Analysis of S Characteristics and Pressure Pulsations in a Pump-Turbine With Misaligned Guide Vanes

Abstract

Growing environmental concerns and the need for better power balancing and frequency control have increased attention in renewable energy sources such as the reversible pumpturbine which can provide both power generation and energy storage. Pump-turbine operation along the S-shaped curve can lead to difficulties in loading the rejection process with unusual increases in water pressure, which lead to machine vibrations. Pressure fluctuations are the primary reason for unstable operation of pump-turbines. Misaligned guide vanes (MGVs) are widely used to control the stability in the S region. There have been experimental investigations and computational fluid dynamics (CFD) simulations of scale models with aligned guide vanes and MGVs with spectral analyses of the S curve characteristics and the pressure pulsations in the frequency and time-frequency domains at runaway conditions. The course of the S characteristic is related to the centrifugal force and the large incident angle at low flow conditions with large vortices forming between the guide vanes and the blade inlets and strong flow recirculation inside the vaneless space as the main factors that lead to the S-shaped characteristics. Preopening some of the guide vanes enables the pump-turbine to avoid the influence of the S characteristic. However, the increase of the flow during runaway destroys the flow symmetry in the runner leading to all asymmetry forces on the runner that leads to hydraulic system oscillations. The MGV technique also increases the pressure fluctuations in the draft tube and has a negative impact on stable operation of the unit.

Pressure Fluctuations in the S-Shaped Region of a Reversible Pump-Turbine

“…Both experimental and numerical studies have revealed complex flow characters in the S-shaped region of pump-turbines. Such vortexes may partially block the flow [7,9,10], or cause stall cells to rotate in the runner channels at 50%-70% of the runner rotation frequency [5,6,8].…”

“…In recent years, both experimental and modelling studies have investigated the unstable flow characteristics of pump-turbines operating in off-design conditions [5][6][7][8][9][10]. Husmatuchi et al [5,6] experimentally investigated pressure fluctuations in the gap between the guide vanes and runner.…”

Optimization of Guide Vane Closing Schemes of Pumped Storage Hydro Unit Using an Enhanced Multi-Objective Gravitational Search Algorithm

“…In order to avoid the operating point from the 'S' characteristic area, Kuwabara et al proposed a curved closing scheme that could effectively decrease the water-hammer pressure [12]. Additionally, appropriate two-phase closing scheme [13,14] and misaligned guide-vane method [15][16][17] also have been applied to control the fluctuation of PSHU. Three-phase valve-closing and curved closing scheme exerted high demands on the governor servomotor, which are difficult to realize in PSP.…”

“…Three-phase valve-closing and curved closing scheme exerted high demands on the governor servomotor, which are difficult to realize in PSP. Meantime, misaligned guide-vane method can significantly increase the pulsating pressure and the runner radial forces during start-up process of PSHU [17]. Therefore, the optimal closing schemes are required to realize optimal coordinated operation of PSP and new energy generation technologies based on deeply analysis of operational mechanism and flow characteristics of PSHU.…”

Review on S-Shape Characteristics of Pump Turbine for Hydropower Generation

“…What brought about this S-Shaped characteristic appear to be in relation with the development of vortexes in the pump-turbine partly blocking the flow [4][5][6]. Established from Some studies indicate that the advancement of stall cells gyrating in the runner channels at 5070% of the rotational frequency of the runner [7][8][9][10].…”