Effect of Inverter-Interfaced Renewable Energy Power Plants on Negative-Sequence Directional Relays and a Solution

Owing to poor overcurrent capacity and high controllability, inverter-interfaced renewable energy power plants (IIREPPs) exhibit exclusive fault characteristics, which are different from those of conventional synchronous sources. This may cause negative-sequence directional relays (NSDRs) of the lines connected to IIREPPs to malfunction. In this study, a methodology for categorizing lines as type-I or type-II is introduced. The influence of IIREPP on the NSDRs of type-I and type-II lines is thoroughly investigated based on the equivalent negative-sequence impedance expression of the IIREPP. Theoretical analysis shows that the NSDRs of the type-I line have a high risk of misidentifying the fault direction; however, the NSDRs of the type-II line can distinguish between forward and reverse faults. An improved control strategy is presented to ensure the correct operation of all the NSDRs. In this strategy, the grid-connected inverters inject negative-sequence current with a limited amplitude and specified phase angle to assist the NSDRs in the determination of the fault direction. The simulation results reveal that when the proposed scheme is adopted, all the NSDRs of the type-I and type-II lines can correctly and sensitively identify the fault direction and operate appropriately under different fault resistances and various types of asymmetrical faults.