Power Swing Generated in Francis Turbines by Part Load and Overload Instabilities

David Valentín,Alexandre Presas,Matias Bossio,Eduard Egusquiza,Mònica Egusquiza,Carme Valero

doi:10.3390/en10122124

Abstract

Hydropower plays a key role in the actual energy market due to its fast response and regulation capacity. In that way, hydraulic turbines are increasingly demanded to work at off-design conditions, where complex flow patterns and cavitation appear, especially in Francis turbines. The draft tube cavitation surge is a hydraulic phenomenon that appears in Francis turbines below and above its Best Efficiency Point (BEP). It is a low frequency phenomenon consisting of a vortex rope in the runner outlet and draft tube, which can become unstable when its frequency coincides with a natural frequency of the hydraulic circuit. At this situation, the output power can significantly swing, endangering the electrical grid stability. This study is focused on the detection of these instabilities in Francis turbines and their relationship with the output power swings. To do so, extensive experimental tests for different operating conditions have been carried out in a large prototype Francis turbine (444 MW of rated power) within the frame of the European Project Hyperbole (FP7-ENERGY-2013-1). Several sensors have been installed in the hydraulic circuit (pressure sensors in the draft tube, spiral casing, and penstock), in the rotating and static structures (vibration sensors, proximity probes, and strain gauges in the runner and in the shaft), as well as in the electrical side (output power, intensity, and voltage). Moreover, a numerical Finite Element Method (FEM) has been also used to relate the hydraulic excitation with the output power swing.

Highlights

Nowadays, hydraulic turbines are increasingly working at off-design conditions in order to extend their range of operation and to compensate the non-constant electricity generation of new renewable energy sources [1,2]
When the vortex rope occurs below the Best Efficiency Point (BEP), the cavitating core is formed with spiral shape in the draft tube with a frequency precession of about 0.25–0.35 the rotating speed of the runner
Three different operating conditions have been plotted in this figure: P/Prated rated = 0.59, which corresponds to the part load instability, P/P

Summary

Introduction

Hydraulic turbines are increasingly working at off-design conditions in order to extend their range of operation and to compensate the non-constant electricity generation of new renewable energy sources [1,2]. When the flow rate in the machine is regulated to generate more or less electricity than in the BEP, a swirl in the flow at the turbine runner outlet appears, especially in Francis turbines, which for certain condition could lead to draft tube surge cavitation or vortex rope [3,4]. This phenomenon has been studied for many years [5,6], experimentally in laboratory and by means of CFD (Computational Fluid Dynamics) simulations. When the frequency of this vortex rope coincides with a Energies 2017, 10, 2124; doi:10.3390/en10122124 www.mdpi.com/journal/energies

Methods

Results

Conclusion