We represent our electrical systems and configuration on a single document in a single-line diagram. This document represents all the connected physical systems. This article discusses the single-line diagram of a substation. We will start from the basics of the single-line diagram and will study the single-line diagram of a sub-station. Let’s start!
| Component | Symbol & Description |
|---|---|
| Generator | Source of electrical power |
| Busbar | Central node where power is collected/distributed |
| Transformer | Changes voltage levels between systems |
| Circuit Breaker | Switches power ON/OFF and isolates faults |
| Isolator/Disconnector | Opens circuit under no load for maintenance |
| Current Transformer (CT) | Measures current, feeds to protection relays |
| Potential Transformer (PT/VT) | Measures voltage for metering and protection |
| Load (Motor, Lights, etc.) | Final power-consuming equipment |
| Relay | Detects faults and commands breakers to operate |
| Earth/Ground | Connection to ground for safety |
What is an SLD (Single Line Diagram)?
It is the representation of different equipment of electrical systems in a substation with the help of lines and the standard symbol of each equipment. We represent the three-phase system with a single line of our substation, so it is called a single or one-line diagram.
Here are different steps in understanding the SLD:
- The first step in understating the SLD is understanding the structure of the document on which the SLD is drawn. This page shows a bill of material (BOM) or bill of quantity (BOQ). This BOM/BOQ side of SLD shows the equipment and its symbol, the rated voltage of the system, the rated current of the system, the STC (Short-time Current) level of the system, and the insulation level of the system.
- The next part shows the title block including details of the owner’s name, project name, on which date the SLD drawing was prepared, and who prepared the drawing.
- It also shows CVT, PT, and CT details including different core ratios in the case of CT. But is not compulsory that this equipment has to be on SLD. It may or may not be present on SLD.
- The second step is to understand the function of equipment of a sub-station for understanding SLD.
- The third step is to understand the symbol of equipment. These are present on SLD for each equipment. You need to get familiar with these symbols.
- The fourth main step is to understand the circuit/bus bar arrangement of the substation. It can be a single bus bar scheme, double bus bar scheme, main or transfer bus bar scheme, and one and half breaker schemes.
- In step five, you need to identify the incoming and outgoing power lines.
These five steps help us to understand and study the SLD of a sub-station.
SLD of a 33/11kV Sub-station
Now we will discuss the single line diagram of the 33/11kV sub-station. Here are some images with symbols of a 33/11kV substation:
As you can see all equipment symbols are provided in the diagram on one side and then we have a parameters table for this 33’/11kV as:
| Sr. No. | Description | Parameters |
| 1 | Rated voltage (nominal) | 33kV |
| 2 | Rated voltage (maximum) | 36kV |
| 3 | Frequency | 50Hz |
| 4 | Basic insulation level (kV) for (lightning impulse) | 170kV |
| 5 | Short circuit level | 31.5kA (for 3 seconds) |
| 6 | Power frequency withstand voltage | 70kV |
| 7 | Control voltage ac | 3 Phase, 433V |
| 8 | Control voltage dc | 60V DC |
| 9 | CREEPAGE distance | 31 mm/KV |
| 10 | Main bus | 800SQMM ACSR Conductor |
33/11kv sub-stations have a single bus bar scheme. It is the most common type of bus bar scheme that re used in 33/11kV sub-stations.
Let’s start reading SLD from the right side of the diagram step by step.
- On the right side of the following SLD, you can see the incoming 33kV HT (high tension).
- This HT cable is then connected to high voltage lightning arrestor. Here is the SLD of the 33/11kV sub-station:
📈 Applications of SLDs
- Substations (LV/MV/HV)
- Industrial plants
- Commercial buildings
- Power generation plants
- Transmission and distribution networks
🧠 Benefits of Single Line Diagrams
✅ Easy visualization of complex systems
✅ Efficient planning and system analysis
✅ Facilitates fault diagnosis
✅ Essential for safety compliance and maintenance
✅ Used in SCADA and energy management systems
🔧 Tools to Create SLDs
- AutoCAD Electrical
- ETAP
- EPLAN
- Microsoft Visio (with electrical templates)
- SmartDraw, PowerFactory, or SKM
SUMMARY
We have discussed the main parameters while discussing the SLD of sub-stations. There is different equipment and SLD defines a specific symbol for each piece of equipment. Equipment includes CT, PT, dis-connectors, lightning arrestors, circuit breakers, isolators, and transformers. This is all about reading a SLD of a sub-station.
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