The relay protection device mounted in the grid line can improve significantly index value of certain voltage transient drop.The voltage transient drop assessment considering power transmission and distribution protection action of the power transmission and distribution grid can improve both accuracy and credibility of the existing assessment method. In this paper

the fault rate calculation formula of sensitive load considering protection action is derived based on feature of relay protection layout proposal of power system

analysis of the influence of step distance protection and inverse time current protection action on voltage transient amplitude and time of duration. The IEEE 30 node system is taken as study target

the voltage transient drop amplitude calculated by Matlab program

result of duration of time and experimental result of PSCAD simulation platform are compared and the sensitive load of 66 node isolated grid system at certain area is assessed and

finally

the result of fault rate with and without relay protection device action is compared.