Robust Power System Stabilizers: a Frequency Domain Approach

Presently, there are two main aspects which allow to reduce the operating as well as the cold reserve power to be kept ready by individual network partners in order to maintain the power system reliability. First, the transmission lines, generators and loads are interconnected into large-scale and complex integrated systems. An important benefit brought about by the interconnected operation is that occurring power system disturbances, eg power plant failures, are jointly intercepted and temporarily compensated for by all participating power systems. Secondly, due to economy and consumers demands, multimachine power systems are being operated closer than ever to their stability limits. Due to stochastically distributed switching actions, electromechanical transients continuously occur in electrical grids. They manifest themselves through local oscillations of individual generators or through interarea oscillations. In case of local oscillations (within the frequency range of 0.7–2.2 Hz) one or several local synchronous generators are involved. Oscillations associated with several generators in one part of the system with respect to the rest of the system are referred to as interarea oscillations . The frequency of these oscillations typically ranges from 0.1 to 0.7 Hz. Multimodal oscillations represent energy exchange between rotors and are characterized by low frequencies (around 0.1–0.2 Hz). Increased power transits are limited on one hand by thermal limit ratings of coupling lines and on the other hand by increasing endangering of the power system stability indicated by poorly damped or even increasing interarea oscillations [4]. Unstable or poorly damped electromechanical oscillation modes in a power system cause stability problems. As one of the most cost-effective methods of enhancing the stability and damping power swings in the power system, power system stabilizers (PSS) are added to the automatic voltage regulator (AVR) subsystem [2, 3, 4, 5, 6]. + + UPSS p