Implementation and settings of negative sequence current protection in distribution systems with integrated inverters

The integration of inverters into modern electrical distribution systems has introduced new challenges in maintaining system stability and protection. Traditional protection mechanisms, such as overcurrent (OC) protection, may not be effective in detecting faults in the presence of inverters, particularly line-to-line faults where the fault current is around 85% of the three- phase fault current. This issue arises because the contribution of inverters to the fault reduces the phase current of the affected phases, leading to the potential failure of overcurrent protection relays to detect the fault, a phenomenon known as protecti on relay blinding. This paper explores negative sequence current protection as an approach to safeguard distribution systems with inverters during line-to-line (LL) and line-to-line-to-ground (LLG) faults. Our research has shown that inverters, by injecting positive sequence currents, reduce the positive sequence current contribution from the grid while keeping the negative sequence current nearly unchanged when inverters are located close to the substation. However, this current increase when inverters ar e connected further from the substation. This indicates that the negative sequence current is favourably influenced from the perspective of negative sequence relay protection. The study provides a comprehensive analysis of the principles underlying negative sequence current protection, along with the implementation and settings of such protection. Simulation results and case studies are presented to demonstrate the effectiveness of negative sequence current protection in enhancing the reliability a nd safety of distribution systems with high inverter penetration.