During the past two decades, the increase in electrical energy demand has presented
higher requirements from the power industry. More power plants, substations, and
transmission lines need to be constructed. However, the most commonly used devices in
present power grid are the mechanically-controlled circuit breakers. The long switching
periods and discrete operation make them difficult to handle the frequently changed loads
smoothly and damp out the transient oscillations quickly. In order to compensate these
drawbacks, large operational margins and redundancies are maintained to protect the
system from dynamic variation and recover from faults. This not only increases the cost
and lowers the efficiency, but also increases the complexity of the system and augments
the difficulty of operation and control. Severe black-outs happened recently in power
grids worldwide and these have revealed that conventional transmission systems are
unable to manage the control requirements of the complicated interconnections and
variable power flow.