IMPACTS OF GFM INVERTERS ON LEGACY DISTANCE PROTECTION RELAYS

This paper analyzes the interactions between conventional Line Distance Protection (LDP) relays and Grid-Forming (GFM) inverters. Various fault types, arc resistances and locations were investigated in a PSCAD model consisting of either a Synchronous Generator (SG) or a GFM inverter connected to the utility grid by a transformer and two transmission lines. A simple distance relay algorithm was coded in Matlab, tuned for the SG and then analyzed together with data obtained from the same system but using a GFM inverter instead. It is found that the inverter can make the relays able to trip correctly for many zone 1 faults, even if the impedance trajectory becomes unstable. However, the inverter causes several misoperations for zone 2 faults. The reasons are that the impedance trajectories either do not settle within zone 2 due to the inverter becoming unstable or they incorrectly enter zone 1. Moreover, the fault arc resistance and fault location can greatly impact the impedance trajectories obtained when the GFM inverter is connected. Additionally, operating the transmission line above its Surge Impedance Loading (SIL) also increases the risk of making the inverter unstable during faults, causing difficulties with zone 2 protection. When transferring power close to the SIL of the line, it can more reliably become stable but the fault current on the high voltage transmission line tends to be of capacitive nature and thereby making the impedance trajectories settle outside the trip zones.