In this paper the zone controller is presented which regulates the reactive power-voltage characteristic (Q-V) of the zone pilot node by altering the reactive power outputs of power plants within the zone. As far as the zone is self-sufficient regarding reactive power needs, the generation is adapted to fit consumption and quasi-independent operation is achieved. As soon as limits are reached the import/export of the zone has to be permitted and voltage is maintained along the defined Q-V characteristics while providing reactive support to surrounding zones. Reactive power is distributed among power plants according to equal reactive reserve margin so maximal voltage stability margin is maintained. The result is minimization of losses due to minimization of reactive power flows among the zones and thus across long distances. Finally simulation results of the proposed controller are given in order to present the controller response in the quasi-independent operation mode.