In IEC 61869-9 every sample carries its quality attribute. Validity is invalid if the quantity is not delivered, and questionable with outOfRange true from the moment there is clipping until accuracy recovers. Failure and inaccurate come from internal supervision. Quality is independent of time error, which is carried in SmpSynch, and the value is always the best estimate of the primary. The merging unit must not emit erroneous data without marking it.
Short answer
A relay receiving sampled values needs more than the numbers. It needs to know whether it can trust them at that instant. IEC 61869-9, 2016 edition, solves this with a quality attribute on each sample, with precise rules on when each bit is set.
Whoever configures or tests a merging unit must know these rules, because the subscribing relay uses them to block functions and avoid undue operations.
The quality attribute per sample
Clause 6.903.9 takes the quality attribute from IEC 61850-7-3 and restricts it. Each value of each ASDU carries its own quality in the same ASDU. If a channel that was accurate becomes inaccurate, the first inaccurate value must already carry the inaccurate attribute set.
The rules per bit are these:
| Attribute | When it is set |
|---|---|
| Validity invalid | The quantity is not delivered. For example, the unused phases of an old format of four TCTR and four TVTR |
| Validity questionable and outOfRange true | From the moment there is clipping until the input returns to the limits and accuracy recovers |
| Validity questionable | Also when inaccurate is true |
| failure | Transformer supervision detected an error, other than loss of synchronism, that makes the value unusable. For example, an internal hardware fault |
| inaccurate | Supervision detected an error, other than loss of synchronism, because of which the value does not meet the nameplate accuracy class, although it may be useful |
| test | According to IEC 61850-7-3. See the test signal section |
The other quality indicators, overflow, badReference, oscillatory, oldData, inconsistent and operatorBlocked, must be false. The source must be process.
The value is always the best estimate the merging unit has of the primary value. Each subscribing application decides how it uses questionable values.
- Does the merging unit deliver that quantity?
- No: Validity invalid and the other attributes at their default value
- Yes: Did the input exceed the limits of the device (clipping)?
- Yes: Questionable and outOfRange true, until it returns to the limits and recovers its accuracy
- No: Did supervision detect an internal error?
- Fault that makes the value unusable: failure true
- Value does not meet the class but may be useful: inaccurate true and validity questionable
- No errors: Good quality
Clipping, outOfRange and recovery
When the fault current exceeds the range of the device, the output is clipped. The standard asks for the outOfRange bit to be set as soon as clipping appears and to stay set until the input is back within the limits and the output has the accuracy of its class again.
While outOfRange is true, the value must be reported somewhere between the clipping limit and the real value of the input. The behavior of the output with the input beyond the limits must be monotonic: it cannot change sign without a corresponding change in the input. In other words, there is no polarity reversal during clipping.
This mechanism lets subscribing applications use the clipping information where appropriate, for example in the record or in a time-overcurrent function.
What each function does with that information is decided by the subscribing relay, not by the merging unit: the standard says each application chooses how to use questionable values.
Quality and time are different things
The quality attribute refers to the value of the sample, regardless of the error in its instant. Time quality is communicated with SmpSynch, defined in clause 6.904.4.
This has a practical consequence. With the merging unit in free-running mode, the time error can be arbitrarily large. Even so, samples can have good quality and be perfectly usable by subscribers that do not compare phase with other samples. A simple overcurrent relay can keep working. A differential between two merging units must look at SmpSynch. Synchronization is covered in the guide on loss of PTP or IRIG-B time synchronization.
Behavior required of the merging unit
Clause 5.901 makes recommendations based on field experience. The most important:
- Defined behavior under all conditions. Above all at power-up, at power-down and when self-diagnosis signals a fault (according to IEC 61869-6, clause 6.604).
- Consistent data. All data of the same ASDU, including the quality bits, must be consistent with each other and represent the same instant.
- Atomic quality update. The bits are updated at the same time as the data they describe and cannot lag behind it.
- No erroneous data without a mark. The merging unit must not emit bad data without the corresponding mark.
- Received Ethernet traffic must not interfere. This holds for any type of traffic, destination address or receive channel load, with 100 % load and full duplex communication.
- Test signal disabled by default. If it exists, it must be disabled when the equipment is shipped and in every installation in service. When it is active, the test values are accompanied by the test bit.
Start-up and shutdown
The standard gives a clear example. An optical current transformer, when powered on, may need to activate thermoelectric coolers, do a controlled laser start-up and wait for the system to stabilize before operating within its accuracy. During that process, the digital output must be disabled.
If it is enabled, all affected values must be marked invalid and with the detail quality set to failure or inaccurate. The same holds during shutdown due to loss of supply and during activation of self-diagnosis, for example on the failure of a digital processing subsystem.
The data stream can disappear at any moment because a component fails. While it exists, its quality bits must faithfully reflect the state of the LPIT according to its self-diagnosis and external alarm inputs. Relays use those bits, and rely on them to avoid undue operations of the protection scheme.
Indications on the device
If the operator can see the device during normal operation, clause 6.901.3 asks for indicators for five states: powered on, in service, alarm or fault, state of the communication link and test condition. Manufacturers must document each indication.
What not to do
- Do not set validity to good when the value is not delivered. Unused phases are marked invalid.
- Do not treat the test bit as decoration: its signal must be disabled by default.
- Do not confuse sample quality with synchronization state.
- Do not block in the relay every questionable sample without reviewing which functions need it.
Related topics
Sample quality relates to synchronization and free-running mode and to processing delay. The sampling rate and LPIT faults complete the picture. The message format is explained in sampled values.
When to call us
At Tiza Group we test what each subscribing relay does with invalid, questionable and clipped samples, and we check that blocking matches your protection philosophy. We offer IEC 61850 engineering and end-to-end testing, in Colombia or elsewhere on request. If a protection blocked or tripped with sampled values, we quote the relay trip analysis, which we can do remotely, worldwide.
Related guides
- Merging unit synchronization, holdover and free-running mode
- Merging unit maximum processing delay
- LPIT faults, reliability and temperature
- Merging unit: IEC 61869-9, 9-2LE and testing
- Quality and time stamp of an IEC 61850 data point
References
- IEC 61869-9, Instrument transformers - Part 9: Digital interface for instrument transformers, edition 1.0 of 2016, clauses 5.901, 6.901.3 and 6.903.9.
- IEC 61850-7-3, to which clause 6.903.9 refers for the quality attribute.
- IEC 61869-6, 2016 edition, clause 6.604.
Frequently asked questions
When is the outOfRange bit of a sample set?
As soon as clipping starts, that is, when the input exceeds the limits of the device. Under IEC 61869-9, validity becomes questionable and outOfRange becomes true until the input is back within the limits and the output recovers its accuracy.
Does loss of synchronization mark the samples as invalid?
No. The quality of the sample is independent of the error in the sampling instant. Time quality is communicated with the SmpSynch attribute. In free-running mode samples can have good quality and serve subscribers that do not depend on the phase difference with other samples.
What must a merging unit publish while it starts up?
Ideally nothing, with the output disabled until the system stabilizes. If the output is enabled, all affected values must be marked invalid and with the detail quality set to failure or inaccurate. The merging unit must not emit erroneous data without marking it.