Unsteady operation and rapid start up of Francis turbines
thesisposted on 2023-05-27, 11:18 authored by Dean GiosioDean Giosio
Hydropower is a key component in a decentralised market with increasing penetration of often intermittent renewable energy resources. There is growing need for large-scaleoperators to develop a more comprehensive understanding of the full dynamic capabilities of a given hydropower plant, while developments in small-scale hydro will aid in waste energy recovery and rural electrification. In particular, greater confidence in the transient response characteristics of hydro generators will in turn enable and encourage increased contributions from other much needed large scale renewable energy sources, such as wind and solar power, without compromising grid stability. The major drawback of current pump-as-turbine (PAT) micro-hydro solutions is the very narrow effective operating band and lack of universal performance prediction models. To address this, the design process and installed performance of a new 6.2 kW PAT microhydro turbine unit and test facility is presented. Steady state testing over the full operating range indicates a best efficiency of 79% and demonstrates near peak efficiency operation over an extended range of head and flow conditions. The developed turbine is a promising prototype of a lower cost alternative for installations where the capital costs associated with conventional Francis turbine units are often prohibitive. The potential for hydraulic turbines to provide rapid reserve power generation is investigated experimentally by transitioning the micro-hydro unit from a modified synchronous condenser mode under varying levels of tail water depression and inlet guide vane (IGV) opening rates. Similar full-scale testing on a 116 MW Francis turbine generating set revealed an undesirable power draw and subsequent output power oscillations during the early stages of transition limiting the potential contribution of the tested unit. Direct shaft power measurements of the micro-hydro unit during transition demonstrate the presence of an inhibiting mechanical torque applied to the runner during low guide vane opening angles, of the order of 2% of rated, which is believed to initiate the electromechanical response seen at full-scale. The temporal location and duration of the applied negative torque was found to be highly dependent on IGV opening rate while the effect of tail water depression level was insignificant under constant head conditions. The magnitude of the power draw was independent of initial tail water depression level, while an increased opening rate marginally reduced the observed power draw at the laboratory scale. An improved one-dimensional numerical model of a Francis turbine hydropower plant for dynamic response studies is presented. The model is based on the equations of motion and continuity, while the conventional representation of the hydraulic turbine as an orifice is improved upon to account for machine behaviour away from design using known inlet flow velocity vectors. The new model remains valid at low IGV angles, where the traditional one-dimensional model breaks down, and is validated against full-scale transient test data. Furthermore, simulation results support the findings of experimental work indicating a high dependence on IGV opening rate in relation to output response during transition from tail water depression mode. Ultimately, the research presented assesses the feasibility of transitioning a Francis turbine unit from tail water depression mode to generation for the purpose of providing rapid load support to the local grid. A three-stage transition mechanism responsible for the observed output response is proposed based on the findings from both experimental and numerical investigations, while key parameters and potential risks are identified. Finally an improved set of operational guidelines are presented for increasing the provision of rapid reserve power generation for maintaining system stability and ensuring a secure electricity supply.
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