Repowering existing HPP infrastructure for the next 70 years: Retrofit of the St. Anton Hydro Power Plant with increased power output and flexibility

In today’s energy market with decreasing energy prices, investments in hydro power plants can hardly be justified, especially if they are private owned. One possibility to raise the annual energy earnings is not only to increase the power output but also the availability and flexibility of an existing power plant. With highly volatile loads in electric grids from renewables like wind and solar grid service tasks become more and more important. Most of the older power plants – in particular storage power plants – are not designed for such operating modes – especially not for rapidly changing and fluctuating load. The St. Anton Power station in northern Italy was constructed between 1948 and 1951 with a head of 600 m and a maximum power output of 72 MW. The original plant design was based on operating conditions which are completely different to the ones actually required. To prove the ability for higher flexibility in power generation, detailed investigation of the power water way, especially surge tank limitations, are necessary. Since the main constructive infrastructure cannot be changed or just in a limited way, only limited additional power generation is possible. The higher the demand for an increasing power output, the higher are the limitations in operational flexibility. An overflow of the surge tank must be prevented in any possible operational or exceptional scenario of the power plant. On the other hand a ventilation of the head race by means of an ineligible empty surge tank must not occur during operation. Flexible operation induces a highly transient fluid flow in the head race and especially the surge tanks of high head hydro power plants. In the planning phase of a refurbishment to increase the power output and/or the flexibility, a reliable prediction of the transient plant behaviour and especially the surge tank performance in unsteady load cases - such as periodic machine starts and stops or switching load cases - is necessary. The present publication presents a way to calculate the occurring waterhammer and mass oscillations with modern techniques in transient system simulation. In order to get a significant and reliable solution, detailed and exact numerical models of complex power plant components like turbines and especially surge tanks are necessary. Since most alpine high head power plants have a unique surge tank concept and design their physical behaviour cannot be simulated accurately with standard models. Hence a custom designed numerical surge tank model was used to describe the real hydraulic behaviour in terms of waterhammer simulations. A validation of the numerical simulation by means of onsite measurements at the existing power plant configuration yields additional confidence for a safe operation after the power increase. This publication contributes to the improvement of performance and flexibility of existing power plant infrastructure based on transient numerical simulations.