  This paper analyzes the delayed voltage recovery phenomenon by simplifying
the Western Electricity Coordinating Council (WECC) composite load model using
the load admittance and the thermal relay dynamics. From this analysis, a
closed form expression approximating the recovery time is derived and the key
load parameters impacting the behavior of voltage recovery are identified. A
monitoring scheme based on the measured load admittance is then proposed to
detect the onset of stalling, even in the presence of voltage oscillations, and
estimate the duration of the delayed voltage recovery. A mitigation scheme
utilizing smart thermostats and offline learning is also derived to ensure that
the voltage recovers to the pre-contingency voltage within a specified time.
Both the monitoring and mitigation schemes only need local measurements at a
substation making them promising for online applications. Results for the
monitoring and mitigation schemes are described in detail for the IEEE 162 bus
system validating the various assumptions used for the analysis and
establishing the connection between the delayed voltage recovery phenomenon and
load admittance.
