  The demand for faster protection algorithms is growing due to the
increasingly faster dynamics in the system. The majority of existing algorithms
require empirically selected set-points, which may reduce sensitivity to
internal faults and cause security problems. This paper addresses these
challenges by proposing a settingless time-domain unit protection algorithm for
medium-voltage lines. The main idea of the algorithm is to identify which model
of a protected line, i.e. healthy or with an internal fault, is more consistent
with the input measurements. This is done by solving a number of small-scale
convex optimization problems, which at the same time determine the
characteristics of an internal fault that best fit the measurements. Thus, the
proposed algorithm merges protection, fault location and fault type
identification functionalities. The algorithm's performance is extensively
tested on a grid model in MATLAB Simulink for different types of generation and
grid operating conditions. The results demonstrate that the algorithm can
operate quickly and reliably, and accurately estimate fault characteristics
even in the presence of noisy measurements and uncertain line parameters.
