About Sample6_selinc.png

About this drawing

What it is: Protection scheme figure (SEL paper, Fig. 6): turn-to-turn fault protection for a grounded 345 kV shunt reactor, fed via breaker 52RX, with CTs CT-RXH (1200:5 C800), CT-RXN (1200:5 C800) and CT-RXG (600:5 C400), bus VT, and relays: (a) 67Q negative-sequence directional element supervising 50G; (b) 67N zero-sequence directional element (Option #1 using VT + CT-RXH, Option #2 using VT + CT-RXG) supervising 50G.
What it does: Detects turn-to-turn faults inside the shunt reactor and trips breaker 52RX, using a directional element to supervise the neutral overcurrent (50G) so it operates only for internal faults.
Analyse next: Build a fault tree with top event "protection fails to trip 52RX for a reactor turn-to-turn fault" (VT/CT failures, relay element failure, loss of directional-control permissive, DC/trip-circuit and breaker failure), and a companion "spurious trip" tree for external faults, VT loss, CT saturation; an FMEA of the scheme comparing options (a), (b)#1 and (b)#2 would also fit. An RBD of the reactor supply path (345 kV bus – 52RX – reactor) could be a later, minor option.
The drawing's function is to trip, not to supply: the relays (67Q/67N, 50G) and instrument transformers form an AND-supervised logic chain (directional control AND overcurrent) whose failure modes are fail-to-operate and false-operate. That is fault-tree/FMEA territory. The only power path (bus → 52RX → reactor) is a single series chain feeding a reactive compensation device, not a load whose continuity is the question, so a supply-path RBD would not answer what this figure is about.

An RBD doesn't fit this drawing, so none was built. The next step above is a recommendation for the engineer: we only build RBDs.

What we still need

Trip path (relay outputs → 52RX trip coil, DC supply): The figure shows only the measurement inputs and the directional-control link. It does not show the trip output contacts, trip coil(s), DC battery/supply, lockout relay, or breaker-failure scheme. These are needed for a "fails to trip" fault tree.
Scheme option chosen (a) 67Q vs (b) 67N Option #1 / #2: The figure gives alternatives. Which one is installed (and whether relays are in one multifunction IED or separate devices) decides which CTs/VT are in the logic and whether failures are common-cause.
Other reactor protection / backup (87, 50/51, 63 sudden pressure): Other reactor protection and backup are not shown. Without them you can't tell whether this scheme is the only turn-to-turn protection or one layer among several, which changes the top-event definition.
VT loss handling and CT details: The figure doesn't show loss-of-potential (LOP) supervision, the VT fusing, or CT saturation and polarity checks. These drive the spurious-trip and blocked-trip modes of the directional element.

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Sample6_selinc.png

draft no RBD r_20261101_165643_e999 · 2026-11-01 16:56:43 UTC

An RBD doesn't fit this drawing: another analysis is recommended (see About this drawing)

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