Coordinated design of under-excitation limiters and power system stabilizers

The inherent dynamical relationship between the under-excitation limiter (UEL) and the power system stabilizer (PSS) control loops in synchronous generators is studied using the frequency response technique. While the stabilizers are effective in introducing electromechanical damping torque components into the generators, it is shown that the limiters should be designed to effect a much slower response characteristics as their main function is to prevent excessive stator end-core heating. The analysis also shows that a reduction in the values of the slope of the boundary curves, which prescribe the operating region of the limiters, is accompanied by a decrease in the damping level of the closed-loop excitation control systems. A method which decouples the design procedure of the UEL from that of the PSS is then proposed.