Comparison of different control structures for Lyapunov-based power system stabilizer

Recently it has been shown that it is possible to design a power system stabilizer (PSS) using a nonlinear multi-machine power system model in conjunction with Lyapunov's direct method. The resulting controller has been shown to be robust. It was also shown that each individual controller acted independently of all the others suggesting that no coordination of settings is necessary. These features should allow a decentralized approach to the design of the PSS. In this paper two control structures implementing the proposed control law have been compared: a hierarchical structure in which the automatic voltage regulator (AVR) is the master controller and the PSS is the slave controller, and a more traditional structure in which the PSS constitutes a supplementary loop to the main AVR. This paper shows that the AVR has to be designed in a different way for each of the structures. Theoretical analysis is supported by the results of simulations tests which highlighted the differences between the way in which the stabilizing signal was obtained for each controller. The tests performed on a multi-machine system model have shown a very good performance of both proposed structures when compared with a classical speed-based PSS.