IEC 61850 substitution lets an operator replace the value and quality of a status or measured data object with ones written by hand, using four attributes with the SV functional constraint. While it is enabled, clients receive the substituted value instead of the one measured from the process, and the quality flags it with the source "substituted". It is a normal operating action, different from test mode.
What substitution is
Short answer. Substitution lets an operator, using a client, replace the value measured from the process with one written by hand, and with it the quality. While it is enabled, all clients receive the substituted value, and the quality warns that it does not come from the process.
Part 7-2 explains the typical case: an operator manually enters the value of a data object of a device, the client writes it, and whoever reads that data object afterwards, by a read or through a report subscription, receives the substituted value. It applies to status attributes (ST constraint) and measured attributes (MX) and to their quality. An example of use is replacing a failing signal with a known value until the input is repaired, so that calculations and logic are not fed an invalid value.
The standard presents it as part of the normal operation of the substation, unrelated to system or equipment testing, which is done with test mode; see test mode and simulation.
The four attributes
The status and measured data classes may include a substitution block, with functional constraint SV (substitution value). It is optional: it is mandatory only if the device declares that it supports substitution.
| Attribute | Function |
|---|---|
| subEna | Enables substitution; true makes the value of the data object always the substitution value, false returns to the process value |
| subVal (or subMag, subCVal) | Value that replaces the process value; its name and type follow those of the data object |
| subQ | Quality that replaces the quality of the data object |
| subID | Address of the device that made the substitution; null if it is not enabled or not known |
See common data classes to see where the block sits in each class.
What it covers and what it does not
Substitution is limited to the attributes with functional constraint ST (status) and MX (measurement), and to the associated quality. It does not substitute commands or settings. The substitution attributes have the SV constraint, and part 7-2 states that, if their value is volatile, the initial value of the enable must be false; if it is not, the value is whatever was set or configured. It is advisable for the manufacturer to document whether, after a restart, the substitution is kept or lost.
Since the substitution attributes are written with the data write services, they can also be written with the dataset services; the service table of the status classes includes the write dataset values service for the SV attributes. See datasets.
How it is applied
The data write services do the whole operation; there is no dedicated substitution service.
- Write value, quality and identification. With a first write service; no effect yet
- Enable substitution. Second service: the data object delivers the substituted value to clients
- Operation with the substituted value. The quality flags the source as "substituted"
- Disable substitution. The data object goes back to delivering the process value
Two rules of the standard deserve attention:
- The values are written before enabling. That way an old value is never enabled, even briefly.
- If the connection from which it was enabled fails, the substitution continues. It changes only by a service or by local means of the device. There may be local automatic functions that disable it, for example when communication is restored or an information block ends.
If the client does not have direct access to the server that acquires the data, as happens with a gateway, handling of the substitution is a local matter of the proxy. Part 8-1 does not add a mapping of its own: substitution is a specialization of the data.
How it appears in the quality
The quality has a source identifier: “process” or “substituted”. With the substituted source, validity is subordinate to the definition of the substituted value. This is intended: substitution serves to swap invalid values for others the client can use as good, and that is why the source flag is what gives the warning. See quality and time stamp.
The standard illustrates the combinations with a three-level chain:
| Case | What happens | What the upper level sees |
|---|---|---|
| A | Input blocked at the process | Questionable and old data |
| B | Substitution at process level | Substituted, and good |
| C | Communication failure between process and bay, with earlier substitution | Substituted, questionable and old data: the substituted value may be old |
| D | New substitution at bay level | Substituted and good, independent of the earlier substitution |
Risks of leaving a value substituted
- False information in operation. A substituted value that no longer reflects the process feeds displays, alarms, interlocks and calculations as if it were real.
- It stays without anyone noticing. Because it survives the loss of the connection, it depends on a later action. Establish who releases it and when.
- HMI visibility. The standard does not guarantee that the HMI shows the substituted source and who did it (subID); it has to be checked in the project.
- Several levels. It can be substituted at the bay and again at the station, independently; review every level of the chain.
What to do in testing and commissioning
- Verify that every substitutable signal can be substituted and released, and that the quality and source change as expected at all levels. See point-to-point signal testing.
- When closing out commissioning, check that no substituted values are left, neither in the devices nor in the gateways. Include the check in the commissioning checklist and in the dossier.
- Record who substitutes and why. The identification (subID) helps, but it does not replace the record of the intervention.
IEC 61850 signal testing with Tiza Group
Are you about to commission a substation and need to check the quality, the substitution and the source of every signal end to end? At Tiza Group we carry out FAT and SAT of IEC 61850 systems, at medium, high and extra-high voltage, in substations and industry, in Colombia or elsewhere on request.
Related guides
- Quality and time stamp of an IEC 61850 data object
- Test mode and simulation in IEC 61850
- Point-to-point signal testing
- Commissioning dossier
- Common data classes (CDC) in IEC 61850
- IEC 61850 test tools
References
- IEC 61850-7-2 (edition 1, 2003): clause 12, substitution model.
- IEC 61850-7-3 (edition 1, 2003): clauses 6.2.4 and 6.2.8, source and relationship between quality identifiers; attributes subEna, subVal, subQ and subID.
- IEC 61850-8-1 (edition 1, 2004): clause 15, mapping of the substitution model.
Frequently asked questions
What happens to a substituted value if communication is lost with whoever substituted it?
It stays in place. The standard states that if the association through which the substitution was enabled fails, the substituted values remain unchanged, and that changes must come from a service or from local means of the server. That is why a value can remain substituted long after the intervention.
In what order is a substitution written?
First the value, the quality and the identification are written; then it is enabled with the enable attribute. The standard recommends doing it with two separate write services, the first for the values and the second for the enable.
Is substitution the same as putting the device in test mode?
No. The standard makes clear that substituting values is part of the normal operation of a substation and has nothing to do with system or equipment testing. Testing is done by putting a logical device in test mode or by activating the test parameter of the control services.