Home / Technical Articles / Protection / Understanding Substation Single Line Diagrams and IEC 61850 Process Bus (Depicting Relay Circuits)

Single Line Diagram (SLD)

The single line diagram (SLD) is the most basic of the set of diagrams that are used to document the electrical functionality of the substation. Its emphasis is on communicating the functions of the power equipment and the associated protection and control system.

Understanding Substation Single Line Diagrams and Their Control and Protection Functions
Figure 1 - Protection engineer setting ABB's relays in power substation (photo credit:

Details about connection and physical location are not as important unless they serve the purpose of communicating function. For example, in Figure 10, the CT polarity marks indicate the direction of the current that a protective element is oriented to, thereby implying a function.

Symbols very similar to Figure 2 and Figure 3 can be seen in Figure 10 which is an example of an SLD.

The challenging task behind the SLD is to include all the necessary data while keeping the diagram easy to read. Therefore the single line may rely on non-intuitive symbology to represent devices since communicating function is so important.

Examples of Symbols Used on One Line Diagrams
Figure 2 – Examples of Symbols Used on One Line Diagrams

Typically, single line or one line diagrams are used to document the configuration of the electrical high voltage circuit of a substation.

Symbols are used to depict the high voltage equipment including: transformers, generators, circuit breakers, fuse, air break switches, reactors, capacitors, instrument transformers, and other electrical equipment. The connections between these pieces of electrical power equipment are shown by solid lines.

On these diagrams the three phase equipment and connections are shown with a single line, thus the basis for the diagram name. Single phase equipment may have the same symbol as a three phase device but will be specifically designated with the phase to which it is connected.

Since three phase devices can be connected in a delta, phase to phase connection, or a wye, phase to neutral connection, symbols are included that indicate the type of connection. This can be a vector representation of the connection or may be indicated by the winding symbol itself.

Three Phase Connection in a Single Line Diagram
Figure 3 – Three Phase Connection in a Single Line Diagram

In some cases a key or basic substation SLD will be used to show just the electrical configuration of the high voltage equipment in the substation. The equipment is shown in the basic physical arrangement but when there are difficulties in showing the equipment in the correct physical orientation and showing the equipment in the correct electrical configuration then the correct electrical configuration is given priority.

Beyond the documentation of the high voltage equipment’s configuration, typically some of the control and protection systems are shown on the SLD in a basic form. The most common additional system to be depicted on the SLDs are the current and voltage transformer circuits.

Both the primary and the secondary of these circuits are shown. In both cases only half of the secondary circuit is shown.

The polarity or delivery half of the circuits for relay operation, not the return circuits, are shown. The secondary circuits for the current transformers are typically shown with solid lines between the devices.

To distinguish difference between the lines for the high voltage circuit and the current transformer circuit, the high voltage circuit is shown with a wider solid line than the current transformer circuit. The devices connected to the current and voltage transformer circuits are often shown with a circle large enough to contain a function number or acronym.

The function numbers and acronyms are listed in the IEEE standard C37.2-2008.


Single Line Diagrams and IEC 61850 Process Bus

Application of IEC 61850 process bus requires a rethinking of how relay circuits are to be shown on an SLD. The merging unit (MU) in a process bus implementation takes analog inputs of voltage and current and digital inputs and converts them to IEC 61850 protocol.

The output is a data stream over a fiber optic connection either to data management equipment or directly to IEDs performing a protection function. In this case, the physical connections to the MU shown on an SLD are unlikely to convey any functional information because the fiber optic connection can carry data concerning voltage, current or digital inputs to the MU.

Knowing what CTs and VTs are feeding an IED can help indicate the protective functions it is performing.

With an MU, you can only tell the set of data that may be feeding the IED, not what data it is using. The protective functions that the IED is performing will not be obvious from the connection alone.

What follows are two examples of how to depict the process bus on SLDs.

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Single Line Process Bus Example A

Previously, there was a one-to-one relationship between the analog measurement (CT or VT) and the input to the IED. Therefore simply showing a connection from a CT to an IED was not only a representation of the physical but also the functional, whatever functions the IED performed had to be based on the analog input.

Now, an MU can have multiple analog signals input to it and then have a single physical output – a fiber optic cable.

So a simple way of showing this is to be consistent with the physical representation, namely CT and VT connections are shown going to the MU but to add text to the fiber input to the IED so the analog input can be followed back to the MU so the function of the IED can be more obvious.

Example A of Merging Unit on Single Line
Figure 5 – Example A of Merging Unit on Single Line

An example of this approach is shown in Figure 5. The MU is labeled as MC#2 and the inputs shown are phase current (CP), ground current (CG), and phase voltage (VP). The IED labeled as 6CB32 is using VP while 3T4 is using CP, CG and VP.

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Single Line Process Bus Example B

Another proposal for representation of the process bus on the SLD is to depict the MU as an optical auxiliary transformer. This retains the practice of showing a one-to-one relationship between analog measurement and the input to the IED.

So the function of communicating the analog voltage or current data to protective relays could be shown as in Figure 6.

These symbols would reflect the physical connection to the current and voltage inputs but would depict the output as data to the subscribing IEDs. Therefore one MU may have both a voltage and current input with output to numerous IEDs. The input to each of these IEDs would be shown separately for each current or voltage.

Symbols for Current and Voltage Output of a Merging Unit and Example B of Current Data Connection to IEDs
Figure 6 – Symbols for Current and Voltage Output of a Merging Unit and Example B of Current Data Connection to IEDs

Figure 6 shows the current data output from an merging unit (MU).

If this is interpreted as a physical depiction it would seem that there were numerous physical connections when in fact there may be a single fiber connection from the MU to the control building.

In addition, because it is current data it is not delivered serially to the IEDs as it would if it were a CT, rather, the data is delivered in parallel to the IEDs. Labeling would allow the association of the function to the correct MU.

In Figure 6, the MU has multiple current and/or voltage inputs therefore labeling will need to address this. Here, it is current element 1 (C1) of merging unit C12 (MUC12) that is being used.

A more detailed representation of the physical connections from the CTs and VTs to the MUs would be shown on the AC schematics and the physical connection from the MU to the IEDs could be show on a process bus architecture drawing.

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Control Functions on the Single Line Diagram

It has been common to show the function of the basic protection circuits and sometimes the control circuits on the SLD by connecting the protective relay circles that enable other devices with dashed lines.

These are the circuits of the response actions, trip and close, which are automatically performed by the protective relays.

An arrow at the receiving end of the dashed lines indicates the direction of the action. The devices that trip or close the high voltage fault interrupting device have dashed lines to the symbols for those devices.

These “control lines” can be seen in Figure 4 pointing at the circuit breakers in the drawing. This method of depicting the relay logic on the SLD has limitations.

The conjunction of two control lines typically depict an OR junction, meaning that either incoming action would result in the same resulting action.

Example A of a Single Line Diagram
Figure 4 – Example A of a Single Line Diagram

The depiction of the logic requiring multiple control actions to be enabled at the same time to accomplish a resulting action, an AND gate, is difficult to depict with this type of documentation. In spite of the shortcoming of this method of logic depiction it has been used for many years and continues to be used.

The advent of user modified control logic in microprocessor based relays challenges the application of this type of relay logic depiction on SLDs.

When the logic of the protection or control circuit is no longer limited to the results of wiring individual relay functions together but is the composite of the user defined logic internal to the relay devices and the external wiring between devices the limitation of the dashed lines to depict the overall protection circuit logic has become unacceptable for many users.

The same evolution in protective relay logic also increased the importance of having a method of detecting the basic overall logic on one diagram.

Prior to the user defined logic in microprocessor based relays, the control schematic provided this overall logic diagram because the logic was created by the wiring of individual functions together.

With the advent of the microprocessor based relay, one output contact can be the composite result of the operation of multiple measuring devices combined with timer and multiple conditional situations. None of this internal complex logic is shown on the typical control schematic.

As a result of these two factors, the limitations of the legacy documentation system and the need to document the internal relay logic along with the external logic, has driven many utilities to change the way that protection relay logic is depicted on the SLDs.

Diagram Comparison of Logic Symbols
Figure 7 – Diagram Comparison of Logic Symbols

One method that has been adopted by some utilities is to depict the basic protection relay logic on the SLD using the traditional Boolean logic symbols or some variation of these symbols.

By using Boolean logic, more complex logic can be depicted than what could be depicted using the dashed line with the arrows and both logic internal and external to the programmable relays can be shown on the same diagram. To facilitate the SLD being understandable by a larger audience at least one utility has adopted symbols used on some generating plants drawings.

These symbols and the more traditional symbols are shown in Figure 7 above.

Figure 8 shows a section of a substation SLD using logic symbols to portray the way the protection and control circuits for the tripping and closing of a circuit breaker are configured.

Section from Substation Single Line
Figure 8 – Section from Substation Single Line (click to expand)

The circuit breaker has two trip coils so the logic for each is shown separately. Both the control logic that is accomplished by inter-device wiring and logic that is accomplish by the custom programming of microprocessor based relays are shown on the same diagram.

Referring to Figure 8 above, the logic inside the dashed box labeled (1M63)62BF5 is custom programmed logic whereas all of the other logic is accomplished with inter-device wiring. The logic shown for device (1M63)62BF5 is a simplification of the complete logic.

The complete logic for this device can be shown on the control schematic for the breaker failure protection. It is important to link the inputs and outputs of this device to the external logic shown on the SLD. There is no local area network (LAN) used for the protection and control circuits at the substation shown in Figure 8.

If there was a LAN the protection and control logic accomplished with signals communicated over the LAN are shown on the same diagram.

Line Relay Symbol for Substation Single Line Diagram
Figure 10 – Line Relay Symbol for Substation Single Line Diagram (click to expand)

The more complex logic like that used in a transmission line pilot scheme are shown in symbols like Figure 9. Figure 9 is the logic for a permissive over reaching transfer trip scheme using relay to relay digital communication.

To simplify the logic for the SLD some of the details of the logic are left out. Some examples of that simplification are the showing of just Zone types and not the individual elements that are combined by logic to detect faults in a Zone and the absence of the timing functions involved in the echo back keying of the permissive trip signal circuit.

With the logic for the protection and control circuits in addition to the primary power circuits, and the current and voltage circuits being shown on the SLD. The SLD can be used to understand the systems being applied in the substation.

The SLD is also a critical link between the schematic diagrams and the relay settings documents in troubleshooting protection and control circuits.

Example B of Single Line Diagram
Figure 10 – Example B of Single Line Diagram (click to expand)

Even though there are commonalities between all single diagrams, any two SLDs from different organizations can look very different. Figure 10 is another example of an SLD but it emphasizes the digital inputs and outputs to each relay along with the use of different texts and additional symbols such as the trip and close descriptions.

But even with these differences, single line diagrams summarize both the power system to be protected and the controls that will operate the power system.

The next level of detail of power system relaying is found in the AC and DC schematics. The AC schematics detail the power system being protected and how it is being measured. The DC schematics detail the controls that operate the power system.

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Reference // Schematic Representation of Power System Relaying by Power System Relaying Committee IEEE Power Engineering Society

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About Author


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 fascilities. Professional in AutoCAD programming. Present on


    Mar 15, 2018

    Can you pl. let me know that according to which Electrical Standard, display of SLD in sub station is a must? You may mail the details on my personal e-mail ID: thanks.

  2. Imtiyaz Rasool Naikoo
    Mar 15, 2018

    Dear sir
    I am an electrical engineer as I m doing six months training in my own state but can u provide me full map information for setting up of 132/33 kVA sub station as I m in search of this

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    Mar 14, 2018

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  4. Zainul haq
    Mar 14, 2018

    I need full advice about low voltage,
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