  Installation of phasor measurement units (PMUs) in a number of substations in
the power grid can help assess a set of its values and parameters, in
particular those related to the dynamics when disturbances occur in the system.
Inertia constant of the power grid is one of the system stability related
parameters that is essential for planning of the system spinning reserve.
Estimates of the grid inertia constant require precise information of the
system frequency at the time of disturbance. In this paper, an improved method
for such estimate is presented, which is based on the derived data of frequency
variations obtained from PMUs. In addition, a way to obtain the appropriate
interval for the lowest error estimate is discussed. The method has been
applied to assess the inertia constant of the Iranian power grid, by obtaining
the rate of change of frequency after a disturbance in the defined interval,
and the rate of specified power imbalance between electrical power input and
output.
