Bill of materials
The bill of materials (BOM) lists the equipment and materials of the substation that the design calculates, with a specification, a unit, a quantity and the spares of each line. A line's quantity is taken from another step of the design: the single-line diagram (SLD), the yard layout, the earthing, the lightning protection or the auxiliary supply. A line names that source, so a quantity can be traced to the drawing or calculation behind it.
Read first: Bays and their equipment
1Sections and columns
The BOM of a new 220 kV project has 40 lines in these sections: HV switchyard, Transformers, 33 kV switchgear, SVG and filters, Structures, Earthing, Lightning protection, Auxiliary supply and Civil. A switching substation adds a lower HV switchyard section after the first. A section appears only when the design has lines for it, and a line appears only when its quantity or its spares are above zero.
In the workbook a line has the columns S.No, Item, Specification, Unit, Design qty, Spares, Total qty and Source in the design, then the makes. The total is the design quantity plus the spares. The specification states the ratings the design chose for the item, for example the highest system voltage, the lightning impulse withstand level (BIL), the short-time current and the rated current of the HV switchgear (Single-line diagram (SLD), The ratings on the SLD).
2Quantities from the design
The BOM counts the equipment of the built bays. The HV switchgear is counted from the bays of the yard layout: circuit breakers (CBs), isolators with and without earth switches, current transformers (CTs), voltage transformers and surge arresters. A future bay is drawn on the SLD and kept in the layout, and the BOM leaves its equipment out (Bays and their equipment, Future bays). The CTs, the capacitor voltage transformers (CVTs) and the surge arresters are single-phase units, 3 per position.
The buses are counted from the layout. A tubular bus is supported on bus post insulators (BPIs) at 9 m spans, with one more insulator than spans on each tube. The aluminium tube is the length of the main buses plus the bay interconnection: the length of each built bay from its first to its last piece of equipment, times 3 phases.
The transformers, neutral grounding resistors and MV switchgear panels come from the SLD. The structures are counted from the equipment they carry: one line gantry per line bay, one support per single-phase unit, one isolator support per pole and one support per BPI. The shield-wire towers and the lightning masts come from the lightning protection design, and the shield wire is its length with an allowance for sag and jumpers (5 wires + 3 % sag and jumpers).
The earthing lines are the quantities of the earth mat design: the buried conductor, the electrodes and the risers (Conductor sizing). The civil lines come from the layout: the road area, the fence's perimeter, the building's floor area and the gravel. The gravel is the earth mat's area times the depth of the surface layer (Surface layer and Cs).
Given a new 220 kV project with the default inputs and its design run. The future bay is in the layout and not in the BOM.
| Item | Unit | Design qty | Spares | Total | Source in the design |
|---|---|---|---|---|---|
| SF6 circuit breaker, 3-pole | set | 4 | 0 | 4 | 4 breakers (double main bus) |
| SF6 circuit breaker spare pole | nos | 0 | 1 | 1 | Mandatory spare |
| Isolator, HCB, 3-phase, motor operated | set | 10 | 0 | 10 | 10 bus isolators |
| Isolator, HCB, 3-phase, with 1 earth switch | set | 3 | 0 | 3 | 3 line / ICT isolators |
| Current transformer, single phase | nos | 12 | 3 | 15 | 4 bays × 3 phases |
| Capacitor voltage transformer, single phase | nos | 9 | 3 | 12 | 3 line / bus CVT positions × 3 |
| Surge arrester, station class, with counter | nos | 9 | 3 | 12 | 3 line / ICT / reactor positions × 3 |
| Bus post insulator | nos | 54 | 6 | 60 | 6 tubes × 72 m at 9 m spans |
| Aluminium tube, 4" IPS (OD 114.3 mm) | m | 916 | 46 | 962 | 6 × 72 m bus + 484 m bay interconnection |
Bus post insulators The yard has 6 tubes of 72 m. Each takes 9 insulators at 9 m spans, so the line counts 54.
Tube The buses take 6 × 72 m = 432 m, and the bay interconnection 484 m, so the line counts 916.
Spares 10 % of 54 insulators is 5.4, rounded up to 6. 5 % of the tube is 45.8, rounded to 46. The CT, CVT and arrester lines each take one set of 3, and the breakers one spare pole.
Result The HV switchyard section lists 4 breakers, 12 CTs and 54 insulators of the built bays, with 15 CTs and 60 insulators in total with the spares.
3Structure steel
The structure steel is an estimate in tonnes from a typical weight per structure, and its line says so. The design counts the structures and does not calculate them, so the weight of a structure is a typical figure, not the result of a structural design.
Given the structures of the default 220 kV project, counted by the BOM.
| Structure | Count | Typical weight each |
|---|---|---|
| Line gantry | 1 | 6.00 t |
| Shield-wire tower or gantry peak | 10 | 2.50 t |
| Lightning mast | 14 | 4.00 t |
| Support of a single-phase unit | 30 | 0.35 t |
| Isolator support, per pole | 39 | 0.50 t |
| Circuit breaker set | 4 | 1.20 t |
| Bus post insulator support | 54 | 0.30 t |
Result The sum of the counts times their weights is the line's 138 t: Structural steel, galvanised (estimate), IS 2062 E250, hot-dip galvanised.
4Spares
The design proposes the mandatory spares of a line:
- one set of 3 single-phase units per rating of CT, CVT and surge arrester, the transformer LV arresters included
- one interrupter pole per type of circuit breaker
- 10 % of the bus post insulators, rounded up to a whole insulator
- 5 % of the aluminium tube, rounded to a whole metre
The other lines have no spares by default. The spares of a line can be edited in the Spares column of the Bill of materials page. An entered figure replaces the design's for that line, is saved with the project, and applies at once to the table and the files, without a design run. The design quantities do not change.
Given the CT line of the default 220 kV project, 12 nos with 3 spares proposed. The user enters 6 spares.
Result The line reads 12 design quantity, 6 spares and 18 in total. The other lines and the drawings are as before.
5Units, specifications and makes
A line's unit is a count, a length in metres, an area in square metres, a volume in cubic metres or a mass in tonnes. A count is in nos for single units, in sets for three-pole or three-phase sets, and in lots for clamps and connectors. The ANSI / IEEE set converts the units, m to ft, m² to ft² and m³ to yd³, and prints ea for nos. It rounds a quantity of 100 or more in its units to a whole unit.
The ANSI / IEEE set also uses its own words: Grounding for Earthing, Station service for Auxiliary supply, and STATCOM for the static var generator (SVG).
Each line belongs to a category of makes, and the BOM shows up to 4 makes per line from the makes kept in the Library. The makes link to their makers' main websites. The BOM prints Verify the makes before use.
6What the BOM leaves out
The BOM carries quantities, specifications and makes, and no prices. It leaves out the equipment of future bays, which keep their space in the layout. It lists the clamps, connectors and spacers of an HV yard as one lot to suit the tube and the terminals. Apart from the HV cables between a gas-insulated switchgear (GIS) and its transformers, cables are not listed, since the design holds no cable routes.
A layout edit changes the quantities that follow the layout, for example the fence's length when a side of the fence moves. A drawing edit in Sketch mode changes the drawings only, and the BOM follows the inputs and the design rules.
7Files
The BOM is delivered as a spreadsheet workbook (XLSX) with a BOM sheet and a Vendors sheet, and as an A4 PDF. Both carry the document number SSL-C-BOM-001 and the document control of the project (Document control). The workbook links each make to its website. Its Vendors sheet lists the categories with their makes and websites.