Coordination of Power System Auxillary Controllers for Damping Torsional Oscillations

Technical advantages of coordinating thyristor controlled static V Ar compensators (SVC) and power system stabilizers (PSS) of turbo-generator excitation systems for the stability enhancement of an AC system during small signal dynamics are presented. The technical feasibility of the coordinated SVC-PSS scheme for simultaneous damping of the inertial and the torsional oscillatory modes in a series compensated ac system is demonstrated. The investigations show that the coordinated scheme not only increases the overall system damping, but also prevents the undesirable interactions between the oscillatory modes. New control schemes for the SVC and the PSS, based on multi-input control systems which utilizes the generator modal speed as the feedback control signals are given. The studies are performed on system-I of the second IEEE benchmark model, using an eigenvalue analysis method. The analytical results are verified by digital computer simulation studies, using the BP A\u27s Electromagnetic Transient Program (EMTP39). The phenomenon of subsynchronous oscillations (SSO) in a series compensated high voltage AC/DC system is investigated. The advantage of employing a supplemental subsynchronous damping controller (SSDC) with the main current controller of the HVDC rectifier station� to damp the SSO modes is illustrated. The effect of reactive power modulation at the rectifier commutating bus by an SVC unit, in order to damp the SSO modes is also investigated. Multi-input control system are used in the design of the SVC auxiliary controller and the SSDC. Modal speeds are used as the feedback signals in both the SVC and the SSDC of the HVDC system. The eigenvalue analysis, the complex torque coefficient and the digital time simulations methods are used in the analysis. The results show that significant damping can be achieved, if both the SVC and the SSDC are utilized in the system