Fundamental concepts of schematic drawings for true field engineers (hands on HV schemes)

Mastering schematic drawings

Schematic drawings, also known as electrical or circuit diagrams, are essential tools for understanding and designing electrical circuits. Mastering schematic drawing is a fundamental skill for field engineers, allowing them to effectively troubleshoot, maintain, and modify electrical schemes. This technical article serves as a guided exploration through the core elements of schematic drawing, starting with the Single Line Diagram (SLD) and branching into detailed drawings that provide a comprehensive view of electrical circuits.

From the standardized identifiers encapsulated in the ANSI Code to the graphical symbols representing various electrical components, we dissect the language of schematics. Crucial details such as Drawing Number, Panel Name, Revision Number, Project Name, and Page Identification are unraveled as indispensable components enriching the narrative of a schematic.

As we navigate the organized world of schematics, the division of pages into rows and columns for efficient addressing becomes a cornerstone skill, complemented by techniques for cross-referencing and tracking inter-panel schematics.

This article delves into the fundamental concepts of schematic drawings, starting from the very basics and gradually building upon them.

It covers a wide range of topics, including:

1. Single-Line Diagrams: Simplified Representations of electrical systems
2. Symbols Used in Single-Line Diagrams
   1. Current Transformer (CT)
   2. Voltage Transformer (VT)
   3. Circuit Breaker
   4. Isolator
   5. Busbar
   6. Transformer
   7. Cable Sealing End
   8. Earthing Switch
3. Insights into the Single-Line Diagram
4. ANSI Code for Device Identifications: Standardized identifiers for electrical devices
5. Relay and Metering Single-Line Diagram
6. Drawing Number, Panel Name, Revision Number, Project Name & Page Identification:
   Essential information for understanding a schematic
7. Division of Page in Rows and Columns for Addressing:
   Organizing a schematic for efficient referencing
8. Addressing Items/Equipment in Schematics:
   Locating specific components within a schematic
9. Representation of Internal and External Schematics:
   Differentiating between internal and external wiring
10. External Wiring Termination Points:
    Identifying termination points for external connections
11. Representation of Different Devices and Components:
    Graphical representations of various electrical elements in detailed drawings
12. Cross-Referencing and Tracking Inter-Panel Schematics:
    Connecting schematics for complex systems
13. BONUS! Download Complete 132kV Line Relay Panel Drawing (30 Pages, PDF)

1. Single-Line Diagrams:

Simplified representations of electrical systems

Before initiating any power project, it is imperative to commence with the finalization of the Single Line Diagram (SLD), an integral component of the front-end engineering design. Serving as a high-level presentation or a bird’s-eye view of the project, the SLD provides essential details that set the foundation for the entire system.

Within the SLD, key specifications are outlined, encompassing voltage levels, the number of line feeders, transformers and redundancy requirements, as well as details pertaining to current and voltage transformers, busbars, short-circuit ratings, and capacitor banks.

It is strongly recommended, especially for new engineers, to thoroughly review and gain command over the Single Line Diagram before delving into the understanding of schematics or detailed drawings. This foundational step ensures a comprehensive grasp of the project’s overarching structure and facilitates a seamless transition to more intricate aspects of electrical design.

Figure 1 – Single-line diagram of 132kV power substation

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2. Symbols Used in Single-Line Diagrams

2.1 Current Transformer (CT)

A Current Transformer (CT) is a crucial component in electrical systems that measures electric current. It transforms high currents into manageable levels for instruments and devices, ensuring safe monitoring and control. CTs are commonly used in conjunction with protective relays to detect abnormal current conditions, such as overloads or short circuits.

Figure 2 – Current transformer in AIS and symbol used in schematics single line diagram

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2.2 Voltage Transformer (VT)

Also known as a Potential Transformer (PT), a Voltage Transformer steps down high voltages to levels suitable for measurement and protection purposes. VTs are essential in ensuring the safety of connected devices and instruments by providing a reduced, proportional voltage output.

They are commonly employed in metering applications, relaying systems, and obtaining voltage references in control circuits.

Figure 3 – Voltage transformer and its symbol

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2.3 Circuit Breaker

A circuit breaker is an essential component in electrical systems designed to interrupt or break the flow of electric current under both normal operating conditions and during fault conditions. It serves as an on-load device, meaning it can operate while the current is flowing through it.

The primary function of a circuit breaker is to provide a means of disconnecting a circuit from its power source, ensuring the safety of electrical equipment and preventing damage in the event of abnormalities.

The letter ‘M‘ represents that the circuit breaker is motor-operated, and the box represents the circuit breaker mechanism. Furthermore, ‘Q‘ is a letter that is commonly used in schematics to represent a switching devices.

Figure 4 – Circuit Breaker in the field and its symbol used in schematics, cross sign represents the current interrupting capacity of circuit breaker

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2.4 Isolator

An isolator is an off-load device designed primarily for the isolation of specific components or sections within a circuit. Unlike a circuit breaker, which can interrupt the flow of current under both normal and fault conditions, an isolator is specifically intended for operations when the associated circuit is not under load, meaning there is no current flowing through it during the isolation process.

The term “offload device” signifies that isolators are intended to operate only when the electrical circuit is not carrying any current.

Figure 5 – Isolator picture in the yard and its representations in schematics

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2.5 Busbar

Busbars are conductive bars or metallic strips used to distribute electrical power within a substation or switchgear. They serve as a common connection point for multiple circuits and devices. Busbars play a crucial role in maintaining the stability and reliability of electrical distribution by efficiently transmitting electrical power from one point to another.

The design and arrangement of busbars vary based on the specific requirements of the electrical system they serve.