Turbine mode start-up simulation of a FSFC variable speed pump-turbine prototype – Part I: 1D simulation

Abstract

Variable speed hydroelectric units equipped with full size frequency converter (FSFC) offer high operational flexibility enabling fast operating point transitions which increase grid regulation capacities. The XFLEX HYDRO H2020 European research project aims to demonstrate flexibility of such technology at prototype scale. The Z’Mutt pumping station, part of the Grande Dixence hydroelectric scheme located in Switzerland, is one of the demonstrators focused on the FSFC technology with a new 5 MW reversible Francis pump-turbine which will be commissioned in 2021. This paper, divided in two parts, aims to simulate the turbine mode fast start-up sequence made possible with the use of a FSFC and to assess the unit damage by means of 1D and 3D CFD simulations. The part I of this paper presents the 1D hydraulic transient simulation results of start-up sequences of unit U5 considering both conventional fixed speed technology and variable speed technology. The time evolution of the unit’s operating point is used as input data for 3D CFD simulations of part II, aiming to assess the impeller damage. Different control strategies to use the FSFC for turbine mode start-up sequence are analysed. Advantages and limits of each strategy are discussed, and recommendation is made for the Z’Mutt prototype demonstrator.

Numerical fatigue damage analysis of a variable speed Francis pump-turbine during start-up in generating mode

“…The global relative damage hill chart with the fixed speed and variable speed BEP tracking trajectories is presented in figure 9. Moreover, the variable speed scenario with linear speed control function as investigated in [4] and [5] is indicated. It must be noticed that the damage peak at n * 11 = 0.48 and Q * 11 = 0.88 could not be detected by the reduced domain setup and therefore, the damage estimations presented in the previous studies turned out underestimated for the variable speed scenario with linear speed increase.…”

“…A new reversible pump-turbine unit that is part of the Z'Mutt power plant forms one of the demonstrators for variable speed technologies. Potential start-up strategies of the 5 MW prototype equipped with a Full Size Frequency Converter (FSFC) have numerically been analyzed in the early project stage employing 1-D hydraulic transient [4] and 3-D CFD/FEA simulations with simplified numerical domain [5]. The preliminary study indicated importantly reduced partial runner fatigue using the FSFC to startup the unit in generating mode.…”

“…2 Numerical methodology 2.1 1-D hydraulic transient simulations A SIMSEN model of the Z'Mutt power plant, as detailed in [4], is used to simulate both hydraulic transients of a conventional and a variable speed turbine start-up of the Z'Mutt pump-turbine prototype. In case of the fixed speed technology, the unit is disconnected from the power network and its rotational speed is increased by the turbine speed regulator acting on the guide vane opening to reach the nominal rotational speed.…”

Assessments of hydropower plants start-up sequences and equivalent runner damage under transient operation

“…Knowing that the starting-up of the unit is one of the most damageable operation for HPPs, variable speed technology can be leveraged to decrease the transient operation impact on the runner damaging. However, how to make good use of the additional degree of freedom offered by variable speed to reduce the equivalent damage during a start-up is not fully understood [17,18].…”

“…This section focuses on the comparison between two considered start-up sequences which have been developed in the framework of XFLEX HYDRO project [17]. The former is the conventional sequence commonly used in fixed speed unit.…”