Variable-speed operation of Francis turbines: A review of the perspectives and challenges

Iliev, Igor; Trivedi, Chirag; Dahlhaug, Ole Gunnar

doi:10.1016/j.rser.2018.12.033

Cited by 69 publications

(28 citation statements)

References 31 publications

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“…This is typically achieved by either using full size frequency converters (FSFC) or doubly fed induction machines to enable decoupling between the power grid and the production unit. Both technologies have certain limitations and introduce additional losses and complexity in the power generation process [17]. In some cases, especially for FSFC operating close to the peak efficiency of the turbine, it is more efficient to switch back to SSO and avoid the additional losses.…”

Section: Optimization Strategy For Variable Speed Operationmentioning

confidence: 99%

“…Owning to the pioneering work done in Japan on variable speed operation (VSO) of reversible pump-turbines [9][10][11][12], several researchers have explored the idea to apply this technology to improve off-design efficiency of conventional Francis turbines [13][14][15]. However, both old and recent studies have shown that for low specific speed Francis turbines operated close to their rated head, such as most of the installed turbines in Norway are, enabling VSO might result in minor efficiency improvements [13,16,17]. This outcome is found to be closely related to the hydraulic design of the runner [16], suggesting that improvements might be expected with a proper redesign/optimization.…”

Section: Introductionmentioning

confidence: 99%

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Optimization of Francis Turbines for Variable Speed Operation Using Surrogate Modeling Approach

Iliev

Tengs²,

Trivedi³

et al. 2020

Journal of Fluids Engineering

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Previous studies suggested variable speed operation of Francis turbines as a measure to improve the efficiency at off-design operating conditions. This is, however, strongly dependent on the hydraulic design and, for an existing turbine, improvements can be expected only with a proper redesign of the hydraulic surfaces. Therefore, an optimization algorithm is proposed and applied to the runner of a low specific speed Francis turbine, with an optimization strategy specifically constructed to improve the variable speed performance. In the constrained design space of the reference turbine, the geometry of the replacement runner is parametrically defined using 15 parameters. Box-Behnken method was used to populate the design space with 421 unique samples, needed to train fully quadratic response surface models of three characteristic efficiencies defined by the proposed objective function. Computational fluid dynamics was used to calculate the responses for each sample. The parametric study showed that the anticipated variation of the shape of the hill-chart, needed to improve the variable speed performance of the turbine, is limited within a narrow range. The presented method is general and can be applied to any specific speed in the Francis turbine range, for both synchronous speed and variable speed optimization tasks.

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Section: Optimization Strategy For Variable Speed Operationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Optimization of Francis Turbines for Variable Speed Operation Using Surrogate Modeling Approach

Iliev

Tengs²,

Trivedi³

et al. 2020

Journal of Fluids Engineering

Self Cite

View full text Add to dashboard Cite

show abstract

“…A SC_inlet inlet area of the spiral casing (SC), m 2 D penstock diameter at the inlet of the spiral casing, m K WK WK coefficient, m 3.5 /kg 0.5 K WK_BEP WK coefficient obtain at BEP flow condition (steady), m 3.5 /kg 0.5 n WK exponent N 1 , N 2 , N 3 number of mesh elements in fine, medium and coarse mesh ∆P pressure difference between outer and inner WK pressure points, Pa Q flow rate (discharge), m 3…”

Section: Nomenclaturementioning

confidence: 99%

“…Consequently, the plants undergo frequent transient events such as start-stops and large ramping rates. Most research is focused on the hydro-mechanical aspects of the transients [1], variable-speed operation [3] and designing flexible hydraulic turbines to support flexible operations [2]. Besides, the instantaneous flow rate in the transients is a key parameter to study the hydro-mechanical effects on the turbine components or to quantify the total volume of water flow during such events.…”

Section: Introductionmentioning

confidence: 99%

Numerical Study of the Winter–Kennedy Flow Measurement Method in Transient Flows

et al. 2020

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This paper explores the possibility of using the Winter–Kennedy (WK) method for transient flow rate measurement in hydraulic turbines. Computational fluid dynamic (CFD) analysis of a numerical model of an axial turbine was carried out for accelerating and decelerating flows. Those were obtained by linearly opening and closing of the guide vanes, respectively, while retaining the inlet pressure constant during the simulations. The behavior of several WK configurations on a cross-sectional plane and along the azimuthal direction of the spiral casing was studied during the transients. The study showed that there are certain WK configurations that are more stable than others. The physical mechanism behind the stability (or instability) of the WK method during transients is presented. Using the steady WK coefficient obtained at the best efficiency point (BEP), the WK method could estimate the transient flow rate with a deviation of about 7.5% and 3.5%, for accelerating and decelerating flow, respectively.

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“…Present day variable-speed operation of a hydro turbine is considered as an alternative option to meet fluctuating energy demand, as it allows a high-ramping rate. New design approaches are focusing on design optimization for the variable-speed operation including high, medium, low, mini and micro head turbines [1][2][3]. Furthermore, mini and micro hydro stations are good options for smart grid configurations.…”

Section: Introductionmentioning

confidence: 99%

Numerical Study of a Francis Turbine over Wide Operating Range: Some Practical Aspects of Verification

2020

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Hydropower plays an essential role in maintaining energy flexibility. Modern designs focus on sustainability and robustness using different numerical tools. Automatic optimization of the turbines is widely used, including low, mini and micro head turbines. The numerical techniques are not always foolproof in the absence of experimental data, and hence accurate verification is a key component of automatic optimization processes. This work aims to investigate the newly designed Francis runner for flexible operation. Unsteady simulations at 80 operating points of the turbine were conducted. The numerical model consisted of 16 million nodes of hexahedral mesh. A SAS-SST (scale adaptive simulation-shear stress transport) model was enabled for resolving/modeling the turbulent flow. The selected time-step size was equivalent to one-degree angular rotation of the runner. Global parameters, such as efficiency, torque, head and flow rate were considered for proper verification and validation. (1) A complete hill diagram of the turbine was prepared and verified with the reference case. (2) The relative error in hydraulic efficiency was computed and the over trend was studied. This allowed us to investigate the consistency of the numerical model under extreme operating conditions, far away from the best efficiency point. (3) Unsteady fluctuations of runner output torque were studied to identify unstable regions and magnitude of torque oscillations.

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Variable-speed operation of Francis turbines: A review of the perspectives and challenges

Cited by 69 publications

References 31 publications

Optimization of Francis Turbines for Variable Speed Operation Using Surrogate Modeling Approach

Optimization of Francis Turbines for Variable Speed Operation Using Surrogate Modeling Approach

Numerical Study of the Winter–Kennedy Flow Measurement Method in Transient Flows

Numerical Study of a Francis Turbine over Wide Operating Range: Some Practical Aspects of Verification

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