Sampled Values (SV) are one of the core technologies behind modern IEC 61850 digital substations. They allow current and voltage measurements from CTs and VTs to be transmitted as digital Ethernet messages instead of analog signals over copper wires.
If GOOSE replaces binary wiring, Sampled Values replace CT/VT analog wiring.
This article explains how SV works, why it exists, where it is defined in IEC 61850, and how it fits into protection and automation systems using clear examples.
Table of Contents
What Are Sampled Values (SV)?
Sampled Values (SV) are digital data packets that contain real-time measurements of:
- Current (from CTs)
- Voltage (from VTs)
These values are digitized by a Merging Unit (MU) and sent to protection relays using Ethernet multicast.
SV is defined in IEC 61850-9-2, which provides:
- The data model
- The sample encoding rules
- The Ethernet transport format (Ethertype 0x88BA)
- The timing and synchronization behavior
Unlike GOOSE—which sends messages only when something changes—SV is a continuous stream of measurement data.
Typical SV sampling rates
- 80 samples per cycle (standard digital substations)
- 256 samples per cycle (high-speed protection)
At 50 Hz:
- 80 samples → 4,000 samples per second
- 256 samples → 12,800 samples per second
This makes SV extremely fast.
Why Sampled Values Exist
Before SV, substations relied on analog CT/VT wiring, often running hundreds of meters from the yard to the control room. This caused several problems:
- Analog signal distortion over distance
- Electromagnetic interference
- Copper wiring cost
- CT burden and saturation issues
- Safety risks during maintenance
- One CT/VT could feed only one relay
With Sampled Values:
- The Merging Unit digitizes the signal at the switchyard.
- Samples travel through fiber/Ethernet, not copper.
- Multiple IEDs can subscribe to the same SV stream.
- No analog distortion or noise occurs.
- Protection accuracy improves dramatically.
This architecture—called the IEC 61850 Process Bus—is the backbone of digital switchyards.
How Sampled Values Work
To understand SV, think of it as a four-step pipeline:
1. The MU converts analog → digital samples
At the switchyard, the Merging Unit receives analog currents/voltages. It converts them into digital samples at 80 or 256 samples/cycle.
Each sample includes:
- SmpCnt – the sample counter that increments with every sample
- ConfRev – the configuration revision of the data set
- allData – the encoded current and voltage values from the data set
- SmpSynch – indicates whether the sampling is synchronized
- RefrTm (optional) – reference time for the sample period
These elements ensure that the protection relay receives correctly timed, synchronized, and consistently structured measurement samples.
2. The MU builds an ASDU
An ASDU (Application Service Data Unit) contains:
- svID — the ID of the SV stream
- datset — reference to the dataset being published
- smpCnt — sample number
- confRev — configuration revision
- sample[] — the actual sampled values
One ASDU = one set of samples for a timestamp.
3. ASDUs are packaged into an APDU
To reduce bandwidth, the MU may combine multiple ASDUs into one frame.
APDU = the “envelope” carrying 1–N ASDUs.
4. Ethernet frame is sent to subscribers
SV uses:
- Ethertype 0x88BA
- Multicast MAC address (01-0C-CD-04-00-xx)
- 802.1Q VLAN
- High priority (typically 4)
Relays subscribe to the SV multicast stream and process the samples in real time.
Sampled Value Control Block (SVCB)
The Sampled Value Control Block (SVCB) is the IEC 61850 object that controls how a Merging Unit (MU) or a protection relay publishes Sampled Values on the network.
It plays the same role for SV that the GOOSE Control Block (GoCB) plays for GOOSE messaging—but instead of sending event-driven messages, it controls a continuous, high-speed CT/VT measurement stream.
The SVCB defines:
- Which DataSet is transmitted
- The multicast MAC address
- The APPID
- The VLAN ID and priority
- The sample rate (e.g., 80 or 256 samples/cycle)
- The number of ASDUs per APDU
- The configuration revision (ConfRev)
- The svID (stream identifier)
- Whether multicast or unicast is used
- Synchronization mode
SV Message Structure (Simple Breakdown)
An SV Ethernet frame contains:
Ethernet Header
- Destination MAC (multicast)
- Source MAC
- VLAN + priority tag
- Ethertype = 0x88BA
SV PDU
- APPID
- Length
- Reserved fields
- APDU → ASDUs → Samples
ASDU Structure
Includes:
- svID
- datset
- smpCnt
- confRev
- smpSynch (sync flag)
- sample values (currents/voltages)
The figure below shows how an SV Ethernet frame is structured, starting from the MAC header down to the APDU field:
Encoding
All elements are encoded using ASN.1 BER, defined in IEC 61850-9-2.
Encoding for the Basic Data Types
This table specifies how each IEC 61850 data type must be encoded inside Sampled Values (SV) messages.
| Data Type (IEC 61850-7-2) | Encoding in Data Set |
|---|---|
| BOOLEAN | 8 bit set to 0 = FALSE, anything else = TRUE |
| INT8 | 8-bit Big Endian signed |
| INT16 | 16-bit Big Endian signed |
| INT32 | 32-bit Big Endian signed |
| INT64 | 64-bit Big Endian signed |
| INT8U | 8-bit Big Endian unsigned |
| INT16U | 16-bit Big Endian unsigned |
| INT24U | 24-bit Big Endian unsigned |
| INT32U | 32-bit Big Endian unsigned |
| FLOAT32 | 32-bit IEEE 754 |
| FLOAT64 | 64-bit IEEE 754 |
| ENUMERATED | 32-bit Big Endian |
| CODED ENUM | 32-bit Big Endian |
| OCTET STRING | 20 bytes ASCII text, null-terminated |
| VISIBLE STRING | 35 bytes ASCII text, null-terminated |
| UNICODE STRING | 20 bytes ASCII text, null-terminated |
| ObjectName | 20 bytes ASCII text, null-terminated |
| ObjectReference | 20 bytes ASCII text, null-terminated |
| TimeStamp | 64-bit timestamp as defined in IEC 61850-8-1 |
| EntryTime | 48-bit timestamp as defined in IEC 61850-8-1 |
Data types according to IEC 61850-8-1
| Data Type | Encoding |
|---|---|
| BITSTRING | 32-bit Big Endian |
Without the SVCB, the MU would not be able to publish any Sampled Values.
Where SV Fits in the IEC 61850 Communication Stack
Unlike MMS (TCP/IP) and SNTP (UDP/IP), Sampled Values run directly on Layer 2 Ethernet.
Stack Summary
| Service | Layer | Transport | Purpose |
|---|---|---|---|
| Sampled Values | L2 Ethernet | Multicast 802.1Q | CT/VT digital measurements |
| GOOSE | L2 Ethernet | Multicast 802.1Q | Fast events, trips, blocking |
| MMS | L5–7 | TCP/IP | SCADA, reports, configuration |
SV and GOOSE both bypass TCP/IP for maximum speed.
Key Characteristics of Sampled Values
1. Continuous streaming
SV never stops sending. Even with no fault, samples continue.
2. Very high bandwidth
A single MU can generate 20–80+ Mbps.
3. Hard real-time timing
SV must be synchronized to microseconds.
4. Deterministic delivery
SV networks use:
- PRP
- HSR
- Dedicated VLAN
- Priority queueing
5. Multicast distribution
One SV stream can feed:
- 1 protection relay
- Backup relay
- Fault recorder
- Meter
- Wide-area monitoring device
All at the same time.
Sampling Rates: 80 vs. 256 Samples/Cycle
80 samples/cycle
- Default for most substations
- Enough for overcurrent, voltage protection
- Lower bandwidth
256 samples/cycle
- Used for differential or distance protection
- Higher accuracy
- Higher bandwidth
The standard allows even higher rates for special applications.
Network Requirements for Sampled Values
Because SV carries essential protection measurements, the network must be engineered carefully.
Mandatory
- VLAN tagging (802.1Q)
- Priority tagging (typically 4)
- Multicast filtering
- Deterministic switching
Recommended / Often Required
- PRP or HSR redundancy
- GPS/PTP time synchronization
- Separate VLAN exclusively for SV
- Industrial Ethernet switches Support PTP hardware timestamping
Avoid
- RSTP (too slow)
- Mixed corporate traffic
- Congested networks
A poorly engineered SV network can cause delayed samples — and relay misoperation.
Use Cases for Sampled Values
Sampled Values are used in:
Protection
- Line differential protection
- Transformer differential
- Busbar protection
- Distance protection
- Feeder protection
Measurement
- Power quality
- Metering
- Phasor estimation
Digital Process Bus
- Replacing CT/VT copper wiring
- Providing data to multiple IEDs
Simulation & Testing in Sampled Values (SV)
In IEC 61850-9-2 Sampled Values, each frame includes a simulation flag called S (Simulate). This flag tells subscribers whether the SV message is coming from:
- The real, configured Merging Unit, or
- A test or simulation device (such as a test set or relay testing tool)
Advantages of Sampled Values
Technical
- Higher accuracy
- No CT saturation through cables
- No analog noise
- Time synchronization
- Fewer wiring errors
Economic
- Lower copper cost
- Faster installation
- Smaller control rooms
Operational
- Safer maintenance
- Easier redundancy
- Multiple devices can share the same sample stream
Practical Fault Example (How SV + GOOSE Work Together)
Here’s what happens during a real fault:
1. Merging Unit digitizes CT/VT signals
SV sends continuous samples to protection relays.
2. Relay detects fault
The protection algorithm processes the waveform.
3. Relay sends GOOSE Trip
Trip command is sent in <3 ms.
4. Breaker opens
XCBR.Pos.stVal changes → sent via GOOSE.
5. System records fault waveforms
SV stream captures the disturbance precisely.
In short:
- SV = measurements
- GOOSE = decisions
Both are required for protection to work properly.
Summary Table — GOOSE vs Sampled Values
| Feature | GOOSE | Sampled Values |
|---|---|---|
| Purpose | Fast events (trips, interlocking) | CT/VT analog measurement |
| Data type | Binary/status | Continuous analog samples |
| Behavior | Event-driven | Periodic streaming |
| Speed | ~1–4 ms | Microsecond timing |
| Bandwidth | Low | Very high |
| Transport | Ethertype 0x88B8 | Ethertype 0x88BA |
| Typical source | Relay | Merging Unit |
| Use cases | Trip, block, permissives | Differential, distance, measurement |
| Can replace wiring? | Binary wiring | Analog CT/VT wiring |
Conclusion
Sampled Values are one of the key technologies enabling IEC 61850 digital substations. They replace analog CT/VT wiring with precise, high-speed, synchronized digital samples delivered over Ethernet.
Thanks to SV, modern substations benefit from:
- Higher accuracy
- Better protection speed
- Improved safety
- Lower wiring cost
- True interoperability
- Full digital Process Bus architectures
Understanding Sampled Values is essential for protection engineers, IED technicians, SCADA integrators, and anyone designing future-proof substations.