Premium Membership ♕

Learn from experienced engineers and study specialized technical articles, guides & courses. Save 50% on courses with the Enterprise Plan.

Home / Technical Articles / Seven design diagrams that every HV substation engineer MUST understand

HV Power Substation

A substation engineer should have a good understanding of the electrical equipment and layout of HV power substation. It’s also important to understand relationship between protection and the other equipment in the substations and the distribution system. Beside this, it’s also important the performance of relays and the criteria for setting these.

Seven design diagrams that every HV substation engineer MUST understand
Seven design diagrams that every HV substation engineer MUST understand

This technical article, although not intended to cover substation design, includes some basic information on substation equipment layout, and other important design diagrams that a substation engineer should be able to handle without difficulty, in order to ensure reliable and safe work of substation equipment and better appreciation of protection schemes and relay settings, and operational procedures.

Apart from the pure electrical aspects, the design of a substation incorporates several engineering fields, among them civil, mechanical and electronic.

Within the electrical design function, the basic diagrams used are the:

  1. Single-line diagram (SLD)
  2. Substation equipment layout drawings
  3. Diagrams of AC connections
  4. Diagrams of DC connections
  5. Secondary wiring diagrams
  6. Logic diagrams
  7. Cabling lists

A brief mention of these is given in the following paragraphs.

1. Single-line diagrams (SLD)

A single-line diagram shows the disposition of equipment in a substation, or network, in a simplified manner, using internationally accepted symbols to represent various items of equipment such as transformers, circuit breakers and disconnectors, generally with a single line being used to represent three-phase connections.

Often the main data for the HV equipment are included in the diagram. More detailed single line diagrams include such items as the instrument transformers and the protection, measurement and control equipment and their associated secondary wiring.

Single line diagram of 110 kV Olympic substation
Figure 1 – Single line diagram of 110 kV Olympic substation (click to expand SLD)

Go back to contents ↑

2. Substation layout diagrams

Substation layout diagrams provide scale drawings of the location of each piece of equipment in a substation, in both plan and elevation.

While individual utilities may have their own format, there is a high degree of standardization of these types of drawings worldwide, for contractual and tendering purposes.

Figure 2a and b shows the plan and elevation drawings for a typical layout of two 115-kV bays, one for a transmission line and the other for the HV side of a local transformer, connected to a single 115-kV busbar. The equivalent single-line diagram is depicted in Figure 2c.

General layout for two 115-kV bays: (a) general layout, (b) elevation A–A’ and (c) single-line diagram
Figure 2 – General layout for two 115-kV bays: (a) general layout, (b) elevation A–A’ and (c) single-line diagram

Although engineers involved in protection relaying may not be directly involved with layout diagrams, these drawings do show the relationship between various items of primary equipment and the location of those items associated with protection systems.

For example, current and voltage transformers that may be located separately from other items of equipment or placed within high voltage equipment such as circuit breakers.

The protection engineer is thus able to ensure that he can safely locate protection equipment within the substation.

Go back to contents ↑

3. Diagrams of AC connections

A diagram of AC connections generally shows the three-phase arrangement of the substation power equipment, and the AC circuits associated with the measurement, control and protection equipment, in schematic form.

The AC diagrams for a typical substation contain information corresponding to bays for incoming transmission lines, bus section and bus couplers, power transformers and MV feeder circuits. In addition, there would also be diagrams containing information on such items as motors and heating that operate on AC.

The layout of AC connections diagrams should be carried out observing the following points:

Each diagram should include all equipment corresponding to a bay, with breakers, disconnectors and transformers represented by schematic symbols. In CT current circuits, only the current coils of the measurement instruments and the protection relays should be drawn, indicating clearly which coils are connected to each phase and which to the neutral. The polarity of equipment should be indicated on the drawings.

High voltage substation - AC-connections
Figure 3 – High voltage substation – AC-connections (click to expand)

It is useful to indicate equipment whose future installation can be foreseen by means of dotted lines.

Solid-state protection relays should be represented schematically by squares, showing the number of terminals and the method of connecting the wiring carrying the voltage and current signals. The points where a connection to earth exists should also be indicated in this diagram, for example, when the neutral of the measurement transformers is connected in star.

The main nominal characteristics should be marked close to each item of equipment.

Protection relay settings and conditions
Figure 4 – Protection relay settings and conditions

For example, for power transformers, the voltage ratio, power rating and vector group should be provided; for power circuit breakers, the nominal and short-circuit current ratings; the transformation ratios for voltage and current transformers, and the nominal voltage of lightning arresters.

Voltage transformer circuits should be drawn physically separated from the rest of the circuits, and the connections to the coils of the instruments that require a voltage signal should also be indicated.

As a minimum, the AC diagram of a transformer should include all the equipment in the bay between the high voltage busbar and the secondary bushings of the transformer.

Go back to contents ↑

4. Diagrams of DC connections

Diagrams of DC connections illustrate the direct current circuits in a substation and should clearly show the various connections to the DC auxiliary services.

These diagrams contain information corresponding to equipment such as:

  • Circuit breakers and disconnectors,
  • Protection and control systems for transformers, busbars, transmission lines and feeders,
  • Annunciator systems,
  • Motor and heating circuits that operate on DC, and
  • Emergency lighting and sockets.

A diagram of connections for all substation equipment that take supplies from the DC system should be provided.

The positive infeeds are normally shown at the top of the diagram, and the negative ones at the bottom, and, as far as possible, the equipment included in the diagrams should be drawn between the positive and negative busbars.

Because of the considerable amount of protection and control equipment within a substation, it is generally convenient to separate out the DC connections into different functional groups such as control and protection equipment, and other circuits such as motors and heating.

Circuit breaker control circuit (motor spring operated)
Figure 5 – Circuit breaker control circuit (motor spring operated)

It is common practice to draw dotted horizontal lines to indicate the demarcation between the equipment located in the switchgear and that located in the protection relay panels.

It is useful if the signaling and control equipment in the relay and control panel is located in one part of the diagram, and the protection equipment in another part. Every terminal should be uniquely identified on the drawing.

As far as possible, the contacts, coils, push-buttons and switches of each mechanism should be drawn together and marked by a dotted rectangle so that it is easy to identify the associated equipment and its role in the circuit.

The internal circuits of the protection equipment are not shown, since it is sufficient to indicate the tripping contacts and the points of interconnection with other equipment inside a dotted rectangle. Given the complexity of distance relays, it might be necessary to make a separate diagram to indicate their connections to the DC system and the interconnection of the terminals. It is also possible that separate diagrams may be required for transformer and busbar differential protection.

Each power equipment bay should have two DC circuits:

One for feeding the protection equipment and a separate one for signaling purposes and controlling breakers and disconnectors. The two supplies should be kept independent of each other, and care should be taken to avoid connecting any equipment across the two DC supplies.

Go back to contents ↑

5. Wiring diagrams

Wiring diagrams show the interconnection of the multicore cables, for example, between the switchgear and the associated control panels, and the routing of individual wires to the equipment installed in the relay and control panels.

These diagrams are required to facilitate the wiring of the measurement, protection and control equipment at the substation construction stage. The wiring should be carried out in accordance with the layout shown in the AC and DC diagrams.

Protection logic schematic for 115-kV line bay
Figure 6 – Protection logic schematic for 115-kV line bay

It is logical that the layout of the different devices on the wiring diagrams should be as seen from the rear of the relay and control panels, as in practice. Each device should be represented by its schematic, with every terminal located in accordance with its actual position on the panel.

Each conductor should be marked with the same identification code as the terminal to which it is connected, and also marked at each end with the location of the far end of the conductor, according to a predetermined code.

Control protection panel front view
Figure 7 – Control protection panel front view

To make the wiring easier to install, the location of the wires on the wiring diagram should correspond to their proposed location inside the relay and control panel.

In the wiring diagrams, the following elements should be uniquely identified: terminals and sets of terminals, multicore cables that go to the switchgear, conductors that go from individual terminals to equipment located in the relay and control panels, and equipment installed in the relay and control panels.

Multicore cables

Each multicore cable should have an identification number. In addition, every conductor in each cable should be numbered. It is useful if the numbering of multicore cables is carried out consecutively by voltage level. With this in mind, an ample range of numbers should be provided, for example, multiples of 100 for each voltage level, thus ensuring that there are sufficient spare consecutive numbers available for any additional cabling in the future.

All the conductors in the wiring diagram should be marked at each end with the location of the far end of the conductor (bi-directional labeling).

Go back to contents ↑

6. Logic diagrams

These diagrams represent the protection schemes for the different substation bays by means of normalized logic structures in order to show in a structured manner the behavior of the substation protection system for any contingency.

An example of such a diagram for a 115-kV line bay at a substation is shown in Figure 2.

Protection logic schematic for 115-kV line bay
Figure 6 – Protection logic schematic for 115-kV line bay

Go back to contents ↑

7. Cabling lists

Cabling lists provide information on the multicore cables that run between various items of equipment and help to make it easier to verify the substation wiring for maintenance work.

The lists should include the following information:

  • Number, length and type of multicore cable;
  • Color or number of each conductor in the multicore cable;
  • Identification of each end of the conductor;
  • Identification of the equipment at each end of the conductor;
  • The function of the conductor.

Go back to contents ↑

Reference // Protection of Electricity Distribution Networks by Juan M. Gers and Edward J. Holmes (Purchase hardcopy from Amazon)

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Ala
    Feb 23, 2022

    What is a difference between substation physical design and substation protection and control design? What do physical designers do v.s P&C?

    • Tim Corrigan
      Sep 01, 2022

      The physical design has to do with clearances and the ability of the station to withstand environmental stress (ice, seismic, wind, fire clearances, electrical clearances.) P&C is involved with sectionalizing the system when there is a fault or other abnormal condition. P&C design docs explain what devices are able to open a breaker, recloser, or sectionalizer, and when they identify what relays or other devices can do this they explain what relay setting files you need to look into to determine how or why some device tripped.

  2. Fana
    May 27, 2019

    Please help need high port tester up to 75kv

  3. Ahmed Sofiane Amziane
    May 20, 2019

    Thank you Edvard for this precious information.
    It’s well organized.

  4. Bratislav
    Feb 22, 2019

    Сигнални табло! :D Чањи краљу

  5. S. Vasekar
    Dec 23, 2018

    Thanks for systematic information and bird eye view of diagrams about which engineers talks oftenly.

  6. Muralidhar
    Nov 15, 2017

    Sir, I’m muralidhar from India I completed my engineering in the year 2014 as an electrical and electronics engineering .
    I had 2 years of experience in erection works I want to upgrade as an electrical engineer suggest me how to upgrade.

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

Enter Captcha Here :

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge