The evolution of digital substations has transformed power system protection and automation.
At the center of this evolution lies IEC 61850-9-2, titled “Specific Communication Service Mapping (SCSM) – Sampled Values over ISO/IEC 8802-3.”
This standard defines how digitized analog values (currents and voltages) are transmitted over Ethernet between process-level devices—such as Merging Units (MUs), protection relays, and IEDs to Centralized protection and control device — enabling what is known as the process bus.
IEC 61850-9-2 replaces the traditional analog copper wiring between CTs/PTs and relays with deterministic Ethernet communication, forming the backbone of modern digital substations.
Scope and Relation to Other IEC 61850 Parts
IEC 61850-9-2 complements the abstract data and service definitions in IEC 61850-7-2 and maps them to Ethernet (ISO/IEC 8802-3) at the data-link layer.
It extends the work of IEC 61850-9-1, which covered serial unidirectional links, by introducing a full Ethernet-based mechanism for Sampled Value (SV) communication.
The relevant stack relationships are:
| Layer | Reference | Function |
|---|---|---|
| Application | IEC 61850-7-2 / 7-3 | Sampled Value data model |
| Presentation / Session | ASN.1 BER | Data encoding |
| Transport / Network | Direct mapping (no TCP/IP) | |
| Data Link | ISO/IEC 8802-3 | Ethernet framing |
| Physical | 100Base-FX (Optical Fiber) | Recommended for EMI immunity |
Core Concept – The Sampled Value Model
IEC 61850-9-2 defines the exchange of Sampled Values (SVs), which represent instantaneous analog measurements—typically currents and voltages—from instrument transformers.
These SVs are produced by Merging Units (MUs), digitized at defined sampling rates (e.g., 80 samples per cycle), and multicast over the Ethernet process bus to subscribing IEDs such as protection relays or measurement devices.
Components Involved
- Merging Unit (MU): Digitizes analog signals and publishes SV streams.
- IED (Subscriber): Subscribes to SV streams and processes data for protection and control.
- Process Bus Network: Ethernet infrastructure carrying SV and GOOSE frames.
Data Structure
SV data sets are defined in Logical Node LLN0 and described in the IED Configuration Description (ICD) file, as specified in IEC 61850-6.
Each dataset may include data objects from multiple logical nodes, supporting flexibility and interoperability.
Communication Profile and Ethernet Mapping
The SV communication profile uses direct Ethernet mapping without TCP/IP overhead for low latency.
VLAN Tagging and Priority
IEC 61850-9-2 adopts IEEE 802.1Q VLAN tagging to prioritize SV frames:
- Default VLAN ID: 0
- Default Priority: 4 (high priority for time-critical traffic)
This ensures SV traffic is separated from non-critical data such as engineering or SCADA communication.
Ethertype and APPID
- Ethertype: 0x88BA identifies IEC 61850 Sampled Values frames.
- APPID (Application Identifier): Distinguishes SV message streams; default range 0x4000 – 0x7FFF.
Each APPID must be unique per MU to avoid misdelivery of frames.
Frame Format
A complete SV Ethernet frame includes:
- Destination MAC (typically multicast 01-0C-CD-04-XX-XX)
- Source MAC (unique per MU)
- VLAN Tag + Priority
- Ethertype (0x88BA)
- APPID + Length
- APDU (Application Protocol Data Unit) encoded in ASN.1 BER
Sampled Value Control Blocks
The Sampled Value Control Block (SVCB) governs how SV data sets are transmitted.
Two types are defined:
| Type | Name | Direction | Typical Use |
|---|---|---|---|
| MSVCB | Multicast Sampled Value Control Block | MU → Multiple IEDs | Process bus SV stream |
| USVCB | Unicast Sampled Value Control Block | MU ↔ Single IED | Dedicated testing, commissioning |
Each SVCB defines parameters like:
SmpRate– samples per periodConfRev– configuration revisionSvEna– enable/disable transmissionSmpSynch– synchronization flagDatSet– linked dataset reference
Transmission is time-controlled and constant to support deterministic protection performance.
Data Encoding (ASN.1 BER)
IEC 61850-9-2 specifies ASN.1 Basic Encoding Rules (BER) for encoding SV messages, harmonized with the MMS syntax from IEC 61850-8-1.
An SV APDU (Application Protocol Data Unit) may contain several ASDUs (Application Service Data Units), each carrying the SV data from one dataset.
A typical APDU includes:
svID– Sampled Value stream identifierdatSet– Dataset referencesmpCnt– Sample counterconfRev– Configuration revisionrefrTm– Reference timestampsmpRate– Sampling ratesequence of data– Measured values (currents/voltages)
This encoding ensures interoperability and compactness, essential for high-speed Ethernet transport.
Process Bus Architectures
Annex B of the standard defines several process bus configurations to balance reliability and performance:
| Alternative | Structure | Notes |
|---|---|---|
| 1 | Separate bus per bay + station-wide backbone | Common for medium substations |
| 2 | Bay segments covering multiple bays | Economical, with moderate traffic |
| 3 | Single station-wide bus | Simpler but high bandwidth requirement |
| 4 | Function-oriented bus | Zones organized by protection function |
In practice, redundant Ethernet rings or PRP/HSR (IEC 62439-3) are implemented for fault tolerance.
Conformance and Testing
IEC 61850-10 defines how conformance tests verify device implementation of IEC 61850-9-2.
Tests ensure interoperability and correct mapping between the abstract model (7-2/7-3) and the real communication stack.
Key test aspects include:
- Verification of SVCB parameters and transmission control.
- Frame format validation (Ethertype, VLAN, APPID, BER encoding).
- Latency and determinism under nominal and load conditions.
- Negative tests for malformed frames and reconfiguration handling.
Test documentation must include:
- Configuration files (ICD/SCD).
- PICS, PIXIT, and MICS statements describing implementation scope.
- Full traceability of test cases and results.
Engineering Considerations
Network Design
- Use dedicated VLANs and switches for SV traffic.
- Apply QoS priority ≥ 4.
- Use optical 100Base-FX or 1000Base-LX links for EMI immunity.
- Ensure end-to-end latency < 1 ms for protection SVs.
Synchronization
- IEEE 1588 PTP or equivalent time reference is mandatory for deterministic sampling.
- Loss of sync should trigger an alarm via
SmpSynch.
Cybersecurity
Although encryption is not defined at process-bus level, isolation and integrity are essential:
- Use dedicated VLANs, firewalls, and access control for process bus traffic.
- Apply IEC 62351 for higher-layer security where applicable.
- Monitor multicast MAC activity to detect anomalies.
Practical Engineering Example
A 132 kV substation bay implementing IEC 61850-9-2:
- 2 Merging Units, each generating SV streams at 80 samples/cycle.
- 6 IEDs subscribing to SV frames.
- Redundant PRP networks.
- Total network load ≈ 12 MB/s per bay.
The design ensures deterministic delivery with < 1 ms latency and no single point of failure.
Conclusion
IEC 61850-9-2 standardizes the transmission of Sampled Values over Ethernet, enabling real-time, interoperable communication between substation devices.
Its key engineering implications include:
- Deterministic, low-latency Ethernet transmission (0x88BA Ethertype).
- Well-defined SV Control Blocks for unicast/multicast streaming.
- VLAN prioritization and process bus segmentation.
- Conformance testing per IEC 61850-10 to ensure interoperability.
IEC 61850-9-2, together with parts 8-1 and 10, provides the foundation for fully digital substations—reducing copper wiring, improving protection speed, and paving the way for future smart-grid architectures.