On September 12, 2019, a pair of SEL-T400L Time-Domain Line Protection relays detected a B-phase-to-ground fault while protecting the Public Service Co. of New Mexico (PNM) 109-mile 345 kV transmission line from San Juan to Cabezon.
The fault occurred on a transmission structure located 42 miles from the San Juan substation. Figure 1 shows the voltage and current signals sampled at 1 MHz captured by the SEL-T400L at the Cabezon substation.
Both SEL-T400L relays successfully tripped their respective circuit breakers. The Cabezon SEL-T400L tripped via the traveling-wave (TW) differential (TW87) scheme, the scheme operated in 1.56 milliseconds, and the fault cleared in 24.36 milliseconds — less than 1.5 cycles at 60 Hz.
The San Juan SEL-T400L tripped via the incremental-quantity ground distance (TD21G) element (TD21G asserted) in 2.21 milliseconds, and the fault cleared in 23.53 milliseconds (1.41 cycles). As a result of the extremely short SEL-T400L trip time, the actual fault-clearing time is essentially the time for the breaker to open, which in this case was under 2 cycles.
In addition to the SEL-T400L relays, PNM applies SEL-411L Advanced Line Differential Protection, Automation, and Control System relays at each terminal that also detected the fault event.
Figure 2 shows the voltage and current signals recorded at the Cabezon terminal along with the performance of the SEL-T400L and SEL-411L elements and schemes.
The TW directional forward (TW32F) element asserted in 63 µs, the incremental-quantity directional forward (TD32F) element asserted in 1.16 milliseconds, the TW differential (TW87) scheme asserted in 1.56 milliseconds, the permissive overreaching transfer trip (POTT) scheme asserted in 1.96 milliseconds, and the incremental-quantity ground distance (TD21G) element asserted in 8.56 milliseconds. The SEL-411L line differential (87OP) element operated in 12.72 milliseconds.
In other words, the SEL-411L tripped 11.16 milliseconds slower or took 8 times longer than the SEL-T400L at this terminal.
In the SEL-T400L at the San Juan terminal, the TW87, POTT, and TD21G elements operated in 2.31 milliseconds, 2.31 milliseconds, and 2.21 milliseconds, respectively.
Figure 3 shows the TWs captured by the SEL-T400L relays at San Juan (black) and Cabezon (blue). We observe that the first TW arrived at San Juan terminal 137.84 µs before the first TW arrived at Cabezon terminal.
This corresponds to a fault location 42.049 mi from San Juan and 67.271 mi from Cabezon. The TWs recorded at the two terminals are in phase, as we expect for an internal (on-the-line) fault or low-energy event.
As these event reports confirm, the SEL-T400L relays provided the expected significantly faster fault-clearing time at both terminals.
