Bus schemes

A bus scheme is the way the circuits of a substation connect to its buses. It sets how many circuit breakers the substation has and the arrangement of the yard's bays. The CEA Construction Regulations 2022 give the bus scheme per voltage level for an air-insulated (AIS) substation (Reg. 44(2)(a), Table 7).

Read first: Single-line diagram (SLD)

1Bus schemes of the IEC + CEA / CBIP setCopy link to Bus schemes of the IEC + CEA / CBIP set

Figure 1 shows the double main bus, one-and-a-half breaker, single sectionalised bus and single bus schemes with their circuit breakers.

• Double main bus. Each bay has an isolator to each of the two buses and one breaker. A bus coupler breaker joins the buses.
• One-and-a-half breaker. Circuits are paired into diameters of three breakers between the two buses. The design builds a diameter with one circuit with two breakers, with space and foundations kept for the third.
• Single sectionalised bus. One bus in two sections, joined by a bus-section breaker with an isolator on each side. The bays alternate between the sections.
• Single bus. One bus, with no coupler and no bus section. A bus outage disconnects the circuits.

2The scheme per levelCopy link to The scheme per level

In the IEC + CEA / CBIP set the design's default per level follows the CEA / CBIP norm. It is the single sectionalised bus at 33 kV and 66 kV, the double main bus at 132 kV and 220 kV, and the one-and-a-half breaker at 400 kV and 765 kV. Single bus is no level's default. A scheme of the set can be chosen at any level, and the compliance checklist then asks for the choice to be confirmed with the transmission utility.

The ANSI / IEEE set adds the ring bus for an AIS yard: a closed loop with a breaker between each two bus sections, each section supplying one circuit, and no main buses (RUS Bulletin 1724E-300 §4.6.4). The design takes 3 or 4 circuit positions on a ring. Its default is the ring bus at 69 kV to 161 kV and the one-and-a-half breaker at 230 kV and 345 kV.