Efficient Reliability Assessment And Weak Part Identification Of Transmission System With Renewable Energy Considering Cascading Failures

In order to adapt to quickly growing power demand and rapid development of renewable energy,a highly reliable transmission system is extremely important in the new generation of power systems.However,the transmission system usually spans vast geographical areas and is exposed to the wild environment.Due to the factors such as the aging of transmission equipment and the occurrence of extreme weather in a wide range,the frequent outages of transmission equipment have become one of the main challenges for power system reliability.Meanwhile,the blackouts in Australia and the United Kingdom showed that the high penetration of renewable generation changes the regulating ability and disturbance tolerance of the grid.To ensure the reliability of electric power,it is of great economic and social significance to investigate the reliability with the consideration of cascading failure of transmission system with high penetration of renewable.With the supports of National Key Research and Development Program of China(2017YFB0902200)and National Science Fund for Distinguished Young Scholars(51725701),this thesis focuses on the reliability assessment of power system.Four issues-the dynamic model and the probabilistic assessment model of cascading failures,reliability assessment and weak part identification of transmission system with renewable energy-are profoundly explored.To assess the blackout risk of power system with high penetration of renewable,the existing cascading failure models need to be improved for capturing the dynamics and relays of renewable generation.In the thesis,a dynamic model of cascading failure considering the utility-scale and distributed renewable energy is proposed.With the solution of dynamic equations for power system,the logics of relaying are simulated for system components such as transmission lines,conventional generators and renewable generations.The failure interactions among sources,networks,and loads are analyzed more comprehensively.In the proposed model,to capture the impact of renewable energy on the system dynamics,the dynamic equations for the utility-scale renewables are constructed with the second generic generation model of WECC(Western Electricity Coordinating Council),and the interactions among distributed renewables and the transmission system are captured through the net load level at buses.And to capture the tolerance of renewables for disturbances,the simulation logic is constructed for the voltage relays and frequency relays of utility-scale renewables and the anti-islanding relay of distributed renewables.The proposed model is applied to the IEEE 39-bus system.The results show that the coupling among sources,networks,and loads becomes tighter with the increase of penetration level of renewable energy.As the available historical data of cascading failures are limited,and the physical models of cascading failures require a high computational complexity,it is challenging to assess the blackout risk with low probability and high impact.To address this problem,a probabilistic assessment model of cascading failure is proposed which only requires a small sample size.Firstly,a coupled interaction matrix is defined to describe the correlation between outage propagation and load loss in cascading failure.Then,the coupled interaction matrix is effectively inferred from a small number of cascading failure samples using the expectation–maximization(EM)algorithm.Finally,a probabilistic assessment model of cascading failure is established with the coupled interaction matrix,in which the propagation paths and the load loss are simulated with the interactions of outage events.It greatly reduces the computational complexity in the analysis of blackout risk.The proposed model is tested on several IEEE standard systems.The results indicate that by using a small number of samples from the steady or dynamic cascading failure models,the proposed model can generate cascading failures with the probability distribution consistent with that from physical models.The simulation complexity of cascading failures is greatly reduced by 90%-97%.To comprehensively assess the reliability of transmission system from the perspectives of independent outages and cascading failures,an efficient screening algorithm for independent outages in transmission system is proposed using an idea of dynamic space pruning.The adequacy index and disastrous index of transmission system can be efficiently evaluated by combining the proposed method and the probabilistic assessment model of cascading failures.Firstly,based on the impact of component outages on adequacy,the root event of independent outages is defined.Then,a dynamic pruning method of N-k independent outage event space is presented,in which the event space of high-order independent outages is divided into subspaces of root event and a complementary subspace.The pruning is to optimize the search direction in contingency screening.Finally,the screening models for the subspaces are established using a mixed integer linear programming method,which can accurately and efficiently screen the root events with high risk.The method is verified on the IEEE RTS-79 system and IEEE 39-bus system.The results show that the proposed method substitutes 76%-82% of high-risk events with root event subspaces.And compared with the contingency screening for high-risk events,the proposed method reduces the screening time by 87% to 96%.The results enable the efficient and comprehensive assessment of the reliability of transmission system.The criticality of transmission components can be measured more comprehensively by the risk allocation for independent outages and cascading failures in transmission system.In this context,a comprehensive risk contribution index of transmission components is proposed with the fuzzy entropy weight method,which can effectively identify weak parts of transmission system for reliability enhancement.Firstly,by screening the root events of independent outages,a reliability tracing method considering load shedding contribution is constructed to quantify the impact of failed lines on the system adequacy.Then,according to the propagation properties of cascading failures,a probabilistic interaction subgraph is constructed,and the contribution of transmission lines to the risk of line outages and load shedding in cascading failure is analyzed.Finally,a fuzzy entropy weight method is used to calculate the risk weights of independent outages and cascading failures,and the comprehensive risk contribution indices of transmission line are established,which can be used to identify the weak parts of transmission system.The proposed method is applied to the IEEE 39-bus system.The results indicate that with the identification of weak parts using the proposed indices and the reduction in the failure rates of weak parts,both adequacy and security of transmission system can be improved.