From copper quantities to digital samples
In a conventional bay, CT and VT secondary circuits run directly to each relay. In a process-bus architecture, a merging unit samples analogue current/voltage—or accepts native sensor signals—and publishes time-related Sampled Values over Ethernet. Protection IEDs subscribe to the streams and calculate functions digitally. Breaker positions and trips may use GOOSE through bay/process interfaces.
1. Components and responsibilities
| Component | Responsibility | Key failure indication |
|---|---|---|
| Sensor/CT/VT | Convert primary quantity with defined ratio/phase performance. | Open/short circuit, scaling or insulation problem. |
| Merging unit/SAMU | Sample, time-tag, quality-mark and publish values. | Self-supervision, loss of input, time or stream. |
| Time source/network | Deliver synchronization and frames within deterministic limits. | PTP clock state, path delay, packet loss, failover. |
| Protection IED | Subscribe, validate quality, execute protection and trip. | SV loss/invalid quality, algorithm alarm, output failure. |
| Engineering system | Maintain SCL stream and subscriber configuration. | Version/configuration mismatch. |
2. Standards chain
IEC 61869-9 specifies the digital interface for instrument transformers; IEC 61850-9-2 maps Sampled Values over Ethernet; IEC 61850 data/communication parts and SCL support engineering; IEC 61869-13 covers a stand-alone merging unit for conventional analogue inputs. IEC TS 60255-216-1:2025 adds protection-focused requirements and tests for functions that subscribe to SV, GOOSE and time synchronization.
Specify the exact supported profile and implementation. Legacy “9-2LE” project profiles and standards-based profiles may differ in sampling, dataset and configuration; a general “IEC 61850-9-2 compliant” statement is incomplete.
3. Time synchronization
Differential, synchro-check and multi-stream functions require samples aligned within the application tolerance. PTP/IEEE 1588 with a suitable power profile is commonly used. Design grandmaster redundancy, boundary/transparent clocks, holdover, clock-quality monitoring and behavior when time becomes uncertain. Some single-stream local functions can continue without absolute time if internal sample consistency remains valid; multi-device functions may need to block or degrade.
Do not test only the clock display. Force grandmaster/path failures and verify the quality flags, IED alarm and protection response.
4. Network sizing and segregation
SV is continuous high-rate multicast traffic, unlike burst/event GOOSE. Calculate bandwidth from streams, sample rate, frame size, replication and engineering margin. Configure VLANs, priorities, multicast filtering and switch queues. Validate latency and packet loss at the final stream count, including redundancy and abnormal bursts.
Separate process traffic logically or physically from office and uncontrolled maintenance traffic. Monitor for duplicate streams, wrong APPID/MAC, oversubscription and switch configuration drift.
5. Redundancy architecture
PRP duplicates frames over independent LAN A and LAN B; HSR sends frames in both directions around a ring. Both can provide zero recovery for a single network failure when correctly implemented. Merging units, clocks, switch power supplies and IED ports must support the selected architecture. Two networks in one cabinet on one DC fuse are not fully independent.
6. Scaling and data quality
Primary/secondary scaling, phase identity, polarity and channel order move into electronic configuration. Verify each subscriber’s interpretation. Protection logic must evaluate SV quality, synchronization status, test/simulation flags and stream timeout. Define whether invalid data blocks a function, transfers to conventional backup or trips; the correct response differs between applications.
7. Reliability and common-mode risk
Process bus reduces copper and enables flexible data sharing, but one merging unit may become the common input to multiple protections. Decide whether critical functions need independent sensor/merging-unit channels, duplicated streams or a conventional backup. Separate device redundancy from functional diversity: two identical IEDs subscribing to one wrong stream are not independent.
8. End-to-end FAT/SAT
| Test layer | Essential tests |
|---|---|
| Sensor/analogue | Primary ratio, polarity, phase angle, burden/interface and saturation/range. |
| Merging unit | Channel mapping, scaling, sample counter, quality, self-supervision and auxiliary-power cycling. |
| Network | Bandwidth, VLAN/priority, latency, packet loss, multicast control, PRP/HSR failover. |
| Time | Grandmaster loss, alternate clock, holdover, time-quality propagation and recovery. |
| Protection | Pickup/timing using physical and SV injection, invalid-stream response, trip path and breaker operation. |
| Lifecycle | SCD/CID consistency, firmware, certificates, backups, replacement procedure and cybersecurity. |
Use IEC 61850 test/simulation mechanisms with a controlled policy. The final acceptance should trace one test from primary quantity or validated digital source to actual breaker interruption and event records.
9. Migration strategy for existing MV switchgear
A SAMU can digitize conventional CT/VT circuits while retaining existing primary equipment. This is useful for staged migration, but it adds a new active layer and does not remove CT secondary hazards. Plan parallel operation, test isolation and rollback. Avoid permanently operating two unsynchronized protection architectures with unclear ownership of trips.
Related protection guides
Engineering limitation
This guide explains a defensible engineering workflow; it is not a project setting calculation. Final protection functions, settings, wiring and trip logic must be based on the approved single-line diagram, short-circuit and coordination studies, equipment data, grid code, relay manual, and verified commissioning results. Changes require formal protection-management control.
References and further reading
- IEC 61850-9-2:2011+A1:2020 — Sampled Values mapping over Ethernet
- IEC 61869-9:2016 — digital interface for instrument transformers
- IEC 61869-13:2021 — stand-alone merging units
- IEC TS 60255-216-1:2025 — digital protection input/output performance and tests
- SEL — Case Studies of IEC 61850 Process Bus Systems — utility implementation experience
Standards must be applied using the edition required by the project, utility and local law. Standards summaries on public pages are not substitutes for the controlled documents.