INVERTER-BASED DISTRIBUTED GENERATORS, MODELING AND CONTROL

In the last few years, electrical power generation is moving from a centralized generation towards distributed generation including on-site Renewable Energy Sources (RESs). In order to overcome RES related issues, as intermittent and fluctuating power, storage elements are included in the on-site generation system. The interaction between distributed generator (DG) units including RES, storage and front-end power converters should be carefully modeled in order to analyze the power system stability. An accurate modeling of DGs is required. In this thesis, a new modeling approach has been proposed for the inverter-based DGs based on the concept of electrostatic synchronous machines. Detailed mathematical expressions have been provided to model the inverter-based DGs. The equivalence between the proposed model and the conventional small-signal model of inverter-based DGs has been clearly stated based on the duality concept. According to the proposed model, each inverter can be replaced by its equivalent electrostatic machine model and the analysis of the whole microgrid can be performed. A performance comparison between the proposed model and the small-signal model of the VSIs has been carried out. Time domain simulation results showed a perfect matching between the two models and thus, validate the proposed modeling approach. As a result, the analysis of large and small-signal stability of microgrids with multi inverter-based DG can be performed by exploiting the swing equation of virtual electrostatic synchronous generators. A modified nonlinear droop control method for three-phase VSI DGs working on islanded LV microgrid has been proposed. The nonlinear droop regulation is based on off-line minimum distribution losses Optimal Power Flow with a new plug-and-play implementation technique, which has been established by constructing a lookup table for the optimized P vs. f droop slope of the VSI DG with the highest power capacity. A comparison between the conventional droop and the proposed nonlinear droop regulation has been carried out. Time domain simulation results showed the advantages of the nonlinear droop regulation method in terms of the distribution line power losses mitigation as well as enhancing the transient response of the system variables.