Research On Typical Strategies Of Fault Current Reduction In Large Power Systems

As the scale of the power system increases and tight interconnections,the fault current level grows rapidly,even exceeding the maximum breaking capacity of the circuit breaker.If the high-level fault current cannot be broken off in time,serious damage will be generated to the power system and large-scale black out may be trig-gered.For now,strategies of limiting the overrating fault current can be classified into two categories,the“blocking”theory and the“dredging”theory,which can be repre-sented by fault current limiters（FCLs）and fault current splitters（FCSs）,respectively.In this paper,optimal placement methods for both techniques are proposed and com-parative analyses are carried out to demonstrate their characteristics.Detailed research works are as follows:The problem of FCL placement is a mixed integer non-linear problem（MINLP）,whose complexity increases with the scale of the power system.To improve the effi-ciency of solving,a straightforward method for placing and sizing the FCL is proposed in this paper without using heuristic technologies or integrated solvers.In the proposed method,the FCL is divided into many impedance units which are added into the net-work one by one.The global sensitivity index（GSI）is proposed to directly determine the location in each iteration considering the global fault current reduction of FCL.The dynamic impedance z _GSI that is used to calculate the GSI is adopted to avoid the pre-determination of locations.Three case studies demonstrate the efficiency and priority of the proposed method.The optimal placement of FCS is also a difficult MINLP but different from the FCL that one must consider the voltage sag caused by the activation of FCS to ensure the security of the power system.Thus,an optimization model for minimum negative effects of voltage sag to loads considering maximum fault current limitation is pro-posed to find an effective solution with acceptable voltage sag.The Computer Business Equipment Manufacturer Association（CBEMA）curve is adopted to evaluate the effect of voltage sag and its duration to the load.The repeat-searching particle swarm algo-rithm（RSPSO）with three developments of strengthened global searching ability,searching space reduction and variable dimension reduction is proposed to solve the problem.Two case studies are carried out to show the effectiveness of the model.Comparative study based on the IEEE 30-bus power system is presented to show the application scope of FCLs and FCSs.The corresponding solutions of FCLs and FCSs are found through the methods mentioned above.Then,comparisons on fault current reduction,during-fault voltage sag,power loss and cost are carried out.The result shows that:when the FCS is applied to limit the fault current,the average fault current reduction on every bus is larger than using the FCL.The cost of FCS is lower than the FCL but the devices are more mature than the FCL.However,the FCS will increase the fault current from generators and cause larger voltage sag than the FCL.In conclusion,the FCS is suitable in the developing and large-scale power systems with good layering and zoning,while the FCL is preferred in developed load center with complicated structure of power system.