Impacts of Solar Photovoltaic on the Protection System of Distribution Networks - A case of the CIGRE low voltage network and a typical medium voltage distribution network in Sweden

Abstract

Solar energy has a massive potential to contribute towards energy security and meet some of the world’s electricity demands. It is a clean and renewable form of energy. There is a lot of research going on in the area of solar. One of the research topics is the integration of solar Photovoltaic (PV) to existing electricity grids. Existing distribution grids are normally radial in nature and have a single direction of current flow from the supply to the customer end. Overcurrent protection coordination for passive radial networks is based upon single direction power flow. When distributed generation (DG) is present in these networks, the current flows from the customer end as well. This current flow distorts the original overcurrent protection coordination by increasing/reducing fault current level and direction of the current flow. This thesis studied the impact on the overcurrent protection coordination of a distribution grid caused by solar DG. A low voltage distribution network and a medium voltage distribution network are modelled and simulated using PSCAD/EMTDC. The performance of the overcurrent protection was studied in both the presence and absence of solar PV. Some of the identified impacts due to the presence of solar PV are false tripping of feeders, nuisance trippings, blinding of protection, and unwanted islanding. The hosting capacity for the medium voltage network was determined to be around 62%. Solar penetration levels of 10% and above caused unwanted tripping for upstream faults. For downstream faults, there was a sharp rise in tripping times at solar penetration levels of between 30 to 62%. It is found that when the DG unit is protected against uncontrolled islanding, the impacts are mitigated. It was found that direct transfer trip, rate of change of frequency and under voltage protection provide good protection for grids with DG units.