In 2022, we had the honour of developing the hydraulics for a diagonal turbine for a long-standing customer. For a new project in Colombia, the turbine design developed back then is now to be reused for an application with a maximum output of more than 2 MW. Unlike the original project, however, variable-speed operation will now be possible. Furthermore, the new turbine will be operated on an approximately 2 km long penstock, which requires extensive consideration of the pressure surge issue. Jaberg & Partner was therefore consulted to analyse the suitability of the originally developed turbine design in terms of variable-speed operation and to investigate the pressure surge risk of the new turbine.
A complete turbine hill chart is required to estimate the runaway speed and the resulting flow rate as well as to perform a pressure surge calculation. For example, in order to be able to check the load rejection at full load and with the guide vane open as part of a 1D transient simulation, at least the operation range up to the runaway curve (= ISO line with 0% efficiency) must be represented.
In a first step, 3D CFD simulations in a full 360° model was used to calculate the required operation range in several steps at different heads and speeds. The extended hill chart was then transferred to the customer in a standardised representation and prepared for the subsequent pressure surge calculation.
In the second step, a pressure surge analysis had to be carried out to determine the transient behaviour of the Colombian hydropower plant, for example in the event of a load rejection of the turbine. This involves modelling the headrace from the water intake to the powerhouse. For this purpose, all hydraulically relevant system components were modelled using the Flomaster simulation software with our extensions for hydraulic systems and hydropower plants. The turbine's behaviour was considered in a specially created numerical model. This required the complete turbine hill chart described above.
The investigation aimed to determine the relevant system and machine parameters (maximum internal pressure on pressurised components, any negative pressure that may occur, maximum turbine speed, maximum flow rate, etc.) in the event of load rejection of the turbine and thus the dimensioning of the turbine operating times.
In the end, we found a solution that enables safe system operation without a cost-intensive surge tank. Our analyses also identified a potential planning error in the penstock.
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