Behaviour of distance protection during off-nominal frequencies in the presence of inverter-based generation

Inverter Based Generators (IBRs) operate as weak sources due to the current limitation their converter control applies during fault condition. The difference between the network short circuit contribution and the IBR one makes the apparent fault resistance increase due to the remote infeed effect. This makes the angle of the apparent fault resistance influence very much the reactance line of distance relays using quadrilateral characteristic. Grid following IBRs follow the grid frequency by using Phase Locked Loop (PLL) units. Errors in PLL units may generate currents with frequency different from the network frequency, creating rotations in the angle of the apparent fault resistance that may cause underreach or overreach effects of the reactance line. It may also generate errors in the directional units, due to the difference between the voltage and current frequency. This paper describes the problems mentioned before and proposes some improvements in both reactance line polarization, to compensate the rotation of the apparent fault resistance, and in the directional units. RTDS simulations generating PLL errors in modelized type 4 wind and PV generators are included, showing the behaviour of both conventional and improved algorithms for reactance line polarization and directional units.