Tricks in designing and analyzing schematics and diagrams of high voltage substations

HV substation schematics & diagrams

High voltage power substations are complex networks of power and control connections, represented by design elements like- Single Line Diagrams, layout and block diagrams, schematics, logic diagrams, schedules, and so many more. Wiring diagrams and schematics, in a sense, are the blueprints of electrical design.

In this article, we aim to familiarize the readers with techniques to design those diagrams and tricks to put them all together to form the core of electrical designs in high voltage power substations. Substation diagrams help ensure that the design norms are followed and serve as a reference for troubleshooting or future extensions.

A case project is used in this article to elucidate the overall design and analytic aspect of substation schematics and diagrams.

Table of Contents:

1. SLD, schematics and wiring diagrams. What’s the difference?
   1. Special addition: Logic diagrams
2. Breakdown of Single Line Diagrams (SLD)
3. Detailing of 132 kV line SLD and protection:
   1. Switchgear operation schematic
   2. Instrument transformer connection schematics
   3. IEDs inputs and outputs
   4. Master trip relay interface
   5. RTU/SCADA interface
   6. Trip signal Logic diagrams
4. Busbar protection scheme
5. Special mention for transformer differential protection
6. Schedules and general arrangement diagrams
7. Bonus! Schematics and diagrams (PDF)

1. SLD, Schematics and wiring diagrams

What’s the difference?

An entire substation design package can be overwhelming, even for seasoned and experienced design engineers. The best trick is to lay a principal concept and build on that. Single Line Diagram, for instance, is the best option to play around with the design aspects and grow into the details.

It is basically a conceptual layout of substations, analogous to a physical component layout that helps the designer illustrate the primary components, basic functionalities, and operational flow without getting too much into details of circuits and wiring diagrams.

Schematics shows the function of circuits rather than replicating the physical layout of wires and connections in the real world.

The next part of the substations design drawing is detailed wiring diagrams that illustrate the actual layout of wires and physical connections between devices. Wiring diagrams try to emulate the essence of the actual components by using their abstract version, retaining the details like orientation, pinouts, connection points, etc.

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1.1 Special addition: Logic diagrams

Logic diagrams are essential parts of the substation drawing package that helps to set up the operation philosophy, which later develops into schematics and wiring diagrams. Multiple tripping and closing functions require a combination or sequence of more than one action, which is where the logic diagrams come in handy.

The main idea is to illustrate the basic logic behind all those operations using common components like AND, OR, and NOR gates.

Suggested Reading – Learn how to analyze wiring diagrams and single line diagram

Learn how to analyze MV switchgear wiring diagrams and single line diagram

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2. Breakdown of Single Line Diagrams

Being the flag bearer of substation design, SLDs require the most attention and time. In terms of function, it is like combining all the decisions of component selection, placement, and functionality in a single piece of paper. Between leaving out some important pieces of information or making it too crowded with redundant information, there is a thin line of balance.

Primary components and protection system requirements depend on the type of function and operation of the substation. For instance, this switching substation has main and transfer bus bars with differential protection as focal point for all operations.

To illustrate primary components and functionality, a detailed principle Single Line Diagram is sufficient.

Figure 1 – Sample SLD for a high voltage switching substation

The selection of busbar schemes, type and number of power transformers, level of protection, etc. influence how a Single Line Diagram shapes up.
Inclusions in sample SLD:

• Description of incoming and outgoing lines
• Switchgear assembly and surge arrestors with ratings
• Description and illustration of CT cores
• ANSI indications for line protection relays and metering
• Arrangement of busbar with the rating
• Arrangement and rating of Bus PTs
• Switchgear assembly for bus coupling
• Switchgear assembly for outgoing lines
• Description and illustration of CT cores
• Power transformer with ratings and vector group illustration
• ANSI indications for differential protection relays and metering
• Medium voltage switchgear assembly with cable ratings

The control and protection sections are only superficially added in the SLD to indicate functionalities without giving too many details and are often covered by separate detailed schematics.