The primary goal for this PhD project has been to investigate instability of reversiblepump turbines (RPTs) as a phenomenon and to find remedies to solve it. The instability occurs for turbines with s-shaped characteristics, unfavourable waterways and limited rotating inertia. It is only observed for certain operation points at either high speed or low load. These correspond to either high values of Ned or low values of Qed. The work done in this PhD thesis can be divided into the three following categories. Investigate and understand the behaviour of a pump turbine: A model was designed in order to investigate the pump turbine behaviour related to its characteristics. This model was manufactured and measurements were performed in the laboratory. By using throttling valves or torque as input the full s-shaped characteristics was measured. When neither of these techniques is used, the laboratory system has unstable operation points which result in hysteresis behaviour. Global behaviour of the RPT in a power plant system was investigated through analytical stability analysis and dynamic system simulations. The latter included both rigid and elastic representation of the water column. Turbine internal flow: The flow inside the runner was investigated by computer simulations (CFD). Two-dimensional analysis was used to study the inlet part of the runner. This showed that a vortex forming at the inlet is one of the causes for the unstable characteristics. Three-dimensional analyses were performed and showed multiple complex flow structures in the unstable operation range. Measurements at different pressure levels showed that the characteristics were dependent on the Reynolds number at high Ned values in turbine mode. This means that the similarity of flows is not sufficiently described by constant Qed and Ned values at this part of the characteristics. Design modifications: The root of the stability problem was considered to be the runner’s geometric design at the inlet in turbine mode. Therefore different design parameters were investigated to find relations to the characteristics. Methods used were measurements, CFD modelling and analytical models. The leading edge profile was altered on the physical model and measurements were performed in the laboratory. Results showed that the profiles have significant influence on characteristics and therewith stability at high speed operation points. Other design parameters were investigated by CFD analysis with special focus on the inlet blade angle.

D. Tech. Electrical Engineering. / Discusses the objective of this study is to develop an embedded generator-pump set topology using a wound rotor induction machine using the doubly fed induction generator concept, and a synchronous machine electrically and mechanically coupled to it, powering its magnetisation circuit. An adjustable pitch pump is also coupled to the generating set on the same shaft to provide an embedded generating-pumping solution that can provide co-incident generating ans pumping functions. The research objectives are as follows: to develop an overall plant topology, to identify plant attributes necessary for proper functionality of the proposed plant, to identify a pumping/generation topology that meets the required electro-mechanical and overall topological layout attribute requirements, to develop a primitive mathematical model of the plant that provides insight into fundamental physical behaviour of the plant, to investigate the stability issues arising from the electromechanical coupling of the two machines used, to establish controllability of the proposed configuration, to identify influencing factors on the stable operation of the proposed plant, to develop an overall system model for simulation. This also entails developing a suitable mathematical model for the variable pitch pump and to simulate the system steady state and dynamic behaviour.