Radial Distribution Grid Research Articles

Integration of large-size photovoltaic systems into the distribution grids: a P–Q chart approach to assess reactive support capability

This study addresses the issue of the control of both active (P) and reactive (Q) power flows injected by grid connected photovoltaic (PV) units participating in the provision of the reactive power compensation ancillary service. A model is derived allowing to define a sort of ‘capability chart’ for the PV unit, that is, the set of points in the P–Q plane which, at steady state, can be reached by properly operating the control system and without exceeding the physical limits of all the involved devices. To this aim, a basic algorithm is developed in the MATLAB environment relying on a simplified description of the system, which neglects the harmonics injected by the voltage source converter inverter used to connect the PV unit with a medium voltage radial distribution grid. The proposed model is used to evaluate all the possible working points in the P–Q chart; a proof of the stability of those points, using the well-known Lyapunov methods, is also annexed. Finally, simulations with the electromagnetic code PSCAD–EMTDC are performed in order to assess the validity of the approximate results and to improve the developed basic algorithm.

Photovoltaic Generating Units as Reactive Supply Ancillary Service Providers

This paper addresses the issue of the application of PhotoVoltaic (PV) Distributed Generation (DG) units for supplying reactive power to the grid. In order to reach this aim, an advanced control scheme to suitably drive the injection of both active and reactive power into a MV radial distribution grid has been developed. Such algorithm allows a decoupled control of active and reactive power by properly adjusting the modulating signals of a PWM interface converter with the grid. A closed loop regulation scheme has been derived and guidelines for the choices of the reference quantities and for the design of the regulators are provided. The control algorithm has been applied to a real MV grid-connected PV power plant, which has been simulated in the PSCAD – EMTDC electromagnetic environment, in order to represent, with high detail, every system component and to thoroughly analyze the occurring electromagnetic transients. The positive effect of reactive power support to the grid from the PV unit has been highlighted, showing benefits on the grid current and voltage profiles with respect to the classical configuration, where the DG system behaves as a sole active power source.