Abstract
A distributed generation steam turbine-generator (simply referred to hereafter as distributed generation turbine-generator) improves the supply reliability of the host installation by reducing the number and the duration of interruptions when it is allowed to island on the local load and operate as a backup supply during islanding. Interconnection standards currently preclude islanding operation that includes the utility load. They recommend that when islanding occurs, the utility load should be removed from the island speedily. However, the transient torques generated at the moments of islanding and removing the utility load from the island may cause shaft fatigue life loss and lead to early fatigue damage. Current research on distributed generation focuses on other issues of islanding while shaft protection has not been addressed, and thus constitutes the gap in this field. Therefore, a two-step shaft protection method for distributed generation turbine-generators that are required to island on the local loads and operate as backup supplies is developed in this thesis. The first step is an operating practice to mitigate the torsional vibrations induced by the transient torques while the second step is a backup to torsional vibrations mitigation. It is based on the predicted risk of shaft fatigue damage. Shaft transient torque during islanding is first modelled. Fatigue study provides the size of the local load required to mitigate shaft torsional vibrations and avoid fatigue life loss during islanding. This is substituted in the torque function to obtain the critical torque. The risk of shaft fatigue damage is predicted by comparing the actual shaft torque with the critical torque. The turbine-generator is shut down when the actual shaft torque exceeds the critical torque to avoid shaft fatigue damage. This operation also prevents islanding on the local load. Conversely, islanding on the local load is allowed when the actual shaft torque is smaller than the critical torque. The developed protection method is simulated in PSCAD (Power Systems Computer-Aided Design) version 5 to evaluate its effectiveness. Established models for the electrical network and shaft torsional vibrations and fatigue are used. The study of shaft fatigue is carried out using stress-number of cycles to failure curves. Results show that the developed protection method ensures islanding operation without endangering the shafts and that it prevents shaft fatigue damage irrespective of the operating conditions. A comparative study of shaft fatigue life loss shows that the developed protection method only yields 0.011 % shaft fatigue life loss under the most adverse islanding condition against 100 % obtained when no protection or when a typical online monitoring and protection system (OMPS) are used. These results show that it is unsafe to operate a distributed generation turbine-generator without shaft protection and that an OMPS is not suitable for islanding protection.