Remote signals become available for designing a wide-area controllers in power system which introduces time delays into the control loop from the latency of communication
networks. Moreover, fault of measurement sensors for local signals can be converted to delays in the signal measurement loop. This paper investigates delay-dependent stability of power systems considering delays introduced from those two sources by calculating the delay margin which denotes a maximal delay a system can tolerate before it becomes unstable. A Lyaponov theory based delay-dependent stability criterion and linear matrix inequalities techniques are proposed to calculate the delay margin, for both constant and time-varying delays. Case studies are based on a single-machine infinite-bus (SMIB) power system equipped with different exciters, including AVR and AVR+PSS exciter controllers. Comparison the impact of time delays for different excitation controllers and relationship between the delay margin and gains of controllers are addressed. Accuracy of delay margin calculated is verified by simulation studies, which also compare the delay tolerance of different exciters.

Time delays,delay-dependent stability,Lyapunov functional,matrix-injection method