Yard layout

The yard layout is the plan of an air-insulated (AIS) high-voltage (HV) switchyard: where each bay stands, how far apart its equipment is, and how the transformers, the roads, the control building and the fence surround it. The design generates it from the single-line diagram (SLD), the bus scheme and the HV level, and the yard plan, its sections, the earth mat, the lightning protection and the bill of materials (BOM) are read from it.

Read first: Bus schemes · Bays and their equipment

1The frame of the planCopy link to The frame of the plan

The plan has its x axis east along the main buses and its y axis north. The lines take off north, on the grid side, and the transformer bays run south, on the plant side. A bay column holds at most one bay north of the buses and one bay south of them, so a line bay and an interconnecting transformer (ICT) bay can share a column.

From north to south the design then places the line gantries, the HV buses, the ICT bays, an internal road, the transformer row with its pits, the ICT road, and the control building with the reactive power area beside it. A peripheral road runs round these, and the fence stands outside the road. Figure 1 shows the arrangement of the default project.

2Bay width, phase spacing and heightsCopy link to Bay width, phase spacing and heights

Each HV level has a fixed geometry: the width of a bay column, the spacing between the three phases, the gap between two main buses, and the heights of the equipment, the main bus and the gantry beam. These distances keep the electrical clearances of the level, and the yard's clearance checks compare them with the minimum clearances. A main bus is three rigid aluminium tubes on post insulators, one per phase, at the phase spacing.

The geometry of the IEC + CEA / CBIP set is the design's table per level (the table). The 33 kV row is the HV side of a 33/11 kV substation, and 765 kV is designed as a switching substation only. The along-bay factor scales the spacings of the 220 kV bay to the other levels (The order of equipment along a bay).

In the ANSI / IEEE set the geometry is derived from the clearances of RUS Bulletin 1724E-300 Table 4-7 for the level's basic impulse insulation level (BIL). The phase spacing is the larger of the rigid-bus spacing and the spacing of horizontal-break disconnect switches. The bay width is 4 phase spacings. The equipment level is the clearance to grade for personal safety, and the main bus is above it by the metal-to-metal clearance and a margin of 1 m. The gantry beam is 1.5 phase spacings above the bus, and two main buses are 2 phase spacings apart. the table lists the result, each distance rounded up as the design rounds it.

In the IEC + CEA / CBIP set the bottom of an insulator stands at least 2.5 m above ground. Above an altitude of 1000 m the design corrects the clearances by the altitude factor of IEC 60071-2, and in the ANSI / IEEE set it takes the RUS row of a higher BIL, whose geometry is larger.

3The order of equipment along a bayCopy link to The order of equipment along a bay

The equipment of a bay stands on its centre line, one phase on each phase line, in the order of the SLD. A line bay runs outward from the bus isolator. Next come the circuit breaker (CB) and the current transformer (CT), then the line isolator with its earth switch. The capacitor voltage transformer (CVT), the surge arrester and the line gantry end the bay. the table gives the spacing between each pair at 220 kV, centre to centre.

At another level each spacing is multiplied by the level's along-bay factor (the table, the table). A transformer bay mirrors the line bay to the south with the same spacings: the bus isolator, the CB, the CT, the isolator with its earth switch, and the surge arrester in the CVT's position. The transformer stands beyond the internal road (The transformer row).

4Columns by bus schemeCopy link to Columns by bus scheme

The bus scheme sets the order of the columns along the buses (Bus schemes). The default 220 kV project's circuits under each scheme of the IEC + CEA / CBIP set show the difference in the table.

• Double main bus. The two buses run east–west, 9 m apart between their nearest phases, so their centre lines are 18 m apart. The first column carries the bus voltage transformers, the bus coupler takes the column after the first bay column, and the other bays follow. North of the buses stand the lines, the bus reactor and then the future bays; south of them, the transformer bays. A bay has an isolator on each bus row.
• Single sectionalised bus. One bus runs along the yard, broken by the bus-section column between its two sections. A bus voltage transformer stands at each end, and the bays alternate between the sections. The bus isolator stands 1.5 m clear of the outer phase.
• Single bus. One bus with one voltage transformer column at its west end and no bus section.
• One-and-a-half breaker. One column per diameter after the column of the bus voltage transformers. The three CBs of a diameter stand in a row between Bus 1 on the north side and Bus 2 on the south side, each with an isolator, its CT and a second isolator.

The fence's east–west size is the same under these schemes, set by the row of the building and the reactive power area. The one-and-a-half breaker yard needs 3 columns, and its diameters make it the deepest of these yards from north to south (Diameters of the one-and-a-half breaker and the ring bus).

5Diameters of the one-and-a-half breaker and the ring busCopy link to Diameters of the one-and-a-half breaker and the ring bus

A diameter carries two circuits between Bus 1 and Bus 2. At 220 kV the centre lines of the two buses are 68 m apart. A line or the bus reactor takes off between the Bus 1 main CB and the tie CB, at a joint 5 m beyond the isolator of the CB group. It crosses over Bus 1 on a strung jumper, and its first isolator stands 2.5 m beyond the bus's outer phase. A transformer takes off between the tie CB and the Bus 2 main CB and crosses over Bus 2.

The jumper hangs at the main bus level plus the phase-to-phase clearance plus a margin of 1 m for sag and the dropper. At 220 kV that is 9.0 m plus 2.1 m plus the margin, a jumper level of 12.1 m. A diameter with one circuit is built with two CBs, and the design keeps the space of the third CB group, extended by 1.5 m at each end, for the future circuit.

The ring bus of the ANSI / IEEE set is a closed loop with a CB between each two bus sections and no main buses (RUS Bulletin 1724E-300 §4.6.4). The design lays it out as two one-and-a-half diameters, without the column of bus voltage transformers. A CB position that the ring does not build is a jumper, with the space kept for the CB that would make the yard a one-and-a-half breaker yard.

6Gas-insulated yardCopy link to Gas-insulated yard

A gas-insulated switchgear (GIS) yard replaces the outdoor buses and bays with an indoor hall along the bus axis. At 220 kV a GIS bay takes 2.0 m of the hall's length, and the hall is 12 m wide and 10 m high. An erection bay of 4 m stands at the west end, and 1.5 m is kept between the end walls and the first and last bays. The lines leave north through bushings from the sulphur hexafluoride (SF6) to air, to an outdoor CVT, surge arrester and gantry, which keep the AIS column width and heights. The transformers are fed south by a gas-insulated bus duct at 220 kV and above, and by cable below.

7The transformer rowCopy link to The transformer row

The internal road runs 2 m south of the southmost item of the HV bays, equipment or kept space, and is 4 m wide. The transformers stand south of it in a row, each in line with its bay. A soak pit extends 1.5 m beyond the tank on each side, and the pits stand 1.5 m south of the road.

The tank and radiator footprint is the size entered in Transformer length and Transformer width, or the design's indicative size. That size is 8 m plus 0.025 m per MVA along the buses, by 5 m plus 0.015 m per MVA across them.

A firewall stands between two transformers side by side whose clear gap is below 10 m. It is 0.3 m thick and extends 0.6 m beyond the tanks. The CEA Safety Regulations 2023 require separation or fire barrier walls between oil-filled apparatus above a stated oil quantity, and soak pits for its oil (Reg. 46(2)(vii)). The separation distance is the design's.

Transformers side by side keep a clear gap of at least 4.3 m: 2 m from each tank to a firewall of 0.3 m between them, after OPTCL Technical Specification E-6 for civil works. A tank longer than its bay column can come closer than that. The design then lays the run of tanks out at this gap, centred on their bays, and each tank keeps its connection from its bay's surge arresters.

A common burnt-oil pit, 5 m square, stands in the first column after the first that has no transformer and is clear of the soak pits. Where no column qualifies, it stands east of the last transformer. The ICT road, 7 m wide for the transformer trailer, runs 1.5 m south of the pits.

8The MV area, the building and the fenceCopy link to The MV area, the building and the fence

The control building stands 3 m south of the ICT road, on the plant side. With indoor medium-voltage (MV) switchgear, the switchgear hall is its west room. With an outdoor MV yard, the yard stands west of the building, 32 m deep, with one bay of 5.5 m per panel position at 33 kV. The static var generator (SVG) area and its harmonic filters stand east of the building in a fenced area. A switching substation and a generating plant have none. Roads, trenches and buildings gives the rooms and their sizes.

The peripheral road runs round the structures, its inner edge 2 m outside the outermost structure, and is 4 m wide. The fence stands 3 m beyond the road's outer edge. The live parts of the HV yard are therefore at least 9 m inside the fence. The ANSI / IEEE set checks the road's width and the fence offset, 7 m, against the safety clearance zone of the fence (NESC 2007 Rule 110A, Table 110-1).

A switching substation has a second HV yard for its lower level south of the ICT row. It is built like the upper yard and mirrored, so its transformer bays face the transformers and its lines leave south. Its control building stands east of the lower yard, clear of the lines.