Nant de Drance – Interview with Mr. Florian Brantschen, NDD

An exclusive insight into a tp installation
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About NDD

NANT DE DRANCE (NDD), the European flagship of the Hydro Power Plant industry, equips its 6 Generator Transformers with the SERGI TRANSFORMER PROTECTOR (TP) in Switzerland.


More than 400 workers, around 10 years of underground construction work in high mountains,17 km of tunnels and 1.7 million m³ of excavated rock. The Nant de Drance construction site is gigantic.

The result is one of the most powerful Pumped Storage Hydro Power Plant (PS HPP) plants in Europe. With an installed capacity of 900 MW, it will generate around 2.5 billion kWh of electricity per year after commercial operation in 2019.

We were able to interview Mr. Florian Brantschen, Electrotechnical Manager of the whole NDD Hydro Power Plant and Mr. Jürgen Wendorff, Project Manager at ABB AG – Transformatoren.

How long have you been working at Nant de Drance?

I have been working at NDD for six years.

When did the project start?

After several years of variant studies and detailed planning, construction work began in 2008 with the excavation of the access tunnels.

Were you there from the beginning or did you join later? What is your function?

No, I joined the project team at a later phase in 2013. The construction work was already well advanced and the main components of the power plant – the pump-turbines and the Generator Transformers – had already been ordered.

My function is to manage the entire electrical equipment of the NDD Hydro Power Plant. In this function I work closely with the Power Plant Planner AF-Consult and the suppliers of the electrotechnical power plant equipment.

Can you tell us more about the Emosson pumped storage hydropower plant? To what extent is this power plant important for Switzerland?

NDD is a holding company of several energy entrepreneurs. The company was founded to build and operate a Pumped-Storage Hydro Power Plant (PS HPP). The new power plant is integrated into the existing Emosson Power Plant complex on the Swiss-French border. Several hydropower plants have been producing electrical energy here for almost 100 years.

The new PS HPP uses the gradient difference between the two existing reservoirs Vieux-Emosson and Emosson. The available gradient varies between 250 and 390 meters. In the PS HPP, there are 6 pump-turbines of 175 MVA power, which are connected to the European extra-high voltage grid of 400 kVolt via one Generator Transformer of the same power.

The whole power plant is in the mountain. It mainly consists of an underground transformer and a machine cavern with the impressive dimensions of 190 x 53 x 35 m. The PS HPP is located in the mountain. The caverns are covered by a 600 m thick rock massif. The access tunnel to the powerhouse has a length of 6 km.

The PS HPP, with its installed capacity of 900 MW, is to play an important role in the liberalized electricity market in the future, both for a secure energy supply in Switzerland and for the grid regulation of the European extra-high voltage grid.

The motor generators are variable-speed and can adapt their output over a wide range to the electricity market requirements very quickly with optimum efficiency in both pump and turbine operations.

The energy stored in the upper storage basin is mainly intended to compensate the volatility of the energy generation from the wind and solar power plants. In addition, the rotating masses of these large PS HPP generators have a stabilizing effect on the transmission grid and are therefore very important for system services.

Can you explain to us why NDD transformers were supplied with the TRANSFORMER PROTECTOR (TP)?

NDD transformers are installed in masonry transformer cells and are equipped with all classic mechanical and electrical protection devices. The individual transformer cells are equipped with conventional gas fire extinguishing systems. According to our safety regulations, such protective equipment would be enough. However, since the Generator Transformers with their high fire capacities in the cavern deep in the mountain have a high damage potential, our planner proposed the TRANSFORMER PROTECTOR (TP) explosion prevention concept from SERGI as additional safety.

The main purpose of the TP is to prevent the transformer from exploding in the event of an internal fault and, as collateral damage, from damaging the transformer cell to such an extent that the gas fire extinguishing system can no longer function correctly.

Therefore, the extremely fast reacting SERGI TP is decisive for our purpose. A special feature of our plant is that we cannot transport the explosive gases outside due to the long distance and therefore have to evacuate them in a controlled environment in the tunnel system.

How did you get to know the company SERGI TRANSFORMER PROTECTOR (TP) and how was the cooperation with SERGI all these years?

The TP has been known to us for a long time through various professional articles and scientific publications. We were able to visit a concrete installation in the neighboring Emosson power plant Vallorcine on French soil. There the system had been installed due to a specification of the insurance company.

Specifically, we did not work directly with SERGI because the protection system was integrated in the transformer delivery. We therefore had the transformer supplier ABB as our primary contact.

Who would you recommend the TRANSFORMER PROTECTOR to?

I would recommend the TP to those operators who must reckon with extremely high consequential damages when their transformer explodes.

Thank you very much for your time and your detailed answers.