Experience In The Design of Custom Protection Schemes For Ungrounded Transmission Lines Interfacing Grid Forming Converters

The increasing penetration of Inverter-Based Resources (IBRs) challenges conventional protection schemes, and the calculation of settings needs to consider that the fault behavior from these sources differs from that of conventional systems governed by synchronous machines. The fault response of IBRs is dictated by control laws, control time constants, thermal limits of switches, and vendor-specific design. Such aspects are not usually fully disclosed by the IBR vendor, which poses challenges to the utility engineers in charge of maintaining these systems. The main issues are related to inconsistent negative-sequence current injected by IBRs and fast frequency variations of these sources. To overcome these challenges, new designs of protection elements have been extensively developed in professional literature, either by implementing time domain-based functions or by improving methods to track fast frequency variations in the power system. However, most of these developments cover typical power systems configurations, and little is discussed about power systems with non-typical topologies fed by inverter-based resources. This paper presents the experience of designing custom protection schemes for an island in Hawaii predominantly fed by inverter-based generation. One end of the transmission line under study is fed by a Grid-Forming (GFM) 14 MVA IBR, while the other end is fed by somewhat weak conventional sources. This power system has two non-typical aspects: 1) A Dyn power transformer with the delta connection on the transmission level, 2) Short transmission lines. This paper presents improvements on local phasor-based protection functions applied for ground and phase faults detection. Local protection plays an important role when communications channels are compromised or offline, thus preventing the use of line current differential protection. Furthermore, they also provide backup zones for adjacent lines in the system. Insights into protection challenges and enhancements to protection functions and settings are provided for utility engineers. The proposed solutions include the use of residual overvoltage elements for ground fault detection and an offset quadrilateral relay for phase fault detection. This paper contributes to sharing experience on designing custom protection schemes for local fault detection in non-typical power systems with GFM IBRs and settings philosophies to improve protection security and dependability.