Electrical Single Line Diagram

 

What is meant by Single Line Diagram (SLD)?

Single Line Diagram (SLD) also called as One line Diagram, is Graphical representation of the electrical power system. Approved/Standard symbols will be used to represent the electrical components such as Transformers, Breakers, Protective relays and bus bars. Mostly, Nowadays SLD is used in the three phase power system applications.

Why Single Line Diagram (SLD) is required?

• Easy to interpret the three-phase electrical system and Power flow
• To calculate the short circuit current and analyze the power system coordination study
• Easy to map and identify the flaws in the system
• Simple method with less drawing space which can be carried out by engineer at site
• Assist to write a System Sequence of Operations with failure modes
• To monitor the entire system operations through PLC and HMI/SCADA screens

Who creates the Single Line Diagram (SLD)?

Single Line Diagram (SLD) is created by Electrical design engineer / Electrical engineering contractor who knows the system very well.

How to create the Single Line Diagram (SLD)?

An electrical engineer should be familiar with all standard electrical symbols before he creates an SLD.

Let’s have a look on the below SLD and learn how to represent the system.

We have divided into two images for easy understanding.

 

Image:1

Image 1 shows a sample SLD with one utility transformer, one incomer breaker shown as 52-M1 and Two feeder breakers 52-F1 and 52-F2 which are connected to the respective loads.

Illustration: When transformer is energized, Power will flow through on the circuit. When incomer / Main breaker 52-M1 gets closed, bus-bar becomes energized. Feeder breakers 52-F1 and 52-F2 can be closed to feed the loads.

Image:2  

Image 2 shows the same SLD with ratings updated. It is important to mention the ratings of the equipment’s in the SLD. Let’s see in detail.

Equipment 1: XFMR

XFMR A1 – Represents transformer name / location

2500KVA – Represents size of the transformer

11kV – 0.433kV – Represents primary and secondary voltage

ONAN – Represents type of transformer (Oil Natural Air Natural)

Z – Represents impedance of the transformer

DYN11 – Represents vector group of the transformer

Equipment 2: Incomer Breaker / Feeder breaker

52-M1 – Represents breaker name

3000A – Represents rating of the breaker

ACB – Represents type of the breaker (Breaker types could be VCB, ACB, MCCB, MCB etc.,)

50kA – Short circuit current rating

Equipment 3: Busbar

3000A – Represents rating of the busbar

Al – Represents busbar material (Could be Aluminum or Copper)

50kA/1 sec – Short circuit current rating per second

Whatever we have illustrated here is with only one utility system. System can be with multiple utilities with Tie breaker, One Utility – One generator with tie breaker, Multiple Generator systems. SLD should be designed accordingly.