A Proposed Approach for Modeling of Power System Uncertanties to Design Robust PSS M.B Abolhasani Jabali Department of Electrical Engineering, Shahed University Iran Grid Management Company(IGMC) Tehran, Iran M.H Kazemi and Saeid Joudaki Department of Electrical Engineering, Shahed University Tehran, Iran Abstract—This There are so many parameters which are effective on calculations of a power system especially for controller design and variations of their values (uncertainties) can influence the performance of the controllers such as the power system stabilizers (PSS). To overcome these problems, H∞ control has been applied to design of robust PSS. One of the difficulties is selection of the weighting function (for uncertainty model) which has a very effective role in H∞ control design. In this paper, an effective approach is presented to determine the weighting function; this approach is based upon transfer functions obtained through various operation scenarios of power systems. To show the efficiency of the proposed approach, the uncertainty model is utilized to design a robust H ∞ PSS by using the feedback control configuration. The simulation results of various operation scenarios show that the obtained H ∞ PSS using this approach is robust enough to damp the power system oscillations I. INTRODUCTION Analysis of power system dynamic behavior requires models with certain parameter values. There are so many parameters which are effective on calculations of a power system especially for controller design and on the other hand variations of the values (uncertainties) can influence the performance of controllers. An important type of these controllers is power system stabilizer (PSS). PSS is designed to add damping to the generator rotor oscillations by proper modulation of its excitation voltage [1]. The PSS provides oscillation damping by producing an electrical torque component in phase with the rotor speed deviations. The basic structure of the PSS comprises a gain, phase compensation blocks, a washout filter and output limiters. With rotor speed employed as the PSS input signal, a torsional filter is also commonly used. The phase compensation blocks are used to provide a phase lead that compensates for the phase lag between the exciter input and the generator electrical torque. In practice, the phase-lead network should provide compensation over the entire frequency range of interest (0.1–2 Hz) and under different operating scenarios. It is generally desirable to have some under-compensation so that in addition to significantly increasing the damping torque, the PSS would promote a slight increase in the synchronizing torque [1]. To design a robust PSS, once the phase compensation blocks are specified, the gains should be coordinated to confer sufficient oscillation damping without having to resort to high values of PSS gain and, consequently, cause excessive adverse transients and interactions among controllers [2]. The conventional PSSs have to be tuned for various operating conditions and some methods are proposed for this object such as non-linear least squares based multivariable root locus following technique for multiple operating scenarios [2, 3]. Since these techniques do not take the presence of system uncertainties e.g. system nonlinear characteristics, variations of system configuration due to unpredictable disturbances, loading conditions etc, the major drawback of these works is the lack of robustness of PSSs against system uncertainties. To attack this problem, the robust control theory is one of the sophisticated countermeasures. To overcome these problems, H ∞ control has been applied to design of robust PSS [4, 5]. In these works, the designed H ∞ PSS via mixed sensitivity approach have confirmed the significant performance and high robustness. In this method, the weighting function (for uncertainty model) which has a very effective role in H ∞ control design is so complicated to select. In this paper, an approach to determine the weighting function for the design of H ∞ PSS is presented; this approach is based upon transfer functions obtained through various operation scenarios of power systems. To show the efficiency of the proposed approach, the uncertainty model is utilized to design a robust H ∞ PSS by using the feedback control according to [6]. The obtained robust PSS guarantees desirable performance requirements imposed on the system. II. UNCERTAINTY AND CONTROLLER DESIGN To perform control designs, different aspects such as robustness, performance and etc, have to be considered. Performance specifications describe how the system behaves in the closed-loop. Several criteria might be of interest such as stability and damping. Robust performance specifications