Upgrading of a Norwegian pressurized sand trap combined with an open air surge tank

Upgrading of a Norwegian pressurized sand trap
combined with an open air surge tank
In 1988 the Tonstad power plant (Norway) was upgraded from 640
to 960 MW. The headrace system to the reservoirs was not upgraded
except for an additional pressure shaft, surge tank and an
additional sand trap. Even with about 50 % higher discharge in the
main tunnel, the sand traps worked adequately after the commissioning.
However, in recent years, higher flexibility of demand is
challenging the power plant and has resulted in events with
flushing of sediments to the turbines. Higher flexibility demands
also challenge the current surge tank design. During one event,
free surface flow is believed to have occurred in the sand trap
since severe damage to two of the turbines was observed. This
contribution presents the background of the power system situation
at the Tonstad power plant and the current flexibility demands
of the power market and its challenges for the hydraulic
system, as well as the proposals to solve the sand trap issues and
the oscillation demands for the surge tank. The paper discusses
the possibility of utilizing a model predictive controller with realtime
flow simulations to solve the challenges without structural
reconstruction, and possible options should structural reconstruction
be necessary. The contribution compares analysed options
for improving the efficiency of sand traps. Finally, an outlook
is offered to future demands for flexibility in hydropower plants
with increasing supply of renewable power sources.