How can I model two transformers controlling the same busbar voltage?
This can be modelled using a master-slave mode available in PowerFactory.
First of all, both transformers shall have a type which includes settings for the tap changer. In addition, the load-flow options for the automatic tap changer control must be active.
Then the transformer element settings have to be defined accordingly: one of the transformers is the master, which means that it sets its tap position. The slave immediately follows, which means that it takes over the tap position of the master. The parameter which defines the controlled node in the slave shall be set to "EXT", with a reference to the tap controller of the master.
In the attached example the transformer on the left side is defined as the master, and it controls the voltage at the LV bus. The transformer on the right side is the slave.