What is a Single Line Diagram? A Detailed Overview

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!

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:

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.

1. On the right side of the following SLD, you can see the incoming 33kV HT (high tension).
2. This HT cable is then connected to high voltage lightning arrestor. Here is the SLD of the 33/11kV sub-station:

• Then we have a motorized double brake type dis-connector with one EARTHING switch. Here M represents motorized brake type (Motor operated are more advantageous than manual operations.
• Next, we have 2 core CT (Current transformer). One core is core measuring and the second core is for protection purposes.
•  Then we have a 33kV circuit breaker as you can see in the above SLD.
• Again we have a motorized brake type dis-connector without any EARTHING switch.
• After this, the connection is given to the single bus bar.
• Again after connecting with the bus bar, we have a 33kV motorized brake type dis-connector without any EARTHING switch.
• Then we have a 33kV circuit breaker as you can see in the above SLD.
• Next, we have 3 core CT (Current transformer). One core is core measuring and the remaining 2 cores are for protection purposes.
• Then we have a high voltage lightning arrestor of 30kV.
• Then we have a step-down transformer of 36kV to 11kV. It is accepting 36kV from the incoming side and stepping it down to the 11kV.
• Then again after the transformer, we have a high-voltage lightning arrestor of 30kV. Then supply is going into the 11kV switchyard.
• 11kV will have CT, PT, and dis-connector, and then from that switchyard, the power will go to consumers.  
• It is useful to use lightning arrestors on both sides of the transformer as it will protect the transformer from switching voltages that may appear in the system. But use of a lightning arrestor on both sides of the transformer is not compulsory but it is useful.  
• This is the bay of transformer-1.
• Similarly, you can find the same equipment on the left side as this SLD has 2 transformers so there are two bays of 2 transformers.
• You need to understand all the equipment symbols and connections from the diagram.
• Then in the middle of two bays, we have a bus dis-connector and a circuit breaker. The advantage of a bus sectionalizer is that in case of fault, it can be disconnected from the healthy part. It is 2 sections of bus.

Similarly, you can study the SLD of 132/33kV sub-station with a double bar scheme. It will have a bus coupler to couple one bus to the other bus and switch the load of one bus to another in case of cutting bus 1 for maintenance or to remove faults without any power loss.  

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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