Influence of Inverter-Interfaced Renewable Energy Generators on Directional Relay and an Improved Scheme

Renewable energy sources are typically interfaced to the grid using power electronics, which can cause their fault current characteristics to display significant low-frequency harmonics and unbalanced sequence impedances. Such current characteristics can lead to the operation failure of fault-component-based directional relays. To demonstrate the influence of inverter-interfaced renewable energy generators (IIREGs) on directional relays in detail, analytical expressions for the IIREG equivalent positive- and negative-sequence superimposed impedances are derived in this paper. Considering various factors, the angular characteristics of the sequence superimposed impedances are investigated. Based on these attributes, it can be concluded that fault-component-based directional relays may be unable to operate in some circumstances. A novel high-frequency impedance-based protection scheme is proposed to manage the adaptability problem by determining the fault direction due to a stable impedance angle. The theoretical analysis and the proposed scheme are tested and verified through real time digital simulation and field testing data.