Error creating Branch of type PowerTransformer

[martinmoraga]

When a branch of type PowerTransformer is created, there is a bug in the function _get_primary_connection. This function shoud not return the PowerTransformerEnd with the higheist nominal voltage but also the PowerTransformerEnd with r>0 and x>0. Independently of the nominal voltage of the PowerTransformerEnd, the parameters x, r can be equal to zero, which results in a singular admittance matrix.