Single-line diagram (SLD)

A single-line diagram (SLD) shows the substation's electrical connections with one line for the three phases. It names each device, its rating and its place in the circuit. The yard plan, the 3D model and the bill of materials follow the same bays and devices.

1One line for three phasesCopy link to One line for three phases

A substation's circuits have three phases. The SLD draws one line for the three and one symbol per device (Graphical symbols). It shows how the circuits meet the buses, which the bus scheme sets (Bus schemes), and the devices in their order along each circuit.

The SLD of a pooling substation shows the high-voltage (HV) switchyard and the medium-voltage (MV) system. The HV switchyard has the line bays, the transformer bays, the bus coupler or bus section and the future bays. The MV system has one section per transformer low-voltage winding, each with its incomer, its feeders from the plant, its reactive power equipment, a harmonic filter and a bus voltage transformer.

2The order in a bayCopy link to The order in a bay

The devices of a bay are drawn in their order along the circuit (Bays and their equipment):

• line bay, from the line to the bus: surge arrester, capacitor voltage transformer (CVT), line isolator with its earth switch, current transformer, circuit breaker, bus isolators
• transformer bay, from the bus to the transformer: bus isolators, circuit breaker, current transformer, isolator with its earth switch, surge arrester, transformer

Each device carries a designation before its name. In the IEC + CEA / CBIP set the designations are letter codes after IEC 81346-2, with the numbering of DIN 40719-2 that Indian utilities keep. The breaker is −Q0, the bus isolators −Q1 and −Q2, and the line isolator −Q9 with its earth switch −Q8. The current transformer is −T1, the CVT −T5 and the surge arrester −F1.

The ANSI / IEEE set prints the device function numbers of IEEE C37.2-1996 as RUS Bulletin 1724E-300 Appendix F reprints them: 52 for the circuit breaker and 89 for the disconnect switches. Its current transformers, voltage transformers and arresters carry their names only.

3The ratings on the SLDCopy link to The ratings on the SLD

The SLD's notes give the ratings of the voltage level. In the IEC + CEA / CBIP set, the CEA Construction Regulations 2022 set the highest system voltage and the lightning impulse withstand per level (Reg. 45(1), Table 9) and the short-time withstand per level (Reg. 43(1), Table 6). The ANSI / IEEE set takes the rated maximum voltage and the basic lightning impulse insulation level (BIL) of its levels from IEEE C37.04-2018 Table 6, and its rated short-circuit currents from Tables 12 and 16.