When you're driving from central Slovenia towards the mountains, the Gorenjska highway crosses the valley of the Sava river. From the viaduct you can see a high dam wedged between sides of a narrow gorge. Behind it is the water reservoir for the Hidroelektrarna Moste.
I've been watching this dam every time I was traveling on that road. For years I wanted to see it up close and the power plant below it. My wish came true when my mum got a contact at the plant and arranged for a guided tour. So my parents and I drove up to Žirovnica last week and visited the facility.
To my surprise, turbines and the generator hall of the power plant are actually located quite a bit down river from the dam, not directly underneath it as I expected. Water from river Sava is routed through a tunnel under the nearby town Moste, from which the facility also got its name.
What you see above is the steep railway with which heavy machinery has been lowered into the valley during construction.
This safety valve on the high-pressure pipe in the tunnel was the first impression of the scale of engineering of this place.
The valve is designed to operate in a fail-safe manner: in case of an emergency, it uses stored potential energy without the need for any external power. The huge red weight shuts off the water supply if electric power is lost or if sensors detect flooding in the facility. Later on in the generator hall we saw emergency buttons similar to fire alarms that would manually trigger it.
Behind the valve is a huge vertical surge tank embedded in rock. Its purpose is to absorb the kinetic energy of the water in the pipeline which could cause damage if the valve would suddenly close.
Even though around 20 m3 of water were flowing per second through the large pipe, there was surprisingly little sound to be heard.
The site is actually home to two power plants:
In 1915 Hidroelektrarna Završnica was built here to become the first public power plant in the area. It used water from the river Završnica which was routed from a reservoir higher up in the mountains and discharged into the river Sava.
Later on, river Sava was dammed in 1952 and Hidroelektrarna Moste built alongside the old power plant. Now HE Završnica is no longer in use and is being slowly converted to a museum. Its water supply however has been routed to one of the turbines in HE Moste and is still used intermittently to generate electricity.
This is one of the three 10 MW Francis turbines currently in operation. In contrast to the quiet flow of water in the pipeline, turbine wells were noisy enough that you had to shout to talk to someone.
While we were visiting, one of the generators had to be stopped because of a problem with one of the oil pumps. Our tour was put on hold while repairs were made. After the pump was fixed, the turbine was spun up, generator synchronized with the grid and control of the power plant handed back to the remote operations center in Ljubljana.
This is one of the modern hydraulic turbine speed governors that are currently in use in the power plant.
All aspects of the power plant are remotely monitored using a SCADA system. Status of every temperature sensor and pump can be accessed from a central control room. Apart from electrical parameters they also measure and record structural status of the dam and its surroundings. They have automatic strain gauges and a grid of reference points where geometers regularly check for any unexpected earth movements.
In theory the plant could be operated entirely by remote control, but we were told that the company will continue to keep the site staffed round-the-clock. Problems can be detected and solved faster and cheaper if experienced engineers are around. Otherwise a maintenance team would have to be dispatched from somewhere else, which could lead to more downtime or a small problem growing into a more expensive one.
In conclusion, I would like to thank the friendly staff of HE Moste for taking the time for us in a busy day and Mr. Pirjevec for guiding us on this tour. We were shown every nook and cranny of the power plant and he gave us a thorough description of the machinery and answered every question we had.
It was a pleasant and informative way to spend a morning and a welcome step away from all the low-power electronics I deal with each day.