The 220-mw Luenersee pumped-storage plant was built in Austria in the early 1950s, to produce electricity during the day when demand was high and to pump water back into the reservoir at night when rates were low. The mode of operation at the plant typically changed only twice a day.
However, owing to the liberalization of the European electricity market and an increase in the use of fluctuating renewable energy sources (particularly wind), beginning in the 1990s, it became important for the country’s hydro plants to provide frequency control to the electrical grid. As a result, hydro facilities such as Luenersee now operate with a high degree of flexibility. This mode of operation results in fast and/or frequent load changes. In turn, these load changes result in significant variation in the level of water in the penstocks.
The Luenersee plant is equipped with five Pelton pump-turbines and is operated remotely. To react quickly to changes in the grid, the units are run in the hydraulic short circuit mode. This means the pumps and turbines run at the same time, in parallel operation. In this mode of operation, the power plant builds a very fast reserve, which can be used in two ways. In the case of too much power in the grid, the turbines can be shut down immediately and the pumps can be left running to remove excess energy. In the case of too little energy in the grid, the pumps can be shut down and the turbines can be operated to produce the needed additional electricity. This setup provides must faster reaction times compared to starting the pumps or turbines from standstill.
However, because this parallel mode of operation was not envisioned when the Luenersee plant was designed, it can result in draining of the surge tank. The periodic opening and closing of the servomotor, which occurs during each change in load requirement, causes the water level in the surge tank to oscillate with increasing amplitude. After only two load cycles, the surge tank is drained. This can result in air flowing into the penstock. This air can be sucked, with the water, through the turbine nozzle, where it suddenly expands like an explosion.
Measures taken to avoid this situation severely restrict operation of the power plant. In the worst case, some of the units have to be taken out of service until the headwater level is increased again — either by pumping or by natural inflow to the upper reservoir.
Solving the problem
The owner of the Luenersee plant, Vorarlberger Illwerke AG, needed a way to prevent the emptying of the surge tank yet still operate the units in the hydraulic short circuit mode. The solution selected was development of an automatic surge tank protection module to be connected to each unit’s digital governor. To design the module, Vorarlberger Illwerke approached the University of Stuttgart in Germany. Researchers at the university developed a computerized model of the power plant, then developed and optimized a tailor-made surge tank protection module for Luenersee.
This module consists of computer software and hardware. Using a computer located at the plant’s remote dispatch center, the dispatcher transmits the load requirement for each turbine’s governor to the module. Based on this load requirement, the module then obtains measurements of the water level in the surge tank, via three pressure transducers installed in the lower surge tank chamber. If the module determines the water level in the surge tank could reach a critical point, it reduces power output from the facility and restricts the speed of operational change of the units. This results in a temporary reduction in the quality of governor control but prevents emptying of the surge tank. Because this situation of restricted speed and reduced power output only happens very rarely and takes just one or two minutes, this situation is not a severe problem for the grid. In the normal mode of operation, the module has no influence on governing of the plant.
The module also contains a fail-safe feature. If the water level in the surge tank reaches the pre-determined lowest possible level, an emergency shutdown of the turbines is initiated. This shutdown is intended to quickly stop outflow from the surge tank and only occurs if the protection module is not working properly.
Under certain conditions, the module allows the plant to operate without restrictions. Those conditions are:
- When water in the surge tank is above a pre-determined level, meaning there is no risk of sudden emptying of the tank; and
- When headwater levels are sufficiently high that there is no risk of drainage of the surge tank.
Installing and testing the module
Before installing the module at Luenersee, university researchers conducted numerical simulations of the behavior of the module. The simulations followed a specific load sequence: increasing from no load to 75 percent load in 100 seconds, followed by a sudden load rejection and again increasing to 75 percent. This sequence was chosen to mimic typical load requirements of the facility. With the module turned off, the surge tank would be emptied after about 12 minutes, and the units would have to be taken off line (see Figure 1 on page 43). With the surge tank protection module on, the turbines are shut down for about 2 minutes. This shutdown reduces outflow from the surge tank.
Once the automatic surge tank protection module was installed at Luenersee, Vorarlberger Illwerke applied load cases similar to those in the numerical simulation, concluding that the module worked as predicted.
— By Klaus Hirtenlehner and Albert Ruprecht, Dr.-Ing. Mr. Hirtenlehner may be reached at Vorarlberger Illwerke AG, Batloggst. 36, Schruns A-6780 Austria; (43) 5556-70186315; E-mail: firstname.lastname@example.org. Dr. Ruprecht may be reached at Institute of Fluid Mechanics and Hydraulic Machinery, University of Stuttgart, Pfaffenwaldring 10, Stuttgart 70569 Germany; (49) 711-68563256; E-mail: albert.ruprecht@ihs. uni-stuttgart.de.