Modelling non-standard over current relay characteristic curves using combined lagrange polynomial interpolation and curve fitting

The characteristic curve of over current relay (OCR) has a very important role in the electrical protection devices. Standard characteristic curve such as Normal Inverse (NI), Extremely Inverse (EI), Very Inverse (VI) and Longtime Inverse (LTI) are often changed at the time of implementation, carried out for the purposes of security protection system due to other devices in the electrical network. The curve changes cause changes of the standard model to be non-standard, in this research proposed combined Lagrange interpolation polynomial and curve fitting for modeling the characteristic curve of the non-standard. It can be shown encouraging results with an average error ranging 0.197 %. It can be used as a reference for modeling the non-standard characteristic curve, especially in the digital protection relay device.