ONAN/ONAF switch feature added

[LunaJansma]

A new feature is added to the transformer thermal model: it is now possible to add an ONAN/ONAF cooling switch. This reflects better how transformers operate under varying load conditions. Until now, the model assumed a fixed cooling regime, but with this update, the switch between ONAN and ONAF can be simulated.
There are two possibilities for the cooling switch:

1. Based on temperature thresholds: the 'switch' is turned on when the model reaches the activation temperature, and switches of when the model reaches the deactivation temperature
2. Based on fan data: if available, a fan schedule can be inputted, showing at every timestep if the fan is ON or OFF.

The feature was validated using one measurement dataset of a power transformer, with the fan (de)activation temperatures estimated from the data. From this validation, we concluded that the switch works well on a high level, keeping the top-oil temperature mostly within the (de)activation temperatures. Some modelled temperature peaks still occurred, but the measured temperature never exceeded the modelled temperature. On a timestep level, some “ping-pong” behaviour showed up in the modelled temperatures, which does not appear in the measurements. See the plot below.

So based on our validation, the switch works well to model temperature on a high level, but timestep-level results should be used with some caution. Also, the switch has not been validated with an inputted fan schedule.
An example notebook on how to use the switch can be found in the examples folder, or via this link: https://github.com/alliander-opensource/transformer-thermal-model/blob/feature/cooling_switch_temperature_based/docs/examples/example_ONAN_ONAF_switch.ipynb

Please let us know how the new feature performs for you and if you have any questions or remarks.