  Motivated by the need for accurate frequency information, a novel algorithm
for estimating the fundamental frequency and its rate of change in three-phase
power systems is developed. This is achieved through two stages of Kalman
filtering. In the first stage a quaternion extended Kalman filter, which
provides a unified framework for joint modeling of voltage measurements from
all the phases, is used to estimate the instantaneous phase increment of the
three-phase voltages. The phase increment estimates are then used as
observations of the extended Kalman filter in the second stage that accounts
for the dynamic behavior of the system frequency and simultaneously estimates
the fundamental frequency and its rate of change. The framework is then
extended to account for the presence of harmonics. Finally, the concept is
validated through simulation on both synthetic and real-world data.
