The authors describe the incremental specification of a power transformer station controller using a controller synthesis methodology. They specify the main requirements as simple properties, named control objectives, that the controlled plant has to satisfy. Then, using algebraic techniques, the controller is automatically derived from this set of control objectives. In our case, the plant is specified at a high level, using the data-flow synchronous SIGNAL language, and then by its logical abstraction, called polynomial dynamical system. The control objectives are specified as invariance, reachability, ...properties, as well as partial order relations to be checked by the plant. The control objectives equations are synthesized using algebraic transformations.