About Sample2.png

About this drawing

What it is: Transformer protection single-line diagram (Fig. 2) for a 765/500 kV autotransformer with a 34 kV tertiary (B3T): breaker-and-a-half style connections (CB7-1/CB7-2 to 765 kV Bus 1/Bus 2, CB5-1/CB5-2 to 500 kV Bus 1/Bus 2). It shows CT and relay assignments for two independent protection systems: System A (blue, 87TA/O) and System B (red, 87TB/B7-REF, 87TB/B5-B3, 87TB/T). It also shows the 49T/RTD thermal inputs, the 63 sudden-pressure relay, and the breaker-failure-initiate (BFI) links.
What it does: It detects transformer and bus-connection faults (differential 87P/87Q, REF, overcurrent 51/50P, tertiary ground 51G, overvoltage 59, V/Hz 24, thermal 49, sudden pressure 63), trips the 765 kV and 500 kV breakers to isolate the transformer, and starts breaker-failure protection (50BF) when a breaker does not open.
Analyse next: Build a fault tree for the top event "transformer fault not cleared" (fails to detect or fails to trip): System A and System B as redundant branches, each with its CT inputs, relay, battery/DC supply (note 2), and trip and BFI paths, plus breaker-failure backup. Add an FMEA of the relays, CT circuits and DC systems to find common-cause failures (shared breaker trip coils, shared CTs such as the series-continued CT circuits in note 1). An RBD of the 765-to-500 kV power path through CB7-x, T and CB5-x could come later as a separate availability study, but this sheet is not drawn for that.
The drawing's purpose is protection. Almost everything on it is CT wiring, relay functions, BF initiation and battery-system separation, so the questions are whether a fault is detected and whether the breakers trip. Those are "fails to operate" questions with AND/OR logic across the two redundant systems, which a fault tree handles. A supply-path RBD ("works while one path is intact") would ignore the relays, CTs and DC systems that this drawing documents.

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

Top event / function definition: An engineer must confirm the top event, for example "transformer internal fault not cleared within X cycles" or "loss of transformer by false trip". We also need to know which faults count (internal, tertiary bus B3T, the 765 kV and 500 kV leads) and the required clearing times.
DC/battery systems A and B, trip coils (note 2, note 4): The battery systems, the breaker trip coils (single or dual per breaker) and the trip and BFI wiring are not shown ("BFI links are not shown except where required"). These decide how independent System A and System B really are, and so the common-cause terms in the fault tree.
Breaker-failure backup (50BF-7-1/7-2/5-1/5-2), adjacent breakers on Bus 1/Bus 2: The breakers that 50BF trips (the middle and bus-side breakers of the breaker-and-a-half bays and the bus protection) are off this sheet. They are needed to model backup clearing when a CB7-x or CB5-x breaker does not open.
CT circuits and shared instrument transformers (note 1, VTs at B7T, Ipol circuit): We need to confirm which CT cores and VT windings are shared between functions or between systems. Examples are the series-continued CT circuits in note 1, the B7T CCVT secondaries, and the 87TB/B5-B3 and 87TB/T inputs. Each shared item is a possible single point of failure for detection.
Device reliability data and test intervals: The sheet gives no failure rates, self-monitoring coverage, or periodic test intervals for the relays, CTs and breakers. A fault tree needs these to quantify the probability of failure on demand.

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Sample2.png

draft no RBD r_20261101_165547_e383 · 2026-11-01 16:55:47 UTC

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

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