Study On The Transient Stability Of The Cascading Failures Considering Fault Intervals

The spatial reverse distribution of power generation and consumption in China is a great problem.With the operation of HVAC and HVDC projects,the scale of synchronized grids expand dramatically and strenthen the electrical connection of different area.While improving the transmission capacity of power grid and energy distribution in time and space,it also poses new challenges to power security and stability control.A number of large-scale blackouts in the world have shown that cascading failures are the main cause of blackout.In recent years,with the wide application of recording equipment,transient stability of cascading failures has attracted the attention of grid operators.It is of great significance to guarantee the transient stability of cascading failures for the safe and stable operation of power system.The traditional cascading failure analysis focuses on the static security analysis,especially the power flow calculation and fault probability analysis after the first fault.Transient stability analysis is only considered as a constraint of fault chain search.Due to the constraints of models,integration techniques and stability analysis tools,the most common treatment method in the past is to ignore the transient process between faults and conduct analysis according to simultaneous faults or fault-by-fault.However,with the increase of permeability and the improvement of intelligence level,power electronic devices and complex controllers lead to the reduction of time interval scale of cascading failures.Traditional methods may not be able to adapt to cascading failures on short time scales.The randomness of the fault interval further increases the difficulty of this problem.Aiming at these problems,this paper focuses on analyzing the transient stability of cascading failures considering fault intervals.We completed following works:A method for calculating transient stability margin of cascading failures based on EEAC is presented.In order to satisfy the requirements of quantitative,rapidity and robustness of transient stability analysis,an analytic expression for calculating transient stability margin of cascading failure is derived in this article based on the EEAC method.This method decouples cascading failures by swings and avoid the misjudgement of stability via the identification of illed-FEP and illed DSP.By comparing the computational speed and accuracy of SEEAC,DEEAC and IEEAC,a coordination research framework is proposed in which we analyze the mechanism by SEEAC and study the influence of various time-varying factors by IEEAC.The variation mechanism of the cascading failure controlling mode with the time interval is analyzed.Based on the basic principle of EEAC controlling mode identification,the pathway of cascading failure affecting controlling mode is explained.Firstly,the relationship between the controlling modes of single fault and cascading failures are investigated with the data-driven research paradigm.Then,based on the stability margin sensitivity of different OMIB images,we exlplain the mechanism of the influence of cascading failures time interval on the controlling mode.Finally,We discuss the relationship between the change of the controlling modes and the optimal emergency control quantity and it is found that when the leading cluster of the controlling mode expands,an appropriate amount of emergency control measures should be added to the cascading failures.The variation mechanism of cascading failure stability margin with the time interval is further studied.The concept of energy injection direction is firstly proposed in this paper and the influence mechanism of the time interval on the stability margin is deeply analyzed based on it.This article firstly excavates the rule of the stability margin variation with the time interval in the Hamiltonian system and proves the periodicity of the stability margin changing with the time interval by avoids the interference of the controlling mode change to the mechanism analysis.Then,the maximum and minimum characteristics of the stability margin are investigate,which shows that if the second fault is a short circuit fault,the DCP time of the first fault is the most(unstable)stable time interval,and if the second fault is an open fault,the beginning time of each swing is the most(unstable)stable time interval.Finally,the complexity of the stability margin varying with the time interval is discussed by taking into account the influence of time varying factors such as damping,controller and mode change.Finally,a method to evaluate the transient stability of cascading failures considering the time interval is proposed.The traditional transient stability evaluation of cascading failures with time interval can only be completed by exhaustive simulation.This article proposes a conservative approach to evaluate cascading failures with the most unstable cascading time interval.In order to solve the deadlock problem of the time interval and the controlling mode in the evaluation,based on the second-order sensitivity index of the stability margin relative to the time interval provided by EEAC,a method for evaluating the transient stability of cascading failures with time interval is proposed.The calculation cost of this method is only about 1% of that of the exhaustive method,which makes the transient stability evaluation of with time interval have practical value in engineering.By comparing this method with those based on simultaneous hypothesis and quasi-stable-state hypothesis,it is pointed out that the traditional methods may get overly-optimistic evaluation results.