Transients in high head Francis turbines

Abstract: On demand energy market and favourable government policies to encourage the usage of renewable energy have led to a dramatic increase of grid connected wind, solar, and other power in recent years. Penetration of the power above manageable limit, produced using intermittent energy sources, to meet the demand has induced instable grid operations. On the other hand, the consumers more – or – less expect to be able to draw greater or lesser amount of grid power, whensoever require, at their discretion, and they expect the grid to accommodate this flexibility. Intermittency in the power production and utilisation flexibility to the end users resulted in increased grid incidents. In order to ensure the uninterrupted grid operation one has to take the responsibility to retrieve the grid parameters within operating band before it collapses. Because of speedy and smooth response to the connected grid network, hydroelectric turbines are most frequently used to bring the grid parameters within stable operating band. In a hydro power plant variation in the parameters is detected by a governor and that governs guide vanes angular movement through servo-actuation mechanism. The guide vanes manage mass flow rate flowing to the runner, i.e., the output torque/power. The guide vanes have to react promptly in a very short time to balance the grid. Thus, most of the hydroelectric units are supposed to operate over a larger operating range irrespective of efficiency and the stability, continuous load variation, load rejections, and increased numbers of start-stop cycles. An incompetent opening/closing of the guide vanes during transients induce aperiodic pressure loading on the blade surfaces resulted in cyclic stresses and fatigue. Unexpected transients led to increased wear, shortening the runner life, increased cost of plant operation, and loss of power production. Transients cannot be stopped or avoided but the damage due to the wear and, therefore, runner life may be improved by minimising the unfavourable pressure loading in the turbine through strategic movement of the guide vanes. This motivated to carry out the investigations on hydraulic turbine during transients and study guide vanes operating strategy. The main scope of the licentiate thesis is to identify the change in operating trend of hydraulic turbines due to injection of intermittent energy to the power grid and how it is affecting the turbine operations. The first paper reviews the change in operating trend and consequences to the hydraulic turbines due to changes in the pattern of turbine operation in the recent years. After drawing conclusions from the referred literature, experimental measurements on a model Francis turbine installed at NTNU, Norway, were performed. Both types of, steady state and transient, operational conditions were investigated including pressure measurements. The second paper presents the observations and investigations of steady state measurements and the corresponding numerical simulations. The third paper presents part of the transient experimental investigations; load variations and rejections. Six transient operational conditions were investigated including time domain frequency variation and rotor-stator interaction. The largest pressure variation compared to steady state operation is obtained during load rejection. Preliminary results indicated that appropriate gate closure may minimize large pressure amplitudes in the blade cascade.