Improved Fast Fault Detection Algorithm Based On Instantaneous Current And Its Variation Ratio

In recent years,with the continuous growth of energy demand and electricity load in our country,the short-circuit capacity of distribution network increases correspondingly,and the short-circuit current level is seriously exceeding the standard due to the concept of "large capacity,few distribution points" adopted in the newly-built and expanded substations.The emergence of fast switching based on fast eddy current drive mechanism provides a simple and practical solution to a series of problems caused by short-circuit current level exceeding the standard in distribution network.In a series of corresponding solutions,in addition to the fast action speed of the fast switch itself,the fast fault identification needs to be realized in milliseconds.In order to increase the fault detection speed to meet the requirement of fast circuit breaker,the basic methodology of fast fault detection algorithm based on instantaneous current and its variation ratio is investigated,the origin and selection of setting parameters are explained..The degradation of its time performance is pointed out,in which,due to the reduction of the initial absolute value of the aperiodic component of short-circuit current and the inductivity of the upstream system.Towards the above problems,an improved method of adding and multiplying adj ustment coefficients is proposed.A fault phase angle scanning module is designed by use of PSCAD/EMTDC simulation software.In order to satisfy that the fault identification time is not greater than the upper limit of the expected identification time under various fault phase angles and that the criterion will not be mistaken when considering the harmonic effect,the further adjustment of the adjustment coefficient in the improved criterion is discussed.Using numerical simulation and theoretical derivation,it is verified that large load input will not cause misjudgment.For the unavoidable inrush current,the degree of its influence on the performance of the improved criterion is given,and two solutions are proposed to increase the delay return and to place the current collector on the high voltage side of the main transformer.On the basis of the above adaptability analysis,in three typical application scenarios of fast fault current identification algorithm and fast switch,the proposed improved method is verified by simulation,the results of which shows its feasibility and effectiveness.