Contents I. Introduction
With the development of the Renewable Energy Sources (RES), the Power Quality (PQ) and grid stability issues are becoming more and more important, due to the intrinsic fluctuations of their power injection. The national Standards provide specifications in regards to the grid codes: [1] is a common reference for PQ in Europe. These ones concern RMS voltage and frequency variations (both slow and rapid), voltage dips, harmonic content of voltage and current signals, unbalance of the threephase systems, flicker disturbances (both short and long term), etc.. About the grid stability, Germany leads the race for the application of the suitable requirements with a norm already operating. A new Italian standard [2] was issued at the end of the previous year and the regulations will become mandatory this year. The main news deal with the ancillary services that the power electronic converters (usually inverters) will give to the grid. In particular, when the voltage dip occurs, the inverters must continue to operate without active power injection if the voltage dip is not too long. In phase of voltage recovery towards the rated value after the complete solving of the fault, the inverters must provide an amount of reactive power to magnetizing the electric machines connected to the local grid. In the past with low penetration of RES plants, when the voltage dip occurred, the inverters were switched off and restarted the operation only at normal conditions. With reference to PhotoVoltaic (PV) systems, in [3] some regulations about harmonic content provide limits referred to the Total Harmonic Distortion (THD) of voltage and current. These limits are defined at the rated power of the devices but, when the devices operate at low power levels, THD indexes increase considerably. In general, if the power rating of the inverter is not negligible with respect to the short circuit power of the grid, the harmonic content in the current injected into the grid can have an impact on the harmonic content of the grid voltage. Below a given limit of harmonic content in current (e.g. ), its influence on harmonic voltage negligible whatever is the rated power of inverter. The active/reactive power control performs the task of unitary power factor within the range 20–100% of rated power; in this interval the electronic converter compensates for the reactive power demand of the grid interface stage. Below this threshold (20–30% of nominal AC power) the reactive power demand and the harmonic distortion determine the power factor's decrease and the increase of , due to the prevailing capacitive behavior on the grid interface stage. In this paper, at first the forthcoming rules for the PV system inverter connection to the LV grid in Europe are presented, taking inspiration from the new Italian standard. In the following the fulfillment of the grid connection requirements during normal operation conditions is verified for a residential PV system with a mono-phase 3 kVA inverter and for a three-phase 230 kVA inverter, part of a large PV plant. The PQ analysis of two inverters is illustrated, taking into account some PQ issues, such as the harmonic content of voltage and current signals, the unbalance of the three-phase systems and the power factor control [4], [5]. Therefore, for a more comprehensive knowledge of the problem, another case-study of a large industrial PV plant is presented, taking into account a 103.6 kVA three-phase inverter. Here, the attention is focused on the PQ consequences of partial shading of the PV modules, in terms of three-phase unbalance, harmonics and power factor.
