Protection systems and stability of distribution networks and microgrids with distributed energy resources

The large-scale integration of Distributed Energy Resources in distribution networks has several technical implications and consequences, which increase in complexity when energy sources are of renewable type. Renewable Energy Sources are characterized by intermittent/unpredictable availability and are connected to the grid through converters, often close to the final users, which means that they are more prone to cause instability issues and potential mis-operation of protection schemes. These effects are the objects of this thesis. A protection system against earth faults in radial and meshed distribution networks with unearthed and compensated neutral is proposed and assessed. The faulty feeder identification algorithm is based on the angle between the zero-sequence voltage and current phasors, estimated at the dominant transient frequency inferred from the transient response of the network within the first milliseconds after the fault. The performances of the protection system algorithm are assessed through a Monte Carlo method that considers the fault resistance, incidence angle and fault location variations. The power system is simulated within the EMTP-RV environment, while the protection algorithm is developed in Matlab. Results of a real-time simulation obtained in the Opal-RT environment further support the applicability of the algorithm. Another important aspect of the large deployment of distributed resources are the diffusion of Microgrids (MGs) which are characterized by faster dynamics than conventional distributions systems. In this context, load dynamics considerably affect the transient stability performance of MGs. The transient stability of a medium voltage MG is analyzed in two different cases: an islanding transition and a fault when the MG is standalone. The exclusion of any rotating generator is expected to heighten the load influence on the system dynamics. Modern controllable loads are also included. The system is implemented in the EMTP-RV simulation environment, in Simulink and real-time simulations are carried out in the Opal-RT environment.