  Real-time monitoring of distribution systems has be-come necessary, due to
the deregulation of electricity markets and the wide deployment of distributed
energy resources. To monitor voltage and current at sub-cycle detail, requires,
typically, major investment undertaking and disruptions to the operation of the
grid. In this work, measurements of the low voltage (LV) side of distribution
transformers (T/F) are used to calculate in real time the waveforms of their
medium voltage (MV) sides, based on a mathematical model of said T/F. This
model is, essentially, the dig-ital twin of the MV side of the T/F. The method
calculates T/F MV waveforms of voltage and current, and active and reactive
power as accurately as an instrument T/F, captures all harmonics con-tent, is
unaffected by asymmetrical loading and identifies most sys-tem faults on the MV
side of the T/F. The digital twin method en-ables monitoring of distribution
T/F that avoids MV instrumenta-tion, does not suffer in accuracy and may be
readily deployable. Field data from an actual MV-LV T/F, agree with simulation
re-sults showcasing the efficacy of the digital twin method.
