Hydroelectric plants get a pretty easy ride in the energy debate. Water comes down a river, spins a turbine and keeps your lights on, and most people never give the machinery inside a second thought.
Hydro plants keep that reputation right up until a turbine needs a rebuild. Then somebody has to take apart a machine that weighs hundreds of tons, fix it, and put it back exactly where it was.
EDF, France’s state-owned power company, has been doing that at Rhinau, a hydro plant on the Rhine in Alsace with Germany on the opposite bank. Rhinau’s Unit 1 spent 18 months out of service while its Kaplan turbine was rebuilt, and it’s been back on the grid since the spring.
I’m less interested in the 18 months than in how tight the fit was. EDF’s team has described the turbine’s runner block, a 243-ton assembly nearly 24 feet across, going back into its pit with only a few millimeters between the blade tips and the ring around them. That ring was brand new, too, set into the powerhouse after crews demolished the original.
So how do you get something that heavy into a hole that tight?
Slowly, I’d imagine. EDF hasn’t published a step-by-step account of the lift, so I can’t tell you exactly how its riggers pulled it off. But ANDRITZ, the Austrian company that handled the turbine work, has laid out the order of operations, and the logic is pretty clear: ring first, runner second, generator last.
So what’s a Kaplan turbine?
A Kaplan turbine is basically a ship’s propeller running in reverse. Instead of the blades pushing the water, the river pushes the blades, and the blades can twist to suit however much water is coming through.
That design makes a lot of sense at Rhinau, where the Rhine only drops about 43 feet through the plant. You don’t get much pressure out of a 43-foot drop, so you make up for it by moving an enormous amount of water, and that’s a big part of why the runner here is so wide. The new seven-blade runners going into Old Hickory Dam in Tennessee measure 264 inches across, while Rhinau’s comes in at about 285 inches.
The blades spin inside a ring-shaped liner called the discharge ring. It’s the fixed surface the blade tips sweep past, and any gap between the two is a gap water can slip through without turning anything. Which means a sloppy fit costs you power pretty much every second the unit runs, for decades.
Replacing the ring meant digging into the powerhouse
Swapping a discharge ring is a lot more work than it sounds like it would be. The Rhinau ring is embedded in concrete, so it’s effectively cast into the structure of the powerhouse. Getting the old one out means breaking up the concrete around it. Putting the new one in means setting it to a tolerance the blades will have to live with for the next few decades. Neither EDF nor ANDRITZ has said publicly what shape the old ring was in when it came out. I’d guess six decades of river water and sediment had something to do with it, but that’s my guess, and it isn’t something either company has stated.
If you read our piece on the Mactaquac dam in New Brunswick, you’ve seen what happens when the concrete around a turbine starts moving: the machine stops fitting its own housing. I’m not suggesting Rhinau had anything like Mactaquac’s swelling concrete, and nobody has said it did. Still, it’s a pretty good reminder that on a hydro plant, the concrete is basically part of the machine.
The runner block went back in first
According to the contractor, the new ring went in first and wrapped up the demolition phase of the job. Months later, the 243-ton runner block was lowered back into the turbine pit, which ended the rehabilitation work on the turbine components themselves.
That’s the lift EDF’s team was talking about with the millimeter clearances. Frankly, it’s the step I’d least want to be in charge of.
Then came the generator. Its rotor, which spins inside the generator’s windings to make electricity, weighs about 303 tons and measures nearly 34 feet across.
Don’t mix the two up. The runner block is the turbine end, down in the water, and the rotor sits up top in the generator. Put the two weights together and you’ve got roughly 545 tons of machinery spinning every time Unit 1 runs.
Unit 1 had to earn its way back onto the grid
Once everything was back together, Unit 1 still had to pass a series of tests before EDF could count on it again. The turbine supplier lists three steps: a run with no load on the generator, a first synchronization with the grid, and a climb up to 45 megawatts.
Synchronizing basically means getting the generator spinning at exactly the right speed and timing to match the grid before the breaker closes. It’s a bit like matching speed before you merge onto a highway, except getting it wrong here can seriously damage the generator. Nobody wants that.
You shouldn’t read that 45-megawatt figure as a power upgrade, though. ANDRITZ lists the runner at 40 megawatts, and neither company has presented the test as an uprate, so I’m treating it as a load test.
Rhinau is one of eight hydro plants EDF runs on the Rhine, according to the company’s guide to its Rhine hydro plants, and it has four Kaplan units in total. EDF hasn’t said publicly whether the other three are in line for the same treatment, so I’m not going to guess.
ANDRITZ, in its commissioning update dated June 2, says Rhinau Unit 1 is the third of seven refurbished Rhine units where it has worked on the discharge ring, and the first where its job covered taking the whole unit apart and putting it back together.
