  Developing effective strategies to rapidly support grid frequency while
minimizing loss in case of severe contingencies is an important requirement in
power systems. While distributed responsive load demands are commonly adopted
for frequency regulation, it is difficult to achieve both rapid response and
global accuracy in a practical and cost-effective manner. In this paper, the
cyber-physical design of an Internet-of-Things (IoT) enabled system, called
Grid Sense, is presented. Grid Sense utilizes a large number of distributed
appliances for frequency emergency support. It features a local power loss
$\Delta P$ estimation approach for frequency emergency control based on
coordinated edge intelligence. The specifically designed smart outlets of Grid
Sense detect the frequency disturbance event locally using the parameters sent
from the control center to estimate active power loss in the system and to make
rapid and accurate switching decisions soon after a severe contingency. Based
on a modified IEEE 24-bus system, numerical simulations and hardware
experiments are conducted to demonstrate the frequency support performance of
Grid Sense in the aspects of accuracy and speed. It is shown that Grid Sense
equipped with its local $\Delta P$-estimation frequency control approach can
accurately and rapidly prevent the drop of frequency after a major power loss.
