes the complete theory system of RCTL.The concept of system frangibility mode of hidden failure is developed by detailed analysis of the hidden failure of backup protection-the main reason of wide spread blackouts in recent years. An evaluating method of hidden failure happening probability of backup protection is also presented based on the RCTL. To eliminate the hidden failure of backup protection caused by the protecting setting, the identification method and criterion system frangibility mode during the normal operation are presented based on the RCTL theory and the mechanism of hidden fault of backup protection.Against the setting principle limitation and defect of traditional backup protection and using the electrical information provided by data acquisition system and modern communication, the widely used distance stage III of backup protection is improved and the method of identifying short circuit and over-load status is also presented. Based on the above achievements, self-adaptability setting principle which can adjust itself according to the power operational mode and over-load protection function are enhanced in distance stage III protection. The setting method of varying time limit is presented on the basis of short circuit fault identification of this line and the neighboring line and the over-load status identification, which can lessen the occurring probability of setting hidden fault effectively.The real time load-shedding controlling strategy of backup protection is presented based on the wide area information, which can realize the corporation between the backup protections of each line and the corporation between backup protection and safety automation devices, which can effectively prevent the power network from cascading failures. The key problems of overall configuration, function allocation and process design of load-shedding controlling system of backup protection are studied. A new real time load-shedding algorithm is put forward and the realizing scheme of wide area backup protection is made up according to the change of load and power flow, which can prevent the power grid from cascading trip effectively.