Experimental Investigation of a 10 MW Prototype Kaplan Turbine during Start-Up Operation

Abstract

An increase in the start/stop cycles of hydraulic turbines due to the penetration of intermittent renewable energy sources is important. Hydraulic instabilities that occur in hydraulic turbines during start/stops may cause structural issues in the turbine components. High-stress fluctuations on the runner blades are expected during start-ups due to the unsteady pressure loading on the runner blades. This paper presents experiments performed on a 10 MW prototype Kaplan turbine at the Porjus Hydropower Center during a start-up cycle. Synchronized unsteady pressure and strain measurements on a runner blade and axial, bending (in two directions) and torsion strain measurements on the shaft were performed. In addition, the general parameters of the turbine (e.g., rotational speed, guide vane opening and runner blade angle) were acquired. Low-frequency fluctuations (0-15 Hz) were observed in the pressure data on the runner blade after opening the guide vanes from the completely closed position. A higher strain value was observed on the strain gauges installed on the runner blade near the hub (200-500 µm/m ) compared to the ones near the shroud at the leading and trailing edge. The strain fluctuation level on the shaft decreased after loading the generator by further opening the guide vanes. Higher fluctuations were observed in the torsion strain compared to axial and bending strain. In addition, the torsion strain peak-to-peak value reached 12 times its corresponding value at 61% guide vane opening.Energies 2019, 12, 4582 2 of 20 slow cycle fatigue [5]. In addition, high amplitude vibrations can occur due to the resonance between the low frequencies developed during the start-up operation and the natural frequencies of the runner. The runner blades experience random and periodic stress that is induced by stochastic and periodic flow during the start-up cycle [5][6][7]. The damage to the runner blade caused by one start-up cycle to a runner blade was observed to be equivalent to thirty years of turbine operation under a partial load operating condition, approximately 60% of the best efficiency point where the pressure pulsations in the draft tube are maximum [8]. In addition, no clear correlation between pressure pulsations in the draft tube and strain values on the runner blade was observed.Guide vane opening limit and opening rate are two parameters that can influence pressure fluctuations in the turbine during the start-up operation. A start-up operation creates a complex flow and stochastic pressure loads in the turbine due to the change in the runner speed and transient pressure variation. The startup and shutdown scheme are defined during the turbine commissioning and can be changed during the refurbishment in power plants. An optimized start-up and shutdown scheme can reduce the damage and increase the life expectancy [9]. Nevertheless, this operation should be performed on a case-by-case basis because each turbine exhibits a unique behavior during a start-up cycle, and general methods have not yet been proposed ...

An Indirect Measurement Methodology to Identify Load Fluctuations on Axial Turbine Runner Blades

“…A dominant frequency of is also observed in Figure 17 f, which is for deep part load operation (OP0). A dominant frequency in the same range was observed in a load increase from speed-no-load in a Kaplan and propeller turbine [ 20 , 23 ]. This frequency, which could be related to the vortical flow structures in the vaneless space, occurred at speed-no-load operation and moved below the runner by increasing the discharge.…”

“…This frequency, which could be related to the vortical flow structures in the vaneless space, occurred at speed-no-load operation and moved below the runner by increasing the discharge. It was previously found that the variation of the dominant frequency related to vortical flow structures ( ) was accompanied by the variation of the low frequency ( ) at the same time period but in the opposite direction [ 20 ].…”

“… ( a ) Position of the pressure transducers on the runner blade pressure side [ 20 ], ( b ) position of the strain gages on the runner blade pressure side [ 20 ], ( c ) the blade runner pressure side before applying the epoxy. …”

“…1. (a) Position of the pressure transducers on the runner blade pressure side [20], (b) po strain gages on the runner blade pressure side [20], (c) the blade runner pressure side b ng the epoxy. ne operational parameters such as guide vane opening, runner blade opening ang adwater level, and tailwater level were simultaneously recorded from the turbi dition, the guide vane opening was measured by a distance transducer of the guide vane hydraulic servomotor to acquire the linear movement of the gu r blades pitch angle and the guide vane angular opening range of this unit is +1 (0-100% opening), respectively.…”

“…Aiming at providing deeper insight into the prototype hydraulic turbines, various operating conditions such as part load operation, load increase, load decrease, startup, speed-no-load, generator excitation, and ramp-up operating conditions have been previously investigated on a Kaplan prototype turbine [ 18 , 19 , 20 ]. Earlier experimental works on prototype and model hydraulic turbines indicated that a signature of hydraulic phenomena in the turbine can be detected by pressure measurement on the draft tube wall or strain measurement on the shaft [ 15 , 20 , 21 ]. However, any comprehensive investigation on the correlation between the load fluctuation on the runner and either the draft tube pressure fluctuation or shaft strain and displacement measurement has not been published.…”