Lost Sampled Values on the Process Bus: Causes

Symptom

In a substation with a process bus, the relay raises a loss-of-sampled-values (SV) alarm for one or more merging units and blocks protection functions: busbar differential, transformer differential or the distance protection of a bay. Sometimes the alarm is permanent; other times it appears and disappears with network load or switching operations.

On the process bus, the merging unit digitizes currents and voltages and publishes them as layer 2 multicast frames according to IEC 61850 sampled values. IEC 61869-9 defines the digital interface of instrument transformers, with a preferred rate of 4800 samples per second independent of the grid frequency. Each frame carries a sample counter (smpCnt), the quality of each value and the smpSynch indicator. The subscribed relay uses those fields to know whether the stream is complete, valid and synchronized.

Possible causes, from the most to the least likely

What to check first

1. Relay diagnostics: which stream is missing, since when, and whether the relay sees it as absent, incomplete, invalid or unsynchronized.
2. Merging unit status: power supply, internal alarms, mode and synchronization state.
3. Time synchronization: if several merging units lost synchronization at the same time, the problem is in the clock or the PTP network. See loss of time synchronization and PTP IEEE 1588. On this point, read ALBEDO xGenius Ethernet and IEC 61850 tester.
4. Subscription parameters in the relay against those of the publisher in the SCD. You can check them with our GOOSE and SV address tool.
5. Network: VLAN, priority, port load and the state of PRP or HSR redundancy.

How it is confirmed

• Capture of the SV stream on a mirror port or with an analyzer connected to the network: address, APPID, identifier, revision, sample counter, quality and smpSynch, frame by frame.
• Comparison with the project SCD: publisher and subscriber must match in all stream parameters and in the dataset structure.
• Measurement of losses and latency with the network loaded, including GOOSE traffic and reconvergences.
• Function test with a simulated stream from a test set that publishes SV: if the function operates with the simulated stream, the problem is in the merging unit or the network.

How to keep it from happening again

• Design the process bus with VLANs and priority for the SV and multicast filters on the switches, so each relay receives only the streams it needs.
• Make the network and the clock redundant: PRP or HSR on the process bus and two PTP grandmasters.
• Keep a single SCD as the reference and load publishers and subscribers from it.
• Supervise smpSynch and quality from the SCADA, not just the final relay alarm.
• Use checklists to take relays and merging units out of test mode after every maintenance.

What not to do

• Do not change the configuration revision of the publisher without updating the subscribers.
• Do not connect switches without PTP support on the process bus.
• Do not leave SV streams on the station bus unfiltered: their volume saturates devices that do not need them. See traffic storm in the substation network.
• Do not disable the blocking of functions on SV loss: it would protect with incomplete data.

Sampling profiles and why they are not mixed

There are two references for sampling on process buses. The 9-2LE profile, born from the UCA implementation guideline, defines 80 samples per cycle (4000 per second at 50 Hz and 4800 per second at 60 Hz) and a 256-samples-per-cycle profile for power quality. IEC 61869-9 defines fixed rates per second: 4800 for measurement and protection, 14,400 for power quality. At 60 Hz the 80-samples-per-cycle profile and the 4800-per-second one coincide; at 50 Hz they do not, because 4800 per second is 96 samples per cycle.

This matters because a relay configured for one rate does not accept a stream of another. If a merging unit is replaced with a model that publishes another profile by default, the relay sees frames that reach the port but do not match its subscription. Also, a frame can carry one or several samples (ASDU), so samples per second and frames per second are not the same; check both in the SCD.

The quality of each value

Each sampled value travels with a quality attribute defined in IEC 61850-7-3. The relay evaluates it before using the data:

• Validity: good, invalid, questionable or reserved.
• Quality detail: overflow, out of range, bad reference, oscillatory, failure, old data, inconsistent or inaccurate.
• Source: process or substituted.
• Test: indicates that the merging unit is in test or simulation mode.

A relay can ignore an entire stream because its values carry the test flag, and the symptom in the relay (“stream absent”) does not distinguish between that cause and a physical disconnection. That is why capturing the frames, which shows quality frame by frame, saves time.

The importance of time

Functions that compare currents from several merging units, such as busbar differential, need all samples to refer to the same instant. At 60 Hz, one microsecond of time error equals about 0.0216° of phase, and the interval between samples at 4800 per second is about 208 µs. Synchronization by PTP according to the power profile of IEC/IEEE 61850-9-3 is designed for accuracies on the order of microseconds; smpSynch tells the function whether the merging unit meets it. A switch without transparent clock or boundary clock support adds delay variations that degrade that accuracy without generating any network alarm.

Frames and addresses on the network

SV travel in layer 2 frames with Ethertype 0x88BA and with multicast addresses that, by IEC 61850-8-1 convention, start with 01-0C-CD-04. This is a recommended use, not an obligation, and some projects may use other addresses. GOOSE uses 0x88B8 and 01-0C-CD-01. With that, a capture filter can separate each type of traffic and count frames per stream.

Illustrative example

After replacing a merging unit, the busbar relay blocks its differential function. The capture shows that the frames arrive with the correct address and APPID, but with a different configuration revision: the new unit was loaded with an earlier version of the SCD. When the current file is loaded, the stream is accepted. It is the same mistake described in the guide on an IED that does not communicate over IEC 61850 MMS: the device is configured with a file that is not the project’s.

When to call us

If a relay loses sampled values and it is not clear whether the cause is the merging unit, the network, the synchronization or the configuration, our team remotely reviews the SCD, the subscriptions and the diagnostics of the main brands’ devices. This is a remote service, available worldwide. Learn about remote relay configuration and our IEC 61850 engineering for digital substations. Contact us.

Related guides

• IEC 61850 sampled values (SV)
• Merging unit
• Process bus
• IEC 61850 digital substation
• Non-conventional instrument transformers
• GOOSE not arriving or arriving late
• Loss of time synchronization
• ALBEDO xGenius: Ethernet, PTP and C37.94 Testing

References

• IEC 61869-9, Instrument transformers – Part 9: Digital interface for instrument transformers.
• IEC 61850-9-2, Specific communication service mapping (SCSM) – Sampled values over ISO/IEC 8802-3.
• IEC/IEEE 61850-9-3, Precision time protocol profile for power utility automation.
• IEC 62439-3, Industrial communication networks – High availability automation networks – Part 3: PRP and HSR.

Frequently asked questions