The main objective of this master thesis is to design a power plant controller for a photo- voltaic (PV) power plant. In a first stage, the current situation of the status of the electrical grid is analysed. The electrical network structure is moving from a conventional system (with centralized power generation, unidirectional power ows, easy control) to a smart grid system consisting on distributed generation, renewable energies, smart and complex control architecture and al- gorithms, bidirectional power ow, etc. Some of the issues regarding the integration of renewable power plants are identified. The increase of renewable penetration into the electrical system can produce instabilities in terms of voltage and frequency. So that, the operation of these power plants must have grid support functionalities. To design a power plant controller for providing PV plants with the capacity to give the required grid support, it is necessary to use a model of a PV power plant. So that, the modelling of the PV power plant is presented. This model contemplates the PV cell, the PV array, the power electronics (PV inverter) and the associated control, the grid layout as well as ancillary devices as FACTS, capacitor banks or energy storage. Once the power plant is modelled, a central controller is designed to coordinate all the PV plant devices in order to deliver the desired active and reactive power to the grid. The controller is capable to perform the following functions: active and reactive power setpoints, voltage and frequency droops, power factor control and ramp rate limitations. These actions are the required by most of the grid codes. The power plant controller is validated with the support of simulations. After that, it has been identified the active power ramp rate problem when the plant is not equipped with auxiliary generation: when the available active power decreases suddenly (for example due to the clouds passing over the PV plant), the ramp rate limitations of active power cannot be respected. So, the installation of an energy storage system has been studied. Considering the PV plant with energy storage system, two control strategies for limiting the active power ramp rate are analysed. After that, possible effects that communication delays can produce are studied and a little modification the controller is proposed in order to improve the PV plant operation.