Investigation of loss of synchronism phenomenon in synchronous machines and protection using trajectory of relative speed

Abstract

The "loss of synchronism" condition in a synchronous machine is a critical state where the rotor fails to stay synchronized with the stator magnetic field. This can be caused by various factors, including system disturbances and power system instabilities. A machine undergoing a loss of synchronism may experiences torsional stresses, increased rotor iron currents and winding stresses in their mechanical systems. Synchronous machine loss of synchronism protection is employed to promptly disconnect a machine operating asynchronously to avoid damage or degradation. This thesis explores the loss of synchronism phenomenon in synchronous generators and introduces a novel loss of synchronism detection method based on the estimated relative speed of the rotor. The proposed algorithm utilizes readily available terminal voltage, current and machine parameters to estimate the rotor speed following a disturbance. It identifies loss of synchronism condition if the estimated relative speed tends to increase during a swing cycle. This method is computationally simple, easy to implement, and faster than impedance-based techniques under certain conditions. The sensitivity and security of this method is evaluated through time-domain simulations under various power system conditions. The performance of the proposed method is also compared against well established loss of synchronism protection schemes. The occurrence of loss of synchronism in synchronous condensers has not been extensively studied, primarily due to its rarity in traditional power systems. However, with the increasing integration of inverter-based resources and associated grid issues, synchronous condensers have become more common. The literature indicates a considerable gap in understanding the loss of synchronism phenomenon in synchronous condensers, particularly under conditions where inverter-based resources dominate the power grid. This thesis examines the loss of synchronism phenomenon in synchronous condensers, including theoretical analysis using phasor diagrams of realistic scenarios. It also investigates the distinction between loss of synchronism and the pole slipping phenomenon, which has led to failures in traditional impedance-based schemes. The proposed relative speed-based method is applied to synchronous condensers and its effectiveness is demonstrated in situations where traditional schemes have failed. Additionally, effectiveness of the proposed method in weak grids with a high penetration of inverter-based resources is analysed using a modified IEEE 39 bus system.