Fault current magnitude in a microgrid depends upon its mode of operation

namely

grid-connected mode or islanded mode. Depending on the type of fault in a given mode

separate protection schemes are generally employed. With the change in microgrid operating mode

the protection scheme needs to be modified which is uneconomical and time inefficient. In this paper

a novel optimal protection coordination scheme is proposed

one which enables a common optimal relay setting which is valid in both operating modes of the microgrid. In this context

a common optimal protection scheme is introduced for dual setting directional overcurrent relays (DOCRs) using a combination of various standard relay characteristics. Along with the two variables

i.e.

time multiplier setting (TMS) and plug setting (PS) for conventional directional overcurrent relay

dual setting DOCRs are augmented with a third variable of relay characteristics identifier (RCI)

which is responsible for selecting optimal relay characteristics from the standard relay characteristics according to the IEC-60255 standard. The relay coordination problem is formulated as a mixed-integer nonlinear programming (MINLP) problem

and the settings of relays are optimally determined using the genetic algorithm (GA) and the grey wolf optimization (GWO) algorithm. To validate the superiority of the proposed protection scheme

the distribution parts of the IEEE-14 and IEEE-30 bus benchmark systems are considered.