A lot of hot air – for nothing? The CFD simulation shows why the nacelles of wind turbines do not always get rid of their heat without problems

We all know it from our childhood when playing with the electric train: The transformer gets quite warm and sometimes it burns out.

The situation is similar for wind turbine transformers, which also become warm.

Only the amount of heat is slightly higher and the damage greater…

The transformer unit of a wind turbine is located in the rear part of the nacelle and has its own cooling air supply.

Figure 1: Nacelle of a wind turbine

During operation, the power losses in the coils cause the transformer to heat up and the resulting heat is released into the ambient air.

In order to maintain the permissible operating temperature of the transformer elements and ensure an uncritical room temperature level, the cooling must be sufficiently dimensioned. If the transformer unit consists of several closely arranged coils, the influence of radiation cannot be neglected and individual locations with increased temperatures may occur.

If the cooling is carried out only by free convection and there is no influence of adjacent coils, an analytical estimation of the cooling properties may suffice under certain circumstances.

On the other hand, if the transformer is actively cooled by several fans, an analytical estimation is no longer possible due to the complex flow field. In this case a thermal analysis of the cooling by a numerical flow simulation is suitable. The use of CFD simulation (Computational Fluid Dynamics = flow simulation) offers the advantage that the complete transformer space in the nacelle including radiation can be considered.

Figure 2: CFD simulation with flow lines in the transformer with active cooling

The fans attached to the underside of the transformer suck air from the surroundings into the nacelle. The positioning of the fans is such that the cooling air is blown directly into the individual annular gaps of the coils to ensure adequate cooling. The heated air is then discharged back into the environment at the top of the nacelle.

When the fans are switched off, the transformer is only cooled by free convection and the coils heat up considerably. Due to the narrow arrangement of the coils, the middle coil experiences an increased heat load compared to the outer coils, as the temperature distribution on the coil housing in Figure 3 shows.

Figure 3: Temperature distribution on the bobbin housing

On the other hand, when cooling is active, the fans on the transformer unit allow a significant reduction in the temperature of the coils (see Figure 4).

Figure 4: Temperature distribution with active cooling

Figure 5 shows the velocity distribution in the annular gap of a coil on different cutting planes.

Figure 5: Speed and temperature distribution on average with active cooling

Already at the outlet planes of the fans there is an inhomogeneous distribution of the velocity, which shows the advantage of CFD simulation in the analysis of such problems.

Now it is possible to optimize in a targeted manner so that no heat nests occur without prototypes having to be built or problems even occurring during use.

After all, nothing is more annoying than if the transformer burns out or you burn your fingers on it… which brings us back to the electric train.

Do you also have to solve similar thermal problems? Something gets too hot and you don’t know if your cooling concept works?

Whether it’s a coil, a battery, a circuit board or the toy transformer of a Märklin railway, we know the physics behind it and can offer you solutions.

Arrange a non-binding appointment for your CFD simulation.

Your Stefan Merkle

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