Stability Limits of Reversible-Pump Turbines in Turbine Mode of Operation and Measurements of Unstable Characteristics

Abstract

Measurements have been performed on a reversible-pump turbine model installed in a closed loop conduit system. The characteristics of the unstable pump turbine in turbine mode show a hysteresis pattern. Hence the output of the system is dependent on the previous state of the flow and not only the input variables. The hysteresis pattern is a characteristic of the whole system, but is caused by the unstable pump turbine. The unstable part of the characteristics was measured by three different methods: 1) by transient sampling of data during the transition between operation modes, 2) by throttling valves that steepens the friction-loss curve, and 3) by switching the causality in the system such that the torque becomes an input parameter and the speed of rotation becomes an output parameter. In the valve throttling measurements a pressure dependency was seen for the characteristics at high nondimensional speeds. This was further investigated by additional measurements of the characteristics at three different pressure levels. A rigid-water-column stability analysis has been conducted. The classic H-Q criterion describes static stability for a pump turbine with constant speed of rotation. With the speed of rotation as a variable, there is a new static stability criterion in addition to the dynamic stability criterion.

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

“…Zhu et al [40] Model pump-turbine (corresponding prototype Hr ¼ 259m, Pr ¼ 255MW) Negative blade lean lead to a weaker S-shaped characteristic Nielsen et al [36] and Olimstad et al [37,38] (ns ¼ 114) Low-specific-speed model pump-turbine…”

“…[36]. [37,38] on the runner geometry effect on the S-shaped characteristics. They observed that there is a major influence on the steepness characteristics by the diameter and speed of rotation.…”

“…Below are the summery of some parameters of pumpturbines which impact the S-shaped characteristics (1) Guide vane opening at speed no-load conditions, the slope of the Q 11 n 11 characteristics generally goes up with cumulative guide vane opening. Hence, the risk of unsteadiness increases with decreasing head [37]. (2) Runner geometric parameters, table two below summarizes the study on the effect of the runner geometric parameters on the S-shaped characteristics.…”

Effects of Pump-turbine S-shaped Characteristics on Transient Behaviours: Experimental Investigation

“…Static tests of pump turbines [1,2] and computational fluid dynamics numerical methods [3,4] can reveal the complex hydraulic phenomena of the internal runner when a pump turbine is operating in the S region, such as stationary vortices, unsteady vortex, and the rotating stall that results in the blockage effect; all these phenomena are fundamental causes for the inversion of the S-shaped characteristic curve. Moreover, the S-shaped characteristics also lead to unstable runaway or no-load operations [5][6][7][8], as well as water-hammer pressure surge after load rejections.…”

“…Due to the S-shaped characteristics, the dynamic processes of a pumped storage station is complicated: 1) The unstable pump turbine S-shaped characteristics make it difficult to measure the Sshaped curves, which can, therefore, be obtained by the transient method [9,10]; 2) pump turbines often show an unstable runaway and no-load operations, which are close to the form of S-shaped curves [5][6][7][8]; 3) in the load rejection process, the water-hammer pressure is also related to the Sshaped curves; 4) in some extreme conditions, such as OAA load rejections, numerical simulations have shown that the water-hammer pressure, especially in the draft tube, is fairly significant [11,12]; 5) to control the water-hammer pressure and runaway speed during the load rejection process, multiphase closing schemes can be used to guarantee the safety of a pumped storage station [13]. Therefore, based on the transient issues described above, the relevant verification experiments were conducted to clarify the effects of S-shaped characteristics on hydraulic transients.…”