Hydropower is the only renewable energy source that can

Hydropower is the only
renewable energy source that can be used as both continuous and intermittently.
Injection of intermittent energy sources (such as solar and wind etc) above a
certain level results in the stability issues of the grid. A certain level of
voltage and frequency must be maintained by the generators and associated grid
to ensure stability in the grid system. Hydroelectric power plants are very
useful in this kind of scenario as they can run with a wide variation of load
which can meet the variations in demand. The dynamic stability of hydro
reaction turbines are sensitive to load variation, start-stop and load
rejections 1. These transients results in cyclic stresses and asymmetric
forces on the runner. As a result of which wear and tear is aggravated reducing
the life of the turbine.

Presently reaction
turbines operates over a wider range and especially running in lower to higher
operating point results in cavitation, vortex break down, high pressure
pulsation increasing wear and structural vibration. Due to the change of load
guide vanes are positioned which ends up forming vortex and destruction of it
very quickly. The flow condition become worst as vortex instability increases. Change
of pressure variation on the suction and pressure side and continuous change of
stagnation point increases fatigue loading detoriating the life of the
component. Experimental investigations also confirm the generation of stall in
such away BEP flow conditions. The shut down and restart is the most damaging
conditions for reaction turbines as large transient forces results in
unfavorable acceleration and deceleration which can cause harm to the
runner.  The most severe transients of
all is total load rejection, where complete disconnection of the generator from
grid occurs due to parameter variation of the generator over the prescribed
range. The runner speed exceeds run away speed and experiences severe
vibrations which damage the components of the turbine 1. Experimental
investigation on a scaled model of a Francis turbine prototype with a specific
speed of 0.27 for a transition to total load rejection indicates unsteady pressure
fluctuation amplitude at the vane less space to be 2.1 to 2.6 time the pressure
amplitude at BEP 2. Investigations on dynamic flow phenomena such as full
load instability, rotor-stator interaction, draft tube instability and channel
vortex generate depending upon operating point are responsible for pressure
oscillations to modify runner meridian contour, runner outflow and flow in
draft tube cone so that pressure pulsation at different operating point is
decreased from original design was performed by Magnoli et al. 3.

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Transient conditions
that prevail are different for different turbines and therefore must be studied
individually along with field measurements to understand the relation between
hydrodynamic forces and mechanical behavior of the component and also to chalk
out optimum operational scheme and guide vane movement to reduce damage and to
maximize the efficiency. Complete transient higher order numerical simulation
along with grid movement is essential to understand the flow physics happening
inside the machine like cavitation, stall, vortex formation and destruction,
pressure impulse etc. The role of turbulence models plays a vital role for this
kind of complex simulation. As the flow is very complicated there may be a need
of new turbulence models to improve the prediction.

The trend of
operating hydro turbines has changed a lot over past years. Previously they
were mostly used as continuous power supplying machine running at best
efficiency point (BEP) whereas presently they are mostly used as intermittent
supply source to meet to base and peak load and also to maintain grid
stability. This has resulted the machine to operate in most complex condition
away from BEP. Therefore experimental, numerical and analytical investigations
of turbine behavior during transients must be studied in detail not only for
design modifications but also for successful retrofitting. 


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