Studies On Total Transfer Capability Calculation Methods And Post-fault Emergency Control Strategies In Interconnected Power Systems

With the deregulation of electricity market in China,interprovincial and interregional transactions have become more common than before,power exchanges between interconnected power grids are more frequent than they used to be,numerous customers purchase electricity from relatively remote power plants with lower cost.Power systems operate close to their operating limits due to the growing level of the power transfer in power grids.Consequently,the modern power system focuses on how to calculate the total transfer capability of interconnected power grids and how to avoid the interruption of power exchanges.Meanwhile,studies of historic blackouts show that some occasional faults may cause an unpredictable cascading failure and will lead to a wide-range of outages.As a result,the modern power system also focuses on how to ensure the safety and the stability of the power system and how to establish post-faults emergency control strategies.For the reasons stated above,the purpose of this dissertation is to research total transfer capability calculation methods and post-fault emergency control strategies for the interconnected power system.Complete and profound researches have been made on establishing a real-time database,total transfer capability calculation method and post-fault emergency control strategies.The works and contributions of this dissertation are summarized as follows:(1)A self-adaptive cubature quadrature Kalman filter(CQKF)for the power system dynamic state estimation is proposed.This method can automatically select an appropriate filter for the state estimation,depending on the current state of the power system.Firstly,for this method is based on the CQKF,it is more accurate than other filters when the power system is in the steady state.Secondly,as the result of the strong tracking filter is combined with the CQKF.This method can rapidly track the system state after sudden changes.Lastly,a process noise estimator is applied to the CQKF.Thus,the filter can avoid divergences which are caused by the inaccurate process noise.(2)An improved Ward external network equivalent based total transfer capability computation method for cross-regional interconnected power systems is proposed.In this study,only boundary bus voltages and predictors need to be exchanged between individual areas during the calculation.And there are four contributions for the proposed method.First of all,it can automatically adjust boundary power injections based on the incremental loads at buyer bus because the proposed external network equivalent is suitable for TTC computation.Secondly,it avoids changing the characteristic of original PV bus.And each area can run its own power flow calculation.That is the result of a fictitious slack bus established in this method.Thirdly,to prevent this method from the lower accuracy or longer computation time which is caused by the fixed pre-dictor step size,the self-adaptive step size is used in repeated power flow calculation.Lastly,distributed power flow calculation is used to correct power flows,it prevents a larger mismatch happening in each area's power flow calculations.(3)A total transfer capability simulation analysis method considering transient stability constraints is proposed.There are five contributions for the proposed method.First of all,this analysis method is based on the time-domain simulation so that it is more accurate than other method.Secondly,various kinds of contingencies are taken into account by this method.It will not miss the fault which leads to the transient instability.Thirdly,the transient stability margin can be quickly deduced via the repeated power flow calculation method with the self-adaptive step size.(4)A sensitivity analysis based emergency control strategy for avoiding cascading overload on transmission lines is proposed.This method can select an appropriate strategy to quickly alleviate the active power in critical lines,depending on the degree of the overload.Meanwhile,the power transfer in other transmission lines can be kept in the normal range.There are three contributions for the proposed method.First of all,the performance of the governor is taken into account.It makes the control strategy easy to be implemented in practice.It also avoids to obtain an infeasible strategy from the proposed method.Secondly,the generator tripping is considered in this method.It makes the proposed method more flexible than others.Generator outputs and active power loads can be quickly kept in balance via the control strategy.Lastly,this control strategy successively activates generation rescheduling,load shedding and generator tripping to alleviate the active power in critical lines,depending on the degree of the overload.It ensures the safety of the critical line and minimizes the effect of the control strategy to the power system.(5)A graph theory based three-stage method for intentional controlled islanding of power systems is proposed.It enables to search out reasonable islanding cutsets,which have the minimum load-generation imbalance or the minimal power flow disruption,without low-voltage problems.In the first stage,a self-adaptive graph simplification algorithm is proposed to obtain a two-terminal graph as a suitable islanding cutset search area from the original power network graph model.In the second stage,an islanding cutset search algorithm is designed to find all of islanding cutsets,including the minimum load-generation imbalance cutset and the minimal power flow disruption cutset,in the two-terminal graph.In the third stage,an islanding scheme checking algorithm is developed to examine the outputs of stage two.It uses the depth first search algorithm to determine reasonable islanding cutsets without low-voltage problems.